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Precision measurement of the 2 (3)S-->3 (3)P transition in 4He.

We report on a precision measurement of the 2 {sup 3}{ital S}--3 {sup 3}{ital P} transition in {sup 4}He. Doppler-free spectra of the transition were observed by exciting metastable helium atoms, produced by a chopped rf discharge, using 389-nm radiation generated by a frequency-doubled Ti-doped sapphire laser. A frequency measurement of the Doppler-free lines was made by interferrometric comparison of the Ti-doped sapphire frequency with that of an iodine-stabilized He-Ne laser using a 1-m evacuated Fabry-Perot etalon. The measurement uncertainty was 2 parts in 10{sup 9} (3 standard deviations), which gives an improvement of a factor of 60 in the term value of the 3 {sup 3}{ital P} level with respect to earlier measurements. This result makes it possible to determine the one electron Lamb-shift correction to the 3 {sup 3}{ital P} state with an accuracy of 0.8%.