Infrared Spectroscopy of CH<sub>5</sub><sup>+</sup> Cations in Helium\nNanodroplets

The methanium CH<sub>5</sub><sup>+</sup> is a prototypical fluxional\nion whose infrared spectra remain unassigned. Here we report on the\ninfrared spectra of CH<sub>5</sub><sup>+</sup> cations and its deuterated\nisotopomer, CH<sub>4</sub>D<sup>+</sup>, in helium droplets at a low\ntemperature of 0.38 K. The ions were produced upon protonation of\nCH<sub>4</sub> molecules, a technique that was developed in this work.\nThe spectra of CH<sub>5</sub><sup>+</sup> around 3000 cm<sup>–1</sup> show two strong and broad infrared bands and a weak shoulder, reflecting\nits highly fluxional nature. The spectrum of CH<sub>4</sub>D<sup>+</sup> shows a much sharper infrared band, indicating a partial quenching\nof the exchange of H/D atoms. This work also reports on the infrared\nspectrum of the methane dimer radical cations (CH<sub>4</sub>)<sub>2</sub><sup>+</sup>.