In Laser-Enhanced Ionization spectrometry (LEI) in flames the measured signal consists of the electrons and ionized atoms which follow a laser excitation due to the enhanced thermal ionization rate of an excited state compared with the ground state. The charged particles are collected by applying a voltage across the flame and measuring the corresponding current increase. The authors report the observation of the linear Stark effect in highly excited states of Li in flames when laser-enhanced ionization is utilized. The experiments show good agreement with the theory for the linear Stark effect. The atoms were excited by ultraviolet light from the ground state (2s) to np states (n = 8-22). Collisional broadening is the dominant broadening mechanism for the lower states while the Stark effect dominates at higher values of n for laboratory electrical field strengths. In the analytical use of LEI for trace element analysis the presence of the Stark effect leads to a reduction of the sensitivity of the method, because it decreases the peak-height of the signal. A new method, which utilizes the Stark effect, is presented for nonintrusive determination of the electrical field distribution in flames.
O. Axner and T. Berglind, "Stark Structure Observation in Rydberg States of Li in Flames by Laser-Enhanced Ionization: A New Method for Probing Local Electrical Fields in Flames," Appl. Spectrosc. 40, 1224-1231 (1986)