Structure and Dynamics from Time Resolved Absorption and Raman Spectroscopy (original) (raw)
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New Journal of Physics, 2008
Time-resolved spectrum after ultrashort pulse excitation revealed fine structure of instantaneous vibronic absorption spectra in a thiophene derivative. The probe photon energy-dependent amplitudes of molecular vibration coupled to the induced absorption were composed of several peaks. An absorbance-change peak-tracking method revealed four vibronic transitions buried in the time-integrated spectra over several vibrational periods of typical molecular vibration. Four vibronic transitions located at 2.024, 1.921, 1.818 and 1.731 eV were found to be correlated among themselves with respect to the photon energies and intensities of the peaks in the difference absorbance change spectra. From the size and sign of the correlation strengths the mechanism of the vibronic coupling was related to non-Condon mechanism and Herzberg-Teller vibronic coupling.
Theoretical Methods for the Analysis of Spectra of Highly Vibrationally Excited Polyatomic Molecules
Laser Chemistry, 1992
The vibrational spectra of classically chaotic systems are usually very complicated and seemingly unassignable. In this paper, two methods for the analysis of spectra of highly vibrationally excited polyatomic molecules are described, and some results for the floppy molecules HCN and LiNC are presented. By application of these methods, relevant information on the underlying dynamics is obtained, thus establishing a bridge between the spectroscopy and the dynamics of these systems.
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Vibrational relaxation in the excited electronic state
physica status solidi (b), 1982
A theory of the temporal evolution of the distribution of the configurational coordinate in the excited electronic state of an impurity centre of a crystal, allowing for the process of pulse excitation, is proposed. The lattice vibrations are considered to be harmonio and the quadratic vibronic interaction is taken into account. I n this model relaxation is caused by the dephasing of phonons conditioned by their dispersion. Formulas are obtained relating the relaxation characteristics regarded with the dynamical Green's function. The present solution takes exactly into consideration the effect of mixing normal modes a t electronic transition and it describes the inf luence of quantum effects upon the classical turning points of the configurational coordinate. The results of the theory are illustrated with the numerical calculations of the distribution of configurational coordinate and optical spectra.
The Journal of Physical Chemistry A, 2011
To investigate the role of quantum effects in vibrational spectroscopies, we have carried out numerically exact calculations of linear and nonlinear response functions for an anharmonic potential system nonlinearly coupled to a harmonic oscillator bath. Although one cannot carry out the quantum calculations of the response functions with full molecular dynamics (MD) simulations for a realistic system which consists of many molecules, it is possible to grasp the essence of the quantum effects on the vibrational spectra by employing a model Hamiltonian that describes an intra-or intermolecular vibrational motion in a condensed phase. The present model fully includes vibrational relaxation, while the stochastic model often used to simulate infrared spectra does not. We have employed the reduced quantum hierarchy equations of motion approach in the Wigner space representation to deal with nonperturbative, non-Markovian, and nonsecular system-bath interactions. Taking the classical limit of the hierarchy equations of motion, we have obtained the classical equations of motion that describe the classical dynamics under the same physical conditions as in the quantum case. By comparing the classical and quantum mechanically calculated linear and multidimensional spectra, we found that the profiles of spectra for a fast modulation case were similar, but different for a slow modulation case. In both the classical and quantum cases, we identified the resonant oscillation peak in the spectra, but the quantum peak shifted to the red compared with the classical one if the potential is anharmonic. The prominent quantum effect is the 1-2 transition peak, which appears only in the quantum mechanically calculated spectra as a result of anharmonicity in the potential or nonlinearity of the system-bath coupling. While the contribution of the 1-2 transition is negligible in the fast modulation case, it becomes important in the slow modulation case as long as the amplitude of the frequency fluctuation is small. Thus, we observed a distinct difference between the classical and quantum mechanically calculated multidimensional spectra in the slow modulation case where spectral diffusion plays a role. This fact indicates that one may not reproduce the experimentally obtained multidimensional spectrum for high-frequency vibrational modes based on classical molecular dynamics simulations if the modulation that arises from surrounding molecules is weak and slow. A practical way to overcome the difference between the classical and quantum simulations was discussed.
Inorganic Chemistry, 1989
The factors that govern the resonance Raman intensities of fundamentals, overtones, and combination bands are quantitatively evaluated. The calculations and interpretation are based on the time-dependent theory of Lee, Tannor, and Heller. From the time-dependent point of view, the Raman intensities are governed by the overlap of the time-dependent wave packet with the final Raman wave function of interest as a function of time. The most important factors are the magnitude of the overlap and the time development of the overlap. The magnitudes of the overlaps for overtones of a given mode are smaller than that for its fundamental, and the magnitude for a combination band is smaller than those of the fundamentals of the modes comprising the combination band. Thus, the overtone and combination bands are weaker than fundamentals. The time development of the overlap depends on both the frequency of the vibration and the displacement of the excited potential surface relative to the ground potential surface along the normal coordinate. The damping of the overlap determines whether short-time or long-time processes dominate the intensities. For large molecules where short-time processes dominate, the larger the initial overlap and the faster the overlap increases with time, the higher the Raman intensity. The intensities of fundamentals, overtone bands, and combination bands will be discussed in terms of the overlap. Qualitative rules for interpreting excited-state molecular properties from the Raman intensities are developed. The spectra of W(CO)5(pyridine) and Rh2(02CCH3)4L2, where L = PPh3 or AsPh3, are analyzed.
2011
Three polycyclic organic molecules in various solvents focused on thermo-dynamical aspects were theoretically investigated using the recently developed statistical quantum mechanical/classical molecular dynamics method for simulating electronic-vibrational spectra. The absorption bands of estradiol, benzene, and cyanoanthracene have been simulated, and most notably, the increase in the spectral intensity for the lowest excited state transition as the temperature is increased observed experimentally is well reproduced. In addition, this method has been extended to treat luminescent processes also, and it is seen that the experimental emission spectrum of cyanoanthracene is also well described. The method still needs further refinement, but results to date, including those presented in this work, document clearly that our model is one which is able to treat the many complex effects that the environment have on electronic absorption and emission spectra. Keywords Organic compounds Á Molecular dynamics Á Photophysical properties Á Electronic spectra 1 Introduction It is well known that chemical photophysics methods can be used to treat the problem of identifying, establishing, and characterizing the interdependencies between polyatomic molecular spatial structures of molecules and the molecules' spectral-luminescent properties: electronic transition energies/frequencies, vibrational frequencies, probabilities for both photo-absorption and photo-emission of various energies and polarization states, and also various radiationless processes, for example, intersystem crossing, and both electronic, vibrational, rotational energy and momentum transfer. Clearly a sound theoretical basis is required for one to rigorously analyze these properties, which are required for one to obtain a better understanding of the processes involved, in particular, for organic, bioinorganic and bioorganic molecules, compounds and complexes. Experimental spectral-luminescent parameters extracted from the spectra should be consistent with those derived from theoretical calculations, which in fact can be described more clearly and precisely. Quantum-mechanical (QM) methods are effective tools to study the photophysical and photochemical properties of various compounds [1, 2]. Calculations of electronic excited states using these approaches give single-point energies corresponding to narrow spectral lines. Quantum mechanical and classical mechanical molecular dynamics (QMMD and Dedicated to Professor Akira Imamura on the occasion of his 77th birthday and published as part of the Imamura Festschrift Issue.