Vibrational Deactivation of Highly Excited Hexafluorobenzene

199

Molecular dynamics simulations were used to study relaxation of a vibrationally excited C6F6* molecule in a N2 bath. Ab initio calculations were performed to develop N2-N2 and N2-C6F6 intermolecular potentials for the simulations. Energy transfer from "hot" C6F6 is studied versus the bath density (pressure) and number of bath molecules. For the large bath limit, there is no heating of the bath. As C6F6* is relaxed, the average energy of C6F6* is determined versus time, i.e., ⟨E(t)⟩, and for each bath density ⟨E(t)⟩ is energy dependent and cannot be fit by a single exponential. In the long-time limit C6F6 is fully equilibrated with the bath. For a large bath and low pressures, the simulations are in the fixed temperature, independent collision regime and the simulation results may be compared with gas phase experiments of collisional energy transfer. The derivative d[⟨E(t)⟩]/dt divided by the collision frequency ω of the N2 bath gives the average energy transferred from C6F6* per collision ⟨ΔE(c)⟩, which is in excellent agreement with experiment. For the ~100-300 ps simulations reported here, energy transfer from C6F6* is to N2 rotation and translation in accord with the equipartition model, with no energy transfer to N2 vibration. The energy transfer dynamics from C6F6* is not statistically sensitive to fine details of the N2-C6F6 intermolecular potential. Tests, with simulation ensembles of different sizes, show that a relatively modest ensemble of only 24 trajectories gives statistically meaningful results.

Section: Introductionmentioning

confidence: 99%

A unified model for simulating liquid and gas phase, intermolecular energy transfer: N2 + C6F6 collisions

Paul

Kohale

Pratihar

et al. 2014

199

“…So far, the larger fraction of the many direct experiments on CET of highly vibrationally excited polyatomic molecules have dealt with monitoring the energy loss of excited donor molecules, among others, e.g., of toluene-d 0 and -d 8 , 4͑b͒-8 azulene-h 8 and -d 8 , [9][10][11][12][13][14][15] benzene-h 6 and -d 6 , 4͑b͒, 16,17 hexafluorobenzene ͑HFB͒, 18,19 and pyrazine. [20][21][22][23][24] Others have monitored the uptake of energy in the bath medium during the relaxation by various techniques [25][26][27][28][29][30][31][32][33][34] or even identified state-specifically the energy transferred, e.g., to CO 2 colliders in single collisions.…”

Section: Introductionmentioning

confidence: 99%

Collisional energy transfer probabilities of highly excited molecules from kinetically controlled selective ionization (KCSI). II. The collisional relaxation of toluene: P(E′,E) and moments of energy transfer for energies up to 50 000 cm−1

Lenzer

Luther

Reihs

et al. 2000

110

156

Complete and detailed experimental transition probability density functions P(E′,E) have been determined for the first time for collisions between a large, highly vibrationally excited molecule, toluene, and several bath gases. This was achieved by applying the method of kinetically controlled selective ionization (KCSI) (Paper I [J. Chem. Phys. 112, 4076 (2000), preceding article]). An optimum P(E′,E) representation is recommended (monoexponential with a parametric exponent in the argument) which uses only three parameters and features a smooth behavior of all parameters for the entire set of bath gases. In helium, argon, and CO2 the P(E′,E) show relatively increased amplitudes in the wings—large energy gaps |E′−E|—which can also be represented by a biexponential form. The fractional contribution of the second exponent in these biexponentials, which is directly related to the fraction of the so-called “supercollisions,” is found to be very small (<0.1%). For larger colliders the second term disappears completely and the wings of P(E′,E) have an even smaller amplitude than that provided by a monoexponential form. At such low levels, the second exponent is therefore of practically no relevance for the overall energy relaxation rate. All optimized P(E′,E) representations show a marked linear energetic dependence of the (weak) collision parameter α1(E), which also results in an (approximately) linear dependence of 〈ΔE〉 and of the square root of 〈ΔE2〉. The energy transfer parameters presented in this study form a new benchmark class in certainty and accuracy, e.g., with only 2%–7% uncertainty for our 〈ΔE〉 data below 25 000 cm−1. They should also form a reliable testground for future trajectory calculations and theories describing collisional energy transfer of polyatomic molecules.

“…The techniques most frequently applied are ultraviolet absorption ͑UVA͒ 31-34 and infrared fluorescence ͑IRF͒. [35][36][37][38][39][40][41] Both methods detect an energy dependent observable of the excited molecules ͑e.g., an absorption coefficient ⑀ or the IR emission intensity I of C-H stretching modes͒ which thus acts as a sort of ''internal thermometer.'' Via a calibration curve of the dependence of ⑀ or I on energy, the change of these quantities during collisional relaxation can be converted into an energy loss profile describing the deactivation rate.…”

Section: Introductionmentioning

confidence: 99%

Collisional energy transfer probabilities of highly excited molecules from kinetically controlled selective ionization (KCSI). I. The KCSI technique: Experimental approach for the determination of P(E′,E) in the quasicontinuous energy range

Hold

Lenzer

Luther

et al. 2000

Collisional energy transfer probabilities of highly excited molecules from kinetically controlled selective ionization (KCSI). II. The collisional relaxation of toluene: P(E ′ ,E) and moments of energy transfer for energies up to 50 000 cm−1 State-resolved collisional energy transfer in highly excited NO 2 . I. Cross sections and propensities for J, K, and m J changing collisionsThe coupled channel density matrix method for open quantum systems: Formulation and application to the vibrational relaxation of molecules scattering from nonrigid surfaces