The feasibility of a novel technique for efficient and selective population transfer from a thermally populated level 1 via an intermediate state 2 to level 3 is experimentally demonstrated. It is shown for sodium dimers that the process of on- or near-resonance stimulated Raman scattering with only partially overlapping laser beams is, in particular, useful for the selective population of high vibrational levels of particles in a molecular beam. This is achieved when the interaction with the Stokes laser, coupling levels 2 and 3, begins earlier than the interaction with the pump laser. The phenomenon, which is closely related to the formation of ‘‘trapped states,’’ is quantitatively explained using the basis of eigenstates of molecules strongly coupled to the radiation fields. The similarity and difference to related techniques such as rapid adiabatic passage phenomena in two-level systems, off-resonant stimulated Raman scattering, or stimulated emission pumping is briefly discussed.
It is shown theoretically and confirmed by experiments that very efficient and highly selective population transfer between atomic or molecular levels can be achieved by stimulated Raman scattering with partially overlapping pulses. It is a particular interesting feature of this method, called STIRAP, that the particles must interact first with the Stokes laser and then with the pump laser, provided there is a sufficient overlap between the interaction with the two lasers. The striking similarity of this transfer process with phenomena well known in atom‐atom collisions is emphasized.
Techniques for population transfer are needed to gain more complete insight into the collision dynamics of thermally unpopulated levels, e.g. highly vibrationally excited states of molecules. Very promising new techniques are now becoming available. They invoke coherence properties of the radiation field and rely on the phenomena of adiabatic following. In this presentation, I will discuss one of them, the STIRAP method, which induces efficient and highly selective flow of population from an initially populated level 1 to a final level 3. These levels are coupled by a pump and a Stokes laser, with both laser frequencies tuned to resonance with an intermediate level 2. Despite the resonance, spontaneous emission, which may populate levels other than level 3, is eliminated. The method can be implemented with cw as well as with pulsed lasers. First results from a study of the effect of initial vibrational excitation of a molecule on a chemical reaction will be presented. It will also be shown how this technique allows the orientation of electronic or molecular angular moments. The prospects of using this technique for efficient transfer of photon momenta to an atom may be mentioned.
Stimulated Raman scattering involving adiabatic passage (STIRAP) is a proven technique for population transfer in three-level systems with strong oscillator strengths. We show that the STIRAP process should also yield high transfer efficiencies when the densities of states near the intermediate and final levels are high, provided certain criteria for the experimentally controllable parameters are met. We also show that high transfer efficiencies may even be possible when the pump and Stokes lasers can access levels outside of the three-level system. The criteria are derived with an approach that emphasizes the adiabatic nature of the transfer process in an eigenvector of the interaction Hamiltonian that resembles a trapped state. The results are compared to density matrix calculations.
We compare the results of full density matrix calculations with recently reported experimental results describing the population transfer in Na2 using Raman scattering with the fundamental laser beam temporally shifted relative to the Stokes laser beam. Our calculations confirm the conclusion that almost total transfer of the population into the terminal level of the Raman transition can be achieved when the laser frequencies are tuned to resonance and the fundamental laser beam is temporally delayed relative to the Stokes shifted laser beam but still partially overlapping it. We analyze our results when the frequencies of the lasers are tuned off-resonance as a function of the Rabi frequencies and the detuning, and compare with experimental and theoretical results. The alignment of the terminal level as a function of time displacement of the Stokes and pump beams is predicted.
Highly efficient and selective population transfer in NO molecules in the electronic ground state (
X2${\mathrm{\ensuremath{\Pi}}}_{1/2}$) from the vibrational level v''=0 to the level v''=6 is demonstrated. It shows, for the first time with pulsed lasers, that a rather generally applicable scheme for complete control over the level population in atoms and molecules is now available. The efficiency relies on a counterintuitive interaction sequence of two lasers with the molecule in a process of stimulated Raman scattering involving adiabatic passage.
We have theoretically investigated the population transfer in a four-level
H2 system by stimulated Raman transition from the ground $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}({\ensuremath{\nu}}_{g}{=0,J}_{g}=0)$ level to higher rovibrational levels $({\ensuremath{\nu}}_{f}{,J}_{f})$ of the $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ state via the excited intermediate $B{}^{1}{\ensuremath{\Sigma}}_{u}^{+}({\ensuremath{\nu}}_{i}{=14,J}_{i}=1)$ and $C{}^{1}{\ensuremath{\Pi}}_{u}^{+}({\ensuremath{\nu}}_{i}{=3,J}_{i}=1)$ levels coupled with each other by nonadiabatic interaction, using time-dependent overlapping pump and Stokes laser fields. The density-matrix treatment, which permits the convenient inclusion of the spontaneous emissions from the intermediate levels, has been employed to describe the dynamics of the two-photon Raman resonance process. The present study performs the calculations of final populations (after both the pulses are over) of the ground and terminal levels for Q-branch
(Jf=0) fundamental $({\ensuremath{\nu}}_{f}=1)$ and first overtone $({\ensuremath{\nu}}_{f}=2)$ transitions and the S-branch
(Jf=2) fundamental $({\ensuremath{\nu}}_{f}=1)$ transition as a function of time delay between the two pulses for the cases of on-resonance as well as off-resonance excitations in a wide range $(2\ifmmode\times\else\texttimes\fi{}{10}^{5}--2\ifmmode\times\else\texttimes\fi{}{10}^{7}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2})$ of peak intensities
IP0 (IS0) of the pump (Stokes) fields. Both fields are assumed to have the same temporal shape, duration, peak intensities, and linear parallel polarizations. The accurate values of spontaneous radiative relaxation rates of the intermediate levels to the initial and final levels, taking into account their J and M dependence, are explicitly included in our calculations. The pulse width (full width at half maximum) ${\ensuremath{\tau}}_{p}$ is taken as 170 ns so that total spontaneous decay can occur during the pulse duration. The transfer efficiency is found to be very sensitive to the peak intensities of the laser pulses in each case of transition considered. Special attention is paid to the effects of the nonadiabatic (NA) interaction between
B(14,1) and
C(3,1) levels on population transfer efficiency. Calculations are also done in some particular cases using the adiabatic Born-Oppenheimer (ABO) approximation. The results with ABO approximation are found to differ remarkably from those obtained including NA interaction. Our calculations for the four-level
H2 system reveal that almost complete population is transferred in counterintuitive pulse order for both on-resonance and off-resonance excitations with intermediate and high values of
IP0 (IS0). For intuitive pulse sequence also a large population transfer is achieved for on-resonance excitation at intermediate values of
IP0 (IS0) and for off-resonance excitation at intermediate and high values of
IP0 (IS0).