Development and implementation of methods for sliced velocity map imaging. Studies of overtone-induced dissociation and isomerization dynamics of hydroxymethyl radical (CH2OH and CD2OH)

An experimental setup for sliced velocity map imaging (SVMI) of H fragments produced in photodissociation of small molecules and radicals is designed, constructed and characterized experimentally. The setup uses an ion-optical arrangement consisting of an accelerator that allows creation of variable electrostatic field geometries near the photodissociation region (where the studied fragments are produced) for initial acceleration and focusing of the ionized fragments and an additional electrostatic lens inside the drift tube for independent control of the optical magnification. This approach permits largely independent control of the radial and temporal characteristics of the velocity mapping, making it possible to achieve time-gated slicing with selectable relative thickness for a very broad range of fragment kinetic energies (from a fraction of an electronvolt to a few electronvolts)—an important capability not available in other SVMI setups described in the literature. At the same time, the kinetic energy resolution (∼1 %) obtainable in a large part of the operating range is comparable to that of the best known SVMI setups.;Two methods for analysis of the raw data produced in VMI and SVMI experiments are developed and implemented. The first method is intended for correction of small distortions of raw velocity map images. It is shown that correction of these distortions, easily achievable by processing of the image using this method, might significantly increase the effective resolution for the speed or kinetic energy distribution extracted from the image. In addition, artifacts in angular distributions, produced by the distortions, are eliminated in the corrected images. The method is universal, being able to correct almost arbitrary distortions, and can be applied to full-projection and sliced velocity map imaging data represented as coordinate lists and as raster images.;The second data-processing method allows reconstruction of SVMI data recorded with slicing pulses of arbitrary shape and relative thickness (including the full-projection VMI as a trivial particular case). This ability permits to exploit the improved signal-to-noise ratio of the SVMI method and at the same time achieve the best resolution over the whole imaged velocity range, impossible with finite slicing without reconstruction. Testing of the method on synthetic and experimental images demonstrated that the results obtained by reconstruction of sliced images outperform the results from sliced images without reconstruction and from Abel inversion of full-projection images in terms of the resulting signal-to-noise ratio and resolution simultaneously.;The developed and implemented setup and methods were successfully used in studies of overtone-induced dissociation and isomerization dynamics of hydroxymethyl radial CH2OH and its isotopolog CD2OH. While these studies, at first glance, largely repeat the previous work, the new experimental capabilities in the present work, significantly improving the resolution and detection efficiency, allowed to obtain quantitative and qualitative results at a completely new level. In particular, interaction of the third O–H stretch overtone (4ν1 level) with a combination of O–H and antisymmetric C–H stretches (3ν1 + ν 2 level) was observed for the first time. Dissociation of the vibrationally excited radical to formaldehyde and hydrogen fragments (CH2OH → CH2O + H) was observed from both of these levels. In case of CD 2OH excitation (for which only the 4ν1 level was observed), in addition to the H products, a small amount of D fragments correlating with CHDO cofragments was observed for the first time, providing an explicit experimental demonstration of the CD2OH → CHD2O → D + CHDO decomposition through isomerization to the methoxy radical. Analysis of the vibrational distributions of the formaldehyde products in all reactions suggests that while O–H bond fission is responsible for the major part of the produced H fragments, a noticeable part of them in CH2OH and CD 2OH decomposition is also due to the isomerization pathway. The relatively high kinetic energy resolution in the present experiments allowed an accurate direct determination of the hydroxymethyl dissociation energies: D0(CH2OH → CH2O + H) = 10 160 ± 70 cm−1, D0(CD 2OH → CD2O + H) = 10 135 ± 70 cm −1, D0(CD2OH → CHDO + D) = 10 760 ± 60 cm−1.