SIMULATION OF EMISSION-SPECTRA OF THE PF2 (B-2(2)-]X(2)B(1)) TRANSITION BY AB-INITIO CALCULATION AND FRANCK-CONDON ANALYSIS

Geometry optimization and harmonic vibrational frequency calculations were carried out on the X(2)B(1) and the lowest B-2(2) states of PF2 a t the MP2/6-31G and MP2/6-311G(2df) levels. CCSD(T)/6-311G(2df)//MP2/ 6311G(2df) calculations were also performed to obtain improved adiabat ic and vertical transition energies for the two combining electronic s tates. In addition, Frandk-Condon analyses were carried out employing the ab initio data obtained to simulate emission spectra of the PF2(B- 2(2) --> X(2)B(1)) transition. Both the computed relative energies and the theoretical spectra confirm that the observed emission spectra by Zhao and Setser were due to the B-2(2) --> X(2)B(1) transition. Furth ermore, the geometry of PF2 in the lowest B-2(2) state was also varied in the vibrational intensity calculations to give the best agreement between the theoretical and observed spectra. The bond length and bond angle thus deduced for PF2 in the B-2(2) state are 1.628 +/- 0.008 An gstrom and 84.9 +/- 0.2 degrees, respectively. Spectra involving excit ed vibrational levels of the B-2(2) State were also generated by assum ing the Boltzmann distribution at selected temperatures. Comparison of these spectra with the observed one suggested that the vibrational po pulation distribution in the upper state does not follow the Boltzmann rule. The potential surface of the upper B-2(2) state under study may be perturbed vibronically by that of the lower, near-linear, (2)A(1) state via the asymmetric stretching mode. This may explain why the the oretical spectra as obtained using the harmonic oscillator model are d ifferent from the experimental one.