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Slow Photoelectron Velocity-Map Imaging of Transient Species and Infrared Multiple Photon Dissociation of Atmospherically Relevant Anion Clusters

  • Author(s): Yacovitch, Tara Irene
  • Advisor(s): Neumark, Daniel M
  • et al.
Abstract

Two different types of vibrationally resolved spectroscopies are used in the experimental study of reactive species: slow-electron velocity map imaging (SEVI) and infrared multiple photon dissociation (IRMPD).

SEVI spectroscopy is used to study the series of vinoxy and substituted vinoxy radicals: vinoxy (H2C=CH-O), i-methylvinoxy (H2C=C(-O)-CH3) and n-methylvinoxy (H3C-HC=CH-O). Vibrational resolution of their ground and first excited electronic states is achieved, leading to accurate measurement of electron affinities, term energies and vibrational frequencies. Radical geometries are deduced and conformational isomers for the larger species are identified. The i-methylvinoxy radical is found to be most stable when the methyl substituent is eclipsed in the ground-state radical and staggered in the excited state radical and ground state anion. Both cis and trans isomers of the n-methylvinoxy radical are observed, with the lower-energy cis isomer contributing to most of the spectral peaks.

The SEVI experiment is also used to study the transition state region of the F + H2 and F + CH4 reactions. The F + H2 results improve on previous spectra, resolving narrow features and suggesting that additional theoretical treatment is necessary to fully describe and assign the experimental results. The entrance valley of the F + CH4 reaction coordinate is measured, showing extended structure attributed to bending or hindered rotation of the methane moiety. The significance of these results in terms of reactive resonances is discussed.

The final SEVI experiments involve a series of alkoxy radicals and their sulfur-substituted analogs: methoxy (CH3O), thiomethoxy (CH3S), ethoxy (CH3CH2O), thioethoxy (CH3CH2S), i-propoxy ((CH3)2CHO) and n-propoxy (CH3CH2CH2O). The two lowest electronic states are close in energy (or formally degenerate) leading to a slew of nonadiabatic effects such as vibronic coupling from the Jahn-Teller or pseudo-Jahn-Teller effect and spin-orbit splitting. Precise determinations for the electron affinities and splittings between the electronic states are made. Variation of the size, symmetry and O/S atoms significantly affects the potential energy landscape of these radicals, leading to drastically altered spectra governed by differing contributions of the various nonadiabatic effects.

IRMPD spectra of negatively charged cluster species containing inorganic acids and water are studied, revealing structural information and size-dependent trends. The small bisulfate-water clusters, HSO4-(H2O)n, show lengthening of the acidic bond in the bisulfate anion, H-OSO3-. This is observed through the characteristic SOH bending vibration. The small mixed clusters of sulfuric and nitric acid, HSO4-(HNO3), NO3-(H2SO4)(HNO3) and HSO4-(H2SO4)(HNO3), show charge localization effects that in some cases counter the structural assumptions made based on the gas phase acidities of the molecular acids. Finally, the clusters containing bisulfate, sulfuric acid and water, HSO4-(H2SO4)m(H2O)n show the recurrence of the triply hydrogen-bound HSO4-(H2SO4) configuration for n = 0, while incorporation of water disrupts this stable motif for clusters with m > 1.

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