R.J. Le Roy

Comprehensive analysis of the A–X spectrum of I2: An application of near‐dissociation theory

High resolution absorption spectra of the A 3Π1u–X 1Σ+g system of I2, consisting of some 9552 lines of some 79 bands spanning the vibrational range v′=0–35 and v″=3–17, have been recorded and analyzed. A fit to them which uses the previously determined accurate molecular constants for the X 1Σ+g state yields an accurate new set of molecular constants for the A state, including the Λ doubling constants. The A‐state vibrational and inertial rotational constants, as well as mechanically consistent centrifugal distortion constants, are represented by near‐dissociation expansions, yielding an accurate representation of the experimental data which also provides a reliable global representation of all observed and unobserved vibration–rotation levels of this state.

A theoretical investigation of the CH2+ di-cation

Abstract The reported observation of CH 2+ ions, with a lifetime of a few microseconds formed as a result of charge-stripping of CH + ions, is examined critically. A number of possible explanations of the observation as an experimental artifact are examined, and found wanting. Ab initio calculations of potential energy curves for CH 2+ show that the repulsive curves correlating with C + + H + are seriously perturbed by bound states of the same symmetry which correlate with C 2+ + H. The resultant potential curve for the 2 Σ + ground state has a slight dip whose depth has an uncertainty of a few tenths of an eV. It is shown that, while the dip in this potential curve cannot support any vibrational levels, scaling of the curve within the limits imposed by the uncertainties can yield such a metastable state, and hence can provide a self-consistent explanation of the experimental observations.

Effect of asymmetric isotopic substitution on atom–diatom potentials

A simple and accurate method is proposed for transforming the Legendre expansion of an atom–diatom potential about the diatom center of mass into a new Legendre expansion about a shifted (by isotope substitution) diatom center of mass. It is found that a simple quadrature procedure yields accurate results throughout the range of intermolecular separation of physical interest, while a previously proposed Taylor series expansion procedure gives comparable accuracy only for intermolecular separations greater than the equilibrium separation. Numerical tests of the method are performed for three model atom‐plus‐rigid diatom systems H2(HD) –He, HCl(DCl) –Ar, and 35ClF(37ClF) –Kr. adequate to determine the residual energy and entropy as well as their volume derivatives. The temperature dependence of the derivatives of the energy and entropy supports the contention that the equation of state is that for segments having a steep repulsive potential of the form e (σ/r)n with a mean field containing the attractive po...

A full analytic potential energy curve for the aΣ+3 state of KLi from a limited vibrational data set

Fourier transform spectra of near-infrared laser-induced fluorescence in (39)K(6)Li show transitions to high vibrational levels of both the X (1)Sigma(+) and a (3)Sigma(+) electronic states. These include 147 transitions into six vibrational levels of the a (3)Sigma(+) state, which lie between 7 and 88 cm(-1) below the dissociation asymptote. Unfortunately, their energies span less than 30% of the well depth. However, fitting those data to eigenvalues of analytical model potential functions whose outer limbs incorporate the theoretically predicted long-range form, V(R) approximately D-C(6)R(6)-C(8)R(8), yields complete, plausible potential curves for this state. The best fits converge to remarkably similar solutions which indicate that D(e)=287(+/-4) cm(-1) and R(e)=4.99(+/-0.09) A for the a (3)Sigma(+) state of KLi, with omega(e)=47.3(+/-1.4) and 44.2(+/-1.5) cm(-1) for (39)K(6)Li and (39)K(7)Li, respectively. Properties of the resulting potential are similar to those of a published ab initio potential and are consistent with those of the analogous states of Li(2), K(2), Na(2), and NaK.

Fourier transform infrared emission spectra of MgH and MgD

High resolution Fourier transform infrared emission spectra of MgH and MgD have been recorded. The molecules were generated in an emission source that combines an electrical discharge with a high temperature furnace. Several vibration-rotation bands were observed for all six isotopomers in the X (2)Sigma(+) ground electronic state: v=1-->0 to 4-->3 for (24)MgH, v=1-->0 to 3-->2 for (25)MgH and (26)MgH, v=1-->0 to 5-->4 for (24)MgD, v=1-->0 to 4-->3 for (25)MgD and (26)MgD. The new data were combined with the previous ground state data, obtained from diode laser vibration-rotation measurements and pure rotation spectra, and spectroscopic constants were determined for the v=0 to 4 levels of (24)MgH and the v=0 to 5 levels of (24)MgD. In addition, Dunham constants and Born-Oppenheimer breakdown correction parameters were obtained in a combined fit of the six isotopomers. The equilibrium vibrational constants (omega(e)) for (24)MgH and (24)MgD were found to be 1492.776(7) cm(-1) and 1077.298(5) cm(-1), respectively, while the equilibrium rotational constants (B(e)) are 5.825 523(8) cm(-1) and 3.034 344(4) cm(-1). The associated equilibrium bond distances (r(e)) were determined to be 1.729 721(1) A for (24)MgH and 1.729 157(1) A for (24)MgD.

Asymptotic potential coefficients for rare gas and alkali atoms and simple molecules interacting with metallic surfaces

Values of the coefficients C3 and C5 which describe the asymptotic potential energy between an atom or molecule and a surface, and the constants Cs1 and Cs2 which characterize the surface mediation of the long-range atom(molecule)-atom(molecule) interaction, are calculated for rare gas and alkali or simple molecules, such as NO, H2, and H2O, adsorbed on noble and transition metals. These calculated results are obtained using analytic representations of the atomic polarizability and a numerical treatment of the substrate dielectric response obtained from measured optical constants.

Isotope effects and Born-Oppenheimer breakdown in excited singlet states of the lithium dimer

Observation of infrared electronic transitions involving the 1 (1)Deltag state of 7Li2 has instigated an investigation of Born-Oppenheimer breakdown in four singlet electronic states correlating with (2s+2s), (2s+2p), and (2p+2p) lithium atoms. The 1 (1)Deltag state, which correlates at long range with (2p+2p) atoms, has been observed in emission from the (5p) (1)Piu Rydberg state and in 1 (1)Deltag-B (1)Piu bands, in both instances following optical-optical double-resonance excitation. The latter transition was observed previously for the lighter isotopomer, 6Li2 [C. Linton, F. Martin, P. Crozet, A. J. Ross, and R. Bacis, J. Mol. Spectrosc. 158, 445 (1993)]. By analyzing multiple-isotopomer data for several electronic systems simultaneously, we have determined the electronic isotope shifts and the leading vibrational and/or rotational Born-Oppenheimer breakdown terms for the X (1)Sigmag+, A (1)Sigmau+, B (1)Piu, and 1 (1)Deltag states of the lithium dimer. This paper also reports Fourier transform measurements of the B-X absorption spectra of 6Li2 and 7Li2, which were required to better define the bottom portion of the B (1)Piu state potential.