Doppler broadening techniques

The ability of angular correlation and Doppler broadening techniques to provide information concerning the momentum of an annihilating electron-positron pair was briefly discussed in section 1.3. Also, it... 

It is possible to combine lifetime measurements with Doppler broadening techniques. In so-called Age-Momentum-Correlated measurements (AMOC), Doppler broadening information is collected as a function of time since positron implantation into the sample [76]. The energy loss mechanism for positronium as it traps in pores can be investigated. This might reveal material dependent effects that have not been included in any porosity studies by positrons. 

Mohamed, H.F.M. (2001) Study on polystyrene via positron annihilation lifetime and Doppler broadened techniques . Polymer, 42, 8013. 

State-resolved inelastic scattering for a wide range of incident conditions ( ), d,) are measured for this system by combining molecular beam techniques with (2 + 1) ion TOF REMPI detection of the scattered molecules [58]. Energy transfer parallel to the surface is measured from the Doppler broadening of the REMPI spectra. Trapping... 

The measurements of 2s — Is transitions in magnetically trapped hydrogen have achieved a relative accuracy of one part in 1012 [21] by means of two-photon spectroscopy which eliminates the first-order Doppler broadening. It is hoped that this technique will allow the measurement of the Is — 2s transition with the accuracy limited only by the shape of the transition line dictated by quantum electrodynamics, i.e. to a few parts in 1015. Further, if the center of the Is — 2s line could be determined with the accuracy of a few parts in 103 of its width, the relative accuracy for this transition would increase to a few parts in 1018. 

In this section we introduce three techniques frequently encountered in positron physics, namely those used to measure annihilation lifetimes and the Doppler broadening (or Doppler shift) and angular correlation of the annihilation radiation. These techniques, or variants thereof, are encountered throughout the rest of this work, and here we briefly describe... 

The experimental techniques involved in measuring the angular correlation and the Doppler broadening of the two annihilation gamma-rays were introduced in section 1.3. These techniques rely on the fact that the motion of the positron-electron pair immediately prior to annihilation causes the two gamma-rays to be emitted in directions differing... 

The positron-trap technique has been used by Surko and coworkers to measure the Doppler broadening of the 511 keV line for positrons in helium gas. This method does not have the drawback of the experiment described above, in which both positronium and free-positron events overlap on the angular distribution curves here the positrons are thermalized prior to the introduction of the gas and therefore cannot form positronium. A comparison of the theoretically predicted and experimentally measured Doppler spectra (Van Reeth et al., 1996) is shown in Figure 6.16. The theoretical results were obtained from the variational wave functions for low energy positron-helium scattering calculated by Van Reeth and Humberston (1995b) see equations (3.75) and (3.77). 

A broad overview of traditional methods and recent developments in experimental positron spectroscopy is presented. A discussion of the generation and detection of positrons and their annihilation radiation is followed by a survey of techniques used for positron lifetime measurement, Doppler broadening spectroscopy and angular correlation of annihilation radiation, and the opportunities presented by combining these methods (e.g. in age-momentum correlation) and/or extending their capabilities by the use of monoenergetic positron beams. Novel spectroscopic and microscopic techniques using positron beams are also described. 

The main techniques used are positron annihilation lifetime spectroscopy (PALS) and the Doppler broadening (DB) or angular correlation (AC) techniques. The PALS parameters are the relative intensities (I j) and the... 

In the field of positronium chemistry AMOC combines the positronlifetime analysis which is especially sensitive to detect long-lived ortho-positronium (o-Ps) with the Doppler-broadening-measurement technique which is particularly suitable for the observation of the para-positronium (p-Ps) state with its characteristic narrow momentum distribution. In addition,... 

Dalby and Bennett " which has given accurate probabilities for a series of transitions. The technique is described briefly onpp. 291-2. Accurate determination of concentrations may still be hindered by self-absorption of the radiation, particularly in the case of the hydroxyl radical. Penner and co-workers have overcome the difficulty by the use of a double path technique, and are able to determine the rotational temperature and concentration of hydroxyl radicals in both flame and shock-tube studies. The single and double path emissivities are compared simultaneously, the double path beam being chopped to give modulation at about 5 sec intervals. The method of correction for line widths and Doppler broadening is discussed . 

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