Measurements and analysis of collisional line-mixing within nuclear hyperfine components of helium broadened HI lines

The absorption by the R (0) and R (1) lines of the fundamental band of HI broadened by He have been measured in the 0-500 mbar pressure range using a tunable difference frequency generation laser spectrometer. The observed transmissions have been fitted and analyzed with a line-coupling model based on the infinite order sudden approximation. It is demonstrated that the three hyperfine components of the R (0) manifold have identical widths and are not coupled by collisions. On the contrary, the profile of the R (1) manifold cannot be modeled without accounting for both line-mixing and for the fact that the nine hyperfine components have different broadening parameters. This is, to our knowledge, the first demonstration of line coupling within hyperfine components.