Energy-dependent dynamics of large-ΔE collisions: Highly vibrationally excited azulene (E=20390 and 38580cm−1) with CO2

An advanced bath model to simulate association followed by ensuing dissociation dynamics of benzene + benzene system: a comparative study of gas and condensed phase results

The role of the environment (N₂ molecules) on the association followed by the ensuing dissociation reaction of benzene + benzene system is studied here with the help of a new code setup. Chemical dynamics simulations are performed to investigate this reaction in vacuum as well as in a bath of 1000 N₂ molecules, equilibrated at 300 K. Bath densities of 20 and 324 kg m^-3 are considered with a few results from the latter density. The simulations are performed at three different excitation temperatures of benzene, namely, 1000, 1500, and 2000 K, with an impact parameter range of 0-12 Å for both vacuum and bath models. Higher association probabilities and hence, higher temperature dependent association rate constants are obtained in the condensed phase. In the condensed phase, when a trajectory takes a longer time for the monomers to associate, the associated complex is formed with a longer lifetime and provides a lower rate of ensuing dissociation. Higher association rate and lower dissociation rate in condensed phase dynamics are due to the energy transfer process. Hence, the energy transfer phenomenon plays a decisive role in the association/dissociation dynamics, which is completely ignored in the same reaction when studied in vacuum.

A Competition between Dissociation Pathway and Energy Transfer Pathway: Unimolecular Dissociation of a Benzene-Hexafluorobenzene Complex in Nitrogen Bath

The unimolecular dissociation of a benzene-hexafluorobenzene complex at 1000, 1500, and 2000 K is studied inside a bath of 1000 N₂ molecules kept at 300 K using chemical dynamics simulation. Three bath densities of 20, 324, and 750 kg/m³ are considered. The dissociation dynamics of the complex at a 20 kg/m³ bath density is found to be similar to that in the gas phase, whereas the dynamics is drastically different at higher bath densities. The microcanonical/canonical dissociation rate constants for the three bath densities are calculated and fitted to the Arrhenius equation. The activation energies are found to be similar to the gas-phase one. However, the pre-exponential factor is lower and decreases with the increase in bath density. The vibrational degree of freedom of the complex more effectively participates in the collisional energy transfer to the N₂ bath, whereas the translational and rotational degrees of freedom of N₂ receive the transferred energy. The energy transfer efficiency increases with the increase in bath density. The time scale of the energy transfer pathway is more than that of the dissociation pathway, and negligible direct dissociation of the complex is observed from the simulation at the highest bath density.

Nascent rotational distribution for LiH(v=0-3,J) states from collisions with H2(E=4300 and 4800cm-1)

Rotationally state selective excitation of H₂(v=1, J=1 or 3) was achieved by stimulated Raman pumping. The full state-resolved distribution of scattered LiH(v=0-3, J=0~13)molecules from collisions with excited H₂(E=4300 and 4800cm^-1) is reported. Nascent rotational and translational energy profiles for scattered LiH(v=0~3) molecules with J=0~13 were measured using high-resolution transient laser induced fluorescence(LIF). The product translational energy for individual J-states increases by 120% for a 13% increase in donor energy. The scattered LiH(v=0, J=0~13) molecules have a biexponential rotational distribution. Fitting the data with a two-component exponential model yields a low-energy distribution and a high-energy distribution. The rotational distribution is sensitive to donor energy. Rotational distributions of scatted LiH(v=1-3) molecules were also measured. The distribution yielded rotational temperatures at 690K for LiH/H₂(E=4300cm^-1) and 730K for LiH/H₂(E=4800cm^-1), respectively. The rate constants for appearance LiH(v=0-3,J) were determined.

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.

Energy-dependence of vibrational relaxation between highly vibrationally excited KH (X1Σ+, ν"=14-23) and H2, and N2

Vibrational state total relaxation rate coefficients, k(ν") (M), for KH (ν"=14-23) by M=H(2) and N(2) have been investigated in an overtone pump-probe configuration. At ν"=14, 15, 16 and 17, the rate coefficients k(ν)(″) (M) increase linearly with vibrational quantum number. The region (ν"=18, 19, 20 and 21) where the dependence is much stronger than linear has significant contribution from multiquantum (Δν≥2) relaxation. For ν"=18, 19, 20 and 21, 0.25, 0.31, 0.38 and 0.31 of the initially prepared population undergo two-quantum (Δν=2) vibrational relaxation in KH (ν")+H(2) collisions. In KH (ν")+N(2), the time profile of ν"=14(15) after preparation of ν"=19(20) was measured. A clear bimodal distribution is observed. The time scale of the first peak is much shorter than the known collisional lifetimes of the intervening vibrational levels and thus a sequential single-quantum relaxation mechanism can be explicitly ruled out. Relaxation of KD with D(2) has been also investigated. The relaxation rate coefficients exhibit distinct maxima for both isotopes (KH and KD). We discuss possible explanation of the experimental results including mass effect, V-R energy transfer and V-V energy transfer.