Resonances in the photodissociation of CO2:: periodic-orbit and wavepacket analyses

The photodissociation of CO2 on the LEPS model potential surface [K.C. Kulander, J.C. Light, J. Chem, Phys. 73 (1980) 4337] is investigated in view of identifying the ultrashort-lived resonances that characterize the 'diffuse' structures in the absorption cross section. To this end, two avenues are pursued: on the one hand, regular and chaotic regimes are identified in the classical phase space, and semiclassical quantization is carried out for both regimes, In particular, the regular regime is amenable to a perturbative treatment which yields three low-energy resonances of symmetric-stretch character, with the associated bending progressions. The chaotic regime is analyzed by periodic-orbit quantization, within the collinear model. We identify partially interleaved progressions of resonances, of both even and odd parity. Periodic orbits of 'off-diagonal' type are found to play the dominant role in generating these progressions, The second avenue to identifying the resonance structures is a quantum-mechanical analysis, Different initial conditions, again of even and odd parity, are used in wavepacket propagation to probe different spectral regions. The resonances are extracted from the spectra obtained by Fourier-transforming the wavepacket autocorrelation function. The agreement between the complementary semiclassical and quantum analyses is found to be very satisfactory. The resonance lifetimes determined in our analysis range from 4 to 60 fs. (C) 1997 Elsevier Science B.V.