Gas Phase LIF Study of the b1Sigma+(b0(+)) left and right arrow X3Sigma-(X10(+), X21) Transition of NI.

First gas phase spectra of the nitrogen monoiodide radical NI are reported. The b1Sigma+(b0(+)) left and right arrow X3Sigma-(X10(+), X21) transition of NI has been studied by laser-induced fluorescence measurements. Ground state NI radicals were generated in a fast-flow system by reaction of microwave-discharged nitrogen with iodine vapor. Vibrational levels v = 0, 1, and 2 of the b1Sigma+ state were excited with a pulsed dye laser. Rotational and vibrational constants of the X3Sigma- and b1Sigma+ states were deduced from analysis of high-resolution excitation spectra. Time-resolved measurements of the fluorescence decay and measurements of relative line and band intensities allowed determination of the radiative lifetime of the b state, of band transition probabilities, and of the ratio of transition moments µ0/µ1 for the b0(+) --> X10(+) and b0(+) --> X21 subtransitions. Comparison of calculated and experimental line intensities and FC factors shows that treatment of the X3Sigma- state in terms of a Hund's case (a) formalism is preferable to use of a case (c) formalism. Copyright 1998 Academic Press.