ultracold polar molecules are attracting 
 considerable attention , 
 and their investigation will extend our knowledge on the physics of 
 ultracold quantum gases into new regimes .
a very interesting class of molecules in this regard is represented 
 by mixed alkali  alkaline earth ( ak  ake ) molecules with 
 both electric and magnetic dipole moments in their 1/2 electronic ground state .
such molecules are proposed for 
 engineering the lattice spin models of condensed matter physics , a 
 state of matter with topological order , that can be used for the realization 
 of a new class of quantum computation .
further applications 
 for ultracold ak  ake molecules are precision metrology with 
 the aim to test fundamental physics constants or 
 quantum state selective ultracold chemistry .
the most promising starting point for the production of ultracold 
 ak  ake molecules are quantum degenerate mixtures of ultracold 
 alkali and alkaline - earth atoms . though mixtures of homo- and heteronuclear 
 alkali molecules have been studied extensively in the past ( see ref ( 1 ) and references therein ) , 
 investigations of ak  ake like mixtures have only been reported 
 for liyb and rbyb .
recent progress in 
 ultracold atomic physics ( e.g. the bose  einstein condensation 
 of alkaline earth metal atoms ca and 
 sr ) suggests that the formation of ultracold ground state 
 ak  ake molecules is within reach , and quantum degenerate mixtures 
 of rb and sr atoms have been successfully realized very recently .
the standard methods to overcome the gap between 
 the interatomic pair distance in an ultracold atomic gas and the final 
 binding length of the diatomic molecule are to convert ultracold atoms 
 to molecules by either photoassociation or magneto - association followed by 
 coherent population transfer to produce ground state molecules .
this 
 approach has been successfully used for the preparation of ground 
 state cs2 , krb , and sr2 molecules by stimulated 
 raman adiabatic passage ( stirap ) . an important requirement for 
 the formation of ultracold molecules 

is the knowledge of their electronic structure , which is crucial for 
 the search for optimal pathways to couple two colliding atoms with 
 their molecular ground state . hence the theoretical as well as
the 
 experimental exploration of excited states can greatly simplify the 
 navigation through complex molecular potentials and can be used for 
 the prediction of transition frequencies and transition probabilities .
the preparation of ak  ake molecules in molecular beams or 
 heat - pipe ovens is complicated , and despite the rising interest in 
 these molecules , only a few spectroscopic data are available . here 

we report on a new method for the production of ak  ake molecules . 

our approach uses helium nanodroplets for the isolation of cold ( 0.37 
 k ) ak  ake molecules in their vibronic ground state in a sequential 
 pickup scheme . in the past 20 years ,
superfluid helium nanodroplets 
 have been established as a matrix for the preparation of tailored 
 molecules .
the method of helium nanodroplet 
 isolation spectroscopy has been used previously for the investigation 
 of a class of molecules very similar to ak  ake molecules , heteronuclear 
 alkali dimers ( and also trimers ) on the surface of helium nanodroplets .
the interaction between 
 the molecule and helium droplet , which manifests itself typically 
 in an asymmetric broadening of vibronic transitions , is relatively 
 small and allows the extraction of molecular parameters of free molecules .
furthermore , the structure of the broadened 
 vibronic transition in the recorded spectrum gives insights into the 
 interaction between molecule and helium droplet .
we demonstrate our approach on the example of lica , a molecule 
 where experimental data as well as calculations are available , which allows a 
 detailed comparison with our results . besides liba , lica is the 
 only system where high resolution spectroscopic data have been obtained .
in addition to the experiment , we present quantum chemical ab initio 
 calculations of the lica molecule .
the majority of available calculations 
 is focused on the ground state because of the outstanding properties and the importance for possible 
 applications of ultracold ground state ak  ake molecules .
gopakumar 
 et al . have recently presented calculations of the lowest two excited 
 states .
the most extensive calculations 
 of lica potential energy curves , including higher states , have been 
 presented by allouche et al . and russon 
 et al . , which can be compared to our 
 results .
the advantage of our approach compared to previous experiments 
 is that the combination of all ak and ake molecules is possible on 
 the droplet with the same experimental arrangement and requires no 
 additional experimental effort .
one of the greatest problems in experiments 
 with heat - pipe oven sources is the large singlet 
 dimer background of the alkali partner of the ak  ake molecule . 

this disadvantage is overcome by helium nanodroplet isolation spectroscopy , 
 which provides a powerful method for the investigation of vibronic 
 transitions of mixed ak  ake molecules .
this manuscript 
 is organized as follows : upon a brief description 
 of the experimental setup and methods we discuss the experimental 
 results .
we present the recorded spectra according to their energetic 
 ordering , starting with the 4  
 x transition . for this transition , 

dispersed emission 
 spectra allow us to draw conclusions on the electronic ground state 
 of lica .
subsequently the 31/2,3/2  
 x transition is discussed where isotope 
 shifts and the spin  orbit splitting could be studied .
the experimental setup is described in detail in refs ( 31 ) and ( 46 ) , and a closer description 
 of the resonance enhanced multiphoton ionization time - of - flight detection 
 can be found in ref ( 47 ) . to give a short overview : the helium droplets are formed by a supersonic 
 jet expansion ; i.e. , precooled helium gas ( t0 = 15 k ) is expanded through a nozzle ( d0 = 5 m ) under high pressure ( p0 = 60 bar ) into the vacuum .
the produced droplet sizes obey 
 a log - normal distribution , and the given 
 source conditions lead to a droplet size distribution maximum of n60,15 = 6000 , which corresponds to a 
 radius of r60,15 = 40  ( assuming 
 spherical droplets ) .
the helium droplet 
 beam is then guided into the pickup chamber where it successively 
 passes through two pickup cells holding small amounts of the corresponding 
 doping elements li and ca .
changing the cell temperature affects the 
 probability of each droplet to pick up one or more atoms .
the optimum 
 temperatures for the formation of lica are tli = 350 c and tca = 370 
 c .
the excitation spectra were recorded using resonance 
 enhanced multiphoton 
 ionization time - of - flight ( rempi - tof ) spectroscopy . in this method 

the lica on the he droplet is excited by a tunable pulsed dye laser 
 ( lambda physik fl 3002 ) and ionized by a fraction of the pump laser 
 power ( radiant dyes rd - exc 200 xecl laser , 26 ns pulse duration , 100 
 hz ) for spectra below 19000 cm or by a second 
 photon of the dye laser for spectra above 19000 cm .
the ion yield is recorded by a time - of - flight mass spectrometer 
 ( jordan d-850 aref ) with angular reflectron .
in addition , the 
 lowest recorded lica transition ( 4  
 x ) was 
 investigated by laser induced fluorescence ( lif ) spectroscopy .
fluorescent 
 light either was monitored with a peltier cooled photomultiplier 
 tube ( hamamatsu r94301 ) or , for dispersed fluorescence emission 
 spectroscopy , was sent through a modified mcpherson eu-700 grating 
 monochromator with an attached ccd camera ( lot - andor idus du401abr - dd ) .
the excitation 
 spectrum of lica 
 on hen , shown in figure 1 , was recorded 
 with rempi - tof spectroscopy in the range 1520025500 cm .
the two lowest recorded band systems ( the 4  x and 31/2,3/2  x transitions ) consist of a progression of vibrational 
 bands with a characteristic asymmetric ( lambda - shaped ) peak form . 
 for transitions into higher states
the vibrational spacing could not 
 be resolved and the bands appear as broadly extended , structureless 
 features in the spectrum . as the density of states increases , the 
 vibrational bands start to overlap and the complexity of the spectra 
 increases . for three transitions ,
experimental data from molecular 
 beam spectroscopy experiments are available and can be compared to 
 our results .
this allows us to draw conclusions 
 on the interaction between the lica molecule and the droplet and the 
 perturbation of molecular states by the droplet . in the following 

we discuss the recorded transitions in detail , followed by a presentation 
 of calculated potential energy curves and transition dipole moments 
 for lica as well as a comparison of calculations and experiments which 
 allows the assignment of the higher excited states .
excitation spectrum of 
 lica on hen as recorded by rempi - tof 
 spectroscopy from 15200 to 25500 cm .
four band 
 systems have been assigned to the 4  x , 31/2,3/2  x , 5  x , and 
 4  x transitions 
 of lica on hen and the peak at 24500 cm is assigned to the two overlapping 6  x and 5 
  x transitions .
the r2pi excitation spectrum of the lica 4  
 x transition is shown in figure 1 in the range 1520016300 cm .
this excited lica molecular state correlates to the li 2s , s + ca 4s3d , d atomic 
 asymptote .
the vibrational levels could be resolved and can be followed 
 from  = 04 .
the absence of hot bands reveals 
 that in the presence of the low - temperature hen environment 
 the lica molecules are cooled efficiently to the lowest vibrational 
 level  = 0 .
hence , upon doping with li and subsequently 
 with ca , the molecule is formed in the vibronic ground state x(=0 ) and the gained bond 
 formation energy is released into the droplet .
the efficient formation 
 of lica demonstrates that the two surface bound species , li and ca , find each other on 
 the droplet surface and a large fraction does not desorb despite the 
 released binding energy of 2605.3(100 ) cm .
the excess energy is carried away by evaporated 
 he atoms , causing a droplet shrinking of about 520 he atoms , if 5 
 cm binding energy of a he atom to the droplet 
 is assumed .
the lica molecule is more 
 strongly bound than the alkali triplet molecules , but weaker than , 
 for example , the sodium singlet dimers ( 5942.6880(49 ) cm  ) , which have been investigated 
 on the helium droplet surface extensively .
the observed peak structure is characteristic for surface located 
 molecules with strongly coupled vibrational motion to the surface 
 of the helium droplet . because of the similarities between li 
 and ca 
helium interaction energies and the similarities between 
 the spectra of surface bound alkali triplet molecules and the lica 
 spectra we expect the lica molecule to reside on the surface of the 
 helium droplet . to increase the signal - to - noise ratio and to 
 avoid saturation effects , we investigated the 4  x transition additionally 
 with lif spectroscopy .
saturation effects can occur due to the relatively 
 high pulse energy of the pulsed dye lasers , which is needed for a 
 reasonable r2pi signal of the molecule on the helium droplet .
the 
 lif excitation spectrum is shown in figure 2 and was recorded with a continuous wave ( cw ) ring dye laser operated 
 with dcm ( 500 mw ) . with lif spectroscopy
the lif spectrum can be compared 
 with the spectrum in figure 1 in ref ( 37 ) .
the intensities of the  = 0 
 and  = 1 peaks in figure 2 match 
 the relative intensities in ref ( 37 ) , which demonstrates that the franck 
condon 
 factors ( fcf ) are not influenced by the interaction with the droplet 
 for this transition .
the vibrational levels obtained for the 4 state can be compared to spectroscopic 
 data of free lica molecules .
band origins for the 
 free molecule are shown as vertical blue lines in figure 2 .
it can be seen that the onset of the rising edge 
 of the lambda - shaped peaks coincides with the free molecule value . 

the broad 
  phonon - wing  is caused by the interaction of the excited 
 molecule with the helium droplet .
the peaks have a fwhm of 3040 
 cm and no zero - phonon line could be observed . 

the peak structure is very similar to the shape of alkali triplet 
 transitions with resolved vibrational levels .
also the bandwidths compare to alkali triplet 
 transitions ( e.g. , for the na2 1g  1u+  30 cm was 
 reported ) . for the case of alkali triplet 
 molecules
it was concluded that a phonon - wing without zero - phonon 
 lines suggests a strong coupling of the vibrational motion of the 
 molecule to the surface of the helium droplet .
hence we conclude that 
 the lica molecule is also strongly coupled to the helium droplet .
the data have 
 been offset corrected , and the red line has been smoothed .
the blue 
 lines denote the free molecule transitions as found in ref ( 37 ) .
the peaks in the lif spectrum were fitted with an asymmetric 
 2-function:1 the 
 maximum of the second derivative of the fit function corresponds 
 to the onset of the rising edge of the peak and hence to the origin 
 of the vibrational band . as has been shown in ref ( 35 ) for lithium triplet molecules 
 on helium droplets , this procedure can be used to determine molecular 
 parameters of free molecules and the values for the origins of the 
 vibrational bands agree within a few cm . in table 1
we compare the experimentally determined vibrational 
 band origins from refs ( 37 ) and ( 39 ) with our 
 fit results .
we include the values obtained from the r2pi spectra 
 for the 30 and the 40 band origins because they agree 
 well with the literature values , indicating that they are not influenced 
 by saturation effects . as shown in table 1 , 
 the results from helium droplet isolation spectroscopy agree very 
 well with the molecular beam and heat - pipe 
 oven experiments , demonstrating the suitability 
 of our method for the determination of molecular parameters and for 
 testing calculated potential energy curves of free ak  ake molecules . 

the molecular parameters te , e , and xee have 
 been calculated from a least - squares fit to the standard expression 
 given in eq 2.2 one standard deviation uncertainties 
 are given in parentheses .
figure 3 shows 
 the 4  x emission of lica 
 molecules upon excitation of the 4  
 = 1 , 2 , and 3 vibrational levels at 15585 , 15857 , and 16130 cm , respectively .
the cw laser was tuned to the phonon - wing 
 maximum of each transition on the helium droplet .
four peaks corresponding 
 to the molecular 01 , 00 , and 11 transitions 
 and the li 2p  2s transition can be seen in the spectra .
the 
 observed fluorescence light in the spectrum originates only from free 
 molecules that leave the droplet upon laser excitation .
fluorescence 
 light could only be detected from the lowest two vibrational levels , 
 indicating a droplet mediated cooling of the lica molecules in the 
 excited states prior to the emission .
similar results have been reported 
 for the emission spectra after the excitation of alkali dimers .
the observation of the li 2p  2s emission indicates that 
 a considerable fraction of the molecules fragments into ca and excited 
 li atoms .
note that excited ca atoms could not be detected in this 
 experiment because , in this energy range , only the lowest metastable 
 triplet p states can be populated upon fragmentation .
their long lifetime 
 ( e.g. , 4.2 s for the ca p1 state , the upper state of the strongest intercombination 
 line ) forbids a detection in our experiment .
a comparison of the three 
 recorded spectra shows that the two signals recorded upon excitation 
 of the  = 1 and  = 2 levels do not differ 
 strongly , except for an increased li atomic emission at the former . 

as can be seen from the spectra , the majority of molecules from which 
 fluorescence light could be detected , are in the vibrational ground 
 state  = 0 .
we conclude that the cold helium environment 
 induces a relaxation of the molecules before they leave the droplet . 

the striking difference at the excitation of  
 = 3 is the absence of transitions originating from  
 = 0
( i.e. , the 00 and 01 line ) accompanied with an 
 increased li atomic emission .
this suggests that in this case for 
 the two competing underlying processes , relaxation and desorption , 
 the latter is faster and prevails .
the observation of an increased 
 li atom signal from the d lines is unexpected because predissociation 
 of the 4 state upon interaction with 
 the crossing 1 state was reported to 
 occur above  = 9 .
we think 
 that this observation is related to the interaction between molecule 
 and helium droplet , where due to the presence of the droplet it could 
 also be possible that the 1 state affects the dynamics 
 of the excited molecule .
our recorded emission spectra demonstrate 
 that the helium droplet isolation technique can be used for the preparation 
 and investigation of free molecules that have desorbed from the droplets 
 upon excitation .
most importantly , the recorded emission spectra give 
 insight into the vibrational levels of the electronic ground state . 

we think that in further experiments these free molecules can be investigated 
 upon formation on the droplet with additional lasers , allowing spectroscopy 
 of cold tailored molecules without restricted resolution due to the 
 interaction with the droplet .
spectra of the 4  x emission of lica molecules 
 formed on helium 
 nanodroplets .
emission was collected upon excitation of  
 = 1 , 2 , and 3 in the 4 state , which 
 is shown as black , blue , and red lines , respectively .
four peaks corresponding 
 to the molecular 01 , 00 , and 11 transitions 
 and the li 2p  2s transition can be seen in the spectra .
figure 4 shows a detailed view of the 31/2,3/2  x transition of lica on 
 hen .
the 3 excited molecular state adiabatically 
 correlates to the same separated atom limit as the 4 state discussed above ( li 2s , s + 
 ca 4s3d , d ) .
the data have been 
 fitted with eq 1 , and the band origins of the 
 vibrational bands correspond to the free molecule transitions and 
 have been obtained by calculating the maximum of the second derivative 
 of the fit .
the fwhm of the peaks is in the range of 80 cm . in table 2
we compare our results with those 
 of a molecular beam experiment , which 
 are , to our knowledge , the only existing experimental data for this 
 transition .
the vibrational spacing as obtained by the fits is within 
 several cm of the literature values .
please note 
 that the values for the vibronic transitions are compared to the 31/2 molecular data , which follows from the 
 fit .
the molecular constants have been obtained by a least - squares 
 fit to the standard expression , eq 2 . in this case
the parameter xee was set to zero , because an inclusion 
 of this parameter resulted in large uncertainties of the molecular 
 parameters .
the free molecule values for the parameters te and e were calculated from the values 
 given in table 2 in ref ( 37 ) .
as can be seen from table 2 , the 
 determined parameters lie well within the one  standard deviation 
 interval and differ only a few cm from the free 
 molecule values .
the relative intensities of the peaks do not allow 
 us to draw conclusions about the franck 
condon factors in this 
 case , because the signal has not been normalized with the relative 
 laser pulse energy over the wavelength range .
close - up of the rempi - tof 
 signal of the 31/2,3/2  x transition . 

plot a shows a comparison of the atomic li and ca ion signals to the molecular lica ion 
 signal .
plot b shows the signal for the different isotopologues of 
 lica , lica and lica , where the latter signal has been scaled by a factor of 15 because 
 of the low abundance of the li isotope . in plot c the 
 effect of spin 
orbit splitting can be seen in the form of a 
 slight kink in the rising edge of each peak , which is situated between 
 the corresponding spin 
orbit split  components 
 of the free molecule transitions , indicated by vertical blue lines .
calculated from values given in 
 table 2 in ref ( 37 ) . to highlight
the effects 
 that can be seen in the recorded signal , 
 figure 4 has been divided into three sections . 

plot a shows a comparison of the atomic li and ca ion signal to the 
 lica molecular ion signal .
atomic calcium follows the trend of lica , 
 whereas the atomic li signal shows only a very weak structure .
the 
 3 potential energy curve is crossed by the 1 curve , which gives rise to a predissociation 
 of the lica molecule , as has been suggested in refs ( 3739 ) .
the 1 curve is purely 
 repulsive above the lowest vibrational level of the 3 
 state ( case c according to mulliken s classification 
 of predissociation cases ) , leading to ground 
 state li atoms and ca atoms excited into their 4s4p , p state in the separated atom limit . as suggested 
 in ref (
we attribute the observation of 
 the large ca ion signal that follows the lica ion signal to predissociated 
 molecules . however , helium droplets are known to induce relaxation 
 processes , which have been observed in the form of intersystem crossings 
 in alkali triplet dimers and quartet 
 trimers or spin relaxation in atoms doped 
 to helium droplets
. the lica molecules will interact 
 with the helium droplets during the predissociation which can lead 
 to atomic fragments in states different from the dissociation products 
 associated with the 1 molecular state . 

hence predissociation of the lica molecule on a helium droplet can 
 give rise to both excited ca and li fragments .
a contribution of ground 
 state li atoms to the li signal is unlikely , because we 
 do not observe a background signal caused by the free atom beam , which 
 is always present in our detection chamber due to our pickup design . 
 in light of this discussion
we think that the following effects are 
 responsible for the signals in figure 4a : the 
 majority of molecules is ionized in a two - photon ionization process 
 and form a stable molecular lica ion .
a fraction of molecules predissociates , 
 resulting in a ca ion signal and also a weak li ion signal caused 
 by the interaction with the droplet . in both cases , for the excited 
 li and ca , two photons are necessary for the ionization .
a competing 
 process is the fragmentation of the molecule upon absorption of a 
 third photon , which will also contribute to both the li and the ca 
 ion signal . at the 3  x lica transition
we were able to separate the 
 signals of lica and lica and thus obtain 
 a spectrum for both isotopologues , which is shown in figure 4b .
despite the very weak lica signal , due to the low abundance of li ( 7.4% ) , the 
 isotope shift can be clearly seen and the trend of an increasing shift 
 for higher excited vibrational levels is obvious .
the isotope shift 
 is well comparable to the values given in ref ( 37 ) , as shown in table 2 .
also the molecular parameters te and e for both isotopologues are in 
 good accordance with the literature values . as has been shown 
 in ref (
orbit 
 ( so ) constant and hund s case ( a ) is appropriate for the description 
 of the coupling of spin and orbital angular momentum with the molecular 
 axis .
the 3 state splits into two spin  orbit 
 sub - bands . due to the broadening of the lines by the interaction with 
 the he droplet , they are hard to resolve . despite this , in our data 
 a small effect of the spin 
orbit splitting ( so constant a0 = 13.3 cm , for  = 0 ) can be seen in the form of a slight kink in 
 the rising edge of each vibrational band in the lica spectrum .
this effect is highlighted in the figure 4c , where the kink can be seen for each vibrational 
 level  = 03 between the band origins of the 
 two spin  orbit split  components of the 
 free molecule , indicated by the vertical blue lines .
it is remarkable 
 that the value of the spin  orbit constant seems to be conserved 
 despite the presence of the droplet . assuming the molecule lies flat 
 on the surface of the droplet , the symmetry of the system will be 
 reduced and an effect on the so constant would be expected .
however , the coupling of spin and orbital angular 
 momentum to the intermolecular axis seems to be much stronger than 
 the influence of the droplet which would make the effect very small . 

if on the other hand the molecular axis of the lica molecule was aligned 
 perpendicular to the droplet surface , the symmetry around the internuclear 
 axis would also be conserved .
this would be another explanation for 
 the observed so splitting . in this section 
 the single structures 
 that can be seen in figure 1 in the energy 
 region above 21250 cm are discussed . the band 
 at 22000 cm has been assigned to the 4  x transition , where 
 the upper level adiabatically correlates to li 2s , s + ca 4s3d , d in the separated 
 atom limit .
the transition extends from 22150 to 23100 cm and shows a steep rising edge and a high signal on the low energy 
 side and a broad shoulder to the high energy side .
the 4 state correlates 
 to a ca singlet state , in contrast to the 4 and 3 states described above , which both 
 correlate to the same ca d state .
calculations have shown 
 that the cahe potential for the ca 4s3d , d state has a pronounced minimum for its molecular  
 substate , whereas all molecular substates corresponding to ca 4s3d , d are repulsive or very weakly 
 bound .
the lack of vibrational resolution 
 in the 4  x excitation spectra indicates a stronger interaction of the excited 
 molecule with the droplet surface .
the steep rising edge of the peak 
 indicates that the laser excitation starts at the lowest vibrational 
 transition , which is confirmed by our calculations .
the onset of the 
 rising edge is shifted to the red by 100 cm as 
 compared to the values found in ref ( 37 ) .
our calculations show that the 4  x transition has 
 a very high transition dipole moment with a maximum of the franck 
condon 
 envelope at the 00 transition , which could cause an easy saturation 
 of this transition .
although the shoulder on the high energy side 
 does broaden with increasing laser energy , the steep rising edge does 
 not show a significant energy dependence .
an analysis of the 
 atomic signals again shows that ca follows the lica signal ; however , 
 at this level of excitation there is a large number of crossings and 
 avoided crossings of molecular potentials that , together with the 
 influence of the hen environment , increase the possibility 
 of the lica molecule being predissociated .
the rempi - tof signal shows 
 a weak cahe ion signal that follows the lica ion signal in the region 
 of the 4  x transition ( not shown ) .
we explain this in accordance with the detection 
 of fragments in the 31/2,3/2  
 x transition : he droplets could act 
 as an intermediate and lead from an excited lica*hen system via a lica*he  hen transition state 
 to the formation of ca*he exciplexes .
the observation that a cahe ion signal is detected at the 4  x transition 
 is in agreement with theoretical results in ref ( 66 ) wherein a relatively strong 
 binding of the ca*(d)he  potential 
 was reported .
two more lica transitions have been found , one 
 weak transition 
 between 21250 and 21500 cm and a strong transition 
 between 24000 and 25250 cm .
the assignment of 
 these states to molecular transitions is based on our calculations 
 and will be treated in detail below .
in addition 
 to the experimental investigation we further examine the excited states 
 of lica by means of molecular - orbital - based quantum chemistry using 
 the molpro software package .
the potential 
 energy curves ( pec ) for the lowest 28 states were obtained from multireference 
 configuration interaction ( mrci ) calculations based on complete active space self - consistent field ( casscf ) wave 
 functions .
orbitals and occupation schemes are 
 referring to the program - specific internal ordering of the irreducible 
 representation ( a1/b1/b2/a2 ) .
the 10 innermost electrons of the ca atom were replaced by an 
 effective core potential ( ecp ) of the stuttgart group ( ecp10mdf ) .
further improvement , especially of the energy 
 spacing , was obtained by applying a core polarization potential ( cpp ) 
 with a static dipole polarizability of 3.522 au and a cutoff radius 
 of 1  .
because the matching ecp - basis 
 set did not provide enough basis functions for an accurate description 
 of the excited states in the experimentally relevant energy range , 
 we tested several larger all - electron basis sets for compatibility 
 with the ecp . in a series of benchmark calculations on the atomic 
 excitations of ca we identified the cc - pv5z basis set as the most suitable compromise between accuracy and computational 
 effort .
the rather unconventional procedure of combining 
 an all - electron basis set with an ecp was justified by the improved 
 reproducibility of atomic excitations deviating from experimental 
 values by less than 3.5% .
basis functions with angular momentum larger 
 than g had to be removed due to program - internal limitations .
this procedure led to a basis set consisting of ( 15s9p6d4f3g)/(26s18p8d3f2 g ) 
 elementary functions and [ 7s6p6d4f3 g ] /[8s7p8d3f2
g ] contracted functions 
 for ca / li . the active space consisted of 19 active orbitals ( 9/4/4/2 ) 
 filled with three electrons .
of the 28 calculated states , ( 8/5/5/2 ) 
 states were calculated in the doublet multiplicity and ( 3/2/2/1 ) states 
 in the quartet multiplicity . in table 3
our calculations tend to slightly overestimate the potential 
 depth , which is a known consequence of the application of effective 
 core potentials .
good agreement is found for our results and the theoretical work 
 of allouche et al . and recent experiments 
 of stein et al .
particularly large deviations between different theoretical 
 approaches 
 occur for the x  ground state .
single 
 reference approaches tend to predict shorter bond lengths and reduced 
 binding energies .
recently , a potential depth of 2260 cm at a distance of 3.395  was determined at the coupled cluster 
 level of theory .
obtained 
 a potential depth of 2607 cm at a distance of 
 3.364  , which is in very good agreement 
 with recent experimental investigations .
a comparison to our result for the ground state demonstrates the 
 overestimation of the potential depth .
benchmark calculations using 
 an all electron basis set at the casscf / mrci level with douglas 
kroll 
 correction of the eighth order yielded a potential depth of 2664 cm at a distance of 3.38  .
a calculation using 
 the ecp gave a potential depth of 3057 cm at a 
 distance of 3.36  .
inclusion of the cpp finally leads to the 
 improved value listed in table 3 .
figure 5 contains the pecs of the doublet 
 manifold . for large internuclear distances the 2 and 3 states approach nearly 
 the same value , although the atomic excitations show a difference 
 of about 300 cm .
one asymptotic value lies within 
 0.6% of the experimentally determined value for the li 1s2p , p excitation and remains 
 independent of minor changes to the basis set and changes to the ecp 
 and cpp .
the two states 
 are very close and exhibit an avoided crossing for large internuclear 
 separations .
the 1 and 2 states 
 show the same behavior for long - range and give the wrong order ( figure 5 ) .
the calculated 
 asymptotes show a slight deviation from experimental atomic excitation 
 energies ( taken from the nist database ) .
all remaining excited states in 
 figure 5 converge to atomic excitations of 
 the ca atom .
the states 5 and 6 show 
 an avoided crossing at 4.5  . because the energies obtained for 
 large internuclear separation differ from the atomic states , our result for the position ( internuclear distance ) 
 and energy of the level crossing are not very accurate .
the same is 
 valid for the avoided crossing of the states 4 
 and 5 at 5.5  . only after adding an appropriate 
 cpp ,
the 5 state shows the strong bonding 
 displayed in figure 5 , without the cpp it appears 
 weakly bound , similar to the 2 state .
the 
 transition dipole moments ( tdms ) for the doublet states are 
 presented in figure 6 .
vertical excitation 
 at the equilibrium distance of the ground state into the 4 state has the largest transition dipole moment 
 in the given range .
this transition gives rise to a strong signal 
 in the experimental measurements . among the  states 
 the 4 state
has the largest tdm for a vertical 
 excitation , which agrees well with the experiment .
transitions into 
 the 2 and 3 states have also strong tdms , but they are outside the range 
 of the experimental investigation .
the tdm for the 1 
 state is very small , the 2 state lies outside the 
 range of the experimental investigation .
transitions into the 5 , 6 , 3 , and 5 states have non - negligible 
 tdms and are experimentally observed .
dipole transitions from the 
 ground state into to  states are forbidden by selection 
 rules and have negligible tdms .
transitions from the x state into quartet states are forbidden and the tdm is zero . 

core polarization 
 effects were neglected , but because there is a good agreement with 
 previous results , the error is expected 
 to be small . for the ground state the permanent dipole moment increases 
 continuously with decreasing internuclear separation until it reaches 
 a value of 0.61 au ( 1.55 d ) at 2.7  , and then the value drops . 
 at the equilibrium distance
the permanent dipole moment has a value 
 of 0.50 au , only slightly deviating from the value determined by kotochigova 
 et al .
betafit 2.1 the pecs of experimentally observed states ( 3 , 
 4 , 4 , 5 , 5 , 6 ) were fitted with an analytical function using 1114 
 parameters .
these potential functions were used in the program level 
 8.0 to determine the vibronic levels 
 and the franck  condon factors .
the latter mostly showed strong 
 transitions for the  = 0 states with franck 
condon 
 factors between 0.95 and 0.7 . with increasing vibrational quantum 
 number the franck 
6 states show a maximum of the franck  condon factor of about 
 0.1 at higher vibrational quantum numbers .
the calculated pecs and tdms 
 for the 4  x transition confirm our 
 interpretation of the experimental data .
we find good agreement between 
 calculated and experimentally determined band origins with deviations 
 below 20 cm .
the trend of decreasing transition 
 probability with increasing vibrational quantum number  
 is in accordance with the experimental findings .
the theoretically 
 predicted transition energy for the 31/2,3/2  x transition is too large : 
 the calculated potential energy curve ( figure 5 ) lies above the atomic value in the separated atom limit . despite 
 this inaccuracy in the absolute position , the vibrational spacing 
 and the isotope shifts are well reproduced by the calculations .
figure 10 shows the experimental and the 
 calculated excitation spectra in the range between 21100 and 25500 
 cm .
the theoretical spectrum was obtained by multiplying 
 the fcf with the tdm of the respective state at 3.4  
 .
the observed bands have been assigned to the 5 , 4 , 6 , 
 and 5 states , respectively .
although calculations 
 predict an extremely small tdm for the 5  x transition , a weak signal 
 originating from this transition is experimentally observed .
the 
 theoretically predicted values are at slightly higher energies than 
 the experimental values . the next structure in the spectra
this state was 
 also investigated experimentally by russon et al . , but due to the interaction with the droplet , the vibrational 
 states could not be resolved in the experiment . for the 4 state
the interaction between the helium and the diatomic 
 molecule is relatively strong , which is indicated by the deep potential 
 between a ca atom in this excited state and a he atom , as can be seen 
 in the diatomic potential curves in ref ( 66 ) .
the deviation between the theoretical and experimental 
 line positions can be explained by taking a look at figure 5 .
the two highest  states show 
 an avoided crossing , which means that the 4 state 
 is associated with the p ca atomic limit .
the potential 
 at large internuclear separation is about 300 cm below the atomic value , which corresponds 
 to the difference between the theoretical and experimental values 
 for the 4  x transition . on the basis of our calculations
, we can assign the 
 structure between 24000 and 25000 cm to the 6 and 5 states .
condon factors are in excellent agreement with the experimentally 
 observed structure and the deviation of the position is less than 
 100 cm .
this good correspondence is probably related 
 to the good reproduction of less than 100 cm of 
 the atomic d ( ca ) state at large internuclear separations . 
 because of an avoided crossing the functions that describe the 5 state at large internuclear separations are the same 
 that describe the 4 state at small internuclear 
 distances .
the calculated asymptotic value ( p ca state ) 
 obtained for the 5 state deviates from the exact 
 value . because of the avoided crossing
, we expect that the 4 potential energy curve shows a similar deviation at small 
 internuclear separations .
the main features of this transition can 
 be explained by the 5 state , which has a strong 
 transition dipole moment .
the vibrational states of this transition 
 are not resolved because of the narrow vibrational spacing of e = 75 cm .
the rising edge deviates from 
 the prediction , which can be explained by the additional contribution 
 of the 6 state to the signal . for this 
 structure a significant li ion - signal
this can be explained 
 by the interaction with even higher states . for the limit of separated 
 atoms
the states li 3s , s + ca 4s , s and li 2p , p + ca 4s4p , p would follow , both including an excited 
 li atom . the pecs of states converging to excited li approach the 
 6 and 5 states and 
 for the 6 state an avoided crossing 
 is indicated by the potential form and a discontinuity in the transition 
 dipole moment ( figures 5 and 6 ) .
the red curve shows the smoothed data , the colored 
 vertical lines 
 represent the calculated franck 
the fcfs have been scaled 
 to fit to the signal ; the scaling factors are given in the legend . 

the transitions into the 5 , 5 , and 6 states have not been 
 observed before and are assigned with the help of our calculations . 

the short black vertical lines above the 4  
 x transition refer to values from ref ( 37 ) . the red vertical line 
 near 22000 cm represents our calculated 00 
 band position .
in addition 
 to the experimental investigation we further examine the excited states 
 of lica by means of molecular - orbital - based quantum chemistry using 
 the molpro software package .
the potential 
 energy curves ( pec ) for the lowest 28 states were obtained from multireference 
 configuration interaction ( mrci ) calculations based on complete active space self - consistent field ( casscf ) wave 
 functions .
orbitals and occupation schemes are 
 referring to the program - specific internal ordering of the irreducible 
 representation ( a1/b1/b2/a2 ) .
the 10 innermost electrons of the ca atom were replaced by an 
 effective core potential ( ecp ) of the stuttgart group ( ecp10mdf ) .
further improvement , especially of the energy 
 spacing , was obtained by applying a core polarization potential ( cpp ) 
 with a static dipole polarizability of 3.522 au and a cutoff radius 
 of 1  .
because the matching ecp - basis 
 set did not provide enough basis functions for an accurate description 
 of the excited states in the experimentally relevant energy range , 
 we tested several larger all - electron basis sets for compatibility 
 with the ecp . in a series of benchmark calculations on the atomic 
 excitations of ca we identified the cc - pv5z basis set as the most suitable compromise between accuracy and computational 
 effort .
the rather unconventional procedure of combining 
 an all - electron basis set with an ecp was justified by the improved 
 reproducibility of atomic excitations deviating from experimental 
 values by less than 3.5% .
basis functions with angular momentum larger 
 than g had to be removed due to program - internal limitations .
this procedure led to a basis set consisting of ( 15s9p6d4f3g)/(26s18p8d3f2 g ) 
 elementary functions and [ 7s6p6d4f3 g ] /[8s7p8d3f2
g ] contracted functions 
 for ca / li . the active space consisted of 19 active orbitals ( 9/4/4/2 ) 
 filled with three electrons .
of the 28 calculated states , ( 8/5/5/2 ) 
 states were calculated in the doublet multiplicity and ( 3/2/2/1 ) states 
 in the quartet multiplicity .
we 
 have presented a comprehensive experimental and theoretical 
 study of the lica molecule .
we show that these molecules can be formed 
 very efficiently on helium nanodroplets by using a sequential pickup 
 scheme .
our results represent the first experimental observation of 
 mixed alkali  alkaline earth molecules on helium nanodroplets . 

a comparison of our experimental results for the x , 4 , 3 , 
 and 4 states with those of previous molecular beam and recent heat - pipe oven experiments reveal that the determined molecular parameters of lica 
 on hen lie within a few cm of the gas 
 phase values .
this demonstrates the capability of helium droplet isolation 
 spectroscopy for the characterization of alkali  alkaline earth 
 molecules .
the interaction between droplet and molecule manifests 
 itself in the appearance of phonon wings in the spectra .
they are 
 caused by the coupling of the vibrational motion of the lica molecule 
 to excitation modes of the helium droplet and extend from the vibronic 
 band origin toward higher energies .
for the 4 and 3 states the vibrational spacing in 
 combination with narrow phonon wings allows the separation of vibrational 
 states
. the narrow , lambda - shaped peak form , which is typical for 
 surface bound molecules , indicates a 
 surface location of lica . ab initio quantum chemical calculations 
 of potential energy curves , 
 transition dipole moments , franck  condon factors , and permanent 
 dipole moments support our spectroscopic study of the lica molecule . 

the 19 lowest lying potential energy curves were determined by using 
 a multireference configuration interaction calculation . on the basis 
 of our calculations
, we were able to identify the previously unobserved 
 transitions into the 5 , 5 , and 6 states .
our results 
 for the lower excited states and the ground state of lica agree well 
 with previous calculations and extend the previous works on lica to higher excited 
 states . despite the perturbation of the molecule by the droplet ,
the 
 resolution of the experimental spectra obtained for lica is sufficient 
 to test calculated potential energy curves .
lica has been taken as 
 an alkali  alkaline earth prototype molecule because of the 
 available experimental and theoretical reference data .
the experimental 
 results serve as a proof of principle and demonstrate that formation 
 of alkali  alkaline earth molecules on helium nanodroplets is 
 possible .
our results indicate that the preparation of various tailor - made 
 alkali  alkaline earth molecules on helium droplet will be possible , 
 opening a new route for the characterization of these molecules
. this 
 could be an important contribution for the preparation of ultracold 
 molecules from ultracold atoms , a process which relies on the knowledge 
 of accurate potential energy curves .
both 
 rb and sr atoms are surface bound species and have been well characterized 
 on helium droplets . on the basis of our results for lica , we conclude 
 that the formation of rbsr molecules for the determination of molecular 
 parameter is feasible . beyond the scope of this article
, our results 
 suggest that the molecules which desorb upon excitation can be further 
 investigated with additional lasers , which would overcome resolution 
 constraints and lead to ro - vibrationally resolved spectra .
 we report on the formation of mixed 
 alkali  alkaline earth 
 molecules ( lica ) on helium nanodroplets and present a comprehensive 
 experimental and theoretical study of the ground and excited states 
 of lica . 
 resonance enhanced multiphoton ionization time - of - flight 
 ( rempi - tof ) spectroscopy and laser induced fluorescence ( lif ) spectroscopy 
 were used for the experimental investigation of lica from 15000 to 
 25500 cm1 . 
 the 42+ and 32 states show a vibrational structure accompanied 
 by distinct phonon wings , which allows us to determine molecular parameters 
 as well as to study the interaction of the molecule with the helium 
 droplet . 
 higher excited states ( 42 , 52+ , 52 , and 62+ ) are not vibrationally resolved and vibronic transitions 
 start to overlap . 
 the experimental spectrum is well reproduced by 
 high - level ab initio calculations . by using a multireference configuration 
 interaction ( mrci ) approach 
 , we calculated the 19 lowest lying potential 
 energy curves ( pecs ) of the lica molecule . 
 on the basis of these calculations , 
 we could identify previously unobserved transitions . 
 our results demonstrate 
 that the helium droplet isolation approach is a powerful method for 
 the characterization of tailor - made alkali  alkaline earth molecules . 
 in this way 
 , important contributions can be made to the search for 
 optimal pathways toward the creation of ultracold alkali  
 alkaline 
 earth ground state molecules from the corresponding atomic species . 
 furthermore , a test for pecs calculated by ab initio methods is provided . 
Introduction
Experimental Section
Results
and Discussion
Theoretical Methods and
Computational Details
Conclusion