State-selective ion mobility measurement for NHn+ (n = 1 - 3) ions in helium gas at low temperature

We have measured ion mobility in cooled helium gas for NH+, NH+2, and NH+3 at 77 and 4.3 K. In the measurement of NH+ and NH+3, we have observed double peaks in the arrival time spectra, which correspond to different electronic states of molecular ions. These results can be regarded as the first observation of the metastable states in mobility measurement for these ions.