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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 7883-7893 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report direct measurements of the state-to-state rotational energy transfer rates for N2 (υ=1) at 298 K. Stimulated Raman pumping of Q-branch (υ=1←0) transitions is used to prepare a selected rotational state of N2 in the υ=1 state. After allowing an appropriate time interval for collisions to occur, 2+2 resonance-enhanced multiphoton ionization is used (through the a 1Πg←X 1Σ+g transition) to detect the relative population of the pumped level and other levels to which rotational energy transfer has occurred. We have performed a series of measurements in which a single even rotational level (Ji=0–14) is excited and the time-dependent level populations are recorded at three or more delay times. This data set is then globally fit to determine the best set of state-to-state rate constants. The fitting procedure does not place any constraints (such as an exponential gap law) on the J or energy dependence of the rates. We compare our measurements and best-fit rates with results predicted from phenomenological rate models and from a semiclassical scattering calculation of Koszykowski et al. [J. Phys. Chem. 91, 41 (1987)]. Excellent agreement is obtained with two of the models and with the scattering calculation. We also test the validity of the energy-corrected sudden (ECS) scaling theory for N2 by using our experimental transfer rates as basis rates (J=L→0), finding that the ECS scaling expressions accurately predict the remaining rates.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 4112-4114 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: When an anisotropic ensemble of isolated atoms or molecules absorbs radiation from a polarized laser beam and the resulting fluorescence is collected independent of wavelength (1+1 unresolved LIF) with optional polarization analysis, the moments of the spatial distribution of the angular momentum J of the initial state in the ensemble may be determined. Previously, work by A. C. Kummel, G. O. Sitz, and R. N. Zare [J. Chem. Phys. 88, 7357 (1988)] correctly derived what possible moments can be found when cylindrical symmetry is present. This work shows that several additional alignment moments, not mentioned in KSZ, can be found in ensembles having noncylindrical symmetry.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 489-489 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4682-4687 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report measurements of the collisional decay of the angular momentum alignment for N2 (v=1) at 298 K. Stimulated Raman pumping of S-branch (v=1←0) transitions is used to prepare a selected rotational state of N2 with an anisotropic spatial J distribution in the v=1 state. After allowing an appropriate time interval for collisions to occur, 2+2 resonance-enhanced multiphoton ionization is used (through the a 1Πg←X 1Σ+g transition) to detect the relative population and alignment of the pumped level and other levels to which rotational energy transfer has occurred. We have performed a series of measurements in which a selected even rotational level (Ji=0–14) is excited and the time-dependent level population and alignment are measured at several delay times. We find the decay of alignment to be no faster than the decay of the population of the pumped level, indicating that pure m-state changing collisions are slower that J-state changing collisions. We have also observed substantial alignment of molecules which have undergone a J-state changing collision. The alignment is this case is found to be inconsistent with a Δm=0 selection rule often used in models of rotational energy transfer rates.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 495-503 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A comparison between experiment and theory is performed for the scattering of (v=1, j=1) H2 from Cu(100) at normal incidence. Experimentally, this system was studied using molecular beam techniques, with stimulated Raman pumping employed to overpopulate (v=1, j=1) in the incident beam, and resonance enhanced multi-photon ionization used to detect the H2 scattered in two (v=1, j) states, and two (v=0, j) states. Theoretically, six-dimensional wave packet calculations were performed, employing a new, extended potential energy surface that was computed with density functional theory, using the generalized gradient approximation and a slab representation of the metal surface. Theory and experiment are in good agreement for the survival probability, i.e., the probability for rovibrationally elastic scattering. However, the theory overestimates the probabilities for rotationally inelastic scattering (to v=1, j=3) and for rovibrationally inelastic scattering (to v=0, j=5 and 7) for channels that could be determined experimentally. The cause of these discrepancies is discussed, as are possibilities for future improvements in the theory as well as the experiment. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2538-2551 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational and vibrational distributions of H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces following atomic permeation are studied using multiphoton ionization combined with time-of-flight mass spectrometry. Rotational distributions are found to be non-Boltzmann and to possess mean rotational energies which are 80%–90% of the surface temperature, Ts. These distributions are identical to within the experimental accuracy for H2 and D2 and also for desorption from the (110) and (111) faces. Moreover, the ortho and para nuclear spin modifications of both isotopes are statistically populated. In contrast, the vibrational population ratio, Pv‘=1/Pv‘=0, is found to be as much as 100 times greater than the ratio corresponding to a Boltzmann vibrational population at Ts. Specifically, the Pv‘=1/Pv‘=0 ratio for H2 (D2) is 0.052±0.014 (0.24±0.20) desorbing from Cu(110), and 0.084±0.030 (0.35±0.20) desorbing from Cu(111). For comparison the Boltzmann-at-Ts ratios would be 0.0009 for H2 and 0.0063 for D2 at T=850 K. Simple models are discussed which attempt to account for the qualitative trends of these results. Detailed balance arguments applied to the vibrational distributions measured in recombinative desorption are unable to predict correctly the dissociative adsorption probability as a function of vibration, indicating that these two processes are dynamically different for this system.
    Type of Medium: Electronic Resource
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