Photofragment infrared emission spectroscopy: Vibrational progression and potential parameters of the CH3(ν2) ‘‘umbrella’’ mode

The out-of-plane bending vibration (ν2) of the free radical CH3 has been detected by time and wavelength-resolved infrared emission spectroscopy following dissociation of CH3I with a pulsed laser at λ = 266 nm. Three distinct Q branch features in the vibrational progression of the ν2 ’’umbrella’’ mode are observed: 3-2 (729±4 cm−1), 4-3 (772±4 cm−1), and 5-4 (811±4 cm−1). These low resolution measurements provide an approximate determination of the harmonic/quartic mixed oscillator potential parameters for the higher levels of the out-of-plane bend. The vibrational assignment is important to studies of radical excitation originating from photodissociation, which is considered in the accompanying paper.