Speaker: Elizabeth DenHartog, University of Wisconsin-Madison
Abstract: In this talk I will describe the laboratory astrophysics program of the Jim Lawler group at the University of Wisconsin – Madison, focusing on the laboratory techniques and instruments used for the measurement of transition probabilities. Transition probabilities are determined from the combination of radiative lifetimes and branching fractions. Radiative lifetimes are measured using time-resolved laser-induced fluorescence on a slow atomic/ionic beam. This experiment has been extremely productive over the 3+ decades of its operation. The atomic beam source at the heart of this experiment was designed by Jim in the early 1980’s, and can produce a gas phase neutral and singly-ionized sample of any metal, and many non-metals. This source is a big reason for the success and productivity of this experiment. Branching fractions are measured in emission using Fourier transform or grating spectroscopy, or a combination thereof. Fourier Transform spectrometers have many advantages for branching fraction work, but have one major drawback. As with any interferometric device, the noise from all lines in the spectrum distribute evenly throughout the spectrum, making the measurement of very weak branches difficult to impossible. I will describe a more recent instrument of Jim’s design – the 3 meter echelle spectrograph – which is ideal for the measurement of the branching fractions of very weak spectral lines.