High-resolution double resonance action spectroscopy in ion traps: vibrational and rotational fingerprints of CH2NH2+

Abstract

By applying various action spectroscopic techniques in a 4 K cryogenic ion trap instrument, protonated methanimine, CH₂NH₂⁺, has been investigated by high-resolution rovibrational and pure rotational spectroscopy for the first time. In total, 39 rovibrational transitions within the fundamental band of the ν₂ symmetric C–H stretch were measured around 3026 cm⁻¹, which were used to predict pure rotational transition frequencies of CH₂NH₂⁺ in the ground vibrational state. Based on these predictions, nine rotational transitions were observed between 109 and 283 GHz using a novel double resonance method, which significantly improved the sensitivity of the rotational measurements. This double resonance method consists of rotational excitation followed by vibrational excitation, which is finally detected as a dip in the number of CH₂NH₂⁺–He complexes formed in the 4 K He bath of the trap. The new measurements and the derived predictions of pure rotational transitions will enable the first radio-astronomical search for CH₂NH₂⁺.