Translational to rotational energy transfer in molecule-surface collisions

A theoretical approach that combines classical mechanics for treating translational and rotational degrees of freedom and quantum mechanics for describing the excitation of internal molecular modes is applied to the scattering of diatomic molecules from metal surfaces. Calculations are carried out for determining the extent of energy transfer to the rotational degrees of freedom of the projectile molecule. For the case of observed spectra of intensity versus final rotational energy, quantitative agreement with available experimental data for the scattering of NO and N(2) from close packed metal surfaces is obtained. It is shown that such measurements can be used to determine the average rotational energy of the incident molecular beam. Measurements of the exchange of energy between translational and rotational degrees of freedom upon collision are also described by calculations for these same systems.     

Vibrational energy transfer in non-reactive molecular collisions: An information-theoretic analysis

Experimental and (trajectory) computed rates of vibrational relaxation of molecules in selected internal states are analyzed. A dynamic constraint is identified. The entire array of (vibrational) manifold-to-manifold rate constants (at a given temperature) is shown to be characterized by this constraint and hence yield a linear surprisal plot. It is argued that bulk measurements of the vibrational relaxation time in a buffer gas will suffice to determine the single parameter in the dynamic constraint.     

Vibrational and rotational energy transfer upon molecular collisions

The cross section of the rotational and vibrational energy transfer is derived by using the first Born approximation which quantizes the translational motion of the colliding particles. The theory developed here integrates the intermolecular potential V(R) over all regions of the internuclear distance R by obtaining a Fourier transform of V(R). This differs from previous semiclassical (impact-parameter) treatments which either considered the short-range repulsive interaction or expanded V(R) into a long-range multipole expansion. The cross section obtained here is expressed in a very simple algebraic expression which can be readily calculated. This will be illustrated by examples of CO₂(001)+N₂(υ=0)=CO₂(000)+N₂(υ=1)+18.6 cm^?1 and CO(υ=1)+CO(υ=1)=CO(υ=0)+CO(υ=2)+27 cm^?1. Calculations have been made both for the exothermic and for the endothermic reactions. The comparison of the present results with experimental results as well as with previously calculated results will be discussed.     

Power-gap law and the dynamical constraints on angular momentum transfer in rotationally inelastic molecular collisions

The model proposed by Dexheimer, Durand, Brunner and Pritchard has been developed and tested on CO₂H₂, CO₂He, Na₂He, Na₂Ne and N₂Ar. This model explains the rapid decrease in the rotationally inelastic integral cross sections with increase in the amount of rotational energy transfer (∣ΔE∣) in the region ∣ΔE∣ > ∣ΔE∣^*, ∣ΔE∣^* is foun to depend on the reduced mass of the system, the moment of inertia of the molecule, the initial rotational state, and the interaction potential. Data for the systems studied show quantitative agreement with the predictions of the model.     

The role of orbiting collisions in vibrational energy transfer

Using a simple model of molecular collisions under a spherically symmetric interaction, it is shown that orbiting collisions can make very large contributions to the inelastic cross sections of non-resonant processes. Calculations for the system HX + CO₂(001) → HX(υ=1) + CO₂(000), where X = F, Cl, I show good agreement with experimental results.     

Dynamics of energy transfer in peptide-surface collisions

Classical trajectory simulations are performed to study energy transfer in collisions of protonated triglycine (Gly)(3) and pentaglycine (Gly)(5) ions with n-hexyl thiolate self-assembled monolayer (SAM) and diamond [111] surfaces, for a collision energy E(i) in the range of 10-110 eV and a collision angle of 45 degrees. Energy transfer to the peptide ions' internal degrees of freedom is more efficient for collision with the diamond surface; i.e., 20% transfer to peptide vibration/rotation at E(i) = 30 eV. For collision with diamond, the majority of E(i) remains in peptide translation, while the majority of the energy transfer is to surface vibrations for collision with the softer SAM surface. The energy-transfer efficiencies are very similar for (Gly)(3) and (Gly)(5). Constraining various modes of (Gly)(3) shows that the peptide torsional modes absorb approximately 80% of the energy transfer to the peptide's internal modes. The energy-transfer efficiencies depend on E(i). These simulations are compared with recent experiments of peptide SID and simulations of energy transfer in Cr(CO)(6)(+) collisions with the SAM and diamond surfaces.     

Bromine-bromine energy transfer by binary collisions

The collisional energy transfer between a highly excited bromine molecule and a non-reactive monatomic (Ar or Br) or diatomic (Br₂) medium has been investigated by binary trajectory calculations over a wide range of medium temperatures and internal energies of the reactant molecule. The efficiency of the energy transfer is compared with expectations based on simple statistical approximations used in unimolecular reaction rate theory. Large deviations are found, particularly with respect to the contributions made by different types of degrees of freedom and to the dependence on the internal energy of the reactant molecule. Transfer to or from vibrational degrees of freedom appears to be very inefficient. Translational energy is most readily transferred. The rate coefficient for energy transfer appears to decrease with increasing internal energy in most cases.     

Vibration-to-vibration energy transfer in COCO collisions

Vibration-to-vibration energy transfer rates for collision between two CO molecules are calculated. Results obtained are compared with recent experiments and theoretical values. It is pointed out that none of the existing calculations gives a complete account of the available experimental data.     

Temperature dependent energy transfer in Ar-O3 collisions

The energy transfer between argon atoms and ozone complexes O3*, excited in the region of the dissociation threshold, is calculated for fixed temperatures (100 K< or =T < or =2500 K) using classical trajectories. The internal energy of ozone is resolved in terms of vibrational and rotational energies. For all temperatures, energy flows from O3* to Ar. The vibrational energy transfer, relative to k(B)T, is very small below 500 K, but gradually increases towards high temperatures. The relative rotational energy transfer, on the other hand, monotonously decreases with T; around 1100 K it falls below the relative vibrational energy transfer. Thermally averaged cross sections for vibrational and rotational energy transfers are also calculated. The implications for the stabilization of ozone complexes in the energy transfer model are discussed.     

Modified statistical theory of energy transfer in ion-molecule collisions

The modified statistical theory developed previously for potentials appropriate to interactions in neutral-neutral collisions, is now extended to more strongly attractive potentials involved in ion-neutral collisions. The model system is the collisional deactivation of C₅H₉⁺ by a variety of both polar and non-polar neutral molecules. A 12 - 6 - 4 potential is used for ion interaction with non-polar neutrals, and a 12 - 6 - 4 - 2 potential, as modified by Su and Bowers to take into account the rotational energy of the neutral, for interaction with polar neutrals. Calculated is (ΔE), the average energy lost by the ion in a collision, and compared with experiment. For C₅H₉⁺-CH₄ collisions, the calculated (ΔE) agrees with experiment within 5%. Predictions of the theory, namely that (ΔE) should increase with excitation energy and should decrease with the size of the excited reactant, are found to be in fair agreement with the somewhat ambiguous experimental evidence.     

Classical theory of vibration energy transfer in collinear collisions

Classical theory of collisions is cast in a form which also includes the uncertainty principle. This theory is used for analyzing the vibration energy transfer in the collinear collision which approximates the He-H₂ system. The results are compared with the quantum calculations and several classical and semiclassical approaches. Very good agreement with quantum theory is found, for all the parameters investigated.     

The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in NO2+-rare gas collisions: an experimental and trajectory study

A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by approximately 4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2+ + Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+ -NO2 charge state.     

Refinement of electron momentum densities of ionic solids using an experimental energy constraint

It is demonstrated how one can refine a given approximate momentum density distribution using a constraint of the experimental electronic energy. The technique developed is based on the calculus of variations. This technique has been applied to ionic solids such as LiF, LiCIl NaF, NACl, MgO, KF and KCl.     

Inelastic molecular collisions with an orientation-averaged interaction energy

The importance of molecular orientations for vibrational-translational energy transfers between diatomic molecules has been investigated. An angle-dependent potential function is assumed, and it is averaged over the orientations and vibrations of colliding molecules. For I₂? I₂ and Cl₂? Cl₂, it is found that the calculated average vibrational transition probability for a colinear collision is over-estimated by large factors (1/γ) compared to that obtained when all possible molecular orientations are considered. At 300⁰K, 1/γ = 34.4 for I₂? I₂ and 17.6 for Cl₂? Cl₂, while it is 6.8 and 5.9 for N₂? N₂ and O₂? ₂, respectively. It is also shown that 1/γ decreases rapidly as temperature increases. At 2000⁰K, 1/γ ≈? 3 for I₂? I₂, Cl₂? Cl₂, and N₂? N₂, while it is ≈? 2.5 for O₂? O₂. In general, when the molecules are large, and when strong attractive forces act between them, 1/γ is very large at low temperatures (<1000⁰K).     

Rovibrational energy transfer in ortho-H2+para-H2 collisions

We present the results of a full-dimensional quantum mechanical study of the rovibrational energy transfer in the collision between ortho-H2 and para-H2 in the energy range of 0.1-1.0 eV. The multiconfiguration time-dependent Hartree algorithm has been used to propagate the wave packets on the global potential energy surface by Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)] and on a modification of this surface where the short range anisotropy is reduced. State-to-state attributes such as probabilities or integral cross sections are obtained using the formalism of Tannor and Weeks [J. Chem. Phys. 98, 3884 (1993)] by Fourier transforming the correlation functions. The effect of initial rotation of the diatoms on the inelastic and de-excitation processes is investigated.     

Near-resonant vibration-to-vibration energy transfer in the NO+-N2 collisions

First principles model calculations of the vibration-to-vibration (VV) energy transfer (ET) processes NO(+)(nu=1)+N(2)(nu=n-1)-->NO(+)(nu=0)+N(2)(nu=n)+(28.64n-14.67) cm(-1) and NO(+)(nu=n)+N(2)(nu=0)-->NO(+)(nu=n-1)+N(2)(nu=1)+(32.52(n-1)+13.97) cm(-1) for n=1-3 in the 300-1000 K temperature range are performed. The VV ET probability is computed for three mechanisms: (1) The charge on NO(+) acting on the average polarizability of N(2) induces a dipole moment in N(2) which then interacts with the permanent dipole moment of NO(+) to mediate the energy transfer. (2) The charge on NO(+) acting on the anisotropic polarizability of N(2) induces a dipole moment in N(2) which then interacts with the permanent dipole moment of NO(+) to mediate the energy transfer. (3) The dipole moment of NO(+) interacts with the quadrupole moment of N(2) to mediate the energy transfer. Because the probability amplitudes of the second and third mechanisms add coherently the ET probability for these two mechanisms is given as a single number. The probability of energy transfer per collision is in the 5 x 10(-3) range. The results of this calculation are compared with the available experimental data. This calculation should help quantify the role of NO(+) in the energy budget of the upper atmosphere.     

Charge transfer in S2++H collisions at intermediate energy

Partial cross-sections for the charge transfer process of S²⁺ ions in collision with atomic hydrogen at impact energies up to 8 keV have been calculated by means of a semi-classical method using ab initio potential energy curves and couplings. The results are in relatively good agreement with experiment and improve significantly previous Landau-Zener calculations.     

Vibrational-translational energy transfer n atom-polyatomic molecule collisions in thermal reaction systems

Vibrational-translational energy transfer probabilities and collisional efficiencies are calculated for atom-polyatomic molecule collisions. It is assumed that a collision complex is formed and that the total internal vibrational energy is statistically distributed among all the modes of the complex. An attractive potential is assumed and account is taken of the centrifugal barrier. Conservation of system angular momentum is imposed. Convolution of the several thermal distribution functions is carried out and completeness and detailed balance are observed. Comparison of calculated quantum statistical quantities with experiment is made for the thermal isomerization of methyl and ethyl isocyanide in the presence of heavy atomic bath gases, such as Xe or Ar, and semiquantitative agreement is found.     

Vibrational energy transfer in N2-N2 collisions: a new semiclassical study

The vibrational energy relaxation in collisions between N2 molecules in the low- and medium-lying vibrationally excited levels was revisited using the semiclassical coupled-state method and the use of two different potential-energy surfaces having the same short-range potential recently determined from ab initio calculations but with different long-range interactions. Compared to the data reported in the classical work by Billing and Fisher [Chem. Phys. 43, 395 (1979)], the newly calculated vibration-to-translation rate constant K(1,0 / 0,0) is in much better agreement with the available experimental data over a large temperature interval, from T = 200 K up to T = 6000 K. Nevertheless, as far as the vibration-to-translation exchanges are concerned, the lower-temperature regime remains quite critical in that the new rate constants do not completely account for the rate constant curvature suggested by the experiments for temperatures lower than T = 500 K. The dependence of the state-selected vibration-to-vibration rate constants, K(v,v-delta v / 0,1), both upon the vibrational quantum number v and the gas temperature are calculated. The substantial deviations from previously found behaviors could have major consequences for the vibrational kinetic modeling of N2-containing gas mixtures.