Combinational Vibration Modes in H₂O/HDO/D₂O Mixtures Detected Thanks to the Superior Sensitivity of Femtosecond Stimulated Raman Scattering

Overtones and combinational modes frequently play essentialrolesin 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 Ramanspectra of H2O and D2O mixtures with femtosecondstimulated Raman scattering (FSRS) and compared the results with calculatedspectra. Specifically, we observed the mode at around 1850 cm(-1) and assigned it to H-O-D bend + rockinglibration. Second, we found that the H-O-D bend overtoneband and the OD stretch + rocking libration combination band contributeto the band located between 2850 and 3050 cm(-1).Furthermore, we assigned the broad band located between 4000 and 4200cm(-1) to be composed of combinational modes of high-frequencyOH stretching modes with predominantly twisting and rocking librations.These results should help in a proper interpretation of Raman spectraof aqueous systems as well as in the identification of vibrationalrelaxation pathways in isotopically diluted water.