Features of magnetically-induced atomic transitions of the Rb D1 line studied by a Doppler-free method based on the second derivative of the absorption spectra.

In this paper we show that the second derivative (SD) technique of the absorption spectra of Rb atomic vapours, confined in a nanocell with a thickness ℓ = λ/2 = 398 nm, allows us to achieve close to Doppler-free spectroscopy. The narrow linewidth and linearity of the SD signal response with respect to transition probabilities allows us to study separately, in an external transverse magnetic field (0.6 to 4 kG), a large number of the atomic transitions of 85Rb and 87Rb atoms. Atomic transitions |Fg, 0⟩ → |Fe = Fg, 0′⟩, for which the dipole moment is null in a zero magnetic field (so-called magnetically-induced transitions), show a gigantic increase in probability with an increasing magnetic field. When a magnetic field is applied to the vapour, we show the possibility of forming a dark resonance on these transitions by adding a coupling laser. We are therefore able to demonstrate a five-fold increase in the transmission of the probe radiation when the coupling laser is on. Theoretical calculations are in very good agreement with the experimental results. [ABSTRACT FROM AUTHOR]