Molecular frame and recoil frame photoelectron angular distributions from dissociative photoionization of NO2

The authors report measured and computed molecular frame photoelectron angular distributions (MFPADs) and recoil frame photoelectron angular distributions (RFPADs) for the single photon ionization of the nonlinear molecule NO(2) leading to the (1a(2))(-1) b (3)A(2) and (4a(1))(-1) (3)A(1) states of NO(2)(+). Experimentally, the RFPADs were obtained using the vector correlation approach applied to the dissociative photoionization (DPI) involving these molecular ionic states. The polar and azimuthal angle dependences of the photoelectron angular distributions are measured relative to the reference frame provided by the ion recoil axis and direction of polarization of the linearly polarized light. Experimental results are reported for the photon excitation energies h nu=14.4 and 22.0 eV. Theoretically the authors give expressions for both the MFPAD and the RFPAD. They show that the functional form in the recoil frame, where an average over the azimuthal dependence of the molecular fragments about the recoil direction is made, is identical to that they have earlier found for the DPI experiments performed on linear molecules. MFPADs were then computed using single-center expansion techniques within the fixed-nuclei frozen-core Hartree-Fock approximation. The computed cross sections for ionization to the (1a(2))(-1) b (3)A(2) state show a strong propensity for ionization with the polarization of the light perpendicular to the plane of the molecule, whereas the ionization to the (4a(1))(-1) (3)A(1) state of the ion is of similar intensity for all orientations of the polarization of the light in the molecular frame. These qualitative features of the MFPAD are also evident in the RFPAD. The RFPAD for ionization leading to the (1a(2))(-1) b (3)A(2) state is strongly peaked in the perpendicular orientation, whereas the RFPAD for ionization leading to the (4a(2))(-1) (3)A(1) state is much more nearly isotropic. Comparison between experimental and theoretical RFPADs indicates that the recoil angle for NO(+) fragments is similar to 50 degrees relative to the symmetry axis of the initial C(2v) symmetry of the NO(2) molecule in the ionization leading to the (1a(2))(-1) b (3)A(2) state and the recoil angle is similar to 120 degrees for the O(+) fragment for ionization leading to the (4a(1))(-1) (3)A(1) state. (c) 2007 American Institute of Physics