10min:

GANG LI, JENNING Y. SETO, PETER F. BERNATH AND ROBERT J. LE ROY, Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada; RAM S. RAM, Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA.

New high resolution Fourier Transform spectra of the 0--3, 0--4, 1--3, 1--4 and 1--5 bands of the B' ^{2 +}-X ^{2 +} transition of MgH, together with lines from sunspot spectra coupling B'--state levels v'=0 & 1 to X--state levels v''=3-8 , combined with older B'-X band data\footnote W.J. Balfour and H.M. Cartwright, Can.J.Phys. 54, 1898 (1976);~ W.J. Balfour and B. Lindgren, Can.J.Phys. 56, 767 (1978). involving v''(X)=3-9 and v'(B')=0-9 and ground-state 2--1 and 1--0 infrared data, yield a description of the ground state for v''=0-2 and v''=3-9 . The v''=2-3 gap was bridged using transitions from the 1--2 and 1--3 bands of the A ² -X ^{2 +} spectrum. In order to avoid complications due to perturbations in the excited state, all of these electronic band data were re-arranged and treated as fluorescence series into the ground state. The resulting data set consisted of a total of 4140 transitions for six isotopomers of MgH and MgD.

We have performed two types of combined isotopomer analyses of these data. (i) A fit to empirical Dunham-type expansions which included hydrogenic Born-Oppenheimer breakdown correction terms required 54 expansion parameters, plus the 701 ``fluorescence series'' origins. However, the resulting empirical centrifugal distortion constants will be unreliable for extrapolation to J values significantly higher than those included in the data set. (ii) An equally good fit to an analytic model potential energy function plus adiabatic and non-adiabatic radial Born-Oppenheimer breakdown correction functions required only 21 potential and correction-function parameters, plus the 701 ``fluorescence series'' origins. The resulting potential function has the MLJ form, and was constrained to have the correct C₆/R⁶ long-range behaviour, so it should prove highly reliable for extrapolations in v or J. The parameter-fit and potential-fit computer programs used for this analysis were DSParFit and DSPotFit.