Ion imaging comes of age

Imaging probes of spectroscopy and dynamics

Pacifichem 2005 Honolulu, HI

December 15–20, 2005

In this 20th anniversary of the invention of the ion imaging technique by David Chandler and Paul Houston, it is fitting to mark the progress of the field, to look back to learn from its origins and look forward to dream what might be. This task is made all the more rewarding by the explosive growth in imaging: astonishing improvements in resolution, the sheer breadth of new applications, and sophisticated developments in analysis and theoretical interpretation of these new experiments offer ample room for our imagination. There is a real sense in which the practice of ion imaging is in transition from a promising adolescence in which it grew, pushing its limits and finding its range, to an adulthood of broad and consistent achievement. Evidence in support of this view is seen in the rich assortment of lectures presented in a symposium entitled “Imaging Probes of Spectroscopy and Dynamics” held at Pacifichem 2005 in Honolulu, HI, last December, highlights of which are compiled in this volume as original work by many of the current leaders in the field.

In their seminal paper,¹ Chandler and Houston used state-selective product ionization with position sensitive detection to record full product velocity distributions for a photodissociation event with single quantum state specificity. The approach combined the multiplex advantage of recording all product velocities simultaneously, with the benefits of spectroscopic detection characteristic of laser-induced fluorescence. It was immediately recognized that the technique could offer a powerful means of studying collision processes, and both groups independently set to work to study crossed-beam scattering. The first success in this arena came from the Houston group,² who reported state-to-state differential cross sections for rotationally inelastic collisions of nitric oxide with argon, a system that has become a mainstay of the imaging world. Soon after, Chandler and coworkers reported the first crossed-beam reactive scattering result, for the very challenging H + D₂ system.³ These early efforts mapped out some of the primary terrain best suited for exploration with the new technique, and current work is more active than ever along these lines. However, a key advance achieved a decade after the original report,⁴ termed “velocity mapping” by its inventors David Parker and Andre Eppink, opened the door to many new avenues of investigation and enticed many new practitioners into the field.

With the passage of another decade, we have seen the advent of “sliced” or 3-D imaging methods,^5–7 often yielding even higher resolution, and one might be tempted to call this now the era of slice imaging. Slicing has allowed velocity-dependent orbital polarization studies to be undertaken in extraordinary detail.⁸ It has enabled studies of oriented molecules to show explicitly the deviations from axial recoil.⁹ Reactive scattering has been transformed by a series of remarkable state-resolved slice imaging experiments, fulfilling the promise of imaging seen in its earliest days.¹⁰ But there are other diverse advances also brought on by velocity mapping, that similarly distinguish activity in the current era. It is now, for example, a time when photoelectron imaging is receiving its due, both in femtosecond time-resolved¹¹ studies and in application to low-energy electron kinetic energy measurement,⁴ where its resolution easily exceeds that of any competing techniques. Multimass imaging has been used to study photochemistry in complex molecules¹² and recently a velocity mapping multimass imaging approach has been demonstrated.¹³ Application of imaging methods to study biomolecules has been pursued, and incorporation of imaging methods with synchrotron radiation to study nanoparticle ionization dynamics has opened another new area to imaging study.¹⁴ Ion photodissociation and ion–molecule reactions are natural subjects for imaging study, and work has recently begun in these areas in earnest.¹⁵ Finally, we note the considerable contributions of the anion community to the study of chemical dynamics. These experiments typically rely on coincidence techniques to record complete momentum distributions for multiple products, so 3-D results are naturally obtained and deep insight into fragmentation mechanisms is achieved.

Essentially all of these developments are featured in the state-of-the-art studies contained in this volume. This represents a valuable snapshot of the field as it now stands, roughly 20 years after the first Polaroid was taken of the detector in the Chandler laboratory at Sandia. To predict where we will be in another decade is another matter, necessarily involving risk, but with a lesson in humility the likely reward. With that reward in view, we take the plunge: We predict ongoing improvement in velocity resolution, with factors other than the imaging system or ion optics becoming the targets of these efforts. We expect further development and broad application of photoelectron imaging to complex molecules and to radicals to improve thermochemical measurements. We expect the lines between the camera-based communities and the time-and-position communities will blur. We anticipate further growth in coincidence studies with ions and electrons using femtosecond (and attosecond) systems, with high harmonics and X-ray sources exploiting imaging to drive new advances. We expect novel combinations of spatial, mass, and velocity imaging to contribute to multidimensional microscopies for biological and materials science. Finally, we expect to be surprised and delighted by the new discoveries made possible by these developments.

Arthur G. Suits

Wayne State University

Masahiro Kawasaki

Kyoto University

Warren Lawrance

Flinders University

Acknowledgements

We wish to thank all of the participants in the Imaging Symposium at Pacifichem, and especially those who have contributed the papers contained herein. We thank Prof. Mike Ashfold for his initial encouragement in developing a special issue of PCCP out of the symposium, and the staff of the Royal Society of Chemistry, in particular Susan Weatherby, Hilary Crichton, and Philip Earis, for their efforts to bring this issue into being.

References

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