Molecular supercollisions: Evidence for large energy transfer in the collisional relaxation of highly vibrationally excited pyrazine by CO2

Abstract

The temperature dependence of the collisional quenching of highly vibrationally excited pyrazine by CO2 molecules has been investigated for the temperature range 243–364 K using high resolution time resolved diode laser spectroscopy. Particular emphasis is placed on vibration to rotation-translation (V→R/T) energy transfer which leaves the CO2 vibrations unexcited and occurs predominantly through short-range repulsive forces. Vibrationally hot pyrazine is prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. For the range of experimental cell temperatures used here, the nascent rotational population distributions of the 0000 ground state of CO2 resulting from collisions with hot pyrazine were probed at short times following excitation of pyrazine by the excimer laser pulse. The CO2 translational recoil velocity was also measured for individual rotational levels of the 0000 state. In addition, temperature dependent rate constants and probabilities were determined for energy transfer from the vibrationally hot pyrazine into individual rotational levels of the 0000 state of CO2. The rotational distributions, velocity recoils, and quenching rates exhibit a very weak temperature dependence for production of CO2 high J states, indicating that the CO2 molecules involved in these energy transfer events originate from rotational levels only slightly greater than the thermal mean J value. Based on these results, values for ΔE, the energy transfer from hot pyrazine to CO2 resulting in final CO2 0000 states J=58 through J=82, are estimated to range from 2550 to 7090 cm−1 in a single collision.

“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

“…11,12 These collisions lead to very large increases in the energy content of the CO 2 bath, occur about 1/10 collisions and contribute significantly to so-called ''supercollision'' events. This type of energy transfer is consistent with mediation through short range repulsive interactions and is the predominant collisional relaxation pathway.…”

“…This approach was pioneered by Flynn and co-workers 10 who used transient infrared absorption of CO 2 and N 2 O following excitation through collisions with highly vibrationally excited NO 2 . 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.…”

“…These results are compared with CO 2 (00 0 0) energy distributions and absolute rate constants which have been reported previously for collisions with excited pyrazine at E vib ϭ40 600 cm Ϫ1 . 12 The magnitudes and probabilities of energy transfer are described in terms of an energy transfer distribution function for the different donor molecule energies. Finally, the energy dependent pyrazine collisional relaxation data are discussed in light of their impact on supercollision energy transfer.…”

Dynamics of molecules in extreme rotational states

“…Rotational motion in molecules can be induced with strong optical fields leading to rotational recurrences, but the amount of rotational energy obtained with this method is fairly modest (19)(20)(21)(22)(23)(24)(25). In some cases, photochemical reactions and inelastic collisions can be used to produce rotationally hot molecules, but the products generally have broad and poorly controlled rotational energy distributions (26).…”

Rotationally Resolved IR-Diode Laser Studies of Ground-State CO2 Excited by Collisions with Vibrationally Excited Pyridine

“…CO 2 line widths also vary nearly linearly with the final rotational angular momentum as was observed in other systems. 7,8,10,21,22 This near linear relationship, illustrated graphically in Figure 3, is an indication of a constant pyridine/ CO 2 effective impact parameter. 7,10 B.…”