Rydberg states via CPmmW spectroscopy

Rydberg-Rydberg transitions of Ca atoms are directly observed by chirped-pulse millimeter-wave spectroscopy, which is a form of broadband, high-resolution, free induction decay (FID) spectroscopy with accurate relative intensities. At moderate to high number densities ($\sim10^6cm^{-3}$), interactions between many Rydberg atoms are mediated by an AC electric field, absorbing and radiating cooperatively. A semiclassical model describes several significant time-domain and frequency-domain cooperative effects in two-level systems and $\Lambda$-type three-level systems. Experimental evidence that supports this model will be discussed. A new experiment, employing the buffer gas cooling technique has been constructed and I expect to present preliminary results. In partically, the $>$100-fold increase in number density will permit study of ``pure electronic'' spectra of Rydberg molecules, such as BaF. We expect to produce $10^8$ state selected core-nonpenetrating Rydberg molecules in a single pulse of a laser-laser-mm-wave excitation sequence.