Doppler-free two-photon spectroscopy in the vacuum ultraviolet: helium 1(1)S-2(1)S transition

We describe techniques for laser spectroscopy in the vacuum-UV (VUV) spectr al region that combine high spectral resolution with high absolute accuracy . A nearly transform-limited nanosecond laser source at 120 nm is construct ed with difference-frequency mixing. This source is used to perform the fir st, to our knowledge, Doppler-free VUV measurement. We measure the inherent ly narrow 1(1)S-2(1)S two-photon transition in atomic helium with a spectra l resolution of 7 parts in 10(8) (180 MHz), the narrowest line width so far observed at such short wavelengths. Careful measurements of optical phase perturbations allow us to determine the absolute frequency of the line cent er to a fractional uncertainty of 1 part in 10(8). Improvements now in prog ress should reduce this uncertainty to 2 parts in 10(9). (C) 2000 Optical S ociety of America [S0740-3224(00)01009-2].