S. V. ONeil

Photoionization of molecular hydrogen

A theoretical formulation of diatomic photoionization is given in terms of an entirely body‐frame Hund’s case b description. Within this formalism, the electronic wavefunctions for both the bound initial state and the free final state of photoionization are described with conventional configuration interaction wavefunctions expanded in a Gaussian basis. The Stieltjes imaging procedure is used to recover the continuum oscillator strength from the discrete eigenspectrum. The continuum oscillator strength is combined with variationally determined vibrational wavefunctions to yield model photoelectron spectra, specific v–v ionization cross sections, a photoion kinetic energy spectrum, dissociative ionization spectra, and a total photoionization spectrum. For a series of photon energies near threshold, cross sections from these calculations are compared with previous theoretical results and with recent experimental measurements.

Slit jet infrared spectroscopy of NeHF complexes: Internal rotor and J-dependent predissociation dynamics

Direct absorption tunable difference frequency IR spectroscopy in a slit jet supersonic expansion has been used to observe complexes of Ne with HF for the first time. Spectra of both the weak HF stretch fundamental (1000)←(0000) and the 10–20 fold more intense bend and stretch combination band (111e,f0)←(0000) transitions are observed, and illustrate several interesting dynamical features. The large ratio of combination band to fundamental intensity is evidence for a highly isotropic potential with respect to HF rotation. The HF bend vibration is thus better thought of as nearly free internal rotor motion with a nearly good space fixed quantum number, jHF =1. Weak anisotropy in the potential permits the jHF=1 (Πebend) levels to relax intramolecularly to jHF =0 (Σ) levels, leading to predissociative line broadening in the sub‐Doppler slit jet spectra. This observed dissociation of NeHF with 44 cm−1 of internal excitation provides a rigorous upper limit to the binding energy. The Πfbend levels, on the other...

Ab initio calculations of radiative transition probabilities in SH, SH+, and SH−

Potential energy and dipole moment functions for the ground states of SH, SH+, and SH− have been calculated from highly correlated electronic wave functions. The electric dipole moments in the vibrational ground states of 32SH, 32SH+, and 32SH− are calculated to be 0.74, 1.29, and 0.27 D, and the rotationless rates of spontaneous emission  A10 to be 1,  52, and 75 s−1, respectively. The predicted transition probabilities between the low lying vibrational states of the electronic ground state of SH and SD are among the smallest so far known for dipole allowed rotation‐vibration transitions. The calculated A–X transition probabilities in SH confirm recent indirect determinations of the radiative lifetimes and absorption oscillator strengths in the predissociating v’=0 level of the A state. The 4Σ− state is calculated to intersect the A 2Σ+ state at R=3.1 a.u., between the classical turning points of v’=0 and 1 in the A state.

Laser photoelectron spectroscopy of CH2−, and the singlet–triplet splitting in methylene

Photodetachment of an electron from CH2− ions using visible light produces CH2 in both its ground 3B1 and lowest excited 1A1 states. In an earlier letter, this process was utilized to determine directly a singlet–triplet splitting of 19.5 kcal/mol. In this paper are reported the details of the early measurement and a number of additional investigations designed to test critically the earlier assignments. Data reported here indicate that one suggested problem with the early results, a misassignment of the 3B1 origin resulting from negative ion hot bands, is not likely. These new results do reveal the presence of an additional peak in the photoelectron spectrum which could possibly be the lowest level of the 3B1 state, allowing a singlet–triplet splitting of 19.5 or 23.2 kcal/mol. The results reported here do not indicate any other inconsistency in the original assignment. Finally, these data are discussed in light of the formidable body of theoretical and indirect thermochemical determinations suggesting a...

Weakly bound NeHF

We have used ab initio methods to characterize the Ne–HF van der Waals complex. The interaction energy was determined using size consistent, correlated CEPA wave functions expanded in a Gaussian basis chosen to represent both intraatomic effects and the low order multipole moments and polarizabilities of Ne and HF. The calculated well depths are −65 cm−1 for linear Ne–HF and −39 cm−1 for linear Ne–FH, with an intervening saddle point at −27 cm−1. The induction contribution to the energy is significantly greater for Ne–HF than for Ne–FH, but dispersion remains the dominant attraction over the region of interest. Converged variational and close‐coupling calculations using the ab initio potential surface reveal three bound levels of the Ne–HF stretch mode, and several metastable levels correlating asymptotically with rotationally excited HF( j=1). Though nearly degenerate, the lifetimes of the two metastable Π (body frame Λ=±1) bending levels differ markedly because of different rotational coupling strengths...

On the bonding in doubly charged diatomics

SummaryThe potential energy of interacting atomic ions A++B+ often shows a shallow local minimum separated by a broad potential barrier from the dissociation products at much lower energy. Early interpretations of dication potential shapes were based on the similarity of the electronic structure between isoelectronic neutral and ionic species and led to a picture of a chemical bond superimposed on a repulsive Coulomb potential. More recently, barriers in dication potentials have commonly been interpreted as avoided curve crossings involving covalent and ionic structures. In this paper, we demonstrate that the former model is the appropriate one except in cases with very small asymptotic ionic/covalent energy splittings. By deriving dication wavefunctions from their neutral isoelectronic counterparts, we obtain upper bound dication potential curves which show all the characteristic features. By further modeling induction effects, we arrive at an almost quantitative fit of accurateab initio dication potentials. The “chemical bond plus electrostatic repulsion’ interpretation of dication interactions also explains why the accurate calculation of potential curves appears to be much more demanding for dications than for isoelectronic neutrals.

Metastable 3Σ−g ground state of F++2 and the bonding in molecular dications

Large multireference configuration interaction (MR‐CI) calculations on the F++2 ion predict a 3Σ−g ground state, metastable with respect to tunneling into the F++F+ nuclear continuum. The potential energy curve displays a 0.40 eV barrier at Rb=1.607 A, between the local potential minimum (Re=1.289 A) and the 3Pg(F+)+3Pg(F+) asymptote at 7.69 eV lower energy. The potential barrier traps four quasibound vibrational levels, with a tunneling lifetime of 16 ms for v=0. A Dunham analysis at the well minimum gives ωe=919.4 cm−1, ωexe=16.31 cm−1, Be=1.073 cm−1, and αe=0.0316 cm−1. In a departure from an earlier viewpoint, the origin of the barrier in this and other dications is interpreted as a sum of the e2/R Coulomb repulsion and the ordinary chemical bonding of the constituent ions. This model also explains the purely repulsive character found for the valence‐excited 1Δg and 1Σ+g states.

Multireference—configuration interaction (MRCI) calculations of HS2+ and experimental observation via electron impact ionization of H2S

Abstract The stability of the HS2+ dication is investigated both theoretically and experimentally. The theoretical analysis involves complete active-space self-consistent field (CASSCF) and multi-configuration reference—configuration interaction (MRCI) theory. A metastable 2П ground state is found with a calculated barrier to dissociation of 1.52 eV. No other metastable states for energies up to 10eV above the ground state are found. In conjunction with the theoretical results, electron impact ionization of H2S is able to generate the HS2+ dication, demonstrating that the species exists on a time scale longer than 10 μs. The appearance potential for HS2+ from H2S is 42.3 ± 1.0 eV. Evidence is also presented for the electron impact production of S2+, H2S2+ and H3S2+.

Moment-trace calculations of atomic and molecular photoabsorption cross sections

Abstract Regularization procedures employing positive integer spectral power moments are described for calculation of atomic and molecular photoionization cross sections. Use of positive moments avoids matrix diagonalizations, and thus appears to have substantial computational advantages.

Low-lying electronic states of PH2+

Abstract All ten electronic states correlating with the four lowest asymptotes of the doubly charged PH 2+ ion have been calculated by the complete active space self-consistent field (CASSCF) and multireference configuration interaction (MR-CI) techniques. In addition to the previously known X 1 Σ + ground state, two more meta-stable states, A 1 Π and a 3 Π, have been found. We find all other states in this energy region to be repulsive.