Coincidence Doppler broadening

A new spectroscopic method for the characterization of surface vacancy clusters is a combination of positron lifetime spectroscopy, which determines the size of vacancy clusters, and coincidence Doppler broadening of annihilation radiation, which gives information on where vacancy clusters are located [5, 6]. If these clusters are located on the surface of gold nanoparticles, namely the interface between the particle and host matrix, the surroundings of the clusters should include both particle atoms and the matrix atoms. Doppler broadening of annihilation radiation (DBAR) with two-detector coincidence should be able to reveal these atomic constituents, and therefore elucidate the location of vacancy clusters. 

Djourelov, N., Suzuki, T., Ito, Y, Shantarovich, V., and Kondo, K., Gamma and positron irradiation effects on polypropylene studied by coincidence Doppler broadening spectroscopy. Radial. Phys. Chem., 72, 687-694 (2005). 

Coincidence Doppler broadening (CDB) measurements are typically obtained using 2 Ge detectors located at an angle of 180° to each other. The... 

Solute content PA coincidence Coincidence Doppler broadening... 

Figure 7.27 Doppler broadening spectrum from two detectors in coincidence as a function of the Doppler shift momentum in atomic units. MSSQ samples with 0% and 40% porogen are shown. The lead filter is used to stop the low energy third photon from reaching a detector. In that case, only two photon events are observed. Statistical errors are of the order of the line width and smaller.

The 2Y-annihilation of positrons produces two correlated photons. On the other hand, hoth lifetime spectroscopy and Doppler-broadening measurements need only one of them. This suggests an evident possibility, that is, the combination of the two measurements. To achieve this goal, a three-photon coincidence system is required. In the system, one of the detectors measures the energy of an annihilation photon, and the other two determine the lifetime of the corresponding positron. 

A laser medium has a Doppler-broadened gain profile of halfwidth 2 GHz. The homogeneous width is 50 MHz, and the transition probability Aik = 1 X 10 s" Assume that one of the resonator modes (L = 40 cm) coincides with the center frequency vq of the gain profile. What is the threshold inversion for the central mode, and at which inversion does oscillation start on the two adjacent longitudinal modes if the resonator losses are 10% ... 

However, when the frequency is coincident with the center frequency of the Doppler profile, the weak probe wave interacts with molecules whose absorption has already been reduced by the strong counterrunning wave. Consequently, the absorption of the probe wave has a resonant minimum equal in width to the homogeneous width and centered exactly on the Doppler-broadened absorption line. This method has been demonstrated in experiments using a CO2 laser operating at 10/um and SFe molecules (Basov et al. 1969), and now it is universally accepted in laser saturation spectroscopy. 

The positron lifetime experiments were carried out with a fast-slow coincidence ORTEC system with a time resolution of about 230 ps full width at half maximum. A 5mCi source of Na was sandwiched between two identical samples, and the total count was one million. The temperature-dependent Doppler broadening energy spectroscopic (DBES) spectra were measured using an HP Ge detector at a counting rate of approximately 800 cps. The energy resolution of the solid-state detector was 1.5 keV at 0.511 MeV (corresponding to positron 2y annihilation peak). The total... 

The Lamb dip is an important manifestation of the saturation of the gain which occurs in inhomogeneously-broadened transitions when the oscillation frequency coincides with the centre of the laser line. A similar phenomenon occurs if the laser frequency is tuned close to the centre of an absorption line of a sample of atoms or molecules interacting with the standing wave field of the laser. This saturated absorption has become an important new technique in atomic and molecular spectroscopy since it removes the limit on the attainable resolution which was formerly imposed by the Doppler broadening of absorption lines. 

In the previous three chapters we have discussed the properties of gas lasers and have shown how they can be designed for single frequency oscillation, and also how the output frequency may be tuned continuously over the bandwidth of the Doppler-broadened gain curve. Unfortunately this tuning range is relatively narrow and the application of these gas lasers to atomic and molecular spectroscopy is restricted to studies of the laser transitions themselves, or to accidental coincidences with molecular absorption lines. It would therefore seem that the new and powerful technique of saturated absorption spectroscopy was also of relatively limited applicability. 

Figure 3.19. Illustration of the double-resonance technique. The pump laser operates at line center on the Fi(4) transition to produce HF molecules in the i = 1, / = 3 state. These molecules have a narrow velocity range compared to that associated with the Doppler-broadened line profile. A probe laser on the 2(3) transition, tuned to coincide with the narrow frequency range associated with the velocity class of laser-excited t = 1, / = 3 molecules, may be used to monitor the laser-excited population. The probe laser can also be tuned to monitor the populations of nearby rotational states in the v = level, as shown.

Herewith, in the formula for the determination of R, instead of the nuclear mass, nif, we should substitute the macroscopic crystal mass then the recoil energy reduces practically to zero, and the y-quantum energy becomes almost precisely equal to the energy difference El-Eq. The Doppler broadening from the thermal motion also vanishes. As a result, the emitting and absorbing lines narrow down to the natural width and coincide with each other. The resonance requirement becomes satisfied. 

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