Influence Of Molecular Vibrations Research Articles

Ultrafast carrier dynamics at organic donor–acceptor interfaces—a quantum-based assessment of the hopping model

We investigate the charge transfer dynamics of photogenerated excitons at the donor–acceptor interface of an organic solar cell blend under the influence of molecular vibrations. This is examined using an effective Hamiltonian, parametrized by density functional theory calculations, to describe the full quantum behaviour of the relevant molecular orbitals, which are electronically coupled with each other and coupled to over 100 vibrations (via Holstein coupling). This electron–phonon system is treated in a numerically quasi-exact fashion using the matrix-product-state (MPS) ansatz. We provide insight into different mechanisms of charge separation and their relation to the electronic driving energy for the separation process. We find ultrafast electron transfer, which for small driving energy is dominated by kinetic processes and at larger driving energies by dissipative phonon emission connected to the prevalent vibration modes. Using this fully quantum mechanical model we perform a benchmark comparison to a recently developed semi-classical hopping approach, which treats the hopping and vibration time scales consistently. We find qualitatively and quantitatively good agreement between the results of the sophisticated MPS based quantum dynamics and the simple and fast time-consistent-hopping approach.

Theoretical study of generalized oscillator strengths for the low-lying electronic excitations of CH3Cl and CF3Cl

We report a theoretical study of electronic excitation in CH3Cl and CF3Cl by electron impact. Momentum-transfer-dependent generalized oscillator strengths (GOSs) are calculated for transitions to low-lying excited singlet-states at the equation-of-motion coupled-cluster singles and doubles level. The influence of molecular vibration is taken into account in the calculation. The theoretical results show reasonable overall agreement with experimental data reported in the literature. The shapes of the GOS profiles reveal that the 1 1E state of CH3Cl has a valence-Rydberg mixed nature, while that of CF3Cl is of a predominant C–Cl antibonding character. A comparison with the experimental GOSs of CH3Cl provides unambiguous evidence that the 3pe state is lower in energy than the 3pa1 state. Optical oscillator strengths are also calculated and comparison is made with available experimental and other theoretical results.

Molecular vibration as a novel explanatory mechanism for the expression of animal colouration.

Animal colouration is characterized by the concentration of pigments in integumentary structures and by the nanoscale arrangement of constitutive elements. However, the influence of molecular vibration on colour expression has been overlooked in biology. Molecular vibration occurs in the infrared spectral region, but vibrational and electronic properties can influence each other. Thus, the vibration of pigment molecules may also affect their absorption properties and the resulting colours. We calculated for the first time the relative contribution of molecular vibration (by means of Raman spectroscopy) and concentration (by means of HPLC) of melanin polymers, the most common animal pigments, to generate diversity in plumage colour in 47 species of birds. Vibrational characteristics explained >9 times more variance in colour expression than the concentration of melanins. Additionally, we modelled melanin Raman spectra on the basis of the chemical structure of their constituent monomers and calculated the Huang-Rhys factors for each vibrational mode, which indicate the contribution of these modes to the electronic spectra responsible for the resulting colours. High Huang-Rhys factors frequently coincided with the vibrational modes of melanin monomers. Our results can be explained by the influence of molecular vibration on the absorption properties of melanins. The colour of organisms may thus mainly result from the vibrational properties of their molecules and only residually from their concentration. As a given melanin concentration can give rise to different colours because different structural melanin conformations can present different vibrational characteristics, vibrational effects may favour phenotypic plasticity and thus constitute an important evolutionary force.

Influence of molecular vibration and rotation on wall heat flux in oscillating rarefied flow

In high temperature environments, molecular vibration and rotation become considerable in flow analysis. We employed unsteady direct simulation Monte Carlo to thoroughly investigate the effects of molecular vibration and rotation on the dynamic response of wall heat flux due to the oscillating flow input in a rarefied environment with high temperature. Three modes of temperature and heat flux (translational, vibrational and rotational) are distinctively quantified to identify the details of the response, in addition to the density and velocity fluctuations. With an increase in the medium temperature, the contribution of internal energy to the heat flux is found to significantly increase; however, the responses vary with the characteristic time of each mode. Besides their usefulness in contributing to the rapid development of space applications, the findings are expected to be significant in designing a thermal protection system for space vehicles in the future.

Ionization behavior of molecular hydrogen in intense laser fields: Influence of molecular vibration and alignment

The alignment- and internuclear-distance dependent ionization of H$_2$ exposed to intense, ultrashort laser fields is studied by solving the time-dependent two-electron Schr\"odinger equation. In the regime of perturbative few-photon ionization, a strong dependence of the ionization yield on the internuclear distance is found. While this finding confirms a previously reported breakdown of the fixed-nuclei approximation for parallel alignment, a simpler explanation is provided and it is demonstrated that this breakdown is not due to vibrational dynamics during the laser pulse. The persistence of this effect even for randomly aligned molecules is demonstrated. Furthermore, the transition from the multiphoton to the quasi-static (tunneling) regime is investigated considering intense 800 nm laser pulses. While the obtained ionization yields differ significantly from the prediction of Ammosov-Delone-Krainov rates, we find a surprisingly good quantitative agreement after introducing a simple frequency-dependent correction to the standard tunneling formula.

Influence of molecular vibration on enhancements in high-order above-threshold ionization of hydrogen molecules

Single ionization of hydrogen and deuterium molecules in strong laser pulses is studied by numerical integration of the time-dependent Schrödinger equation. We investigate the effect of the molecular vibrational energy scale on the yield in the rescattering plateau of above-threshold ionization spectra. Strong correlations between ejected electrons and the vibrational motion of the remaining ions are found.

Inelastic effects in molecular junction transport: scattering and self-consistent calculations for the Seebeck coefficient

The influence of molecular vibration on the Seebeck coefficient is studied within a simple model. Results of a scattering theory approach are compared with those of a full self-consistent non-equilibrium Green's function scheme. We show, for a reasonable choice of parameters, that inelastic effects have a non-negligible influence on the resulting Seebeck coefficient for the junction. We note that the scattering theory approach may fail both quantitatively and qualitatively. The results of calculations with reasonable parameters are in good agreement with recent measurements [Science 315, 1568 (2007)].

Molecular beam investigation of hydrogen dissociation on Si(001) and Si(111) surfaces.

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2 x 1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for nu=1 and nu=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7 x 7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2 x 1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.

Coherent Phonon Scattering in Molecular Devices

In the present work we investigate the influence of molecular vibrations on the tunneling of electrons through a molecule sandwiched between two metal contacts. The study is confined to the elastic scattering only, but beyond the harmonic approximation. The problem is tackled both from a classical and a quantum-mechanical point of view. The classical approach consists in the computation of the time-dependent current fluctuations calculated at each step of a molecular dynamics (MD) simulation. On the other hand, the vibrational modes are treated quantum-mechanically and the tunneling current is computed as an ensemble average over the distribution of the atomic configurations obtained by a suitable approximation of the density matrix for the normal mode oscillators. We show that the lattice fluctuations modify the electron transmission. At low temperatures the quantum-mechanical treatment is necessary in order to correctly include the zero-point fluctuations. However, for temperatures higher than few hundreds Kelvin the simple harmonic approximation which leads to the phonon modes breaks because the oscillation amplitudes of the lowest energy modes become large.

Electron–phonon interactions in poly(para-phenylene) oligomers

The influence of molecular vibrations on the low-lying electronic levels of short oligomers of poly(para-phenylene) is studied. Within an interacting molecular orbital basis, the complete electron–phonon Hamiltonian is diagonalized numerically. Energy shifts and equilibrium phonon displacements are calculated for the totally symmetric vibrational modes. For the 1 1Ag+–1 1Bu− transition, Huang–Rhys and Franck–Condon factors, as well as the change of equilibrium geometry, are computed. The importance of vibrations with a frequency of about 0.2 eV is confirmed. The intrinsic Stokes shift is found to be zero, in agreement with experimental data.

Influence of molecular vibration and transport model on computation of the wall heat flux

Zuppardi and Verde (Improved Fay–Riddell prcoedure to compute the stagnation point heat flux, Journal of Spacecraft and Rockets 35(3) (1998) 403–405) introduced some operating improvements in the Fay–Riddell computing procedure for the solution of the laminar boundary layer and for the computation of the heat flux at the stagnation point of spherical bodies in high speed, non-equilibrium dissociating air. Zuppardi and Verde pointed out that the most important improvements were related to the computation both of vibrational temperatures and of Prandtl and Lewis numbers. Vibrational temperatures were computed only as functions of the free stream thermodynamic parameters. Prandtl and Lewis numbers were computed as functions of both the free stream and the local thermodynamic parameters in the boundary layer. The Chapman–Enskog theory with the Lennard–Jones collision integrals were used to compute the transport coefficients. In the present paper the effects of the variability of the vibrational temperatures also in the boundary layer, and of using the more updated collision integrals by Yun and Mason (Collision integrals for the transport properties of dissociating air at high temperature, The Physics of Fluid 5(4) (1962) 380–386) have been evaluated. The results showed that the influence of these factors on the profile of the thermo-fluid–dynamic parameters in the boundary layer was negligible, but it was important on the wall heat flux. The inclusion of the variability of molecular vibration improved the matching both with Navier–Stokes results and experimental data.

Influence of molecular vibrations on dissociative adsorption

The influence of molecular vibrations on dissociative adsorption is studied by six-dimensional quantum dynamical calculations. For the system H 2 at Pd (100), which possesses non-activated pathways, it is shown that large vibrational effects exist and that they are not due to a strongly curved reaction path and a late dissociation-hindering minimum barrier, as was previously assumed. Instead, they are caused by the lowering of the H-H vibrational frequency during the dissociation and the multi-dimensionality of the potential energy surface. Still there are quantitative discrepancies between theory and experiment identified.

Vibrations and sticking

The sticking of a particle on a surface requires ultimate dissipation of the translational energy via the phonon bath. This may occur via a direct transfer of energy to phonons on impact but also via a complex route involving some form of intermediate behaviour. This article outlines some ways in which vibrational degrees of freedom influence the sticking behaviour of systems with different types of interactions. These include resonant and non-resonant phonon processes in physisorption systems, rotational-vibrational coupling in non-dissociative chemisorption systems and the influence of molecular vibrations in dissociative chemisorption.

Calculation of isotropic hyperfine coupling constants by the symmetry adapted cluster expansion configuration interaction theory

The isotropic hyperfine coupling constants (HFCCs) are calculated for H2O+⋅, CH2O+⋅, CH3O⋅, CH3NH⋅, CH3CH2⋅, and CH3OCH+⋅3, using the method of SAC (symmetry adapted cluster expansion)-CI. After examining various basis sets, we found that the double-zeta quality basis sets of Dunning are the best among the sets examined. The calculated values agree fairly well with the experimental values. We point out that the conventional configuration selection based on the energy criterion has an inherent limit for the calculation of HFCCs. The influence of molecular vibrations on the HFCCs has been semiquantitatively examined for CH2O+⋅, CH3O⋅, CH3NH⋅, and CH3CH⋅2.

Chlorine-35 NQR study of molecular motion of polycrystalline insecticide p,p'-DDT. 2. The influence of molecular vibrations and reorientation on the chlorine-35 NQR frequency and linewidth in p,p'-DDT

Chlorine-35 NQR study of molecular motion of polycrystalline insecticide p,p'-DDT. 2. The influence of molecular vibrations and reorientation on the chlorine-35 NQR frequency and linewidth in p,p'-DDT

The Influence of Molecular Vibration on Brayton-Cycle Performance

By means of the harmonic-oscillator approximation, the influence of molecular vibration on Brayton-cycle performance is demonstrated.