Pulsed field ionization-photoelecton bands for CO_2~+(a~2II_u and B~2sum_u~+)in the energy range of 17.2-19.0 eV:An experimental and theoretical study

摘要

The vacuum ultraviolct (VUV) pulsed field ionization—photoelectron (PFI—PE) spectra for CO2 have been measured in the energy range of 17.2—19.0 eV, showing complex vibronic structures for the CO~(A 2H~ andB 2~±) states. The PFI—PE spectra for CO~(A 211k andB 2~+) are dominated by the v j’ (symmetric stretching) vibrational progressions, and weak bands due to excitation of both even and odd quanta of the v (bending) and v ~ (antisymmetric stretching) modes are observed in the VUV—PFL—PE spectra. The simulation of rotational contours resolved in the PFI—PE vibronic bands associated with excitation to CO~(A 2113/21/2,1 ;v 0—5.v~0,vi=0) and CO(B2~ 0,0,0) has yielded accurate ionization energies for the formation of these states from C02(X 1~)· Three-dimensional potential energy functions (PEFs) for CO~(B 2~+) have also been generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Based on these PEFs, vibrational energy levels for CO~(B 2~t) together with the Franek—Condon factors for their formation from C02(X ‘~ ~), have been calculated. With the guide of these theoretical predictions, the vibrational bands resolved in the PFI—PE spectrum for CO~(B 2~+) have been satisfactorily assigned. This assignment reveals the nature of many vibrational PFI—PE bands as originated from anharmonic resonance interactions and members of Fermi polyads.