Sub-Doppler slit jet infrared spectroscopy of astrochemically relevant cations: Symmetric (v1) and antisymmetric (v6) NH stretching modes in ND2H2+.

Sub-Doppler infrared rovibrational transitions in the symmetric (v1) and antisymmetric (v6) NH stretch modes of the isotopomerically substituted ND2H2+ ammonium cation are reported for the first time in a slit jet discharge supersonic expansion spectrometer. The partially H/D substituted cation is generated by selective isotopic exchange of ND3 with H2O to form NHD2, followed by protonation with H3 + formed in the NHD2H2/Ne slit-jet discharge expansion environment. Rotational assignment for ND2H2+ is confirmed rigorously by four line ground state combination differences, which agree to be within the sub-Doppler precision in the slit jet (∼9 MHz). Observation of both b-type (v1) and c-type (v6) bands enables high precision determination of the ground and vibrationally excited state rotational constants. From an asymmetric top Watson Hamiltonian analysis, the ground state constants are found to be A" = 4.856 75(4) cm-1, B" = 3.968 29(4)cm-1, and C" = 3.446 67(6) cm-1, with band origins at 3297.5440(1) and 3337.9050(1) cm-1 for the v1 and v6 modes, respectively. This work permits prediction of precision microwave/mm-wave transitions, which should be invaluable in facilitating ongoing spectroscopic searches for partially deuterated ammonium cations in interstellar clouds and star-forming regions of the interstellar medium. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]