Imaging molecular dynamics

The advent of charged particle imaging methods has led to extraordinary advances in our appreciation of the structure and dynamics of excited state molecules and of molecular reactivity in the gas phase; many such advances are show-cased in this themed issue.¹ The utility of ion imaging methods for studying gas phase reaction dynamics was first demonstrated by Chandler and Houston in 1987,² who reported a squashed (two-dimensional (2-D) projection) of the 3-D speed distribution of the CH₃ fragments formed following pulsed laser photolysis of a jet-cooled sample of CH₃I. The CH₃ radicals were ionised at their point of creation by resonance enhanced multiphoton ionisation, and then accelerated onto a time and position sensitive detector to yield the image of interest. Photoelectron imaging studies began appearing a few years later.³ Illustrations of the improvements in image resolution that could be achieved using centroiding and event counting methods started to surface in 1996,^4–6 the velocity mapping break-through appeared in 1997⁷ and variants of slice imaging started to appear in 2003.^8–10 Methods for processing such images also advanced over this period, with the early analysis methods involving inverse Abel transforms¹¹ or forward convolution methods¹² progressively supplemented by basis set expansion¹³ and Fourier moment analysis¹⁴ methods. Several authoritative reviews map the evolving state-of-the-art for both ion and photoelectron imaging.^15–18

The collection of papers in this themed issue devoted to Imaging Molecular Dynamics is testament to the ever growing impact of imaging methods in advancing gas phase chemical physics. The short history of ion imaging provides an object lesson in the ways in which advances in scientific understanding progress hand-in-hand with technical and methodological breakthroughs. This issue contains several exemplars of recent exciting breakthroughs, in fast sensors that allow imaging of several different masses in the time-of-flight spectrum of ions formed in a single laser pulse (Brouard), in new routes to image inversion based on maximal entropy methods (Dick) and in the derivation of (the square of) a many-body wavefunction from careful measurements of the linear momentum of the fragments formed upon dissociation (Fechner).

The impact of imaging methods on advancing the range and depth of our understanding of photoinitiated unimolecular and bimolecular processes cannot be overstated. Examples of the former appearing in this issue include detailed studies of the excited state photophysics – fragmentation and/or alternative radiationless decay pathways – of radicals (North), isolated small molecules (Ashfold, Banares, Hochlaf, Kitsopoulos, Reid, Song, Stavros), molecules in or on clusters (Farnik, Hochlaf) and clusters (Castleman, Mackenzie). Imaging photoelectrons can be equally rewarding, as illustrated here by examples exploring photodetachment (and, in some cases, photodissociation) of singly and doubly charged anions (Culberson, Mabbs, Verlet), and by photoelectron–photoion coincidence imaging studies designed to reveal and quantify molecular chirality (Powis) and to distinguish bromobutyne isomers via their dissociative ionisation (Bodi). The issue is equally successful at capturing the diversity of bimolecular collision processes that benefit from careful interrogation by imaging methods, with examples spanning Cl + alkane reactions (Suits), the effects of reagent vibrational excitation in the F + CH₄ reaction (Cheng), ion–molecule reactions (Wester) and exquisitely detailed measurements of state-to-state resolved differential cross-section for rotationally inelastic scattering of a symmetric top molecule (ND₃) in collision with He (Orr-Ewing).

Several of the imaging studies described in this themed issue might well have been considered the stuff of science fiction at the time of the PCCP Perspective we co-authored in 2006.¹⁷ The field is certainly showing no loss of vibrancy. Looking forward, therefore, it would be most surprising if we do not witness similarly impressive advances in the next few years in, for example, the application of new universal ionisation schemes and of multimass imaging methods, in the range of wavelengths and timescales used in the excitation and/or probe steps, in the range and complexity of gas phase species amenable for study and in the development and application of species-selective spatial imaging methods.

References

1. This themed issue was initiated by participants of the EU-ITN network ICONIC-238671.
2. D. W. Chandler and P. L. Houston, J. Chem. Phys., 1987, 87, 1445.
3. See, for example, W. K. Kang, Y. S. Kim and K.-H. Jung, Chem. Phys. Lett., 1995, 244, 183.
4. L. J. Rogers, M. N. R. Ashfold, Y. Matsumi, M. Kawasaki and B. J. Whitaker, Chem. Phys. Lett., 1996, 258, 159.
5. Y. Tanaka, M. Kawasaki, Y. Matsumi, H. Fujiwara, T. Ishiwata, L. J. Rogers, R. N. Dixon and M. N. R. Ashfold, J. Chem. Phys., 1998, 109, 1315.
6. B. Y. Chang, R. C. Hoetzlein, J. A. Mueller, J. D. Geiser and P. L. Houston, Rev. Sci. Instrum., 1998, 69, 1665.
7. A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum., 1997, 68, 3477.
8. C. R. Gebhardt, T. P. Rakitzis, P. C. Samartzis, V. Ladopoulos and T. N. Kitsopoulos, Rev. Sci. Instrum., 2001, 72, 3848.
9. J. J. Lin, J. G. Zhou, W. C. Shiu and K. P. Liu, Rev. Sci. Instrum., 2003, 74, 2495.
10. D. Townsend, M. P. Minitti and A. G. Suits, Rev. Sci. Instrum., 2003, 74, 2530.
11. Imaging in Chemical Dynamics, ed. A. G. Suits and R. E. Continetti, American Chemical Society, Washington, DC, ACS Symposium Series, 2001, vol. 770, and references therein.
12. M. J. J. Vrakking, Rev. Sci. Instrum., 2001, 72, 4084.
13. V. Dribinski, A. Ossadtchi, V. A. Mandelshtam and H. Reisler, Rev. Sci. Instrum., 2002, 73, 2634.
14. M. J. Bass, M. Brouard, A. P. Clark and C. Vallance, J. Chem. Phys., 2002, 117, 8723.
15. A. J. R. Heck and D. W. Chandler, Annu. Rev. Phys. Chem., 1995, 46, 335.
16. Imaging in Molecular Dynamics, ed. B. J. Whitaker, Cambridge University Press, 2003.
17. M. N. R. Ashfold, N. H. Nahler, A. J. Orr-Ewing, O. P. J. Vieuxmaire, R. L. Toomes, T. N. Kitsopoulos, I. Anton-Garcia, D. Chestakov, S.-M. Wu and D. H. Parker, Phys. Chem. Chem. Phys., 2006, 8, 26.
18. T. Suzuki, Int. Rev. Phys. Chem., 2012, 31, 265 , and references therein.

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