State-resolved collisional relaxation of highly vibrationally excited pyridine by CO2: Influence of a permanent dipole moment

Abstract

The quenching of highly vibrationally excited pyridine through collisions with a 300 K bath of CO2 is investigated using high resolution transient infrared laser spectroscopy. Highly excited pyridine (Evib=37 950 cm−1) is prepared using pulsed ultraviolet (UV) excitation at 266 nm, followed by radiationless coupling to the ground electronic state. Energy gain into CO2 resulting from collisions with highly excited pyridine is probed using transient absorption techniques. Distributions of nascent CO2 rotational populations in both the ground (0000) state and the vibrationally excited (0001) state are determined from early time absorption measurements. Translational energy distributions of the recoiling CO2 in individual rovibrational states are determined through measurement of Doppler-broadened transient line shapes. These experiments investigate the influence of a large permanent dipole moment (μpyridine=2.2 D) on the collisional quenching dynamics of molecules with very large amounts of internal energy. A kinetic model is developed to describe rates for appearance of CO2 states resulting from collisions with excited pyridine as well as for quenching of excited pyridine at early times. These experiments show that collisions resulting in CO2 (0000) are accompanied by substantial excitation in rotation (Trot=1200 K for J=56–82) and translation (Ttrans=2900 K for J=78) while the vibrationally excited CO2 (0001) state has rotational and translational energy distributions near the initial 300 K distributions. Rate constants for the two energy transfer pathways are compared with previously published data on quenching collisions of excited (nonpolar) pyrazine, revealing only minor relative enhancement (∼2) in the vibrational excitation channel in pyridine relaxation. Overall quenching rates for excited pyridine are determined for both CO2 states investigated. These data show that the rotational and translational energy gain in CO2 is much more sensitive to collisional depletion of excited pyridine.