In this chapter we follow the chemical change as it unfolds in time. We have three primary motivations. The first is that the time-dependent view matches the way most of us think about a reaction. We have an image of atoms moving, changing partners, etc., rather like a movie on a molecular scale. Time-resolved experiments provide insights that are intuitively appealing. On a more technical level, working in the time domain allows us not only to access the transition state region but also to probe the system as it exits from the transition state. We can see how things evolve rather than just the integrated effect of the dynamics as revealed by a post-collision analysis. Thirdly, time-resolved experiments reveal what happens at short times and this kind of information is otherwise hard to come by experimentally.

In earlier chapters we discussed experiments at a well-defined energy; we know where we start before the event and we can probe what happens well after it. But we have to infer what happens in the middle. In this chapter we discuss time-resolved experiments, experiments that are able to probe what the nuclei are doing throughout. This then paves the way for studies in the condensed phase where time-resolved experiments are a main tool.

To implement our program we have first to address two key issues, one of principle and one of practice. The uncertainty principle inherently imposes a loss of energy resolution when the time resolution becomes better.