PROBING THE NO2-]NO-STATE VIA TIME-RESOLVED UNIMOLECULAR DECOMPOSITION(O TRANSITION)

Time resolved, subpicosecond resolution measurements of photoinitiated NO2 unimolecular decomposition rates are reported for expansion coole d and room temperature samples. The molecules are excited by 375-402 n m tunable subpicosecond pulses having bandwidths greater-than-or-equal -to 20 cm-1 to levels which are known to be thorough admixtures of the B-2(2) electronically excited state and the 2A1 ground electronic sta te. Subsequent, decomposition is probed by a 226 nm subpicosecond puls e that excites laser-induced fluorescence (LIF) in the NO product. Whe n increasing the amount of excitation over the dissociation threshold, an uneven, ''step-like'' increase of the decomposition rate vs energy is observed for expansion cooled samples. The steps are spaced by app roximately 100 cm-1 and can be assigned ad hoc to bending at the trans ition state. Relying on experimental estimates for the near threshold density of states, we point out that simple transition state theory pr edictions give rates that are consistent with these measured values. T he rates are sufficiently rapid to question the assumption of rapid in tramolecular vibrational redistribution, which is implicit in transiti on state theories. In contrast to expansion cooled samples, room tempe rature samples exhibit a smooth variation of the reaction rate vs phot on energy. By comparing rates for rotationally cold and room temperatu re NO2, the ON-O bond is estimated to be approximately 40% longer in t he transition state than in the parent molecule.