Experimental control of excitation flow produced by delayed pulses in a ladder of molecular levels

We study a method for controlling the flow of excitation through decaying levels in a three-level ladder excitation scheme in Na₂ molecules. Like the stimulated Raman adiabatic passage (STIRAP), this method is based on the control of the evolution of adiabatic states by a suitable delayed interaction of the molecules with two radiation fields. However, unlike STIRAP, which transfers a population between two stable levels g and f via a decaying intermediate level e through the interaction of partially overlapping pulses (usually in a Λ linkage), here the final level f is not long lived. Therefore, the population reaching level f decays to other levels during the transfer process. Thus, rather than controlling the transfer into level f, we control the flow of the population through this level. In the present implementation a laser P couples a degenerate rovibrational level in the ground electronic state X ¹Σ_g⁺, v″=0, j″=7 to the intermediate level A ¹Σ_u ⁺, v′ = 10, J′ = 8, which in turn is linked to the final level 5 ¹Σ_g⁺, v = 10, J = 9 by a laser S, from which decay occurs to vibrational levels in the electronic A and X states. As in STIRAP, the maximum excitation flow through level f is observed when the P laser precedes the S laser. We study the influence of the laser parameters and discuss the consequences of the detection geometry on the measured signals. In addition to verifying the control of the flow of population through level f we present a procedure for the quantitative determination of the fraction κ_f of molecules initially in the ground level which is driven through the final level f. This calibration method is applicable for any stepwise excitation.