A measurement, accurate to 3 ppm, is reported of the 2 3 Pi-2 3 P 2 fine structure in He 4 . An atomic-beam optical-microwave resonance technique was used. Two sets of measurements were made on the transition 2 3 P 1 (mj=0)<->2 3 P 2 (mj = 0). The first set was in a magnetic field of about 100 G ("low" field), where the uncertainties arose from field-dependent asymmetries in the line shape. The second set of measurements was in a field of about 500 G ("high" field), and here the main uncertainties were in the field measurements and the calculation of g factors. The results of the two sets were in good agreement: low field: £(2 3 Pi)-E(2 3 P 2 ) = 2291.188±0.015 MHz; high field: £(2 3 P 1 )-E(2 3 P 2 ) = 2291.197±0.008 MHz; weighted mean: E{2*Pi)-E{2 Z P 2 ) = 2291.195±0.007 MHz. The advantages of the atomic-beam technique and the possible importance of this fine-structure measurement are discussed. measurement of hydrogen hyperfine structure); S. D. Drell and J. D. Sullivan, Phys. Rev. 154, 1477 (1967) (most recent discussion of the theoretical hyperfine structure).
A series of Zeeman resonances has been measured in the 2 P state of He, using the opticalmicrowave atomic-beam magnetic-resonance technique. The results were analyzed in terms of relativistic and motional contributions to the Zeeman effect, and are presented in the form of corrected g factors: gs =gs -(76.0+2.4) &&10 6, gz, =gl +(3.8+9.0) &&10 6, and an additional factor g"=(4.0 +25.0) &&10 6. These are compared with theoretical calculations using quasihydrogenic radial wave functions which give gs'=gs-(79.9+3.5) &&10, gl~--gL, + (1.1+1. 5) &10 and g"= -(3.2 +4. 4) x10 . The values of the experimental g factors are gs = 2.002 243 2 +0. 000 022 4, and gi, =0. 999 867+0. 000009. The validity of the theory and the consistency of the data are discussed. The experimental results were used to reduce the quoted uncertainty in a previously reported fine-structure measurement.The new result is E(2 P&) -E(23P&) = 2291. 196 + 0. 005 MHz.
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