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  • American Association for the Advancement of Science (AAAS)  (117)
  • American Institute of Physics (AIP)
  • 2015-2019  (81)
  • 2000-2004  (107)
  • 1
    Electronic Resource
    Electronic Resource
    Dynamics of a nonlinear master equation: Low-dimensional manifolds and the nature of vibrational relaxation (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 7828-7838 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of nonlinear master equations describing vibrational relaxation in shock-heated molecules are studied. The nonlinearity results principally from inclusion of self-collisions. The master equations were derived in a previous paper by fitting experimental data and besides being nonlinear they vary according to changes in the bath temperature. It is demonstrated that, except for brief transients, the dynamics lie on one-dimensional, nonlinear manifolds, including the full time of experimental observation. The one-dimensional nature of the dynamics allows for an in depth study of vibrational relaxation. It is shown that vibrational distributions cannot be characterized accurately by a vibrational temperature until they are close to equilibrium and that one-dimensional rate laws accurately describe the dynamics on the one-dimensional manifold. The latter characteristic is important, because it allows results generated from master equations which include self-collisions to be easily incorporated into kinetic modeling. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1467905
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  • 2
    Electronic Resource
    Electronic Resource
    Probing potential surfaces for hydrogen bonding: Near-infrared combination band spectroscopy of van der Waals stretch (ν4) and geared bend (ν5) vibrations in (HCl)2 (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 6132-6145 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High resolution infrared spectra of the two "low" frequency intermolecular modes—van der Waals stretch (ν4) and geared bend (ν5) of (HCl)2—have been characterized in HCl-stretch excited states using a slit jet spectrometer. In a high resolution high sensitivity search covering the range between 2880 and 3070 cm−1 four (HCl)2 combination bands associated with in-plane vibrations ν4 and ν5 have been observed. The vibrational assignment of these bands is based on comparison between observed intermolecular mode energies and predictions from recent six-dimensional (6D) quantum mechanical (QM) calculations [Y. Qiu, J. Z. H. Zhang, and Z. Bacic, J. Chem. Phys. 108, 4804 (1998)], though additional confirmation is provided by ancillary spectroscopic information such as rotational constants, predissociation linewidths, and 35Cl/37Cl isotopic band shifts. The experimentally observed intermolecular energies agree with theoretical predictions to (approximate)2–4 cm−1 out of 60–90 cm−1, suggesting that the 6D potential energy surface can describe combination band excitation in these lower frequency intermolecular coordinates fairly well. Three of the four observed combination bands arise from the upper tunneling level (B+), and all four bands are built exclusively on bound HCl stretch (ν2). To account for these striking intensity anomalies, a simple model for three-dimensional QM calculation of transition moments is introduced, which correctly reproduces the experimental trends. In this model, the propensity for ν2 based combination bands arising from upper tunneling levels can be successfully ascribed to the unusually "floppy" nature of the intermolecular vibrations, which results in a "harmonic oscillator" Δv=+1 propensity for excitations between tunneling levels along the geared bend coordinate. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1436105
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  • 3
    Electronic Resource
    Electronic Resource
    Beyond the Born–Oppenheimer approximation: High-resolution overtone spectroscopy of H2D+ and D2H+ (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 6146-6158 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transitions to overtone 2ν2 and 2ν3, and combination ν2+ν3 vibrations in jet-cooled H2D+ and D2H+ molecular ions have been measured for the first time by high-resolution IR spectroscopy. The source of these ions is a pulsed slit jet supersonic discharge, which allows for efficient generation, rotational cooling, and high frequency (100 KHz) concentration modulation for detection via sensitive lock-in detection methods. Isotopic substitution and high-resolution overtone spectroscopy in this fundamental molecular ion permit a systematic, first principles investigation of Born–Oppenheimer "breakdown" effects due to large amplitude vibrational motion as well as provide rigorous tests of approximate theoretical methods beyond the Born–Oppenheimer level. The observed overtone transitions are in remarkably good agreement (〈0.1 cm−1) with non-Born–Oppenheimer ab initio theoretical predictions, with small but systematic deviations for 2ν2, ν2+ν3, and 2ν3 excited states indicating directions for further improvement in such treatments. Spectroscopic assignment and analysis of the isotopomeric transitions reveals strong Coriolis mixing between near resonant 2ν3 and ν2+ν3 vibrations in D2H+. Population-independent line intensity ratios for transitions from common lower states indicate excellent overall agreement with theoretical predictions for D2H+, but with statistically significant discrepancies noted for H2D+. Finally, H2D+ versus D2H+ isotopomer populations are analyzed as a function of D2/H2 mixing ratio and can be well described by steady state kinetics in the slit discharge expansion. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1458244
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  • 4
    Electronic Resource
    Electronic Resource
    Erratum: "Femtosecond stimulated emission pumping: Characterization of the I2− ground state" [J. Chem. Phys. 112, 8847 (2000)] (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8854-8854 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1318753
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  • 5
    Electronic Resource
    Electronic Resource
    Laser spectroscopy of jet-cooled ethyl radical: Infrared studies in the CH2 stretch manifold (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1823-1834 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A glow discharge, slit supersonic expansion in conjunction with direct infrared laser absorption methods has been utilized to record high resolution vibration–rotation spectra of the CH3–CH2 ethyl radical. The slit supersonic expansion results in efficient rotational cooling from discharge temperatures down to Trot(approximate)14 K, permitting unambiguous rotational assignment and spectral analysis for the first time. Furthermore, a discharge on/discharge off data collection scheme permits clean discrimination between spectral contributions from radical vs precursor absorption. Spectra for both symmetric and asymmetric CH2 stretch manifolds are observed. Least-squares fits of transition frequencies out of the K=0 ground state manifold to a near prolate top model Hamiltonian reproduce the data to within the 7 MHz experimental uncertainty and provides rotational constants for both ground and vibrationally excited symmetric/asymmetric CH2 stretch states. The band origins for the CH2 stretch vibrations [3037.018 96(12) cm−1 and 3128.693 69(13) cm−1] are in reasonable agreement with ab initio theory; though predictions for relative intensities of the two bands are off by nearly an order of magnitude and indicate that the transition moment vector is tilted 33° away from each C–H bond toward the C–C bond axis. Structural analysis based on the measured B and C rotational constants imply a C–C bond distance of 1.49 Å. This is consistent with partial ((approximate)15%) double bond character for the ethyl radical carbon frame and in excellent agreement with theoretical predictions. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480746
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  • 6
    Electronic Resource
    Electronic Resource
    Femtosecond stimulated emission pumping: Characterization of the I2− ground state (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 8847-8854 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Femtosecond stimulated emission pumping in combination with femtosecond photoelectron spectroscopy is used to characterize the potential energy function of the I2−(X˜ 2Σu+) ground state up to vibrational energies within 2% of the dissociation limit. The frequency and anharmonicity of this state are measured at a series of vibrational energies up to 0.993 eV by coherently populating a superposition of ground state vibrational levels using femtosecond stimulated emission pumping, and monitoring the resulting wave packet oscillations with femtosecond photoelectron spectroscopy. The dissociative I2−(A˜′ 2Πg,1/2) state is used for intermediate population transfer, allowing efficient population transfer to all ground state levels. Using the measured frequencies and anharmonicities, the X˜ 2Σu+ state has been fit to a modified Morse potential with the β-parameter expanded in a Taylor series, and the bond length, well depth, and υ=0–1 fundamental frequency set equal to our previously determined Morse potential [J. Chem. Phys. 107, 7613 (1997)]. At high vibrational energies, the modified potential deviates significantly from the previously determined potential. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481499
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  • 7
    Electronic Resource
    Electronic Resource
    Modeling of nonlinear vibrational relaxation of large molecules in shock waves with a nonlinear, temperature-varying master equation (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 7814-7827 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We model recent experiments on the vibrational relaxation of oxirane in a shock tube. A master equation is developed which includes self-collisions of the oxirane, leading to a nonlinear master equation. This master equation is also applied to a more limited study of vibrational relaxation for cyclopropane in a shock tube. The time variation of the temperature dependence of the bath is also included in the calculations. Good agreement between the modeling and experiments are obtained through a fit to the energy transfer parameters. These fits demonstrate that self-collisions are dominant in promoting the relaxation even for mixtures of Kr and oxirane where the oxirane is 2% and 4% dilute. This dominance comes from two sources: (1) much larger energy transfer per collision for oxirane–oxirane collisions and (2) resonant energy transfer effects. For cyclopropane, some of the good fits show smaller energy transfer characteristics for self-collisions than buffer gas collisions. Even in these cases self-collisions are an important part of the energy transfer process through resonant energy transfer effects. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1467904
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  • 8
    Electronic Resource
    Electronic Resource
    Electronic relaxation dynamics of carbon cluster anions: Excitation of the C˜ 2Πg←X˜ 2Πu transition in C6− (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 11185-11192 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Anion femtosecond photoelectron spectroscopy (FPES) has been used to monitor intramolecular electronic relaxation dynamics following the excitation of the C˜ 2Πg←X˜ 2Πu 000 electronic transition in C6−. The time-dependent photoelectron spectra provide a detailed picture of the relaxation dynamics in which the initially excited C˜ 2Πg (v=0) level evolves into highly vibrationally excited C6− in its ground electronic state. The spectra show evidence for a two-step relaxation mechanism: internal conversion (IC) to vibrationally excited B˜ 2Σu+ and A˜ 2Σg+ states, occurring on a time scale of 730±50 fs, followed by IC from these intermediate states to highly vibrationally excited levels in the X˜ 2Πu ground state with a time constant of 3.0±0.1 ps. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1421378
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  • 9
    Electronic Resource
    Electronic Resource
    Vibrational relaxation of the free terminal hydroxyl stretch in methanol oligomers: Indirect pathway to hydrogen bond breaking (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9352-9360 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational relaxation of methanol-d (MeOD) in carbon tetrachloride has been investigated via ultrafast infrared pump–probe experiments. Exciting at 2690 cm−1, only the free O-D (where the D is not H-bonded) stretching mode is initially populated. For MeOD mole fractions ≤0.025, a 2.15 ps single exponential decay is observed. At mole fractions ≥0.0375, the signal decays (2.15 ps decay time) below zero (increased absorption) and then recovers on time scales of 22 ps and (very-much-greater-than)300 ps. The increased absorption indicates the formation of additional free ODs caused by the breaking of H-bonds that are not directly coupled to the initially excited vibration. The two-time scale recovery of this signal arises from geminate and nongeminate recombination. The data are fit with a set of kinetic equations that accurately reproduce the data. The results suggest that vibrational relaxation of the initially excited free OD stretch into intramolecular modes of the methanol leads to H-bond breaking. This contrasts studies that suggest direct relaxation of a vibrationally excited OH stretch into an H-bond stretch is responsible for H-bond breaking. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1415447
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  • 10
    Electronic Resource
    Electronic Resource
    The effects of a dynamic lattice on methane self-diffusivity calculations in AlPO4-5 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3345-3350 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Canonical ensemble molecular dynamics simulations were conducted for methane diffusion in AlPO4-5 in order to assess the role of the lattice motion on adsorbate diffusivity in straight pore zeolites. Both a static lattice model and a full dynamic lattice model were used at a loading of 1.5 methane/unit cell at 295 K. Although recent simulation work has asserted that there should be a difference, we show that there is little difference in the observed methane diffusivity (1.26×10−7 m2/s) and passing frequency (0.305) when a static lattice approximation is used over a full dynamic lattice (1.33×10−7 m2/s and 0.328). Furthermore, we introduce a methodology for handling lattice motion in molecular simulations by utilizing the normal vibrational modes in a harmonic crystal approximation. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480917
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