Slice imaging study of NO2 photodissociation via the 12B2 and 22B2 states: the NO(X2Π) + O(3PJ) product channel

The state-resolved photodissociation of NO2 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(P-3(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 (beta) are derived from the O(P-3(J)=(2,1,)0) images. For the 1(2)B(2) state photodissociation of NO2, 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 beta values show a gradual decrease with the photolysis wavelength increasing except for a sudden increase at 357.38 nm. The results suggest that the NO2 photodissociation via the 1(2)B(2) state proceeds via the non-adiabatic transition between the 1(2)B(2) and (X) over tilde (2)A(1) states, leading to the NO(X-2 Pi) + O(P-3(J)) products with wavelength-dependent rovibrational distributions. As for photodissociation of NO2 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 beta 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 (X) over tilde ground state.