State-resolved collisional quenching of highly vibrationally excited pyridine by water:: The role of strong electrostatic attraction in V→RT energy transfer

We report experiments that investigate the influence of long-range attractive forces on collisional energy loss from highly vibrationally excited molecules. State-resolved studies of energy transfer from highly vibrationally excited pyridine (mu=2.2 D) to water (mu=1.8 D) in a low-pressure environment at 298 K have been performed using high-resolution transient absorption spectroscopy of water at lambda approximate to 2.7 mu m. Pyridine in its ground electronic state with 37 900 cm(-1) of vibrational energy was prepared by absorption of pulsed ultraviolet light (lambda=266 nm) to the S-1 state, followed by rapid internal conversion to the S-0 state. Collisions between vibrationally excited pyridine and water that result in rotational and translational excitation of the ground vibrationless state of H2O (000) were investigated by monitoring the populations of individual rotational states of H2O (000) at short times following pyridine excitation. The infrared probe of water was the highly allowed asymmetric stretching (000 --> 001) transition. The nascent distribution of rotationally excited H2O (000) states is well described by a thermal distribution with a rotational temperature of T-rot=770 +/- 80 K. Doppler-broadened transient linewidth measurements yield the velocity distributions of the recoiling H2O (000) molecules that correspond to center-of-mass translational temperatures of T(trans)similar to 515 K for all water rotational states investigated. Additionally, rate constants for energy gain in individual water states were determined, yielding an integrated rate constant of k(2)(int)=1.1x10(-11) cm(3) mol(-1) s(-1) for the appearance of H2O (000) with E-rot=1000-2000 cm(-1). These results are compared with previous relaxation studies of excited pyrazine (mu=0 D) with water and of excited pyridine with CO2 (mu=0 D), and the influence of electrostatic attraction on the relaxation dynamics is discussed. (C) 1999 American Institute of Physics. [S0021-9606(99)00432-8].