HIGH-RESOLUTION NEAR-INFRARED SPECTROSCOPY OF He/N$_2$/H$_2$ POSITIVE-COLUMN PLASMAS

Abstract

{M. Okumura, B. D. Rehfuss, B. M. Dinelli, M. G. Bawendi, and T. Oka, J. Chem. Phys. \textbf{90{Y. Kabbadj, T. R. Huet, D. Uy, and T. Oka, J. Mol. Spectrosc. \textbf{175{G. Osmann, P. R. Bunker, P. Jensen, and W. P. Kraemer, J. Mol. Spectrosc. \textbf{186Author Institution: Department of Chemistry, Department of Astronomy $\&$ Astrophysics,; and the Enrico Fermi Institute, University of Chicago, Chicago, IL 60637Like its isoelectronic cousins BH$_2^-$ and CH$_2$, the amidogen cation NH$_2^+$ has a quasilinear ground state $\widetilde{X}$$^3$B$_1$ with a low barrier to linearity (155 cm$^{-1}$) and metastable excited electronic states $\widetilde{a}$$^1$A$_1$ and $\widetilde{b}$$^1$B$_1$ that become degenerate ($^1\Delta$) at linearity. In addition to its theoretical interest (due to the quasilinearity and the Renner effect), NH$_2^+$ is one of the most fundamental molecular ions that exist abundantly in laboratory plasmas containing hydrogen and nitrogen. Despite this, only two high-resolution experimental detections of NH$_2^+$ have been reported (the observation of the antisymmetric N-H stretch}, 5918 (1989).} at 3360 cm$^{-1}$, and four hot bands}, 277 (1996).} from 2900-3500 cm$^{-1}$). \vspace{1em} In an attempt to observe the predicted near-infrared electronic absorption spectrum of NH$_2^+$,}, 319 (1997).} we have recently obtained new spectra of positive ions in a liquid-nitrogen-cooled positive column He/N$_2$/H$_2$ plasma. The spectra were recorded using a high-resolution, high-sensitivity spectrometer based on a Ti:sapphire laser (11,000-13,000 cm$^{-1}$) and incorporating velocity modulation, phase modulation with heterodyne detection, noise subtraction, and optical multi-passing. The observation and assignment of the spectra is complicated by the presence of thousands of lines from the $A^2\Pi_u - X^2\Sigma_g^+$ system of N$_2^+$. We will report the results of our analysis of the new spectra