The empirical determination of Born–Oppenheimer breakdown (BOB) correction functions from the analysis of high resolution diatomic molecule spectra is becoming increasingly common. However, in virtually all applications to date, the analytic expressions used to represent those functions have unphysical limiting behaviour which makes the resulting overall potential energy functions unusable at long-range. This paper delineates the limiting physical constraints, which should be imposed on the form of such BOB correction functions, and presents flexible new expressions incorporating those constraints. The utility of these new forms is illustrated by a re-analysis of recent high resolution IR data for the ground electronic state of AgH, which yields an updated potential energy function and more meaningful adiabatic (potential) and non-adiabatic (centrifugal) BOB correction functions for this system. Further examination of this AgH system also illustrates the problem of model dependence associated with efforts to determine physically unique potential energy and centrifugal BOB correction functions.

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Representing Born -Oppenheimer breakdown radial correction functions for diatomic molecules

The potential energy curve for the B 1Πu state of Li2 has a rotationless barrier which protrudes above its energy asymptote. A direct fit to spectroscopic data for all three isotopomers of this species, including Λ-doubling splittings and tunneling predissociation line widths, is used to determine an accurate analytic potential energy function plus Born–Oppenheimer breakdown and Λ-doubling perturbation radial strength functions for this system. This analysis introduces an analytic model for representing a potential function with a rotationless barrier, and shows that a radial perturbation function treatment can determine the symmetry of the perturbing state giving rise to Λ-doubling splittings.

Potential energy, Λ doubling and Born–Oppenheimer breakdown functions for the B 1Πu “barrier” state of Li2

Isotopic dependence of Born-Oppenheimer breakdown effects in diatomic hydrides: The X1Σ+ states of and

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 A, re(H35Cl)=1.27454677±0.00000006 A, re(H79Br)=1.4144292±0.0000001 A, and re(HI)=1.6290588 ± 0.0000004 A.

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

Isotopic dependence of Born-Oppenheimer breakdown effects in diatomic hydrides: The B1Σ+ and X1Σ+ states of HCl and DCl

http://www.cchem.berkeley.edu/rjsgrp/publications/papers/1988/89_gruebele_1988.pdf

Alan Stein

Papers

Experimental potential functions for open and closed shell molecular ions: Adiabatic and nonadiabatic corrections in X3Σ− OH+ and X1Σ+ ArH+