Effect Of Vibrational Excitation Research Articles

Vibrational mode effects, scattering dynamics, and energy disposal in reaction of C2H+2 with methane

The effects of collision energy and mode-selective vibrational excitation on the reaction of C2H+2 with CH4 and CD4 have been measured, along with the corresponding product velocity distributions. Two distinct reaction mechanisms are active in the energy range below 5 eV. At low energies, a long-lived C3H+6 complex forms efficiently, then decomposes primarily to C3H+5+H and C3H+4+H2. The RRKM lifetime of this complex is estimated to range between ∼10 ns and ∼10 ps over the experimental energy range, and this is sufficient time to allow substantial H-atom scrambling. Complex formation is strongly inhibited by collision energy, weakly inhibited by CC stretching, and enhanced by bending excitation. Competing with the complex-mediated mechanism is a direct H-atom abstraction reaction, producing C2H+3+CH3 with little atom scrambling. This reaction is shown to have a ∼150 meV activation barrier and is strongly enhanced by collision energy, becoming the dominant channel above 0.4 eV. CC stretching provides a weaker enhancement than collision energy, while bending enhances the reaction ∼10 times more efficiently. As collision energies increase, the C2H+3 product is increasingly forward scattered with an increasing fraction of the available energy going into recoil. Energy put into reactant vibration mostly is retained as internal energy of the products. Over the collision energy range from 0.4 to 2.8 eV, the collision time in the direct reaction varies from ≥1.3 ps to ≤70 fs.

Observation of an anomalous increase in total cross sections with high vibrational excitation in the Li2 (v=0–21)+Na reactive system

The effect of initial vibrational excitation on Li2(v,J)+Na collisions was studied experimentally and theoretically. The relative integral cross sections σ(v,J) were measured in a crossed beam experiment for vibrational states v populated thermally or by optical pumping, and for final states detected by laser induced fluorescence. It is found that σ(v,J) increases by 35±7% between v=0 and v=20, while the dependence on the initial rotational state J is insignificant, within ±5%. The effect of vibrational enhancement is about twice as large as that found for the Li2–rare gas systems for the same amount of vibrational excitation. The effect is about an order of magnitude larger than predicted by our quasiclassical trajectory calculations on a LEPS potential energy surface, thus suggesting that additional long-range potential terms ought to be included. We also attempted to measure state-selectively the LiNa (v′,J′) products from the thermal reaction and found an upper limit of the reactive cross section of 1 Å2. This contrasts with the trajectory calculations that predict a 20 times larger cross section; we thus conclude that the reactive part of the potential also needs to be re-evaluated.

Role of mass renormalization in adiabatic quantum tunneling.

We introduce an adiabatic expansion to derive a formula for a macroscopic quantum tunneling probability that takes the mass renormalization due to the coupling to fast environmental degrees of freedom into account. The formula is applied to studying the effects of nuclear vibrational excitations on the fusion cross section in heavy-ion collisions at energies below the Coulomb barrier. It is shown that mass renormalization plays an important and useful role to discuss the effects of vibrational excitations of moderate energy. Also a general proof is given to show that the renormalized part of the effective mass appearing in our formula is the cranking mass.

Etching of Si surfaces with hot chlorine beams: Translational and vibrational excitation of the incident chlorine particles

A systematic study of etching of Si(100) and Si(111) and Ar+ bombarded Si(100) (surface with defect sites) with chlorine has been carried out. We have compared the reactivity of atomic and molecular chlorine, and studied the effect of translational and vibrational excitation of the etchant on the etch rate. Similar etch rates were obtained on the Si(100) and Si(111) single crystal surfaces. On both surfaces the major etch products were SiCl4 at Ts<500 K and SiCl2 at Ts≳600 K. Some SiCl desorption was obtained at Ts≳1100 K. We have outlined the conditions under which the etch rate, at constant incident flux, is adsorption and desorption limited. On the Si(100) and Si(111) surfaces we found that when the etch rate is reaction limited, higher etch rate can be obtained with atomic chlorine than with molecular chlorine. In the adsorption limited regime, translational excitation of both atomic and molecular chlorine results in a higher etch rate than that obtained with slow atomic and molecular chlorine. Vibrational excitation of Cl2 does not result in an enhancement of the etch rate on the single crystal surfaces studied. In the desorption limited regime the etch rate is primarily determined by the surface temperature. The nature of the etchant particles does not have a significant effect. On Ar+ bombarded Si(100), we obtained a small increase in the etch rate with translationally and vibrationally excited Cl2, and comparable enhancement is obtained with Cl2 that is only translationally excited. The difference in the etch properties of the single crystalline and Ar+ bombarded silicon surfaces points to the importance of surface defects in etching. The present results show that at room temperature the low etch rate of silicon single crystal surfaces with chlorine, as compared to etching with fluorine, is due to the low formation rate of volatile surface products.

A shock tube study of the CO+OH→CO2+H reaction

The rate coefficient for the reactionCO+OH→CO2+H has been determined using mixtures of nitric acid (HNO3), carbon monoxide (CO), and argon in incident shock wave experiments. Upon shock heating, the nitric acid rapidly decomposes into OH and NO2. The OH subsequently reacts predominantly via reaction (1). Quantitative OH time histories were obtained by continuous-wave (cw) narrow-linewidth UV laser absorption of the R1(5) line of the A2Σ+«X2Πi (0,0) transition at 32,606.56 cm−1 (vacuum). In some experiments, helium was added to the reactant mixture to examine CO vibrational excitation effects on the rate coefficient determination. It was found that the rate of excited CO (v=1) with OH is less than the rate of ground-state CO (v=0) with OH, which is in agreement with previous state-dependent work. The experiments were conducted over the temperature range 1090–2370 K and the pressure range 0.19–0.82 atm. The second-order rate coefficient was determined to be k1(T)=2.12×1012 exp[−2630/T (K)] (cm3 mol−1 s−1) with overall uncertainties of +16, −12% at high temperatures and +19, −22% at low temperatures. These results are in good agreement with recent studies of reaction (1) and are well fit by a chemically activated intermediate model. The current work also provides a link to previous low-temperature data.

Non-additive intermolecular forces from the spectroscopy of Van der Waals trimers: the effect of monomer vibrational excitation in Ar2–HF and Ar2–HCl

Calculations on the vibration–rotation energy levels of the Van der Waals trimers Ar2–HF and Ar2–HCl are carried out, in order to investigate the role of three-body (non-pairwise-additive) forces. The present calculations focus on the lowest Van der Waals vibrational state formed from HX molecules in different intramolecular vibrational states, v. The calculations use both pairwise-additive potentials, derived from the accurately known Ar–Ar, Ar–HF and Ar–HCl potentials, and various different contributions to the three-body forces. All five intermolecular degrees of freedom are included. The red shift observed in the Ar2–HF fundamental band is fairly well predicted (within 0.5 cm–1) by the pairwise-additive potential, but the agreement with experiment is improved when non-additive forces are included. The rotational constants are found to be very sensitive to non-additive terms, and especially to the ‘exchange quadrupole’ term. Non-additive forces of the type usually used in studies of atomic forces, such as triple dipole forces, are found to be inadequate to reproduce the observed spectra.

Effects of vibrational excitation in photostimulated desorption

Excitation of vibrational and rotational degrees of freedom, which occurs in the desorption process of NO and CO molecules from Pt(111) and Pt(001) surfaces with ultraviolet nanosecond-pulsed laser irradiation, is investigated by resonance-enhanced multiphoton ionization detection. Desorbed NO molecules are highly excited in vibration and rotation, whereas CO desorbed from Pt is rotationally cold and vibrationally excited. On the basis of the substrate-mediated desorption mechanism, the population in the first vibrational excited state and the rotational energy distribution can be consistently interpreted. In particular, we propose a simple scheme interpreting the rotational energy distribution.

Molecular electric properties and nuclear and vibrational relaxation

The effects of nuclear relaxation and change in vibrational molecular energy are accounted for in the theoretical determination of the molecular properties of HF, CH4, and C2H4. A method based on a finite-difference technique is employed. An analysis of the influence of these contributions versus the purely electronic contribution is made, together with an appraisal of the effect of vibrational excitation.

A quasiclassical trajectory study of mode specific reaction dynamics in the Cl + HOD and H + HOD reactions

We present the results of extensive quasiclassical trajectory studies of the reaction Cl + HOD → OH + DCl, OD + HCl and its counterpart with Cl replaced by H, based on the H3O potential surface of Schatz and Elgersma. Emphasis in this study has been given to studying the effect of vibrational excitation of the reagents on the branching between the OH and OD product channels. The specific initial vibrational states considered are (000), (100), (001), and (004), and we consider initial translational energies up to 2.7 eV. In the quasiclassical calculations, the initial vibrational states are defined using good actions which have been calculated using a Fourier series expansion method. We demonstrate that the initial states defined this way are close to being stationary for each vibrational mode. The resulting reactive cross sections for the Cl and H atom reactions are comparable in magnitude, with the Cl cross sections being systematically higher than the H cross sections. For both reagent atoms, branching to the OH product is enhanced when the (100) initial state is considered, and branching to OD is enhanced when the initial state is (001). Branching to OD is very strongly enhanced if the initial state is (004). The branching ratio is similar in magnitude for Cl and H atom reactions, though the energy dependence is different. No evidence is found that the Cl causes enhanced collision induced scrambling of the HOD vibrational excitation while the reagents approach. Product state distributions for both reactions are also presented at selected energies and compared with experiment and with previous calculations. Effects of zero point violation are studied, and it is found that requiring each product diatomic to have zero point energy leads to unphysically small cross sections and inaccurate cross section ratios. However, constraining just the zero point energy of the newly formed diatomic leads to accurate ratios and realistic cross sections.

The trans-bending and F–F stretching vibrations of HF dimer in νHF=1: The influence of intermolecular vibrational excitation on the predissociation dynamics

The optothermal detection method has been used to obtain near infrared spectra for combination bands of HF dimer associated with the F–F stretching (ν4) and trans-bending (ν5) vibrations, built upon the HF fundamental stretches (ν1 and ν2). These spectroscopic results are used to estimate the intermolecular trans-bend and F–F stretch frequencies, as well as the associated tunneling frequencies, all corresponding to the excited HF stretch vibrational states. Photofragment angular distributions have also been obtained for these excited states. These provide new insights into the effects of intermolecular vibrational excitation on the vibrational predissociation dynamics of this dimer. The initial state dependence of the final rotational state distributions suggests that the intermolecular stretch and bend are decoupled, with the energy in the F–F stretch evolving directly into relative translation of the photofragments, while excitation of the bend has no effect on the average kinetic energy release.

Collision-induced first overtone band of gaseous hydrogen from first principles.

In previous work [Phys. Rev. A 40, 6931 (1989)] the interaction-induced dipole moments of ${\mathrm{H}}_{2}$ pairs have been obtained by treating the complex of the two molecules like one molecule in the self-consistent-field and size-consistent, coupled-electron pair approximations; from this dipole surface, the binary collision-induced absorption spectra have also been computed for the rototranslational and the fundamental band. In the present work, the radial transition matrix elements of the induced dipole components are obtained for the first overtone band of ${\mathrm{H}}_{2}$ at 1.2 \ensuremath{\mu}m. Two cases are here considered: ${\mathit{v}}_{1}$=0\ensuremath{\rightarrow}0, ${\mathit{v}}_{2}$=0\ensuremath{\rightarrow}2 (``single'' vibrational transition) and ${\mathit{v}}_{1}$=0->1, ${\mathit{v}}_{2}$=0\ensuremath{\rightarrow}1 (``double'' transition), where the ${\mathit{v}}_{\mathit{i}}$ are the vibrational quantum numbers of two interacting ${\mathrm{H}}_{2}$ molecules (i=1 or 2). The dependence of these dipole elements on the most important initial and final rotational states (j=0,. . .,3) is also evaluated. From these results, the spectral profiles of the collision-induced absorption of molecular hydrogen pairs in the infrared 1.2-\ensuremath{\mu}m (the first ${\mathrm{H}}_{2}$ overtone) band are obtained. The calculations are based on a proven isotropic potential model which we have extended to account for the effects of vibrational excitations. The comparison of the computated spectra with the measurements available at temperatures from 24 to 300 K shows agreement within the estimated uncertainties of the best measurements (\ensuremath{\approxeq}10%). This fact suggests that theory is capable of predicting these spectra reliably at temperatures for which no measurements exist, with an accuracy that compares favorably with that of good laboratory measurements.

Adiabatic representations for the study of flux redistribution during photodissociation involving coupled electronic states: The effect of vibrational excitation on the photofragmentation of CH3I

In this paper we exploit a method for the time-independent study of the growth of photofragment flux, governed by photon absorption, and the subsequent redistribution of the flux, governed by the Hamiltonian in the excited, unbound state. For dissociation involving multiple electronic states, each with internal (vibrational or rovibrational) degrees of freedom, from a mechanistic standpoint it is more revealing to analyze the flux in a basis which is adiabatic in the nuclear degrees of freedom but diabatic in the electronic degrees of freedom. This approach is pursued here in application to a two-dimensional model for the photodissociation of CH3I.

Quasiclassical trajectory study of Li2(v⩽ 25,j⩽ 100)-Na exchange reaction

The effect of high initial vibrational (v = 0-25) and rotational (j = 0-100) excitation of the reactant molecule on the cross-section σR for the reaction Li2 + Na → LiNa + Li at a relative translational energy (E trans) of 0·5 eV has been investigated using the three-dimensional quasiclassical trajectory method on the recently proposed Varandas-Morais-Pais (VMP) potential. σR increases almost linearly with increase in v, but it remains unaltered, within the statistical error, with increase in j for j = 0-30. For the higher j states, there is an increase in σR with increase in j, and the rotational enhancement gets larger with increase in v, becoming comparable to the vibrational enhancement at v = 20. The dependence of σR on E trans over the range 0·18 - 0·85 eV has been examined for v = 0, j = 0. For E trans ⩽ 0·25 eV, all trajectories get trapped. For higher E trans, σR rises to a maximum and then levels off. Comparison of our results with those on the Whitehead-Grice (WG) potential reveals that the reactivity is larger on the VMP potential than on the WG for the low v while the opposite is true for the higher v.

Double many-body expansion of the two lowest potential-energy surfaces for Li3 and dynamics of the Li + Li2(v) reaction. Initial orientation and vibrational excitation effects

The semiempirical double many-body expansion method has been used to obtain consistent analytic representations of the two lowest doublet potential-energy surfaces for Li3. A recently reported semiempirical formulation of the three-body dynamical correlation energy has been used, which accounts for the proper dependence on the diatomic intramolecular coordinate of the Li–Li2 long-range dispersion energy. Besides providing an accurate description of the most recent, and reliable, ab initio energies for both sheets of the potential-energy surface of the title system at the strong interaction regions, the lowest sheet of the Li3 doublet potential-energy surface shows the correct long-range behaviour for the isotropic and leading anisotropic terms in the Legendre analysis of the Li–Li2 interaction as a function of the Li2 stretching coordinate. Based on the premise that only this lowest sheet is necessary, quasiclassical trajectories have been run to explore the effect of vibrational bond stretching in the Li + Li2(v) exchange reaction. The effect of the reagent initial orientation and rotational excitation as a function of the collision energy on the lithium atom exchange dynamics has also been investigated.

Comparison of bending, C–C stretching, and collision energy effects on the reaction of C2H+2 with D2

We report the effects of vibrational excitation and collision energy on the cross sections and branching ratios for reaction of acetylene cations with D2, using two different guided-ion-beam instruments. Two major reaction channels are observed, both of which are nearly thermoneutral. Hydrogen atom exchange is slightly exoergic due to zero point energy, and is inhibited by both collision and vibrational energy. Formation of the two isotopic ‘‘C2H+3 ’’ products is enhanced by collision energy and C–C stretching vibration, but not by bending vibration. The branching ratios at low collision energies are consistent with reaction via an intermediate complex, and Rice–Ramsberger–Kassel–Marcus (RRKM) analysis is used to extract further information. At collision energies above 1 eV, D-atom transfer by a direct mechanism is also observed as a route to C2H2D+ production. Comparison of our results using both the Stony Brook and Freiburg instruments is made with the state-selected experiments of Honma, Kato, Tanaka, and Koyano [J. Chem. Phys. 81, 5666 (1984)], who previously studied both the C2H+2+D2 and C2D+2+H2 isotopic reactants. Our results for C2H+2+D2 are consistent with their C2D+2+H2 data for all collision energies and with their C2H+2+D2 data at 0.2 eV collision energy. We do not reproduce the anomalous vibrational effects they reported for 1 and 2 eV collision energies.

Calculation of real-gas effects on blunt-body trim angles

The effect of vibrational excitation and dissociation at high temperatures on the trim angle of attack of a blunt lifting body is calculated for a nonequilibrium flow regime in air using a CFD technique. The vibrational-electronic temperature and the species densities are calculated assuming the flow to be in a nonequilibrium state. The forebody flow of a two-dimensional blunt body of the shape of the Apollo Command Module at a finite angle of attack is calculated. The results show that the pitching moment around a reference point is larger and the trim angle of attack is smaller for a reacting gas than for a perfect gas. The calculated shift in the trim angle due to the real-gas effect is of the same order as that seen during the Apollo flights.

The effect of reagent excitation on the dynamics of the reaction O(1D2)+H2→OH(X 2Π)+H

The effect of H2 translational, rotational, and vibrational excitation on the dynamics of the O(1D2)/H2 reaction are explored in a semiclassical trajectory study involving both of the energetically accessible potential energy surfaces of the system. Landau–Zener probabilities determine surface hopping. At low reagent excitation, the deep H2O potential minimum dominates the dynamics, causing the reagents to reorient towards a H–O–H (insertion) configuration and form the H2O intermediate, irrespective of the initial approach geometry. High vibrational excitation enhances the probability for transitions onto the excited state potential during the interaction. Reactions which sample the excited state potential have fundamentally different dynamics from those which remain on the lower state. For reactions involving H2(v=4), the OH product has a bimodal vibrational distribution, peaking in OH(v′=2) and OH(v′=9). The lower peak is due to reactions which access the excited state potential; the higher peak results from those which remain on the lower state for the entire interaction. High translational excitation shortens the interaction time and reduces the effect of the potential minimum to reorient the reagents. Rotational excitation also reduces the effect of the potential minimum by causing the system to rotate out of the insertion geometry before entering the potential minimum.

Effect of vibrational excitation on electron transport in gases

The thermal electron mobility was found to increase with increasing temperature T for CH 4 and to decrease with T for C 2F 6. These findings can be understood on the basis of the momentum transfer cross section σ m(ϵ) of the two molecules and the effect of vibrational excitation on electron transport.

The effects of vibrational mode, spin–orbit state, and collision energy on collision-induced dissociation and predissociation of OCS+

We report the first study of the effects of mode specific vibrational excitation on collision-induced dissociation of a polyatomic cation. Resonance-enhanced multiphoton ionization was used to produce OCS+ ions with variable excitation in each of the three vibrational modes, in combination with either spin–orbit electronic state. Cross sections for fragmentation of the state-selected ions in collision with Xe and Ar atoms were measured over the collision energy range from 0.5 to 9.0 eV, using a guided ion beam tandem mass spectrometer. Both spin-allowed and spin-forbidden channels are observed for both collision partners, and the cross sections in both cases are enhanced by collision energy and all modes of vibrational excitation. Vibration is more effective than collision energy, but the effect is nonmode specific. The absolute magnitude of the dissociation cross sections and the relative vibrational effects are bigger for collisions with Xe than for Ar. The spin–orbit state of the primary ion has no effect on dissociation of ground vibrational state OCS+, but there appears to be some synergy between vibrational and spin–orbit excitation. The results are discussed in view of the electronic structure of OCS+ and possible scattering mechanisms.

A study of the reactions between Ba(1S) and N2O, O2, and CO2

A kinetic study is presented of the reactions between Ba(1S) atoms and N2O, O2, and CO2 over an extended temperature range. Ba atoms were produced in an excess of each oxidant and He bath gas by the pulsed 2-photon (193.3 nm) dissociation of BaO, produced in situ by the reaction between Ba vapor and the oxidant, and then monitored by time-resolved laser induced fluorescence spectroscopy at λ=553.55 nm (Ba(61P−61S)). The following results were obtained (at the 2σ level of uncertainty): k(Ba+N2O, 242<T<850 K) =(2.99±0.14)×10−10 exp[−(5.7±0.17) kJmol−1/RT]; k(Ba+O2, 254<T<566 K) =(4.89±0.38)×10−10 exp[−(7.06±0.23) kJmol−1/RT]; k(Ba+CO2, 257<T<849 K)=(3.97±0.21)×10−10 exp[−(7.66±0.18) kJmol−1/RT] cm3 molecule−1 s−1. These results are in good agreement with recent molecular beam studies. In the case of Ba+N2O, two line-of-centers collision models are employed to demonstrate that the effect of vibrational excitation of N2O is to enhance the chemiluminescence quantum yield as well as the reaction cross section. Comparison with the chemiluminescence cross section for the reaction Ba+N2O →BaO(A1Σ+ and A′1Π ), obtained in a beam-scattering experiment, indicates a branching ratio of (18.3±6.7)% into these states.