Time-resolved vibrationally mediated photodissociation of HNO3: Watching vibrational energy flow

Ultrafast excitation of an O-H stretching vibrational followed by photodissociation of the energized molecules allows direct observation of the time for intramolecular energy redistribution in isolated nitric acid. We excite the first overtone of the O-H stretch vibration in HNO3 with a 100 fs laser pulse. A second, time-delayed pulse preferentially photodissociates molecules having vibrational excitation in modes orthogonal to the O-H stretch, The photodissociation yield increases as a function of time because energy flows out of the initially excited O-H bond into other more efficiently dissociated vibrations. The single exponential time constant for this intramolecular vibrational relaxation is 12 ps, consistent with moderate coupling of the O-H stretch to states close in energy. (C) 1997 American Institute of Physics.