International Workshop on Atomic Interactions in Laser Fields - Abstracts

Anomalous depolarization of the magnetic dipole-electric quadrupole transitions in atomic ⁸⁷Rb

^lS.B. Bayram, ¹M.D. Havey, ²D.V. Kupriyanov, and ²J.M. Sokolov

^lPhysics Department, Old Dominion University, Norfolk, VA 23529
²St. Petersburg State Technical University, St. Petersburg, Russia

For atomic systems, levels which radiatively decay solely by a forbidden process typically have lifetimes greater than a microsecond or so. Such transitions are normally those that have multipole character other than electric dipole. Experimental determination of forbidden transition strengths in atomic systems is a challenging enterprise, and new approaches, as well as refinement of existing ones, are required to make progress in this area. Generally, the strength of these weak processes is also difficult to accurately compute, and so reliable measurements of them provide challenging benchmarks to atomic structure calculations. In the present paper, we report on an experimental study of the two-photon 5s²S_1/2→5p²P_3/2→8p ²P_j' transitions in atomic Rb. The p-p transition is electric-dipole forbidden, but has allowed magnetic dipole (M1) and electric quadrupole (E2) amplitudes. We exploit the fact that the E2 and M1 contributions to the transition probability have different linear polarization dependence on the excitation rates. For the 5p²P_3/2→8p²P_1/2 transition, P_L = -60% (M1) and P_L = +100% (E2), while for the 5p²P_3/2→8p²P_3/2 transition, P_L = +75% (M1) and P_L = 0% (E2). Measurement of P_L for the transitions allows the relative M1 and E2 contributions to be determined. A linear polarization degree for each transition was measured for a range of laser power, Rb density, and for variations in applied electric and magnetic fields. For the 5p²P_3/2→8p²P_1/2 transition, it was found that P_L increased with increasing Rb density. For the 5p²P_3/2→8p²P_3/2 transition, P_L decreased with increasing Rb density. The anomalous density dependence is likely due to variation of effective laser 1 power with density. The strong resonance radiation induces mixing of the np levels, an effect which mainly increases the M1 part of the transition amplitude, and can lead to the observed effect. For high density, when there is strong absorption of laser 1, excitation takes place in the near wings of the transition, reducing the effective laser 1 coupling strength. Then values for the relative M1 and E2 transition strengths were determined to be I_T^E2/I_T^M1 = 4.3(3) for the 5p²P_3/2→8p²P_l/2 transition.

This work is supported by the U.S. Civilian Research and Development Foundation and the National Science Foundation.