Quantum interference effect in collision-induced intramolecular energy transfer within singlet-triplet mixed states

The principle of wave-particle duality of quantum mechanics ascertains that any microscopic particle must also exhibit wave properties. The matter wave, or de Broglie wave, was first evidenced by Davisson and Germer in 1927 in the electron diffraction by crystals. In recent years, numerous fascinating examples of the quantum interference effect (QIE) have been discovered in the molecular systems in their excitation, dissociation and ionization by photons as well as in collision processes. Our group was the first to obtain the experimental evidence of QIE in a collision, specifically for the singlet-triplet mixed state of a diatomic species, and to derive an explicit expression for its energy transfer cross-section. In this expression, the interference phase angle (thetast) that describes the phase angle difference between singlet and triplet energy transfer channels is defined and experimentally measured for CO(A (1)Pi, v = 0/ e (3)Sigma(-), v = 1)-M collision system with M = rare gases (He, Ne, Ar), homonuclear diatomics (H-2, N-2, O-2) and heteronuclear diatomics (HCl) via the optical-optical double resonance multiphoton ionization (OODR-MPI) technique. We have also observed QIE in Na-2( A (1)Sigma(u)(+), v = 8 / b(3) Pi(0u), v = 14)-Na collision. More recently, we have carried out quantum scattering calculations of the interference angle based on the first order Born approximation of time dependent perturbation theory. For atom-diatom collision, the anisotropic Lennard-Jones interaction potential was adopted, and for polar diatom-diatom collision, the long-range dipole-dipole interaction proportional to R-3 was shown to be a proper potential for the calculation. All the calculated theta(ST) at T = 77, 253 and 470 K for CO(A Pi(1), v = 0/ e (3)Sigma(-), v = 1)-M, for M = He(theta(ST) = 58degrees similar to 65degrees), Ne (66degrees similar to 69degrees), Ar (72degrees similar to 90degrees) and HCl (101degrees similar to 110degrees), are in good agreement with the experiments. Our calculated differential theta(ST) are in the range of 48degrees similar to 70degrees for CO-He and 93degrees similar to 112degrees for CO-HCl collision for all v and b values that are physically significant. These values are close to those experimental theta(ST)'s obtained in the gas cell, implying that the "average effect" is not serious. The calculation also gives an effective collision time of 0.3 ps for CO - He and 1.5 ps for CO - HCl collision, which explains why the experimental theta(ST) for the former is much smaller than that of the latter. These results show that theta(ST) should provide important information on the singlet-triplet mixed state intermolecular potential, which is difficult to obtain by other experimental or theoretical methods.