Near-infrared electronic transitions of Pb-2

Emission spectra of two new electronic transitions of Pb, have been measure d with a Fourier transform spectrometer in the 5200-8500 cm(-1) range. The emissions were observed from the afterglow of a microwave discharge in a mi xture of Pb, vapor with hydrogen and argon carrier gas. By comparison with the results of ab initio calculations, the spectra are assigned to transiti ons from the lowest 1(g) and 1(u) states to the X(l)0(g)(+) ground state. T he X(2)1(g) --> X(1)0(g)(+) transition between the fine-structure component s of the X(3)Sigma(g)(-) ground state must be magnetic dipole in nature and thus is the first purely magnetic dipole fine-structure transition observe d in the optical region. The I,, state is mostly a component of the low-lyi ng inverted A (IIu)-I-3 state and so is denoted A(2)1(u). Vibrational analy ses have yielded the following electronic energies and vibrational constant s for (208)pb(2) (in cm(-1)): X(l)0(e)(+): omega(e) = 110.20(2), omega(e)x( e) = 0.341(2); X(2)1(g) : T-e = 5304.9(1), omega(e) = 120.57(7), omega(e)x( e) = 0.254(14); A(2)1(u): T-e = 7817.5(2), omega(e) = 126.45(6), omega(e)x( e) = 0.399(11), where the numbers in parentheses are the standard deviation s of the parameters. (C) 2000 Academic Press.