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  • PHY 350 Laser Spectroscopy of Rubidium

    Significance of the experiment

    This experiment explores many aspects of atomic spectroscopy in a vapor cell. In a quantum mechanical system, confining electrons results in discrete energy levels. In the case of atoms, electrons are confined in a potential created by the Coulomb interaction with protons in the nucleus. An atom with more than one proton and electron can quickly get complicated. Each electron has an interaction term with every other electron as well as proton. Apart from the basic Coulomb interaction, electrons have both intrinsic and orbital angular momentum that shifts and split the discrete energy levels. The nucleus itself can have intrinsic angular momentum further shifting and splitting the energy levels.

    To explore these energy levels, a laser beam with defined, tunable photon energy is used. When the photon energy matches the energy of the atomic transition, the atomic system will absorb a photon from the laser and get transferred to the higher energy states. Scanning the photon energy in a linear controlled fashion allows the experimenter to determine all of the energy levels and splittings. Using quantum theory, we can then extract information about the atomic system such as nuclear spin and the angular momentum of the electrons in each of the states.

    In addition to all of these interactions, the atoms are also have thermal energy. If an atom has a velocity, the atomic resonance is shifted according to classical Doppler theory.

    In this lab, you will learn about all of the mechanisms that shift and split the energy levels of the atom as well as Doppler theory to account for (and remove) those Doppler effects.

    Conceptual Introduction

    Please carefully read the Introduction to Laser Spectroscopy Experiments on the TeachSpin website. You will be doing an enhanced version of experiments like those described in the TeachSpin introduction. Our enhanced version uses much of the same optical apparatus, but replaces the somewhat troublesome diode laser with a tunable Ti- Sapphire laser from Will William’s research lab.

    In addition, familiarize yourself with the detailed description of Saturated Absorption Spectroscopy of Rubidium in (Melissinos 2003) on pages 243 - 250.

    Questions for first lab class

    A few short questions about this experiment and the apparatus are listed below. You will find it helpful to review the figures on the TeachSpin webpage and in (Melissinos 2003) Section 6.6 to answer these questions. Write short answers to these questions in your lab book and bring the lab book to class.

    1. What is an absorption peak?

    2. Why does an absorption peak look like a dip in a graph of transmitted light versus laser frequency?

    3. What is the difference between a pump beam and a probe beam?

    4. What does saturated absorption mean?

    5. How could you detect saturated absorption, experimentally?

    6. What determines the frequency width of an absorption peak? What is the mathematical form of the peak, and what is the physical significance behind the shapes?