In this paper we extend the existing theory of stimulated Raman adiabatic passage (StiRAP) in three-level systems to examine the nature of selective photoexcitation using counterintuitively ordered pulses. Our goal is to develop a version of StiRAP that permits control of product selectivity in a chemical reaction. We analyze the case of selective excitation to one state in a pair of degenerate target states in a four-level system, and find that one cannot control the ratio of the populations in the two states in a resonant two-photon process. However, the extension of the system to include a fifth state and a third laser field makes selective excitation to one of the degenerate states possible. It is found that, subject to reasonable restrictions, one may accomplish complete population transfer to a single target state of a degenerate pair of states in a five-level system.
A five-level four-pulse phase-sensitive extended stimulated Raman adiabatic passage scheme is proposed to realize complete control of the population transfer branching ratio between two degenerate target states. The control is achieved via a three-node null eigenstate that can be correlated with an arbitrary superposition of the target states. Our results suggest that complete suppression of the yield of one of two degenerate product states, and therefore absolute selectivity in photochemistry, is achievable and predictable, even without studying the properties of the unwanted product state beforehand.
A five-level four-pulse phase-sensitive extended stimulated Raman adiabatic passage scheme is proposed to realize complete control of the population transfer branching ratio between two degenerate target states. The control is achieved via a three-node null eigenstate that can be correlated with an arbitrary superposition of the target states. Our results suggest that complete suppression of the yield of one of two degenerate product states, and therefore absolute selectivity in photochemistry, is achievable and predictable, even without studying the properties of the unwanted product state beforehand.
We describe the exploitation of adiabatic transfer of population between selected states of a molecule as a means for the control of product selectivity in a branching unimolecular reaction. The particular procedures employed to achieve that adiabatic transfer of population depend on the character of the spectrum of states of the molecule; the ones we consider are either extensions of, or are illuminated by, the STIRAP method. The results of studies of efficient selective population of one of a pair of nearly degenerate states in the thiophosgene molecule, and of efficient promotion of the HCN → CNH isomerization reaction, are reported. In the former case the adiabatic population transfer is achieved by use of the Kobrak-Rice extended STIRAP method, while in the latter case it is achieved by use of consecutive STIRAP processes. In both cases the efficiency of selective population transfer is shown to be insensitive to radiative
We examine the conditions needed to accomplish stimulated Raman adiabatic passage (STIRAP) when the three levels (ground
g, excited
e, and final
f) are degenerate, with arbitrary couplings contributing to the pump-pulse interaction $(g\text{\ensuremath{-}}e)$ and to the Stokes-pulse interaction $(e\text{\ensuremath{-}}f)$. We show that in general a sufficient condition for complete population removal from the
g set of degenerate states for arbitrary, pure or mixed, initial state is that the degeneracies should not decrease along the sequence
g,
e, and
f. Using a sequence of unitary transformations we demonstrate that when this condition holds it is possible to achieve the degenerate counterpart of conventional STIRAP, whereby adiabatic passage produces complete population transfer. Indeed, the system is equivalent to a set of independent three-state systems, in each of which a STIRAP procedure can be implemented. We also examine the cases when this degeneracy constraint does not hold and show what can be accomplished in those cases. For example, for angular momentum states when the degeneracy of the
g and
f levels is less than that of the
e level we show how a special choice for the pulse polarizations and phases can produce complete removal of population from the
g set. Our scheme can be a powerful tool for coherent control in degenerate systems, because of its robustness, when selective addressing of the states is not required or impossible. We illustrate the analysis with several analytically solvable examples, in which the degeneracies originate from angular momentum orientation, as expressed by magnetic sublevels.
We propose a selective, efficient, and robust way to realize control of coherent population transfer in a $\ensuremath{\Lambda}$-type four-level system with a closely spaced doublet in the final state with time-separated chirped pump pulse and Stokes pulse pair by merging stimulated Raman adiabatic passage, temporal coherent control, and chirped adiabatic passage techniques. Moreover, an arbitrary coherent superposition between the final doublet, or between the intermediate state and either of the doublets can be created. This method holds the ability to ``control with control'' and has potential applications in coherent control of chemical reactions and quantum information processing.