Emission spectroscopy of photodissociating N 2 O 4 excited near 200 nm to the p nb , O p NO 2 * / n s N – N * avoided crossing

These experiments and complementary electronic structure calculations seek to probe the early dissociation dynamics of N2O4 excited in the strong 186 nm ultraviolet absorption band. Laser photons of 199.7, 203, and 205 nm are used to dissociate N2O4 molecules expanded in a free jet. The emission from the dissociating molecules is dispersed in a spectrometer and collected with an optical multichannel analyzer ~OMA!. We observe a strong progression in n3 , the N–N stretching mode, consistent with electronic structure calculations and a previous observation of N–N bond fission leading to NO2 photoproducts in this band. We also observe emission to combination bands in n4 , the torsion, and n5 , the out-of-phase antisymmetric NO2 stretch, which we attribute to vibronic coupling to a ss* excited state configuration. Analysis of our data allows for an assignment of the torsional frequency of N2O4. Around 200 nm nitric acid, nitromethane, and N2O4 all exhibit a similar absorption to an adiabatic surface that changes electronic character from pnb,OpNO2 * at short O2N–R (R5CH3, OH, NO2) internuclear distances to ns* at extended internuclear distances. We compare our present emission spectra of N2O4 with previous emission experiments on nitromethane to understand how the character of the Franck–Condon region of the excited states in these two molecules differ. © 1999 American Institute of Physics. @S0021-9606~99!01442-7#