Highly resolved measurements of Stark-tuned Förster resonances between Rydberg atoms

We report on experiments exploring Stark-tuned F?rster resonances between Rydberg atoms with high resolution in the F?rster defect. The individual resonances are expected to exhibit different angular dependencies, opening the possibility to tune not only the interaction strength but also the angular dependence of the pair state potentials by an external electric field. We achieve a high resolution by optical Ramsey interferometry for Rydberg atoms combined with electric field pulses. The resonances are detected by a loss of visibility in the Ramsey fringes due to resonances in the interaction. We present measurements of the density dependence as well as of the coherence time at and close to F?rster resonances.     

Effect of photoions on the line shape of the Förster resonance lines and microwave transitions in cold rubidium Rydberg atoms

Experiments are carried out on the spectroscopy of the Förster resonance lines Rb(37P) + Rb(37P) → Rb(37S) + Rb(38S) and microwave transitions nP → n′S, n′D between Rydberg states of cold rubidium atoms in a magneto-optical trap (MOT). Under ordinary conditions, all spectra exhibit a linewidth of 2–3 MHz irrespective of the interaction time between atoms or between atoms and microwave radiation, although the limit resonance width should be determined by the inverse interaction time. The analysis of experimental conditions has shown that the main source of line broadening is the inhomogeneous electric field of cold photoions that are generated under the excitation of initial nP Rydberg states by broadband pulsed laser radiation. The application of an additional electric-field pulse that rapidly extracts photoions produced by a laser pulse leads to a considerable narrowing of lines of microwave resonances and the Förster resonance. Various sources of line broadening in cold Rydberg atoms are analyzed.     

The theoretical and experimental investigations of Stark structure in sodium Rydberg states atoms

The experimental results of the Stark structure of Na in the vicinity of n~*=21-23 with the external electric field from 0 to 380V/cm are reported. The theoretical calculation of the stark levels of Na based on the atomic potential model are in good agreement with experiment. The limiting error between the theory and experiment turns out to be due to experimental uncertainty.     

Specifics of the stochastic ionization of a Rydberg collision complex with Förster resonance

The physics of the formation of dynamic nonlinear resonances in an isolated Rydberg collisional complex is described. The development of the stochastic instability of Rydberg electron trajectories due to charge exchange in the complex is considered. The realization of the resonance in external statistic magnetic and electric fields is predicted to be accompanied by a significant narrowing of areas of stochastic motion with a concurrent decrease in the rates of the ionization of real quasimolecular systems proceeding through the migration over the Rydberg crowding of quantum states.     

Chemical states of localized Fe atoms in ethylene matrices using in-beam Mössbauer spectroscopy

The reaction products of isolated single iron atoms in a low concentration matrix of ethylene were studied using in-beam Mössbauer spectroscopy with a short-lived ⁵⁷Mn (T _1/2=1.45 m) beam. The in-beam Mössbauer spectrum of ⁵⁷Fe arising from ⁵⁷Mn in a matrix of ethylene and argon measured at 16 K was analyzed with four components. Density functional theory calculations were carried out to confirm the assignments. It was suggested that the reaction produced monoiron species of Fe(C ₂ H ₄) with a spin state of S = 2.     

Interaction of excitons in indirect solid solutions of In1−xGaxP at high levels of excitation

The cathode luminescence spectra of GaP and solid solutions of In_1–xGa_xP (with an indirect energy band structure) obtained by liquid-phase epitaxy are investigated at T=77 and 9°K. It is established that the recombination of excitons on neutral donors and the pure exciton recombination with the participation of TA (=13±1 meV), LA (=31±1 meV), and TO (=43±2 meV) phonons. dominate in the low-temperature luminescence spectra of GaP and In_1–xGa_xP with a small concentration of photon impurities. The complex luminescence band that arises for sufficiently high levels of excitation and at T = 9°K is attributed to the luminescence of an electron-hole drop (EHD). The transformation of spectral form of the EHD band with the transition from GaP to In_1–xGa_xP is attributed to a simultaneous reduction in the contribution of the TA and TO replicas and to the appearance of a phononless component in the luminescence of the EHD.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 37–40, September, 1985.     

Mössbauer spectroscopy with high velocity resolution in the study of ordinary chondrites

Mössbauer spectroscopy with high velocity resolution was used to study of 11 ordinary chondrites from L and H chemical groups. Mössbauer spectra were recorded in 4,096 channels and then presented in 1,024 channels. An increase of velocity resolution allowed us to carry out more detailed analysis of ordinary chondrites and to decrease experimental error for evaluation of hyperfine parameters in comparison with previous chondrite Mössbauer spectra measured in 512 channels or less. Variations of hyperfine parameters were observed for corresponding iron bearing phases in chondrites. Two crystallographically non-equivalent octahedral sites M1 and M2 in olivine and pyroxene were revealed in Mössbauer spectra of bulk chondrite samples.     

Pulsed photoconductivity in diamond upon quasi-continuous laser excitation at 222 nm at the formation of an electron–hole liquid

An order-of-magnitude enhancement of the pulsed photocurrent in a polycrystalline diamond sample synthesized by chemical vapor deposition is observed under the conditions of formation of an electron–hole liquid. Nonequilibrium charge carriers are excited by laser pulses at a wavelength of 222 nm with FWHM pulse duration of 18 ns and peak intensity above 2.5 MW/cm² upon cooling the sample to 90 K. For peak intensities of laser excitation lower than 1 MW/cm², sample cooling from 300 to 90 K leads to a decrease in pulsed photocurrent by about a factor of 5. The observed increase in pulsed photocurrent is attributed to the formation of the electron–hole liquid.     

Saturated Förster resonance energy transfer microscopy with a stimulated emission depletion beam: a pathway toward single-molecule resolution in far-field bioimaging

We theoretically demonstrate that the spatial resolution of stimulated emission depletion (STED) microscopy can be substantially enhanced without increasing the intensity of the STED beam. In our scheme, tiny nanobeads codoped with donor and acceptor molecules are used as fluorescent probes, in which F?rster resonance energy transfer (FRET) can occur with an ~100% efficiency between the donors and acceptors. Enhancement of the depletion of acceptors in the nanobeads with the doughnut-shaped depletion beam can lead to an increase of FRET efficiency in the outer area of the excitation spot, which itself is used for deexciting donor molecules and, consequently, enhancing the optical resolution.     

Changes of valence state of Yb in YbCuAl at high pressure: A Mössbauer study

The 84.3 keV Mössbauer transition in¹⁷⁰Yb was used to investigate the intermediate valent intermetallic compound YbCuAl at pressures up to 130 kbar and in the temperature range between 1.6 K and 77 K. The electric quadrupole interaction parameters |eQV _zz | and , as well as their temperature variation are drastically dependent on pressure: |eQV _zz | is at 4.2 K larger by almost a factor of three at 130 kbar when compared to ambient pressure. On the other hand, |eQV _zz | drops by more than a factor of two at 130 kbar when the temperature is raised from 1.6 K to 77 K. A much smaller temperature dependence of quadrupole coupling is present at ambient pressure. Both these effects give strong evidence for a valence transition towards the 4f ¹³ (Yb³⁺) configuration under applied pressure. At 4.2 K this transition is completed near 50 kbar. No magnetic order could be detected down to 1.6 K over the whole pressure range. Some line broadening is observed at high pressures and low temperatures which most probably is related to paramagnetic relaxation phenomena and supports the claim that the Yb ion is in a magnetic state under these conditions.Work supported by the Deutsche Forschungsgemeinschaft     

Mössbauer spectroscopy with high velocity resolution in the study of ferritin and Imferon: preliminary results

An increase of velocity resolution in Mössbauer spectroscopy allows increasing accuracy and decreasing experimental error in determination of hyperfine parameters. Lyophilized samples of human liver ferritin and Imferon (Fisons UK) were measured at room temperature by Mössbauer spectroscopy with high velocity resolution. Spectra were measured in 4,096 channels for further presentation in 256, 512, 1,024, 2,048 and 4,096 channels. The differences of quadrupole splitting and isomer shift revealed for both ferritin and Imferon Mössbauer spectra presented in 512 and 2,048 channels, respectively, were much better distinguished in case of higher velocity resolution. Therefore, Mössbauer spectroscopy with high velocity resolution may be very useful to reveal small differences of hyperfine parameters for biological subjects in biomedical research.     

Förster Energy Transfer in Arrays of Epitaxial CdSe/ZnSe Quantum Dots Involving Bright and Dark Excitons

Using time-resolved photoluminescence (PL) spectroscopy, we establish the presence of the Förster energy transfer mechanism between two arrays of epitaxial CdSe/ZnSe quantum dots (QDs) of different sizes. The mechanism operates through dipole–dipole interaction between ground excitonic states of the smaller QDs and excited states of the larger QDs. The dependence of energy transfer efficiency on the width of barrier separating the QD insets is shown to be in line with the Förster mechanism. The temperature dependence of the PL decay times and PL intensity suggests the involvement of dark excitons in the energy transfer process.     

Calculation of the Förster Resonance Energy Transfer Parameters in Nanospheres Containing CdSe/ZnS Quantum Dots and Diarylethene

Journal of Applied Spectroscopy - An analysis is presented for theoretical model studies of photochromic systems with reversible fluorescence modulation derived from polymer nanospheres containing...     

Study of visually different areas in the Chinga iron meteorite fragment using Mössbauer spectroscopy with a high velocity resolution

Visually different areas on the saw-cut surface of Chinga metal meteorite fragment were observed. Study of metal samples from these areas was carried out using scanning electron microscopy, X-ray diffraction and Mössbauer spectroscopy with a high velocity resolution. The obtained differences in Mössbauer parameters may be related to the differences in the microstructural Fe–Ni phase composition.     

Narrow α + <Superscript>28</Superscript>Si elastic-scattering states at high excitation in <Superscript>32</Superscript>S

The excitation function and angular distributions of elastic α-particle scattering on ²⁸Si have been measured in the laboratory energy range 6-28 MeV using a backscattering technique on a thick target, yielding a continuous energy distribution. More than 200 narrow states are observed, with widths in the range ∼ 30-100 keV at excitation energies E ^* = 13-32 MeV. Angular distributions at backward angles were measured, and angular momentum values of more than 83 states have been deduced. The analysis gives spin-parities J ^π, α-partial widths Γ_α and reduced widths of the narrow high-lying resonant states in ³²S. The experimentally observed states display both the negative- and the positive-parity rotational-like sequences with seemingly no parity splitting, a finding which is at variance with most potential-model predictions. The deduced effective moment of inertia indicates a more extended structure than the ground-state configuration. The observed strength of each ℓ-value is analyzed in terms of an underlying split doorway state of Lorentzian form, which yields an interpretation as fragmented rotational α + ²⁸Si states. Received: 26 June 2000 / Accepted: 16 September 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: kkallman@abo.fi RID="b" ID="b"Present address: Swedish Polytechnic, FIN-65200 Vasa, Finland. Communicated by D. Guerreau     

Tunnelling splittings in the high resolution microwave and UV spectra of the benzonitrile—water complex: modelling the internal motion in its S0 and S1 states

Refined values of the barriers to internal motion in the 1:1 complex between benzonitrile and water in the gas phase have been determined from analyses of its fully resolved microwave and ultraviolet spectra. Both spectra exhibit tunnelling splittings associated with this motion. Modelling this behaviour using a semi-rigid Hamiltonian for an internal rotation of water around its C ₂ axis yields values of V ₂ = 440 ± 30 cm^?1 and 450 ± 30 cm^?1 in the ground and excited electronic states, respectively. These relatively high barriers are a consequence of two hydrogen bonds between the interacting species.