High sensitivity Doppler broadening

In a typical Doppler measurement only one of the two annihilation photons with on average half of the Doppler shift is observed. With second detector opposite to the first one and operated in coincidence with the first one the full Doppler shift is observed. The signal to noise ratio improves by a factor of-1000. The shell structure of tightly bound core electrons of atoms does not change much when the atoms form a solid. Doppler shifts from these electrons can be detected and permit the identification of specific elements next to the annihilation site [70]. 

Since the positron traps in vacancy like open volume and positronium in pores, it will be possible to selectively detect impurities next to vacancies [71]. For metal indiffusion experiments one can design test structures of silicon, metal, low-/ layer samples. Two detector coincident measurements would be performed as a function of temperature and time to observe the chemical signature of the metal in the low-/ layer. The effectiveness of diffusion barriers can be tested by depositing the barrier prior to the low-/ layer. 

Para positronium two photon annihilations cause the narrow sharp feature in the center at zero momentum. The events at large momentum ( 4a.u.) are due to three photon annihilations. By adding lead filters between the sample and the detectors, the third photon is absorbed and only two photon events meet the sum energy restriction. 

The very large signal to noise ratio of 1 in 106 and better makes this method highly sensitive to small fractions of three photons, where the standard 3-to-2 photon method fails. It has been proposed to use this technique to observe rare 3 photon decays of positrons in metals [72], 

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The Doppler-free techniques, which have been explained in Chap.10, eliminate the bothering Doppler width and therefore already small collision broadening can be sensitively monitored. One example is given by the investigation of broadening or shifts of Lamb dips in saturation spectroscopy (see Sect. 10.2), which can be measured with an accuracy of a few kilohertz if stabilized lasers are used. This high accuracy allows detection of even small interaction forces such as those experienced by atoms or molecules without permanent dipole moments. The interaction potential at large internuclear distances can be described in these cases by a van der Waals potential V(r) = -ar where the constant a depends on the polarizability of the collision partners. 

Even if such high power levels cannot be sustained, it should still be possible to observe two-photon excitation with the help of more sensitive detection methods. Towards this end, U. Boesl and E. Hildum in our laboratory have recently completed construction of a hydrogen atomic beam apparatus, which permits the detection of 2S atoms via photoionization. The resulting charged particles are observed with a time-of flight mass spectrometer. Despite transit time broadening and uncompensated relativistic second order Doppler shifts, we hope to achieve line widths on the order of 1 Mhz in this... 

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