Rovibrational Energy Transfer in Ne−Li₂(A¹Σ_u⁺,v=0): Comparison of Experimental Data and Results from Classical and Quantum Calculations

Abstract

Absolute rate constants for rotational and rovibrational energy transfer in the system Ne-Li2(A1Sigmau+) were measured by a dispersed fluorescence technique following excitation of the (v = 0,j = 18) initial level of Li2(A1Sigmau+). The rate coefficients for Deltav = 0 processes decline monotonically with increasing |Deltaj|. The Deltav = 1 rate coefficients are also peaked near Deltaj = 0 but show a broad shoulder extending to approximately Deltaj = 30. Classical trajectory calculations and accurate quantum mechanical close-coupled calculations were used to compute theoretical rate constants from an ab initio potential surface. The agreement between the classical and quantum calculations is very good. The calculations slightly overestimate the measured rate constants for Deltav = 0, Deltaj <or= 6 processes but underestimate those for Deltav = 0, Deltaj >or= 20, implying that the anisotropy of the ab initio surface is too small at short range and too large at long range. For Deltav = 1 collisions, the calculations agree well with experiment for Deltaj <or= 0 and show the correct qualitative behavior for positive Deltaj, including both the peaking at Deltaj = 0 and the shoulder extending to positive Deltaj. However, they underestimate rate constants for Deltav = 1, Deltaj > 0 collisions, disagreeing with experiment by a factor of 2 for Deltaj approximately 20 but agreeing better at higher and lower Deltaj. Analysis of classical trajectories indicates that the vibrationally inelastic collisions fall into two groups corresponding to equatorial and near-end impacts; the former generally produce small Deltaj while the latter produce large Deltaj. Studies of a simple model potential show that this dual mechanism may be a general phenomenon not limited to the particular potential surface employed here. Criteria controlling the relative importance of the two vibrational excitation routes are enumerated.

Rovibrationally Inelastic Atom-Molecule Collision Cross Sections from a Hard Sphere Model

“…where µ is the B-C reduced mass, L is the angular momentum of the molecule, r is the distance between the two atoms, and k is the spring constant of the molecule. Then the e↵ective potential of the molecule is given by 3) and the Lagrangian is given by…”

“…This will allow us to study T-shaped collisions, in which A collides with the molecule on its side, and incites vibrational energy by pushing B and C apart from one another other. It has been shown in our lab [3] that this mechanism dominates the vibrational energy transfer process for Li 2 -X collisions where X is a non-reactive noble gas atom, and thus it is important to understand it in the context of hard spheres.…”

Extraction of Atom-Diatom Collision Properties by Least Squares Fitting

“…If the molecule undergoes an inelastic collision, there is satellite fluorescence. [3] If the molecule undergoes a rotationally inelastic collision, ∆j, the change in the rotational quantum state is always even. [4] As we see in figure 2, fluorescence is not emitted as a series of delta functions, but rather is emitted as a spectral line shape with finite width.…”

“…Here, n f is the population of a given collisionally populated final state, n i is the population of the initial excited state and n X is the population of the collision partner; Γ f is the rate of decay from the final state f and k if is the thermally averaged rate constant of the collision leading to final state f . [3] If we assume a steady state solution, we can solve for the ratio of final state population to initial state population in terms of the rate constant and two quantities as below:…”

“…Particularly, comparison of calculations with experimental data implies that it is insufficiently anisotropic at long range and has an anisotropy that increases too quickly as R decreases. [3]…”