Isotope shift of the 23S1-23P transition in helium

Abstract
The 4He-3He isotope shift of the 23S1-23P transition is directly measured by Doppler-free absorption spectroscopy with a colour-centre laser operating in single mode around lambda =1.08 mu m. The splitting between the (23S1-23P0) line of 4He and the (23S1, F=3/2-23P0) line of 3He is (811+or-3) MHz, leading to a shift of (33668+or-3) MHz for the transition 23S1-23P between 4He and 3He. This value compares well with previous experimental data but is more accurate. The agreement with the calculated isotope shift is good.