Kinetically controlled selective ionization study on the efficient collisional energy transfer in the deactivation of highly vibrationally excited trans-stilbene

Direct measurements of the gas-phase collisional energy transfer parameters are reported for the deactivation of highly vibrationally excited trans-stilbene molecules, initially prepared with an average energy of about 40 000 cm(-1), in the bath gases argon, CO2, and n-heptane. The method of kinetically controlled selective ionization (KCSI) has been used. Complete experimental collisional transition probability density functions P(E',E) are determined, which are represented by a monoexponential form with a parametric exponent in the argument, P(E',E) proportional to exp[-{(E - E')/(C0 + C1E)}Y] (for downward collisions), well established from earlier KCSI studies. A comparison of the first moments of energy transfer rate constants, kE,1, or of resulting first moments of energy transfer, , for trans-stilbene with those for azulene and toluene clearly shows the considerably more efficient deactivation of trans-stilbene for all bath gases, presumably due to the much greater number of very low-frequency modes of trans-stilbene. However, on a relative scale this gain in deactivation rate of excited trans-stilbene is clearly collider dependent and decreases distinctly with the growing collision efficiency of the larger bath gas molecules.     

Selection rules for collisional energy transfer in homonuclear diatomics. rotationally inelastic collisions

We have made a precise study of the circular polarisation of rotationally resolved features of laser-excited iodine. The J′ = 19, ν′ = 16 level of ³II⁺_ou was excited using circularly polarised dye-laser fluorescence and a quantitative data on polarisation features representing inelastic transfer of ΔJ′ = 30 was recorded. The experimental circular polarisation ratios were compared to those predicted by two totally conserving models, ΔM = 0 and Δθ = 0. The agreement between experimental points and the predictions based on the former lead to the formation of a new selection due on rotationally inelastic transfer namely, ΔM = 0.     

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.     

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.     

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.     

Semiclassical extension of the Landau-Teller theory of collisional energy transfer

A semiclassical version of the quantum coupled-states approximation for the vibrational relaxation of diatomic molecules in collisions with monatomic bath gases is presented. It is based on the effective mass approximation and a recovery of the semiclassical Landau exponent from the classical Landau-Teller collision time. For an interaction with small anisotropy, the Landau exponent includes first order corrections with respect to the orientational dependence of the collision time and the effective mass. The relaxation N(2)(v=1)-->N(2)(v=0) in He is discussed as an example. Employing the available vibrationally elastic potential, the semiclassical approach describes the temperature dependence of the rate constant k(10)(T) over seven orders of magnitude across the temperature range of 70-3000 K in agreement with experimental data and quantum coupled-states calculations. For this system, the hierarchy of corrections to the Landau-Teller conventional treatment in the order of importance is the following: quantum effects in the energy release, dynamical contributions of the rotation of N(2) to the vibrational transition, and deviations of the interaction potential from a purely repulsive form. The described treatment provides significant simplifications over complete coupled-states calculations such that applications to more complex situations appear promising.     

A Modified statistical theory of collisional energy transfer in thermal reactions

A fully statistical kernel describing the probability of energy transfer in collisions between polyatomic reactant (A) and heat bath (M) molecules in a thermal system is developed, proceeding through the formation of an intermediate collision complex (AM) whose internal degrees of freedom are assumed to exchange energy. After pointing out that this kernel does not give a quantitatively useful answer, the kernel is modified by introducing the concept that the collision complex lifetime is due to orbiting collisions, and that the (AM) lifetime must equal collision duration. This puts two constraints on the internal degrees of freedom of (AM): (1) those that correlate with relative translation and intrinsic rotation of separated A and M (= transitional modes) can contain only an amount of energy not exceeding E_*, which is the maximum energy for which orbiting can occur; (2) those that correlate with internal degrees of freedom of M must have a density of states such that, subject to constraint (1), the lifetime of (AM) is equal to collision duration. It turns out, quite unambiguously, that the appropriate density of states is equivalent to just one oscillator of M participating in energy exchange. Calculations of average amount of energy transferred (Δ E>) in the system CH₃NC + M show good quantitative agreement with experiment for both polar and non-polar M. The modified theory does not give any appreciable dependence of Δ E> on the size of M because collision duration is assumed to depend only on the long-range part of the potential.     

Relaxation of internal energy and collisional energy transfer

Using the exponential model for the collisional transition probability, it is shown that relaxation of average internal energy is a measure of bulk-average energy transfer ?ΔE?. This is a macroscopic property which is a complicated function of both time and initial excitation and is only distantly related to average energy transferred per collision ?ΔE?, a microscopic property.     

Quasi-resonance energy transfer in molecular collisions. Vibrational energy exchange between N2 and CO2 molecules

Possible reasons for the non-monotonous temperature dependence of the quasi-resonance energy transfer probability and relative contributions of short-range and long-range forces are discussed. Particular calculations are made for the case N₂-CO₂(ν₃) vibrational energy exchange.     

The collisional deactivation of O(1D) atoms by molecular oxygen

Vibrationally excited ¹⁶O¹⁸O(³Σ_g^-) has been observed at short delay times during the flash photolysis of mixtures of ¹⁶O₃ and ¹⁸O₂. This species can only be formed by the reaction of an ¹⁶O(¹D) atom with an ¹⁸O₂ molecule, indicating that O₂(³Σ_g^-)_{ν″ > 0} is a product of the collisional deactivation of O(¹D) by O₂.     

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.     

Translational energy disposal in molecular collisions: The transfer of momentum constraint

A Franck—Condon type argument, which requires the least transfer of momenta to the nuclei during a collision is outlined and applied to the analysis of translational energy disposal and its dependence on the initial translational energy. Using the maximal entropy procedure of information theory we are able to proceed directly from the assumed (model) constraint to the product state distribution.     

Measurements of collisional energy transfer in unimolecular processes

It is shown that the optimal means of tabulating collisional energy transfer parameters in gas-phase uni- and ter-molecular reactions is as the average downward energy transfer, rather than the total energy transferred or the collision efficiency.     

Velocity dependence of vibrational branching ratios in electronic energy transfer collisions of metastable argon atoms with nitrogen

Measurements have been made of the vibrational branching ratio (υ′=0)/(υ′=1) in N^*₂ (C³Π_u) formed in electronic energy transfer collisions between argon metastable atoms and ground state nitrogen molecules, using crossed molecular beams. In the relative collision energy range, 0.08–0.20 eV, this ratio is 3.5±0.2.     

The deactivation of the asymmetric stretch mode of carbon dioxide by hydrogen

The deactivation of CO₂(00⁰1) by H₂ is shown to involve a near-resonance transfer of vibrational to rotational energy.     

M-dependence of collisional transfer of rotational energy in methanol

An M-resolved microwave double resonance experiment on methanol is described. Relative signal intensities and M-selection rules in pure CH₃OH are consistent with a dipole—dipole collisional interaction, while those for CH₃OHHe and CH₃OHH₂ mixtures indicate more complex interactions.     

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.     

70 years of Landau-Teller theory for collisional energy transfer. Semiclassical three-dimensional generalizations of the classical collinear model

This article, in historical retrospective, describes the development of the celebrated Landau-Teller (LT) model of 1936 for vibrational-translational energy exchange in collisions of an atom with a diatomic molecule. We discuss semiclassical generalizations of the classical LT model and generalizations of the collinear LT model to account for the effects of rotation of the diatom on the vibrational relaxation rate. The former is based on the recovery of the Landau semiclassical exponent from the classical LT encounter time, and the latter on the definition of a 1-D driving mode within the manifold of the translational and rotational degrees of freedom of the colliding partners. The utility of generalized LT models is illustrated by three case studies that exemplify weak and strong effects of the rotation as well as the efficiencies of different driving modes in the vibrational relaxation of highly asymmetric diatoms.     

Energy transfer between polyatomic molecules. 1. Gateway modes, energy transfer quantities and energy transfer probability density functions in benzene-benzene and Ar-benzene collisions

We report collisional energy transfer, CET, quantities for polyatomic-polyatomic collisions and use excited benzene collisions with cold benzene bath, B-B, as our sample system and compare our results with the CET of excited benzene with Ar bath. We find that the gateway mode for both systems is the out-of-plane modes and that in B-B CET, vibration to vibration, V-V, is the dominant channel. Rotations play a mechanistic role in the CET but the net rotational energy transfer is small compared to V-V. The shape of the down side of the energy transfer probability density function, P(E,E'), is convex for B-B collisions and it becomes less so as the temperature increases. In Ar-B collisions, P(E,E') is concave and it becomes less so as the temperature decreases. We report average vibrational, rotational, and translational energy transferred, , as function of temperature for various initial conditions.