Experimental ground-state combination differences of CH_5~+

摘要

Protonation of methane (CH_4), a rather rigid molecule well described by quantum mechanics, produces CH_5~+, a prototypical floppy molecule that has eluded definitive spectroscopic description. Experimental measurement of high-resolution spectra of pure CH_5~+ samples poses a formidable challenge. By applying two types of action spectroscopy predicated on photoinduced reaction with CO_2 and photoinhibition of helium cluster growth, we obtained low-temperature, high-resolution spectra of mass-selected CH_5~+. On the basis of the very high accuracy of the line positions, we determined a spectrum of combination differences. Analysis of this spectrum enabled derivation of equally accurate ground state-level schemes of the corresponding nuclear spin isomers of CH_5~+, as well as tentative quantum number assignment of this enfant terrible of molecular spectroscopy.