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The Journal of Physical Chemistry A

Optimizing Raman Ladder Climbing: Theory and Application in Na2

Bo Y. Chang, Ignacio R. Solá, Jesús Santamarı́a

Published Sep 6, 2001• 7 citations
Publisher

Abstract

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.

DOI
10.1021/jp011077s

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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.
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The use of cesium Rydberg atoms as a test system for quantum ladder climbing with chirped laser pulses is described. Quantum ladders can be used in the coherent control of chemical reactions; for instance, by exciting a specific molecular bond by climbing its vibrational ladder. A Rydberg atom is an easy test system because of the large transition dipole moments between Rydberg states. The Rydberg states are easy to access and to analyze. Picosecond far-infrared laser pulses around a wavelength of 50 \ensuremath{\mu}m, with peak intensities of 109W/cm2,{10}^{9}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},109W/cm2, from the free-electron laser for infrared experiments (FELIX) at Nieuwegein, The Netherlands, were used. It is found that the complexity of the system puts severe limits on the allowed intensities that are used to drive the ladder. At high intensity the ladder is distorted due to coupling with states outside the ladder. This distortion frustrates efficient climbing.
Abstract We have theoretically investigated the population transfer and the selective excitation of vibrational states of the target state in a three-state Na 2 molecule using the chirped adiabatic passage. The population transfer among electronic states is studied. Compared with the single negative chirped pulse, complete population transfer can be achieved by using a positive chirped pulse. The effects of laser parameters on vibrational population distribution are discussed, and the selective excitation of vibrational states can be achieved by adjusting the central frequency. Finally, we give the method of selective excitation of specific vibrational states.
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.