Abstract
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.
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