Resolving the energy and temperature dependence of C6H6∗ collisional relaxation via time-dependent bath temperature measurements

Abstract

The relaxation of highly vibrationally excited benzene, generated by 193 nm laser excitation, was studied using the transient rotational-translational temperature rise of the N2 bath, which was measured by proxy using two-line laser induced fluorescence of seeded NO. The resulting experimentally measured time-dependent N2 temperature rises were modeled with MultiWell based simulations of Collisional Energy Transfer (CET) from benzene vibration to N2 rotation-translation. We find that the average energy transferred in benzene deactivating collisions depends linearly on the internal energy of the excited benzene molecules and depends approximately linearly on the N2 bath temperature between 300 K and 600 K. The results are consistent with experimental studies and classical trajectory calculations of CET in similar systems.

Non-statistical intermolecular energy transfer from vibrationally excited benzene in a mixed nitrogen-benzene bath

“…The collision frequency for each bath component may be expressed as ω = ω P × P, where P is the pressure of the bath gas, i.e., 31.0 atm and 1.5 atm for N 2 and C 6 H 6 , respectively. With parameters used to interpret experiments, 32 ω P is 1.14 × 10 11 and 1.74 × 10 11 atm −1 s −1 for N 2 and C 6 H 6 , respectively. The resulting value of ω is 3.79 × 10 12 s −1 for the simulations reported here.…”

“…The simulation results may be compared with those determined experimentally. 32 The experiments contain a small percentage of NO in the bath. N 2 and NO are expected to have the same rotational temperature, and the rotational temperature of NO is determined by monitoring its rotational level populations.…”

“…Given in Fig. 5 are the plots of ∆E c versus E obtained from experiment, 32 which may be compared with the simulation. A fit to experimental results is given for ∆T = 130 K, where the initial and final bath temperatures were 300 and 430 K, respectively.…”

“…The simulation reported here model experiments 32 in which a fraction of C 6 H 6 molecules in a mixed N 2 -C 6 H 6 bath, initially at 300 K, are vibrationally excited by S 2 laser excitation and then undergo S 2 → S 0 internal conversion, followed by intermolecular energy transfer and heating of the bath. The vibrationally excited C 6 H 6 molecules are identified as C 6 H 6 * .…”

“…The experimental procedure for the experiments modeled here has been described in detail previously. 32 Laser excitation at 193 nm, i.e., 148.1 kcal/mol, is used to excite benzene molecules in the bath by a S 0 → S 2 transition. S 2 then undergoes rapid internal conversion to form the vibrationally excited ground state S 0 * , with small amounts of intersystem crossing (5%) and fluorescence (2%).…”