Angular Time-Correlation Functions from Spectra for Some Molecular Liquids

Tests of models of molecular reorientation in liquids

Presentation d'un modele simple pour la reorientation fortement empechee de molecules anisotropes et utilisation dans le calcul des fonctions de correlation temps de reorientation

External electric field effects on absorption, fluorescence, and phosphorescence spectra of a series of unsubstituted diphenylpolyynes have been examined in a PMMA film. The analysis of the electroabsorption spectra indicates that the shorter diphenylpolyynes exhibit only the change in molecular polarizability, whereas the longer ones exhibit the change both in dipole moment and in molecular polarizability following absorption. The finding of the change in dipole moment following absorption of centrosymmetric diphenylpolyynes is interpreted in terms of the symmetry distortion upon doping a polymer film. When the external electric field is applied, the fluorescence yield is reduced and enhanced, respectively, in diphenylacetylene and diphenyloctatetrayne, indicating that the rate of the nonradiative process from the fluorescence state is accelerated in diphenylacetylene and decelerated in diphenyloctatetrayne by an external electric field. All of the diphenylpolyynes used in the present study exhibit the change in molecular polarizability following the phosphorescence process.

The equilibrium geometries of the methyl halides

Stark effect (electroabsorption) spectroscopy of photosynthetic reaction centers at 1.5K: Evidence that the special pair has a large excited-state polarizability

Infrared Study of the Adsorption and Reaction of MethylChloride and Methyl Iodide on Silica-Supported Pt Catalysts

The results of a previous simulation for a fluid of rigid heteronuclear diatomic molecules using a shifted force, site-site potential [1] are extended to evaluate several time correlation functions for four state points for the orthobaric liquid. Time autocorrelation functions are reported for the centre of mass velocity, the force, the torque, the angular velocity and the orientational functions P 1 (cos ϑ) and P 2 (cos ϑ). We also report the self and distinct parts of the Van Hove time dependent pair correlation function. The orientational correlation times and functions are compared with experimental results from nuclear magnetic resonance, Raman scattering and far infrared absorption and also with two popular simple models of molecular reorientation; the simulation results are in only moderate agreement with experiment.

Within the framework of probability distributions on projective Hilbert space a scheme for the calculation of multitime correlation functions is developed. The starting point is the Markovian stochastic wave function description of an open quantum system coupled to an environment consisting of an ensemble of harmonic oscillators in arbitrary pure or mixed states. It is shown that matrix elements of reduced Heisenberg picture operators and general time-ordered correlation functions can be expressed by time-symmetric expectation values of extended operators in a doubled Hilbert space. This representation allows the construction of a stochastic process in the doubled Hilbert space which enables the determination of arbitrary matrix elements and correlation functions. The numerical efficiency of the resulting stochastic simulation algorithm is investigated and compared with an alternative Monte Carlo wave function method proposed first by Dalibard et al. [Phys. Rev. Lett. {\bf 68}, 580 (1992)]. By means of a standard example the suggested algorithm is shown to be more efficient numerically and to converge faster. Finally, some specific examples from quantum optics are presented in order to illustrate the proposed method, such as the coupling of a system to a vacuum, a squeezed vacuum within a finite solid angle, and a thermal mixture of coherent states.

On the calculation of correlation functions in nuclear matter

We systematically investigate the underlying relations among different levels of approximation for simulating electronic de-excitations, with a focus on modeling X-ray emission spectroscopy (XES). Using Fermi's golden rule and explicit modeling of the initial, core-excited state and the final, valence-hole state, we show that XES can be accurately modeled by using orbital optimization for the various final states within a Slater-determinant framework. However, in this paper, we introduce a much cheaper approach reliant only on a single self-consistent field for all the final states, and show that it is typically sufficient. Further approximations reveal that these fundamentally many-body transitions can be reasonably approximated by projections of ground state orbitals, but that the ground state alone is insufficient. Furthermore, except in cases where the core-ionization induces negligible changes in polarization, linear-response approaches within the adiabatic approximation will have difficulty in accurately modeling de-excitation to the core level. Therefore, change in the net dipole moment of the valence electrons can serve as a metric for the validity of the linear-response approximation.

Electric field-induced fluorescence quenching has been measured for a series of conjugated polymers with applications in organic light-emitting diodes. Electrofluorescence measurements on isolated chains in a glassy matrix at 77 K show that the quenching efficiency for poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) is an order of magnitude larger than that for either a ladder-type polymer (MeLPPP) or polyfluorene (PFH). This effect is explained in terms of the relatively high probability of field-enhanced internal conversion deactivation in MEH-PPV relative to either MeLPPP or PFH. These data, obtained under dilute sample conditions such that chain-chain interactions are minimal, are contrasted with the much higher quenching efficiencies observed in the corresponding polymer films, and several explanations for the differences are considered. In addition, the values of the change in dipole moment and change in polarizability on excitation (|Δμ| and tr(Δα), respectively) are reported, and trends in these values as a function of molecular structure and chain length are discussed.

Electroabsorption spectroscopy was used to determine the change in polarizability $(\mathbf{\ensuremath{\Delta}}\mathbf{\ensuremath{\alpha}})$ and the change in dipole moment $(\mathbf{\ensuremath{\Delta}}\mathbf{\ensuremath{\mu}})$ upon photoexcitation of $\ensuremath{\beta}$-carotene homologs with an increasing number $(n)$ of conjugated $\mathrm{C}\mathrm{C}$ double bonds. The $\mathbf{\ensuremath{\Delta}}\mathbf{\ensuremath{\alpha}}$ values increased systematically with increasing $n$. Although the magnitude of the intrinsic $\mathbf{\ensuremath{\Delta}}\mathbf{\ensuremath{\mu}}$ values of the homologs was negligible because of their centrosymmetric structures, an anomalous increase in the value of $\mathbf{\ensuremath{\Delta}}\mathbf{\ensuremath{\mu}}$ was observed at $n=9$. The stark-broadening mechanism originally proposed by Kulakov and Parschuk [Chem. Phys. Lett. 325, 517 (2000)] is a possible origin that can generate this anomaly. It is proposed that either the $3\phantom{\rule{0.2em}{0ex}}^{1}A_{g}^{\ensuremath{-}}$ or another intermediate state whose group symmetry is ${A}_{g}$ should be located close in energy to the $1\phantom{\rule{0.2em}{0ex}}^{1}B_{u}^{+}$ state at $n=9$. This increase in the value of $\mathbf{\ensuremath{\Delta}}\mathbf{\ensuremath{\mu}}$ leads to an enhancement of the third-order nonlinear polarizability. We suggest that the energy gap between the gerade- and ungerade-type excited states is an important factor controlling the magnitude of the optical nonlinearity of carotenoids.

Influence of fluorine substitution in nitrosoethylene and their vibrational spectra, calculated with Møller-Plesset perturbation theory of second order

The electronic structure of donor/acceptor polyenes (merocyanines) is studied by means of electroabsorption (Stark) spectroscopy. Molecules belonging to this class exhibit an extreme variability of their linear and nonlinear optical properties. Depending on the nature of the donor and acceptor end groups and the polarity of their solvent environment they can adopt ground state molecular structures between limits with mostly localized double bonds (polyene-like) or fully delocalized double bonds (cyanine-like). By comparing the results obtained for the change in dipole moment, Δμ, and change in polarizability, Δα, upon optical excitation of 12 donor/acceptor polyenes with theoretical predictions, each molecule's location in between these limits can be assigned. The results demonstrate that this position can be understood by considering only the relative electron donating and accepting strengths of the end groups. The solvent dependence of the electronic structure is also studied by measuring Stark spectra for two compounds in different frozen solvents. The results are consistent with a change of the molecule's ground state toward more dipolar structures in polar solvents. Taken together, the donor, acceptor, and solvent dependences of the electronic structure of donor/acceptor polyenes can be described by a simple model based on the gradual change of the molecule’s ground state between polyene-like and cyanine-like structures.

In the simplest kinetic models of collisional narrowing or reduction of the Doppler contribution to a spectral line width, the narrowing process is related to the usual diffusion constant of transport theory. Dicke narrowing requires a correlation between the pre- and post-collisional absorber or emitter electric dipole moment. Pressure broadening on the other hand results from at least a partial destruction of this correlation so that in general pressure broadening and Dicke narrowing are statistically dependent on and correlated with each other. It follows that a spectroscopic diffusion constant is required. A classical phase description (which is easily converted to a semiclassical one) is used here to derive a kinetic equation for which the approximate line shape is obtained by It velocity moment method. The spectroscopic diffusion constant closely resembles the Chapman-Enskog first approximation for the diffusion constant but has mixed in an extra function (the memory) which represents the correlation between collision-induced changes of the dipole moment and velocity changes and the correlation between the pre- and post-collision electric dipole moment. Dicke narrowing can be used to obtain information about the line broadening amplitude SB(b, w) for strong velocity-changing collisions. The Galatry ('weak' collision) and 'strong' collision line-shape functions are obtained as different cutoff approximations in the velocity moment analysis. The present analysis, however, is not limited to specifically weak or strong collisions. The two line-shape formulae are shown to be virtually identical sufficiently far from the line centre and at sufficiently high densities. Convenient, approximate analytical formulae for the half-width are obtained using two different definitions.