Abstract

The study of the electronic spectrum of atomic fluorine is extremely difficult since all the single photon transitions from the ground state require wavelengths that are below the LiF window cutoff at 106 nm. This situation presents an ideal application for the techniques of multiphoton excitation, followed by either fluorescence or ionization detection. We report here the first observation of three-photon resonant excitation of the ²P_1/2,3/2 excited states in atomic fluorine at 105,000 cm^-1. This state is then ionized by the absorption of two additional photons forming a five-photon process. The experiment consists of focusing a doubled YAG-pumped dye laser at 286.3 nm into a flowing cell containing a mixture of F₂ and He. The UV radiation completely dissociates the F₂ in the focal volume and simultaneously ionizes the atomic fluorine. The ions are collected with a pair of biased electrodes and the charge is amplified. We estimate that ~10⁴ ions are produced in the focal volume for a 1:1 signal/noise ratio. We also observe three-photon resonant four-photon ionization of molecular F₂ in the same region. This is the first observation of the MPI spectrum of F₂ and most of the observed features have been assigned using previous data.¹ We have carefully measured the intensity of our laser in the focal volume and present estimates of the effective five-photon ionization cross section for detection applications.

© 1985 Optical Society of America