Upper limit on the magnetic dipole contribution to the 5p-8p transition in Rb by use of ultracold atom spectroscopy

We report on hyperfine-resolved spectroscopic measurements of the electric-dipole-forbidden 5p3/2→8p1/2 transition in a sample of ultracold Rb atoms. The hyperfine selection rules enable the weak magnetic dipole M1 contribution to the transition strength to be distinguished from the much stronger electric quadrupole E2 contribution. An upper limit on the M1 transition strength is determined that is about 50 times smaller than an earlier experimental determination. We also calculate the expected value of the M1 matrix element and find that it is less than the upper limit extracted from the experiment.