High Resolution Leak-Out Spectroscopy of the ν ₂ Symmetric CH ₃ Stretching Mode of Protonated Acetonitrile, CH ₃ CNH ⁺

The symmetric CH₃ stretching mode ν₂ of protonated acetonitrile (CH₃CNH⁺), located around 3.4 μm, has been measured at high resolution in a cryogenic 22-pole ion trap apparatus via leak-out spectroscopy. In addition to this fundamental vibrational mode, several close-lying combination bands interfering through Fermi-resonances were detected in its proximity. Two of those have been assigned as the ν₅ + 2ν₈ and ν₅ + ν₇ + ν₉ combination bands, and three more likely arising from the four-quanta combination mode, ν₇ + 2ν₉ + ν₁₀. The formation of ground-state combination differences permits the spectroscopic assignment of 227 out of the 261 detected lines. A thorough treatment of the Fermi resonance system yields spectroscopic constants for three of the bands and approximate spectroscopic constants for all remaining ones.