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  • 1
    Electronic Resource
    Electronic Resource
    Femtosecond time-resolved ionization spectroscopy of ultrafast internal-conversion dynamics in polyatomic molecules: Theory and computational studies (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7806-7822 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A framework for the theoretical description of two-pulse time-resolved ionization spectroscopy of ultrafast excited-state dynamics of polyatomic molecules is developed. The radiation–matter interaction as well as intramolecular couplings in the excited-state manifold are treated nonperturbatively by solving the time-dependent Schrödinger equation. The numerical solution is based on a discretization of the ionization continua which becomes particularly efficient for ultrashort laser pulses. With this method converged computations of ionization signals become possible even for complex molecular systems. Computer simulations are performed for a model system representing three-dimensional non-Born–Oppenheimer excited-state dynamics on conically intersecting potential-energy surfaces (the S1 and S2 surfaces of pyrazine). The dependence of the observable time-resolved ionization signals (total ion yield as well as photoelectron spectrum) on the properties of the laser pulses (carrier frequency and pulse duration) is explored. It is demonstrated that ultrafast electronic decay processes as well as coherent vibrational motion in excited states can be monitored by pump–probe ionization with suitable pulses. The dependence of the time-resolved ionization signals on properties of the cation (ionization potentials and potential-energy surfaces) is also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461816
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  • 2
    Electronic Resource
    Electronic Resource
    Resonance Raman spectroscopy of the S1 and S2 states of pyrazine: Experiment and first principles calculation of spectra (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6851-6860 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: New experimental and theoretical data on the resonance Raman (RR) spectroscopy of the S1 and S2 states of pyrazine are presented. Based on recent ab initio CASSCF (complete- active-space-self-consistent-field) and MRCI (multireference configuration interaction) calculations of Woywod et al. [J. Chem. Phys. 100, 1400 (1994)], we construct a vibronic coupling model of the conically intersecting S1 and S2 states of pyrazine, which includes the seven most relevant vibrational degrees of freedom of the molecule. Employing a time-dependent approach that treats the intramolecular couplings in a nonperturbative manner, we calculate RR cross sections for this model, taking explicitly into account the nonseparability of all vibrational modes. The combination of high-level ab initio calculations and multimode propagation techniques makes it possible, for the first time, to make first-principles predictions of RR spectra for vibronically coupled electronic states of an aromatic molecule. The theoretical data are compared to experimental gas-phase RR spectra which have been obtained for five different excitation wavelengths. The comparison reveals that the ab initio predictions match the experimental results in almost every detail. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470689
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  • 3
    Electronic Resource
    Electronic Resource
    Nonperturbative approach to femtosecond spectroscopy: General theory and application to multidimensional nonadiabatic photoisomerization processes (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 3998-4011 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general nonperturbative approach to calculate femtosecond pump-probe (PP) signals is proposed, which treats both the intramolecular couplings and the field-matter interaction (numerically) exactly. Experimentally as well as in a perturbative calculation it is straightforward to distinguish between different spectroscopic processes through the direction of the wave vector of the emitted radiation. A nonperturbative calculation, on the other hand, yields the overall polarization of the system, which is the sum of all these contributions. We present a general and practical method that allows to extract the individual spectroscopic signals, which are resolved in time, frequency, and direction of the emission, from the overall polarization. We briefly derive the basic expressions for the time- and frequency-resolved PP signals under consideration, and discuss in detail the simplifications that arise when the usual assumptions (i.e., weak laser fields, nonoverlapping pulses, slowly-varying envelope assumption and rotating-wave approximation) are invoked. The computational procedure is illustrated by nonperturbative calculations of the polarizations and PP signals for a one-dimensional shifted harmonic oscillator. To demonstrate the capability of the approach we have evaluated the polarization as well as PP signals for a three-dimensional model system with vibronically coupled potential-energy surfaces, which describes ultrafast nonadiabatic isomerization dynamics triggered by the twisting of a double bond. We consider various wavelengths and pulse durations of the laser fields and study integral and dispersed PP spectra as well as coherent photon-echo signals. It is shown that the time- and frequency-resolved PP signals reflect in real time the disappearance of the reactants and the delayed appearance of the products. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469586
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  • 4
    Electronic Resource
    Electronic Resource
    Path-integral treatment of multi-mode vibronic coupling (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 926-937 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A path-integral (PI) approach to real-time quantum dynamics is developed which is suitable to treat the short-time dynamics of vibronic-coupling systems involving many degrees of freedom. The theory is formulated for the case of two electronic states which are coupled by a single active vibrational mode and whose energy separation is modulated by many so-called tuning modes. Time-dependent correlation functions are expressed as sums over all possible paths in the space of two electronic states in discretized time. For each electronic path, the multi-mode vibrational propagator factorizes into a product of single-mode propagators. Introducing the concept of classes of approximately equivalent paths, the summation over paths is replaced by a summation over classes and the computation of propagator averages within each class. It is shown that the propagator averages can efficiently be calculated by a recursive scheme. The performance of the PI method has been tested for a two-state four-mode model representing S1–S2 vibronic coupling in pyrazine. The PI results (time-dependent correlation functions and absorption spectra) are compared with numerically exact reference data which are available for this model. To demonstrate the potential of the path-integral approach for multi-mode problems, calculations are reported for a twenty-four-mode vibronic-coupling model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467253
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  • 5
    Electronic Resource
    Electronic Resource
    Path-integral treatment of multi-mode vibronic coupling. II. Correlation expansion of class averages (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 6499-6510 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A path-integral approach to real-time quantum dynamics is presented which is suitable to treat the dynamics of vibronic coupling or spin boson models. In these models the vibrational dynamics is nonseparable as a consequence of the electronic inter-state coupling. The sum over all possible paths in electronic-state space generated by the usual Trotter procedure is expressed in terms of single-mode averages over classes of paths and statistical mode correlations. The averages for classes of a given length can be calculated iteratively from averages over shorter paths. This expansion is formally exact and finite for a finite number of modes. Usually only a limited number of terms has to be evaluated in order to obtain converged results. The scaling of the computational effort with respect to the number of time steps and the number of modes is given by a low-order power law, depending on the chosen class structure and the order of the expansion. The usual time-dependent wave-packet propagation and the full path enumeration, which exhibit an exponential scaling behavior with respect to either the number of modes or the number of time steps, can be considered as opposite limiting cases of the correlation expansion (CE) of the path integral. The convergence of the CE is tested by application to a two-state four-mode model representing S1-S2 vibronic coupling in pyrazine, for which exact references (time-dependent correlation functions) are available. The potential of the CE approximation for the treatment of multi-mode problems is demonstrated by application to an extended 24-mode vibronic-coupling model. This model is suitable to provide a microscopic description of ultrafast optical dephasing processes in large molecules. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469364
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  • 6
    Electronic Resource
    Electronic Resource
    Characterization of the S1–S2 conical intersection in pyrazine using ab initio multiconfiguration self-consistent-field and multireference configuration-interaction methods (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 1400-1413 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Potential-energy surfaces of the three lowest singlet states of pyrazine have been calculated as a function of ab initio determined ground-state normal coordinates, using complete-active-space self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) techniques. The conical intersection of the S1 and S2 adiabatic potential-energy surfaces has been mapped out in selected subspaces spanned by the most relevant vibrational coordinates. A unitary transformation from the adiabatic to a quasidiabatic electronic representation is performed, which eliminates the rapid variations of the wave functions responsible for the singularity of the nonadiabatic coupling element. Transition-dipole-moment functions have been obtained in the adiabatic and in the diabatic representation. The leading coefficients of the Taylor expansion of the diabatic potential-energy and transition-dipole-moment surfaces in terms of ground-state normal coordinates at the reference geometry have been obtained at the CASSCF/MRCI level. Using a vibronic-coupling model Hamiltonian based on this Taylor expansion, the absorption spectrum of the interacting S1–S2 manifold has been calculated, taking account of the four spectroscopically most relevant modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466618
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  • 7
    Electronic Resource
    Electronic Resource
    Femtosecond spectroscopy of ultrafast nonadiabatic excited-state dynamics on the basis of ab initio potential-energy surfaces: the S2 state of pyrazine (1993)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 97 (1993), S. 12466-12472 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100150a004
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  • 8
    Electronic Resource
    Electronic Resource
    Model study of near-threshold photoionization of large molecules: The effect of vibrational relaxation (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 3174-3185 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The influence of intramolecular vibrational relaxation in large molecules on near-threshold photoabsorption and photoionization processes is investigated. The vibrational relaxation results from coupling of the initially excited modes to a large number of inactive bath modes. Starting from a model Hamiltonian including all vibrational modes, the bath degrees of freedom are eliminated within the Markov approximation using Hilbert-space projection-operator techniques. Additional Feshbach projection techniques and a threshold expansion of Coulomb Green's function are used to cast the resulting expressions into a numerically tractable form. Predissociation channels are included in a phenomenological manner. The numerical results allow us to study the characteristic effects of vibrational relaxation on absorption and ionization spectra. In particular, the competition between autoionization, predissociation and vibrational relaxation of the ion core is investigated. The suppression of the ionization quantum yield above the lowest ionization threshold as a result of these two decay channels is demonstrated. It is shown that the quenching of autoionization by vibrational relaxation or predissociation can be experimentally distinguished on account of the different scaling behaviour of both processes. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473059
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  • 9
    Electronic Resource
    Electronic Resource
    Multilevel Redfield description of the dissipative dynamics at conical intersections (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 263-274 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Redfield theory is applied to investigate the photoinduced dynamics at a conical intersection (the so-called system) which is weakly coupled to a thermal environment (the so-called bath). The dynamics of the system is described by a two-state three-mode model Hamiltonian, chosen to represent the S1(nπ*)–S2(ππ*) conical intersection in pyrazine. Dissipative effects are introduced through a bilinear coupling of the system vibrational modes with a harmonic bath, which represents the remaining vibrational degrees of freedom of the molecule and/or interactions with a condensed-phase environment. The Redfield equations for the reduced density matrix are solved numerically without further approximations. From the reduced density matrix the time evolutions of electronic-state populations and vibrational coherences are obtained, as well as time-dependent probability densities of individual vibrational modes. The results provide a visualization of the essential features of the ultrafast (time scale of 10 fs) internal-conversion process at the conical intersection and the ensuing vibrational cooling process on the lower adiabatic potential-energy surface. The effect of vibrational damping on the linear optical absorption spectrum is also investigated. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1423326
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  • 10
    Electronic Resource
    Electronic Resource
    Symmetriebrechung und Nicht-Born-Oppenheimer-Effekte in Radikalkationen (1983)
    Weinheim : Wiley-Blackwell
    Zeitschrift für die chemische Industrie 95 (1983), S. 221-236 
    Details
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Die verschiedenen elektronischen Zustände, in denen Moleküle existieren können, werden oft als voneinander unabhängig betrachtet. Thematik des vorliegenden Aufsatzes ist es, die Grenzen dieser Annahme zu verdeutlichen, nämlich die Wechselwirkung zwischen verschiedenen elektronischen Molekülzuständen durch die Kernbewegung. Diese Wechselwirkung kann mehrere wichtige Konsequenzen haben, von denen zwei im Detail diskutiert werden. Die erste ist eine Verzerrung des Kerngerüstes des Moleküls, was zu einer Symmetrieerniedrigung in angeregten oder ionischen Zuständen gegenüber dem Grundzustand der neutralen Spezies führt. Allgemeine Merkmale dieser Symmetrieerniedrigung werden anhand typischer Beispiele interpretiert. Die andere Konsequenz der Wechselwirkung ist die Möglichkeit, daß die Atomkerne während ihrer Schwingungsbewegung zwischen verschiedenen Potentialflächen des Moleküls hin- und herspringen (Nicht-Born-Oppenheimer-Effekte). Das Wesen dieses Verhaltens wird untersucht, und es wird gezeigt, daß das “Springen” sehr schnell vor sich gehen und die Kernbewegung vollständig beherrschen kann. Zur Veranschaulichung unserer allgemeinen Ideen benutzen wir die Photoelektronen-Spektren von Ethylen und verwandten Verbindungen und weisen in ihnen das Auftreten starker Nicht-Born-Oppenheimer-Effekte nach. Es stellt sich heraus, daß hier das Franck-Condon-Prinzip bei der Analyse der Schwingungsstruktur versagt.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/ange.19830950306
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