Doppler-free two-photon spectroscopy in the vacuum ultraviolet: Helium 1¹S-2¹S transition

We describe techniques for laser spectroscopy in the vacuum-UV (VUV) spectral region that combine high spectral resolution with high absolute accuracy, A nearly transform-limited nanosecond laser source at 120 nm is constructed with difference-frequency mixing. This source is used to perform the first, to our knowledge, Doppler-free VUV measurement. We measure the inherently narrow I¹S-2¹S two-photon transition in atomic helium with a spectral resolution of 7 parts in 10⁸ (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 center to a fractional uncertainty of 1 part in 10 ⁸. Improvements now in progress should reduce this uncertainty to 2 parts in 10⁹.