Magnetic field modulated infrared laser spectroscopy of molecular ions: The ν1 band of DCO+

The ν1 (C–D stretch) fundamental band of the DCO+ molecular ion has been studied using a tunable diode laser spectrometer and a liquid‐nitrogen‐cooled hollow‐cathode absorption cell with magnetic field modulation. A total of eight lines were measured, and their analysis yielded the ν1 band origin, 2584.5618(14) cm−1, and the change in rotational constant, α1=226.3(11) MHz. The relatively small value of α1 as compared to theory and to the isoelectronic molecules DN+2 and DCN suggests the possibility of mixing between the (v1v2l2v3)=(1000) and (0400) vibrational states. The observation of ν1 for DCO+ means that 11 of the 12 fundamental bands have been observed for the isoelectronic family of molecular ions HCO+, DCO+, HN+2, and DN+2, leaving only ν2 of DCO+ still undetected.