Shift and broadening of resolved hydrogen Balmer-αfine-structure lines in helium

Accurate values for the density shift and broadening of five isolated ${\mathrm{H}}_{\alpha }$ 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% ${\mathrm{H}}_2$. 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 ${\mathrm{H}}^{*}$ and He. Consequences of a comparison with the positive shift of the unresolved ${\mathrm{H}}_{\alpha }$ 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 ${\mathrm{H}}_{\alpha }$ fine-structure lines and to a collisional narrowing effect analogous to that observed in nuclear magnetic resonance.