Current Research in the Suits Group (page one)

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Overview: Velocity Map Imaging
Velocity map imaging is a recent, high-resolution variant of the ion imaging technique.  The approach is schematically illustrated below.  Neutral products of a photodissociation or reactive event are ionized by a probe laser and projected by ion optics onto an imaging microchannelplate detector.  A CCD camera captures the 2-D image and established techniques allow reconstruction of the full 3-D image.  The use of ion lenses in place of grids eliminates the spatial blurring of the image, allowing velocity resolution on the order of 1% or better.  It is a powerful, versatile experimental technique, as illustrated in the snapshots of current work presented below.

A.G. Suits and R. E. Continetti, "Imaging in Chemical Dynamics: the State of the Art," in Imaging in Chemical Dynamics , A. G. Suits and R. E. Continetti, eds.,  ACS Symposium Series 770, (American Chemical Society, Washington DC, 2000).

DC Slice Imaging Spectroscopy

D. Townsend, M P. Minitti and A. G. Suits, “Direct current slice imaging,” Rev. Sci. Instrumen. 74(4), 2530 (2003)

We have developed a DC version of slice imaging that allows the central section of the distribution to be recorded exclusively. This removes the need to perform mathematical reconstruction of the projected image, leading to enhanced resolution and S/N. The approach is summarized below:

Application to Cl2 photodissociation at 355 nm shows considerably improved resolution compared with the reconstructed unsliced image. The 2.4% velocity resolution is limited by the Cl recoil from the electron in the REMPI probe step and the velocity spread from the 35/37 isotopes in the undetected fragment.

Ion Pair Imaging Spectroscopy (IPIS)

M. Ahmed, D. S. Peterka, P. Regan, X. Liu and A. G. Suits, "Ion pair imaging spectroscopy: CH₃Cl -> CH₃⁺ +Cl^-,"  Chem. Phys. Lett. 339, 203-208 (2001).

X. Liu, R. Gross and A. G. Suits, “‘Heavy electron’ photoelectron spectroscopy: rotationally resolved ion pair imaging of CH3+,” Science 294 2527 (2001).

An exciting new direction involves imaging the dissociation of molecules to give Ion-pair products, schematically illustrated below.  If one of these is a structureless particle, such as a Cl^- anion, then the kinetic energy release provides spectroscopic information on the other particle in a way analogous to photoelectron spectroscopy.  This approach has several advantages, however:  It may be used to gain insight into radicals that are difficult to probe using conventional techniques.  Furthermore, since it is a slower process, one is not confined to Franck-Condon favored transitions, so that lower levels of the ion may be accessed.

Results for ion-pair imaging of methyl chloride at 118nm are shown below.  On the left is the DC sliced image of the methyl radical cation produced in this process, and on the right is the kinetic energy release spectrum obtained from the image, revealing rovibrational resolution dominated by a progression in the umbrella mode, as well as combination band excitation involving in-plane bend.

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