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.