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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new cell design for off-axis amplification of femtosecond dye lasers which uses total internal reflection is reported. Compared to the conventional on-axis configuration, these amplifiers produce less amplified spontaneous emission in the far field and maintain the input beam profile. © 1995 American Institute of Physics.
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Methods of implementation of classical molecular dynamics simulations of moderate size molecule vibrational energy relaxation and analysis of their results are proposed. Two different approaches are considered. The first is concerned with modeling a real nonequilibrium cooling process for the excited molecule in a solvent initially at equilibrium. In addition to the solute total, kinetic, and potential energy evolution, that define the character of the process and the rate constant or relaxation time, a great deal of important information is provided by a normal mode specific analysis of the process. Expressions for the decay of the normal mode energies, the work done by particular modes, and the vibration–rotation interaction are presented. The second approach is based on a simulation of a solute–solvent system under equilibrium conditions. In the framework of linear nonequilibrium statistical thermodynamics and normal mode representation of the solute several expressions for the rate constant are derived. In initial form, they are represented by integrals of the time correlation functions of the capacities of the solute–solvent interaction atomic or normal mode forces and include the solute heat capacity. After some approximations, which are adequate for specific cases, these expressions are transformed to combinations of those for individual oscillators with force–force time correlation functions. As an attempt to consider a strongly nonequilibrium situation we consider a two-temperature model and discuss the reason why the rate constant can be independent on the solute energy or temperature. Expressions for investigation of the energy redistribution in the solvent are derived in two forms. One of them is given in the usual form of a heat transfer equation with the source term describing the energy flux from the excited solute. The other form describes the energy redistribution in the solvent in terms of capacity time correlation functions and can be more convenient if memory effects and spatial dispersion play an important role in energy redistribution in the solvent. © 1999 American Institute of Physics.
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results of nonequilibrium molecular dynamics simulations of vibrational energy relaxation of azulene in carbon dioxide and xenon at low and high pressure are presented and analyzed. Simulated relaxation times are in good agreement with experimental data for all systems considered. The contribution of vibration–rotation coupling to vibrational energy relaxation is shown to be negligible. A normal mode analysis of solute-to-solvent energy flux reveals an important role of high-frequency modes in the process of vibrational energy relaxation. Under all thermodynamic conditions considered they take part in solvent-assisted intramolecular energy redistribution and, moreover, at high pressure they considerably contribute to azulene-to-carbon dioxide energy flux. Solvent-assisted (or collision-induced) intermode energy exchange seems to be the main channel, ensuring fast intramolecular energy redistribution. For isolated azulene intramolecular energy redistribution is characterized by time scales from several to hundreds of ps and even longer, depending on initial excitation. The major part of solute vibrational energy is transferred to the solvent via solute out-of-plane vibrational modes. In-plane vibrational modes are of minor importance in this process. However, their contribution grows with solvent density. The distribution of energy fluxes via azulene normal modes strongly depends on thermodynamic conditions. The contribution of hydrogen atoms to the overall solute-to-solvent energy flux is approximately two to three times higher than of carbon atoms depending on the system and thermodynamic conditions as well. Carbon atoms transfer energy only in the direction perpendicular to the molecular plane of azulene, whereas hydrogen atoms show more isotropic behavior, especially at high pressure. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 8380-8390 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collisional deactivation of vibrationally highly excited azulene was studied from gas into compressed liquid phase by pump-and-probe picosecond laser spectroscopy. Collisional deactivation rates were compared with solvatochromic shifts Δν of the azulene S3←S0 absorption band under identical conditions. Employing supercritical fluids at pressures between 0.03 and 4000 bars and temperatures between 298 and 640 K, measurements covering the complete gas–liquid transition were performed. For the energy transfer experiments, azulene with an energy of ∼20000 cm−1 was generated by laser excitation into the S1- and internal conversion to the S0*-ground state. The subsequent loss of vibrational energy was monitored by following the transient absorption at the red wing of the S3←S0 absorption band near 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all conditions, the energy decays were found to be exponential with phenomenological deactivation rate constants kc. kc and spectral shifts Δν showed quite similar density dependences: the low pressure linear increase of both quantities with density ρ at higher densities starts to level off, before it finally becomes stronger again. The parallel behavior of energy transfer rate constants and solvent shifts becomes particularly apparent near to the critical point: measurements in propane at 3 K above the critical temperature showed that kc and Δν are essentially constant over a broad density interval near to the critical density. These observations suggest that both quantities are determined by the same local bath gas density around the azulene molecule. By Monte Carlo simulations it is shown that kc(ρ) follows an isolated binary collision (IBC) model, if the collision frequency Z is related to the radial distribution function g(r) of an attractive hard-sphere particle in a Lennard-Jones fluid. Within this model, average energies 〈ΔE〉 transferred per ethane–azulene collision are temperature independent between 298 and 640 K and pressure independent between 0.03 and 4000 bars. By means of radial distribution functions the density dependence of Δν can be represented as well. © 1997 American Institute of Physics.
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  • 5
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoisomerization of cis-stilbene in liquid solution was studied by time-resolved excited-state absorption spectroscopy using 306 nm pump and 612 nm probe pulses of 100 fs width. Transient absorption signals were found to decrease exponentially with time. Decay rate constants were determined over the pressure range 1–4000 bars at temperatures of 295 and 390 K in a series of alkane solvents as well as in methanol, acetonitrile, and in polymethylmethacrylate (PMMA). The viscosity dependence confirms the existence of two pathways of the reaction, one leading to ground-state cis- and trans-stilbene (C/TS), the other to dihydrophenanthrene (DHP). Whereas the DHP component shows only little viscosity dependence up to 4 kbar, the C/TS component is characterized by rate constants which are inversely proportional to the solvent viscosity. This is in contrast to earlier conclusions from studies with solvents at 1 bar. The C/TS process shows practically no temperature dependence apart from that of the viscosity, whereas the DHP process has an apparent activation energy of 8.5 kJ mol−1. The results are interpreted in terms of a Kramers–Smoluchowski equation for low-barrier processes as well as by solvent modifications of the low-barrier potential. Alternative interpretations by rate theory for barrierless processes are considered as well.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The pressure and temperature dependence of the photoisomerization rate coefficient of trans-stilbene in the S1 state have been measured in the solvents C2H6, C3H8, C4H10, Xe, Co2, SF6, and CHF3. At constant temperature, the pressure dependences up to 6 kbar can be well represented by the Kramers–Smoluchowski model. The comparison of results in different solvents clearly indicates the importance of reactant–solvent cluster formation modifying the height and imaginary frequency of the barrier. The change of the temperature dependence with pressure points towards a multidimensional barrier of nonseparable character. Multidimensional barrier effects manifest themselves most clearly via the temperature dependence of the rate coefficient in the Kramers–Smoluchowski limit.
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoisomerization of diphenylbutadiene was studied by picosecond absorption spectroscopy over wide pressure and temperature ranges in liquid and supercritical alkanes, CO2, SF6, and He. The reaction shows typical features of a thermal unimolecular reaction on the S1 potential energy surface. The rate can be expressed by a combination of standard unimolecular rate theory and Kramers–Smoluchowski theory. However, multidimensional behavior manifests itself in the transition to the gas phase low pressure range as well as to the high density Kramers–Smoluchowski range: in the former case, the low pressure limit of a unimolecular reaction of the polyatomic molecule is approached; in the latter case, the effective imaginary barrier frequency shows a marked apparent temperature dependence. The experiments also suggest contributions of reactant–solvent cluster interactions, which modify the barrier height even in nonpolar solvents.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The expressions for vibrational energy relaxation (VER) rates of polyatomic molecules in terms of equilibrium capacity time correlation functions (TCFs) derived in the first paper of this series [J. Chem. Phys. 110, 5273 (1999)] are used for the investigation of VER of azulene in carbon dioxide at low (3.2 MPa) and high (270 MPa) pressure. It is shown that for both cases the VER times evaluated on the basis of the same potential model via solute–solvent interaction capacity TCFs by means of equilibrium molecular dynamics (EMD) simulations satisfactorily agree with the nonequilibrium (NEMD) molecular dynamics [J. Chem. Phys. 110, 5286 (1999)] and experimental [J. Chem. Phys. 105, 3121 (1996)] results as well. Thus it follows that these methods can complement each other in characterizing VER from different points of view. Although more computational power and refined methods of dealing with simulated data are required for EMD simulations, they allow the use of powerful tools of equilibrium statistical mechanics for investigating the relaxation process. To this end, an analysis of VER mechanisms on the basis of normal mode and atomic representations is carried out. The influence of temperature and CO2 pressure on azulene normal mode spectra and solvent assisted intermode coupling in connection with the eigenvector structure is investigated in great detail. The normal mode capacity cross-correlation matrix reveals the significance of intermode coupling, which significantly contributes to intramolecular vibrational energy redistribution (IVR). As a new concept, partial normal mode relaxation rates are introduced. It is shown that these rates demonstrate similar properties as the energy exchange rates through particular normal modes in nonequilibrium simulations. Atomic spectra and friction coefficients are characterized by a complicated frequency dependence due to contributions from many normal modes. Atomic capacity TCFs and partial relaxation rates are analyzed and reveal a similar picture to that obtained from NEMD simulations. These results show that VER and IVR cannot be separated from each other and have to be considered as mutually connected processes. © 1999 American Institute of Physics.
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  • 9
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results from nonequilibrium molecular-dynamics simulations of collisional energy transfer from vibrationally highly excited azulene in compressed CO2 are compared with experimental results from our laboratory obtained under comparable physical conditions. As observed in the experiment, the cooling rates show a purely monoexponential decay of the excess energy. The influence of the microscopic solvent shell structure on these processes is investigated using the full three-dimensional anisotropic CO2 structure around azulene obtained from the simulation. The analysis shows that local heating effects of any kind do not play a role in our model system. Predictions of the pressure dependence of the energy transfer rates by the isolated binary collision model are compared with results from the simulations using two different definitions of the collision frequency in dense fluids. © 1998 American Institute of Physics.
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  • 10
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collisional deactivation of vibrationally highly excited azulene was studied from the gas to the compressed liquid phase. Employing supercritical fluids like He, Xe, CO2, and ethane at pressures of 6–4000 bar and temperatures ≥380 K, measurements over the complete gas–liquid transition were performed. Azulene with an energy of 18 000 cm−1 was generated by laser excitation into the S1 and internal conversion to the S0*-ground state. The subsequent loss of vibrational energy was monitored by transient absorption at the red edge of the S3←S0 absorption band near 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all conditions, the decays were monoexponential. At densities below 1 mol/l, collisional deactivation rates increased linearly with fluid density. Average energies 〈ΔE〉 transferred per collision agreed with data from dilute gas phase experiments. For Xe, CO2, and C2H6, the linear relation between cooling rate and diffusion coefficient scaled collision frequencies ZD turned over to a much weaker dependence at ZD(approximately-greater-than)0.3 ps−1. Up to collision frequencies of ZD=15 ps−1 this behavior can well be rationalized by a model employing an effective collision frequency related to the finite lifetime of collision complexes. © 1996 American Institute of Physics.
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