Spatial distribution of ions in a linear octopole radio-frequency ion trap in the space-charge limit

We have explored the spatial distribution of an ion cloud trapped in a linear octopole radio-frequency (rf) ion trap. The two-dimensional distribution of the column density of stored silver dimer cations was measured via photofragment-ion yields as a function of the position of the incident laser beam over the transverse cross section of the trap. The profile of the ion distribution was found to be dependent on the number of loaded ions. Under high ion-loading conditions with a significant space-charge effect, ions form a ring profile with a maximum at the outer region of the trap, whereas they are localized near the center axis region at low loading of the ions. These results are explained quantitatively by a model calculation based on equilibrium between the space-charge-induced potential and the effective potential of the multipole rf field. The maximum adiabaticity parameter \eta_max is estimated to be about 0.13 for the high ion-density condition in the present octopole ion trap, which is lower than typical values reported for low ion densities; this is probably due to additional instability caused by the space charge.Comment: 8 pages, 5 figure

Collision Dynamics of Argon Cluster Ions, Ar^+_n (n=3-23) : Molecular Dynamics Simulation Based on Diatomics-in-Molecules Method

The molecular dynamics method combined with a quantum mechanical calculation has been used to simulate the collision between an argon atom and an argon cluster ion, Ar^+_n(n=3-23), which contains a given amount of internal energy. Two pathways were observed; (i) Evaporation after collisional energy transfer to the internal degrees of freedom vs. (ii) fusion via complex formation. The total reaction cross sections were compared with those experimentally obtained. It is found that the branching fractions of the evaporation and the fusion depend critically on the internal energy and the impact parameter

Caractéristiques Agronomiques et Identification des Facteurs Déterminant la Faible Productivité des Agroforêts à Cacaoyers (Theobroma Cacao L.) au Togo

Les agroforêts à cacaoyers du Togo sont caractérisées par leur faible productivité. L’objectif de cette étude est d’identifier les caractéristiques agronomiques et les facteurs déterminant la faible productivité des cacaoyers. Une enquête a été menée auprès de 385 cacaoculteurs suivie d’une prospection réalisée dans 88 vergers de cacaoyers répartis en quatre tranches d’âge : les jeunes plantations entrant en production (5 à 10 ans), les cacaoyers adultes en pleine production (10 à 20 ans), les cacaoyers vieillissants dont la production est en déclin (21 à 30 ans) et les vieilles plantations (>30 ans). Les résultats ont montré que la tranche d’âge moyenne des vergers est comprise entre 10 et 20 ans. Ces vergers sont essentiellement installés sur les anciens champs de cacaoyers (42,6%) et sur précédent forestier (40,3%) avec du matériel végétal constitué en grande partie d’hybrides sélectionnés (82,9%). Les fréquences de désherbages, des traitements phytosanitaires (insecticides et fongicides) et d’applications de fumure (organique et minérale) sont inférieures aux normes recommandées par la recherche. Les rendements moyens sont faibles et sont compris entre 218,64 kg/ha dans les vieilles plantations (> 30 ans) et 450,77 kg/ha dans les plantations adultes en pleine production (10 à 20 ans). En ce qui concerne les différentes sous zones agroécologiques, les résultats ne révèlent aucune différence significative pour les rendements. Ces rendements varient de 305,76 kg/ha dans la sous zone de plateaux à 376,59 kg/ha dans la sous zone de Kloto-Litimé. Cette faible productivité est liée au faible niveau de gestion et d’entretien des vergers, aux caractéristiques des peuplements cacaoyers et des peuplements associés. Cocoa agroforests in Togo are characterized by their low productivity. The objective of this study is to identify the agronomic characteristics and factors determining the low productivity of cocoa trees. A survey was conducted among 385 cocoa farmers followed by a survey carried out in 88 cocoa orchards and four age groups were considered: young plantations entering production (5 to 10 years), adult cocoa trees in production (10 to 20 years), aging cocoa trees in production (21 to 30 years) and old plantations (>30 years). The results showed that the mean age group of orchards is 10 to 20 years old. These orchards are mainly installed on old cocoa fields (42.6%) and on previous forest (40,3%) with plant material consisting largely of selected hybrids (82,9%). The frequencies of weeding, phytosanitary treatments (insecticides and fungicides) and manure applications (organic and mineral) are below the standards recommended by research. Average yields are low and range between 218,64 kg/ha in old plantations (> 30 years) and 450,77 kg/ha in adult plantations in production (10 to 20 years). With regard to the different agro-ecological sub-zones, the results show that yields vary from 305,76 kg/ha in the plateau sub-zone to 376,59 kg/ha in the Kloto-Litimé sub-zone. This low productivity is linked to the low level of management and maintenance of orchards, to the characteristics of cocoa stands and associated stands. &nbsp

Optical pumping by a laser pulse traveling in a cavity

We have developed a general method to perform optical pumping by a pulsed laser with the aid of an optical cavity ͑cavity-assisted optical pumping͒. Optical pumping is achieved by repetitive interaction of a single laser pulse with a target material in the cavity. This method is demonstrated for manganese ions, Mn + , stored in a linear radio-frequency ion trap; about 10 8 ions are spin polarized by a 5-ns laser pulse via 7 P J ← 7 S 3 ͑J =3 or 4͒ transition in the ultraviolet region. The linewidth of the pulsed light source is broad enough to transfer the populations of lower hyperfine levels to the highest one ͑F =11/ 2͒ in the 7 S 3 ground state; the nuclear spin is polarized as well. DOI: 10.1103/PhysRevA.77.033417 PACS number͑s͒: 32.80.Xx, 32.10.Fn, 37.10.Ty, 37.20.ϩj Optical pumping ͓1͔ is a powerful technique for spin polarization widely used since the first idea of Kastler in 1950 ͓2͔. It is operated by repeated cycles of absorption of circularly polarized light and spontaneous emission back to the initial state. This cycle transfers angular momentum of photons to target atoms. The atoms eventually reach a nonstatistical population distribution, where only one of the magnetic sublevels is populated. Spin-polarized atoms and nuclei thus produced have a variety of applications ͓3-5͔: Highly precise spectroscopy especially with double resonance techniques ͓6,7͔, spin-exchange collisions ͓8͔, manipulation and statecontrol of atoms ͓9͔, sensitive magnetometry ͓10͔, and so forth. Although the advent of tunable lasers greatly expanded the application of optical pumping, the light source has been limited to continuous-wave ͑cw͒ lasers. This is due to the long interaction time needed to repeat the pumping cycles until the spin-polarization process is completed; the time scale is typically longer than several microseconds. Therefore, standard nanosecond laser pulses are not suitable for the light source. Recently, several schemes have been proposed for pulsed lasers to generate spin-polarized atomic ions without relying on optical pumping ͓11͔. The elaborate schemes, however, have been applied only to alkaline-earth elements; the ground-state atoms in 1 S 0 are excited by a circularly polarized laser pulse to 3 P 1 ͑M J = +1͒, and further ionization results in spin-polarized ions in the 2 S 1/2 ground state. The advantage of pulsed lasers over cw ones, particularly in the tunability in short wavelengths, urges us to develop a new method with broader applicability. In this paper, we present "cavity-assisted optical pumping," which allows us to perform optical pumping by a laser pulse with the aid of an optical cavity. It is shown that repeated interaction of a single laser pulse produces highdegree spin polarization of target materials in the cavity. This method provides a general technique for using pulsed lasers in a manner similar to cw light sources. The broad linewidth inherent to pulsed lasers enables nuclear spins to be polarized as well by exciting transitions split by hyperfine structures. In the experiment, we have created a spin-polarized ensemble of about 10 8 ions of manganese, Mn + , stored in a linear ion trap. Mn + has a nuclear and an electron spin of I =5/ 2 and J = 3, respectively, in the ground state ͑ 7 S 3 ͒ with an electronic configuration of 3d 5 4s 1 . We have observed spin polarization of Mn + ions in the highest angular momentum of F = J + I =11/ 2; the ions are forced to populate in the sublevel of the magnetic quantum number M = +11/ 2 by interaction with a laser pulse of + circular polarization

Local vibration modes and nitrogen incorporation in AlGaAs: N layers

Raman scattering measurements in dilute AlGaAs:N films grown by plasma-assisted molecular beam epitaxy on (100) GaAs substrates reveal strong local vibration modes (LVM) associated to N complexes. The LVM observed frequencies between 325 and 540 cm–1 are in good agreement with density functional theory supercell calculations of AlnGa4–nN complexes (n = 1,2,3,4). We find that the observed LVMs correspond to all n values including Al4N. The LVMs spectra are resonant at energies around 1.85 eV. The values of the extended phonon frequencies of the ternary compound (GaAs and AlAs-like) reveal changes in the N distribution depending on the growth conditions: A transition from random- to nonrandom nitrogen distribution is observed upon increasing the growth temperature. Our results confirm the preferential bonding of N to Al in AlGaAs:N, due to the higher Al-N bond strength as compared to the Ga-N bond

Landau levels and magnetopolaron effect in dilute GaAs:N

The magnetic-field dependence of the energy spectrum of GaAs doped with nitrogen impurities is investigated. Our theoretical model is based on the phenomenological Band Anticrossing Model (BAC) which we extended in order to include magnetic field and electron - phonon interaction. Due to the highly localized nature of the nitrogen state, we find that the energy levels are very different from those of pure GaAs. The polaron correction results in a lower cyclotron resonance energy as compared to pure GaAs. The magneto-absorption spectrum exhibits series of asymmetric peaks close to the cyclotron energy

Breathing Vibration of Ar Clusters Analyzed by Molecular Dynamics Calculation : Cluster-Shape Dependence of the Mode-Separation

The vibrational motion of Ar clusters (Ar_n, n=20 and 30) having isomers in a variety of shapes was simulated by use of the molecular dynamics method and the mode-separation of the breathing vibration from the quadruple spheroidal vibration was investigated. It was found that these modes of highly spherical isomers were almost fully separated from each other, while coupling between these modes were significant in non-spherical isomers. The relation between the cluster shape and the mode-separation was elucidated

Gain studies of 1.3-μm dilute nitride HELLISH-VCSOA for optical communications

The hot electron light emitting and lasing in semiconductor heterostructure-vertical-cavity semiconductor optical amplifier (HELLISH-VCSOA) device is based on Ga0.35In0.65 N0.02As0.08/GaAs material for operation in the 1.3-μm window of the optical communications. The device has undoped distributed Bragg reflectors (DBRs). Therefore, problems such as those associated with refractive index contrast and current injection, which are common with doped DBRs in conventional VCSOAs, are avoided. The gain versus applied electric field curves are measured at different wavelengths using a tunable laser as the source signal. The highest gain is obtained for the 1.3-μm wavelength when an electric field in excess of 2 kV/cm is applied along the layers of the device

GaInNAs-based Hellish-vertical cavity semiconductor optical amplifier for 1.3 μm operation

Hot electron light emission and lasing in semiconductor heterostructure (Hellish) devices are surface emitters the operation of which is based on the longitudinal injection of electrons and holes in the active region. These devices can be designed to be used as vertical cavity surface emitting laser or, as in this study, as a vertical cavity semiconductor optical amplifier (VCSOA). This study investigates the prospects for a Hellish VCSOA based on GaInNAs/GaAs material for operation in the 1.3-μm wavelength range. Hellish VCSOAs have increased functionality, and use undoped distributed Bragg reflectors; and this coupled with direct injection into the active region is expected to yield improvements in the gain and bandwidth. The design of the Hellish VCSOA is based on the transfer matrix method and the optical field distribution within the structure, where the determination of the position of quantum wells is crucial. A full assessment of Hellish VCSOAs has been performed in a device with eleven layers of Ga0.35In0.65N0.02As0.08/GaAs quantum wells (QWs) in the active region. It was characterised through I-V, L-V and by spectral photoluminescence, electroluminescence and electro-photoluminescence as a function of temperature and applied bias. Cavity resonance and gain peak curves have been calculated at different temperatures. Good agreement between experimental and theoretical results has been obtained

Improved Optoelectronic Properties of Rapid Thermally Annealed Dilute Nitride GaInNAs Photodetectors

We investigate the optical and electrical characteristics of GaInNAs/GaAs long-wavelength photodiodes grown under varying conditions by molecular beam epitaxy and subjected to postgrowth rapid thermal annealing (RTA) at a series of temperatures. It is found that the device performance of the nonoptimally grown GaInNAs p-i-n structures, with nominal compositions of 10% In and 3.8% N, can be improved significantly by the RTA treatment to match that of optimally grown structures. The optimally annealed devices exhibit overall improvement in optical and electrical characteristics, including increased photoluminescence brightness, reduced density of deep-level traps, reduced series resistance resulting from the GaAs/GaInNAs heterointerface, lower dark current, and significantly lower background doping density, all of which can be attributed to the reduced structural disorder in the GaInNAs alloy.© 2012 TMS