Precise measurement of hyperfine structure in the 5P

We demonstrate a technique to measure hyperfine structure using a frequency-stabilized diode laser and an acousto-optic modulator locked to the frequency difference between two hyperfine peaks. We use this technique to measure hyperfine intervals in the $5{\rm P}_{3/2}$ state of 85Rb and obtain a precision of 20 kHz. We extract values for the magnetic-dipole coupling constant A=25.038(5) MHz and the electric-quadrupole coupling constant B=26.011(22) MHz. These values are a significant improvement over previous results