INFRARED SPECTROSCOPY OF JET COOLED ND$_2$H$_2^{+}$ MOLECULAR IONS: THE SYMMETRIC AND ANTISYMMETRIC NH STRETCH MODES

Abstract

Rovibrational progressions in the symmetric ($v$$_6$) and antisymmetric ($v$$_1$) NH stretching modes of the ND$_2$H$_2^{+}$ molecular ion are observed for the first time, exploiting the i) high ion density and ii) high resolution capabilities of our slit jet discharge infrared spectrometer. These isotopomeric ions are generated by striking a modulated (50 KHz) electrical discharge in a mixture of ND$_3$/H$_2$O/H$_2$ gases, achieving a modulated ion density suitable for time-gated, lock-in detection in the throat of a long path slit-jet expansion. Assignment of both $b$-type and $c$-type bands enables high accuracy determination of the rotational constants ($A$$^{\prime\prime}$=4.85598(19), $B$$^{\prime\prime}$=3.96811(11), and $C$$^{\prime\prime}$=3.44661(40) cm$^{-1}$), with band origins for $v$$_1$ and $v$$_6$ modes determined to be 3297.54367(34) and 3337.90456(33) cm$^{-1}$, respectively. The results prove to be in good agreement with anharmonically corrected predictions from ${ab}$ initio quartic force fields of Martin and Lee.