Observation of the 6S1/2-7S1/2 single-photon transition of cesium induced by an external d. c. electric field

2014 We have observed the direct single-photon electric dipole excitation of the 7 2S1/2 state of cesium, by exciting the 6 2S1/2 ground state in an external d. c. electric field of 10 to 1 000 V/cm with a single-mode, c. w., tunable dye laser. The resonance was detected by monitoring the decay fluorescence from the excited state. We verified the selection rules obeyed by the hyperfine transitions which depend on the relative orientation of the linear polarization of the beam with respect to the field. When they are orthogonal, the line pattern reflects the existence of a spindependent part of the polarizability induced by spin-orbit effects, the size of which was measured with respect to that of the scalar part. Results in very weak fields show that the oscillator strength of the 6S-7S transition is smaller than 2 x 10-13. We have detected the hyperfine structure of the 681/2-781/2 radiative transition of neutral cesium atoms placed in an external d. c. electric field using single-photon electric dipole excitation from the 681/2 ground state by a single-mode c. w. tunable dye laser. In the conditions of our experiment the transition probability was smaller by 8 to 12 orders of magnitude than that of the resonance line. In spite of the Doppler broadening the linewidth (0.75 GHz in our experiment) was sufficiently small to resolve fully the excitedstate and ground-state hyperfine structures. We were able to verify the different selection rules obeyed by the transition versus the angle between the applied electric field and the linear polarization of the exciting beam. We believe this is the first observation of a direct excitation from an alkali Sij2 ground-state to the first-excited 5~~2 state and of the typical selection rules for electric dipole transitions induced by an electric field between hyperfine levels of two S states. Our measurements also yield a stringent upper limit to the transition rate in zero field. High members (n N 15-30) of the forbidden series no Sl/2 ~ n81/2 have been observed in the absorption (*) Laboratoire associe au Centre National de la Recherche Scientifique. spectrum of K, Rb and Cs in electric fields of 300 to 1 000 V/cm [1]. For low quantum numbers the mixing of states of different I is much less effective, and direct excitation using conventional sources does not seem feasible. Recent theoretical investigations joint to the new possibilities offered in spectroscopy by tunable lasers have given a renewal of interest to those transitions. In a search for parity violating effects induced in atomic physics by weak neutral currents, particular attention was paid to twice forbidden magnetic dipole transitions nSI/2-n’ SI/2 of heavy alkali atoms [2]. The present paper, although devoted to effects induced by an external electric field, can be considered as a preliminary step into the rather difficult experimental investigation that consists in probing parity conservation in the excitation of a nSt~2-n’51~2 radiative transition. Our experiment was performed on Cs vapor at high density (10-2 to 10-1 torr), contained in a glass cell without buffer gas ; the cell was equipped with two plane parallel electrodes made of stainless steel and distant of 0.5 cm. The d. c. voltage applied could reach at least 500 V without discharge in the cell. The tunable light source was a Coherent Radiation model 490 jet-stream dye laser using fluorescein added Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:01975003607-8018900