The dependence of intermolecular interactions upon valence coordinate excitation: The upsilon(HF)=4 levels of ArHF

The valence state dependence of the Ar-HF interaction potential is extended to upsilon(HF)=4. Three new ArHF (upsilon(HF)=4) states, (4000), (4100), a nd (4110), are observed between 14 780 and 14 880 cm(-1) using intracavity laser induced fluorescence. The term values and rotational constants of the se states are the following: (4000) nu(0)=14 783.603 23(30) cm(-1), B=0.103 606 8(68) cm(-1); (4100) nu(0)=14 867.419 06(70) cm(-1), B=0.102 612(27) c m(-1); and (4110) nu(0)=14 875.048 30(39) cm(-1), B=0.103 217(19) cm(-1), r espectively. The spectral red shifts of ArHF (upsilon 000) dramatically inc rease from 9.654 cm(-1) at upsilon=1 to 48.024 cm(-1) at upsilon=4. The rot ational constant of ArHF(4000) increases essentially linearly with HF valen ce excitation, becoming 1.3% (40 MHz) greater than that observed at upsilon =0. At upsilon=4, the outer classical turning point of HF is extended by 0. 4 Angstrom from r(e), and there is no evidence for Ar-H repulsion. The spec tral red shift for linear hydrogen bonded Ar-HF(upsilon 000) indicates a st rong enhancement of binding energy upon HF valence bond excitation, while t he rotational constant reveals an almost surprising decrease in heavy atom separation. Both the T-shaped ArHF(upsilon 110) and antilinear Ar-FH(upsilo n 100), however, show very little dependence of binding energy upon upsilon (HF) valence excitation. These observations are in good accord with the ab initio intermolecular potential surface. (C) 2000 American Institute of Phy sics. [S0021-9606(00)00134-3].