Isotope shift in the neon ground state by extreme-ultraviolet laser spectroscopy at 74 nm

Narrow-band extreme-ultraviolet (xuv) radiation in the range 73.5--75.5 nm has been produced in Xe by generating the third harmonic of light from a frequency-doubled, pulsed blue-dye laser. In a one-xuv-photon plus one-uv-photon ionization process both 2p^6-2p⁵3s[3/2]₁ and 2p^6-2p⁵3s'[1/2]₁ transitions in ²⁰Ne and ²²Ne were observed. Frequencies were calibrated using a tellurium absorption spectrum of the fundamental blue light. Frequencies of 134 459.33(5) cm^-1 for the 2p^6-2p⁵3s[3/2]₁ and 135 888.73(5) cm^-1 for the 2p⁶-2p⁵3s'[1/2]₁ transition in ²⁰Ne are obtained. In both transitions the isotope shift (²⁰Ne-²²Ne) was measured. The values are -0.014(3) cm^-1 and +0.003(3) cm^-1, respectively. Combined with existing values for the 3s[3/2]₁ and 3s'[1/2]₁ level shifts, a value for the ground-state specific mass shift of 9.54(15) GHz is deduced, in good agreement with a theoretical value of 10.1 GHz.