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 Na(2) 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 Lambda 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 1Sigma(g)+, v" = 0, j" = 7 to the intermediate level A 1Sigma(u)+, v' = 10, J' = 8, which in turn is linked to the final level 5 1Sigma(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 kappa(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.

Control of population flow in coherently driven quantum ladders.

A technique for adiabatic control of the population flow through a preselected decaying excited level in a three-level quantum ladder is presented. The population flow through the intermediate or upper level is controlled efficiently and robustly by varying the pulse delay between a pair of partly overlapping coherent laser pulses. The technique is analyzed theoretically and demonstrated in an experiment with Na2 molecules.