Vibrational Excitation Of Polyatomic Molecules Research Articles

State-to-state vibrational kinetics of diatomic molecules in laser-induced ignition of a syngas-air mixture: Modeling study

A thermally nonequilibrium model was created to describe the processes occurring in a syngas-air mixture when exposed to intense CO laser radiation. The model consistently describes the reactions of syngas oxidation in air and the state-to-state vibrational kinetics of CO, N2, O2, H2, and OH molecules and also takes into account the nonequilibrium vibrational excitation of polyatomic molecules in the mode approximation. Using the constructed model, the possibility of laser-induced ignition of syngas in air was analyzed. Special attention was paid to the influence of the efficiency of vibrational energy of CO molecules in overcoming the energy barriers of reactions on the simulation results. For the key reactions involving CO molecules, the values of the coefficients of vibrational energy usage were estimated employing the method of classical single-trajectory calculations. It was shown that these coefficients are the critical parameters determining the possibility of using the CO laser radiation to accelerate the ignition of syngas. In addition, we have shown that in the problem considered, because of the strong nonequilibrium excitation of CO vibrations, it is extremely important to take into account the deviation of the vibrational distribution functions of molecules from the Boltzmann one, which requires the use of a state-to-state model of vibrational nonequilibrium. The inaccuracy in calculating the laser-induced ignition time of syngas in air by means of a mode model can reach an order of magnitude.

Electron-impact vibrational excitation of cyclopropane.

We report a very detailed test of the ab initio discrete momentum representation (DMR) method of calculating vibrational excitation of polyatomic molecules by electron impact, by comparison of its results with an extensive set of experimental data, covering the entire range of scattering angles from 10° to 180° and electron energies from 0.4 to 20 eV. The DMR calculations were carried out by solving the two-channel Lippmann-Schwinger equation in the momentum space, and the interaction between the scattered electron and the target molecule was described by exact static-exchange potential corrected by a density functional theory (DFT) correlation-polarization interaction that models target's response to the field of incoming electron. The theory is found to quantitatively reproduce the measured spectra for all normal modes, even at the difficult conditions of extreme angles and at low energies, and thus provides full understanding of the excitation mechanism. It is shown that the overlap of individual vibrational bands caused by limited experimental resolution and rotational excitation must be properly taken into account for correct comparison of experiment and theory. By doing so, an apparent discrepancy between published experimental data could be reconciled. A substantial cross section is found for excitation of the non-symmetric HCH twisting mode ν4 of A1 (″) symmetry by the 5.5 eV A2 (') resonance, surprisingly because the currently accepted selection rules predict this process to be forbidden. The DMR theory shows that the excitation is caused by an incoming electron in an f-wave of A2 (') symmetry which causes excitation of the non-symmetric HCH twisting mode ν4 of the A1 (″) symmetry and departs in p- and f-waves of A2 (″) symmetry.

Relaxation Processes at High Levels of Vibrational Excitation of Polyatomic Molecules in the Ground Electronic State

By the spectral and kinetic characteristics of the luminescence of vapors of polyatomic molecules (anthracene, anthraquinone, fluorenone) initiated by selective IR multiphoton excitation (IR MPE) of molecules in the ground electronic state S0 the relaxation processes proceeding under vibrational excitation of molecules to energies exceeding the energies of the lower excited electronic states have been investigated. The changes in the spectral and kinetic characteristics with increasing CO2 laser energy density and vapor Pv and foreign gas pressure PFG are analyzed. They are similar to the characteristics obtained for normal fluorescence of these molecules with changing vibrational energy Evib content. On the basis of experimental data and model calculations it has been concluded that at the laser radiation densities used in the case of IR MPE the molecules reach energies considerably exceeding the energies of the electronic levels. It is shown that a nonadiabatic connection between the electronic states leads to the population of mixed electronic states isoenergetic to the vibrational levels of the ground electronic state and to emission of delayed luminescence spectrally identical to the normal luminescence of these molecules. It has been found that when high vibrational levels are populated, new relaxation channels, such as reverse electron relaxation, emission from high vibrational levels of the ground electronic state, and multiquantum vibrational energy transfer at collisions leading to a rapid establishment of vibrational equilibrium become important.

Electron-impact vibrational excitation of polyatomic molecules

Vibrational excitation in electron collisions with polyatomic molecules is reviewed. Theoretical treatments based on the fixed-nuclear-orientation approximation are summarized. Results of the theoretical studies of H O, H S and CH are presented as an example and compared with experimental data. A calculation in the Born approximation with a long-range electrostatic interaction is also shown. In particular, the usefulness of the Born approximation with a dipole interaction is examined in detail. Finally possible further improvement of the theory is discussed.

A lowest-order theory of vibrational excitation of polyatomic molecules upon resonant electron impact. Vibrational excitation in CH3Cl

The author derives equations giving the differential cross section for excitation of vibrations by resonant electron impact in polyatomic molecules. The description 'lowest order' applies because many of the quantities are represented by Taylor's expansions in the normal coordinates with the series truncated after the first term that produces excitation. The discussion shows the importance of the phases of the scattering amplitude matrix in influencing the angular scattering in vibrational excitation. In particular, the linear Jahn-Teller effect between a doubly degenerate resonant state and a degenerate vibrational mode produces more scattering in the backward direction than in the forward direction in the case of the v=0 to v=1 excitation. The theory is applied to methyl chloride, CH3Cl, giving a comparison of the results with experiment.

Multiphonon vibrational excitation of polyatomic molecules in mixtures with foreign gases

The role of collision effects in multiphoton vibrational excitation (MVE) of molecules is an active area of research at the present time [1--5]. Interest in this question stems from the little-studied characteristics of relaxation processes with high levels of vibrational excitation of polyatomic molecules, and also from possible practical applications in laser photochemistry. For polyatomie molecules, for which the region of discrete vibrational levels is narrower and the cross sections of multiphoton absorption differ less from the sections in the 0 ~ 1 transition than in the case of small molecules, the influence of collisions on MVE has not been studied as thoroughly. Of particular interest is the influence of collisions on MVE in two-component mixtures of absorbing molecules and foreign gases. When the molecules of such mixtures are excited by radiation from a CO s laser, a number of complex effects are observed; and no satisfactory and unambiguous interpretation has yet been given. Analysis of these effects may be greatly complicated by thermal processes, which often determine the effective time of photochemical reactions initiated by laser radiation in a confined volume [3--4]. In the work reported here, we investigated changes in absorbed energy, rates of relaxation processes, and intensities of induced absorption from high vibrational levels of the ground electronic state, as related to increases in the pressure of foreign gases in mixtures with polyatomic molecules of different degrees of complexity. The molecules that were investigated were aerolein (CaH40), acetophenone (CallaO), and diacetyl (C4HeOs); in the region of CO~. laser radiation, they have IR absorption bands with Vm~ x = 959,954, and 945 cm -1 and comparatively small absorption cross sections at a frequency of 944 cm -1 (P20) (5.5, 1.5, 1.15).10 -19 cm 2, respectively. The gas mixtures were admitted to a heated cuvette with a length of 40 cm. The absorbing molecules were diluted with a large quantity of He, N s, or Ar. In measuring the absorbed energy in the pressure interval from 0 to 2.0.106 Pa, we used a specially designed high-pressure steel cuvette. The IR radiation pulse had an energy up to 5 J, duration of peak 80 nsec at half-height, and a "tail" of about 1.5 #see. We used a collimated laser beam with a diameter (1.5 cm) that was smaller than the diameter of the cuvette (4 cm). The changes in cross section of the laser beam through the length of the cuvette were no greater than 30%. The absorbed energy was determined by means of standard calorimeters or pyroelectric sensors. The cuvette length and the vapor pressure were selected so that the vapor would absorb no more than 15% of the laser energy, so that we were able to consider the intensity of IR radiation as identical over the entire length of the cuvette, and the temperature distribution in the laser beam as uniform. The kinetics of change in populations of vibrational levels of the ground electronic state were monitored on the basis of electronic absorption. These observations were performed in the long-wave absorption bands of acrolein (320--390 nm), diacetyl (370--460 nm), and acetophenone (300--360 nm), and also in regions where there is no absorption under ordinary conditions. At various levels of CO s laser energy density Ecos, we determined the dependence of the absorbed energy (average per molecule) on the pressure of foreign gas Peg (Fig. 1). We found that, for these particular polyatomie molecules, the changes in cross section of MVE with increasing Peg are either very small or zero. Oscillograms of induced absorption on the long-wave shoulder of the electronic spectra, in the region where there is little or no original absorption, demonstrate that this absorption increases with increasing pressure of N~. or Ar for all of the molecules that were investigated. In Fig. 2 we show the influence of Pfg on the relative intensities I(t)/Io(t), where I(t) and I0(t) are the intensities at the maxima of the induced absorption pulses in the mixtures and in the pure vapor, respectively. An increase in electronic absorption when vapor with a pressure no higher than l0 s Pa is highly diluted with a foreign gas is observed only with pressures Pfg < 1.10 s Pa. Subsequently, the curves come out to saturation more

Symmetry selection rules vs. vibronic coupling in resonant selective vibrational excitation of polyatomic molecules by electron impact

In the unsaturated polyatomic molecule series: C 6H 6, C 2H 4, CH 3CHO, H 2CO, (CE 3) 2CO, CH 2CHCHCH 2, C 2H 2, HCN and CH 3CN, all presenting an overlap between a low energy π ★ (1-2 eV) and a very broad σ ★ (4–8 eV) shape resonance, we show that the vibronic coupling process between the π ★ and the σ ★ resonances can explain the resonant excitation of the non-totally-symmetric vibrational modes without invoking an s( l=0) wave in the scattering process.

Statistical theory of multiphoton excitation of polyatomic molecules based on the Wigner function

This paper is aimed at establishing a statistical theory of rotational and vibrational excitation of polyatomic molecules by an intense IR laser. Starting from the Wigner function of quantum statistical mechanics, we treat the rotational motion in the classical approximation; the vibrational modes are classified into active ones which are coupled directly with the laser and the background modes which are not coupled with the laser. The reduced Wigner function, i.e., the Wigner function integrated over all background coordinates should satisfy an integro-differential equation. We introduce the idea of ‘‘viscous damping’’ to handle the interaction between the active modes and the background. The damping coefficient can be calculated with the aid of the well-known Schwartz–Slawsky–Herzfeld theory. The resulting equation is solved by the method of moment equations. There is only one adjustable parameter in our scheme; it is introduced due to the lack of precise knowledge about the molecular potential. The theory developed in this paper explains satisfactorily the recent absorption experiments of SF6 irradiated by a short pulse CO2 laser, which are in sharp contradiction with the prevailing quasi-continuum theory. We also refined the density of energy levels which is responsible for the muliphoton excitation of polyatomic molecules.

Selective vibrational excitation of polyatomic molecules by stimulated emission pumping: The 523O2 level (εvib = 2036 cm−1) of p-difluorobenzene

The method of stimulated emission pumping (SEP) is used to populate excited vibrational level (εvib = 2036 cm−1) in the ground state of a medium-sized polyatomic molecule, namely p-diflurobernzene. The state specificity of the SEP process is demonstrated by generating single vibronic level (SVL) fluorescence from an S1 level (302) via absorption from the SEP-prepared level, 523O2. The wavelength resolved SEP-SVLF fluorescence spectrum establishes the integrity of the prepared S0 level and provides a means for monitoring its population dynamics.

Small-angle ion-molecule vibrational excitation

The strong coupling correspondence principle is formulated to provide an approximation to the T-matrix elements for vibrational excitation of polyatomic molecules in rotationally sudden collisions. The theory is valid in the forward diffraction zone and at wider angles. Using the long-range form of the interaction potential this theory is applied to diatoms and linear triatoms. It is shown that there are three ranges of scattering angle in each of which different subsidiary approximations are valid. Particular application is made to proton collisions with CO, NO and N2; the author obtains approximate analytic expressions for the cross section in the forward direction and at larger scattering angles (greater than 0.2 degrees ). It is shown how various parts of the potential affect the cross section. Theory and experiment agree satisfactorily. The excitation of the infrared-active modes of CO2 by protons is also considered. In particular, an analytic approximation to the ratio of the bend to the asymmetric stretch cross sections is obtained and is shown to agree well with experiment.

Molecular collisions. XVIII. Restricted distorted wave approximation to rotational and vibrational excitation of polyatomic molecules

Perturbation expressions are obtained for a scattering matrix and four scattering cross sections describing rotational and vibrational excitation in collisions of polyatomic molecules. Computationally convenient versions of the expressions are discussed.