Translational and rotational excitation of the CO[sub 2](00[sup 0]0) vibrationless state in the collisional quenching of highly vibrationally excited 2-methylpyrazine: Kinetics and dynamics of large energy transfers

State-resolved collisional relaxation of highly vibrationally excited CsH by CO2

Quenching of highly vibrationally excited CsH(X(1)Σ(+), v=15-23) by collisions with CO2 was investigated. A significant fraction of the initial population of highly vibrationally excited CsH(v=22) was relaxed to a low vibrational level (Δv=-5). The near-resonant 5-1 vibration-to-vibration (V-V) energy was efficiently exchanged. The rate constants for the rotational levels of CO2(00(0)0) [J=36-60] and CO2(00(0)1) [J=5-31] from the collisions with excited CsH were determined. The experiments revealed that the collisions resulting in CO2(00(0)0) were accompanied by substantial excitation in rotation and translation. The vibrationally excited CO2(00(0)1) state exhibited rotational and translational energy distributions near those of the initial state. The total quenching rates relative to the probed state of excited CsH were determined for both CO2 states. The corresponding data indicated that the gains in the rotational and translational energies in CO2 were sensitive to the collisional depletion of excited CsH.

High resolution IR diode laser study of collisional energy transfer between highly vibrationally excited monofluorobenzene and CO2: the effect of donor fluorination on strong collision energy transfer

Collisional energy transfer between vibrational ground state CO2 and highly vibrationally excited monofluorobenzene (MFB) was studied using narrow bandwidth (0.0003 cm(-1)) IR diode laser absorption spectroscopy. Highly vibrationally excited MFB with E' = ∼41,000 cm(-1) was prepared by 248 nm UV excitation followed by rapid radiationless internal conversion to the electronic ground state (S1→S0*). The amount of vibrational energy transferred from hot MFB into rotations and translations of CO2 via collisions was measured by probing the scattered CO2 using the IR diode laser. The absolute state specific energy transfer rate constants and scattering probabilities for single collisions between hot MFB and CO2 were measured and used to determine the energy transfer probability distribution function, P(E,E'), in the large ΔE region. P(E,E') was then fit to a bi-exponential function and extrapolated to the low ΔE region. P(E,E') and the biexponential fit data were used to determine the partitioning between weak and strong collisions as well as investigate molecular properties responsible for large collisional energy transfer events. Fermi's Golden rule was used to model the shape of P(E,E') and identify which donor vibrational motions are primarily responsible for energy transfer. In general, the results suggest that low-frequency MFB vibrational modes are primarily responsible for strong collisions, and govern the shape and magnitude of P(E,E'). Where deviations from this general trend occur, vibrational modes with large negative anharmonicity constants are more efficient energy gateways than modes with similar frequency, while vibrational modes with large positive anharmonicity constants are less efficient at energy transfer than modes of similar frequency.

Accurate Calculations of Rotationally Inelastic Scattering Cross Sections Using Mixed Quantum/Classical Theory

For computational treatment of rotationally inelastic scattering of molecules, we propose to use the mixed quantum/classical theory, MQCT. The old idea of treating translational motion classically, while quantum mechanics is used for rotational degrees of freedom, is developed to the new level and is applied to Na + N2 collisions in a broad range of energies. Comparison with full-quantum calculations shows that MQCT accurately reproduces all, even minor, features of energy dependence of cross sections, except scattering resonances at very low energies. The remarkable success of MQCT opens up wide opportunities for computational predictions of inelastic scattering cross sections at higher temperatures and/or for polyatomic molecules and heavier quenchers, which is computationally close to impossible within the full-quantum framework.

Alkylation effects on the energy transfer of highly vibrationally excited naphthalene

The energy transfer of highly vibrationally excited isomers of dimethylnaphthalene and 2-ethylnaphthalene in collisions with krypton were investigated using crossed molecular beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques at a collision energy of approximately 300 cm(-1). Angular-resolved energy-transfer distribution functions were obtained directly from the images of inelastic scattering. The results show that alkyl-substituted naphthalenes transfer more vibrational energy to translational energy than unsubstituted naphthalene. Alkylation enhances the V→T energy transfer in the range -ΔE(d)=-100~-1500 cm(-1) by approximately a factor of 2. However, the maximum values of V→T energy transfer for alkyl-substituted naphthalenes are about 1500~2000 cm(-1), which is similar to that of naphthalene. The lack of rotation-like wide-angle motion of the aromatic ring and no enhancement in very large V→T energy transfer, like supercollisions, indicates that very large V→T energy transfer requires special vibrational motions. This transfer cannot be achieved by the low-frequency vibrational motions of alkyl groups.

Mixed quantum-classical theory for the collisional energy transfer and the rovibrational energy flow: application to ozone stabilization

A mixed quantum-classical approach to the description of collisional energy transfer is proposed in which the vibrational motion of an energized molecule is treated quantum mechanically using wave packets, while the collisional motion of the molecule and quencher and the rotational motion of the molecule are treated using classical trajectories. This accounts rigorously for quantization of vibrational states, zero-point energy, scattering resonances, and permutation symmetry of identical atoms, while advantage is taken of the classical scattering regime. Energy is exchanged between vibrational, rotational, and translational degrees of freedom while the total energy is conserved. Application of this method to stabilization of the van der Waals states in ozone is presented. Examples of mixed quantum-classical trajectories are discussed, including an interesting example of supercollision. When combined with an efficient grid mapping procedure and the reduced dimensionality approximation, the method becomes very affordable computationally.

Rotationally resolved IR-diode laser studies of ground-state CO2 excited by collisions with vibrationally excited pyridine

Relaxation of highly vibrationally excited pyridine (C5NH5) by collisions with carbon dioxide has been investigated using diode laser transient absorption spectroscopy. Vibrationally hot pyridine (E' = 40,660 cm(-1)) was prepared by 248 nm excimer laser excitation followed by rapid radiationless relaxation to the ground electronic state. Pyridine then collides with CO2, populating the high rotational CO2 states with large amounts of translational energy. The CO2 nascent rotational population distribution of the high-J (J = 58-80) tail of the 00(0)0 state was probed at short times following the excimer laser pulse to measure rate constants and probabilities for collisions populating these CO2 rotational states. Doppler spectroscopy was used to measure the CO2 recoil velocity distribution for J = 58-80 of the 00(0)0 state. The energy-transfer distribution function, P(E,E'), from E' - E approximately 1300-7000 cm(-1) was obtained by re-sorting the state-indexed energy-transfer probabilities as a function of DeltaE. P(E,E') is fit to an exponential or biexponential function to determine the average energy transferred in a single collision between pyridine and CO2. Also obtained are fit parameters that can be compared to previously studied systems (pyrazine, C6F6, methylpyrazine, and pyrimidine/CO2). Although the rotational and translational temperatures that describe pyridine/CO2 energy transfer are similar to previous systems, the energy-transfer probabilities are much smaller. P(E,E') fit parameters for pyridine/CO2 and the four previously studied systems are compared to various donor molecular properties. Finally, P(E,E') is analyzed in the context of two models, one indicating that P(E,E') shape is primarily determined by the low-frequency out-of-plane donor vibrational modes, and the other that indicates that P(E,E') shape can be determined from how the donor molecule final density of states changes with DeltaE.

Vibrational energy relaxation of the OH(D) stretch fundamental of methanol in carbon tetrachloride

The lifetimes of the hydroxyl stretch fundamentals of two methanol isotopomers, MeOH and MeOD, in carbon tetrachloride solvent are calculated through the use of the perturbative Landau-Teller and fluctuating Landau-Teller methods. Examination of these systems allows for insight into the nature of the vibrational couplings that lead to intramolecular vibrational energy transfer. While both systems display energy transfer to nearly degenerate modes, MeOD also displays strong coupling to an off-resonant vibration. The relaxation of MeOH and MeOD occurs through transitions involving a total change in the vibrational quanta of 4 and 3, respectively. We calculate vibrational energy relaxation lifetimes of 4-5 ps for MeOH and 2-3 ps for MeOD that agree well with the experimentally determined values.

Energy transfer of highly vibrationally excited azulene. III. Collisions between azulene and argon

The energy transfer dynamics between highly vibrationally excited azulene molecules (37 582 cm(-1) internal energy) and Ar atoms in a series of collision energies (200, 492, 747, and 983 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. The angular resolved collisional energy-transfer probability distribution functions were measured directly from the scattering results of highly vibrationally excited azulene. Direct T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational/rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). Significant amount of energy transfer from vibration to translation was observed at large collision energies in backward and sideway directions. The ratios of total cross sections between T-VR and V-T increases as collision energy increases. Formation of azulene-argon complexes during the collision was observed at low enough collision energies. The complexes make only minor contributions to the measured translational to vibrational/rotational (T-VR) energy transfer.

Energy transfer between azulene and krypton: Comparison between experiment and computation

Trajectory calculations of collisional energy transfer between excited azulene and Kr are reported, and the results are compared with recent crossed molecular beam experiments by Liu et al. [J. Chem. Phys. 123, 131102 (2005); 124, 054302 (2006)]. Average energy transfer quantities are reported and compared with results obtained before for azulene-Ar collisions. A collisional energy transfer probability density function P(E,E'), calculated at identical initial conditions as experiments, shows a peak at the up-collision branch of P(E,E') at low initial relative translational energy. This peak is absent at higher relative translational energies. There is a supercollision tail at the down-collision side of the probability distribution. Various intermolecular potentials are used and compared. There is broad agreement between experiment and computation, but there are some differences as well.

Energy transfer of highly vibrationally excited azulene: Collisions between azulene and krypton

The energy-transfer dynamics between highly vibrationally excited azulene molecules and Kr atoms in a series of collision energies (i.e., relative translational energies 170, 410, and 780 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. "Hot" azulene (4.66 eV internal energy) was formed via the rapid internal conversion of azulene initially excited to the S4 state by 266-nm photons. The shapes of the collisional energy-transfer probability density functions were measured directly from the scattering results of highly vibrationally excited or hot azulene. At low enough collision energies an azulene-Kr complex was observed, resulting from small amounts of translational to vibrational-rotational (T-VR) energy transfer. T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational-rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). The shapes of V-T energy-transfer probability density functions were best fit by multiexponential functions. We find that substantial amounts of energy are transferred in the backward scattering direction due to supercollisions at high collision energies. The probability for supercollisions, defined arbitrarily as the scattered azulene in the region 160 degrees <theta<180 degrees and DeltaEd>2000 cm(-1) is 1% and 0.3% of all other collisions at collision energies 410 and 780 cm(-1), respectively.

Trajectory Study of Supercollision Relaxation in Highly Vibrationally Excited Pyrazine and CO2

Classical trajectory calculations were performed to simulate state-resolved energy transfer experiments of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) and CO(2), which were conducted using a high-resolution transient infrared absorption spectrometer. The goal here is to use classical trajectories to simulate the supercollision energy transfer pathway wherein large amounts of energy are transferred in single collisions in order to compare with experimental results. In the trajectory calculations, Newton's laws of motion are used for the molecular motion, isolated molecules are treated as collections of harmonic oscillators, and intermolecular potentials are formed by pairwise Lennard-Jones potentials. The calculations qualitatively reproduce the observed energy partitioning in the scattered CO(2) molecules and show that the relative partitioning between bath rotation and translation is dependent on the moment of inertia of the bath molecule. The simulations show that the low-frequency modes of the vibrationally excited pyrazine contribute most to the strong collisions. The majority of collisions lead to small DeltaE values and primarily involve single encounters between the energy donor and acceptor. The large DeltaE exchanges result from both single impulsive encounters and chattering collisions that involve multiple encounters.

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, <DeltaE>, as function of temperature for various initial conditions.

PECT Model Analysis and Predictions of Experimental Collisional Energy Transfer Probabilities P(E‘,E) and Moments 〈ΔE〉 for Azulene and Biphenylene

Experimental collisional energy transfer data from kinetically controlled selective ionization (KCSI) and ultraviolet absorption (UVA) experiments are analyzed in the framework of the partially ergodic collision theory (PECT). Collisions of azulene and biphenylene with different colliders are investigated as case studies. The downward wings of the P(E',E) energy transfer distributions obtained from the PECT model are fitted to the recently introduced "variable-shape"-exponential 3-parameter functional form of P(E',E) obtained from KCSI experiments, P(E',E) proportional, variant exp[-{(E - E')/(C(0) + C(1)E)}(Y)]. The PECT model is able to reproduce the characteristic dependence of the KCSI "shape parameter" Y on the choice of collider, the energy dependent width of the KCSI P(E',E) distributions, described by alpha(E) = C(0) + C(1)E, and the temperature dependence of the UVA data above room temperature. The statistical approach of PECT obviously captures the essence of large molecule energy transfer at chemically significant energies without the need of knowing specific features of the detailed collision dynamics. It therefore shows promise for predicting the shape of P(E',E) in master equation kernels for larger molecules.