## Schematic illustration of various cases where the partial photodissociation cross section σ does not start abruptly at the threshold energy ((a), (b), (e)) and where it does ((c), (d)), depending on the shapes and relative position of the potential energy curves and on the initial vibrational level

**Figure 4.** Schematic illustration of various cases where the partial photodissociation cross section σ does not start abruptly at the threshold energy ((a), (b), (e)) and where it does ((c), (d)), depending on the shapes and relative position of the potential energy curves and on the initial vibrational level.

**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.