Measurement of photoionization cross-sections from 6s5d ¹D₂ excited state of ytterbium at and above the first ionization threshold.

Experimental investigations of photoionization cross sections from the even-parity excited state 6s5d 1D2 of atomic ytterbium are reported. A heat pipe-cum-linear thermionic diode ion detector, working in space charge limited mode, has been used for generating the atomic vapors of Yb. An Nd:YAG pumped narrow bandwidth (~ 0.2 cm-1) Hanna-type dye laser, charged with LDS-698 dye and tuned at 722.6 nm, was used to excite electrons via two-photon transition 6s2 1S0 → 6s5d 1D2. The excited state population is then promoted to the ionization threshold at 439.2 nm and above the threshold at 355 nm and 266 nm wavelengths. To measure the photoionization cross section from the excited state, the saturation technique has been employed, in which the intensity of the exciting laser (722.6 nm) 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 the threshold region was recorded as a function of ionizing laser intensity, which yield the cross section in the range 230± 13 Mb to 32.6 ±1.2 Mb. To the best of our knowledge, measurement of photoionization cross sections from the 6s5d 1D2 excited state of atomic ytterbium is the first time reported.