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Measurement of the He ground state Lamb shift via the two-photon 11S - 21S transitionWe have extended two-photon Doppler-free spectroscopy to the vacuum ultraviolet spectral region, to accurately measure the He 11S - 21S transition at 120 nm. Our result is 4 984 872 315(48) MHz. This yields a ground state Lamb shift of 41 104(48) MHz, in fair agreement with theory and other experiments. This approach has the potential for significantly better accuracy once improvements in the laser and the wavelength metrology are implemented.The He ground state provides an ideal testing ground for quantum electrodynamics (QED) calculations of the Lamb shift in two-electron, three-body systems. The "Lamb shift" refers to all QED and finite mass corrections beyond those of order (Z a)2. The 120 nm radiation is generated by harmonic upconversion in crystals and gases. We pulse amplify a cw laser to 50 mJ/pulse at 842 nm. The 4th harmonic is generated by doubling twice in BBO crystals. The 7th harmonic at 120 nm is generated by sum-difference frequency mixing in a Kr/Ar gas cell. The 120 nm radiation is focused into a supersonic He expansion. A small mirror retroreflects the 120 nm radiation in a standard Doppler-free arrangement. The figure below shows an average of 3 scans over the Doppler-free He transition at 120 nm. The solid circles are the data. The heavy solid line is a model lineshape fit to the data. The thin solid line at the top shows fringes from a marker etalon. At 120 nm theses fringes appear every 525 MHz. This scan shows the narrowest observed atomic transition in the VUV to date, with a fractional width of 7×10-8. 
The article describing this measurement has been published in Phys Rev. Lett. 80(16). Download the article (85 pdf format) describing the measurement. Or contact Thomas Lucatorto. to have a printed copy mailed to you.
Online: May 1998 - Last update: June 2000 |
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