A femtosecond midinfrared pump-probe study of hydrogen-bonding in ethanol

We present a femtosecond midinfrared pump-probe study of hydrogen bonding. It is shown that upon excitation of the OH-stretching vibration of hydrogen-bonded ethanol dissolved in CCl₄, the hydrogen bonds are predissociated on a femtosecond time scale. The measured predissociation time constant depends strongly on the excitation frequency, and ranged from ~250 fs at 3330 cm^-1 to ~900 fs at 3450 cm^-1. The time constant of the subsequent reassociation of the hydrogen bonds was found to be 15 ps, in accordance with previous picosecond studies. Furthermore, polarization-resolved measurements show that orientational relaxation takes place on a time scale much shorter than the pulse length of ~200 fs. This rapid orientational relaxation can be explained from the fast delocalization of the O-H stretching excitation over the hydrogen-bonded ethanol oligomers. The orientational anisotropy R reaches a value of 0.15 instantaneously, and remains constant for all delays.