15min:

ROBERT J. LE ROY, Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.

Although the Ross et al. -Bunker-Watson expression for the vibration-rotation levels of isotopomers-\alpha of diatomic molecule A--B\,, ~ E^\alpha ( v ,J) = \sum_l,m U_l,m\over(µ_\alpha )^{m + l/2}\big ( 1+m_e\overm_A^\alpha\,_l,m^A +m_e\over m_B^\alpha\,_l,m^B \big )\,( v+1/2 )^l\,[J(J+1)]^m~,\, is fairly widely used, it has a number of deficiencies. In particular:~ (i) it is not clear how to treat the (l,m)=(0,0) term,~ (ii) the U_l,m constants have inconveniently varying units depending on fractional powers of the mass,~ (iii) fits to data for only two isotomers cannot yield ``true" mass-independent U_l,m values, (iv) for non-hydrides, the magnitudes of the U_l,m constants are not even qualitatively similar to those of the analogous familiar Dunham Y_l,m constants, and~ (v) direct fits to this expression are impossible, so non-linear fits or indirect methods must be used to determine the _l,m^a values and their uncertainties. Moreover, this formulation is incompatible with the use of non -Dunham expressions (such as near-dissociation expansions) for the vibrational energies and rotational and centrifugal distortion constants. The present paper presents an alternate description of the Born-Oppenheimer and JWKB breakdown corrections which resolves all of these difficulties, and illustrates its use by a combined-isotopes analysis of the best available microwave, infrared and electronic data for the ground electronic states of HF and DF.