ROTATIONAL STRUCTURE AND SPLITTING PHENOMENA IN ELECTRONIC-SPECTRA OF MATRIX-ISOLATED NH

The forbidden transition a 1-DELTA <-- --> X 3-SIGMA- of matrix-isolated NH has been analysed in Ne, Ar, Kr, and Xe. The rotational structure including levels up to N" = 4 strongly supports the rotation-translation-coupling (RTC) model of Friedmann and Kimel. Translational mode frequencies v(t) of NH in the electronic ground state are 68.5 cm-1 in Ar, 54 cm-1 in Kr, and 39 cm-1 in Xe. Similar frequencies v(t) in the a 1-DELTA state, which show up as sharp features in the phonon sideband of the RTC-induced b 1-SIGMA+-->a 1-DELTA transition, were identified in Ar and Kr. A narrow matrix-induced quartet/quintet splitting of the a 1-DELTA state has been utilized to identify two trapping sites. NH radicals in these sites are nearly degenerate in the a 1-DELTA and b 1-SIGMA+ states (DELTA-E in the state is only 0.05 cm-1 in argon), but can be individually excited and detected at sufficient spectral resolution due to the splitting. The width of the splitting decreases by less than 30% in the order Ar > Kr > Ne > Xe, and is totally unaffected by deuteration. It is proposed that the NH radical is trapped with about equal probability in substitutional sites of O(h) and D3h symmetry. However, trapping in two interstitial O(h) sites, which differ in the second coordination sphere in fcc and local hcp lattices, cannot be excluded in Ar, Kr and Xe.