Multiphoton ionization spectroscopy of alkaline earth atoms

Abstract

By using multiphoton ionization spectroscopy, hundreds of previously unobserved states have been identified in the alkaline earth atoms Ca, Sr and Ba. In this technique, pulsed, tunable dye lasers are focused into a pipe containing the atomic vapor. A simple wire probe, inserted into the hot vapor, readily detects ions produced as a result of the multiphoton excitation. Excitation spectra from various well-defined initial states are obtained by recording the ion signal as a function of laser wavelength1S0. In a single laser experiment, one obtains the two-photon excitation spectrum from, the ground state of the alkaline earths, to the Rydberg series of bound even-parity 1S01D2 and 3D2 states, most of which have not previously been identified by conventional spectroscopy. In a two-laser version of this experiment, we have made the first observation of Rydberg series of even-parity autoionizing states. In a three laser variation, the excitation spectrum from the first excited 3S1 levels (of Ca, Sr and Ba) reveals the P Rydberg series. Since transitions from the 1S0 ground state to these series are spin-forbidden, they have not previously been seen in absorption spectra.