Laser induced stepwise and two-photon ionization studies of strontium in the air acetylene flame

Abstract

Using strontium atoms present as a trace constituent in an air-acetylene flame as an example, a rich laser enhanced ionization spectrum was obtained. One pulsed tunable dye laser was tuned to a transition originating from the ground state (460.73 nm) and another scanned over different spectral regions. The lines obtained were spectroscopically characterized as to the type of absorption process, which included non-resonant processes as well as single wavelength, two-wavelength, and two-photon resonant processes. With a maximum irradiance of 100 MW cm −2, two-photon transitions resulting in collisionally assisted ionization included the 5 sn s and 5 dn d Rydberg series (up to 37 s and 15 d) together with a strong auto-ionizing transition at 431.10 nm. The complexity of the observed ionization spectrum when the irradiance is high indicates that spectral interferences need to be carefully considered in analytical applications.