Vibrational Energy Loss Research Articles

Determination of adsorbate molecular orientation from resonance electron-scattering angular distributions.

We have measured the angular profile of electrons inelastically scattered from oriented molecular ${\mathrm{O}}_{2}$, physisorbed on graphite at 20 K. Electrons ejected after vibrational energy loss from a temporary negative-ion resonance observed near 9 eV have an angular profile which peaks at 15\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} from the normal to the crystal surface, independent of the incident beam angle. From an analysis of these angular distributions we are able both to identify the ${\mathrm{O}}_{2}$ negative-ion state (as $^{2}\ensuremath{\Pi}_{\mathrm{u}}$) and to determine the orientation of the ${\mathrm{O}}_{2}$ molecular axis (as 25\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} from the vertical) in what we believe to be the monolayer $\ensuremath{\zeta}2$ phase.

Erratum: Selection rules for vibrational energy loss by resonant electron impact in polyatomic molecules

Erratum: Selection rules for vibrational energy loss by resonant electron impact in polyatomic molecules

Selection rules for vibrational energy loss by resonant electron impact in polyatomic molecules.

Approximate expressions for inelastic elements of the S matrix are given in terms of integrals over vibrational functions and R-matrix electron states. The magnitude and symmetry of these integrals determine the vibrations that are excited by various resonances, and the present analysis successfully predicts the spectra measured for benzene, ethylene, and formaldehyde. The recently observed s-wave component in the rotational excitation of ${\mathrm{N}}_{2}$ is also explained by the treatment. The theory shows how vibrational excitation can give information about the symmetries of resonant electronic states. This can be particularly useful when angular distribution measurements are unavailable.

Studies of the amidogen-nitric oxide reaction by infrared kinetic spectroscopy

Infrared kinetic spectroscopy using excimer laser flash photolysis and color center laser probing has been used to study the NH/sub 2/ + NO reaction. The amidogen radical, NH/sub 2/, was produced by ArF photolysis of NH/sub 3/. Infrared absorptions of OH and H/sub 2/O were measured to determine the absolute contributions of the OH and H/sub 2/O product channels. It was found that the OH channel accounts for 13 +- 2% of the reaction. Using two different pairs of NH/sub 3/ and H/sub 2/O lines, we measured values of 0.85 +- 0.09 and 0.66 +- 0.03 for the ratio of H/sub 2/O formed to NH/sub 3/ photolyzed. All of the H/sub 2/O signals exhibit a pronounced induction period suggesting that H/sub 2/O is produced in very high vibrational states. The time evolution of low-lying vibrationally excited and ground vibrational state H/sub 2/O lines is adequately simulated by a model in which a stepwise sequential loss of vibrational energy occurs with quenching cross sections for each step proportional to excess energy.

Rapid-scanning constant-energy synchronous fluorescence spectrometer as a liquid chromatographic detector

A rapid-scanning fluorimeter based on constant-energy synchronous scanning is described. It is used as a liquid chromatographic detector for polyaromatic hydrocarbons (PAHs). The computerized system scans at a rate of 200 nm s −1 and collects data in the forward and reverse directions. Data are processed in real time. The system is tested for 3-component and 9-component mixtures of PAHs. A synchronous fluorescence spectrum is obtained for each PAH being eluted in the chromatogram. The spectral peaks observed in the rapid scan correspond to the absorption/fluorescence transitions of the PAHs with an overall vibrational energy loss equal to the Δν value chosen. Limits of determination for the PAHs are in the pg to low ng range. The system provides a reduced signal/noise ratio compared to the variable-wavelength fluorescence system for most PAHs examined.

A quasiclassical trajectory study of vibrational energy transfer in collisions involving intermolecular attraction of moderate strength

Abstract Monte Carlo selected, quasiclassical trajectories have been computed on six potential energy hypersurfaces possessing potential minima or “wells” up to 50 kJ mol −1 deep. The aim of the investigation has been to examine how vibrational energy transfer in A + BC(υ = 1) collisions is promoted by intermolecular attraction of moderate strength. Here results are reported for the mass combination m A = 20 u, m B = 1 u, m C = u. The results show that even quite slight intermolecular attraction can enhance energy transfer, as long as the attraction does not just depend on the separation of A from the center-of-mass of BC. The mean loss of vibrational energy does not depend only the well depth but also on its “location” (in particular, the difference in r BC at the minimum and in isolated BC) and on the angular anisotropy of the potential. Large transfers of energy do not occur only in complex-forming collisions; indeed, a high fraction of trajectories on all surfaces are direct but show similar transfer of energy as in the more complex trajectories on the same surface. The results of the calculations are discussed in relation to the mechanisms and rates of vibrational relaxation in collisions between radicals and between species. such as HF + HF, capable of forming hydrogen bonds.

Vibration-absorbing properties of petroleum asphalts

This article investigates the effectiveness of using asphaltic materials as binders in various vibration-absorbing formulations for transport equipment. The vibration absorption properties of the asphalts were determined on the basis of the vibrational energy loss factor at 21 + or - 1/sup 0/C over a frequency range of 70-1200 Hz. Vibrations in transport equipment are the greatest in the 100-1200 Hz frequency region. The high cost of vibration-absorbing polymeric materials and the complications involved in applying them to surfaces necessitate the search for bituminous materials that are competitive with polymeric materials in damping properties. It is concluded that straight bituminous materials can provide effective absorption in the frequency range 100-1200 Hz.

Vibrational energy transfer from highly excited anharmonic oscillators. Dependence on quantum state and interaction potential

In order to elucidate the general features of vibrational deactivation of highly excited anharmonic oscillators, we present quasiclassical trajectory calculations on prototype collinear I2 (v)-inert gas collision systems. The results for vibrational-translational energy transfer reveal several interesting trends as a function of initial vibrational quantum state, projectile mass, and projectile–oscillator interaction potential. (1) Vibrational deactivation is inefficient from all quantum levels and for all projectile masses. The average energy transfer per collision ΔE is strongly peaked at intermediate vibrational levels (v≊80) and is observed to be at most ≊−kbT. Furthermore, when scaled to h/ω(E), the ’’local’’ oscillator energy spacing, ΔE can be accurately represented by a simple power law in vibrational quantum number over a wide range of bound states. (2) Energy transfer is progressively less efficient from levels in the neighborhood of and approaching dissociation. (3) Vibrational energy loss for high levels of initial vibrational excitation (v≳90) is rather insensitive to the nature of the interaction potential. Smooth exponential and hard-sphere interaction results differ by less than an order of magnitude. This observed insensitivity motivates the development of an analytic collision model, in which simple hard-sphere geometry and dynamics are used to calculate ΔE. The model results are in qualitatively good agreement with trajectory calculations and also indicate that nonuniform sampling of the anharmonic oscillator velocity and phase are responsible for decreased energy transfer efficiency from high vibrational states.

State-to-state relaxation processes for XeCl(B, C)

The XeCl (B–X) and (C–A) emission spectra obtained from reaction of Xe (3P2 or 3P1) with Cl2, CCl4, and COCl2 in the presence of He, Ne, Ar, Kr, and N2 bath gases were used to study the vibrational relaxation and transfer between the B and C states of XeCl. By using the different Cl donors, different ranges of vibrational energy were emphasized. The bound–free emission spectra were simulated for various pressures of bath gas to obtain vibrational distributions. Numerical modeling of the XeCl(B) and XeCl(C) vibrational populations and the B/C intensity ratio as a function of pressure gave rate constants for vibrational relaxation and transfer, as well as the model for the state-to-state processes. For Ar as the bath gas, vibrational relaxation can be characterized by an exponential gap model Pij ∝ e−0.1ΔE/kT, with rate constants of (1–6), (6–12), and (20–30) × 10−11 cm3 molecule−1 sec−1 for the v ranges of 0–30, 30–70, and 70–130, respectively. The rate constants for electronic state transfer are (3–11), (11–15), and (15–15) × 10−11 cm3 molecule−1 sec−1 for the same v ranges. The vibrational energy loss upon electronic state transfer was best described by a Poisson-type function displaced to lower energy from the initial energy. These basic models also describe the relaxation in the other gases with He and Ne being less efficient and Kr and N2 more efficient than Ar. The magnitudes of the rate constants and the models are discussed.

Optical propagation and vibrational energy loss in gaseous waveguide lasers

It is observed that the condition for minimum optical attenuation in a dielectric waveguide laser simultaneously maximizes long range vibrational energy transfer from the upper laser level molecules to the waveguide wall. It is suggested that a specifically chosen thin coating on the waveguide wall could increase gain by both decreasing deactivation of the upper laser level and increasing deactivation of the lower laser level. These ideas are applied to the CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser.

Survey of Advanced Gasdynamic Laser Concepts

Introduction E is transported between the thermal heat source and the optical cavity of a gasdynamic laser (GDL) in the vibrational modes of the nitrogen molecule. Pure nitrogen exhibits relatively long vibrational relaxation times, making it well suited to the formation of an inversion during a rapid gasdynamic expansion. The carbon dioxide and catalyst that are necessary to allow efficient extraction of this stored energy in an optical cavity act as deactivants during the gasdynamic expansion, causing significant loss of stored vibrational energy. Management of this energy loss has required the use of very rapid expansions in nozzles with very small throats. The overall efficiency of GDLs is the product of the fraction of the invested thermal energy stored in the nitrogen vibrational modes, the freezing efficiency of the nozzle, and the extraction efficiency of the optical cavity. The stored energy fraction increases with stagnation temperature; however, the freezing efficiency falls with increased stagnation temperature. Consequently, an optimum stagnation temperature occurs near 2000 K for the conventional premixed GDL. The specific energy that can be stored in the vibrational levels of nitrogen as the temperature is raised is shown in Fig. 1. The conventional premixed GDL falls far short of attaining the full potential of the gasdynamic laser concept. Nozzle freezing efficiency increases as the carbon dioxide and catalyst concentrations are lowered. However, a certain concentration of both is required to allow good extraction efficiency. A small throat size provides a rapid rate of expansion which increases the freezing efficiency. However, the viscous losses increase with the lower Reynolds numbers associated with smaller nozzles, limiting the extraction efficiency in the cavity. The development of the conventional premixed GDL has been a process of optimization within these several mutually exclusive constraints, resulting in an overall efficiency of 0.1 -1.0%. Early in the development of the GDL it was suggested that the mixing of the carbon dioxide and catalyst after expansion to low static temperature and pressure would reduce the collisional loss of stored vibrational energy and provide greater flexibility of operating conditions. Vibrational energy can thus be frozen into the pure nitrogen donor gas well upstream of the nozzle throat at levels near the stagnation conditions. Calculations for stagnation conditions of 2000 K, 10 atm show a 1% loss of vibrational energy in the expansion of pure nitrogen as compared to the 40-50% loss that occurs principally in the throat region of a conventional nitrogen/carbon dioxide/catalyst GDL expansion. Mixing of the excited nitrogen with cold carbon dioxide is accompanied by near resonant collisional transfer of the vibrational energy to the lasing gas molecules, resulting in a population inversion with respect to the two laser levels. The thermal excitation of the nitrogen with subsequent turbulent mixing of the carbon dioxide and catalyst allows an increase in stagnation temperature to above 4000 K before molecular dissociation becomes important. The stored vibrational energy can be raised by an order of magnitude to the premixed case as seen in Fig. 1. The kinetic constraints on vibrational freezing are not as critical without the carbon dioxide and catalyst in the primary gas stream, allowing the use of larger nozzle throats resulting in lower viscous losses. The flexibility of operation with various mixtures of nitrogen, carbon dioxide, and catalyst permits more efficient extraction of power in the cavity region. These characteristics enable the mixing GDL to attain 5-10 times the efficiency of the conventional premixed GDL. Overall efficiencies of as high as 6% have been reported for the mixing GDL. The origins of the mixing gasdynamic laser can be found in the early research of Patel concerning energy transfer from nitrogen excited in a low-pressure electric discharge to carbon dioxide mixed subsonically 20 cm away from the discharge. His subsonic mixing laser was 10 times as efficient as the contemporary conventional CO2 discharge laser. Several other mixing lasers excited by electric discharges were subsequently developed' using both subsonic and supersonic

Vibrational kinetics in CO electric discharge lasers: Modeling and experiments

Numerous models have been developed to predict the performance of CO electric discharge lasers. Although many comparisons of predicted and observed laser power have been made, few comparisons exist at the more fundamental level of predicted and measured CO vibrational population distributions. Such comparisons provide a critical test of the vibrational kinetic mechanisms assumed in the models and may help explain discrepancies between predicted and measured laser output. In the present study, a model of CO laser vibrational kinetics is developed, and predicted vibrational distributions are compared with measurements. The experimental distributions were obtained at various flow locations in a transverse cw discharge in supersonic (M=3) flow. Good qualitative agreement is obtained in the comparisons, including the prediction of a total inversion at low discharge current densities. The major area of discrepancy is an observed loss in vibrational energy downstream of the discharge which is not predicted by the model. This discrepancy may be due to three-dimensional effects in the experiment which are not included in the model. Possible kinetic effects which may contribute to vibrational energy loss are also examined.

Dependence of the inelastic electron tunneling intensity on adsorbate concentration

Contrary to previous theories, it is shown that the intensity of molecular vibrational energy loss peaks in Inelastic Electron Tunneling varies as n 1.3 rather than as n, where n is the concentration of adsorbate molecules on the insulator surface in metal-insulator-metal junctions. Even though obtained with a simple model for the junction, this dependence agrees quantitatively with experimental results of Langan and Hansma.

Electron cooling by excitation of carbon dioxide

The cooling of a heated electron gas by electron impact excitation of CO2 is believed to be an important process in the neutral atmospheres of Mars and Venus. Electron energy loss rates are calculated for a variety of excitation processes as a function of electron temperature at a CO2 gas temperature of 200 K. Vibrational excitation is found to be the most efficient cooling mechanism for the range of electron temperatures studied. The contributions of various vibrational energy loss processes to electron cooling are discussed.

Radiationless transitions between excited singlet states of biacetyl

AbstractEmission processes from lower excited states S1 (fluorescence) and T1 (phosphorescence) have been studied in the gas and liquid phases when biacetyl is excited into the second singlet state S2. (In agreement with Kasha's rule no fluorescence is observed from the S2 state.)In the liquid phase, when biacetyl is excited into the singlet states S1 and S2, no difference is observed between these emission processes. This phenomenon certainly results from an efficient nonradiative transition between the second excited singlet state S2 and the first excited state S1 with practically no excess vibrational energy. The quantum yield of this transition is almost unity and does not depend on the nature of the solvent.In the gas phase no emission processes are observed when biacetyl is excited into the S2 state at low pressure (less than 10 mm Hg). High pressure of inert gas is necessary in order to observe these processes. As for excitation into the S1 state with vibrational energy, loss of vibrational energy through collisions occurs from the S2 state. The quantum yield of the S2 → S1 transition by excitation at 290 nm is estimated around 0.5–0.6 at 6 atm of inert gas (ethane, ethylene, or carbon dioxide).

Performance of a large, CW, preexcited CO supersonic laser

The performance of a CO laser having electrical excitation by a continuously operating, self-sustained glow discharge in a high-pressure gas mixture followed by a supersonic expansion to the cavity region has been studied. A threshold for lasing was found which corresponded to 0.035 eV of input energy per CO molecule. Beyond this, nearly 15 percent of the additional energy was extracted by the resonator. The total power reached 940 W with a maximum electrooptical efficiency of 13.7 percent and a specific power of 17 kJ/lb. The vibrational population distribution of the CO in the cavity, with and without lasing, was determined from the overtone emission spectrum. The data indicated a significant loss of vibrational energy during expansion.

Heterogeneous losses in gasdynamic lasers

A theoretical analysis is made of the main forms of vibrational energy losses in heterogeneous media used in gasdynamic lasers. Calculations show that the most important losses in a CO2 gasdynamic laser are optical (absorption of light by particles); these losses increase with the gas pressure in the resonator and with the weight concentration of the particles. The most important losses in a CO gasdynamic laser are those associated with the deactivation of the excited molecules on the surfaces of condensated particles.

Electron vibrational energy transfer under MHD combustion generator conditions

An analysis of the energy transfer rate between an electron gas and the vibrational modes of diatomic gases has been made. The electron vibrational energy transfer rate may be characterized by δ v , the electron vibrational energy loss factor. For MHD combustion generator conditions, the electron vibrational energy transfer rate dominates and δ v is approximately equal to δ, the total electron energy loss factor. Detailed numerical predictions have been obtained for N 2 and CO. These results indicate that the energy transfer rate is strongly dependent upon the electron velocity distribution function. In particular, for a Maxwellian electron distribution function (which prevails in MHD combustion generators) δ v has been found to exceed published experimental values of δ by a factor as large as 50 for N 2. Consideration of the conditions under which the experimental values of δ were obtained shows that the electron distribution function was non-Maxwellian. An analytical re-determination of δ v and δ, using appropriate non-Maxwellian distribution functions, resolves the discrepancy between the calculated values of δ v and the measured values of δ.

Dominance of methyl groups in picosecond vibrational relaxation in hydrocarbons

Picosecond relaxation times of CH stretching vibrations in a series of liquid hydrocarbons have been measured using the Raman scattering technique. The results indicate that the vibrational energy loss takes place primarily through the methyl groups in these molecules.

Vibrationally Inelastic Low-Energy CO+- Ar Collisions

Examination of relative differential cross sections for inelastic scattering of CO(+) by Ar with a high-resolution ion-beam apparatus in which a CO(+) beam interacts with a neutral-Ar beam, and the energy, mass, and angular distribution of scattered ions are measured. Maxima in the inelastic energy-loss spectra occur at energies corresponding to CO(+) spectroscopic vibrational spacings. Weakly inelastic processes are observed below the threshold for vibrational energy loss, corresponding to rotational excitation with the relative importance of rotational transitions increasing with decreasing energy and scattering angle.