Ab initio calculations on PO2 and anharmonic Franck-Condon simulations of its single-vibrational-level emission spectra

Geometry optimization and harmonic vibrational frequency calculations were carried out on some low-lying electronic states of PO2 at the CIS, CASSCF, MP2, and RCCSD(T) levels with various standard basis sets of at least valence triple-ζ quality. Relative electronic energies, including vertical excitation energies from the X̃2A1 state {with the electronic configuration of ...(7a1)2(8a1)1} and Te values, were computed at the RCCSD(T) and CASSCF/MRCI levels with basis sets of up to aug-cc-pVQZ quality. These computed results, particularly the computed Te values, suggest that the upper electronic state of the laser induced fluorescence (LIF) and single-vibrational-level (SVL) emission spectra of PO2 reported recently by Lei et al. [J. Phys. Chem. A 2001, 105, 7828] is the 22A1 state of PO2 {with the electronic configuration of ...(7a1)1(8a1)2}. RCCSD(T)/ aug-cc-pVQZ and CASSCF/MRCI/aug-cc-pVQZ(no g) energy scans were carried out on the X̃2A1 and 22A1 states of PO2, respectively, in the symmetric stretching and bending coordinates. Franck-Condon factors (FCFs) between the two states, which allow for the Duschinsky and anharmonic effects, were calculated employing the potential energy functions obtained from the ab initio scans. Comparison between the simulated spectra based on the computed FCFs and observed SVL emission spectra led to reassignments of the vibrational designations of the emitting SVLs in the upper state. On the basis of the excellent agreement between the simulated spectra for the revised SVLs and the observed emission spectra, the electronic transition involved in the LIF and SVL emission spectra reported by Lei et al. is confirmed to be 22A2-X̃2A1 of PO2. Following the revised vibrational assignments of the upper electronic state in the SVL emissions, the vibrational assignments of the LIF excitation bands given by Lei et al. are revised and a revised T0 value of 30660 cm-1 is estimated for the 22A1 state of PO2. In addition, employing the iterative Franck-Condon analysis (IFCA) procedure in the simulation of the SVL emission spectra, the equilibrium geometry of the 22A1 state of PO2 is derived for the first time (re = 1.560 Å; θe = 116.5°).