beam.html

Julie Rapoport '97 designed and put together a new atomic beam source and vacuum system which will ultimately provide a dense collimated beam of atomic thallium (photo of current system, simplified schematic). With this tool we plan to measure Stark interference and Stark shifts in the ground-state transitions of thallium. Peter Nicholas '98 worked to bring the new beam apparatus on-line (current top view of source showing oven, nozzle, and collimation cone), and Rob Lyman ('99) and Andrew Speck ('00) completed construction of the final beam line elements. Here is a current schematic of the finished product.
In 1999-2000, Andrew Speck '00 installed our laser locking system whereby we lock the diode laser by comparing its frequency to that of a known, stable HeNe reference laser. At this point the only frequency changes occuring are caused by synthesizer tuning of the AOM. Using double-pass geometry (and after frequency doubling), the AOM frequency shif is multiplied four-fold in the UV, allowing us sufficient tuning range to the maximum Stark shift achievable in our system (90 MHz @ 30 kV/cm electric field). Our measurement scheme involves locking the laser to the side of the atomic absorption, then applying simultaneous E-field and AOM frequency changes such that we stay at the same relative point on the side of the line.

In Jan 2001 we obtained the first spectra from the atomic beam apparatus thanks to the hard work of many students, and most recently to postdoc David Richardson, and Paul Friedberg '01 (see here)!
[The width is nearly 10 times narrower than the equivalent data taken using the vapor cell (subject of Rob's thesis and our most recent paper). The thallium oven/source inside the beam unit was at 740 C for this data (our max is about 800 C). We superimpose the cell data for comparison! It is very encouraging to see absorption equivalent to almost one full optical depth in the beam. For this data we 'chopped' both the laser beam and the atomic beam.]

During 2001-2002, with Charlie Doret '02 completing his thesis and at the helm (and PKM on sabbatical leave), we completed apparatus testing, took extensive data and have now completed a new Stark shift measurement with 0.5% accuracy. Among various other honors, Charlie was selected to give an invited talk at the 2002 APS/DAMOP meeting (Williamsburg, VA, May 29-June1). We are happy that he will be around in the lab to help launch a new set of students in the summer of 2002 before heading off to Harvard this fall.: [pdf version of Charlie's thesis defense talk, May 2002]
II. Future M1 spectroscopy in the Atomic Beam
One goal of the atomic beam project is to create a dense beam, and utilize signal processing techniques to allow transmission spectroscopy for the case of the weak M1 transition. In this way the Faraday rotation of 1.28 micron light as it passes through the beam in a known magnetic field can be used as a density normalization method as we measure the absorptivity change upon application of a very large static electric field. Precise measurements of the Stark shift and of the scalar and tensor polarizability in this transition offer another excellent way to independently test the atomic structure calculations. A possible way to increase the IR absorption to more measurable levels would be to introduce a ring enhancement cavity (see below) into this experiment. The use of the chopping wheel and lock-in detection of small absorption signals will be essential here. We are also pursuing the idea of FM-modulation of the diode laser frequency and subsequent demodulation to extract a clean, background-free absorption signal.