State-Resolved Studies of Collisional Quenching of Highly Vibrationally Excited Pyrazine by Water:  The Case of the Missing V → RT Supercollision Channel

The quenching of highly vibrationally excited pyrazine through collisions with H2O at 300 K in a low-pressure environment was investigated using high-resolution transient absorption spectroscopy of water at λ ≈ 2.7 μm. Highly vibrationally excited pyrazine with E vib = 37 900 cm-1 was prepared by absorption of 266 nm light to the electronically excited S2 state, followed by rapid radiationless decay to the ground electronic state. Collisions between highly excited pyrazine and water that result in rotational and translational excitation of the vibrationless ground state of H2O (000) were investigated by measuring the state-resolved appearance of individual rotational states of H2O (000). Transient absorption measurements have been made on numerous rotational states to determine the nascent distribution of rotational energy gain in water. Doppler-broadened transient absorption line shapes were collected for a number of rotational levels in the (000) state in order to measure velocity distributions of the scattered water molecules. The nascent distribution of water rotational states with E rot > 1000 cm-1 is well described by T rot = 920 K, and the velocity distributions correspond to T trans ≈ 560 K, independent of the rotational state. Rate constants for energy gain into individual quantum states of H2O (000) from collisions with hot pyrazine provide a measure of the high-energy part of the energy-transfer probability distribution function. The quenching of hot pyrazine through collisions with water displays a significant reduction in the bath translational energy gain when compared to earlier studies on the quenching of hot pyrazine (E vib = 37 900 cm-1) by CO2 {Wall, M. C.; Mullin, A. S. J. Chem. Phys. 1998, 108, 9658}. A comparison of the two systems provides insights into the molecular properties that influence the relaxation of highly vibrationally excited molecules.