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  • 1
    Electronic Resource
    Electronic Resource
    A model for ultrafast vibrational cooling in molecular crystals (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 830-841 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model is presented to describe vibrational cooling (VC) in crystals of large molecules. Vibrational cooling is the process by which a vibrationally excited crystal returns to the ground state. This process may consist of many sequential and parallel vibrational relaxation (VR) steps. The model describes a highly excited, vibrationally dense molecular crystal at zero and finite temperatures. An initially excited vibration relaxes via anharmonic coupling by sequential emission of many lattice phonons until all vibrational energy is destroyed. The time evolution of vibrational excitation probability is described with a Master equation. Various models for the phonon density of states, which exerts primary control over the VR process, are considered. It is found that VC occurs on a much slower time scale than VR, and that the rate of VC is only weakly dependent on temperature, even in systems where VR is highly temperature dependent. An important conclusion of this work is that vibrational cooling is described by an ensemble averaged vibrational population distribution function which moves to lower energy states and broadens as time increases. The motion to lower energy is described by a "vibrational velocity'' (emitted energy per unit time) which is independent of temperature, while the width of the distribution increases with increasing temperature. The model is then used to calculate experimental observables including time resolved absorbance, emission, and Raman scattering following excitation of a high frequency vibration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455206
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  • 2
    Electronic Resource
    Electronic Resource
    Theory of vibrational cooling in molecular crystals: Application to crystalline naphthalene (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 842-858 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The process of vibrational cooling (VC) is theoretically investigated in the molecular crystal naphthalene. Specificially we consider the process where a highly excited vibration cools by emitting lower energy vibrations (vibrational relaxation, or VR) and phonons. We also consider the subsequent cooling of emitted optic phonons by emission of acoustic phonons. Using previously determined vibrational lifetimes [J. R. Hill et al., J. Chem. Phys. 88, 949 (1988)], a consistent transition rate matrix is obtained which describes VR of all vibrations and optic phonons at all temperatures. Then a Master equation is solved numerically to obtain the time dependent vibrational populations of all states following impulse excitation of a high frequency vibration. These results are compared to a previously derived analytic model for VC in molecular crystals [J. R. Hill and D. D. Dlott, J. Chem. Phys. 89, 830 (1988)]. In that theory, which is shown to be in good agreement with the naphthalene calculation, the excess vibrational excitation moves to lower energy states and broadens as time increases. The motion toward lower energy states is described by a temperature independent "vibrational velocity'' (emitted energy per unit time). In naphthalene, the vibrational velocity is V0 ≈9 cm−1 /ps. The VC process occurs on a time scale as much as an order of magnitude longer than an individual VR step. Although VR is highly temperature dependent, VC is not. The VC calculations are used to predict the decay from the initial state, the time dependent populations of transient vibrational excitations, and the return to the vibrationless ground state. All these quantities are directly related to experimental observables such as incoherent anti-Stokes Raman scattering and hot luminescence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455207
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  • 3
    Electronic Resource
    Electronic Resource
    Vibrational relaxation of guest and host in mixed molecular crystals (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 2361-2371 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational relaxation (VR) of dilute impurity molecules (naphthalene, anthracene) in crystalline host matrices (durene, naphthalene) is studied with the ps photon echo technique. The results obtained by echoes on vibrations in the electronically excited state are compared to previous ps time delayed coherent Raman studies of ground state vibrations of the pure host matrix. The relaxation channels for guest and host, and the effects of molecular and crystal structure on VR rates are determined.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454019
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  • 4
    Electronic Resource
    Electronic Resource
    Ultrafast microscopy of shock waves using a shock target array with an optical nanogauge (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 4975-4983 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A large area shock target array was fabricated. By moving the array through a ps pulsed laser beam, shock waves could be reproducibly generated at a high repetition rate of up to ten shocks per second. The dynamics of shock wave propagation through various layers of the array were studied using optical nanogauges. A nanogauge is a sub micron thick layer whose optical properties are affected when the shock front passes through the layer. Since shock velocities are typically a few nm/ps, nanogauges can be used to study picosecond time scale shock dynamics. Using picosecond optical microscopy on targets with different thickness aluminum layers, it was found that the shock required 0.5 ns to form and then it propagated for a few ns with a constant velocity of 8.3 km/s (8.3 nm/ps), indicating a shock pressure of 49 GPa. The arrival time jitter of many hundreds of shocks, at an aluminum/polymer interface was found to be ±50 ps. The shock propagation through a polymer, polyester, was studied by observing the arrival of the front at a 50 nm thick nanogauge embedded in the polymer. When the shock was transmitted from the aluminum to a polymer layer, its velocity was 5.5 km/s, indicating a shock pressure of 14 GPa, in good agreement with shock impedance calculations. The shock target array is a flexible method of studying picosecond time scale dynamics of materials at and just behind the shock front. The use of different optical nanogauges, such as dye-doped polymer films, which can sense the temperature, pressure, and which indicate multiphonon up pumping, is briefly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355788
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  • 5
    Electronic Resource
    Electronic Resource
    Molecular dynamics observed 60 ps behind a solid-state shock front (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 8313-8321 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microfabricated monolithic shock target arrays with embedded thin layers of dye-doped polymer films, termed optical nanogauges, are used to measure the velocity and pressure (Us=3.5 km/s; P=2.1 GPa) of picosecond-laser-driven shock waves in polymers. The 60 (±20) ps rise time of absorbance changes of the dye in the nanogauge appears to be limited by the transit time of the shock across the 300 nm thick gauge. The intrinsic rise time of the 2 GPa shock front in poly-methyl methacrylate must therefore be ≤60 ps. These measurements are the first to obtain picosecond resolution of molecular dynamics induced by the passage of a shock front through a solid. Good agreement was obtained between the nanosecond time scale shock-induced adsorption redshift of the dye behind the P=2 GPa shock front, and the redshift of a nanogauge, under conditions of static high pressure loading in a diamond anvil cell at P=2 GPa. Transient effects on the ≈100 ps time scale are observed in the dye spectrum, primarily on the red absorption edge where hot-band transitions are most significant. These effects are interpreted as arising from transient overheating and subsequent fast cooling of the dye molecules behind the shock front. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470143
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  • 6
    Electronic Resource
    Electronic Resource
    Vibrational relaxation and vibrational cooling in low temperature molecular crystals (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 949-967 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The processes of vibrational relaxation (VR) and vibrational cooling (VC) are investigated in low temperature crystals of complex molecules, specifically benzene, naphthalene, anthracene, and durene. In the VR process, a vibration is deexcited, while VC consists of many sequential and parallel VR steps which return the crystal to thermal equilibrium. A theoretical model is developed which relates the VR rate to the excess vibrational energy, the molecular structure, and the crystal structure. Specific relations are derived for the vibrational lifetime T1 in each of three regimes of excess vibrational energy. The regimes are the following: Low frequency regime I where VR occurs by emission of two phonons, intermediate frequency regime II where VR occurs by emission of one phonon and one vibration, and high frequency regime III where VR occurs by evolution into a dense bath of vibrational combinations. The VR rate in each regime depends on a particular multiphonon density of states and a few averaged anharmonic coefficients. The appropriate densities of states are calculated from spectroscopic data, and together with available VR data and new infrared and ps Raman data, the values of the anharmonic coefficients are determined for each material. The relationship between these parameters and the material properties is discussed. We then describe VC in a master equation formalism. The transition rate matrix for naphthalene is found using the empirically determined parameters of the above model, and the time dependent redistribution in each mode is calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454175
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