Vibrational Förster transfer in ice Ih
We have studied Förster energy transfer between O−H vibrations in H2O/D2O ice Ih using femtosecond, two-color, mid-infrared pump−probe spectroscopy. We found that as a result of couplings to nearby O−H stretch modes, the vibrational relaxation time decreases from 480 fs for dilute HDO in D2O down to 300 fs for pure H2O ice. The anisotropy shows an initial 140 fs decay down to a concentration-dependent end level. This end level for low concentrations can be explained from the limited rotational freedom (20°) of a water molecule in the ice lattice over time scales > 15 ps. The decreasing end levels for higher concentrations of H2O result from Förster energy transfer to the next-nearest six O−H groups. No Förster transfer beyond these neighbors is observed. Variation of the ice temperature between 200 and 270 K was found to have negligible effect on the dynamics.