Time-delayed detection in level anticrossing spectroscopy

Summary In the present paper we discuss the time-delayed detection of the fluorescence signal in a level anticrossing experiment showing that, unlike what occurs in other methods, it does not lead, in general, to a signal profile narrowing as the delay time increases. However, we find conditions under which this technique provides the possibility of enhanced spectral resolution, as well as gives a direct measurement of the difference between the decay rates of the levels involved.     

Quantum optics with nuclear gamma radiation

Some features of quantum optics related to inversionless amplification are translated to nuclear systems. Nuclei and nuclear transitions differ in several ways from atomic systems: gamma radiation has a much shorter wavelength (of the order of lattice-distances), nuclear transitions are many orders of magnitude weaker than atomic ones (making nuclear pumping extremely hard), no coherent gamma-ray sources are available to produce bichromatic gamma radiation. All this makes it very hard to merely translate quantum optical results to nuclear systems. We show that under very specific conditions destructive quantum interferences can occur in the nuclear absorption probability, while the stimulated emission probability is not affected. Nuclear level mixing plays a crucial role in this. This revised version was published online in August 2006 with corrections to the Cover Date.     

Phase modulation of recoilless gamma radiation by nuclear zeeman effect

The use of an alternating magnetic field for phase modulation of recoilless gamma radiation is studied. The measurements were performed with the Mössbauer resonances of Fe-57 and Zn-67. Novel time-domain data are presented from experiments on the nuclear Zeeman effect at high modulation frequencies.     

Observations of elastic and quasi-elastic nuclear gamma resonance absorption in hemoglobin crystals

Mössbauer spectra of carbonmonoxy-liganded human hemoglobin crystals, selectively enriched with ⁵⁷Fe in the α-chains, were measured at 260 K and at 101 K a quadrupole doublet of lorentzians with the usual, narrow line width was observed, while at 260 K a superposition was observed of one doublet with a narrow and a second one with a broadened line width. The narrow doublet is attributed to elastic absorption processes and the broadened doublet to quasi-elastic ones. The spectrum at 260 K indicates that fluctuational processes between conformational substates of the quarternary “R” structure do exist.     

On the duration of the resonance absorption and emission of nuclear gamma rays

The Krylov-Fock theorem is applicable to describing the decay of an excited quantum-mechanical system in the case where the time of excited-state formation is much shorter than the characteristic decay time. The results obtained by calculating, on the basis of this theorem, the average lifetime t_av of an excited state for nuclei that resonantly scatter γ rays are in serious contradiction with available data on t_av including experimental data. This disproves the assumption that the resonance absorption and emission of γ rays proceeds within times much shorter than τ=?/Γ (Γ is the intrinsic width of a nuclear level). A qualitative explanation of this effect is proposed.     

Observation of level anticrossing effects in the laser-Stark spectra of POF3

Nonlinear level anticrossing effects have been observed in saturated absorption in transitions in the ν₄ band of POF₃ by the method of laser-Stark spectroscopy. The anticrossings occur as narrow additional absorptions superimposed on the Doppler-broadened transitions. From the resonant fields of the anticrossings, values of A₀-B₀ have been calculated which agree well with values obtained previously. The widths of the anticrossings were found to be independent of sample pressure in the 10- to 50-m Torr range. A procedure for calculating the lineshape for level crossing or level anticrossing that has a greater range of validity than the conventional perturbation theory is described.     

Inelastic nuclear resonant absorption of synchrotron radiation in thin films and multilayers

Inelastic nuclear resonant absorption of synchrotron radiation is an efficient and unique method for the direct measurement of vibrational density of states (VDOS) of thin films and interfaces that contain Mössbauer isotopes. This is demonstrated for the ⁵⁷Fe nuclear resonance in the case of amorphous and crystalline Tb–Fe alloy thin films and buried Fe/Cr interfaces in epitaxial α-Fe(0 0 1)/Cr(0 0 1) superlattices.

     

Optically induced level anticrossing in undoped GaAs/AlGaAs coupled double quantum wells

We report a photoluminescence detected anticrossing of the energy levels in an undoped asymmetric coupled-double-quantum-well buried in a p-i-n structure. Due to the built-in electric field, the quantum wells are tilted in such a way that the symmetric energy level is higher than that of the antisymmetric one in the conduction band. Keeping the laser excitation energy below the barrier, with increasing laser power, the level anticrossing and the quantum confined Stark effect were observed due to decreasing built-in electric field by the photogenerated electron and hole pairs.     

132Cs gamma radiation

The decay of¹³²Cs was studied by means of Ge(Li) detectors and a magnetic beta-ray spectrometer. The energies and intensities of the transitions in¹³²Xe and¹³²Ba were determined. Of the decay schemes proposed hitherto, that of Johnson et al. [Phys. Rev.138 (1965), B 520] was confirmed.     

Controlling two-dimensional electron localization via phase-controlled absorption and gain in the three-coupled quantum wells

We investigate two-dimensional (2D) electron localization via phase-controlled absorption and gain of a weak probe field in an asymmetric semiconductor three-coupled quantum well (TCQW) with a closed loop under the action of two orthogonal standing-wave fields. It is found that we can achieve high-precision and high-resolution 2D electron localization via properly varying the parameters of the system. The influences of direct one-photon transition and indirect three-photon transition on the precision of probe absorption–gain spectra are also discussed in details. Thus, the proposed scheme shows the underlying probability for the formation of the 2D electron localization in a solid.     

Quantum-interference effects for gamma radiation under crossing-anticrossing conditions for nuclear levels in an RF field

A new approach to observing the effect of electromagnetically induced transparency in gamma optics is proposed. The propagation of a resonant photon in a ⁵⁷Fe magnetic medium in an applied rf field is considered for this purpose. It is shown that, under crossing-anticrossing conditions, a resonant rf field substantially changes the gamma-optical properties of the medium, which become dependent on the parameters of the field. This opens the possibility for exercising a coherent control of the photon group velocity and a controllable filtration of unpolarized gamma radiation in a sample.     

Two-quantum stimulated emission of gamma radiation

Various schemes exploiting two-quantum nuclear transitions for the generation of coherent gamma radiation (stimulated anti-Stokes and stimulated two-quantum de-excitation of isomeric nuclei in an active medium with the hidden population inversion of nuclear states and the ignition of an avalanche-like stimulated gamma-emission by counterpropagating X-rays) are discussed.     

Temporal optics of recoil-free gamma radiation

A discussion is given of the time-behavior of recoil-free γ-radiation passing through a resonant medium. The classical dispersion theory of Hamermesh is extended to account for different linewidths in source or absorber so that a synchrotron-radiation source can be simulated. A result, essentially the same as that of Kagan et al., follows in the limit of large source width. A review is given of the quantum beats which result from the absorption of one line of a frequency-modulation spectrum. An example is given of the measurement of second-order Doppler shift in Fe−Be by the method of quantum beats. Finally, there is a discussion of the measurement by quantum beats of the very small energy shift in a⁵⁷Co−Cu source by a magnetic field.     

Cherenkov-transition radiation in gamma frequency region

The spectral and angular distributions, as well as the total number of photons of gammaray Cherenkov-transition radiation (GCTR) produced by charged particles in the photon energy region (0.8–2) MeV, are calculated using the results of the recent discovery that in the above-mentioned region the refraction index of some materials is greater than unity, n(ω) > 1, due to Delbruck scattering on Coulomb field of nuclei. Using the results of the numerical calculations carried out, we propose an experimental arrangement for observation and study of GCTR produced by electrons with energy much higher than the threshold energy of GCTR (～10 GeV). Some applications of GCTR, in particular, the search for new materials with n(ω) > 1 in gamma region with the help of this method, are discussed.     

Hard gamma as probe of nuclear dynamics

Experimental results concerning the ⁵⁸Ni + ⁵⁸Ni at 30 MeV/u and ⁵⁸Ni + ¹⁹⁷Au at 30 and 45 MeV/u reactions are presented. The emission mechanism of hard photons has been exploited to get information on the time evolution of the reactions. From the measured hard gamma multiplicity associated to different classes of heavy residues a quantitative estimate of the reaction centrality has been obtained. Moreover, evidence of a prompt Intermediate Mass Fragment (IMF) emission is shown for the Ni + Au reaction at 45 MeV/u for central collisions exploring the correlation function between thermal photons and IMF’s.