Spectroscopy of H-Like Atoms

Saturated-absorption, or “Doppler-free,” spectroscopy is a widespread and useful tool for modern atomic physics. As the name suggests, it allows the effective removal of the Doppler broadening in absorption lines of an atomic vapor. The basic idea is to use counter-propagating “pump” and “probe” beams of the same frequency (generated by the same laser) to select for and probe only those atoms whose velocity component parallel to the beams is zero, i.e. those atoms moving at right angles to the laser beams. This allows us to do spectroscopy limited only by the natural linewidth of the atomic transitions, revealing features of the absorption lines that would otherwise be unobservable in a gas at room temperature.

In first part of this lab, participants will use a Teachspin laser-diode apparatus and a cell of Rubidium vapor to learn how to do Doppler-free spectroscopy in hydrogen-like atoms. After learning the basics of diode lasers and saturated-absorption spectroscopy, they will calibrate the system using an asymmetric Michelson interferometer and measure the hyperfine splitting of the P3/2 levels of both 85Rb and 87Rb.

Optical resonance in atoms forms the basis of the second part of this lab. Even though the effect is strictly a quantum one, it can be modeled semi-classically. Thus, the system exhibits all of the familiar features of a driven harmonic oscillator, including resonance and the associated phase shift. In this case, the phase shift can be observed interferometrically, and doing so provides an opportunity to cover several useful and interesting topics in experimental optics and atomic physics.

Participants will use the same Teachspin laser-diode apparatus and Rubidium cell they used in the first part to observe both the absorption and refractive index change of a gas as they tune the frequency of the laser through atomic resonance. First, they will measure the absorption of the gas as a function of temperature and extract the latent heat of vaporization of atomic Rubidium using the Clausius-Clapeyron relation. Then they will place the cell in one arm of a Mach-Zender interferometer and, by observing the resulting fringe pattern, measure the change in index of refraction due to resonance. The absorption and index of refraction are fundamentally related through the Kramers-Kronig relations, and we will cover some of the theory of this as well as the measurement technologies.