Direct and indirect processes represent the two major classes of photodissociation. In the direct case, the fragmentation proceeds too fast for the molecule to develop a complete internal vibration in the upper electronic state before it breaks apart. The wavepacket leaves the FC region and never returns to its starting place with the consequence that the autocorrelation function decays rapidly to zero. The resulting absorption spectrum is broad and without any vibrational structures. In indirect photodissociation, on the other hand, the excited complex in the upper electronic state lives for a sufficiently long time to allow the development of internal vibration. The wavepacket oscillates in the inner region, frequently recurs to its place of birth, and continuously leaks out into the exit channel. The autocorrelation function exhibits many recurrences and decays slowly to zero. The resulting absorption spectrum is composed of narrow lines which reflect the quasi-bound (resonance) states of the complex and their widths reflect the coupling to the continuum.

The transition from direct to indirect photodissociation proceeds continuously (see Figure 7.21) and therefore there are examples which simultaneously show characteristics of direct as well as indirect processes: the main part of the wavepacket (or the majority of trajectories, if we think in terms of classical mechanics) dissociates rapidly while only a minor portion returns to its origin. The autocorrelation function exhibits the main peak at t = 0 and, in addition, one or two recurrences with comparatively small amplitudes. The corresponding absorption spectrum consists of a broad background with superimposed undulations, so-called diffuse structures. The broad background indicates direct dissociation whereas the structures reflect some kind of short-time trapping.