Measurement of Absolute Photoionization Cross Sections from the 7s and 7p Excited States of Atomic Ytterbium

New studies of the absolute photoionization cross sections from the two excited states (6s7s ¹S₀ and 6s7p ³P₂) of neutral ytterbium (Yb I) are reported. The saturation technique has been implemented, in which the intensity of the exciting laser is kept fixed while that of the ionizing laser is varied using neutral density filters. The photo-ion signal from the excited state at and above threshold region were recorded as a function of ionizing laser intensity. Two Hanna-type dye lasers along with a thermionic diode detector working in space-charge limited mode were used for the measurements. In the first set of experiments, the population from the ground state (6s^{2 1}S₀) was promoted to the 6s7s ¹S₀ excited state (34350.65 cm⁻¹) via two-photon transition using the dye laser wavelength tuned at 582.2 nm. The ionization limit above the excited state lies at 50443.08 cm⁻¹ which is accessed via single-photon transition using 621.4 nm dye laser wavelength. A total of four channels in the continuum at 0.07 eV, 0.11 eV, 0.34 eV, and 1.49 eV excess photoelectron energies above the ionization threshold (I.P.) are accessed. For the former two, dye lasers tuned at 600 nm and 590 nm are used whereas for the later ones, frequency-doubled (532 nm) and –tripled (355 nm) outputs of the Nd:YAG laser are used. The photoionization cross sections at and above the I.P. have been measured using the saturation technique as employed in the third set, photoionization cross sections from the 6s7p ³P₂

excited state have been measured at and above the I.P. For 6s7p ³P₂ state (38551.2 cm⁻¹), the I.P. is accessed via 840 nm dye laser wavelength and all the continuum channels are accessed using 720 nm, 590 nm, and 355 nm ionizing wavelengths at 0.25eV, 0.63 eV, and 2.02 eV photoelectron energies, respectively. To the best of our knowledge, all the photoionization cross section measurements are reported for the first time.