Multiphoton ionization of CH₃ radicals in the gas phase

The technique of mass-selected multiphoton ionization (MPI) has been applied to the detection of gas-phase methyl radicals. Methyl radicals were produced by pyrolysis of various CH₃ sources in a resistively heated tantalum oven. The output of a Nd:YAG pumped dye laser was focused into the ionization region of a quadrupole mass spectrometer. The mass-selected multiphoton ionization spectrum was obtained by measuring the mass 15 ion signal as a function of dye laser wavelength. Methyl radicals were identified by observing CH₃ Rydberg-state resonances in the MPI spectrum. The three-photon Rydberg states are assigned along with the associated vibronic transitions. The results are discussed in terms of the vibronic structure of the electronically excited Rydberg states. These results indicate the general applicability of multiphoton ionization to the detection of nonfluorescing free radicals.