ISOMERIC STRUCTURES OF THE ELECTRONICALLY EXCITED ACETYLENE AR COMPLEX - SPECTROSCOPY AND POTENTIAL CALCULATIONS

Acetylene . Ar complex in the S, state has been characterized through laser fluorescence excitation spectra in the acetylene ($) over tilde A<--($) over tilde X, 3(0)(n) (n=0-4) bands region. Two isomeric struc tures have been determined for the acetylene(($) over tilde A) Ar comp lex from rotational band analysis, even though only one structure was known to exist for the ground state acetylene(($) over tilde X) Ar. Th e in-plane isomer has the Ar atom situated in the molecular plane of t he trans-bent acetylene, 3.77 Angstrom from the acetylene center of ma ss and tilted from the Il atoms. The out-of-plane isomeric structure, directly inverted from the rotational constants, has argon 3.76 Angstr om away from the acetylene center of mass and 18 degrees tilted from t he Ct rotational axis. This structure is most likely due to large ampl itude bending motions away from the equilibrium position which is alon g the C, axis, Axis switching effect in the rotational band analysis f or both isomers has been examined and found to be negligible, (Formula s for calculating the three-dimensional axis switching angles are deta iled in the Appendix.) Three van der Waals vibrational mode frequencie s have been determined from the vibrational progressions in the spectr a; the stretching fundamental of the out-of-plane isomer is 28 cm-l; t he in-plane bending fundamental, and the out-of-plane be?ding first ov ertone of the in-plane isomer are II and 17 cm(-1), respectively. The isomeric structures have been compared with the results from a painwis e-atom potential calculation with parameters transferred from the ones previously derived for C2H4, Ar potential calculations, It was found that when the set of parameters that most closely reflects the electro nic density distribution in C2H2(A) orbitals was used, two potential m inima mimicking the two isomeric structures were generated. This poten tial calculation can even qualitatively reproduce the complex spectral shift induced;by the v, mode excitation in acetylene. Combining the o bserved spectral shifts and previous experimental and theoretical stud ies of acetylene(X) Ar, we have estimated the binding energy of the ou t-of-plane C2H2(($) over tilde A).AT isomer to be 179 cm(-1), and that of the in-plane isomer to be 170 cm(-1). (C) 1995 American Institute of Physics.