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.