Edmond Murad

On the existence of silicon nanodust near the sun

We study the sublimation sequence of silicate dust in the inner solar system and show that only metal oxides survive at small distances from the Sun, while silicon sublimates at moderate temperatures. Consideration of laboratory studies showing the generation of Si nanocrystals suggests that their conditions do not resemble those in the inner solar system. We conclude that it is not reasonable to suggest the presence of Si nanoparticles in the vicinity of the Sun.

Optical observations of water in Leonid meteor trails

[1] Two simultaneous filtered images (589 and 423 nm) of a meteor trail were recorded during the 2002 Leonid storm. The first image shows Na atoms and the second Ca and Fe atoms and signals at altitudes much higher than can give rise to ablation of metals, in agreement with other observations of high altitude visible trails [Spurný et al., 2000a; Spurný et al., 2000b]. Ablation models [McNeil et al., 1998] and analysis of the history of the 2002 Leonid meteoroids [McNaught and Asher, 1999] support the conclusion that the high altitude emissions are due to H 2 O + and H α,β,γ formed through the decomposition in the hyperthermal collision between H 2 O from meteoroid ice [Kresak, 1973] and atmospheric N 2 [Dressier et al., 1992].

The solar cycle effect on the atmosphere as a scintillator for meteor observations

We discuss using high solar cycle atmospheric conditions as sensors for observing meteors and their properties. High altitude meteor trails (HAMTs) have sometimes been observed with HPLA (High Power Large Aperture) radars. At other times they are not seen. In the absence of systematic studies on this topic, we surmise that the reason might be differing atmospheric conditions during the observations. At EISCAT HAMTs were observed in 1990 and 1991. Very high meteor trails were observed with Israeli L-band radars in 1998, 1999 and 2001. Through the Leonid activity, around the latest perihelion passage of comet Tempel-Tuttle, optical meteors as high as 200 km were reported. This was partly due to new and better observing methods. However, all the reported periods of high altitude meteors seem to correlate with solar cycle maximum. The enhanced atmospheric and ionospheric densities extend the meteoroid interaction range with the atmosphere along its path, offering a better possibility to distinguish differential ablation of the various meteoric constituents. This should be studied during the next solar maximum, due within a few years.

An ab initio and experimental study of vibrational effects in low energy O++C2H2 charge-transfer collisions

Theoretical and experimental studies are performed to elucidate the low energy charge-transfer dynamics of the reaction, O+(4S)+C2H2(Xu200a1Σg+)→O+C2H2+. In particular, the role of the low-frequency acetylene bending modes (612 and 730 cm−1) in promoting charge transfer was examined. High-temperature guided-ion beam measurements are carried out over the energy range from near-thermal to 3 eV at 310 and 610 K. The charge-transfer cross sections are found to decrease up to 0.5 eV, to have a constant value at intermediate energies between 0.5 and 1.5 eV, and then to dramatically increase above a threshold of a spin-allowed process determined to be at 1.7 eV. A bending vibrational enhancement of ∼8 is observed at intermediate energies. Thermal energy rate co-efficients are measured in a variable temperature-selected ion flow drift tube apparatus from 193 to 500 K. At each temperature, a negative energy dependence is observed. In order to elucidate the reaction mechanism in detail, high level ab initio calculation...

GUIDED-ION BEAM STUDY OF THE O2++C2H2 CHARGE-TRANSFER AND CHEMICAL REACTION CHANNELS

Guided-ion beam cross section and product ion time-of-flight (TOF) measurements are presented for the O2+(2Πg)+C2H2 reaction over the center-of-mass collision energy range of 0.05–12 eV. The reaction exhibits a strong charge-transfer (CT) channel and several weak chemical reaction channels leading to the following product ions: C2H2O+, CH2+, COH+/HCO+, CO+, CH+, and C+. The magnitude of the charge-transfer cross section is comparable to the capture cross section at low collision energies. The charge-transfer products are characterized predominantly by a direct, long-range mechanism that is accompanied by smaller impact parameter collisions involving longer-lived complexes, whose lifetimes are several times shorter than the complex rotational period. The TOF analysis indicates that the complexes also decay to chemical reaction products at higher energies. A density functional survey of intermediates is conducted. The chemical reaction products are postulated to be formed via an endothermic transition state...

Collision energy dependence and product recoil velocity analysis of O+(4S)+C2H2 charge-transfer and chemical reaction channels

Guided-ion beam cross section and product ion time-of-flight (TOF) measurements are presented for the O+(4S)+C2H2 reaction over the center-of-mass collision energy range of 0.05–18 eV. Despite a large number of exothermic channels, the total reaction cross section at low energies is more than two orders of magnitude smaller than the capture cross section. A common energy onset for charge-transfer, CH+, and COH+/HCO+ products is observed at 1.70±0.10u2009eV, above which the total cross section for these channels rapidly rises with energy, eventually exceeding 5 A2. Above 4 eV, the C2H+ and CO+ products also become significant, and weaker channels producing C+, C2+, and CH2O+ are also identified. The C2H+ fragment is interpreted as being primarily a dissociative charge transfer (CT) channel at collision energies above ∼3.79 eV, while the threshold for forming CO+ can be associated with a CO++H+CH channel. The TOF measurements demonstrate that efficient C2H2+u2002A state formation occurs above ∼2.7 eV. The onset at ...