Revealing Molecular Dynamics Through DC Sliced Ion Imaging
West, Niclas A.
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Velocity mapped ion imaging is a well-established technique for studying photolysis dynamics on the molecular level. Mapping the velocity of a spherical cloud ensemble of photofragments provides enough information to characterize how the fragment velocities, angular momenta, and transition dipole moments are correlated during dissociation. Here we dissociate methyl nitrite with 355 nm linearly polarized light, probe the NO fragments with a 1+1′ REMPI scheme, and velocity map the NO+ fragments to learn where photodissociation reaction energy is partitioned. Conventional ion imaging maps the NO+ ions by sending the 3D Newton ion sphere via >2 electrodes toward a 2D microchannel plate (MCP)/phosphor detector to be crushed into a 2D ion image. The newer DC slicing technique uses >3 electrodes of a special ratio 1:0.93:0.81:0.75 of voltages to elongate the Newton sphere several hundred ns along the time of flight axis. The MCP is then gated by application of a voltage pulse to accept only ~40 ns of the center of the elongated Newton sphere. This collects only the central slice of the Newton sphere. The crushed and sliced ion image data for methyl nitrite photolysis yielded comparable stories of the stereodynamics of the reaction. Sliced ion imaging had the advantages that the bipolar moments were determined without the need for mathematical deconvolution and that fitting the 2D distributions was much simpler. Disadvantages of DC sliced ion imaging were that much of the Newton ion sphere (signal) was thrown away and that the lower electrode voltages used made the ions more susceptible to stray fields.