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A new method for investigating fluctuations in macroscopic coherences is suggested and demonstrated by measurement of radiator reorientation kernels in Stark-tunable infrared transitions of ^^CH3F. The dominant (AM= 1) kernel is obtained by a scale transformation from the shape of a curve of two-pulse photon-echo intensity versus Stark voltage, at fixed large echo time delay.PACS numbers: 33.80.-b, 34.50.-s, 34.90. + q Laser-induced transients have wide application as probes of relaxation phenomena in vapors and condensed media. However, the fluctuations which are the underlying cause of the relaxation are often difficult to characterize in detail. In this Letter we describe a new method for studying fluctuations in macroscopic coherences. The method singles out those processes which transfer coherence between superposed states whose energy separations have been made identical by means of a tunable external field. Such fluctuations have special significance because they tend to preserve the phase of the macroscopic coherence for long observation times.We demonstrate this idea by measuring radiator reorientation velocity-changing kernels in Starktunable infrared transitions of ^^CH3F. The dominant kernel is obtained as the shape of a curve of two-pulse photon-echo intensity versus Stark voltage, at fixed large echo time delay. The method works by singling out those collisions for which the collision-induced Doppler frequency shift is compensated by the (tunable) Stark frequency change of the oscillator which accompanies infrared radiator reorientation, A simple scale transformation of the applied Stark voltage to velocity units yields the reorientation kernel.Radiator reorientation is a form of (infrared) coherence transfer, and is described by the quantum transport equation.^""^ However, the physical picture and conditions for which collision-induced coherence transfer can occur are not well established and presently there is no experimental information on the correlation between radiator reorientation and velocity changes.'^ Processes of this type have been analyzed theoretically in some detail with neglect of the accompanying velocity changes.^' ^ Previous experiments have been confined to Zeeman coherence and microwave coherence rotational transfer (intramolecular) ,^ for which velocity changes were not measured.In our experiments, two-pulse photon echoes are used to study infrared radiator reorientation kernels for molecules with a first-order Stark effect, by application of a small dc electric field parallel to the z direc-tion. A pulse of linearly polarized (f) laser radiation then excites a number of uncoupled {JM -^ J'M) infrared transitions, so that a molecule initially in the state M becomes a dipole radiator, t^r, with an orientation labeled by M. The electric field causes the dipolar radiation frequency to be orientation (M) dependent, the Stark shift being V5M, where v^ is the transitionfrequency shift for M = L Under the assumption that the input pulse bandwidth and dc Stark shifts are small compare...

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