A PAIR POTENTIALS STUDY OF MATRIX-ISOLATED ATOMIC ZINC - 1 - EXCITED P-1(1) STATE DYNAMICS IN SOLID AR

The pair-potentials calculations of McCaffrey and Kerins [J. Chem. Phy s. 106, 7885 (1997)] used with success in simulating the emission spec troscopy of the Zn-RG matrix systems are extended to examine the diffe rent temporal decay characteristics exhibited at low temperature, T < 13 K, by the singlet emission bands in the Zn-Ar matrix system. The 23 8 nm band, assigned in the earlier theoretical work to the body mode Q (2), exhibits a 0.1 ns risetime, the 219 nm band assigned to the waist mode Q(3), is prompt. By extracting the gradients and the second deri vatives of the Q(3) and Q(2) mode potentials of a Zn . Ar-18 cluster, decay rates of 3 and 2 ps, respectively, are calculated at the Franck- Condon regions of these potentials accessed in absorption, leading to effective competition between the Q(2) and Q(3) modes for relaxation o f excited-state population and thereby to the coexistence of the 238 n m emission with the 219 nm band. A quasi-bound region is located at 0. 32 Angstrom in the body mode, Q(2), which slows down the relaxation on this mode and is identified as responsible for the recorded risetime on the 238 nm emission. The temperature dependence exhibited in the Zn -Ar system at higher temperatures (T > 14 K) in which the intensity of the 219 nm band can reversibly be put into the 238 nm band, was exami ned by generating the (PES) potential-energy surface for coupled Q(2) x Q(3) vibronic modes. The theoretically predicted activation energy b arrier is 380 cm(-1) which is only in qualitative agreement with the v alue of 130.6 cm(-1) extracted in the kinetics study. Possible reasons for the overestimation in the theoretical value are discussed. (C) 19 98 American Institute of Physics. [S0021-9606(98)00432-2]