Non-perturbative Probe Laser-induced Fluorescence Spectroscopy of Laser-cooled Ca-Plasma in a Linear Radiofrequency Ion Trap

A non-perturbative probe laser induced fluorescence (LIF) system has been developed to observe a laser-cooled one-component plasma. Since the undistorted spectra can be obtained using this measurement system, a quantitative evaluation of the spectrum shape of the laser-cooled plasma was achieved. The spectra are decomposed into the Doppler and Lorentzian components using a Voigt function. The ion temperature is calculated as 4.5 K from the Doppler component, and the Coulomb coupling parameter Gamma is calculated as 0.5 using the ion temperature. On the other side, the Lorentz width is unexpectedly larger than the natural linewidth by 70 MHz or more, and much larger than the ion-ion collisional broadening derived from the Debye screening assumption. The results show that the Debye screening assumption is already incorrect at Gamma = 0.5.