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  • 1
    Electronic Resource
    Electronic Resource
    The sensitivity of absorption experiments with phase locked ultrashort pulses to the excited state potential energy surface (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 2600-2608 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is often believed, with some justification, that the excitation of a molecule with an ultrashort pulse generates wave packets whose behavior is well approximated by classical mechanics. Therefore, it seems that such pulses are not particularly useful for accurate studies of quantum dynamics of an excited molecule; at least not when compared to cw spectroscopy. In this paper we show that this shortcoming can be overcome if one studies the emission of a molecule which absorbs one photon by interacting with two phase locked ultrashort pulses. The excited state population is affected by quantum interference and the dependence of the emission on the delay time between the pulses is very sensitive to small changes in the excited state potential. This sensitivity is as high at room temperature as it is at zero degrees Kelvin. The use of phase locked pulses can distinguish between two potential energy surfaces for which the traditional pump–probe experiments give practically identical results. These statements are documented by exact numerical calculations for the case of the I2 molecule and by analysis. We also discuss the reasons why the rotational motion affects absorption from two phase locked pulses at very short delay times between the pulses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462011
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  • 2
    Electronic Resource
    Electronic Resource
    A time-dependent interpretation of the absorption spectrum of CH3ONO (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1-13 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use time-dependent quantum theory to interpret the absorption spectrum of CH3ONO in terms of the nuclear motion on the upper potential surface. The model uses one excited potential energy surface and two nuclear coordinates: the NO stretch and the CH3O–NO bond. The latter bond breaks upon excitation leading to dissociation. The spectrum consists of a broad band and two progressions corresponding to predissociation resonances. The band width is inverse proportional to the time scale on which the NO bond length increases to adjust to the longer equilibrium bond length of the upper potential energy surface. The progression of intense narrow resonances corresponds to the NO stretching motion. The other progression is due to oscillations along the reaction coordinate of the wave function temporarily trapped in the predissociation well. Our calculations show that important dynamic information can be obtained by "smearing off'' the high resolution spectrum to generate a series of low resolution versions which reveal the time scales on which various spectral features develop in the spectrum. We also show that time-dependent theory can be used efficiently to calculate the lifetime of relatively long lived resonances.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458463
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  • 3
    Electronic Resource
    Electronic Resource
    Vibrational coherence effects in the pump–probe studies of photochemical predissociation (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 3444-3455 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine how the results of pump–probe experiments with short pulses are affected by the coherence of the excitation process, in the particular case when the pump causes the predissociation of the molecule. To document and understand this process we perform exact quantum mechanical calculations and examine simple models. The exact calculations are performed on a model of CH3ONO photodissociation. We examine the effect of coherence when the probe excites the transition complex and when it excites the NO molecule produced by photodissociation. In both cases the vibrational coherence leads to oscillations in the dependence of the two pulse laser-induced fluorescence (LIF) signal on the delay time between the pulses. In the first case the effect is due to the coherent excitation of the predissociation resonances. In the second the important factor is the coherence of the NO vibrational state produced by photodissociation. The physical factors affecting this process are discussed in detail. One of the interesting findings is that the relative translational energy distribution affects in an important way the manner in which vibrational coherence modifies the two pulse LIF signal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460848
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  • 4
    Electronic Resource
    Electronic Resource
    A quantum mechanical study of predissociation dynamics of NaI excited by a femtosecond laser pulse : Chemical Reaction Dynamics Meeting (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6116-6128 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This work was stimulated by the experiments of Rosker, Rose, and Zewail (RRZ) who used a pump–probe sequence of femtosecond pulses to study the predissociation of NaI. We calculate quantum mechanically the nuclear wave function created by the pump pulse and its subsequent evolution. To make contact with the experiments we assume that, depending on its wavelength, the probe pulse is absorbed by either the free sodium atoms or by the bound molecule in the neutral state, and that the measured fluorescence induced by the probe pulse is proportional to the populations of these two "species.'' The similarity between computed populations and the observed signal confirms this conjecture. We study how various factors, such as the pulse length and shape, the initial vibrational state, the temperature, and the diabatic coupling strength affect the populations (and thus the LIF signal). We also show that the RRZ analysis based on classical trajectories and the Landau–Zener formula agrees semiquantitatively with the exact quantum results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456377
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  • 5
    Electronic Resource
    Electronic Resource
    Excitation of symmetric and asymmetric stretch in a symmetric triatomic molecule: A time-dependent (collinear) analysis of the IHI− photodetachment spectrum (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 16-22 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present time-dependent quantum calculations for the collinear IHI−→IHI transition, which has been studied before by time-independent methods by Schatz [J. Chem. Phys. 90, 4847 (1990)] and Bowman and Gazdy [J. Phys. Chem. 93, 5129 (1989)]. The mass ratio of the atoms involved determines the time scales of the symmetric stretch on the one hand and the asymmetric stretch on the other. Both modes are excited in the transition complex. We demonstrate how resonances in the spectrum can be related to the motion of the ionic wave function on a potential energy surface for IHI. The dynamical picture is complementary to the one which is obtained from the stationary state calculations. The system under consideration differs in many aspects from other examples of symmetric triatomic molecules. The dynamics corresponding to structures in the spectrum reveals the peculiarities of heavy–light–heavy systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460391
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  • 6
    Electronic Resource
    Electronic Resource
    CH3ONO predissociation by ultrashort laser pulses: Population transients and product state distribution (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2317-2327 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use a two-dimensional potential energy surface for CH3ONO to model the behavior of the molecule interacting with an ultrashort laser pulse. All the results presented here are obtained by solving accurately the time-dependent Schrödinger equation; when possible we also develop a simple model to explain the exact observations. We calculate the lifetimes of the predissociative resonances, the transient populations induced by semi-infinite pulses, and the time evolution of the product state distribution after excitation with a very short pulse. The behavior of the excited state population created by the pulse is explained in terms of the interference between the amplitudes representing photon absorption at different times when the pulse–molecule interaction is not zero. The time evolution of the final state distribution is explained in terms of interference between the wave packets at different times from the interaction region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457973
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  • 7
    Electronic Resource
    Electronic Resource
    Two-photon excitation of NaI with femtosecond laser pulses (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1596-1602 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have performed a quantum analysis of the two-pulse excitation of NaI, pertinent to the experiments of Rosker, Rose, and Zewail. The model for the NaI molecule is based on early work by Faist, Levine, Grice, and Herschbach. The quantum calculation uses a time-dependent fast Fourier transform method and is exact. We examine the dependence of the laser-induced fluorescence (LIF) signal on the properties of pump and probe pulses and on the dynamics of the excited molecule. Several important differences between the LIF method with ultrashort pulses and the cw case are pointed out.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457118
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  • 8
    Electronic Resource
    Electronic Resource
    The relative kinetic energy distribution of the hydrogen atoms formed by the dissociation of the electronically excited H2 molecule (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1986-1993 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We develop a model describing the relative kinetic energy distribution of the hydrogen atom pair formed by spontaneous photon emission in a transition from the bound state g 3Σ+g of H2 to the repulsive state b 3Σ+u. The results are compared to the experiments of Koot, van der Zande, and Los. The calculations are based on a time dependent theory of the dissociation process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455096
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  • 9
    Electronic Resource
    Electronic Resource
    A theoretical study of I2 vibrational motion after excitation with an ultrashort pulse (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5693-5699 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We calculate the population created by a short pump pulse exciting the I2 molecule to the bound region of the B state, followed by excitation with a short probe pulse to the state E (or F). The nuclear state produced by the pump oscillates in the well of the B state and the probe is absorbed to populate the E (or F) state only when the wave function passes through the Franck–Condon region of the B→E (or B→F) transition. Because of this, the population on the E (or F) state oscillates with the delay time between the pump and the probe. The calculations agree with the experiment in the case when the probe excites the E state. When the F state is excited the theory predicts a doublet structure which is not observed; moreover, in some cases the experiment and theory differ at the shortest delay times. We discuss the dependence of the LIF signal on the pulse width and the initial state, the long time behavior of the LIF signal, and illustrate the role of the population transients on the B states at early times, during or immediately after the pump pulse acted on the molecule.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459563
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  • 10
    Electronic Resource
    Electronic Resource
    Periodic orbits and diffuse structures in the photodissociation of symmetric triatomic molecules (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3252-3257 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We reinvestigate the collinear photodissociation of a symmetric triatomic molecule studied a decade ago by Kulander and Light [J. Chem. Phys. 73, 4437 (1980)], within the time-dependent formalism. The diffuse structures in the absorption spectrum are uniquely related to three, well separated recurrences which, in turn, are explained by three generic periodic orbits of the corresponding classical Hamiltonian. One of these orbits describes pure symmetric stretch motion while the other two orbits combine symmetric stretch and hyperspherical motion. The applicability of simple one-dimensional, zeroth-order pictures is reviewed in the light of these new results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458858
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