Doppler modulation

Fig. 2.6 Schematic illustration of a Mossbauer transmission experiment in five steps. The Absorption bars indicate the strength of recoilless nuclear resonant absorption as determined by the overlap of emission and absorption lines when the emission line is shifted by Doppler modulation (velocities Uj,. .., 1)5). The transmission spectrum T v) is usually normalized to the transmission T oo) observed for v oo by dividing T(v)IT(oo). Experimental details are found in Chap. 3...

Fe which have full width 2r at 0.2 mm s . Other isotopes are less demanding, e.g., Au, for which the lines are ten times wider. Most spectrometers are equipped with electromechanical Mossbauer velocity transducers of the loudspeaker type. This technique is suitable for velocity variations ranging from less than 1 mm s full scale up to several cm s and covers the whole reach of hyperfine splitting for most of the common isotopes. Kalvius, Kankeleit, Cranshaw, and others [1-5] have been pioneers in the field, who laid foundations for the development of high-precision drives with feedback amplifiers for proper linear velocity scales with high stability and low hum. Other techniques for Doppler modulation have been developed for isotopes with extremely narrow hyperfine lines, e.g., Zn. For such isotopes, piezoelectric transducers are mostly used [6, 7], more details of which are found in Sect. 7.2.1. 

Most Mossbauer experiments are currently performed with commercially available radioactive sources. For some applications, however, a so-called source experiment may be useful, in which the sample is labeled with the radioactive parent-isotope of the Mossbauer nucleus such as Co. The y-radiation of the radioactive sample is then analyzed by moving a single-line absorber for Doppler modulation in front of the detector. 

H.S. Schweda, G.K. Chawla, R.W. Field, Highly excited, normally inaccessible vibrational levels by sub-Doppler modulated gain spectroscopy. Opt. Commun. 42, 165 (1982)... 

A typical Mossbauer experiment thus involves an oscillating radioactive source that contains a parent isotope (e.g., "Co for Fe), a stationary absorber that is usually the sample, and a detector. The Mossbauer spectrum consists of a plot of y-ray counts (relative absorption) as a function of the velocity of the source. In the source the radioactive isotope feeds the excited state of the Mossbauer isotope, which decays to the ground state. The energy of the recoil-free emitted radiation is Doppler modulated. Resonant absorption occurs when the energy of the y-ray just matches the nuclear transition energy for a Mossbauer atom in the absorber. This is detected by the decreased... 

Figure 9.26 (a) Doppler line shape with a Lamb dip. (b) As in (a) but with modulation and phase-... 

Chapter 3 is devoted to pressure transformation of the unresolved isotropic Raman scattering spectrum which consists of a single Q-branch much narrower than other branches (shaded in Fig. 0.2(a)). Therefore rotational collapse of the Q-branch is accomplished much earlier than that of the IR spectrum as a whole (e.g. in the gas phase). Attention is concentrated on the isotropic Q-branch of N2, which is significantly narrowed before the broadening produced by weak vibrational dephasing becomes dominant. It is remarkable that isotropic Q-branch collapse is indifferent to orientational relaxation. It is affected solely by rotational energy relaxation. This is an exceptional case of pure frequency modulation similar to the Dicke effect in atomic spectroscopy [13]. The only difference is that the frequency in the Q-branch is quadratic in J whereas in the Doppler contour it is linear in translational velocity v. Consequently the rotational frequency modulation is not Gaussian but is still Markovian and therefore subject to the impact theory. The Keilson-... 

A phase modulation can also be expressed as frequency modulation. The corresponding frequency deviation is the time derivative of the modulated phase angle (Pm t). According to the basic relationships Afrequency deviation Af(f) with respect to the carrier frequency fg, commonly known as the Doppler frequency shift... 

Figure 4.16. To cover all possible transitions in the absorbing nucleus, the energy of the source radiation is modulated by using the Doppler effect, such that the emitted radiation has an energy E v) = Eo(l + vjc). For Fe the required velocities fall in the range (1 to t-1 cm s k In Mossbauer emission spectroscopy, the sample under investigation is the source, and a single line absorber is...

There have been recent studies on the importance of NO in modulating skin blood flow in both normal animals and in inflammatory models. Khan etiU. (1993), using laser-Doppler techniques, showed that the NOS inhibitor L-NAME inhibited rabbit ear blood flow. It was possible to do this chronically for up to 2 weeks using implanted osmotic pumps. Pons et id. (1993) also used laser Doppler to show that the vasodilator eflFect of LPS in rabbit skin, which mimics the efiect of Gram-negative bacteria, was likely to involve both i-NOS and IL-1. We have already discussed the damaging eflPects of neutrophils... 

In real gases and liquids, however, atoms are never at rest. If y-emission takes place while the nucleus (or atom) is moving at velocity Un in the direction of the y-ray propagation, the y-photon of energy Ey is modulated by the Doppler energy Ep, [3] ... 

In an actual Mdssbauer transmission experiment, the radioactive source is periodically moved with controlled velocities, +u toward and —d away from the absorber (cf. Fig. 2.6). The motion modulates the energy of the y-photons arriving at the absorber because of the Doppler effect Ey = Eq + d/c). Alternatively, the sample may be moved with the source remaining fixed. The transmitted y-rays are detected with a y-counter and recorded as a function of the Doppler velocity, which yields the Mdssbauer spectrum, r(u). The amount of resonant nuclear y-absorption is determined by the overlap of the shifted emission line and the absorption line, such that greater overlap yields less transmission maximum resonance occurs at complete overlap of emission and absorption lines. 

Mossbauer spectra are usually recorded in transmission geometry, whereby the sample, representing the absorber, contains the stable Mossbauer isotope, i.e., it is not radioactive. A scheme of a typical spectrometer setup is depicted in Fig. 3.1. The radioactive Mossbauer source is attached to the electro-mechanical velocity transducer, or Mossbauer drive, which is moved in a controlled manner for the modulation of the emitted y-radiation by the Doppler effect. The Mossbauer drive is powered by the electronic drive control unit according to a reference voltage (Fr), provided by the digital function generator. Most Mossbauer spectrometers are operated in constant-acceleration mode, in which the drive velocity is linearly swept up and down, either in a saw-tooth or in a triangular mode. In either case. 

Micro-Doppler analyses can also be used for people identification. In that case, it is possible to detect Doppler frequency modulation caused by heartbeats, breathing and body motion (walk, run, head turns etc.). 

Figure 6.3. Schematic of interference fringe region, measurement volume, Doppler burst signals, and effects of particle size on signal modulation.

By modulating the electric field and using phase-sensitive detection methods, Uehara et al. 8 ) were able to increase the sensitivity considerably and they could even detect Stark splittings of less than the doppler width of the components. Fig. 3 shows the Stark spectrum of HDCO for different electric field strengths. Because of the Stark modulation technique the absorption lines appear differentiated the zero points represent the center of each line. 

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