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  • 1
    Electronic Resource
    Electronic Resource
    Direct measurement of polymer temperature during laser ablation using a molecular thermometer (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 2440-2448 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Photothermal laser ablation is studied using poly-(methyl methacrylate) films doped with a dye, IR-165, which functions as a molecular heater and thermometer. Direct optical measurements of temperature are performed on samples heated by 100 ns near-IR pulses at 1.064 μm, at rates dT/dt≈5×109 deg/s. Below ablation threshold, the heat capacity measured by optical calorimetry is precisely the value obtained by conventional calorimetry. At ablation threshold, the peak surface temperature is Tabl=600 °C. The increase in heat capacity observed above threshold, together with the results of a conventional thermal analysis, is used to determine the weight fraction of material decomposed at ablation time χth=0.02. With increasing pulse energy, the fraction decomposed increases and a more forceful ablation is observed, but the surface temperature does not continue to increase past Tlim=715 °C, which is determined to be the limiting temperature for thermal decomposition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351589
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  • 2
    Electronic Resource
    Electronic Resource
    Shocked molecular solids: Vibrational up pumping, defect hot spot formation, and the onset of chemistry (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 3798-3812 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model and detailed calculations are presented to describe the flow of energy in a shocked solid consisting of large organic molecules. The shock excites the bulk phonons, which rapidly achieve a state of phonon equilibrium characterized by a phonon quasitemperature. The excess energy subsequently flows into the molecular vibrations, which are characterized by a vibrational quasitemperature. The multiphonon up pumping process occurs because of anharmonic coupling terms in the solid state potential surface. Of central importance are the lowest energy molecular vibrations, or "doorway'' modes, through which mechanical energy enters and leaves the molecules. Using realistic experimental parameters, it is found that the quasitemperature increase of the internal molecular vibrations and equilibration between the phonons and vibrations is achieved on the time scale of a few tens of picoseconds. A new mechanism is presented for the generation of "hot spots'' at defects. Defects are postulated to have somewhat greater anharmonic coupling, causing the vibrational temperature in defects to briefly overshoot the bulk. The influence of the higher defect vibrational temperature on chemical reactivity is calculated. It is shown that even small increases in defect anharmonic coupling have profound effects on the probability of shock induced chemistry. The anharmonic defect model predicts a size effect. The defect enhanced chemical reaction probability is reduced as the particle size is reduced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457838
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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    Preexponential-limited solid state chemistry: Ultrafast rebinding of a heme–ligand complex in a glass or protein matrix (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1825-1836 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ultrafast spectroscopy is used to investigate the temperature dependence of a bimolecular chemical reaction occurring at reaction centers embedded in a glycerol:water glass. The reaction centers consist of carbon monoxide bound to protoheme (PH–CO), or to myoglobin at pH=3 (Mb3–CO), a protein containing PH–CO with a broken proximal histidine–Fe bond. These systems have in common a small energetic barrier for rebinding of the photodissociated ligand. In the glass, the ligand is caged, so that only geminate rebinding is possible. Rebinding is not exponential in time. For t(approximately-greater-than)20 ps, the survival fraction of deligated heme N(t)∝t−n(n≥0). Below 100 K, rebinding is dominated by an inhomogeneous distribution of activation enthalpy P(ΔH
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459957
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  • 5
    Electronic Resource
    Electronic Resource
    Diffusion can explain the nonexponential rebinding of carbon monoxide to protoheme (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8771-8776 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The recombination after flash photolysis of carbon monoxide (CO) to protoheme (PH) in glycerol: water is studied over ten decades in time (1 ps to 10 ms). The rebinding consists of an initial nonexponential geminate phase followed by a slower exponential bimolecular phase. The entire time course of this reaction between 260 and 300 K can be explained in a unified way using a simple, analytically tractable diffusion model involving just three parameters: the relative diffusion constant, the contact radius, and the intrinsic rate of reaction at contact.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459265
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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    Theory of ultrahot molecular solids: Vibrational cooling and shock-induced multiphonon up pumping in crystalline naphthalene (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1695-1710 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method is presented for calculating ultrafast vibrational energy redistribution in anharmonic solids composed of large molecules. It is an improvement over the previous weak coupling model of Hill and Dlott [J. Chem. Phys. 89, 842 (1988)] because the emitted phonons are now allowed to act back on the excited vibrations. The model is used to investigate the dynamics of "ultrahot'' molecular solids, materials with enormous levels of vibrational or phonon excitation. Ultrahot solids are produced in laser ablation and shock-induced detonation. Using model parameters for crystalline naphthalene, we investigate multiphonon up pumping after a 40 kbar shock and vibrational cooling after strong excitation of a high frequency vibrational fundamental. In both processes, the phonons attain a state of internal equilibrium characterized by a time-dependent phonon quasitemperature θp(t) within a few ps. Energy redistribution among the phonons is efficient because phonons are more anharmonic than molecular vibrations. In up pumping, there is a large excess of phonons at t=0, which decreases as vibrations are pumped by phonons. Under these conditions, the rates of anharmonic scattering processes are maximum at t=0 and the lower levels of the ladder of molecular vibrations are pumped before the higher levels. The vibrational population distribution then rapidly attains an approximate state of quasiequilibrium, characterized by a vibrational quasitemperature θv(t). Thermal equilibrium where θp(t) = θv(t) is achieved in ∼100 ps. In vibrational cooling, there is initially a large excess of high frequency vibrations and few phonons. Because phonons accumulate as the vibrations cool, the rates of anharmonic scattering processes are a minimum at t=0. Under these conditions, the vibrations are far from a state of quasiequilibrium until thermal equilibrium is attained at ∼1 ns.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459097
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  • 8
    Electronic Resource
    Electronic Resource
    Vibrational cooling in large molecular systems: Pentacene in naphthalene (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 3590-3602 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ultrafast laser experiments are conducted on low temperature crystals of pentacene in naphthalene (PTC/N) to study the process of vibrational cooling. A vibration of the excited singlet state, denoted Sν1, is excited, and the decay out of this state, as well as the subsequent arrival at the vibrationless ground state S01, are monitored by photon echoes, absorption recovery, and a new technique, pump-induced coherent Stokes Raman scattering [T.-C. Chang and D. D. Dlott, Chem. Phys. Lett. 147, 18 (1988)]. Eight vibrational modes of PTC, ranging from 260 to 1350 cm−1 are studied. The experimental results are interpreted using a previously developed model of vibrational cooling [J. R. Hill and D. D. Dlott, J. Chem. Phys. 89, 830 (1988)]. This model predicts the dependence of the vibrational cooling rate on the amount of excess vibrational energy and the temperature. The motion of the vibrational probability distribution toward the ground state is predicted to occur with a temperature independent "vibrational velocity'' which describes the rate of vibrational energy dissipation. Using the model, we fit all eight data sets with a single adjustable parameter, the vibrational velocity, and we obtain the value V0=10±2 cm−1/ps. The prediction of a nearly temperature independent V0 is confirmed over the temperature range 1.5 to 35 K. Finally, we discuss the application of these measurements to the problem of heme cooling in optically excited heme proteins.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455818
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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    Real time ultrafast spectroscopy of shock front pore collapse (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 5139-5146 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Shock-wave induced nanopore collapse (average diameter 100 nm) at 4.2 GPa in a 3-μm-thick poly-methyl methacrylate (PMMA) layer is measured in real time using coherent anti-Stokes Raman spectroscopy (CARS). Pore collapse is monitored via CARS transitions of a dye probe embedded in the porous medium. A pore collapse time constant of 3 ns in PMMA is in poor agreement with hydrodynamic pore collapse models but in excellent agreement with a viscoplastic model that uses the "shock viscosity" determined from the PMMA viscoelastic response to shock. The shock viscosity is more than 12 orders of magnitude smaller than the ordinary viscosity. A downstream gauge of polycrystalline anthracene monitors changes in the steeply rising shock front (〈25 ps rise time) after passing through the porous medium or a scattering medium with 100-nm-diam scatterers. The anthracene is a two-dimensional (2D) shock gauge that provides a time sequence of CARS spectra S(t,λ). The 2D gauge is shown to be capable of discriminating between a shock front that gradually rises with time constant tr or a bunch of steeply rising shocklets with an arrival time spread equal to tr. The transmitted shock front is shown to consist of a bunch of steep shocklets with an arrival time spread of 550 ps. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1412831
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