Temperature and pressure control of cold helium gas above liquid helium for laser spectroscopy of atoms and molecules at 3-30 K

We have developed a novel and simple technique to control the temperature and pressure of cold helium gas independently in a designated space above liquid helium. The technique functions through adjustment of both the pumping speed of helium gas and the evaporation of liquid helium using a heater immersed in the liquid helium. This technique enables us to investigate the peculiar temperature dependence of collision cross sections of Ba^+* and He, or Ba^+*-He and He at temperatures of 3-30 K through observation of laser-induced fluorescence spectra of Ba^+*-He, or exciplexes.