Ultrafast vibrational predissociation of hydrogen bonds: Mode selective infrared photochemistry in liquids

Using intense ultrashort excitation pulses in the infrared (IR) tuned to the CH or oligomeric OH absorption bands of ethanol dissolved in CCl ₄, vibrational predissociation and partial reassociation of hydrogen bonds are observed on the picosecond time scale. Induced absorption at the OH stretching frequency 3500 cm^-1 of proton donor end groups serves as a spectroscopic probe of the broken H bridges and is studied as a function of time by the help of delayed, tunable interrogation pulses. Different values for the effective dissociation rates and total quantum yields provide strong evidence for mode specifity of the IR photodissociation in the liquid at ambient temperature. The experimental results on vibrational population decay and energy transfer rates suggest a V-V transfer mechanism for CH to OH modes for the vibrational predissociation after CH excitation.