Abstract(4/21/05) Optical Pumping of Rubidium Advanced Laboratory, Physics 407 University of Wisconsin Madison, WI 53706 Abstract A Rubidium high-frequency lamp is used as the pumping light source to excite Rubidium atoms in a sample cell. The pumping source light is filtered to transmit the D1 Rb line and polarizers are used to produce circular polarization. The circular polarization pumping mechanism selectively repopulates the sample cell ground state Rb Zeeman hyperfine levels away from thermal equilibrium populations according to whether the pumping light is left or right circularly polarized. The sample cell is then irradiated using RF coils with the frequency of the hyperfine levels, changing the transparency of the sample cell Rb vapor. The change in sample transparency is measured as a function of RF frequency in a Silicon photodiode detector. This technique is used to determine the nuclear spin and magnetic moments of the Rb87 (I=3/2) and Rb85 (I=5/2) isotopes. 1Optical pumping of Rubidium Dr. Johannes Recht and Dr. Werner Kiein Optical pumping is a process used in high frequency spectroscopy which was developed by A. Kastler. It allows the spectroscopy of atomic energy states in an energy region which is not accessible by means of direct, optical observation. Kastler was awarded the Nobel Prize for Physics for this in 1966. Transitions are induced in a low density atomic vapour by means of high-frequency irradiation. These transitions can be detected through a change in the optical absorption which occurs during this process. With the help of high-frequency spectroscopy, it is possible to observe transitions between the Zeeman levels of hyperfine states in weak magnetic fields, where the spacing between neighbouring Zeeman states is less than 810− eV. When the levels of these states are known - the energies can be calculated with 1st order quantum mechanical perturbation calculation [1] - one also obtains a method for measuring weak magnetic fields with almost the same accuracy as that with which the irradiating frequency can be determined, i.e. with an accuracy of 1 in . 810 In addition, this process allows the experimental observation of the anomalous Zeeman effect. It was considered appropriate to develop an easy-to-handle measuring system, at least for higher education establishments, if not for secondary school instruction as well. 1 Physical fundamentals 1.1 Energy level scheme Rubidium is an alkali metal in the first main group of the periodic table. The low energy states of this element can be described very well using Paschen notation. The rubidium atom consists of a spherically symmetrical atomic residue with orbital spin 0, and one optically-active electron with an orbital angular momentum 0, 1, 2… and an electron spin of 1/2. The line structure of the energy states is illustrated in Fig. 1. It is caused by the half-integral electron spin, which leads to a multiplicity of 2, i.e. a doublet system. The ground state is an S-state; the orbital angular momentum here is L =0. The spin-orbit coupling results in a total angular momentum quantum number J = 1/2. Due to this coupling, the first excited level with L=1 splits up into a and a state. Both states can be easily excited in a gas discharge. During the transitions from the first excited states and into the ground state , the doublet D21/2P23/2P21/2P23/2P21/2S1 and D2 characteristic of all alkali atoms is emitted. For rubidium, the wavelengths of the transitions are 794.8 nm (D1 line) and 780 nm (D2 line). 2Fig. 1 Energy Level Scheme for Rb87 The hyperfine interaction, caused by the coupling of the orbital angular momentum J with the nuclear spin I, leads to the splitting of the ground state and the excited states. The additional coupling provides hyperfine levels with a total angular momentum F = I±J. For 87Rb with a nuclear spin I = 3/2, the ground state 2 and the first excited state split up into two hyperfine levels with the quantum numbers F = 1 and F = 2. Compared with the transition frequency of Hz between and , the resulting splitting of the ground state and the excited states is much smaller. For the ground state, this hyperfine splitting of Hz is approx. 5 powers of 10 smaller than the fine structure splitting. 1/2S21/2P14410×21/2P21/2S96.8 10× In the magnetic field, an additional Zeeman splitting (Fig. 1) into 2F +1 sub-levels respectively is obtained. For magnetic fields of approx. 1 mT, the transition frequency between neighbouring Zeeman levels of a hyperfine state is Hz, i.e. another 3 powers of 10 smaller than the hyperfine splitting. The energy or frequency relationships between the individual states are of particular significance in understanding optical pumping. 6810× 1.2 Optical pumping The process can be explained in more detail using the energy level scheme of 87Rb (Fig.1) as A reference. The transitions from 21/2S to and are electrical dipole transitions. They are only possible if the selection rules 21/2P23/2PFm0∆= or Fm1∆=± have been fulfilled. The transitions between the Zeeman levels are detected using a method discovered by A. Kastler in 1950 which will be described in the following [1]. The D1 line emitted by the rubidium lamp displays such a high degree of Doppler broadening that it can be used to induce all permissible 3transitions between the various Zeeman levels of the 21/2S and states. If an absorption cell located in a weak magnetic field and filled with rubidium vapour is irradiated with the σ21/2P+ circularly-polarized component of the D1 line, the absorption taking place in the cell excites the various Zeeman levels which are higher by Fm1∆=+. However, the excited states decay spontaneously to the ground state and re-emit π, σ+ or σ− light in all spatial directions in accor-dance with the or selection principle. Fm∆=0 1Fm∆=± The irradiating, circularly-polarized light effects a polarization of the atomic vapour in the absorption cell. This can be interpreted as follows: during the process of absorption, the polarized light transmits angular momentum to the rubidium atoms. The rubidium vapour is polarized and thus magnetized macroscopically. Without optical irradiation, the difference between the population numbers of the various Zeeman levels in the ground state is infinitesimally small, due to its low