Eigenstate resolved infrared and millimeter-wave–infrared double resonance spectroscopy of methylamine in the N–H stretch first overtone region

Abstract

The eigenstate resolved, near-infrared spectrum of methylamine, CH₃NH₂, has been measured at 5 MHz resolution in the region near 6615 cm^-1 using a molecular-beam laser spectrometer with optothermal detection. Part of the spectrum of the NH 2ν₁ (symmetric stretch first overtone) has been assigned by applying a millimeter-wave–infrared double resonance technique. The spectra are very fractionated by intramolecular vibrational energy redistribution with an observed density of states between 20 and 30 states per cm^-1 which is about four times the density of vibrational states calculated in the harmonic approximation. The coupling between the bright states and the bath is found to be rather strong (0.1 cm^-1), and the lifetime for the redistribution is determined to be 3 ps for the 2ν₁ vibration.