Excitation Of Molecular Vibrations Research Articles

Selective excitation of molecular vibrations by interference of Floquet states

The authors develop a mechanism for the selective excitation of molecular vibrational states by short strong laser pulses. This mechanism is based on constructive interference of Floquet states. A phase functional is constructed which allows the theoretical prediction of the parameters of optimal pulses. The role of anharmonicities of the molecular binding potential and their effect on the structure of the instantaneous Floquet states is clarified.

Vibration Dynamics in Scattering of NO/Ag: Adiabatic versus Nonadiabatic Processes

Vibrational excitation in NO/ Ag scattering is discussed within the adiabatic approxima­ tion. Taking into account the observed change of NO stretch frequency on adsorption the observed magnitude and energy dependence of molecular vibrational excitation can ·be described with reasonable parameters. At low energies the observed temperature depen­ dence can be explained semi-quantitatively by taking the coupling to phonons into account. It is pointed out that the observed temperature dependence poses problems in both the adiabatic as well as the nonadiabatic models. Further investigations are needed to obtain unambiguous results concerning the relative importance of adiabatic and nonadiabatic effects in NO/ Ag scattering.

Molecular vibration dynamics in molecule-surface interactions

A quantum mechanical coupled channels approach to associative or recombinative desorption and scattering of diatomic molecules is described. The formulation is based on the concept of a reaction path and allows prediction of the vibrational excitation of desorbing molecules. We first consider very light molecules such as H2 and D2 desorbing via a Langmuir-Hinshelwood reaction. In a simple model neglecting rotations and substrate vibrations, the dependence of molecular vibrational excitation on incident energy, the curvature of the reaction path and the position and height of the saddle point are discussed. Various experimental results can be described with reasonable parameters. Vibrational excitation in Eley-Rideal reactions and rotational excitations in general are discussed only in a semiquantitative way. For heavier molecules the coupling to substrate vibrations in principle will become more important. Arguments will be presented that for the problem of vibrational excitation in desorption and scattering this coupling may still be neglected approximately. Results for vibrational excitations of CuF desorbing from Cu are in support of this simple point of view.

A theory of low-energy electron impact excitation of dipole-allowed molecular vibrations. An application to acetylene, C2H2

The article outlines a theory for the direct excitation of dipole-allowed vibrations, v=0 to v=1, in polyatomic molecules. The molecule is modelled by the Hartree-Fock function in contrast to earlier treatments using a point dipole model. The discussion retains the practice of these earlier treatments in using the Born approximation for the scattering amplitude. The theory is applied to the acetylene molecule. The results show an improvement over the point dipole model between values of the vibrational transition moments inferred from infrared intensity measurements and values of differential cross sections for low-energy electron impact excitation of these vibrations.

Selective resonance population in electron scattering by adsorbed molecules.

We have investigated vibrational excitation of physisorbed ${\mathrm{O}}_{2}$ by electron impact in the 4--14-eV range, where a single broad resonance is observed in gas-phase studies. We show that by orienting the molecule in two different alignments on the surface of graphite it is possible to selectively populate two different negative-ion resonance states in this energy range, the $^{4}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}$ state (at \ensuremath{\sim}8.5 eV) and the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}$ state (at \ensuremath{\sim}6 eV), each of which leads to molecular vibrational excitation. Calculations show that this selective resonance population arises from the distinctive differential electron-capture cross sections of the two molecular states, into which electrons are ``fed'' by the surrounding surface structure.

Analysis of semiconductors and insulators by high‐resolution electron energy loss spectroscopy—prospects for quantification

AbstractHigh‐resolution electron energy loss spectroscopy (HREELS) is now widely known and recognized for its very significant contributions to the characterization of molecules adsorbed on well‐defined surfaces: many chemical surface reactions were understood successfully (adsorption sites, reaction scheme, time‐resolved experiments) through the analysis of electron‐induced vibrational spectra. Semiconductors and insulators form other classes of materials that contribute to vibrational spectra by intrinsic vibrations: compound semiconductors, and insulators present acoustic and optical phonons, i.e. collective lattice vibrations, whereas organic insulators (like polymers) offer numerous molecular groups to the vibrational excitation by the electron beam (extrinsic vibrations). Quantitative analysis of the surface of those materials rests on the understanding of the interaction mechanisms between monochromatized electrons and intrinsic or extrinsic vibrations.Three scattering processes have been recognized to contribute to HREEL spectra: dipole interaction, impact scattering and resonance scattering. They have been characterized theoretically and experimentally through their dependence versus variation of the electron beam energy and/or of the scattering angular geometry.Recent experiments have shown that the analysis of intrinsic phonons is very well understood for insulators. As a consequence, quantitative analysis of compound semiconductors is possible and brings additional information on the surface stoichiometry (of AlxGa1−xAs, for example) and on the doping level. For organic polymers, however, the excitation of molecular vibrations does not—up to now—appear to follow a simple scheme: all the scattering mechanisms (dipolar, impact and resonance) appear to play a significant role; a true quantification will be possible only through a systematic study of model compounds, and will require further theoretical treatments.

Electron affinity and structure of Langmuir-Blodgett films of cadmium arachidate by means of low-energy electron transmission

The results of low-energy electron transmission experiments on Langmuir-Blodgett (LB) films of cadmium arachidate deposited on copper substrates precoated with ferric stearate are described. The energy location of the bottom of the conduction band ( V 0) for the ordered alkyl tails in the LB film was directly determined to be 0.46±0.02 eV above the vacuum level at −120 to −130°C, that is, the electron affinity ( V A = − V 0) is −0.46±0.02eV. Further, it is demonstrated that the band structure is significantly blurred by the thermal excitation of molecular vibrations and by poor crystallinity. The crystallinity of the films is dependent on the number of layers deposited, and the results are consistent with those obtained by angle-resolved photoemission experiments. The present results strongly suggest that the electron affinities (2.25 and 2.3 eV) obtained by other workers for a metal/monolayer/metal junction, using fatty acid salts, are not those of the ordered parts of the alkyl tails.

Assessment of two-temperature kinetic model for ionizing air

A two-temperatur e chemical-kinet ic model for air is assessed by comparing theoretical results with existing experimental data obtained in shock tubes, ballistic ranges, and flight experiments. In the model, one temperature (T) is assumed to characterize the heavy-particle translational and molecular rotational energies, and another temperature (Tv) the molecular vibrational, electron translational, and electronic excitation energies. The theoretical results for nonequilibrium flow in shock tubes are obtained using the computer code STRAP (shock-tube radiation program) and for flow along the stagnation streamline in the shock layer over spherical bodies using the newly developed code SPRAP (stagnation-point radiation program). Substantial agreement is shown between the theoretical and experimental results for relaxation times and radiative heat fluxes. At very high temperatures, the spectral calculations need further improvement. The present agreement provides strong evidence that the two-temperature model characterizes principal features of nonequilibriu m airflow. New theoretical results using the model are presented for the radiative heat fluxes at the stagnation point of 6 m radius sphere, representing an aeroassisted orbital transfer vehicle, over a range of freestream conditions. Assumptions, approximations, and limitations of the model are discussed. Nomenclature = average molecular speed ^/$kT/nm, cm s ~ ! = pre-exponential factor in reaction rate coefficient, cm3mole~ 1 s~ * - average vibrational energy per particle, erg = average vibrational energy per particle under equilibrium, erg = reaction energy, erg

Mechanisms of change in the refractive index during propagation of λ = 2.8 μm radiation in a damp atmosphere

An analysis is made of the influence of various mechanisms on changes in the refractive index accompanying the absorption of λ = 2.8 μm radiation by atmospheric water vapor. Conditions are obtained under which the influence of changes in the polarizability of the medium as a result of the excitation of molecular H2O vibrations, accompanied by changes in the density of the medium, can be neglected in calculations of the refractive index. It is found that a focusing thermal lens may be formed in the beam channel.

UV spectroscopy of molecular “hot” bands

We consider both the theory and the experimental measurement of the temperature and frequency dependences of absorption cross sections of molecular “hot” bands. In order to calculate the molecular UV spectra, a generating function method applicable to molecules with an arbitrary vibrational distribution (VD) was used. In this approach the Gaussian (central) part of the spectrum was found to be insensitive to the shape of VD, depending on the average molecular vibrational excitation energy and the spectroscopic parameters. In contrast, the long-wavelength wings depend on peculiarities of the VD shape. Thus spectroscopic measurements directly probe the molecular distribution of highly excited vibrational states.

Theoretical study of electrical breakdown in nitrogen induced by laser radiation at 1.06μm

The electron cascade model original proposed by Evans and Gamal (1980) to study atomic gas breakdown is extended to molecular gas breakdown calculations, including molecular electronic excitations and vibrational excitation effects. The validity of this model is studied for nitrogen at pressures ranging from 7.6*102 to 3.3*104 Torr irradiated by laser radiation of wavelength 1.06 mu m and 10 ns pulse duration. The calculated threshold intensities are in good agreement with the experiment. Calculations of the electron energy distribution function have also been performed to reflect the important electron-molecule energy exchange processes in molecular gases. It has been found that vibrational losses cause a 'precipitate decrease' in the electron energy distribution function near 2.0 eV and a very marked reduction in mean electron energy.

Erratum: Chaos and incoherence in a model of multiple-photon excitation of molecular vibrations

Erratum: Chaos and incoherence in a model of multiple-photon excitation of molecular vibrations

Chaos and incoherence in a model of multiple-photon excitation of molecular vibrations.

We consider in detail our earlier classical model of multiple-photon excitation of molecular vibrations in an infrared laser field, wherein the observed fluence dependence of the process is associated with chaotic variations of a pumped mode. We focus our attention first on a quasicontinuum model of background vibrational modes, which provides us with a dynamical system in which the role of chaos is neatly interpretable. Then we consider a model with only a finite number of background modes in order to corroborate the conclusions drawn from the quasicontinuum model.

Impulsive stimulated scattering: General importance in femtosecond laser pulse interactions with matter, and spectroscopic applications

It is shown that ‘‘impulsive’’ stimulated Raman scattering (ISRS) should occur, with no laser intensity threshold, when a sufficiently short laser pulse passes through many types of matter. ISRS excitation of coherent optic phonons, molecular vibrations, and other excitations (including rotational, electronic, and spin) may play important roles in femtosecond pulse interactions with molecules, crystals, glasses (including optical fibers), semiconductors, and metals. Spectroscopic applications of ISRS, including time-resolved spectroscopy of vibrationally distorted molecules and crystals, are discussed.

The structure factor of liquid chlorine from 0.4 to 11 Å −1

The structure factor for liquid chlorine on the coexistence curve at room temperature (296 K) was measured out to a q of 11 Å −1 using neutrons of 88 meV. The inelasticity correction to the differential scattering cross section included terms due to the excitation of molecular vibrations which were important at large q. Comparisons were made to other published results for S( q) and some differences were noted, especially at low q and in the peak heights.

Effective overcoming of frequency detuning and resonance effects under molecular vibrational excitation by picosecond infrared laser pulses

A number of frequency detunings, in particular those caused by vibrational anharmonicity, can be compensated under coherent excitation of molecules by picosecond infrared laser pulses. Multi-photon excitation by a sequence of picosecond pulses is much more effective than nanosecond pulse excitation with the same energy fluence.

Off-resonant excitation of molecular vibrations in liquids, investigated by picosecond probe scattering

The coherent excitation of molecular vibrations in liquids via off-resonant stimulated Raman amplification is shown to consist of two parts with vastily different spectral and temporal properties. One component situated at the frequency differenceωL−eωS of the driving fields follows almost instantaneously the system response function. The second part at the eigenfrequency eω0 carries information on the vibrational dephasing. Detecting both frequency components simultaneously by coherent anti-Stokes probe scattering a novel 2-channel method is demonstrated. The vibrational dephasing time of thev3-mode of CH3I is directly measured to beT2=2.6±0.15 ps. This result gives experimental support to the deconvolution techniques required in conventional spectroscopy.

Non-linear coherent Raman spectroscopy

A general introduction to several coherent Raman methods, which are based on the third-order non-linear susceptibility is given. These methods are described basically under a common point of view, which is the excitation of coherent molecular vibrations in the field of two strong laser beams operating at different frequencies. Most of these methods can be applied successfully also under electronic resonance conditions. As a particular example, studies of coherent anti-Stokes continuum resonance Raman scattering in iodine vapour will be presented and discussed in detail.

Excitation of molecular vibration on collision with an atom (or a molecule) in the presence of an infrared laser beam

A general theory of excitation of a diatomic molecule due to collision with an atom (or a molecule), for example the He-HF system, in the presence of an infrared laser beam is presented. The effect of collision on the radiative excitation process is investigated by changing the various collision parameters, for example the impact parameter, collision velocity etc., with different values of molecule-field detunings. For a complete description, the quantum dynamics of excitation of a diatomic molecule due to a laser field is investigated in detail by using a quasi-energy (non-perturbative) formalism.

Electron–Molecular Vibration Coupling in Trimerized Organic Ion‐Radical Semiconductors

AbstractThe totally‐symmetric (ag) internal molecular vibrational excitations in a trimer are considered. The mode corresponding to the charge transfer (CT) between non‐centric molecules is shown to be responsible for the indirect IR activity. The trimer MO calculations in the site occupation representation are simplified and the symmetry of the basis states is taken into account. Eigenstates and eigenvalues of the molecular trimer with two radical electrons are calculated as a function of the transfer integral, t, and the Coulomb „on‐site”︁ repulsion, U. The following interpretation of the absorption spectrum of the cesium salt of tetracyanoquinodimethane Cs2(TCNQ)3 is given: the 1.4 eV peak is attributed to intratrimer charge transfer excitation, the less than 0.6 eV peak corresponds to the electronic transition between neighbouring trimers. The electron–molecular vibration coupling constants and Hubbard parameters U and t are evaluated for Cs2(TCNQ)3.