## Computed photodissociation cross section of the *v*'' = 5 level of the *b* ^{3}Σ^{+} state (black) towards the H^{+} + He(1s2p ^{3}P^{o}) channel (red)

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**Figure 5.** Computed photodissociation cross section of the *v*'' = 5 level of the *b* ^{3}Σ^{+} state (black) towards the H^{+} + He(1s2p ^{3}P^{o}) 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.

**Abstract**

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* ^{3}Σ^{+} state of HeH^{+} towards the nine other ^{3}Σ^{+} and 6 ^{3}Π *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.