Coherent population trapping probed by charge exchange reactions

Coherent trapping of atomic population in sodium induced by two-frequency laser light has been studied by using a charge exchange reaction for probing the upper level population. When the two-photon resonance condition occurs, the signal due to electron capture from excited ${\mathrm{Na}}^{\mathrm{*}}$(3p) atoms by ${\mathrm{He}}^{2+}$ projectile ions exhibits the characteristic ‘‘black resonance’’ dip, indicating (by nonoptical means) that the fraction of Na atoms in the excited state actually drops to zero.