Precision Laser Spectroscopic Measurement of HELIUM(1S2S Singlet-S Binding Energy

The purpose of this thesis is to precisely measure and compare with theory the absolute binding energy of the 1s2s ^1S state of ^4 He atom, relative to the ground state of the ^4He^+(1s) ion. The Doppler-free two-photon transitions from the 2 ^1S to the n^1D states (n from 7 to 20) are excited from a singlet metastable helium beam for the first time. The energy intervals between the 2^1S and the n^1 D states are precisely determined by comparison of the absolute wavelength of the transition with a standard laser (iodine stabilized He-Ne laser with an accuracy of 1.6 parts in 10^{10}) in a measuring etalon. By using the very well known binding energies of the n^1D states precisely calculated by Drake, the binding energy of the 2 ^1S state is 32 033.228 831(5) cm ^{-1}. The accuracy of 1.6 parts in 10^{10} is over 30 times better than any other experiment. The experimental result disagrees with the theoretical calculation by over 600 times the error. The binding energy of 3^1 P state, derived from a combination of this result and the NIST measurement, is 12 101.304 037(45) cm ^{-1}. which is in agreement with Drake's theory within the error.