A number of sources of picosecond optical pulses and the means by which they may be used to investigate fast molecular processes have been described. In addition, it should be pointed out that most of these techniques can be extended to take advantage of other properties of the laser pulses; in particular, the polarization is of use in measuring time-dependent antisotropy. The state of the art is now the generation of 30fs pulses; it seems likely that this is all the time resolution that one is likely to need for investigating biological and other molecular processes as at such short times the uncertainty principle leads to a considerable loss of spectral resolution.

Picosecond lasers are ideal for the study of biochemical processes which occur in the subnanosecond time domain, some of which are identified in Table 1. It is impractical to consider here all of these, and some have in any case been reviewed extensively. Instead, attention is focused on one area, namely photosynthesis, where picosecond studies have led to considerable improvement in understanding this most important of all photochemical processes. It would be as well to recall here the overall pattern of natural photosynthesis. It consists of a chain of electrontransfer reactions which, in green plants and algae, results, at the electron donor end, in the oxidation of water to oxygen, and, at the electron acceptor end, in reduction processes such as the conversion of carbon dioxide to carbohydrate, of nitrogen to ammonia or of protons to hydrogen. These involve two photochemical steps and two separate, but linked, photochemical systems. Photosystem I1 oxidizes water and produces a reduced intermediate (e.g. a hydroquinone). Photosystem I oxidizes the hydroquinone and