Quasi-Resonant Vibration-Rotation Transfer in Inelastic Li 2 *-Ne Collisions † Brian Stewart* Department of Physics, Wesleyan UniVersity, Middletown, Connecticut 06549 Peter D. Magill Lucent Technologies, Murray Hill, New Jersey 07974 David E. Pritchard Department of Physics and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed: April 17, 2000; In Final Form: August 15, 2000 We present the results of a detailed study of the influence of rotational angular momentum on vibrotational transfer in the system Li 2 *(V i ,j i ) + Ne f Li 2 *(V f ,j f ) + Ne, where V i,f and j i,f indicate initial and final vibrational and rotational levels, respectively, and Li 2 * is in its first electronically excited 1 Σ u + state. Level-to-level inelastic rate constants for j i up to 76 have been measured. The measurements span 4 orders of magnitude in size and include |ΔV| e 5 and |Δj| e 50. The results extend the range of previous measurements in this system and further document the phenomenon of quasiresonant vibrotational transfer. This process, induced by high rotational angular momentum, results in large rate constants for vibrational transfer and a systematic correlation of Δj and ΔV according to the rule Δj )-4ΔV. At j i g 64, the total vibrationally inelastic rate constant is found to be larger than the total rotationally inelastic rate constant. A fully classical treatment of the dynamics on an ab initio potential surface results in rate constants that agree remarkably well with the data. I. Introduction Inelastic atom-diatom transfer under ordinary conditions is dominated by rotational transfer; the probability of vibrationally inelastic transfer is generally orders of magnitude smaller. This is because, for most collisions at thermal velocities, the atom- molecule force contains only a small component at the oscillator frequency and the collisions are vibrationally adiabatic. For this reason, increased collision energy generally enhances vibra- tionally inelastic transfer. 1 However, molecules with small moments of inertia can have, at large j, rotational frequencies and energy level spacings comparable to those of the vibration. In such cases, the vibrational and rotational motions can become strongly coupled by the collision, and the probability of collisionally induced vibrational transfer can be greatly en- hanced. In fact, rotational enhancement of vibrational transfer was proposed as early as 1962 by Cottrell and Matheson 2,3 to account for the observation that pure methane undergoes vibrational relaxation more rapidly than tetradeuteromethane. These authors reasoned that the larger average angular velocity of methane at a given temperature results in a greater effective collision velocity and, hence, an enhanced probability for vibrationally inelastic collisions. Work at M. I. T. has resulted in the first level-resolved measurements of vibrational transfer at sufficiently high j i to observe the onset of resonant vibration-rotation transfer. We present here a detailed experimental study of vibrotational transfer in the system Li 2 *(V i ,j i ) + Ne f Li 2 *(V f , j f ) + Ne, where 7 Li 2 * is in its first electronically excited 1 Σ u + state, V and j are its vibrational and rotational quantum numbers, and i and f refer to its initial and final quantum levels. This study extends the scope of previous experimental studies 4-6 of the effect of j i on the vibrotationally inelastic transfer process and permits us to fully document the phenomenon known as quasiresonant vibrotational transfer that occurs at high j i in Li 2 collisions. These level-to-level measurements span a large dynamic range and are unprecedented in the range of final quantum levels resolved. A short account of this work has appeared previously; 7 in this report we give full details of our many high-j inelastic rate constant measurements. Renewed interest in the phenomenon, 8-10 particularly at the extremely low temperatures now attainable in magneto-optic traps, 9 prompts us to give a detailed account of our data at this time. In addition, we present results of classical trajectory calculations on an ab initio potential surface that was not available at the time of our initial report. The remainder of this section summarizes the hallmarks of the quasiresonant transfer process and reviews previous calcula- tions and experiments that are relevant. Section II provides a brief description of the experiment and a summary of the technique we use to obtain level-to-level rate constants from collision spectra. Section III presents the rate constants and a description of their main features. Section IV contains results of classical trajectory calculations on an ab initio potential surface that agree remarkably well with the data, as well as a discussion of some of the issues that arise when binning quasiclassical trajectories for a process that is sharply peaked with respect to the final action. Section V is devoted to a brief discussion of the mechanism of quasiresonant transfer and a review of recent relevant literature. A. Quasiresonant Vibrotational Transfer. The first level- to-level rate constant measurements in the similar system Li 2 *- Xe were made by Saenger et al. 4 They found that increased † Part of the special issue “C. Bradley Moore Festschrift”. * Corresponding author. E-mail: bstewart@wesleyan.edu. 10565 J. Phys. Chem. A 2000, 104, 10565-10575 10.1021/jp001445c CCC: $19.00 © 2000 American Chemical Society Published on Web 10/25/2000