Overtones and combinational modes frequently play essential
roles
in ultrafast vibrational energy relaxation in liquid water. However,
these modes are very weak and often overlap with fundamental modes,
particularly in isotopologues mixtures. We measured VV and HV Raman
spectra of H2O and D2O mixtures with femtosecond
stimulated Raman scattering (FSRS) and compared the results with calculated
spectra. Specifically, we observed the mode at around 1850 cm–1and assigned it to H–O–D bend + rocking
libration. Second, we found that the H–O–D bend overtone
band and the OD stretch + rocking libration combination band contribute
to the band located between 2850 and 3050 cm–1.
Furthermore, we assigned the broad band located between 4000 and 4200
cm–1to be composed of combinational modes of high-frequency
OH stretching modes with predominantly twisting and rocking librations.
These results should help in a proper interpretation of Raman spectra
of aqueous systems as well as in the identification of vibrational
relaxation pathways in isotopically diluted water.