Resonant Coherent Excitation Research Articles

Phonon modes involved in triggering threshold phenomenon characterising sliding-velocity dependence of wearless nanofriction

ABSTRACT Wearless nanofriction between nonmetallic solids is investigated in terms of molecular dynamics (MD). The results of this study concern wearless-nanofrictional characteristics of the MD models in which the thermal-buffer zone is located far enough from the sliding interface so as not to disturb the heat generation mechanism. An enormous number of simulation runs and frequent validation of their results were carried out so that the pressure and the interfacial temperature, which directly affect the heat generation processes, were made common at different sliding velocities even under the above constraint. Sliding-velocity dependence of the ensemble-averaged frictional force density under the steady sliding condition is characterised by the threshold phenomenon. The dramatic increase in rate of the frictional-heat generation at sliding speeds above the threshold is triggered by resonant coherent excitation of certain phonon modes. Spatiotemporal patterns of such elastic waves hidden in random thermal motion of atoms near the sliding interface were clarified by the sampling and averaging. Features of these patterns are discussed in relation to the threshold characteristics of wearless nanofriction.

Resonant coherent excitation and energy loss of slow channeling helium ions in AlN

By performing real-time time-dependent density-functional theory dynamic simulations, we report a velocity-dependent charge resonance on slow initial bare ions when channeling through the rocksalt crystalline AlN. Such charge resonance is accompanied by tremendously enhanced stopping at narrow velocity windows, which is attributed to the depletion in electronic screening by bound electrons, when the formation of defect state on projectile is depressed dramatically by the periodical field from lattice arrangement. The enhancement of stopping is not found in simulations with initial neutral incident ion, suggesting the effect of such charge resonance process mainly resides in the charge accumulation during the preequilibrium state, while making marginal influence on the excitation of bound state charge. We also investigated the charge resonance with revised lattice parameter, confirming the periodic atomic arrangement of lattice plane along ion trajectory is the main cause of such charge resonance.

Effect of resonant coherent excitation on the electronic stopping of slow channeled ions

We calculated the nonlinear screening and the electronic stopping of energetic helium ions in a III-V compound semiconductor at low speeds below stopping maximum under channeling conditions. For the range of velocity considered, it is found the energy loss of the intruding ions show pronounced dependence on the electronic screening from the induced charge. Different with the prevalent scenario described by free-electron gas model that the induced charge distribution lags behind the projectile forming the so-called wake potential. We found through real-time time-dependent density functional theory that the induced charge by the channeling ion keeps oscillating back and front when channeling through the zinc-blende crystalline GaN, which is interpreted as a consequence of temporally oscillating Coulomb field arising from the periodical atomic arrangement along ion trajectory. When one of the frequencies coincides with the transition energy of the ionic charge, it would be resonantly excited by the oscillating field, which further gives rise to a depletion in electronic screening by bound electrons to the ion, resulting in odd enhanced stopping trend at narrow velocity windows.

6.2.3 Interaction between Heavy Ion Beams at Several Hundreds MeV/u and Crystal Targets —Three-dimensional Resonant Coherent Excitation—

6.2.3 Interaction between Heavy Ion Beams at Several Hundreds MeV/u and Crystal Targets —Three-dimensional Resonant Coherent Excitation—

Applications of ion channeling in accelerator mass spectrometry

Three applications based on the passage of channeled ions through a crystalline membrane are proposed for the accelerator mass spectrometry (AMS) technology. (i) A channeling degrader for the separation of isobars suppresses the angular straggling even if a sufficient energy difference is created between the AMS nuclide and its isobars using a relatively thick membrane. A proof-of-principle experiment showed its potential comparative advantage. Feasibility will be discussed for two other potential applications; the experimental demonstrations are under consideration. (ii) A channeling stripper simultaneously provides a sufficient stripping function to remove interfering molecules in the AMS and low gas pressure to prevent ion scattering. This helps in overcoming a crucial problem in the development of an (A)MS systems in the range of 50 keV. Surface channeling by grazing incidence onto a stack of mono-crystalline substrates is introduced to address the high particle load. (iii) A novel isobar separation technique, using resonant coherent excitation based on periodical field induced by ions passing through the lattice, assists in controlling the charge state of the AMS nuclide and its isobars directly. For practical use, the thickness of the crystalline membrane should be reduced to satisfy the charge state distribution in the non-equilibrium states.

Generation of ultrafast magnetic fields from molecular coherent electron currents

Coherent electron currents in circular charge transport are of great importance in photoinduced processes, magneto-optics, and reaction dynamics of molecules. We present ultrafast magnetic field generation by circularly polarized attosecond ultraviolet laser pulses in molecular media. Simulations in a one electron molecular model ${\mathrm{H}}_{2}^{+}$ show that the induced magnetic field exhibits periodic oscillations on attosecond time scales, which depends on the laser pulse and molecular alignment. We attribute it to the coherent electronic currents, arising from resonant coherent excitation between the ground and excited electronic states. The temporal evolution of the generated magnetic fields and the induced electronic currents relies on the molecular orbital superposition in the electron coherence, providing a way for controlling ultrafast magnetic field generation.

Potentialities of fast ion neutralization at grazing incidence angles from crystal surfaces for development of new generation of uv laser sources

Abstract. The paper considers processes of electron capture by fast ions scattered from metal surfaces at grazing incidence angles. It discusses the possibilities of excitation of high atomic levels in neutralization process and laser generation. The Doppler compression effect for ion beams is shown to be rather essential for laser generation in the ion energy range of several tens keV. It is shown that the four-level laser generation scheme is the preferable one. The values of critical currents for laser generation are evaluated. The experimental setup for ion beam neutralization is observed. The experimental results on nitrogen ion beam neutralization on silicon (001) wafers at grazing incident angles are presented. The dependence of the neutralization coefficient on the grazing incidence angle is measured, a good coincidence between peak maximum and Lindhard angles is demonstrated. It is shown that in case of neutralization without special vacuum surface cleaning the neutralization coefficient makes approximately 30% for ion energy of 40 keV. The one-electron resonant neutralization to nitrogen 2P1/2, 2D3/2, 4S3/2 atomic terms is discussed. The mechanism of resonant coherent excitation of fast nitrogen atoms in front of a Si (001) surface is observed, the resonant energy being estimated about 70 keV.

Potentialities of fast ion neutralization at grazing incidence angles from crystal surfaces for development of new generation of uv laser sources

Abstract. The paper considers processes of electron capture by fast ions scattered from metal surfaces at grazing incidence angles. It discusses the possibilities of excitation of high atomic levels in neutralization process and laser generation. The Doppler compression effect for ion beams is shown to be rather essential for laser generation in the ion energy range of several tens keV. It is shown that the four-level laser generation scheme is the preferable one. The values of critical currents for laser generation are evaluated. The experimental setup for ion beam neutralization is observed. The experimental results on nitrogen ion beam neutralization on silicon (001) wafers at grazing incident angles are presented. The dependence of the neutralization coefficient on the grazing incidence angle is measured, a good coincidence between peak maximum and Lindhard angles is demonstrated. It is shown that in case of neutralization without special vacuum surface cleaning the neutralization coefficient makes approximately 30% for ion energy of 40 keV. The one-electron resonant neutralization to nitrogen 2P1/2, 2D3/2, 4S3/2 atomic terms is discussed. The mechanism of resonant coherent excitation of fast nitrogen atoms in front of a Si (001) surface is observed, the resonant energy being estimated about 70 keV.

Time-Resolved Coherent Anti-Stokes Raman Scattering of Graphene: Dephasing Dynamics of Optical Phonon.

We report dynamics of the G-mode in graphene probed with time-resolved coherent anti-Stokes Raman scattering measurements. By applying BOXCARS excitation geometry with three different excitation wavelengths, various nonlinear processes can be selectively detected due to energy and momentum conservation and temporal sequence of the pulses. The Raman signal due to resonant coherent excitation of the G-mode shows exponential decay with lifetime of ∼325 ± 50 fs. This decay time is shorter than expected based on the line width of the G-mode in the Raman spectrum. We propose that the unexpectedly short dephasing time is a result of dynamic variation of nonadiabatic coupling of the photoexcited electron distribution and the G-mode.

Electronic stopping power of slow-light channeling ions in ZnTe from first principles

Nonadiabatic dynamics simulations are performed to investigate the electronic stopping power of a helium ion moving through ZnTe crystalline thin films under channeling conditions. Using ab initio time-dependent density-functional theory, we found by direct simulation that electronic stopping power versus projectile velocity deviates from velocity proportionality and displays a transition between two velocity regimes for helium ions channeling along middle crystalline axes in $\ensuremath{\langle}100\ensuremath{\rangle}$ and $\ensuremath{\langle}111\ensuremath{\rangle}$ channels and also in a $\ensuremath{\langle}110\ensuremath{\rangle}$ channel with low-impact parameters. This transition causes a change in the slope of the energy loss versus ion velocity curve at a characteristic velocity related to the impact parameter and the lattice plane spacing. It may be an indication of extra energy loss channel beyond the electron-hole excitation. To analyze it, we checked the charge transfer between the moving projectiles and host atoms. It is found that the soft transition between two velocity regimes can be attributed to the resonant coherent excitation stimulated by the time-periodic potential experienced by the channeling ion and also the charge exchange in close encounters between Helium ion and host atoms.

Enhancing the energy resolution of resonant coherent excitation using the cooled U89+ beam extracted from the ESR

SynopsisWe report on the resonant coherent excitation (RCE) of the 2s-2p3/2 transition in Li-like U89+ with an enhanced energy resolution, which was achieved by reducing the projectiles momentum spread. The kinetic temperature of the beam was decreased by electron cooling in the ESR, and the collisional momentum broadening in the target was suppressed by the use of thin crystal (1.0 and 2.5 μm-thick). The resonance width was observed to be ∼1.4 eV in FWHM, which is three-times narrower than that from the previous work.

Metastable Ar17+ (2 s) production by Stark-assisted resonant coherent excitation

We report on the selective production of the 2s metastable state of 390-MeV u−1 H-like Ar17+ ions by the resonant coherent excitation (RCE). The RCE to the 2s state was achieved by using Stark mixing of n = 2 states under planar channeling conditions in a 1-m thick Si crystal. The population of the 2s state was quantitatively estimated from the ionization probability in a stripper foil installed 50 mm downstream of the crystal. The 2s population was found to be 0.05 of the total number of Ar17+ ions at the stripper. The result is in good agreement with a theoretical calculation performed with the density matrix approach.

Resonant Coherent Excitation of Hydrogenlike Fe25+ in a Ge crystal

An extremely-thin Germanium crystal (0.8 μm-thick) has been firstly exploited in the resonant coherent excitation (RCE) experiment. The 460 MeV/u H-like Fe25+ ions were incident on the Ge crystal under the (220) planar channeling as well as non-channeling conditions. The RCE from 1s to 2p1/2 and 2p3/2 states were successfully observed as an enhancement of the x-ray fluorescence.

Effects of fine-structure depolarization studied by resonant coherent excitation of H-like Ar17+

We studied the anisotropy of Ly-α x-rays from H-like Ar17+ ions excited by polarization-controlled resonant coherent excitation. The effects of fine-structure depolarization on the 2p1/2 and 2p3/2 states were experimentally demonstrated as a reduction in anisotropy of decay radiation.

Stark effect in resonant coherent excitation of 2s electron of Li-like Fe23+ ions channeling in a Si crystal

We measured resonant coherent excitation of a 2s electron of 83.3MeV/u Fe23+ ions planar-channeling in the (2¯20) plane of the silicon crystal. A silicon surface barrier detector (SSD) was used as a crystal target in order to obtain information on the ion trajectory in the channel since the energy deposit (ΔE) to the SSD gives information on the ion trajectories where the resonant transitions occur. For the low ΔE, i.e., near the channel center, optically allowed 2s–3p transitions were much stronger compared with other transitions. Increasing ΔE, i.e., increasing the amplitude of ion trajectory, the optically forbidden 2s–3s transition rapidly became strong. On the other hand, the optically forbidden 2s–3d transitions did not become strong as rapidly as 2s–3s transition. Furthermore, it was found that the transition energies to the n=3 states changed with ΔE. The shifts of the transition energies were consistent with the estimation for the energy levels of the Stark-mixed n=3 states depending on the distance from the channel center.

Convoy electron emission following ionization of highly-charged ions excited by resonant coherent excitation

Projectile ionization of highly-charged Ar and Fe ions in the excited states passing relativistically fast through a thin crystalline foil was experimentally studied. We selectively controlled the population of the excited states of the projectiles, and their alignment by choosing a specific m-state through three-dimensional resonant coherent excitation technique by periodical electric fields in a crystalline. We measured energy-differential spectra of electron emission released from projectiles at zero degree. Under the resonance condition, we found an evident enhancement of the convoy electron yield, which reflects the electron momentum distribution of the initial bound state of the excited ions.

Resonant coherent excitation of 191.1 MeV/u U89+ ions planar-channeled in a silicon crystal

In an experiment with relativistic highly charged ions (β=v/c=0.56) channeled through a silicon crystal the resonant coherent excitation of the 2s1/2-2p3/2 transition in 191 MeV/u Li-like uranium ions was observed. The resonant condition was identified by measuring the x-ray emission as a function of the ion incidence angle with respect to the [110] crystal axis in the (220) plane. The position and the width of the resonance where determined from the dependence of the x-ray yield from the 2p3/2-2s1/2 decay on the angular orientation of the crystal.

X-ray surface waves generated by the resonant coherent excitation of relativistic ions in crystals

X-ray ion radiation, caused by resonant coherent excitation (RCE) in a crystal, arises at the crystal-vacuum interface and propagates along the surface, generating a so-called surface wave. The dependence between the surface-wave intensity and the departure angle of characteristic X-ray radiation upon RCE is investigated in a kinematic approximation of diffraction theory. It has been shown that the angular dependence for the intensity of this radiation leaving the crystal surface differs substantially from the radiation intensity inherent to the decay of ion states excited by RCE.

Optically erasing disorder in semiconductor microcavities using polariton–polariton interactions

AbstractThrough resonant coherent optical excitation of upper branch polaritons in semiconductor microcavities, an effective screening of the disorder potential experienced by lower branch polaritons is predicted. The effect is due to polariton–polariton interactions, which generate bistability for the upper‐polariton states and introduce an effective potential for lower‐polaritons. The effect represents a simple tool to improve the characteristics of devices operating in the low density regime.

Resonant coherent excitation of ions in planar channel of crystal with inclined beam incidence

Resonant coherent excitation of relativistic heavy ions in crystals under planar channeling conditions is considered at beam incidence inclined with respect to the channeling plane. The beam inclination leads to effective exclusion of contribution of ions with small energy of transverse motion from observables of resonant coherent excitation. This effect can be used for resolution of the observables by individual ion trajectories.