The complex vibronic structure resolved in the twohyphen;photon absorption spectrum of the 3s1Ersquo;Rydberg state ofsymhyphen;triazine is quantitatively assigned in terms of a simple Jahnndash;Teller twohyphen;state electronic Hamiltonian coupled to second order by a single active mode, ngr;6. For Jahnndash;Teller linear and quadratic terms,k=2.14, andg=0.046 (in units of the zerothhyphen;order frequency), eigenvalues of this Hamiltonian fit positions and splittings of more than 25 measured bands to within an average deviation of 0.5percnt;. Eigenvectors show evidence of strong mixings of adiabatic states and of linear Jahnndash;Teller wave functions by quadratic (localization) effects, both of which are confirmed by quantitative agreement between measured and calculated band intensities. Adiabatic potential energy surfaces are calculated, and exact nonadiabatic quantum mechanical results are compared with various levels of approximation. This comparison shows that the simple model of an adiabatic free rotor/radial oscillator serves well to qualitatively describe the structure and dynamics of the lowest few states. Lower surface adiabatic Bornndash;Oppenheimer (or Bornndash;Huang) calculations also give a good approximate account of energy level structure for these deep states. Interestingly, compared with exact results, wave functions of the adiabatic approximations appear to underestimate potentialhyphen;energy localization of nuclear density over surface depressions, and overestimate above barrier reflection. Cone resonances are identified for high energy states of triazinersquo;s linear coupling parameters but it is shown that higher order coupling tends to disrupt such localizations.
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