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  • Articles  (106)
  • Chemistry and Pharmacology  (106)
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  • Articles  (106)
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  • 1
    Electronic Resource
    Electronic Resource
    Dependence of intramolecular vibrational relaxation on central atom substitution: .nu.1 and 2.nu.1 molecular beam optothermal spectra of 3,3-dimethyl-1-butyne and trimethylsilylacetylene (1991)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 95 (1991), S. 8282-8293 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100174a050
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  • 2
    Electronic Resource
    Electronic Resource
    The onset of intramolecular vibrational energy redistribution and its intermediate case: The ν1 and 2ν1 molecular beam, optothermal spectra of trifluoropropyne (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 3891-3916 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the optothermal method for molecular beam, infrared spectroscopy, we have measured both the fundamental and first overtone of the acetylenic C–H stretch in CF3CCH. In the fundamental we observe a spectrum which shows only few perturbations. The majority of lines can be successfully fit to a model assuming an anharmonic coupling of the C–H stretch to a single, near-resonant background state with a coupling matrix element of 0.006 cm−1. We have observed other perturbations in this spectrum, including a state coupled by a weak perpendicular Coriolis interaction. All observed couplings are very weak and local in nature. In the overtone, where the density of background states increases by a factor of 100, we observe a spectrum characteristic of a system in the intermediate case of IVR (intramolecular vibrational energy redistribution). Analysis of the R(0) and P(1) transitions provides a homogeneous IVR lifetime of about 2 ns, which is long compared to lifetimes generally quoted for overtone vibrational relaxation. The root-mean-square coupling matrix element in the overtone is about 0.0008 cm−1. The higher J transitions in the overtone suggest that Coriolis interactions are present in the spectrum. The interpretation of these spectroscopic results in the context of IVR is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460796
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  • 3
    Electronic Resource
    Electronic Resource
    The rotationally resolved 1.5 μm spectrum of the HCN–HF hydrogen-bonded complex (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8896-8905 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the overtone spectrum of the CH stretching mode in HCN–HF. The vibrational predissociation rate is approximately twice that previously determined for fundamental excitation, whereas the complexation induced frequency shift is only marginally larger than that of the fundamental spectrum. These results are discussed in terms of a first-order perturbation theory treatment as set forth by LeRoy, Davies, and Lam [J. Phys. Chem. 95, 2167 (1991)]. We suggest that the frequency shift observed here might not only be due to complexation, but also to a long-range anharmonic interaction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463364
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  • 4
    Electronic Resource
    Electronic Resource
    Statistical intramolecular vibrational relaxation and its hindrance: The fundamentals of (CH3)3C–C≡CH and (CH3)3Si–C≡CH (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2152-2153 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the fundamental acetylenic C–H stretch near 3.0 μm of (CH3)3C–C≡CH and (CH3)3Si–C≡CH using an optothermal, molecular beam spectrometer. We find that the individual R(J) lines of the hydrocarbon are Lorentzian with a FWHM of 800 MHz indicating statistical intramolecular vibrational relaxation (IVR) with a 400 ps lifetime. The R(J) lines of the silicon compound are clearly asymmetric and, in addition, show a FWHM of about 150 MHz indicating a much longer (〉2 ns) lifetime. The increase in IVR lifetime in the larger density of states molecule may be due to reduced kinetic coupling resulting from the heavier Si atom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459042
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  • 5
    Electronic Resource
    Electronic Resource
    Numerical Laplace transform density of states calculation for medium and large molecules (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 6437-6444 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel implementation of the Laplace transform method for the calculation of the density of states of molecules, for which the partition function can be explicitly given is described. It consists of doing the inverse Laplace transform numerically after multiplying the integrand by a smoothing factor. This evaluation is more accurate than the method of steepest descent, and the computation can still be done on a PC in a few minutes. By first order expansion of the partition function in the anharmonic parameters we have been able to calculate the density of states for a model molecule composed by anharmonically coupled anharmonic oscillators, which cannot be treated by the well known Beyer–Swinehart algorithm.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464808
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  • 6
    Electronic Resource
    Electronic Resource
    Ring-down cavity absorption spectroscopy of the very weak HCN overtone bands with six, seven, and eight stretching quanta (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6287-6301 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A nonstandard, high sensitivity, absorption detection technique has been applied to the investigation of the very weak fifth, sixth, and seventh overtones of HCN at 100 Torr and 296 K. The frequency range covered is from 17 500 to 23 000 cm−1. We report high resolution, absolute absorption spectra with a noise equivalent sensitivity as low as ∼2×10−9/cm (recently improved to 7×10−10/cm). Band origins, rotational constants, and band intensities are reported and compared with calculated values. The HCN overtone spectra in the present study are not affected by any kind of perturbation, despite the high excitation energy involved.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465866
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  • 7
    Electronic Resource
    Electronic Resource
    The ν1 vibrational predissociation lifetime of (HCN)2 determined from upperstate microwave-infrared double-resonance measurements (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8559-8570 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have successfully applied upper state microwave-infrared double-resonance spectroscopy, with optothermal molecular beam detection, to the determination of the (HCN)2 vibrational predissociation lifetime upon excitation of the outside CH stretching mode (ν1). The measured lifetime is 1.7(5) μs. At this time this is the longest lifetime for a hydrogen bonded complex determined with such precision. We have also been able to accurately measure several upper state transition frequencies, therewith revealing the presence of a perturbation in the spectrum that shifts at least one of the rotational levels by an amount on the order of 1 MHz, and appears to decrease the lifetime of the level measurably. A rate equation approach predicts that this experiment will have a negligible signal to noise ratio for upper state lifetimes much shorter than the infrared (IR) laser–molecular beam interaction time, such as is the case here. In order to explain the presence of a double resonance signal, we have proposed a new mechanism that is based upon a coherent interaction model. The technique of microwave–IR double resonance should be widely applicable to the measurement of dissociation lifetimes in the presently difficult-to-access range from about 100 ns to 100 μs. This is a range in which a significant number of complexes are known to have vibrational predissociation lifetimes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465579
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  • 8
    Electronic Resource
    Electronic Resource
    Alkali–helium exciplex formation on the surface of helium nanodroplets. I. Dispersed emission spectroscopy (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9686-9693 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dispersed emission spectra collected upon the 4 2P3/2,1/2←4 2S1/2 optical excitation of K atoms attached to helium nanodroplets include broad, structured, red-shifted features which are shown to be due to K*He exciplex formation, paralleling our former observation of Na*He [J. Reho, C. Callegari, J. Higgins, W. E. Ernst, K. K. Lehmann, and G. Scoles, Discuss. Faraday Soc. 108, 161 (1997)]. The exciplex formation is demonstrated by the agreement obtained in comparing the K*He A(1) 2Π→X(1) 2Σ emission spectra with the predictions derived from available ab initio potential energy surfaces. Recent analysis of both exciplex emissions also points to the possibility of triatomic (Na*He2 and K*He2) exciplex formation for a small fraction of the alkali atoms. The lack of fluorescence quenching, which is present when the spectra are taken in bulk liquid helium, is due to the surface location of the alkali atoms on the helium droplets that allows the nascent Na*He and K*He exciplexes to desorb from the droplet and emit as isolated molecules. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1321033
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  • 9
    Electronic Resource
    Electronic Resource
    Alkali–helium exciplex formation on the surface of helium nanodroplets. II. A time-resolved study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9694-9701 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have monitored the time evolution of the fluorescence of K*He exciplexes formed on the surface of helium nanodroplets using reversed time-correlated single photon counting. In modeling the present data and that from our previous work on Na*He, we find that partial spin–orbit coupling as well as the extraction energy of helium atoms from the droplet contribute to the observed dynamics of both K*He and Na*He formation, which differ considerably after either D1(n 2P1/2←n 2S1/2) or D2(n 2P3/2←n 2S1/2) excitation for both K(n=4) and Na(n=3). Our quantitative prediction of the Na*He formation dynamics coupled with preliminary data on and modeling of the formation dynamics of K*He allow for extrapolation to the case of Rb*He. Spin–orbit considerations combined with a simple model of helium atom extraction from the matrix reveal the following predicted trend: as the choice of the alkali guest atom is moved down the periodic table, alkali atom–He exciplex formation along the 1 2Π3/2 surface occurs faster while formation along the 1 2Π1/2 surface occurs more slowly, ceasing to occur at all in the case of Rb. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1321034
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  • 10
    Electronic Resource
    Electronic Resource
    Finite size effects and rotational relaxation in superfluid helium nanodroplets: Microwave-infrared double-resonance spectroscopy of cyanoacetylene (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4636-4646 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microwave-infrared double-resonance spectroscopy has been used to probe the solvation environment and its influence on the rotational relaxation of a cyanoacetylene molecule embedded in a superfluid 4He nanodroplet. The results support a model in which (within any given rotational state) the guest molecules are distributed over a set of spectroscopically inequivalent states which are most likely "particle-in-a-box" states originating from the confinement of the guest molecule within the droplet. Revisitation of previously collected microwave–microwave double-resonance data suggests that transitions between these states occur at a rate which is comparable to the rotational relaxation rate, but not fast enough as to produce motionally narrowed, homogeneous absorption lines. The relative intensities of the rotational lines in the microwave-infrared double-resonance spectra are observed to depend strongly on the average droplet size. In the large droplet limit we can explain the observed pattern by invoking a "strong collision" regime, i.e., one in which the branching ratios of the rotational relaxation do not depend on the initial rotational state. For small droplets we speculate that, because of finite size effects, the density of (surface) states may become discontinuous, producing deviations from the "thermal" behavior of the larger systems. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1288604
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