Laser Spectroscopic Measurement of the ^87Rb–^85Rb D_1-Line Isotope Shift

High-resolution (few MHz) absorption spectroscopy was demonstrated by using an atomic beam and a laser. The Doppler absorption width of natural-abundance Rb was reduced from ≈570 to 15 MHz (5.7-MHz natural width) by using an atomic beam. A magnetic field of about 75 kOe was used to scan the six 85Rb and four 87Rb mI components of the D1 line ( 52S12 to 52P12; 7947.64 Å in zero field) into coincidence with the 7944.66-Å 1-μs 202Hg ii laser pulses. From the known ground- and excited-state hyperfine constants and the Breit–Rabi formula, we conclude that the 87Rb absorption spectrum is shifted up by 77±3 MHz relative to that of 85Rb. This uncorrected isotope shift can be compared with previous measurements of 105±15 and 77.7±2.1 MHz. Improved stability and a narrower atomic beam would permit reduction of the isotope-shift and hyperfine-constant errors to below 1 MHz.