Unraveling the energy dependence in large ΔE(V→RT) energy transfer: Separation of ΔE and probability in the collisional relaxation of highly vibrationally excited pyrazine (E_vib=36 000 to 41 000 cm^-1) by CO₂

The energy dependence of collisional relaxation of highly vibrationally excited pyrazine through collisions with CO₂ has been investigated for pyrazine vibrational energies E_vib = 36 000-41 000 cm^-1. Highly excited pyrazine was prepared by absorption of tunable pulsed UV laser light at four wavelengths between 251 and 271 nm. Data for pyrazine excitation at 246 and 266 nm have been published previously and are included with the data presented here. We have used high resolution transient absorption spectroscopy to measure nascent distributions of rotational and translational energy gain in CO₂ molecules in their ground vibrationless (00⁰0) state that are populated through collisions with excited pyrazine molecules. Our results reveal that substantial amounts of rotational and translational energy are imparted to CO₂(00⁰0) molecules for all pyrazine energies studied and that the magnitude of this energy changes very little for a 13% change in the pyrazine vibrational energy. In contrast, state-resolved rates for appearance of individual CO₂(00⁰0) rotational states show a strong dependence on the precise value of the donor energy and increase by nearly an order-of-magnitude for a 5000 cm^-1 increase in pyrazine energy. The high energy part of P(E,E′) is obtained from our energy gain measurements for each pyrazine excitation energy and these distributions are compared with experimental measurements of 〈ΔE〉 and its energy dependence.