Weak bond stretching for three orientations of Ar–HF at vHFÄ3

Three new ArHF (vHF53) states, ~3001!, ~3101!, and ~3111!, have been observed between 11 350 and 11 420 cm by the hot band transitions from ~0001! using intracavity laser induced fluorescence. The term values and rotational constants of these levels are: ~3001! n0 511 385.928 98(28) cm, B50.095 546(32) cm; ~3101! n0511 444.258 12(68) cm , B 50.090 617(37) cm; and ~3111! n0511 456.076 51(36) cm , B50.091 863(14) cm. Observation of the ArHF ~3001! state provides the van der Waals stretching frequency for ArHF at v53, namely 46.8945(4) cm5(3001) – (3000). This value shows an increase of 8.208 cm ~21%! upon HF v53←0 valence excitation. The stretching frequency for the T shaped ArHF is (3111) – (3110)533.7055(5) cm. This value is only 7% greater than that observed at v51. The (vHF101) S bend-stretch combination state, corresponding to (ns51) of the Ar–FH configuration, has not been observed at vHF50 – 2. The stretching frequency here is (3101) – (3100) 531.8178(8) cm. The soft-mode frequencies reveal strong bend-stretch coupling in the complex. Excellent agreement ~within 0.3 cm! is found between experiment and prediction from Hutson’s H6~4, 3, 2! potential @J. Chem. Phys. 99, 9337 ~1993!#, for the three new levels. Large basis set coupled cluster calculations @CCSD~T!# of the Ar–HF intermolecular potential surface, V(R ,u ,r), are presented for r50.6– 2.0 Å and u50 – 180° on a grid with 15° spacing. This is an enlargement of the HF valence coordinate of more than double the equilibrium value. The dependence of the intermolecular potential upon the HF valence coordinate, r, is very anisotropic, being maximal for u50° and becoming essentially independent of r for u>45°. © 2000 American Institute of Physics. @S0021-9606~00!00516-X#