Photon beat experiments

Experimentally, one finds that the hydrodynamic radius does scale like in 6 solvents, but in good solvents the situation is less clear. Dififri-sion or sedimentation experiments on polystyrene in toluene give D where the apparent exponent Vai is of order 0.53-0.54. More recent photon beat experiments give I aw = 0.55 0.02 for polystyrene in benzene. 

Quantum beats have been extensively studied in beam foil spectroscopy, but only recently have results been reported for their observation following electron impact excitation.For beats to be observed the excitation to the participating levels must be coherent and the modulation period of the decay must be greater than the instrumental resolution of about 10 s obtainable in electron photon coincidence experiments. Since the internal relaxation times from fine or hyperfine interactions of coherently excited states are T 10" s and beats between... 

Up to now, we have given a general theoretical development of the self-beat technique. As a practical illustration of the experimental apparatus used to detect autocorrelation functions in scattering experiments, the equipment currently used in our laboratory will now be described. While our treatment of the autocorrelation function has been in terms of an analog signal, the computer that measures this function is actually a digital device. This is based on the fact that it is also valid to count the scattered photons in order to calculate Ci(r) as the optical intensity signal is essentially determined by the number of photons that strike the photocathode per unit time. We have then... 

Quantum beats have been observed in a variety of experiments, particularly in beam—foil measurements. Teubner et al. (1981) were the first to observe quantum beats in electron—photon coincidence measurements, using sodium as a target. The zero-field quantum beats observed by them are due to the hyperfine structure associated with the 3 Pii2 excited state (see fig. 2.20). The coincidence decay curve showed a beat pattern... 

A typical layout of a squeezing experiment based on a Mach-Zehnder interferometer (Sect. 4.2.3) is shown in Fig. 14.64. The output of a well-stabilized laser is split into two beams, a pump beam bi and a reference beam b2. The pump beam with the frequency co] generates by nonlinear interaction with a medium (e.g., four-wave mixing or parametric interaction) new waves at frequencies o l /. After superposition with the reference beam, which acts as a local oscillator, the resulting beat spectrum is detected by the photodetectors D1 and D2 as a function of the phase difference A0, which can be controlled by a wedge in one of the interferometer arms. The difference between the two detector output signals is monitored as a function of the phase difference A0. Contrary to the situation in Fig. 14.62, the spectral noise power density p(/, 0) (= Pnep per frequency interval d/ = 1 s ) shows a periodic variation with 0. This is due to the nonlinear interaction of one of the beams with the nonlinear medium, which preserves phase relations. At certain values of 0 the noise power density Pn(/, 0) drops below the photon noise limit... 

While new applications of the electron-photon coincidence method, such as zero field quantum beats, will continue the full analysis of the experimental results is complex. The determination of parameters, such as the spin orbit phase parameters e and A, which allow specific dynamical features of theoretical models to be tested will take on a greater significance. For heavy atoms this is of particular significance since there is only limited theoretical results available and effects, such as spin orbit interaction, must be taken into account. Perhaps also the "ultimate" electron impact experiments are starting where polarised electrons are used as the projectile in coincidence experiments. 

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