Electronic states and transitions of the TeX (X = Cl, Br, I) radicals

Ab initio multireference configuration interaction calculations including spin-orbit coupling have been carried out for the first time for valence electronic states of the TeX (X = Cl, Br, I) radicals and compared with the results for the isovalent TeF and IO systems obtained earlier at a similar level of theoretical treatment. The calculated spectroscopic constants are in good agreement with experimental data in the rare cases when the latter are available. It is shown that the X (2)Pi(sigma(s) pi(4)pi*(3)) ground state bonding becomes consistently weaker down the TeX group (calc. D = 25 480 cm(1) for TeF, 12 100 cm(1) for TeI) due to the more covalent character of bonding in the heavier radicals. The first excited state, A (4)Sigma (pi* --> sigma*), is calculated to be bound in all systems. It is split into Ohm 1/2 and 3/2 components, with regular ordering in the Franck-Condon region, opposite to that of the ground X (2)Pi state. For larger internuclear distances, the A(1) (4)Sigma(1/2)(-) state undergoes an avoided crossing with X-2 (2)Pi(1/2), which causes a shoulder in the X-2 potential curve and also leads to a crossing between the A(1) and A(2) curves and large distinctions in their vibrational frequencies. The pi* --> sigma* B (2)Sigma, C (2)Delta, and 1 (2)Sigma(+) states are calculated to lie next in energy. They are all bound in the lightest of the TeX radicals, TeF, but successively lose their bonding character down the group. In contrast to oxygen monohalides, the 2 (2)Pi(sigma(2)pi(4)pi*(3)) state has a repulsive potential curve. Electric dipole transition moments and radiative lifetimes have also been calculated for the low lying bound states in all systems. Most of them are found to be quite weak. The A(1,2) --> X-1,X-2 spectra are dominated by parallel contributions, with the A(2) --> X-1 being the strongest one. The tau values for this transition are quite similar and lie in the 17-30 mus range. Radiative lifetime values for the B --> X-1,X-2 transitions demonstrate very irregular behaviour for various TeX radicals, due to strong admixture of A (4)Sigma character to the X-1,X-2 states near the B (2)Sigma potential minimum. The A(1,2) (4)Sigma(1/2,3/ 2)(-) and B (2)Sigma(1/2)(-) states of TeX (X = Cl, Br, I) still await their experimental observation.