Chemical Dynamics

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Position sensitive detection of products has witnessed a tremendous amount of popularity in recent years [[1]].   In particular imaging methods using optical relaying of the ion signals such that the spatial distribution of the particles can be captured using a digital camera, has started to rival time of flight (TOF) applications.  The original work of Chandler and Houston [[2]] has undergone several modifications that lead to this increase in popularity.  In their seminal work, a beam of molecules was photodissociated using a polarized laser beam.  The products were state selectively ionized using a resonant enhanced multiphoton ionization (REMPI) method, and the nascent ions were extracted using a homogeneous extraction field formed by a flat plate (repeller) and a flat fine grid (extractor).  The principle of the method was very appealing but its limited resolution compared to TOF impeded its popularity.  The main resolution problem was the physical dimensions of the molecular beam (~1 or 2 mm) which would be transferred over to the detector (typically 20 mm in radius) so at best the resolution achieved would be ~10% in velocity. 

This limitation was overcome with the development of velocity mapping by Eppink and Parker in 1997 [[3]].  In this ingenious scheme the homogenous field was replaced by an inhomogeneous field produced by 3 annular plates: the repeller, the extractor and the ground electrode.  By adjusting the relative voltage between the repeller and extractor, the field contours took on a shape that ensured a focusing of ion velocities “independent” of where they were produced in a plane perpendicular to the TOF axis (extraction field).  This development elevated the imaging technique into great popularity replacing slowly but surely most TOF or magnetic bottle machines for gas phase dynamics studies.  The resolution has now improved by an order of magnitude down to 1-2 % in velocity, making it competitive to TOF and additionally allows for enhanced angular distribution measurements.  The only limitation remaining was the fact that these optically coupled imaging experiments relied on cylindrical symmetry of the measured distribution and on a noisy mathematical transform (the inverse Abel) to extract a “slice” through the three dimensional distribution. 

The remedy to this cylindrical symmetry requirement was offered by the development of slice imaging in 2001 by Kitsopoulos and coworkers [[4]].  The basic principle is to introduce a temporal spread in the ion cloud along the TOF axis, such as to allow a narrow detector gate to slice out the central part.  There are presently two ways of doing this.  The original design of Kitsopoulos and coworkers where the extraction field is pulsed ON after a short delay following the photofragment ionization. 

The alternative method developed in 2003 concurrently by Suits and coworkers [[5]] and independently and Liu and coworkers [[6]  relies on using a very weak acceleration field in the interaction region followed by strong acceleration.  In both DC slicing approaches, the temporal spread is achieved in one electric field while a second field is used to achieve velocity mapping.  In this study we introduce a new electrode design so as to that eliminate the need for the second field.  In this new design a single repeller and a grounded extractor is sufficient to achieve both velocity mapping and slicing.

 


[[1]]  Imaging in Molecular Dynamics, Ed. B. J. Whitaker, (Cambridge University Press, Cambridge 2003).

[[2]]   D.W. Chandler and P.L. Houston, J. Chem. Phys. 87, 1445 (1987).

[[3]] A.T.J.B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477 (1997).

[[4]]  C.R. Gebhardt, T.P. Rakitzis, P.C. Samartzis, V. Ladopoulos, T.N. Kitsopoulos, Rev. Sci, Instrum. 72, 3848  (2001).

[[5]]  D. Townsend, M.P. Minitti, and A.G. Suits, Rev. Sci. Instrum. 74, 2530 (2003).

[[6]]   J.J. Lin, J. Zhou, W. Shiu, and K. Liu,  Rev. Sci. Instrum. 74, 2495 (2003).



Contact Person(s):
Prof. Theofanis Kitsopoulos


Last Updated:  1/3/2009
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