Atoms and ions in superfluid helium. I: optical spectra of atomic and ionic impurities

Optical spectra of atomic impurities in liquid helium have been investigated. Comparison is made between the wavelength of the free atomic and ionic lines and those in the liquid heluim matrix. Simultaneously, the line width and a possible asymmetry is recorded. Presence and absence of radiative transitions depend on the species of the atom implanted in the quantum fluid. The absence of any optical transitions from states lying as low as 1, 8 eV velow the ionization limit will be explained in this paper