Abstract. Most of the extraterrestrial dust entering the Earth's atmosphere ablates to produce metal vapours, which have significant effects on the aeronomy of the upper mesosphere and lower thermosphere. A new Chemical Ablation Model (CAMOD) is described which treats the physics and chemistry of ablation, by including the following processes: sputtering by inelastic collisions with air molecules before the meteoroid melts; evaporation of atoms and oxides from the molten particle; diffusion-controlled migration of the volatile constituents (Na and K) through the molten particle; and impact ionization of the ablated fragments by hyperthermal collisions with air molecules. Evaporation is based on thermodynamic equilibrium in the molten meteoroid (treated as a melt of metal oxides), and between the particle and surrounding vapour phase. The loss rate of each element is then determined assuming Langmuir evaporation. CAMOD successfully predicts the meteor head echo appearance heights, observed from incoherent scatter radars, over a wide range of meteoroid velocities. The model also confirms that differential ablation explains common-volume lidar observations of K, Ca and Ca + in fresh meteor trails. CAMOD is then used to calculate the injection rates into the atmosphere of a variety of elements as a function of altitude, integrated over the meteoroid mass and velocity distributions. The most abundant elements (Fe, Mg and Si) have peak injection rates around 85 km, with Na and K about 8 km higher. The more refractory element Ca ablates around 82 km with a Na:Ca ratio of 4:1, which does therefore not explain the depletion of atomic Ca to Na, by more than 2 orders of magnitude, in the upper mesosphere. DiffusionCorrespondence to: J. M. C. Plane (j.m.c.plane@leeds.ac.uk) of the most volatile elements (Na and K) does not appear to be rate-limiting except in the fastest meteoroids. Nonthermal sputtering causes ∼35% mass loss from the fastest (∼60-70 km s −1 ) and smallest (10 −17 -10 −13 g) meteoroids, but makes a minor contribution to the overall ablation rate.

Abstract. Most of the extraterrestrial dust entering the Earth's atmosphere ablates to produce metal vapours, which have significant effects on the aeronomy of the upper mesosphere and lower thermosphere. A new Chemical Ablation Model (CAMOD) is described which treats the physics and chemistry of ablation, by including the following processes: sputtering by inelastic collisions with air molecules before the meteoroid melts; evaporation of atoms and oxides from the molten particle; diffusion-controlled migration of the volatile constituents (Na and K) through the molten particle; and impact ionization of the ablated fragments by hyperthermal collisions with air molecules. Evaporation is based on thermodynamic equilibrium in the molten meteoroid (treated as a melt of metal oxides), and between the particle and surrounding vapour phase. The loss rate of each element is then determined assuming Langmuir evaporation. CAMOD successfully predicts the meteor head echo appearance heights, observed from incoherent scatter radars, over a wide range of meteoroid velocities. The model also confirms that differential ablation explains common-volume lidar observations of K, Ca and Ca+ in fresh meteor trails. CAMOD is then used to calculate the injection rates into the atmosphere of a variety of elements as a function of altitude, integrated over the meteoroid mass and velocity distributions. The most abundant elements (Fe, Mg and Si) have peak injection rates around 85 km, with Na and K about 8 km higher. The more refractory element Ca ablates around 82 km with a Na:Ca ratio of 4:1, which does therefore not explain the depletion of atomic Ca to Na, by more than 2 orders of magnitude, in the upper mesosphere. Diffusion of the most volatile elements (Na and K) does not appear to be rate-limiting except in the fastest meteoroids. Non-thermal sputtering causes ~35% mass loss from the fastest (~60–70 km s−1) and smallest (10−17–10−13g) meteoroids, but makes a minor contribution to the overall ablation rate.

[1] Lidar observations, conducted at the South Pole by University of Illinois researchers, are used to characterize the seasonal variations of mesospheric Na and Fe above the site. The annual mean layer abundances are virtually identical to midlatitude values, and the mean centroid height is just 100 m higher for Na and 450 m higher for Fe compared with 40°N. The most striking feature of the metal profiles is the almost complete absence of Na and Fe below 90 km during midsummer. This leads to summertime layers with significantly higher peaks, narrower widths, and smaller abundances than are observed at lower latitudes. The measurements are compared with detailed chemical models of these species that were developed at the University of East Anglia. The models accurately reproduce most features of these observations and demonstrate the importance of rapid uptake of the metallic species on the surfaces of polar mesospheric clouds and meteoric smoke particles. The models show that vertical downwelling in winter, associated with the meridional circulation system, must be less than about 1 cm s À1 in the upper mesosphere in order to avoid displacing the minor constituents O, H, and the metal layers too far below 85 km. They also show that an additional source of gas-phase metallic species, that is comparable to the meteoric input, is required during winter to correctly model the Na and Fe abundances. This source appears to arise from the wintertime convergence of the meridional flow over the South Pole.

Excitation of the H-Si bond on the Si͑100͒-͑2 3 1͒:H surface at 157 nm leads to the desorption of atomic hydrogen. Quantitative measurement using polarized light shows that the transition dipole moment is oriented at ϳ18 ± from the surface normal, in agreement with the H-Si bond direction. This result unambigously establishes the direct photodesorption mechanism. A comparison of the isotope effect in the photodesorption cross section with a time-dependent quantum mechanical simulation reveals an excited-state lifetime of ϳ0.4 fs. [S0031-9007 (99)08569-5] PACS numbers: 82.50.Fv, 82.65.My

Thiolate self-assembled monolayers (SAMs) on metal surfaces have been explored recently to address the assembly and connection issue in molecular electronics. In these systems, the molecule-metal contact is detrimental to electron transport. This is manifested not only in contact resistance, but also in the nature of the molecular device, which depends on the extent of wave function mixing between the molecule and the metal surface. We probe interfacial electronic structure, particularly unoccupied electronic states, in thiolate SAMs on Cu(111) using laser two-photon photoemission spectroscopy, in conjunction with ab initio calculations of model molecules. We find that the interfacial electronic structure is dominated by two virtual orbitals localized to the thiolate anchor and strongly coupled to the metal substrate. The shapes and energies of these interfacial σ*-like orbitals are independent of the nature of the hydrocarbon group (conjugated aromatics or saturated alkyls). As low-lying acceptor orbitals, they may play important roles in electron transport through self-assembled molecular wires.