Improved direct potential fit analyses for the ground electronic states of the hydrogen halides: HF/DF/TF, HCl/DCl/TCl, HBr/DBr/TBr and HI/DI/TI
The potential energy and Born-Oppenheimer breakdown functions for the X1Σ+ ground electronic states of the hydrogen halides HF, HCl, HBr, and HI are reported in full analytic form. All available pure rotational and vibrational-rotational spectroscopic data for the various isotopologues of the four HX molecules, as well as the B1Σ+→X1Σ+ emission band system data for HF/DF and HCl/DCl, were employed in direct potential fit determinations of the various radial functions. Significant improvements over previous work have been made to the mathematical models for these functions, particularly with respect to the behavior of the potential energy in the long-range region where dispersion forces between the component atoms dominate. The MLR3 model for the potential energy is employed, allowing for constraint of the three leading dispersion coefficients C6, C8, and C10, for the X states of HF and HCl. Quantum-mechanical rotational and centrifugal distortion constants are calculated for all isotopologues considered in the non-linear least-squares fitting procedures. Computer code is provided in order that precise calculation of all functions for all isotopologues is possible for a user-specified radial grid. Precise estimates are obtained for X state equilibrium internuclear separations for the principal isotopologues of all four halides, namely re(HF)=0.91683897±0.00000004 Å, re(H35Cl)=1.27454677±0.00000006 Å, re(H79Br)=1.4144292±0.0000001 Å, and re(HI)=1.6290588 ±0.0000004 Å.