Coherent dynamics of multilevel systems

A theoretical analysis of high-gain Compton free electron laser dynamics when the radio frequency (rf) field evolves self-consistently with single resonant particles representing a pre-bunched electron beam is presented and the single particle phase space is studied. Using a set of universally scaled equations the optimal parameters of excitation for a free electron laser (FEL) in the single particle approximation are found. Using a new approach the relations between the initial rf field amplitude, the output field amplitude and the particle’s initial detuning from resonance are also found. A set of equations describing the self-consistent evolution of the rf field with e-bunch macro-parameters, such as bunch width and bunch mean phase, is derived from the universally scaled equations under the condition of the uniform initial distribution of electrons in the bunch. The analysis of optical field generation and amplification by compact bunches of electrons is provided. The saturation regime and the physical reason for the saturation in the high-gain Compton FEL driven by pre-bunched electrons are studied and discussed.

There were examined photoionization spectra of benzene cooled in supersonic molecular beams. Compounds of benzene vapor with several buffer gases were used -- He, Ne, Ar, Kr, N<SUB>2</SUB>, CH<SUB>4</SUB>. The photoionization was performed by frequency tuning KrF laser (linewidth 0.8 cm<SUP>-1</SUP>; tuning range 120 cm<SUP>-1</SUP>). The process is step-by-step, with the real intermediate level. The tuning range envelops two vibronic bands 6<SUB>0</SUB><SUP>1</SUP>1<SUB>0</SUB><SUP>1</SUP>16<SUB>1</SUB><SUP>1</SUP> and 6<SUB>1</SUB><SUP>1</SUP>1<SUB>0</SUB><SUP>3</SUP>, belonging to electron transition <SUP>1</SUP>A<SUB>1g</SUB> - <SUP>1</SUP>B<SUB>2u</SUB>. The ionization spectrum is mainly dependent on the absorption spectrum at the first step. The both bands are hot, but nevertheless the ionization efficiency at supersonic beam conditions is rather high; it's caused by large 'gap' of vibronic temperature from rotational and translative ones. As it was experimentally defined with various buffer gases, that bands intensity relation varies too. This fact testify about vibronic cooling dependence on the sort of buffer. Besides, a significant difference in the bands envelope contour widths was discovered. That is rather unusual fact, as the band width mainly depends on rotative temperature in the low state (rotative band structure wasn't resolved in that experiments). Based on their data the rotative temperature estimation shows for benzene, that rotative temperature of molecules with excited 6<SUP>th</SUP> vibronic mode is always lower (approximately equals 4.5 K with Ne buffer), than molecules with 16<SUP>th</SUP> one (approximately equals 15 K at the same conditions). This fact was interpreted as the rotationally-vibronic interaction result. As degenerated 6<SUP>th</SUP> mode has vibrational angular momentum, which is absent in the 16<SUP>th</SUP>, degenerated as well, so rotative-vibronic interaction in the first case is significantly strong, and leads to levels splitting comparable with the rotative structure. As the result rotative levels density for 6<SUP>th</SUP> mode twice more. The interpretation moment for rotative relaxation, i.e., the time, or characteristic distance from the nozzle cut-off, where rotative relaxation in the molecular beam forming on translative temperature and molecular density as well as sublevels energy interval. The last is twice less for molecula with the excited 6<SUP>th</SUP> mode, that is the reason of their more intense rotative cooling.

Resonance spectra are presented for electron-molecule scattering using supersonic molecular beams. The supersonic beam is crossed at right angles by an electron beam. Results for N2 and CO are presented. (AIP).

Application of a modified version of high-temperature high-pressure all-metal pulsed source of supersonic molecular beam is demonstrated in a production of van der Waals (vdW) complexes. The vdW complexes are produced possessing controllable rotational temperature (Trot) in the range from 3 K to 19 K. An effective control over Trot is illustrated employing excitation spectrum recorded using the B31(53P1) ← X10+(51S0) transition in CdAr. First-time resolved rotational structure in the profile of the υ′ = 2←υ′′ = 0 vibrational component is reported. The control over Trot is crucial in a dissociation of the (111Cd)2 isotopologue in the supersonic beam. For the process, excitation at well defined J′← J′′ rotational transition within the (υ′,υ′′) = (40,0) vibrational band of the A10u+(51P1) ← X10g+(51S0) transition is employed. It is followed by the dissociation using A10u+(υ′ = 40,J′′) → X10g+ bound → free transition. An analysis and simulation of the (40,0) vibrational band rotational structure are presented. Parameters describing conditions in the supersonic beam, degree of rotational cooling, Doppler broadening and spectral bandwidth of the laser beam are used.

Microwave assisted magnetic recording (MAMR) has been proposed as one of the prospective recording technologies for forthcoming tera-bit-class magnetic recording. In this paper, the technological and related physical issues of MAMR are reviewed. MAMR is the combined technology of microwave assisted switching (MAS) and spin torque oscillator (STO). MAS is a magnetization switching technology in which magnetization switches through excitation of its precessional motion by a radio frequency (rf) field. The MAS behavior in which nonlinear large angle precession is excited has been a challenging issue from scientific and technological viewpoints. Recently developed theories successfully predict the MAS behavior of a single macrospin in which all spins in a magnet are assumed to behave coherently. For a magnetic element with a size larger than the exchange length, spatially non-uniform magnetization precession modes become dominant instead of the coherent precession mentioned above, resulting in significant enhancement of the MAS effect. This size dependent MAS effect is experimentally very well verified for nanosized dots with perpendicular magnetization. Meanwhile, the STO is used as an rf field generator in MAMR technology. The aforementioned large angle magnetization precession dynamics are also a crucial problem for the stable operation of STO. While various types of STOs have been proposed so far, the optimum structure of STO for MAMR is still under investigation. The combination of the MAS and STO may provide frequency domain recording architecture which enables us to realize a new recording scheme, such as multilevel recording.

This study concerns the thallium‐205 cation in aqueous solution in the presence of a calixarene molecule. Although the measurement of the self‐diffusion coefficient of pure thallium (without calixarene in the aqueous solution) does not pose any particular problem, major difficulties are encountered with the standard method using gradient strength increment as soon as thallium is partly complexed by calixarene. With static magnetic field gradients, the NMR signal is so weak that it prevents any reliable measurement, whereas radio frequency (rf) field gradients lead to an unrealistic value of the diffusion coefficient. This failure is explained by the fact that thallium is in fast exchange between two sites (complexed and free thallium) thus exhibiting a single NMR signal although, in the course of the experiment, two signals, with an important difference in resonance frequencies (due to the large thallium chemical shift range), are effectively involved. With the objective to understand these quite unexpected observations, the theory underlying NMR diffusion experiments is first reviewed, and criteria of fast exchange are discussed for three parameters: chemical shifts, relaxation rates, and diffusion coefficients. It turns out that off‐resonance effects are responsible for unwanted defocusing due to rf pulses in the static magnetic field gradient method and for time‐dependent gradients in the rf field gradient method. Concerning the latter, a remedy is proposed which consists in applying the stronger gradient and incrementing the gradient pulse durations. After correction for relaxation, the expected value of the diffusion coefficient is retrieved. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 127–137, 2010.

The electron transport collision frequency in a radio frequency (rf) field νen and the effective rf field frequency ωeff have been found as appropriate integrals of the electron energy distribution function measured in an argon inductively coupled plasma (ICP) at 6.78 MHz. Also, the effective electron collision frequency νeff and ωeff have been found from the plasma conductivity measured with a magnetic probe. It is shown that due to the Ramsauer effect and collisionless power absorption in the ICP, νeff and νeff/ωeff vary unexpectedly slowly with gas pressure.

Magnetic resonance imaging (MRI) is a well established diagnostic technique, one from which all patients should be able to benefit, including those with implanted medical devices. This paper describes an experimental and numerical study of the temperature rise near the ends of wires by the radio frequency (rf) field in MRI. These wires simulate long wires which may be part of a medical implant. Temperature rise as a function of time was measured for wires of different lengths and diameters in a phantom exposed to the 64 MHz rf field inside a MRI body coil. For wires with no or thin insulation, the maximal rise was about seven times background for a wire length of 20 cm; wires with longer and shorter lengths exhibited less temperature rise. The temperature rise was greater for wires with thicker insulation. Computer simulations using a quasistatic model are in reasonable agreement with the measurements on shorter wires. It is concluded that medical implants with long conducting lead wires may result in potentially unsafe temperature rise during MRI imaging.

Measurement of short transverse relaxation times by pseudo-echo nutation experiments

In several MRI and MRS applications it is necessary to select a well-defined region of interest (ROI) with optimized signal-to-noise ratio and radio frequency (RF) field homogeneity. We have shown, theoretically and experimentally, that transverse field RF surface coils comprising a multitude of parallel linear elements are characterized by improved B1 spatial homogeneity and sensitivity in the central ROI, with respect to the standard square loop design. Finite-element modelling of RF coils tuned to 64.0 MHz was used to study the effect of the number of linear current elements and relative positioning with respect to B1 field homogeneity and sensitivity in the central ROI, both when empty and in the presence of a homogeneous tissue model. Workbench B1 field calibrations obtained with RF coil prototypes tuned to 64.0 MHz are in good agreement with our theoretical finite-element modelling. We have shown that the number of linear current elements and their relative positioning can be carefully selected to optimize either the B1 field homogeneity or sensitivity in the central ROI. We have shown that a number of multi-element RF coil geometries can be chosen to obtain an optimized RF field distribution and/or field amplitude. For example, we found that a six-element RF coil with 6 mm element separation is a good compromise for optimizing the B1 sensitivity (up to a factor 2) and the spatial homogeneity (about 30 mm with ΔB1/B1 ⩽ 20%) in the central ROI, with respect to the standard square loop or the two-element FO8 design. MRI images obtained at 2.35 T (proton frequency 100.34 MHz) with standard GE pulse sequences showed that the six-element FO8 prototype significantly increases the RF field homogeneity and RF field amplitude, with respect to the SL coil design.

A comprehensive theoretical description is given for the effects of magnetic resonance on the angular distribution of radiation emitted from oriented nuclear states. The formulation is made in a general way. It may be applied to an ensemble of nuclei oriented by any method: For example, nuclear reactions, angular correlations, and low-temperature nuclear orientation may be treated. In fact the theory can also be applied to optical-double-resonance experiments. Statistical tensors are defined to describe nuclear crientation in the "resonant" state. Interactions of the oriented ensemble with extranuclear fields are then considered, and the effect of a radio-frequency (rf) field on the angular distribution of radiation is given. Two formulations are given for the "pure magnetic" case, for which numerical calculations were done. One employs an angular correlation formalism, following the evolution of the density matrix in the laboratory frame $S$, while the other is more closely related to conventional NMR. In the latter approach the transformation into the frame ${S}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}$, wherein the statistical tensors are time-invariant, is described in terms of a "generalized torque equation" governing the motion of a unit vector along the symmetry axis in ${S}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}$. Both formulations are exact. Time-dependent distribution functions are worked out in detail, with both fixed and random-phase angles between the rf field and the initial symmetry direction. Fast oscillations due to the constant magnetic field are modulated by slow oscillations due to the rf field. Time-integral curves were calculated. These show great sensitivity to the rank of the relevant statistical tensor, to geometry, and to the phase of the rf field. Multipole structure is predicted for certain geometries, with the resonance line showing a number of maxima equal to the rank of the statistical tensor. Under certain conditions two types of asymmetry are observable. A "transient" asymmetry appears for low-rf-field values: This asymmetry is sensitive to the sign of the nuclear moment, but it disappears in high-rf fields. Odd-rank statistical tensors can also give response functions with "persistent" asymmetry that remains at high-rf fields. This is a parity effect and is not sensitive to the sign of the nuclear moment. Effects of relaxation are also discussed briefly.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTResonant two-photon ionization spectroscopy in supersonic beams for discrimination of disubstituted benzenes in mass spectrometryChung Hang. Sin, Roger. Tembreull, and David M. LubmanCite this: Anal. Chem. 1984, 56, 14, 2776–2781Publication Date (Print):December 1, 1984Publication History Published online1 May 2002Published inissue 1 December 1984https://pubs.acs.org/doi/10.1021/ac00278a033https://doi.org/10.1021/ac00278a033research-articleACS PublicationsRequest reuse permissionsArticle Views110Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

Test particle motion and acceleration has been explored in strong radio frequency (RF) fields, for which quasilinear ponderomotive force approximation is not valid. By nonlinear acceleration in spatially varying wave amplitude of RF travelling wave, electrons may be accelerated to time averaged velocities significantly larger than the RF wave phase velocity, and than the boundary plasma thermal velocity, in RF fields of several Volts per centimeter at wave frequency of 7 MHz. It is also demonstrated that even weak spatial gradients, much weaker than those expected in experiments, of the RF wave field amplitude, have significant consequences for the particle motion. Estimates are presented of the total energy transferred from the near antenna RF field to the plasma due to the nonlinear electron acceleration effects.

We explore properties of atoms whose magnetic hyperfine sublevels are coupled by an external magnetic radio frequency (rf) field. We perform a thorough theoretical analysis of this driven system and present a number of systematic approximations which eventually give rise to dressed adiabatic radio frequency potentials. The predictions of this analytical investigation are compared to numerically exact results obtained by a wave packet propagation. We outline the versatility and flexibility of this class of potentials and demonstrate their potential use to build atom optical elements such as double wells, interferometers, and ringtraps. Moreover, we perform simulations of interference experiments carried out in rf induced double-well potentials. We discuss how the nature of the atom-field coupling mechanism gives rise to a decrease of the interference contrast.

Frequency-shifted excitation by phase-incremented pulses of arbitrary shape can be solved by introducing a second rotating frame. The results show that a phase-incremented pulse can be decomposed into an infinite number of unsymmetrically amplitude-scaled and phase-shifted effective radio frequency (rf) fields, which are responsible for the unsymmetrical and phase-shifted centerband and sideband excitations. In addition to a universal phase shift associated with each band of excitation, a phase inversion occurs when the scaling factor of the effective rf field becomes negative. Also if the total phase increment of the pulse is not equal to 2kπ (k integer) an additional phase shift equal to the total phase increment will be introduced. By properly choosing the total phase increment the phase shift of the centerband can be compensated. A computer program based on the Bloch equations is developed to calculate directly the excitation profiles of phase-incremented pulses, which agrees well with the theoretical predications.