Time correlation function approach to liquid phase vibrational energy relaxation: H[sub 2] and D[sub 2] solutes in Ar solvent.

The theoretical treatment in Paper I [D. W. Miller and S. A. Adelman, J. Chem. Phys. 117, 2672, (2002), preceding paper] of the vibrational energy relaxation (VER) of low-frequency, large mass dihalogen solutes is extended to the VER of the high-frequency, small mass molecular hydrogen solutes H[sub 2] and D[sub 2] in a Lennard-Jones argon-like solvent. As in Paper I, values of the relaxation times T[sub 1] predicted by the theory are tested against molecular dynamics (MD) results and are found to be of semiquantitative accuracy. To start, it is noted that standard Lennard-Jones site-site potentials derived from macroscopic data can be very inaccurate in the steep repulsive slope region crucial for T[sub 1]. Thus, the H-Ar Lennard-Jones diameter σ[sub uv] is not taken from literature values but rather is chosen as σ[sub UV] = 1.39 Å, the value needed to make the theory reproduce the experimental H[sub 2]/Ar gas phase VER rate constant. Next, by MD simulation it is shown that the vibrational coordinate fluctuating force autocorrelation function 〈&Ftilde;(t)&Ftilde;〉[sub 0] of Paper I decays roughly an order of magnitude more rapidly for the molecular hydrogen solutions than for the dihalogen solutions. This result implies a relatively slow decay for the molecular hydrogen friction kernels β(ω) = (k[sub B]T)[sup -1] ∫[sup ∞, sub 0] 〈&Ftilde;(t)&Ftilde;〉[sub 0] cos ω tdt, yielding for the H[sub 2]/Ar and D[sub 2]/Ar systems at T= 150 K physical millisecond values for T[sub 1] =beta;[sup -1](ω) despite the high liquid phase vibrational frequencies ω[sub 1] of H[sub 2] and D[sub 2]. The rapid decay of 〈&Ftilde;(t)&Ftilde;&ran;[sub 0] is due to both the steepness of the repulsive slope of the H-Ar potential and the small masses of H and D. Thus, the small value chosen for σ[sub UV] is needed to avoid unphysically long T[sub 1]'s. Next, an analytical treatment of the H[sub 2]/D[sub... [ABSTRACT FROM AUTHOR]