On the compressive response of carbon nanotube tangles

The nonlinear bulk compressibility of entangled multiwalled carbon nanotubes is studied. The analogy with textile fibre assemblies is explored by means of the well established van Wyk model. In view of the small diameter of the nanotubes, the possible effect of adhesive van der Waals interactions at tube-tube contacts is analysed. It is found, however, that the contribution of adhesive contacts to the bulk stress should be negligible. Compression experiments are performed on multi-walled carbon nanotubes and show that van Wyk's model is able to describe the response, although the values of the dimensionless parameter k of van Wyk's model were lower than expected. There is indeed no indication that van der Waals interactions play any significant role.     

A coincidence technique for the study of intense laser atom interactions

A new device for charged particle coincidence experiments in strong-field, short pulse laser-atom/molecule interactions is presented. The device consists of a single time of flight spectrometer, common for both positive and negative charge detection. Experimental parameters required for the use of the device in the high intensity regime are discussed. A demonstration of electron-ion coincidence measurements in the interaction of Xe atoms with 60 fs laser pulses at 800 nm and an intensity of W/cm² is reported. Received 22 November 1999     

Polymorphism of sorbitol

The polymorphism of sorbitol was investigated, confirming the existence of four anhydrous crystalline phases plus the hydrate. The crystallised melt (CM), the alpha form, and the gamma form were obtained via a dry route. The CM was confirmed to be a crystalline state with a spherulite morphology. The alpha form was obtained via direct conversion from the CM, in contrast to more complicated routes previously reported, and was found to have a very high crystallinity. Gamma crystals were obtained by seeding the melt at high temperature; however, crystallinity was clearly less than for alpha crystals.     

Field emission behavior of cuboid zinc oxide nanorods on zinc-filled porous silicon

Single-crystalline zinc oxide (ZnO) nanorods with cuboid morphology have been prepared on the zinc-filled porous silicon substrate using a vapor phase transport method. Field-emission measurements showed that the turn-on field and threshold field of the cuboid ZnO nanorods film were about 3.2 and 8.2 V/μm respectively. From the emitter surface, a homogeneous emission image was observed with emission site density (ESD) of ∼10⁴ cm⁻². The better emission uniformity and the high ESD may be attributed to a large number of ZnO nanocrystallites as emitter on the surface of the nanorod end contributing to emission.     

Demineralisation of semi-anthracite char with molten salts/HCl: Effects on the porous texture and reactivity in air

The effects of chemical heat treatments of a semi-anthracite char (AC) on textural properties and reactivity in air of the material are investigated. The starting char was first treated with a mixture of LiCl/KCl or LiCl/KCl/CaO at 743, 873 or 1173 K and the products obtained were then washed thoroughly with distilled water. A small fraction of these samples were treated with 10⁻³ M HCl solution. Valuable information on textural modifications produced in the material was derived from the adsorption isotherms for CO₂ at 273 K. The reactivity tests were carried out at 808, 823 and 823 K. The microporosity developed and the reactivity in air increased in the partially demineralised products. The former effect was stronger for the LiCl/KCl/CaO-treated samples and the latter for a larger number of the LiCl/KCl-treated samples.     

Fast Rydberg gates without dipole blockade via quantum control

We propose a scheme for controlling interactions between Rydberg-excited neutral atoms in order to perform a fast high-fidelity quantum gate. Unlike dipole-blockade mechanisms already found in the literature, we drive resonantly the atoms with a state-dependent excitation to Rydberg levels, and we exploit the resulting dipole-dipole interaction to induce a controlled atomic motion in the trap, in a similar way as discussed in recent ion-trap quantum computing proposals. This leads atoms to gain the required gate phase, which turns out to be a combination of a dynamic and a geometrical contribution. The fidelity of this scheme is studied including small anharmonicity and temperature effects, with promising results for reasonably achievable experimental parameters.     

Temporal coherent control induced by wave packet interferences in one and two photon atomic transitions

The interaction of a sequence of two identical ultrashort laser pulses with an atomic system results in quantum interferences as in Ramsey fringes experiments. These interferences allow achievement of temporal coherent control of the excitation probability. We present the results of a temporal coherent control experiment on two different atomic systems: one-photon absorption in K (4s-4p) and two-photon absorption in Cs (6s-7d). In K, the quantum interferences between the two excitation paths associated with the laser pulses are revealed through rapid oscillations of the excitation probability as a function of the time delay between the two pulses. These oscillations take place at the transition frequency (period T = 2.56 fs). The interferences are modulated by beats (at about 580 fs) resulting from the doublet structure of the excited state (4p (² P _1/2 , ² P _3/2 )). Three complementary interpretations of this experiment are presented: in terms of beats of quantum interferences, of variation in the spectrum intensity, and of wave packet interferences. Whenever the two laser pulses are temporally overlapped, optical interferences are superimposed on to the quantum interferences. The distinction between these two types of interference is clearly revealed in the two-photon excitation scheme performed on Cs (6s-7d (² D _3/2 , ² D _5/2 )) because quantum interferences occur at twice the frequency of the optical interferences. Received: 30 December 1997 / Revised: 28 February 1998 / Accepted: 4 March 1998     

Laser-induced breakdown of krypton near five-photon ionization threshold

Selective excitation of laser-induced breakdown in krypton in the pressure range 1–4 bar and the spectral range 420–620 nm at a laser intensity of 10¹¹ W/cm² has been studied. It is shown that the breakdown of krypton can be induced selectively due to resonantly enhanced multiphoton ionization via excited atomic states. A number of four- and five-photon atomic resonances have been identified in breakdown excitation spectra. The role of different factors determining the selectivity of the breakdown has been studied. Possible analytical applications of a selective breakdown are discussed.     

Three-photon resonance enhanced excitation spectrum of atomic thallium

We present first measurements on the resonance enhanced three-photon excitation in thallium, using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. The even-parity 6s²ns²S_1/2 (15 ? n ? 31) and nd ²D_5/2 (13 ? n ? 42) Rydberg states have been observed. The measured level energies reveal a dynamic shift from the photoabsorption values, which is decreasing with increasing n, while the asymmetry in the line profile is observed to be increasing with increasing n. In addition, an autoionising level (sp²⁴P_3/2) adjacent to the ionization threshold has been observed and quantitatively analyzed using the Fano’s photoionization cross-section relation for an isolated autoionising resonance.     

On the origin of light emission from porous anodic alumina formed in sulfuric acid

We have investigated the photoluminescence (PL) property of porous anodic alumina membranes (PAAMs) formed on bulk Al foils in 0.3 M sulfuric acid. Different from that from the PAAMs formed in oxalic acid, the obtained PL spectra show two emission bands which have different origins. One centered at ∼465 nm (α-band) weakens its intensity in the PAAM annealed in O₂ and is thus attributed to optical transition in oxygen vacancies. The other in the blue (β-band) redshifts with increasing excitation wavelength. On the basis of spectral examinations and analyses, we ascribe the β-band to radiative recombination of carriers in the isolated hydroxyl groups at the surface of the pore wall, whereas the photogeneration of carriers takes place in oxygen vacancies in the pore wall. This work improves the understanding of the light-emitting property of the PAAMs formed in sulfuric and oxalic acid.