ROTATIONAL ELECTRONIC SPLITTING OF MATRIX-ISOLATED NH/ND A (1)DELTA IN ARGON CAGES OF O(H) AND D(3H) SYMMETRY - SPECTROSCOPIC ANALYSIS AND THEORETICAL INTERPRETATION

Electronic state-dependent Ar-NH(X,a,b) potential surfaces from ab initio calculations have been combined with a classical model of a doped rare-gas matrix to calculate electronic matrix shifts and local-mode frequencies which compare favorably with spectroscopic observations. It is shown that NH/ND substitutes one Ar atom in sites of O(h) and D3h SYMmetry which occur with equal probability over a wide range of experimental conditions, while interstitial trapping and impurity effects could be excluded. The previously reported site-dependent rotational-electronic (rotronic) splitting in the a 1DELTA state of NH is shown to be insensitive to deuteration. The partial lifting of the 10-fold degeneracy in the lowest rotronic level of the a 1DELTA state could be modeled for both site symmetries by evaluating the effect of the rare-gase cage in first-order perturbation theory. The NH(a.Ar(n) interaction is constructed from the average of a recently published pair of ab initio NH(a1A').Ar and NH-(a1A'').Ar potential surfaces. Electronic splitting is introduced via a ''difference' potential surface of NH-(a).Ar12 which was obtained by quantum mechanical methods, as described in detail in an accompanying paper. The perturbation gives rise to rotronic splittings which differ in cages of O(h) and D3h symmetry. The selection rules governing transitions from the electronic ground state to the sublevels of NH a 1DELTA in cages of O(h) and D3h symmetry are consistent with the observed spectra.