“…However, the γ of several second-step transitions for alkali metal elements have been measured by two-color two-photon resonance absorption spectroscopy using a gas cell and two tunable lasers. The reported broadening coefficients of the second-step transitions show the same tendency as that observed in this study, which is that the coefficient of the second-step transition (P → D) 57,58 is larger than that of the first-step transition (S → P). 59 For example, the broadening coefficients of the first-step (5 2 S 1/2 → 5 2 P 1/2 ) and second-step (5 2 P 1/2 → 5 2 D 3/2 ) transitions for Rb atoms under helium buffer gas are reported to be 20.8 MHz per torr 59 and 54.64 MHz per torr, 57 respectively.…”