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Physical Review A

Adiabatic climbing of vibrational ladders using Raman transitions with a chirped pump laser

Szczepan Chelkowski, George N. Gibson

Published Nov 1, 1995• 101 citations
Publisher

Abstract

We present detailed calculations of a new scheme for the adiabatic transfer of population in molecular vibrational ladders and for controlling chemical reactions: Raman chirped adiabatic passage. The scheme makes use of far-off-resonant Raman transitions with one chirped and one monochromatic laser pulse. We show that this method can be used to climb vibrational ladders in molecular systems and, with sufficiently large chirping, can vibrationally dissociate diatomic molecules.

DOI
10.1103/physreva.52.r3417

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Enhanced population transfer from the lowest to the highest level in the three-state ladder 5s->5p->5d of the rubidium atom by frequency swept ultra short laser pulses is observed if the frequency sweep first makes the pulse resonant with the 5s->5p transition, and later with the 5p->5d transition. With this intuitive scheme it is possible to achieve complete population inversion via the intermediate level by two successive adiabatic passages. Complete inversion is also obtained by a {open_quote}counterintuitive{close_quote} direction of the frequency-sweep, in which case population of the intermediate level during the pulse is strongly reduced. The atomic system can be seen as a model system for multistep vibrational excitation in an anharmonic molecular potential.

Bulletin of the American Physical Society•May 1, 1993•0 citations
We present detailed calculations of a new scheme for the adiabatic transfer of population in molecular vibrational ladders and for controlling chemical reactions: Raman chirped adiabatic passage. The scheme makes use of far-off-resonant Raman transitions with one chirped and one monochromatic laser pulse. We show that this method can be used to climb vibrational ladders in molecular systems and, with sufficiently large chirping, can vibrationally dissociate diatomic molecules.
Calculations indicate that high vibrational states of oxygen and chlorine can be excited using a series of nonresonant Raman pulses, where both the pump and the Stokes pulses are chirped with linear frequency sweeps. Most of the previously reported coherent processes (such as simple adiabatic passage) are seriously degraded when rotational effects are included. However, we find that the laser pulse parameters (intensity and bandwidth) required to invert population into high vibrational states via Raman chirped adiabatic passage are achievable using technology that is currently available. Applications to homonuclear diatomic molecules are discussed in detail.
Chirped pulses allow for non-adiabatic passage between dressed states of Raman processes. It is shown that using an appropriately chirped pulse, one can control the interference between resonant and non-resonant pathways in stimulated Raman processes. A three-level system is studied in detail and application to complete ro-vibrational level inversion to high vibration quantum numbers (v > 10) in H2 via RCNAP (Raman chirped non-adiabatic passage) is presented. Copyright © 2000 John Wiley & Sons, Ltd.
The theory of vibrational ladder climbing excitation by coherent stimulated nonresonant Raman using chirped laser pulses is developed. We analytically obtain the conditions for inverting the population to a final preselected vibrational state and the restrictions that apply in the linear chirp regime. By controlling both the shape of the laser pulses and the chirp profile, the ladder climbing process can be accelerated without reducing the yield of the selective excitation. Numerical results are presented for selection of moderately excited vibrational levels in Na 2, where the important contribution of several excited electronic states is also clarified.
Efficient population transfer by adiabatically chirping through a multiphoton resonance in microwave driven and kicked Rydberg atoms has recently been reported both experimentally and theoretically. Here we report on our simulations in which we have exploited this mechanism to vibrationally excite a diatomic molecule up to |ν = 4, J from the ground state by chirping through a four-photon resonance condition. This is an efficient means of population transfer, which is an alternative to the ladder climbing scheme requiring chirping through a sequence of states in the correct order. We discuss and compare one-dimensional quantum and classical models where there is no rotational degree of freedom. This comparison suggests that for the lowest laser intensity we consider, the process is classically forbidden and the transition occurs through tunnelling. We show that for larger peak intensities, the transfer can be looked upon as a classical transition in phase space, similar to that observed in the atomic case. We extend our simulations to fully three-dimensional quantum calculations and investigate the effect of coupling between different rotational pathways. We finally discuss the effect of thermal averaging over the initial J-states using a temperature for which the first few rotational levels inside the ν = 0 manifold are populated.
Enhanced population transfer from the lowest to the highest level in the three-state ladder 5s->5p->5d of the rubidium atom by frequency swept ultra short laser pulses is observed if the frequency sweep first makes the pulse resonant with the 5s->5p transition, and later with the 5p->5d transition. With this intuitive scheme it is possible to achieve complete population inversion via the intermediate level by two successive adiabatic passages. Complete inversion is also obtained by a {open_quote}counterintuitive{close_quote} direction of the frequency-sweep, in which case population of the intermediate level during the pulse is strongly reduced. The atomic system can be seen as a model system for multistep vibrational excitation in an anharmonic molecular potential.