We report the first experimental evidence of variable collision-broadening parameters within the hyperfine multiplet of a rotational line. A similar behavior is observed in the Stark pattern of one hyperfine line. All the experimental results, obtained on ${\mathrm{CH}}_3$I, are explained in terms of a theoretical model accounting for line coupling. The same approach is successful in interpreting the literature data for the Stark lines of ${\mathrm{CH}}_3$Br and previous null results for other molecules.

• Received 15 December 1987

DOI:https://doi.org/10.1103/PhysRevA.37.3790