SHIFT AND BROADENING OF RESOLVED HYDROGEN BALMER-ALPHA FINE-STRUCTURE LINES IN HELIUM

Accurate values for the density shift and broadening of five isolated Hα fine-structure transitions have been measured. Doppler-free polarization and double-quantum saturation spectroscopy with a cw dye laser are used in a He dc discharge with 1% H2. The negative shifts and the huge broadening values are correctly predicted by an impact-approximation theory taking into account the large cross sections for fine-structure transfer (quenching) collisions and only Van der Waals interaction between H* and He. Consequences of a comparison with the positive shift of the unresolved Hα line and of the alkali-metal D lines in He buffer gas are discussed. For pressures below 0.1 Torr hfs decoupling collisions give rise to a positive shift of single Hα fine-structure lines and to a collisional narrowing effect analogous to that observed in nuclear magnetic resonance. © 1979 The American Physical Society.