Non-Monotonicity of Excited State Populations Observed in a Cu--He Hollow Cathode Discharge

An interesting non-monotonic structure in the distribution of excited state populations is observed in a Cu--He hollow cathode discharge, and this is explained by l-changing collisions of the excited states with background gas atoms. At helium pressure of 0.1 Torr and cathode current of 200-300mA, relative populations of He I 1snp ¹P (n= 2-16) states are measured with the corresponding VUV radiation intensities, and are plotted against excitation energies. As energy levels increase, populations of high-n (n> 10) states are found to decrease much more quickly than low-n (n< 7) populations. For intermediate states (n= 7-10), the declining tendency is interfered by population transfers from 1sns ¹S states due to l-changing collisions, and an obvious non-monotonic structure is formed at relatively low electric current. Measurements have also been carried out for He II np ²P (n= 2-14) series, in which the l-changing collisions are overwhelmed by Stark quenching of the n²S states and thus population interference does not occur.