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Journal of the Optical Society of America B

Molecular radiator reorientation collision kernels from Stark sublevel photon echoes

J. M. Liang, Ramachandra Rao Dasari, Michael S. Feld, J. E. Thomas

Published Apr 1, 1986• 5 citations
Publisher

Abstract

Radiator reorientation kernels are measured in Stark-tunable infrared transitions of 13CH3F. The dominant (ΔM = 1) kernel is obtained by a scale transformation from the shape of a two-pulse photon-echo intensity versus Stark voltage curve, at fixed large echo time delay. The quantum-mechanical transport equation in the presence of coherence M transfer and a dc electric field is reviewed, and the applications of photon echoes in a tunable Stark field to measure reorientation collision kernels are discussed.

DOI
10.1364/josab.3.000506

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Molecular Collision Kernels by Stark Sublevel Photon Echoes: Radiator Reorientation and Population Transfer.

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Radiator reorientation kernels are measured in Stark-tunable infrared transitions of 13CH3F. The dominant (ΔM = 1) kernel is obtained by a scale transformation from the shape of a two-pulse photon-echo intensity versus Stark voltage curve, at fixed large echo time delay. The quantum-mechanical transport equation in the presence of coherence M transfer and a dc electric field is reviewed, and the applications of photon echoes in a tunable Stark field to measure reorientation collision kernels are discussed.
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We describe a new Stark sublevel photon echo technique for measuring fluctuations in macroscopic coherences. This idea is applied to the Stark tunable infrared (λ = 9 μm) R(4, 3, V3 = 0 → 1) transition of 13CH3F to measure, for the first time, the radiator reorientation velocity changing kernels WΔ M (Δv) which describe the rates (per unit velocity) at which the radiator orientation M changes by an amount ΔM accompanied by a velocity change Δv.1 In the experiments, two pulse photon echoes are obtained using a laser field polarized along an applied weak de Stark field ( z ^ direction) which shifts each M transition by Mvs (Hz).
Experiments exploiting similarities for coherence collisional evolution in velocity space and in M space are reported. Generally, there is a total outgoing rate from any state and incoming rates (kernels) from neighboring states. Collision-induced velocity changes, ΔV, are studied with lasers by measuring the Doppler frequency Δ V/λ (λ = optical wavelength). To obtain information about coherence M transfer, it is only necessary to associate a frequency with each ΔM. This idea is demonstrated by measuring two-pulse photon echo intensity Ie for 13CH3F in a small dc Stark field. Acoustooptic modulation of infrared cw laser radiation, polarized parallel to the Stark field, is used to excite a number of uncoupled M → M transitions on the R(4,3) line. Each transition has a resonance frequency, vo + M· vs (vs is M = 1 Stark shift, Hz). A most remarkable result is obtained for large echo delays T with ΔVT/λ >> 1, all ΔV. The shape of the curve ln[ Ie(o)/ Ie(V)] vs Stark voltage V tends to that of the dominant coherence ΔM transfer kernel itself. Physically this arises because, for large T, the macroscopic coherence is preserved only for collisions where the frequency changing due to M transfer compensates the Doppler shift due to the ΔV, i.e., Δ V/λ + ΔM-·vs = 0. Generalizations are discussed.
Collision-induced coherence transfer between infrared molecular transitions is studied using a technique that permits the complete distribution of accompanying velocity changes to be determined in a simple way. The experiments are carried out by measuring the intensity of two pulse photon echoes as a function of applied Stark field at fixed large echo time delay. The velocity change distribution for coherence-transfer collisions is obtained directly as the shape of the echo intensity versus Stark field curve. Previously unpublished data for dipole-quadrupole and van der Waals interactions are presented, and a theory of coherence-transfer velocity-changing kernels is developed that is in good agreement with the data.
We report the investigation of molecular M changing kernels using new Stark sublevel photon echo techniques to obtain, for the first time, the entire distribution of discrete M transfer rates. The method exploits the analogy between velocity space, where inhomogeneous broadening is due to Doppler frequency shifts, and M space where an artificial inhomogeneity can be created with a small Stark field. In the presence of the Stark field, random collision induced coherence transfer between M sublevels of an infrared transition of moderate J leads to Stark field dependant degradation of the molecular macroscopic polarization during the echo dephasing-rephasing process. The effects of inelastic (J changing) collisions and elastic velocity changing collisions which are field independent can be eliminated by measuring the ratio of the echo signals with the field on and off as a fuction of either echo time delay or Stark field. The nonexponential decay curve which is obtained is related to the M changing collisions kernel by discrete transformation with the frequency components $\\nu{_{s}}\\Delta $M, where DeltaM\\Delta MDeltaM is an integer and nus\\nu_{s}nus​ is the Stark shift for unit M. In the experiment, the (V=0,J=4,K=3)longrightarrow(v3=1,J=5,K=3)(V=0, J=4, K=3) \\longrightarrow (v_{3}=1, J=5, K=3)(V=0,J=4,K=3)longrightarrow(v3​=1,J=5,K=3) transition in CH3PCH_{3}PCH3​P is studied. Acoustooptic (A/0) intensity modulation of 9P32 c.w. CO2CO_{2}CO2​ laser radiation is used for input pulse generation. A novel digital oscillator driver is employed which has no RF leakage, so that A/0 rejected ratios are scattered light limited at −106:1^{-}106:1−106:1. Preliminary results yield echo signals with very high signal to background ratio and show completely new nonexponential behavior of the echo decay curve. Applications of both two and three pulse (stimulated) sublevel experiments will be described."