 (top X-axis) of N2 ice deposited at 8 K, black solid trace. The blue dashed trace is the VUV-absorption cross-section spectrum of gas phase N2 adapted from Mason et al. 38. The fit, red dotted (single bands, see Table 2) and dashed-dotted trace, is the sum of twenty Gaussian profiles. It has been offset for clarity.
VUV-absorption cross section as a function of photon wavelength (bottom X-axis) and VUV-photon energy (top X-axis) of O2 ice deposited at 8 K, black solid trace. The blue dashed trace is the VUV-absorption cross-section spectrum of gas phase O2 adapted from Lu et al. 36. The fit, solid red dotted (single bands, see Table 2) and dashed-dotted trace, is the sum of two Gaussians. It has been offset for clarity.
The ground state of O2 is X and its bond energy is Eb(O−O) = 5.1 eV (Okabe 1978).
The VUV-absorption cross section as a function of wavelength and photon energy is shown in Fig. 5. The O2 ice thickness estimation based on the fringes method, described in Sect. 3, requires a thick ice of about 60 ML. But the VUV-absorption cross-section measurement only works well for thin ices below 200 ML. Therefore, several experiments were performed to ensure that the ice thickness estimation was correct in the case of O2. Solid O2 presents a broad band centered on 141 nm (8.79 eV) in the 118−162 nm (10.50−7.65 eV) region (attributed to the X transition, named Schumann-Runge band). It also presents a feature centered on 177 nm (7.0 eV), according to Mason et al. 38 and Lu et al. 35, which is beyond our spectral range. The resulting fit and Gaussians involved are represented in Fig. 5 by a red dashed-dotted trace. Following Lu et al. 35, there is no obvious requirement for a broad line centered near 161 nm (7.70 eV), but without this band the resulting fit in the 150−162 nm range would not be as good.
The average VUV-absorption cross section has a value of cm2 in the 120−162 nm (10.33−7.65 eV) range. The total integrated VUV-absorption cross section has a value of cm2 nm ( cm2 eV) in the same spectral region. The VUV-absorption cross sections of the O2 ice at Ly-α (121.6 nm) and the molecular hydrogen band emissions (157.8 and 160.8 nm) are cm2, cm2, and cm2. Gas-phase data from Lu et al. 36 were used for comparison with our solid phase data, see Fig. 5. Solid- and gas-phase data have a broad absorption band, but only the gas-phase spectrum presents discrete transitions. The VUV-absorption cross section of O2 in the gas phase has an average value of 4.0 × 10-18 cm2. The O2 gas spectrum was integrated in the 118−162 nm range, giving a value of 3.3 × 10-16 cm2 nm (2.0 × 10-17 cm2 eV), which is higher than the solid-O2 value. The VUV-absorption cross sections of O2 gas at 121.6 nm, 157.8 nm and 160.8 nm are 2.5 × 10-18 cm2, 6.5 × 10-18 cm2, and 4.7 × 10-18 cm2, which is also higher than the solid-phase measurements, with the exception of the Ly-α wavelength (121.6 nm).
Figure 6 shows a comparison of the VUV-absorption cross section for all the species represented in the same linear scale. Because of its low VUV-absorption cross section, the N2 spectrum was multiplied by a factor of 200 for better appreciation. It can be observed that the most highly absorbing molecule around the hydrogen Ly-α (121.6 nm) position is CH4. Solid O2 is most highly absorbing around the Lyman band system. N2 gives the lowest absorption in both cases. All the species absorb at the Ly-α wavelength, with the exception of N2, for which an upper limit was given in Sect. 3.3. CH4 and N2 absorb VUV-photons with wavelengths lower than 150 nm, while O2 absorbs throughout the scanned range. Table 6 compares the VUV-absorption cross sections of all the species in the gas and solid phase as well as an average VUV-absorption cross section in the 120.8−122.6 nm and 132−162 nm range normalized by the Ly-α and Lyman band system photon flux of our MDHL.
Comparison between the VUV-absorption cross sections in the 120−160 nm range of all the species in the gas and solid phase.
VUV-absorption cross section as a function of wavelength (lower X-axis) and photon energy (top X-axis) of all the species in the solid phase at 8 K. The CO2 absorption only occurs at wavelengths shorter than 133 nm, the absorption at longer wavelengths is due to the photoproduced CO during spectral adquisition.
Interstellar and circumstellar ice mantles are composed of several species such as H2O, CO, NH3, CO2, CH4, and CH3OH (Mumma & Charnley 2011, and ref. therein). In some cases, these molecular ice components are mixed in the ice mantle.
The VUV-absorption cross-section spectrum of CO2 presented in this work has the disadvantage of being mixed with the CO produced by the VUV-irradiation of the MDHL used to acquire the VUV-spectroscopy of the species. But on the other hand, our data show that a binary ice mixture containing CO2 and CO can lead to a mixed VUV-absorption spectrum where mixture effects can be appreciated, using this new way of performing VUV-spectroscopy (cf. Wu et al. 2010, 2012), where the band profiles and positions are altered to some extent by mixture effects.
As is commonly observed for example in infrared spectra, multicomponent ice mantles most likely display VUV-absorption features that correspond to the various species, where CO is the most highly absorbing molecule in the VUV among the common ice components. VUV-irradiation of CO2 ice led to a 22% of CO with respect to the starting CO2 column density. CH3OH ice led to 16% of CO for the same UV-fluence, see Paper I. The two production rates are similar but only the VUV-absorption cross-section spectrum of CO2 ice presents CO features. On the other hand, the average VUV-absorption cross section of CH3OH ice is almost ten times larger than the average VUV-absorption cross section of CO2 in the 133−160 nm range.
The VUV-spectrum of CO in a CO2 ice matrix is different from that of pure CO ice, with shifted positions that occur half-way between the gas and solid CO values, see Table 4 and Fig. 3. Therefore, CO in a solid CO2 matrix can be treated as a highly porous CO ice mantle. Infrared spectroscopy of CO2:CO = 20:1 ice shows a similar trend, the CO band shifts from 2138.6 cm-1 (for pure CO ice) to 2139.7 cm-1 and the FWHM increases from 2.5 cm-1 (pure CO ice) to 5.8 cm-1 (Sandford et al. 1988).
Formation of N3 from N2 irradiation with VUV-photons is extremely difficult (Hudson & Moore 2002). Proton and electron bombardment is more effective in the production of the azide radical than VUV-photons (Hudson & Moore 2002; Jamieson & Kaiser 2007). The likely explanation is the low VUV-absorption cross section of N2. Detection of N3 in space might be indicate a cosmic-ray field, or alternatively VUV-photons with energies higher than Ly-α (10.2 eV) that we were unable to study in this work. The latter possibility is very unlikely because much more intense absorption is not expected in the short interstellar radiation field range, from 91.2 to 114 nm, which was not measured in the laboratory.
Regarding the two homonuclear molecules studied, O2 and N2, the former presents a broad absorption band in the 118−160 nm range with a VUV-absorption cross section on the order of 10-18 cm-2, while N2 presents a vibrational structure in the 118−150 nm range with a VUV-absorption cross section on the order of 10-20 cm-2. Using the Pontoppidan 50 approximation of the abundances of nitrogen and oxygen, these two orders of magnitude differences can determine different photochemistry efficiencies for N2 and O2. This supports the hypothesis that a photochemistry richer in O than in N is expected in icy grain mantles, because a significant fraction of the N atoms might be locked as solid N2.
The ice penetration depth of photons with a given wavelength, or the equivalent absorbing ice column density of a species in the solid phase, can be calculated from the VUV-absorption cross section following (6)which can be converted into an ice thickness using Eq. (3), inserting the volumetric density of the ice provided in Table 1: (7)where It(λ) is the transmitted intensity for a given wavelength λ, I0(λ) the incident intensity, N(λ) is the absorbing column density in cm-2, and σ(λ) is the cross section in