Abstract

We have performed high-resolution spectroscopic measurements on the negative molecular ion NH⁻. The infrared beam of a Burleigh FCL-20 color-center laser overlaps a parallel mass-selected 3-keV ion beam and induces vibrational-rotational transitions. The laser-excited ionic state autodetaches, yielding a fast (keV) collinear neutral NH, which is detected by secondary electron emission. Resonances are observed by scanning the ion velocity with fixed laser frequency. Our results more than quadruple the number of high-precision measurements in this system. Our results include the important Q and P branches for the first time. We have observed transitions in the isotopically substituted species ¹⁵H⁻. Analysis of the combined isotopic data yields the anharmonicity of the vibrational potential well. Because of a kinetic velocity compression effect, this is one of the few one-photon Doppler-free spectroscopic techniques. The 3-MHz (30-ppb) resolution is equivalent to a temperature of 0.04 K. We can measure autodetachment lifetimes and resolve lambda-doubling and hyperfine structure arising from the interaction of the nuclear magnetic moments with the electronic magnetic fields. This is the first observation of hyperfine structure in this molecular system, and only the second observation of hyperfine structure in any negative molecularion.

© 1986 Optical Society of America