Carbon nanotubes (CNT) are an important new class of technological materials that have numerous novel and useful properties. The forecast increase in manufacture makes it likely that increasing human exposure will occur, and as a result, CNT are beginning to come under toxicological scrutiny. This review seeks to set out the toxicological paradigms applicable to the toxicity of inhaled CNT, building on the toxicological database on nanoparticles (NP) and fibers. Relevant workplace regulation regarding exposure is also considered in the light of our knowledge of CNT. CNT could have features of both NP and conventional fibers, and so the current paradigm for fiber toxicology, which is based on mineral fibers and synthetic vitreous fibers, is discussed. The NP toxicology paradigm is also discussed in relation to CNT. The available peer-reviewed literature suggests that CNT may have unusual toxicity properties. In particular, CNT seem to have a special ability to stimulate mesenchymal cell growth and to cause granuloma formation and fibrogenesis. In several studies, CNT have more adverse effects than the same mass of NP carbon and quartz, the latter a commonly used benchmark of particle toxicity. There is, however, no definitive inhalation study available that would avoid the potential for artifactual effects due to large mats and aggregates forming during instillation exposure procedures. Studies also show that CNT may exhibit some of their effects through oxidative stress and inflammation. CNT represent a group of particles that are growing in production and use, and therefore, research into their toxicology and safe use is warranted.

The nonphotochemical laser-induced nucleation of aqueous supersaturated solutions of potassium chloride is demonstrated. We have observed that a single, 7 ns pulse of near-infrared (1064 nm) laser light can be used to grow a single crystal of potassium chloride. The experimental results are analyzed using a model in which nucleation is enhanced through the isotropic electronic polarization of subcritical crystal nuclei by laser radiation and the associated reduction in free energy of the nuclei. Classical nucleation theory is used to calculate the fraction of subcritical nuclei, initially in zero field, which become supercritical in the laser field; this fraction is correlated with the crystallization yield and is shown to successfully describe the dependences of the experimentally observed yields upon laser power and supersaturation. The experimental results are analyzed to obtain a phenomenological value of the crystal-solution interfacial tension, γ ) 2.19 ( 0.03 mJ m -2 .

Spatial and temporal control of crystal nucleation is demonstrated by nonphotochemical laser-induced nucleation of an aqueous agarose gel prepared with supersaturated potassium chloride. The location of nucleation was controlled by means of an optical mask; crystals were only observed in the area exposed to near-infrared laser radiation. The dependence of nucleation on laser power was measured, and the results suggest that the agarose gel reduces the effective supersaturation of the aqueous potassium chloride.

Non-photochemical laser-induced nucleation (NPLIN) is the formation of a new phase from a metastable phase by the action of light on matter. Using millijoule, nanosecond laser pulses at visible and near-infrared wavelengths, it is possible to form the new phase localized in the volume of the beam. In the case of nucleating molecular solids, the laser polarization may have an effect on the particular polymorph that is formed. Despite the huge potential for applications of NPLIN, there is uncertainty regarding the molecular-scale mechanism, and various possible scenarios may well be relevant to nucleation in general and not just NPLIN. In this Perspective, the discovery and phenomenology of NPLIN are described, putative mechanisms are outlined, and some observations on the broader class of nucleation phenomena are given.

We have studied the complete Cl-atom molecular-frame photofragment angular momentum distributions from the photodissociation of Cl 2 and ICl in the 320-560 nm region using time-of-flight mass spectroscopy with laser detection. The experimental signals were analyzed using the polarization-parameter formalism described in the preceding paper. These experiments study three distinct cases. The first case is the 470 nm dissociation of Cl 2 through the B 3 ⌸ 0 ϩ u state accessed via a parallel transition, yielding Cl-atom photofragments with polarizations described by the single parameter a 0 (2) (ʈ)ϭϪ0.7Ϯ0.2. The second case is the 320 nm dissociation of Cl 2 through the C 1 ⌸ 1u state accessed via a perpendicular transition, yielding Cl-atom photofragments with polarizations described by the two parameters a 0 (2) (Ќ)ϭϪ0.50Ϯ0.10 and a 2 (2) (Ќ)ϭϪ0.32Ϯ0.06. The third case is the dissociation of ICl in the 490-560 nm region in which dissociative states of both parallel and perpendicular character are accessed. In this wavelength region, the polarizations of the resulting Cl-atom photofragments are completely described by the approximately constant incoherent parameters, a 0 (2) (ʈ)Ϸϩ0.4, a 0 (2) (Ќ)ϷϪ0.2, and a 2 (2) (Ќ)ϷϪ0.2, whereas the interference contributions to the polarization, the Im͓a 1 (1) (ʈ ,Ќ)͔ and Re͓a 1 (2) (ʈ ,Ќ)͔, oscillate sinusoidally with excitation wavelength in a fashion that is sensitive to the shapes of the dissociative surfaces.

Molecular chlorine was photolyzed using circularly polarized radiation at 310 and 330 nm, and orientation moments of the chlorine-atom Cl(2 P j) photofragment distributions were measured by resonance enhanced multiphoton ionization using circularly polarized light with Doppler resolution. The product atoms were found to be strongly oriented in the laboratory as a result of both incoherent and coherent dissociation mechanisms, and the orientation moments contributed by each of these mechanisms have been separately measured. The experimental results can be explained by nonadiabatic transitions from the C 1 ⌸ 1u state to higher states of ⍀ϭ1 u symmetry, induced by radial derivative coupling. Ab initio calculations indicate strong Rosen-Zener-Demkov noncrossing-type radial derivative couplings between states of 1 u symmetry. The observed angular distribution ͑␤ parameter͒ indicates that 88% of Cl*(2 P 1/2) fragments produced at 310 nm originate from a perpendicular transition to the C state. The orientation measurements suggest that 67Ϯ16% of 35 Cl*(2 P 1/2) atoms dissociate via the 1 u (3 ⌺ 1u ϩ) state, and 21Ϯ6% dissociate via the 1 u (3 ⌬ 1u) state.

We are grateful to Dr Philip Camp for many useful discussions, and to an anonymous reviewer for pointing out a published correction to Eq. (5). We wish to acknowledge the Science and Technology Facilities Council (STFC) and the Engineering and Physical Sciences Research Council (EPSRC) for supporting this work (EP/G067546/1), and to the Royal Society (London) for a research grant.

Vibrationally state-resolved differential cross sections ͑DCS͒ and product rotational distributions have been measured for the ClϩHD(vϭ1, Jϭ1)→HCl͑DCl͒ϩD͑H͒ reaction at a mean collision energy of 0.065 eV using a photoinitiated reaction ͑''photoloc''͒ technique. The effect of HD reagent rotational alignment in the ClϩHD(vϭ1, Jϭ2) reaction has also been investigated. The experimental results have been compared with exact quantum mechanical and quasiclassical trajectory calculations performed on the G3 potential energy surface of Allison et al. ͓J. Phys. Chem. 100, 13575 ͑1996͔͒. The experimental measurements reveal that the products are predominantly backward and sideways scattered for HCl(vЈϭ0) and HCl(vЈϭ1), with no forward scattering at the collision energies studied, in quantitative agreement with theoretical predictions. The experimental product rotational distribution for HCl(vЈϭ1) also shows excellent agreement with quantum-mechanical calculations, but the measured DClϩH to HClϩD branching ratio is near unity, which is at variance with the theoretical calculations that predict about 3 times larger yield of HClϩD at these collision energies. The reactivity shows a marked dependence on the direction of the HD(vϭ1, Jϭ2) rotational angular momentum, and experimental measurements of this reagent alignment effect are in good agreement with theoretical predictions.