Heinz-Peter Liebermann

An ab initio study of the CH3I photodissociation. I. Potential energy surfaces

The multireference spin-orbit (SO) configuration interaction (CI) method in its Lambda-S contracted SO-CI version is employed to calculate two-dimensional potential energy surfaces for the ground and low-lying excited states of CH3I relevant to the photodissociation process in its A absorption band. The computed equilibrium geometry for the X A1 ground state, as well as vibrational frequencies for the nu2 umbrella and nu3 symmetric stretch modes, are found to be in good agreement with available experimental data. The 3Q0+ state converging to the excited I(2P1/2o) limit is found to possess a shallow minimum of 850 cm(-1) strongly shifted to larger internuclear distances (RC-I approximately 6.5a0) relative to the ground state. This makes a commonly employed single-exponent approximation for analysis of the CH3I fragmentation dynamics unsuitable. The 4E(3A1) state dissociating to the same atomic limit is calculated to lie too high in the Franck-Condon region to have any significant impact on the A-band absorption. The computed vertical excitation energies for the 3Q1, 3Q0+, and 1Q states indicate that the A-band spectrum must lie approximately between 33,000 and 44,300 cm(-1), i.e., between 225 and 300 nm. This result is in very good agreement with the experimental findings. The lowest Rydberg states are computed to lie at >or=49,000 cm(-1) and correspond to the ...a(1)2n3a1(6sI) leading configuration. They are responsible for the vacuum ultraviolet absorption lines found experimentally beyond the A-band spectrum at 201.1 nm (49,722 cm(-1)) and higher.

Spin–orbit configuration interaction study of the potential energy curves and radiative lifetimes of the low‐lying states of bismuth hydride

An ab initio configuration interaction (CI) study including the spin–orbit coupling interaction is carried out for the lowest 23 states of the bismuth hydride molecule by employing relativistic effective core potentials for the bismuth atom. The computed spectroscopic constants are in good agreement with corresponding experimental data, although there is a tendency to overestimate bond lengths by 0.05–0.10 A and to underestimate the vibrational frequencies accordingly. The B0+ excited state is found to have no dissociation barrier, and its radiative lifetime is computed to be 4.3 μs, with parallel transitions to X10+ being significantly stronger than the perpendicular B–X21 species. The experimental E0+ state is assigned as the third root of this symmetry and its potential curve possesses a dissociation barrier of 1840 cm−1. This result explains the predissociation characteristics observed for this state and is also consistent with the failure to observe emission from it when attempts are made to form it ...

Comparison of spin-orbit configuration interaction methods employing relativistic effective core potentials for the calculation of zero-field splittings of heavy atoms with a 2Po ground state

Computational strategies for the treatment of relativistic effects including spin-orbit coupling at a highly correlated level are compared for a number of heavy atoms: indium, iodine, thallium, and astatine. Initial tests with perturbation theory emphasize the importance of high-energy singly excited configurations which possess large spin-orbit matrix elements with the ground state. A contracted basis consisting of L–S CI eigenfunctions (LSC–SO–CI) is found to give an accurate representation of both spin-perturbed 2Po components as long as key np→pi* singly excited configurations are included. Comparison is made with a more extensive treatment in which all selected configurations of various L–S symmetries form the basis for the multireference–spin-orbit–configuration interaction (MR–SO–CI). Good agreement is obtained with experimental SO splittings for the In, I, and At atoms at a variety of levels of treatment, indicating that the L–S contracted SO–CI approach can be implemented quite effectively with r...

On the ultraviolet photofragmentation of hydrogen iodide

An ab initio configuration interaction (CI) study including spin-orbit coupling is carried out for the ground and low-lying excited states of the HI molecule by employing a relativistic effective core potential for the iodine atom. The computed spectroscopic constants for the X 1Σ+ ground and b 3ΠΩ Rydberg states are in good agreement with available experimental data, as are the vertical excitation energies for the repulsive a 3Π1, a 3Π0+, and A 1Π1 states of the A band. The a 3Π0+ state is found to possess a shallow minimum of 600 cm−1 depth outside the Franck–Condon region, at ≈5.1 a0. The electric-dipole moments have also been calculated for transitions from the ground to the A band states. Contrary to what is usually assumed, the a 3Π1, A 1Π1←X0+ transition moments are found to depend strongly on internuclear distance. Employing the computed potential energy and transition moment data, partial and total absorption spectra for the A band are calculated and the I* quantum yields, ΦI*(ν), are determined ...

An ab initio study of the CH3I photodissociation. II. Transition moments and vibrational state control of the I* quantum yields

Multireference spin-orbit configuration interaction calculations of transition moments from the X A1 ground state to the 3Q0+, 3Q1, and 1Q excited states responsible for the A absorption band of CH3I are reported and employed for an analysis of the photofragmentation in this system. Contrary to what is usually assumed, the 3Q0+(A1), 3Q1(E), and 1Q(E)<--X A1 transition moments are found to be strongly dependent on the C-I fragmentation coordinate. The sign of this dependence is opposite for the parallel and perpendicular transitions, which opens an opportunity for vibrational state control of the photodissociation product yields. The computed absorption intensity distribution and the I* quantum yield as a function of excitation energy are analyzed in comparison with existing experimental data, and good agreement between theory and experiment is found. It is predicted that significantly higher I* quantum yield values (>0.9) may be achieved when vibrationally hot CH3I molecules are excited in the appropriate spectral range. It is shown that vibrational state control of the I*/I branching ratio in the alkyl (hydrogen) iodide photodissociation has an electronic rather than a dynamic nature: Due to a different electron density distribution at various molecular geometries, one achieves a more efficient excitation of a particular fragmentation channel rather than influences the dynamics of the decay process.

AB INITIO RELATIVISTIC CONFIGURATION INTERACTION CALCULATIONS OF THE SPECTRUM OF BISMUTH OXIDE : POTENTIAL CURVES AND TRANSITION PROBABILITIES

A series of configuration interaction calculations employing relativistic effective core potentials including the spin–orbit interaction is reported for the X1 2Π1/2 ground and numerous low‐lying excited states of the bismuth oxide molecule up to 30 000 cm−1. Special difficulties connected with the treatment of open‐shell systems and double‐group irreducible representations are discussed and a feasible computation scheme is developed for dealing with them. The spin–orbit interaction is found to cause a high level of mixing between a variety of low‐lying λ–s states, producing a number of avoided crossings which play a key role in determining the character of the BiO spectrum. A comparison with existing experimental data for both the energy locations and intensities of a large number of band systems indicates that the present calculations are capable of predicting Te values to an accuracy of 0.1–0.2 eV. Corresponding radiative lifetime results generally agree within a factor of 2, with the best experience o...

Theoretical study of the energies and lifetimes of the low-lying states of bismuth fluoride

Abstract A series of CI calculations has been carried out for various low-lying electronic states of the bismuth fluoride molecule by employing relativistic effective core potentials including spin-orbit effects. It is found that the lowest 0 + excited state (A0 + ) of this system contains a large contribution from the π*→σ* 3 Π λ- s state, especially at bond distances which are equal to or greater than the equilibrium value for the X 1 0 + ground state. The B0 + state at somewhat higher energy is found to contain the largest portion of the b 1 ∑ + character arising from the π 4 π* 2 configuration, in disagreement with earlier calculations by Balasubramanian who only reports a single excited state of this (0 + ) symmetry. Results of the latter study for the X 1 0 + , X 2 1 and a2 states of lower energy are in good agreement with those of the present work, However. The relatively large r e value observed for the A0 + state is quite consistent with the present theoretical description, as well as the correspondingly lower vibrational frequency compared to that of X 1 0 + . Radiative lifetimes have also been obtained for a number of excited states and the results are found to be in reasonably good agreement with recent measured data. An explanation is also provided for the anomalous μ 0 /μ 1 ratio for the A0 + → X transitions, again based on the large amount of 3 Π character in the upper state. The present data also provide a clear assignment for the A′ and A″ states recently found for this system (Ω = 1 and 0 − ) respectively. The lifetime of the X 2 1 state is computed to be 1.05 ms and a zero-field splitting of 6937 cm −1 is obtained, both of which results are in good agreement with the corresponding measured values of 1.4 ms and 6768 cm −1 . The next-lowest-energy state (a2) is predicted to have a liftime which is 80 times larger, a result which is consistent with the failure to date to observe emission bands initiating from this state.

Complex self-consistent field and multireference single- and double-excitation configuration interaction calculations for the Πg2 resonance state of N2−

Self-consistent field and multireference single- and double-excitation configuration interaction calculations employing the complex basis function technique are carried out for the (2)Pi(g) resonance state of the N(2) (-) molecule as well as several other anionic resonance states in the neighboring energy region. The results of calculations employing the same method for the (1)S (2s(2)) state of the He atom and the (1)Sigma(g) (+) (sigma(u) (2)) state of the H(2) molecule are found to be in good agreement with those of earlier work. The present theoretical treatment has succeeded for the first time in satisfying the rigorous criterion of the complex variational principle in computing the N(2) (-) resonance states, namely, a cusp in the plots of real versus imaginary components of the corresponding complex energies has been located at each internuclear distance. On this basis, it is found that the open-shell orbital in the lowest-energy adiabatic N(2) (-) resonance state of (2)Pi(g) symmetry changes its character from quite compact at large internuclear distance to relatively diffuse for r<2.3a(0). This is in contrast to all previous theoretical treatments of this system that have not rigorously satisfied the complex variational principle in their determination of this wave function.

Radiative Charge Transfer in Collisions of O with He

Radiative charge transfer has been investigated for collisions of O with He+ with both a fully quantum mechanical theory and an optical potential method. Cross sections and rate coefficients are presented for the process O(3P) + He+ → O+(4So,2Po,2Do) + He + ω and are compared to those of direct charge transfer. The relative collision energies considered range from 0.1 meV to ~3 eV with a semiclassical extension to 10 keV and temperatures between 10 and 2.0 × 106 K. The results demonstrate that radiative charge transfer is the dominant process over the energy and temperature region considered. Total emission spectra for the two strongest of the ten possible transitions are given for several collision energies, and the origin of resonance-like structures in the spectra is discussed.

Role of the electric dipole moment in positron binding to the ground and excited states of the BeO molecule

Self-consistent-field and multireference single- and double-excitation configuration interaction (CI) calculations have been carried out for various electronic states of the beryllium oxide molecule and their positron-attached counterparts. Particular emphasis is placed on the correlation between the polarity of a given BeO state and the magnitude of the positron binding energy as the internuclear distance is varied. Potential curves are computed for all BeO states that correlate with the first three atomic limits for this system and good agreement is found between the experimental and calculated spectroscopic constants in all cases. The present level of CI treatment is known to underestimate the positron affinities of atoms by at least several tenths of an eV, and this fact needs to be taken into account in evaluating the results for positron binding to molecules. The lowest BeO excited states (3,1Pi) are not found to bind with a positron in the Franck-Condon region due to their comparatively small dipole moments caused by O to Be charge transfer relative to the X 1Sigma+ ground state, which in turn does have a fairly sizeable positron affinity. The situation changes significantly as dissociation proceeds, however, with both 4,2Pi and 2Sigma+ positronic states lying several tenths of an eV lower than their neutral counterparts over a broad range of internuclear distance.