Slice imaging study of NO 2 photodissociation via the 1 2 B 2 and 2 2 B 2 states: the NO(X 2 Π) + O( 3 P J ) product channel.

The state-resolved photodissociation of NO 2 via the 1 2 B 2 and 2 2 B 2 excited states has been investigated by using time-sliced velocity-mapped ion imaging technique. The images of the O( 3 P J =2,1,0 ) products at a series of excitation wavelengths are measured by employing a 1 + 1' photoionization scheme. The total kinetic energy release (TKER) spectra, NO vibrational state distributions and anisotropy parameters ( β ) are derived from the O( 3 P J =2,1,0 ) images. For the 1 2 B 2 state photodissociation of NO 2 , the TKER spectra mainly present a non-statistical vibrational state distribution of the NO co-products, and the profiles of most vibrational peaks display a bimodal structure. The β values show a gradual decrease with the photolysis wavelength increasing except for a sudden increase at 357.38 nm. The results suggest that the NO 2 photodissociation via the 1 2 B 2 state proceeds via the non-adiabatic transition between the 1 2 B 2 and X̃ 2 A 1 states, leading to the NO(X 2 Π) + O( 3 P J ) products with wavelength-dependent rovibrational distributions. As for photodissociation of NO 2 via the 2 2 B 2 state, the NO vibrational state distribution is relatively narrow with the main peak shifting from v = 1, 2 at 235.43-249.22 nm to v = 6 at 212.56 nm. The β values exhibit two distinctly different angular distributions, i.e. , near isotropic at 249.22 and 246.09 nm and anisotropic at the rest of the excitation wavelengths. These results are consistent with the fact that the 2 2 B 2 state potential energy surface has a barrier, and the dissociation process is fast when the initial populated level is above this barrier. A bimodal vibrational state distribution is clearly observed at 212.56 nm, in which the main distribution (peaking at v = 6) is ascribed to dissociation via an avoided crossing with the higher electronically excited state while the subsidiary distribution (peaking at v = 11) likely arises due to dissociation via the internal conversion to the 1 2 B 2 state or to the X̃ ground state.