Optical Pumping of Rb Vapor
About the experiment
This experiment is quantum mechanics in action. In a sample of Rb in a magnetic field, certain excited Zeeman levels are selectively populated using circularly polarized light at the transition frequency. Application of microwave radiation at the frequency of the Zeeman splitting will depopulate the "pumped" level via stimulated emission, and the frequency can be precisely measured by watching for the change in absorption of the pumping light.
Optical pumping has been widely used to exploring atomic energy states, atomic transitions, and atomic collisions. Several Nobel Prizes have been awarded for work in this area (eg. Alfred Kastler 1966). When a sample of gaseous atoms is placed in a static magnetic field, the electronic states undergo Zeeman energy level splittings in addition to fine-structure and hyperfine-structure splittings. By applying polarized light at the proper frequency, we can induce transitions from ground state levels to excited state energy levels. The atoms then decay to higher ground state levels until we have "pumped" all of them into the same (highest) ground state energy level. At this point we can see an increase in light passing through the sample because no more photons can be absorbed by the gas. However, when we apply a radiofrequency signal of just the right energy to stimulate transitions to a lower level, we see a sudden decrease in the light signal as photon absorption reoccurs. By determining the frequency of the RF signal we gain information about the atomic energy levels.
Our apparatus uses electromagnetic waves at optical- and radio-frequencies in the presence of a uniform, constant magnetic field. The atom you will be studying is rubidium because of its simple hydrogen-like qualities - a "one-electron atom" with core electrons forming closed shells. You will measure the energy splittings of the two isotopes of rubidium, the nuclear moments of these isotopes, and the strength of the earth's magnetic field in the laboratory. You will also observe such interesting phenomena as the recovery time for pumping and "Rabi oscillations".
What you will learn
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Basic Physics for the operation of lasers and atomic clocks
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A precise technique for measuring atomic energy levels and nuclear spins
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Techniques that will prepare you to perform the Diode Laser Spectroscopy (Doppler Free Spectroscopy) experiment
Preparation
Required reading:
The writeup describes a measurement using a laser to pump the cell. However, it's perfectly feasible to do it with a rubidium lamp.
Recommended reading:
- Reference Rubidium Diagrams Rb Energy Levels
- Polarization Tutorial
- Optical Pumping Experiment - Adv. Lab at UC Berkeley
- Optical Pumping Experiment - Junior Lab at MIT
- Breit-Rabi Diagram (Mathematica Demonstrations)
- Optical Pumping by Arnold L. Bloom, Scientific American, Oct. 1960, p 72
- Optical Pumping by Robert L. de Zafra, American Journal of Physics, 28 (1960), pp 646-654
- Optical Pumping Theory and Experiment by Reuben Benumof, American Journal of Physics, 33 (1966), p 151
Related Experiments:
Supplementary materials
Instruments:
