The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide: Excitation of the infrared inactive (100), (020), and (0220) bath vibrational modes

Abstract

The collisional quenching of highly vibrationally excited pyrazine, C4H4N2, by CO2 has been investigated using high resolution infrared transient absorption spectroscopy at a series of cell temperatures. Attention is focused on collisions which result in excitation of the Fermi-mixed bath vibrational states (1000) and (0200), along with the unmixed overtone bend state (0220). The vibrationally hot (Evib≈5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the CO2 molecules in each vibrationally excited state are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions and velocity recoils, strongly suggest that the vibrational excitation of CO2 is dominated by a long-range electrostatic interaction despite the fact that the dipole transition matrix elements connecting the CO2 ground state to the excited states vanish for the isolated molecule. The vibrational energy transfer is accompanied by very little rotational and translational excitation and displays the characteristic strong, inverse temperature dependence (probability of transfer increases with decreasing temperature) expected of energy transfer mediated by a long range attractive interaction. A number of possible explanations for this apparent anomaly are considered, of which energy transfer mediated by dipole/quadrupole forces appears to be the most consistent with the data.

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

“…From the point of dynamics of inelastic scattering, an experiment studying a single collision event, like in CO 2 diode laser spectroscopy, [35][36][37][38][39][40] is of course closer to the heart of physics. However, in these investigations only the initial collisions with low rotational temperature are observed which differ from the typical ones in relaxations under thermal conditions.…”

“…[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. [35][36][37][38][39][40][41][42] The vast majority of those data are on ͗⌬E͘, the average amount of energy transferred per collision. Among the reported energy dependent ͗⌬E͘, many rather surprising differences and unusual variations are reported, even for the same relaxing species in standard bath gases, and the reasons for such cases are usually not clear.…”

“…Very valuable and detailed experimental data on inelastic collisions have recently been obtained from diode laser spectroscopy of the bath gas, which allows full rotational, vibrational and translational information on the result of a single collision. [35][36][37][38][39][40][41]47 Fully state-resolved observation of the energy transfer on the acceptor side, which characterized part of the far downward wing of P(EЈ,E) ͑i.e., at a restricted range of large ⌬E values͒, have so far been possible for CO 2 as a collider. To put the measured ''partial'' P(EЈ,E) data from these measurements on an absolute scale involves a non trivial problem of suitable normalization.…”

“Supercollision” energy dependence: State-resolved energy transfer in collisions between highly vibrationally excited pyrazine (Evib=37 900 cm−1 and 40 900 cm−1) and CO2

“…The first of these is vibration to vibration energy exchange in which collisions of the bath molecule with the highly vibrationally excited donor result in excitation of bath vibrations. 13,15 Strikingly, very little rotational and translational excitation of the bath accompanies the vibrational excitation. This type of energy transfer is consistent with energy exchange mediated by long range intermolecular forces.…”

“…Subsequent experiments have focused on the collisional deactivation of other highly excited donors, such as pyrazine [11][12][13][14] and hexafluorobenzene. 15 By choosing an energy-accepting molecule with well-resolved rotational and vibrational states such as CO 2 , individual energy transfer pathways can be identified and characterized. The complete energy profile for the acceptor can be determined, because in addition to rotational and vibrational energy content, the translational energy of the acceptor can be measured from Doppler broadened rotational lines.…”