Introduction

One of the most exciting advances in chemical physics in recent years has been the emergence and development of femtochemistry. This has been brought about largely because of advances in ultrafast laser technology, particularly the discovery of self-mode locking in Ti:sapphire and the development of chirped pulse or regenerative amplifiers. Another important innovation has been the development of a variety of linear and nonlinear spectroscopic techniques to probe electronic and nuclear dynamics. Nonlinear methods have been particularly useful in the study of solvation dynamics in the condensed phase. In the gas phase, where the density of molecules is much lower, ionization techniques such as pump-probe mass spectrometry have more often been employed. However, mass spectrometry can only provide the time-dependent population of a chemical species, in other words, kinetic information. In order to extract more detailed information on the reaction dynamics, measurements of the velocity vectors of the photoelectrons and fragment ions produced upon ionization are required. As we have seen in the preceding chapters, an imaging detector placed at the end of a time-of-flight mass spectrometer can easily accomplish such measurements. In this chapter we explore how ultrafast lasers can be coupled with charged particle imaging to develop experimental probes of ultrafast dynamic processes in molecules, such as electronic dephasing (radiationless transitions) and intramolecular vibration energy redistribution (IVR).

Introduction

Charged particle imaging provides us with very beautiful pictures that offer graphic insight into chemical dynamics. Although it is often the case that general dynamical information can be deduced by simple inspection of the primary data, the images obtained in the typical imaging experiment are, in fact, projections of a three-dimensional (3-D) object onto a two-dimensional (2-D) screen. In order to extract all the information potentially available to us we need to consider what data recovery techniques are available to reconstruct the 3-D velocity distribution of the charged particles created in the experiment from the image we actually record.

There are two fundamentally different approaches; inversion methods and forward convolution methods. Inversion methods make use of the fact that if the original (3-D) distribution has an axis of cylindrical symmetry its (2-D) projection parallel to this axis contains enough information to unambiguously reconstruct the full (3-D) distribution. As we have seen in the previous two chapters, such an axis of symmetry in laboratory space can be found in many photodissociation or bimolecular scattering experiments. However, if there is no cylindrical symmetry in the experiment, a forward convolution method is generally necessary. Here, the experiment is simulated in a computer model that produces (2-D) data that are then compared with the experimental data. By iteratively optimizing parameters in the computer model the best reconstruction of the experimental data is sought.

The field of molecular reaction dynamics has made enormous progress since the pioneering experiments of Yuan Lee, Dudley Herschbach and John Polanyi. The intervening years have seen numerous developments in both experimental techniques and theoretical methods. For the authors of this book one of the most exciting of these advances was the introduction of charged particle imaging by Dave Chandler and Paul Houston described in their 1987 paper ‘Two-dimensional imaging of state selected photodissociation products detected by multiphoton ionization’ published in the Journal of Chemical Physics.

I was extremely fortunate to be able to join Paul Houston in Ithaca in 1988/89 where we constructed the second imaging machine (Dave Chandler's original machine in Sandia having been temporarily put out of action in an unfortunate accident that Paul describes in the first chapter). It was an extraordinarily exciting experience to be involved in those earlier experiments and I am extremely grateful to Paul for the opportunity. The early data showed the power of the technique to provide graphic insight into chemical mechanism but it was difficult to obtain quantitative information because of instrumental problems to do with the arrangement of the ion optics. These were overcome by André Eppink and Dave Parker working in Nijmegen.