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ABSTRACT: Reported here is a study of the effects of liquid helium cooling on the fragmentation of ions formed by electron impact mass ionization. The molecules of interest are picked up by the helium nanodroplets as they pass through a low pressure oven. Electron impact ionization of a helium atom in the droplet is followed by resonant charge transfer to neighboring helium atoms. When the charge is transferred to the target molecule, the difference in the ionization potentials between helium and the molecule results in the formation of a vibrationally hot ion. In isolation, the hot parent ion would undergo subsequent fragmentation. On the other hand, if the cooling due to the helium is fast enough, the parent ion will be actively cooled before fragmentation occurs. The target molecule used in the present study is triphenylmethanol (TPM), an important species in synthetic chemistry, used to sterically protect hydroxyl groups. Threshold PhotoElectron PhotoIon COincidence (TPEPICO) experiments are also reported for gas-phase TPM to help quantify the ion energetics resulting from the cooling effects of the helium droplets.
Journal of the American Chemical Society 10/2004; 126(36):11283-92. · 9.91 Impact Factor
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ABSTRACT: Much recent progress has been made theoretically and computationally towards understanding the importance of conical intersections for chemical reactions. Nonetheless, experimental characterization of conical intersections has proven extremely difficult with one striking exception: the Jahn-Teller conical intersection. This article overviews the fundamental similarity of a variety of conical intersections and demonstrates how the spectroscopy of Jahn-Teller active molecules can be used to characterize them. Specific results are reviewed for four representative Jahn-Teller active molecules, C5H5, C6H6+, Ag3 and CH3O.
Chemical Society Reviews 02/2003; 32(1):38-49. · 28.76 Impact Factor
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ABSTRACT: Ab initio calculations have been performed for the cations of benzene, C6H6, and its fluorinated analogs, C6F6 and C6H3F3. Calculated molecular parameters characterizing the Jahn-Teller potential energy surface (PES) are very consistent with those derived from the spectra of C6F6+ and C6H3F3+. However the calculated Jahn-Teller stabilization energy for the benzene cation is roughly three times greater than that previously reported experimentally. With the aid of the calculated values, a more complete analysis of the available spectral data for C6H6+ and C6D6+ is performed, with an emphasis on the data from ZEKE experiments and IR spectra of the Ar⋅C6H6+, Ne⋅C6H6+, and Ar⋅C6D6+ complexes. The comprehensive analysis reveals Jahn-Teller activity in 3 e2g modes for C6(H/D)6+ and provides values for their vibrational frequencies, linear and quadratic Jahn-Teller coupling constants, as well as quadratic coupling constants for several other degenerate modes. These new molecular parameters are generally in good agreement with the corresponding values derived from the ab initio calculations and produce a total Jahn-Teller stabilization energy in good agreement with the computed value. © 2002 American Institute of Physics.
The Journal of Chemical Physics 12/2002; 117(23):10654-10674. · 3.33 Impact Factor
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ABSTRACT: The growth of water clusters in liquid helium droplets results in the formation of cyclic structures up to and including the hexamer. In view of the sequential nature of the molecular pick-up process, the formation of water rings involves the insertion of water monomers into preformed cyclic water clusters. The implication of this observation is that the barriers to the ring insertion process are low enough to be overcome during the experiment. This paper presents a combined experimental and theoretical effort to explore the insertion process in detail. Our results provide important new insights into the dynamics of hydrogen-bonded networks. We map out the cluster potential energy surfaces and visualize them using disconnectivity graphs. Nonequilibrium walks on these surfaces show that ring water clusters can be formed during sequential addition of water molecules by surmounting small barriers that are thermally accessible even at the low temperature of the experiment. We find that the effects of zero-point energy are significant in making these processes feasible.© 2002 American Institute of Physics.
The Journal of Chemical Physics 07/2002; 117(3):1109-1122. · 3.33 Impact Factor
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ABSTRACT: Ab initio calculations are performed for the 2E1″ and 2A2″ states of the cyclopentadienyl radical. An important goal of these calculations is to guide the analysis of the experimentally observed 2A2″– 2E1″ electronic spectrum. Vibrational frequencies for both the and state are reported. Large changes in frequency between the states for out-of-plane vibrations are found, leading to the expectation that overtones of these modes will appear strongly in the spectrum. Additionally, spectroscopically obtainable parameters describing the Jahn–Teller effect are calculated for the state. Using all this information the − electronic spectrum is predicted for both C5H5 and C5D5. © 2001 American Institute of Physics.
The Journal of Chemical Physics 03/2001; 114(11):4855-4868. · 3.33 Impact Factor
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ABSTRACT: The radiative and nonradiative decay of the 2E electronic state of the CH3S radical has been investigated using a variety of experimental techniques. Lifetimes have been measured for a number of vibrational levels; these data have been analyzed along with similar results previously obtained for other methoxy radical family members, CH3O, CD3O, and CF3S. It is concluded that the totality of the data is best described by a model which postulates mode-selective fragmentation into a methyl (or fluoromethyl) radical and an O or S atom. For CH3S there appears a second nonradiative decay channel, possibly producing H+CH2S. © 2000 American Institute of Physics.
The Journal of Chemical Physics 11/2000; 113(21):9649-9657. · 3.33 Impact Factor
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ABSTRACT: The Ã2A1−X̃2E electronic spectrum of the CD3O radical has been investigated in detail. By using a variety of techniques, including rotational structure analysis, over 65 bands terminating on different vibronic levels of the à state have been assigned. The branching ratios for fluorescence decay vs photofragmentation of these same levels have also been determined. The detailed spectroscopic mapping of the à state vibronic structure is used to interpret the dynamical observations. The fragmentation is found to be rather mode specific, with ν3, the C−O stretch, being a major promoting mode, while ν2, the umbrella motion, is not. Unexpectedly the methyl rock, ν6, also appears to be an effective promoting mode.
02/1999;
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ABSTRACT: The natural lifetimes of a large number of the vibrational levels of the excited 2Σ+ electronic state of the family of rare gas complexes, R⋅SH (R=Ne, Ar, and Kr) and their deuterides, are reported. It is well known that the natural lifetime of the 2Σ+ state of isolated SH/D is markedly shortened by a photofragmentation process. Our results for the complexes show that the rare gas atom plays an important role in inhibiting this process. From a classical model of the molecular system we are able to explain the trends observed in our lifetime data. The data from the R⋅SD complexes where for some vibrational levels the deuterium atom appears to be trapped between the rare gas and sulfur atoms allows us to establish a radiative lifetime for these complexes and the SH/D monomer. © 1998 American Institute of Physics.
The Journal of Chemical Physics 06/1998; 109(1):162-169. · 3.33 Impact Factor
