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.

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 as a function of excitation energy. The maximal ΦI*(ν) values are calculated to be 0.55–0.59 and lie at 39 000–40 000 cm−1, which agrees well with experimental results. The influence of the t 3Σ1+ state and of the nonadiabatic effects on the ΦI*(ν) values is found to be negligible in the essential part of the A band. Finally, it is shown that significantly higher I* quantum yield values (up to 0.8–0.9) may be achieved when vibrationally hot HI molecules are excited in the appropriate spectral range.

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.

Multireference spin-orbit configuration interaction calculations have been carried out for the valence and low-lying Rydberg states of CH(3)I. Potential energy surfaces along the C-I dissociation coordinate (minimal energy paths with respect to the umbrella angle) have been obtained as well as transition moments for excitation of the Rydberg states. It is shown that the B and C absorption bands of CH(3)I are dominated by the perpendicular (3)R(1),(1)R (E)←X̃ A(1) transitions, while the (3)R(2)(E), (3)R(0(+) )(A(1))←X̃ A(1) transitions are very weak. It is demonstrated that the bound Rydberg states of the B and C bands are predissociated due to the interaction with the repulsive E and A(2) components of the (3)A(1) state, with the (3)A(1)(E) state being the main decay channel. It is predicted that the only possibility to obtain the I((2)P(3/2)) ground state atoms from the CH(3)I photodissociation in the B band is by interaction of the (3)R(1)(E) state with the repulsive (1)Q(E) valence state at excitation energies above 55,000 cm(-1). The calculated ab initio data are used to analyze the influence of the Rydberg state vibrational excitation on the decay process. It is shown that, in contrast to intuition, excitation of the ν(3) C-I stretching mode supresses the predissociation, whereas the ν(6) rocking vibration enhances the predissociation rate.

The generalized relativistic effective core potential (GRECP) approach is employed in the framework of multireference single-and double-excitation configuration interaction (MRD-CI) method to calculate the spin-orbit (SO) splitting in the 2 P o ground state of the Tl atom and spectroscopic constants for the 0 + ground state of TlH. The 21-electron GRECP for Tl is used and the outer core 5s and 5p pseudospinors are frozen with the help of the level shift technique. The spin-orbit selection scheme with respect to relativistic multireference states and the corresponding code are developed and applied in the calculations. In this procedure both correlation and spin-orbit interactions are taken into account. A [4,4,4,3,2] basis set is optimized for the Tl atom and employed in the TlH calculations. Very good agreement is found for the equilibrium distance, vibrational frequency, and dissociation energy of the TlH ground state (R e = 1.870 Å, ω e = 1420 cm −1 , D e = 2.049 eV) as compared with the experimental data (R e = 1.868 Å, ω e = 1391 cm −1 , D e = 2.06 eV).SHORT NAME: GRECP/MRD-CI calculations on Tl and TlH