The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and support the appealing properties of this velocity mapping technique.

The methyl iodide A-band photodissociation process CH3I+hν→CH3(v,N,K)+I(2P3/2), I*(2P1/2) has been studied in a cold molecular beam. Full three-dimensional state-specific speed and angular distributions of the nascent fragments were recorded using (2+1) resonance-enhanced multi-photon ionization (REMPI) and velocity imaging, a new variant of ion imaging. By combining the I* quantum yield and anisotropy parameters for both I and I* channels, the relative absorption strength to the contributing electronic states (3Q0, Q13 and Q11) as well as the probability for curve crossing (3Q0→1Q1) are determined for excitation wavelengths across the full A band (240–334 nm). Parallel excitation to the Q03 state turns out to dominate the A band even more than previously thought.

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

Translational and internal energy partitioning in the methyl and iodine fragments formed from photodissociation of methyl iodide in the A-band region is measured using velocity mapping. State-selective detection combined with the very good image quality afforded by the two-dimensional imaging technique allow a detailed analysis of the kinetic energy and angular distributions. Product vibrational energy is, as previously known, mainly partitioned into ν2, the umbrella mode of the methyl fragment, but a substantial fraction of molecules is also excited with one quantum of ν1, the symmetric C–H stretch, especially at higher dissociation energies. Preliminary evidence is also presented for excitation of several quanta of ν4, the asymmetric deformation mode. Rotational energy partitioning is similar for CH3 products formed in both the ground-state I(2P3/2) and the spin–orbit excited I*(2P1/2) channel for photodissociation across the full A-band spectrum. Dissociation of vibrationally excited molecules plays an increasingly important role at longer dissociation wavelengths. Two CH3I modes remain populated in the pulsed beam expansion, ν2(a1), the C–I stretch, and ν6(e), the methyl rock. Each reactant vibrational mode couples in a very specific manner into the I and I* dissociation channels. Trends in vibrational and rotational energy disposal are compared with recent theoretical predictions. Readjustment of many aspects of the ab initio multidimensional potential energy surfaces which have recently been calculated for CH3I appears to be necessary. The improved resolution offered by velocity mapping also allows a more accurate determination of the C–I bond energy. A dissociation energy of 2.41±0.02 eV is found.

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