Experimental and theoretical study of the electronic states and spectra of TeH and TeLi

Gas-phase emission spectra of the hitherto unknown free radical TeLi have been measured in the NIR range with a Fourier-transform spectrometer. The emissions were observed from a fast flow system in which tellurium vapor in argon carrier gas was passed through a microwave discharge and mixed with lithium vapor in an observation tube. Two systems of blue-degraded bands were measured at high spectral resolution in the ranges 8000-9000 and 570-6700 cm(-1) and vibrational and rotational analyses were performed. In order to aid in the analysis of the experimental data, a series of relativistic configuration interaction calculations has been carried out to obtain potential curves for the low-lying states of TeLi and the isovalent TeH and also electric dipole transition moments connecting them. As in the TeH system, the ground state of TeLi is found to be X(2)Pi (i), but with a remarkably smaller spin-orbit splitting. The TeLi calculations indicate a strongly bound A(2)Sigma (+) state, while in TeH the analogous state is computed to lie significantly higher at approximate to 32000 cm(-1) and it is strongly predissociated. Based on the theoretical analysis, the observed TeLi band systems are assigned to the transitions A(2)Sigma (+)(A1/2) --> X(1)(2)Pi (3/2)(X(1)3/2) and A(2)Sigma (+)(A1/2) --> X(2)(2)Pi (X(2)1/2). Analysis of the spectra has yielded the molecular constants (in cm(-1)) X(1)(2)Pi (3/2):omega (e) = 457.49(3), omega (e)x(e) = 2.482(9), B-0 = 0.408908(8); X(2)(2)Pi (1/2): T-e = 2353.44(3), omega (e) = 456.28(4), omega (e)x(e) = 2.635(8), B-0 = 0.414954(8), p(0) = 1.00637(4); A(2)Sigma (+): T-e = 8574.64(2), omega (e) = 437.81(3), omega (e)x(e) = 2.581(8), B-0 = 0.423903(8), p(0) = -0.19915(2), where the numbers in parentheses are the standard deviations of the parameters. Comparison of the isovalent TeLi and TeH systems emphasizes that the difference in bonding character (ionic in TeLi vs covalent in TeH) is responsible for qualitative differences in the electronic spectra of these two molecules. (C) 2001 Academic Press.