Knowledge Bank

Despite early successes with velocity modulation spectroscopy$\mathrm{<mrow\; data-mjx-texclass="ORD">,</mrow>}$of ammonium (NH$\mathrm{4<mrow\; data-mjx-texclass="ORD">+</mrow>}$ ) and perdeuteroammonium (ND$\mathrm{4<mrow\; data-mjx-texclass="ORD">+</mrow>}$) cation, corresponding studies of any of the mixed H/D isotopomers (NHD) have proven elusive. In this talk, we present first high resolution results on the lone NH stretch fundamental mode for the jet cooled ND$3$H$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ ion, based on tunable IR difference frequency absorption spectroscopy in a slit jet, sub-Doppler resolution infrared spectrometer. Supersonically cooled ND$3$H$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ ions are generated by modulated (50 KHz) discharges (650V, 500 mA) in ND$3$ doped H$2$/Ne slit jet expansions, monitored by time-gated, lock-in detection methods and with absorption sensitivities near the quantum shot noise limit. Jet cooled (30 K) $P$, $Q$, and $R$ branch rovibrational progressions in the $a$-type NH stretch band are observed and unambiguously assigned by four line ground state combination differences, with additional confirmation by nuclear spin statistical weights for the 3 identical D atoms (I = 1). Least squares fits to this parallel band yield precision rotational constants and an NH stretch vibrational band origin at 3316.8347(19) cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. These high resolution spectroscopic results prove to be in generally excellent agreement with high level $ab$ initio theoretical predictions by Martin and Lee.