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  • 11
    Electronic Resource
    Electronic Resource
    Photofragment excitation spectroscopy of the formyl (HCO/DCO) radical: Linewidths and predissociation rates of the A˜(A‘) state (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1796-1802 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photofragment excitation (PHOFEX) spectra of the jet-cooled formyl (HCO and DCO) radical have been measured by monitoring laser-induced fluorescence of the CO fragment. The following A˜(A‘)←X˜(A') vibronic transitions were measured: (v1,v2,v3)←(0,0,0) where for HCO v2=6–16 with v1,v3=0; v2=7–12 with v1=1, v3=0; v2=9–12 with v1=0, v3=1 and for DCO v2=14–18 with v1,v3=0. The PHOFEX technique provides a powerful method for discriminating against strong transitions so as to allow assignment and measurement of many weaker Franck–Condon bands. The linewidths of the diffuse transitions lie in the 16–150 cm−1 (FWHM) range; they increase strongly with K' (the projection of the total angular momentum onto the a axis) and decrease slightly with v'2 (the number of HCO/DCO bending quanta). The linewidths are interpreted as radiationless transition rates and yield upper A˜-state lifetimes ranging from 70 to 700 fs. These data are in excellent agreement with a dynamical model of HCO/DCO curve crossing developed by R. N. Dixon [Mol. Phys. 54, 333 (1985)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459953
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  • 12
    Electronic Resource
    Electronic Resource
    Analysis of a model for the ripple phase of lipid bilayers (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6070-6080 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report two-dimensional Monte Carlo, mean-field, and exact one-dimensional studies of a statistical-mechanical model for the ripple (Pβ') phase of hydrated phosphatidylcholine lipid bilayers. The model is a p-state chiral clock model coupled to an Ising model through the chiral field. The microscopic parameters of the model are fixed by independently obtained values for pairwise molecular interactions. The one-dimensional model possesses a "floating fluid phase'' characterized by exponentially decaying, spatially modulated correlations. Solutions of the mean-field equations for the model include modulated phases but these are found to be metastable states, and the mean-field phase diagram is dominated by phases characteristic of the p-state chiral clock model. Monte Carlo simulations also reveal clock model phases. However, when unphysical effects due to mod p counting in the Hamiltonian are eliminated, the Monte Carlo simulations reveal a modulated phase, intermediate in temperature between a high-temperature disordered phase and a low-temperature ordered phase which is identified with the chain tilt (Lβ') phase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459495
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  • 13
    Electronic Resource
    Electronic Resource
    Photodissociation dynamics of acetone at 193 nm: Photofragment internal and translational energy distributions (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7498-7513 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photofragment internal and translational energy distributions resulting from the 193 nm photolysis of acetone have been measured. Vacuum-ultraviolet laser-induced fluorescence was used to probe the CO fragment, and multiphoton ionization time-of-flight mass spectrometry was used to probe the CH3. A Boltzmann distribution was observed to fit each degree of freedom with the following characteristic temperatures: CO@B: Tvib =2700 K, Trot =3000 K, Ttrans =3000 K; CH3@B: Tvib =800 K, Trot =500 K, Ttrans =3500 K. No evidence was found for two distinct CH3 populations, as might be characteristic of a stepwise reaction. Energy partitioning between the fragments was fit well by a simple impulsive model in which the available energy is divided equally between the two dissociating C–C bonds, the two bonds cleaving in rapid succession on a time scale short enough to allow little redistribution of energy into the methyl degrees of freedom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457274
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  • 14
    Electronic Resource
    Electronic Resource
    Unimolecular reactions in isolated and collisional systems: Deuterium isotope effect in the photoisomerization of stilbene (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6697-6707 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The isomerization of t-stilbene (stilbene h12 ) and three deuterated derivatives has been studied in a supersonic expansion, the thermal gas phase, and solution. In the jet we find that almost all effect of full deuteration (stilbene d12 ) is produced by deuteration of the two ethylinic hydrogens only (stilbene d2 ). Complete deuteration of the phenyl rings (stilbene d10 ) has rather little influence on the decay of the jet-cooled molecule. Nonexponential decays are found at intermediate excess energies in the jet-cooled system, with the degree of nonexponentiality decreasing with increasing excess energy. The ordering of the decay rates observed in the jet is not consistent with previous RRKM calculations of the isomerization rates of stilbene h12 and d2. Using similar parameters the calculations consistently place the stilbene d2 and stilbene d10 curves in the wrong order. Our results suggest extensive but not complete vibrational relaxation in the isolated molecule. Vibrational redistribution rapidly becomes complete in the presence of buffer gas. In thermal samples the isomerization rates of stilbene h12 and stilbene d10 are identical over a wide range of solvents and temperatures. By contrast the isomerization rates in stilbene d2 and stilbene d12 are 1.4 and 1.5 times slower than in stilbene h12. Again, these ratios appear constant over a wide range of experimental conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455342
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  • 15
    Electronic Resource
    Electronic Resource
    Is there resonance enhancement of the cross section for vibrational relaxation induced by very low energy collisions? The I2–He system revisited (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6777-6784 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rate coefficients for state-to-field vibrational relaxation of I2 (B 3Π0u+, υ'=16) induced by collisions with He at translational temperatures of 2–12 K are measured using state-selected time-resolved dispersed fluorescence spectroscopy in a supersonic free jet expansion. These vibrational relaxation rate coefficients in the 2–12 K regime are found to be an order of magnitude smaller than comparable rate coefficients at 300 K. The rate data are compared with calculated classical I2–He encounter rates and with rates computed using quantum mechanical cross sections for I2–He calculated by Schwenke and Truhlar. The role of scattering resonances in determining vibrational relaxation rates at low translational temperatures is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455352
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  • 16
    Electronic Resource
    Electronic Resource
    Collisional excitation of CO by 2.3 eV H atoms (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1141-1149 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational and rotational distributions of CO excited by collisions with 2.3 eV H atoms have been obtained by monitoring the products with vacuum ultraviolet (VUV) laser induced fluorescence. Translational-to-vibrational (T→V) transfer is dominated by the dynamics of collisions occurring in the two wells on the H+CO potential energy surface, one characterizing the HCO radical and the other characterizing COH. The measured vibrational distributions agree well with the results of trajectory calculations performed on the ab initio potential energy surface of Bowman, Bittman, and Harding (BBH). The measured rotational distributions show two significant differences from the calculated ones. First, for v=0 the experiments find more population in J〈15 than predicted. This discrepancy may be due to errors in the repulsive part of the BBH surface that is outside the HCO and COH wells, but inside the van der Waals well. Second, for v=1, the experimental distribution is flat from J=0 to J=10, whereas the calculated one rises from near zero at J=0 to a peak at J=12. This discrepancy appears to be the result of an excessively high ab initio estimate (by a few tenths of an eV) of the barrier for H atom addition to CO to form COH.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460020
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  • 17
    Electronic Resource
    Electronic Resource
    The photochemistry of the formyl radical: Energy content of the photoproducts (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6332-6333 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458311
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  • 18
    Electronic Resource
    Electronic Resource
    Temperature dependence of state-to-state vibrational relaxation from the 441(1B2u) state of naphthalene induced by very low energy collisions with argon (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4766-4778 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: State-to-state vibrational relaxation rate coefficients have been obtained for naphthalene colliding with argon in the very low energy collision range. A single vibronic level v'44=1(ν44 is an in-plane ring distortion) is prepared by laser excitation of naphthalene expanded in a supersonic free jet of argon. Relaxation to specific destination states is monitored using time resolved dispersed fluorescence spectroscopy. The observed state-to-state relaxation rate coefficients vary substantially among the available pathways but are explained satisfactorily by simple propensity rules when the destination state involves only out-of-plane vibrational motion. Transfer to the sole in-plane destination state (v24=1) is found to be less facile by more than an order of magnitude relative to that expected from simple propensity rules. Measured propensities are found to be relatively independent of collision energy for the temperature range studied here (3–20 K). This suggests that the translational energy dependence of the state-to-state cross section σif is similar to that for the total inelastic cross section σi, where from previous studies of polyatomic vibrational relaxation in this temperature range it has been demonstrated that σi scales with the Lennard-Jones cross section.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458667
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  • 19
    Electronic Resource
    Electronic Resource
    Temperature dependence of vibrational relaxation in the very-low-energy collision regime: The ground electronic state of p-difluorobenzene prepared by stimulated emission pumping (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3151-3159 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Stimulated emission pumping is used to selectively populate the υ‘3=1 vibrational level (εvib=1258 cm−1) in the S0 state of p-difluorobenzene cooled in a supersonic free-jet expansion of argon. The time-dependent population of the υ‘3=1 level, as it is depleted by collisions with the argon carrier gas, is probed using single vibronic level fluorescence. By varying the point along the expansion axis (X/D) at which state preparation and population probing are carried out, the rate coefficient for vibrational relaxation of the 31 state is measured as a function of temperature in the range 1–12 K. The temperature dependence of the rate coefficient is compared with the temperature dependence of hard-sphere (HS) and Lennard-Jones (LJ) elastic encounter rates. Comparison with data for the same collision system (p-difluorobenzene–Ar) at room temperature in both the S1 and S0 electronic states suggests that the temperature dependence of the Lennard-Jones elastic rate provides a useful means of scaling the temperature dependence of polyatomic vibrational relaxation over a wide range of temperatures, i.e., from 300 K down to near absolute zero.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458849
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  • 20
    Electronic Resource
    Electronic Resource
    Collision partner and level dependence of vibrational relaxation in S0 p-difluorobenzene. Stimulated emission pumping combined with single vibronic level fluorescence spectroscopy (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 4748-4764 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rate coefficients have been measured for vibrational relaxation in S0 p-difluorobenzene vapor under assault from eight collision partners. The dissipation of vibrational energy is monitored from each of four vibrational states with energies in the range ∼1500–3300 cm−1. Rate coefficients are found to vary between 1.5×10−10 cm3 molecule−1 s−1 for helium colliding with p-difluorobenzene in the lowest state explored, to 12×10−10 cm3 molecule−1 s−1 for p-difluorobenzene self-relaxation from the highest level studied. The dependence of collision partner on the vibrational relaxation efficiency for each initially prepared state is studied, revealing a dependence on the reduced mass of the collision pair, and/or the well depth of the system. In addition the dependence of the efficiency on the initially prepared state is investigated for any one collision partner using a modified version of the Parmenter–Tang propensity rule model. A comparison between these data for vibrational relaxation in the S0 state of p-difluorobenzene and previous data for vibrational relaxation in the S1 state of the same molecule is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454688
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