Ultrafast vibrational energy relaxation of the water bridge

We report the energy relaxation of the OH stretch vibration of HDO molecules contained in an HDO:D₂O water bridge using femtosecond mid-infrared pump-probe spectroscopy. We found that the vibrational lifetime is shorter (∼630 ± 50 fs) than for HDO molecules in bulk HDO:D₂O (∼740 ± 40 fs). In contrast, the thermalization dynamics following the vibrational relaxation are much slower (∼1.5 ± 0.4 ps) than in bulk HDO:D₂O (∼250 ± 90 fs). These differences in energy relaxation dynamics strongly indicate that the water bridge and bulk water differ on a molecular scale.