Three-colour four-photon resonant excitation of the even-parity autoionizing resonances in YbI

We report new data on the even-parity autoionizing resonances due to the 4f inner-shell excitation in ytterbium using a three-colour four-photon reson ant excitation technique. The spectra were observed using three dye lasers simultaneously pumped by a common Nd:YAG laser in conjunction with a thermi onic diode ion detector. More than 50 new levels corresponding to the 4f(13 )6s(2)nl Rydberg series have been detected. The data were analysed in the l ight of the spectral lineshapes, parent ion configurations and quantum defe ct approach.