1(6/1/09)Optical Pumping of RubidiumAdvanced Laboratory, Physics 407University of WisconsinMadison, WI 53706AbstractA Rubidium high-frequency lamp is used as the pumping light source to excite Rubidium atomsin a sample cell. The pumping source light is filtered to transmit the D1Rb line and polarizersare used to produce circular polarization. The circular polarization pumping mechanismselectively repopulates the sample cell ground state Rb Zeeman hyperfine levels away fromthermal equilibrium populations according to whether the pumping light is left or right circularlypolarized. The sample cell is then irradiated using RF coils with the frequency of the hyperfinelevels, changing the transparency of the sample cell Rb vapor. The change in sampletransparency is measured as a function of RF frequency in a Silicon photodiode detector. Thistechnique is used to determine the nuclear spin and magnetic moments of the Rb87(I=3/2) andRb85(I=5/2) isotopes.2Optical pumping of RubidiumDr. Johannes Recht and Dr. Werner KieinOptical pumping is a process used in high frequency spectroscopy which was developed byA. Kastler. It allows the spectroscopy of atomic energy states in an energy region which isnot accessible by means of direct, optical observation. Kastler was awarded the Nobel Prizefor 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 duringthis process. With the help of high-frequency spectroscopy, it is possible to observe transitionsbetween the Zeeman levels of hyperfine states in weak magnetic fields, where the spacingbetween neighbouring Zeeman states is less than810eV.When the levels of these states are known - the energies can be calculated with 1st orderquantum mechanical perturbation calculation [1] - one also obtains a method for measuring weakmagnetic fields with almost the same accuracy as that with which the irradiating frequency canbe determined, i.e. with an accuracy of 1 in810.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 highereducation establishments, if not for secondary school instruction as well.1 Physical fundamentals1.1 Energy level schemeRubidium is an alkali metal in the first main group of the periodic table. The low energy states ofthis element can be described very well using Paschen notation.The rubidium atom consists of a spherically symmetrical atomic residue with orbital spin 0, andone optically-active electron with an orbital angular momentum 0, 1, 2… and an electron spin of1/2. The line structure of the energy states is illustrated in Fig. 1. It is caused by the half-integralelectron 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 totalangular momentum quantum number J = 1/2. Due to this coupling, the first excited level withL=1 splits up into a21/2Pand a23/2Pstate. Both states can be easily excited in a gas discharge.During the transitions from the first excited states21/2Pand23/2Pinto the ground state21/2S, thedoublet D1and D2characteristic of all alkali atoms is emitted. For rubidium, the wavelengths ofthe transitions are 794.8 nm (D1line) and 780 nm (D2line).3Fig. 1 Energy Level Scheme for Rb87The hyperfine interaction, caused by the coupling of the orbital angular momentum J with thenuclear spin I, leads to the splitting of the ground state and the excited states. The additionalcoupling provides hyperfine levels with a total angular momentum F = IJ. For87Rb with anuclear spin I = 3/2, the ground state21/2Sand the first excited state21/2Psplit up into twohyperfine levels with the quantum numbers F = 1 and F = 2. Compared with the transitionfrequency of144 10 Hz between21/2Pand21/2S, the resulting splitting of the ground state and theexcited states is much smaller. For the ground state, this hyperfine splitting of96.8 10 Hz isapprox. 5 powers of 10 smaller than the fine structure splitting.In the magnetic field, an additional Zeeman splitting (Fig. 1) into 2F +1 sub-levels respectively isobtained. For magnetic fields of approx. 1 mT, the transition frequency between neighbouringZeeman levels of a hyperfine state is68 10 Hz, i.e. another 3 powers of 10 smaller than thehyperfine splitting. The energy or frequency relationships between the individual states are ofparticular significance in understanding optical pumping.1.2 Optical pumpingThe process can be explained in more detail using the energy level scheme of87Rb (Fig.1) asA reference. The transitions from21/2Sto21/2Pand23/2Pare electrical dipole transitions. Theyare only possible if the selection rulesFm 0 orFm 1 have been fulfilled.The transitions between the Zeeman levels are detected using a method discovered by A. Kastlerin 1950 which will be described in the following [1]. The D1line emitted by the rubidium lampdisplays such a high degree of Doppler broadening that it can be used to induce all permissible4transitions between the various Zeeman levels of the21/2Sand21/2Pstates. If an absorption celllocated in a weak magnetic field and filled with rubidium vapour is irradiated with the +circularly-polarized component of the D1line, the absorption taking place in the cell excites thevarious Zeeman levels which are higher byFm 1 . However, the excited states decayspontaneously to the ground state and re-emit or light in all spatial directions in accor-dance with theFm 0 orFm 1 selection principle.The irradiating, circularly-polarized light effects a polarization of the atomic vapour in theabsorption cell. This can be interpreted as follows: during the process of absorption, thepolarized light transmits angular momentum to the rubidium atoms. The rubidium vapour ispolarized and thus magnetized macroscopically.Without optical irradiation, the difference between the population numbers of the variousZeeman levels in the ground state is infinitesimally small, due to its low energy spacing inthermal equilibrium. However, the irradiation with light results in a strong deviation from thethermal equilibrium population. In other words, the population distribution changes as a result ofoptical