The structure and stability of mass-selected bisulfate, sulfuric acid, and water cluster anions, HSO4(-)(H2SO4)m(H2O)n, are studied by infrared photodissociation spectroscopy aided by electronic structure calculations. The triply hydrogen-bound HSO4(-)(H2SO4) configuration appears as a recurring motif in the bare clusters, while incorporation of water disrupts this stable motif for clusters with m > 1. Infrared-active vibrations predominantly involving distortions of the hydrogen-bound network are notably missing from the infrared multiple-photon dissociation (IRMPD) spectra of these ions but are fully recovered by messenger-tagging the clusters with H2. A simple model is used to explain the observed "IRMPD transparency".

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Vibrational Spectroscopy of Bisulfate/Sulfuric Acid/Water Clusters: Structure, Stability, and Infrared Multiple-Photon Dissociation Intensities

Mode-selective chemistry has been a dream of chemists since the advent of the laser in the 1970s. Despite intense effort, this goal has remained elusive due to efficient energy randomization in polyatomic molecules. Using ab initio molecular dynamics calculations, we show that the interaction of molecules with intense, ultrashort mid-infrared laser pulses can accelerate and promote reactions that are energetically and entropically disfavored, owing to efficient kinetic energy pumping into the corresponding vibrational mode(s) by the laser field. In a test case of formyl chloride ion photodissociation, the reactions are ultimately complete under field-free conditions within 500 fs after the laser pulse, which effectively overcomes competition from intramolecular vibrational energy redistribution (IVR). The approach is quite general and experimentally accessible using currently available technology.

A Reaction Accelerator: Mid-infrared Strong Field Dissociation Yields Mode-Selective Chemistry.

Isotopically selective infrared multiple‐photon dissociation of CTCl3 in CDCl3 was investigated using a pulsed CO2 laser‐pumped NH3 laser in the 12 μm region. The wavelength dependence of CTCl3 MPD in the ν4 absorption band was studied and the observed MPD peak was found to be shifted ∼10 cm−1 to the red of the absorption peak at 835 cm−1, consistent with the anharmonicity of the CTCl3 ν4 mode. The CTCl3 dissociation probability per pulse was investigated under various experimental conditions, including laser energy, pulse duration, and CDCl3 and buffer gas pressure. A rate equation model was used to analyze the CTCl3 experiments assuming various multiple‐photon absorption and collisional relaxation mechanisms; this modeling effort was extended to a brief, more general, investigation of multiple‐photon absorption by molecules with prespecified dissociation probability profiles. Lower limit single step T/D enrichment factors in chloroform MPD exceeding 15 000 were achieved under optimized conditions of the...

M.V. Kuzmin

Papers

Multiple photon dissociation of polyatomic molecules under vibrational deactivation by a buffer gas