A FEMTOSECOND MIDINFRARED PUMP-PROBE STUDY OF HYDROGEN-BONDING IN ETHANOL

We present a femtosecond midinfrared pump-probe study of hydrogen bond ing. It is shown that upon excitation of the OH-stretching vibration o f hydrogen-bonded ethanol dissolved in CCl4, the hydrogen bonds are pr edissociated on a femtosecond time scale. The measured predissociation time constant depends strongly on the excitation frequency, and range d from similar to 250 fs at 333 cm(-1) to similar to 900 fs at 3450 cm (-1). The time constant of the subsequent reassociation of the hydroge n bonds was found to be 15 ps, in accordance with previous picosecond studies. Furthermore, polarization-resolved measurements show that ori entational relaxation takes place on a time scale much shorter than th e pulse length of similar to 200 fs. This rapid orientational relaxati on can be explained from the fast delocalization of the O-H stretching excitation over the hydrogen-bonded ethanol oligomers. The orientatio nal anisotropy R reaches a value of 0.15 instantaneously, and remains constant for all delays. (C) 1997 American Institute of Physics.