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.     

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.     

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.     

Intramolecular vibrational energy redistribution involving the torsion in CF3CH3: a molecular dynamics study

Classical trajectory calculations on intramolecular vibrational energy redistribution (IVR) involving the torsion in 1,1,1-trifluoroethane (TFE) are reported. Two potential energy functions (PEFs) are used to describe the potential energy surface. The "full" PEF gives excellent agreement with the experimental vibrational frequencies. The "simple" PEF omits nondiagonal interaction terms, but still gives very good agreement with the experimental frequencies. The "simple" PEF is intended to minimize mode-mode coupling. Neither PEF includes the HF elimination reaction. Calculations are carried out both with nominal microcanonical selection of initial coordinates and momenta, and with a modified selection method that places controlled amounts of energy in the torsion. Total (classical) vibrational energies from 0.005 to 140 kcal mol(-1) are investigated. The calculated time constants describing energy flow out of the torsional mode are <10 ps for classical vibrational energies near the classical reaction threshold energy (approximately 75 kcal mol(-1)) and greater. It is found that the rate of decay from the torsion largely depends on the amount of energy in the other vibrational modes. Analysis using power spectra shows that the torsional mode in TFE is strongly coupled to the other vibrational modes. These results strongly suggest that vibrational energy in TFE will not be sequestered in the torsion for time periods greater than a few tens of picoseconds when the molecule has enough energy to react via HF elimination.     

The dependence of low-energy electron attachment to CF3Br on electron and vibrational energy

In a joint experimental and theoretical effort, we have studied dissociative electron attachment (DEA) to the CF3Br molecule at electron energies below 2 eV. Using two variants of the laser photoelectron attachment method with a thermal gas target (T(G) = 300 K), we measured the energy dependent yield for Br- formation over the range E = 3-1200 meV with resolutions of about 3 meV (E < 200 meV) and 35 meV. At the onsets for excitation of one and two quanta for the C-Br stretching mode nu3, downward cusps are detected. With reference to the recommended thermal (300 K) attachment rate coefficient k(A)(CF3Br) = 1.4 x 10(-8) cm3 s(-1), absolute cross sections have been determined for Br- formation. In addition, we studied Br- and (CF3Br)Br- formations with a seeded supersonic target beam (10% CF3Br in helium carrier gas, with a stagnation pressure of 1-4 bars and nozzle temperatures of 300 and 600 K) and found prominent structure in the anion yields due to cluster formation. Using the microwave pulse radiolysis swarm technique, allowing for controlled variation of the electron temperature by microwave heating, we studied the dependence of the absolute DEA rate coefficient on the mean electron energy E over the range of 0.04-2 eV at gas temperatures T(G) ranging from 173 to 600 K. For comparison with the experimental results, semiempirical resonance R-matrix calculations have been carried out. The input for the theory includes the known energetic and structural parameters of the neutral molecule and its anion; the parameters of the resonant anion curves are chosen with reference to the known thermal rate coefficient for the DEA process. For the gas temperature T(G) = 300 K, good overall agreement of the theoretical DEA cross section with the experimental results is observed; moreover, rate coefficients for Br- formation due to Rydberg electron transfer, calculated with both the experimental and the theoretical DEA cross sections, are found to agree with the previously reported absolute experimental values. At T(G) = 300 K, satisfactory agreement is also found between the calculated and experimental attachment rate coefficients for mean electron energies E = 0.04-2 eV. The strong increase of the measured rate coefficients with rising gas temperature, however, could be only partially recovered by the R-matrix results. The differences may result from the influence of thermal excitations of other vibrational modes not included in the theory.     

Spatially resolved vibrational energy transfer in molecular monolayers

We have shown that it is possible to input heat to one location of a molecule and simultaneously measure its arrival in real time at two other locations, using an ultrafast flash-thermal conductance technique. A femtosecond laser pulse heats an Au layer to approximately 800 degrees C, while vibrational sum-frequency generation spectroscopy (SFG) monitors heat flow into self-assembled monolayers (SAMs) of organic thiolates. Heat flow into the SAM creates thermally induced disorder, which decreases the coherent SFG signal from the CH-stretching transitions. Recent improvements in the technique are described, including the use of nonresonant background-suppressed SFG. The improved apparatus was characterized using alkanethiolate and benzenethiolate SAMs. In the asymmetric 2-methyl benzenethiolate SAM, SFG can simultaneously monitor CH-stretching transitions of both phenyl and methyl groups. The phenyl response to flash-heating occurs at least as fast as the 1 ps time for the Au surface to heat. The methyl response has a faster portion similar to the phenyl response and a slower portion characterized by an 8 ps time constant. The faster portions are attributed to disordering of the methyl-substituted phenyl rings due to thermal excitation of the Au-S adbonds. The slower portion, seen only in the methyl SFG signal, is attributed to heat flow from the metal surface into the phenyl rings and then to the methyl groups.     

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.     

Theory of collisional energy transfer - bromine in low-density inert gases

Classical binary collision trajectory calculations have been carried out to study the energy transfer efficiency between the internal degrees of freedom of highly energized bromine (Br₂) and the translation degrees of freedom of an inert gas (He, Ne, Ar, Kr, Xe) in the low-density limit. The dependence on species (mass, strength of attraction), temperature of the gas (T = 160, 300, and 1500 K) and internal energy of the bromine (14, 28, 43 kcal/mole; E_d = 45.5 kcal/mole) is considered. Global statistical theories overestimate the average energy transferred per collision by an order of magnitude or more. A simple impulsive collision theory is developed and found to account for the magnitudes (typically within a factor of 2–3) and the gross-trends to reasonable accuracy.     

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.     

A crucial demonstration of strong collisional behavior of vibrational energy for gas-surface collisions

Application of the variable encounter method (VEM) has been made to the thermal cyclobutene decomposition system. Vibrational energy transfer in the cyclobutene-seasoned quartz surface system has been studied. The occurrence of strong collisions is demonstrated in crucial manner at temperatures below 400 K.     

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.     

Chemical effects of radical vibrational energy content on the abstraction reaction: CF3 + Br2 → CF3Br + Br

We have studied the effect of vibrational mode activation in the CF₃ radical on the bromine abstraction reaction; CF₃ + Br₂ → CF₃Br + Br. Excess vibrational energy resides in the symmetric modes of the radical after 248 nm photolysis of the parent molecule, CF₃I. Our data indicate that the hot radicals react no faster than thermalized CF₃, and may actually have a lower cross-section for reaction. Dynamical factors that result in poor coupling of the vibrational energy to the reaction coordinate, as well as other similar considerations, could be responsible for the experimental observations. In addition, we have made an independent determination of the rate for the bromine abstraction reaction of (1.08 ± .13) × 10¹² s^?1 cm³ mol^?1.     

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.     

Coherent vibrational energy transfer along a peptide helix

To measure the transport of vibrational energy along a peptide helix, Hamm and co-workers [J. Phys. Chem. B 112, 9091 (2008)] performed time-resolved vibrational experiments, which showed that the energy transport rate increases by at least a factor of 4, when a localized C=O mode of the peptide instead of an attached chromophore is excited. This finding raises the question if coherent excitonic energy transfer between the C=O modes may be of importance for the overall energy transport in peptides. With this idea in mind, nonequilibrium molecular dynamics simulations as well as quantum-classical calculations are performed, which qualitatively reproduce the experimental findings. Moreover, the latter model (an exciton Hamiltonian whose matrix elements depend on the instantaneous positions of the peptide and solvent atoms) indeed exhibits the signatures of coherent quantum energy transport, at least within the first few picoseconds and at low temperatures. The origin of the observed decoherence, the absence of vibrational self-trapping, and the possibility of quantum interference between various transport paths are discussed in some detail.     

Influence of temperature on thymine-to-solvent vibrational energy transfer

At the instant following the non-radiative deactivation of its ππ* electronic state, the vibrational modes of thymine possess a highly non-equilibrium distribution of excitation quanta (i.e., >4 eV in excess energy). Equilibrium is re-established through rapid (5 ps) vibrational energy transfer to the surrounding solvent. The mechanisms behind such vibrational cooling (VC) processes are examined here using femtosecond transient grating and two-dimensional photon echo spectroscopies conducted at 100 K and 300 K in a mixture of methanol and water. Remarkably, we find that this variation in temperature has essentially no impact on the VC kinetics. Together the experiments and a theoretical model suggest three possible mechanisms consistent with this behavior: (i) vibrational energy transfer from the solute to solvent initiates (directly) in intramolecular modes of the solute with frequencies >300 cm(-1); (ii) the relaxation induced increase in the temperature of the environment reduces the sensitivity of VC to the temperature of the equilibrium system; (iii) the time scale of solvent motion approaches 0.1 ps even at 100 K. Mechanism (i) deserves strong consideration because it is consistent with the conclusions drawn in earlier studies of isotope effects on VC in hydrogen bonding solvents. Our model calculations suggest that mechanism (ii) also plays a significant role under the present experimental conditions. Mechanism (iii) is ruled out on the basis of long-lived correlations evident in the photon echo line shapes at 100 K. These insights into photoinduced relaxation processes in thymine are made possible by our recent extension of interferometric transient grating and photon echo spectroscopies to the mid UV spectral region.     

Rotational mode participation in long-distance vibrational energy transfer in solids

It is shown that nonresonant vibrational energy transfer between molecules in solids can be greatly enhanced by participation of the rotational mode. This mechanism is predicted to dominate over multiphonon-assisted energy transfer for systems with a large mismatch (?10² cm^?1 between the vibrational energy spacings of the donor and the acceptor. Calculations based on this mechanism on the processes NH,ND(³Π,υ = 1) → ¹²CO, ¹³CO (in solid Ar) yield results in excellent agreement with the experimental isotope effects.     

Surface modification of polypropylene with CF4, CF3H,CF3Cl,and CF3Br plasmas

ESCA and contact angle measurements were used to characterize the surfaces of polypropylene and glass substrates exposed to CF₄, CF₃H, CF₃Cl, and CF₃Br plasmas. The use of both organic and inorganic substrates allowed clear distinction between treatments which led to plasma polymerization and treatments which caused grafting of functional groups directly to the substrate surfaces. CF₄ plasmas were the only treatments studied which fluorinated polypropylene surfaces directly, without the deposition of a thin, plasma-polymerized film. CF₃H polymerized in a plasma, while CF₃Cl and CF₃Br plasmas caused chlorination and bromination of polypropylene surfaces, respectively. Correlations were made between the active species present in the plasmas and the surface chemistry observed on the treated polypropylene substrates.     

Semiclassical time-correlation function approach to collisional energy transfer into many-atom systems

Cross sections for energy transfer into many-body systems can be expressed in terms of time-correlation functions (TCFS ) of transition operators. A semiclassical version is presented by treating internal motions as quantized and relative motions as classical. The time evolution of internal motions can be calculated in the Heisenberg picture and avoids expansions in target states. The decoupling of fast and slow internal motions is treated and applied to vibrational–rotational decoupling in polyatomic molecules. Results are presented for Li⁺-CO₂ collisions.     

Semiclassical three-dimensional model for vibrational energy transfer in diatomic molecules

A semiclassical model for calculation of rate constants for vibrational excitation in diatomic gases at low temperatures (below 1000 K) is suggested. The model has been tested by its ability to predict the relaxation times of hydrogen (τ_H1 in the temperature region 40–1000 K. The agreement with experimental values is excellent. The isotopic ratio τ_D2/τ_H2 as a function of temperature is predicted.     

Experimental and theoretical vibrational study of 2,2,2-trifluoroethyl trifluoromethanesulfonate, CF3SO2OCH2CF3

The infrared spectra of 2,2,2-trifluoroethyl trifluoromethanesulfonate (CF3SO2OCH2CF3) were obtained in the gaseous, liquid and solid states as well as the Raman spectrum of the liquid. Quantum chemistry calculations using the density functional theory were used to predict the most stable geometry and conformation of the studied molecule. Subsequently, the harmonic vibrational frequencies and force field were calculated. An assignment of the observed spectral features made after comparison with the related molecules and with the predicted frequencies was used as the basis of a scaling of the original force field in order to reproduce as well as possible the experimental frequencies. With this purpose a set of scale factors was calculated by a least square procedure, leading to a final root mean square deviation (RMSD) of 9.7 cm(-1).