Studies of narrow autoionizing resonances in gadolinium

The autoionization (AI) spectrum of gadolinium between the first and second limits has been investigated by triple-resonance excitation with high-resolution cw lasers. A large number of narrow AI resonances have been observed and assigned total angular momentum J values. The resonances are further divided into members of AI Rydberg series converging to the second limit or other ‘interloping’ levels. Fine structure in the Rydberg series has been identified and interpreted in terms of Jcj coupling. A number of detailed studies have been performed on the interloping resonances: these include lifetime determination by line-shape analysis, isotope shifts, hyperfine structure, and photoionization saturation parameters. The electronic structure of the interloping levels is discussed in terms of these studies. Line-widths generally decrease with increasing total angular momentum and the J=7 resonances are extremely narrow with Lorentzian widths ranging from <1 MHz up to 157 MHz. The strongest resonances are found to have cross-sections of ∼10−12 cm2 and photoionization can be saturated with powers available from cw diode lasers.