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 ...

Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen

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.

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Imaging the dynamics of gas phase reactions

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 241±002 eV is found

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Energy partitioning following photodissociation of methyl iodide in the A band: A velocity mapping study

Ab initio contracted spin–orbit configuration interaction (SOCI) calculations have been carried out to obtain potential energy surfaces of 3Q0 and 1Q1 excited states of methyl iodide as functions of all the geometrical parameters except for the three C–H stretches. The results are fitted to six‐dimensional diabatic potential functions and their couplings. Classical trajectory calculations have been performed using these potential functions. The rotation of the CH3 product in the I channel has been calculated to be perpendicular to the top axis and to have a peak at N=5 and extend up to N=8, whereas it is cold in the I* channel, in good agreement with recent experiments. The CH3 rotation is excited by the time trajectories arrive at the conical intersection region; this excitation is retained in the I‐channel product because the 1Q1 surface has a small bending force constant outside the conical intersection, whereas it is damped in the I* channel because 3Q0 still has a large bending force constant. The ca...

Ab initio potential energy surfaces and trajectory studies of A‐band photodissociation dynamics: CH3I*→CH3+I and CH3+I*

Advances in the time propagation of multidimensional wave packets are exploited to present the A‐band photodissociation dynamics of methyl iodide for five active vibrational modes on the three relevant excited ab initio potential surfaces. The five modes considered represent all of the experimentally observed dynamical activity. The only modes neglected are the asymmetric C–H stretch and the asymmetric deformation of the methyl group. The kinetic energy operator corresponding to these five degrees of freedom is derived. The fully quantum mechanical calculation was implemented upon grids using 2880 distinct time‐dependent configurations, determined by the multiconfigurational time‐dependent Hartree algorithm, for each electronic state. All of the currently known experimental results regarding the umbrella vibration, symmetric C–H stretching vibration, perpendicular rotation, and parallel rotation of the photodissociated methyl radical fragment are well reproduced. The full wavelength dependence of all of t...

Time‐dependent photodissociation of methyl iodide with five active modes

Photofragment imaging: The 266 nm photodissociation of CH3I

The technique of photofragment imaging is described, and several examples of the
power of the technique are presented. Two-dimensional images of state-selected
photofragments from the photodissociations of CD3I and H2S illustrate how photofragment
imaging reveals β parameters, brancing ratios, Doppler profiles and vector
correlations. Comparisons are made with Doppler profiling and one-dimensional
time-of-flight techniques.

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John W. Thoman

Papers

Photofragment imaging: The 266 nm photodissociation of CH3I

Two-dimensional Imaging ofPhotofragments