Computed photodissociation cross section of the v'' = 5 level of the b 3Σ+ state (black) towards the H+ + He(1s2p 3Po) channel (red)
Figure 5. Computed photodissociation cross section of the v'' = 5 level of the b 3Σ+ state (black) towards the H+ + He(1s2p 3Po) channel (red). The Fourier transform of the autocorrelation not only yields the direct photodissociation cross section (green, full line), but also a contribution from predissociation (green, dotted line) which incorrectly appears as a smooth envelope for short propagation times.
We illustrate some of the difficulties that may be encountered when computing photodissociation and radiative association cross sections from the same time-dependent approach based on wavepacket propagation. The total and partial photodissociation cross sections from the 33 vibrational levels of the b 3Σ+ state of HeH+ towards the nine other 3Σ+ and 6 3Π n = 2, 3 higher lying electronic states are calculated, using the autocorrelation method introduced by Heller (1978 J. Chem. Phys. 68 3891) and the method based on the asymptotic behaviour of wavepackets introduced by Balint-Kurti et al (1990 J. Chem. Soc. Faraday Trans. 86 1741). The corresponding radiative association cross sections are extracted from the same calculations, and the photodissociation and radiative association rate constants are determined.