Mechanism Of Vibrational Relaxation Research Articles

Perturbation theory approach to dynamical tunneling splitting of local mode vibrational states in ABA molecules

We introduce a point of view for treating the dynamical tunneling splitting of symmetric local mode vibrational states in ABA molecules (A=H typically) which is the one we have employed in treating the vibrational spectroscopy of CH overtones in molecules such as (CX3)3YCCH. Namely, the vibrational coupling corresponding to the dynamical tunneling in semiclassical mechanics via many intermediate off-resonance weak transitions between initial and final states can be treated by a standard high-order perturbation theory. We apply that method to the present simpler problem of tunneling splittings in ABA molecules, and compare the results with those of exact diagonalization, the semiclassical method, and the periodic orbit quantization. Of all the approximate methods, the perturbation theory was found to provide the best approximation to the results of exact diagonalization for the system treated. The relationship between these three methods and application to the problem of vibrational relaxation in polyatomic molecules with tunneling mechanism of intramolecular vibrational relaxation is discussed.

Mechanisms and rate constants of the collisional deactivation of CO+(A 2Π,v) by He and Ar

Collisional deactivation processes of CO+(A 2Π,v) by He and Ar are investigated in detail using both a time-resolved laser induced fluorescence and a threshold electron-secondary ion coincidence technique. Three deactivation mechanisms, pure vibrational relaxation, internal conversion, and charge transfer reaction, are identified. The time-resolved fluorescence from both the initially photoexcited and vibrationally relaxed levels of CO+ A 2Π makes clear the two distinct vibrational relaxation mechanisms: indirect two-step population of the lower levels in the CO+ (A)+He system and direct transition to the lower levels in the CO+(A)–Ar system. Threshold electron-secondary ion coincidence (TESICO) experiments demonstrate that the charge transfer reactions CO+ (A,v)+Ar→Ar++CO contribute significantly to the overall fluorescence quenching by Ar.

Molecular dynamics in liquid CD 3NO 2 from Raman bandshapes

Vibrational bandshapes of totally symmetric ν 3, ν 4 and ν 5 motions of liquid deuterated nitromethane have been studied as a function of temperature and, for ν 5, also of concentration in CH 3NO 2 solutions. Correlation functions for ν 4 and ν 5 modes are calculated, and hypotheses about vibrational relaxation mechanisms are presented.

Molecular dynamics and vibrational relaxation of ν 4 mode in methylcyanide.

Raman vibrational bandshape of the ν 4 mode of methylcyanide has been studied as a function of temperature in both CH 3CN and CD 3CN isotopomers. The secondary structure of such bands in these molecules has been interpreted and analyzed as a sequence of hot bands: spectral fittings and a suitable vibrational relaxation mechanism are proposed.

Raman study of xenon difluoride spectra in aprotic solvents

There are different theoretical and experimental aspects of solvent influence on solute. Microwave spectroscopy, depolarized Rayleigh scattering and nuclear magnetic resonance are sensitive to this interaction. The most significant information concerning the behaviour of condensed systems in the picosecond region can be obtained from data on vibrational spectroscopy, and Raman spectroscopy in particular. The present work is devoted to the study of the vibrational and rotational motion of xenon difluoride molecules in aprotic solvents such as CH 3CN, CCl 4 and (CH 3) 2SO. Xenon difluoride is the most widely used fluorinating agent in the chemistry of fluorine. Calculations of vibrational G v(t) and reorientational G R(t) correlation functions as well as the corresponding correlation times were carried out. The mechanisms of vibrational and rotational relaxation for ν 1(∑ + g) mode of the XeF 2 molecule in the stated solvents are discussed. A comparison of relaxation process rates was done by taking into account the solvent molecular structure.

Molecular-orbital calculations of the lifetimes of the vibrational modes of CO on Cu(100).

A nonempirical molecular-orbital theory of the relaxation of excited vibrations at metal surfaces by energy transfer to conduction electrons is described. With use of ab initio calculations on cluster models of the adsorbate-surface system, the lifetimes of all four local modes of CO on Cu(100) are obtained. The calculated lifetimes are in qualitative agreement with experiment, and can be understood in terms of simple orbital interpretations of the nonadiabatic vibrational relaxation mechanisms.

Mechanism of vibrational relaxation and intersystem crossing within excited ion-pair states of I2

Visible and ultraviolet fluorescence of I2, following excitation by ArF/193nm excimer laser pulses, was recorded for different pressures of argon buffer gas in a flow system. Dispersed fluorescence spectra due to the transitionsD’(2g) → A’(2y andD(0 n + )→X0 g + ) were analysed by inversion and spectral simulations. Thus vibrational distributions in the emitting states were obtained as a function of pressure to determine the mechanism of relaxation to populate the lowest quantum levels of theD’ state, which are the emitting states in the iodine laser. Fast intersystem crossing is found to occur from initially populated vibrational levels of theD state to other ion-pair states correlating with the ground state ions, followed by rapid relaxation, involving both direct vibrational relaxation within individual states and intersystem crossing between states.

A time-dependent approach to the intramolecular vibrational relaxation mechanism. An application to the Ar…Cl 2(B) complex

In this paper, a time-dependent and a time-independent formalism based on the diabatic configuration interaction (DCI) are presented as a way of studying the intramolecular vibrational relaxation (IVR) phenomenon in the Ar…Cl 2 van der Waals complex. Both formalism show the importance of the mixing of different vibrational states in the metastable initial wavefunction, mainly of the v + 1 and v−1 channels. By so doing, the IVR is explained by means of a pure quantal interference mechanism.

CO on NaCl(100): Model system for investigating vibrational energy flow

Vibrational energy flow from CO physisorbed on NaCl(100) has been investigated. Infrared spectroscopy of monolayer CO reveals the narrowest linewidth reported for any surface-bound molecule. The vibrational band at 2154.98 cm−1 has a full-width at half-maximum (FWHM) of 0.09 cm−1 at 4 K. We believe that this residual linewidth is due to imperfections of the crystal face. Nevertheless it places a lower limit for all vibrational relaxation processes of 60 ps. The linewidth increases above 10 K, a behavior which can be accounted for by a vibrational phase relaxation mechanism. Attempts to photodesorb CO from its substrate by laser excitation into the vibrational absorption band reveal an exceedingly inefficient quantum yield of Φ≤10−7. Finally laser excitations of a multilayer of CO on NaCl(100) at 22 K demonstrate that vibrational energy can be stored in the adsorbed CO long enough to allow fluorescence however relaxation to the substrate reduces the fluorescence lifetime from that of the bulk crystal. These measurements (bandwidth, vibrational desorption quantum yield, and fluorescence lifetime) of monolayer and multilayer CO on NaCl(100) indicate that the vibrational motions are poorly coupled to this alkali halide substrate.

Chaotic passive Q-switching pulsation in a N_2O laser with a saturable absorber

A single-mode N2O laser with NH3 as a saturable absorber is found to exhibit chaotic passive Q switching with multipeaked pulses. The chaotic dynamics is strongly correlated with vibrational relaxation mechanism in the laser medium. The phenomenon is predicted, and its characteristic behavior is well explained, by the rate-equation analysis recently developed for the case of instability in CO2 lasers.

On the Mechanism of Vibrational Relaxation of Two‐Centre Excitions in Solid Neon

AbstractThe population of vibrational levels of the lowest‐lying excimer term in solid neon is reproduced in the experimental luminescence spectra. The analysis of the kinetic equation suggests that the reproduced population can be caused by the vibrational relaxation only in the case of a great heat relase due to the two‐centre excitation jumps through the lattice.

Dynamics for CH3F trapped in rare gas crystals and spectroscopic consequences

The dynamics of CH3F molecule trapped in a rare gas crystal is determined in order to explain the infrared spectra and vibrational relaxation data which had led to controversial qualitative interpretations. It is shown that the orientational motions of the molecular axis are strongly coupled to the translational dynamics of the molecule and, in a smaller extent, to the lattice vibrations. As a first consequence, the molecular axis remains nearly anticollinear to the axis joining the molecular center of mass and the site center, and the molecule behaves as a slightly hindered rotor implying both the molecular axis and its center of mass; the spinning motion appears to be a tunneling motion which considerably narrows the splitting between the k=0 and k=1 levels. As a second consequence, the orientational signals are strongly broadened by the translational dynamics of the molecule and of the crystal. So, the only observed sharp signal with a large foot is interpreted as the superimposition of a pure vibrational Q(1) branch and of broadened rotational structures connected to R( j0) and R( j1) ( j=0,1,2) signals. Moreover, there is not a dominant channel for the vibrational relaxation mechanism of CH3F trapped in argon matrix, since the transfers to the orientational modes, to the local or to the bulk phonon modes are shown to be equally efficient, with times (∼10 μs) in agreement with experimental data. In xenon matrix, the direct transfer to the lattice vibrations seems to be the most efficient mechanism of relaxation.

Mechanism of vibrational relaxation in the nonequilibrium plasma of a vacuum arc

Mechanism of vibrational relaxation in the nonequilibrium plasma of a vacuum arc

On the phonon mechanism of vibrational relaxation of adsorbed particles

The simple perturbation theory or the adiabatic approximation are generally used to describe the phonon mechanism of vibrational relaxation. These approaches are shown to be equivalent to each other provided that the adiabatic wave functions for various normal modes are constructed in the framework of the perturbation theory. The relative role of various terms of an anharmonic coupling between normal modes is discussed. The anharmonic terms that are proportional to the displacement of the absorbed particle from equilibrium, are found to make the main contribution to the one- and two-phonon relaxation. The anharmonic terms that are proportional to the displacements of the surface atoms from the equilibrium positions, yield the dominant contribution to the multiphonon relaxation.

Intra‐ and Intermode VibrationalEnergy Flow in CH3F Excited byIrradiation With an Intense CO2Laser: A Non‐Linear VibrationalRelaxation

The vibrational relaxation mechanism of CH3F whose ν3 mode is excited by a transversely excited atmospheric (TEA)CO2 laser‐pulse has been discussed on the basis of observation of the laser‐induced fluorescence (LIF) of the ν3 overtones and of the C–H stretching modes and kinetic analyses. The time‐evolved LIF in the 3‐μm region was found to be dependent significantly on the laser fluence; at a low fluence (<0.01 J cm−2), the emission intensity increased almost exponentially with a rate similar to the one determined in the experiment with a Q‐switched laser, while at a higher fluence (>0.1 J cm−2) the emission rose much faster to form a peak which decayed quickly before a second broad peak appeared. In the wavelength‐resolved fluorescence measurement, it was found that the kinetics of population in the ν4 level were similar to those in the 3ν3 level, while those in the ν1, 2ν5, ν2 + ν5 and 2ν2 levels behaved in a way different from those in the ν4 level. These observations lead to the conclusion that the laser energy poured into the ν3 mode flows to the ν4 level directly through 3ν3 as well as by the successive inter‐ and intramode V–V energy transfers, i.e., ν3→ν6→ν2, ν5→2(ν2, ν5)→ν1→ν4. A model calculation of the relaxation kinetics, including the direct V–V energy transfer between two 3ν3 and ν4 levels, could reproduce the 3‐μm emission data. Another significant finding is a non‐exponential depopulation in the 2ν3 level, the ν4, ν1 and other vibrational levels in the 3‐μm region in the final stage of the relaxation. The deactivation rate is larger for a higher level and is a decreasing function of time and is much larger than the V–T/R energy transfer rate. determined in the experiment of weak laser excitation. This may be attributed primarily to the successive intermode V–V energy transfers from the 3ν3 to the ν4, ν1, 2ν5, . . levels which lie between the 2ν3 and 3ν3 levels. This process may be repeated if the intramode V–V energy transfer from the 2ν3 to the 3ν3 level occurs. The deactivation mechanism found in this experiment is called ‘catastrophic cyclic path’ which is effective until the relative population distribution in relevant levels between 2ν3 and 3ν3 becomes that in equilibrium at the translational temperature of the CH3F gas.

On the mobility of aldehyde groups

Abstract The complex permittivities of o-, m- and p-toluylaldehydes are measured in the microwave and FIR-bands in very diluted solutions of cyclohexane, n-heptane and decaline at various temperatures. It shows that the p-configuration resembles more the model of a rotating group, whereas the o- and m-compounds are blocked in potential wells, tending to the mechanism of vibrational relaxation.

A mechanism for carbon-hydrogen vibrational relaxation in alkanes

A mechanism for carbon-hydrogen vibrational relaxation in alkanes

Theoretical investigation of the vibrational relaxation process Ar + OH(ν = 1) → Ar + OH(ν = 0)

For the system Ar + OH electronic adiabatic as well as non-adiabatic vibrational relaxation mechanisms have been studied on the basis of the classical trajectory and the surface-hopping trajectory method, respectively, using simple model potentials for the two lowest electronic states. The collision processes are discussed in detail, especially regarding the influence of the initial rotational angular momentum of the OH molecule which greatly enhances the vibrational deactivation cross sections. Furthermore, thermal rate constants are calculated and compared to recent results from transition state theory; considerable disagreement is found for both mechanisms.

Isotropic and anisotropic Raman scattering study in liquid p-dioxane

The isotropic and anisotropic profiles of the 835 and 2965 cm −1 Raman lines of p-dioxane in the neat liquid and in solution have been studied as a function of temperature and concentration. From the correlation functions obtained by Fourier inversion of band contours, the possible interaction responsible for the vibrational dephasing of the oscillators and their reorientational relaxation are considered. It is shown that the p-dioxane molecule tumbling about the C 2 axis in the molecular plane perpendicular to the oxygen-oxygen direction proceeds by small-step Brownian diffusion associated with an Arrhenius activation energy of 9.0 kJ mol −1. The vibrational relaxation mechanism of the two modes is interpreted in terms of pure dephasing due to weak collisions.

Isotropic Raman band shape and vibrational relaxation for molecules in the liquid state: A Critical study of some band shape functions

AbstractA band shape function for the isotropic Raman scattering described in terms of the band width in the weak coupling limit and the relaxation time of the bath mode has been proposed by summing the results of Berne et al. [J. Chem. Phys. 45, 1086 (1966)], Wang [Mol. Phys. 33, 207 (1977)] and Knauss [Mol. Phys. 36, 413 (1978)]. The results of band analyses with this function have been compared with those obtained from other conventionally used band shape and correlation functions. The present function is preferable for precise interpretation of vibrational relaxation mechanisms, particularly for those cases where temperature and concentration dependence of the band parameters are involved.