ClOOCl (ClO dimer) photolysis is believed to dominate the catalytic destruction of polar stratospheric ozone during springtime through the production of atomic chlorine. Decomposition by an alternate pathway to form ClO would not catalyze ozone loss. Molecular beam experiments have demonstrated that photoexcitation of ClOOCl at both 248 and 308 nm leads to dissociation via multiple dynamical pathways, producing ClO + ClO and 2Cl + O 2 . At 248 nm, both concerted and sequential dissociation to 2Cl + O 2 were observed. The primary dissociation channels occurred within a rotational period at both excitation wavelengths. The relative Cl:ClO product yields are 0.88:0.12 and 0.90:0.10 at 248 and 308 nm, respectively. Lower limits on these ratios were determined. These results substantially confirm the importance of ClOOCl photolysis in catalyzing springtime polar ozone depletion.) 108.4 kJ/mol (7)

The interaction of energetic fluorine atoms with a fluorinated silicon surface has been studied by monitoring energy and angular distributions of scattered fluorine atoms. Two beams with average translational energies of 284 and 544 kJ/mol were directed onto the SiF x layer, known to exist during steady-state etching. While thermal scattering of unreacted fluorine atoms is observed, nonthermal scattering dominates and includes both single- and multiple-bounce collisions at the complex surface. Multiple-bounce collisions often lead to a near-thermal (cosine) distribution of exit angles but incomplete thermalization of translational energy. A hard-sphere kinematic model, based on single atom−surface collisions, can be used to predict the overall average energy transfer as a function of deflection angle, indicating that the complex scattering events at the surface can appear on average like single collisions.

Scattering of energetic F atoms on a fluorinated Si surface is studied by molecular beam methods. The energy transfer closely follows hard-sphere collision kinematics. Energy and angular distributions of unreacted F atoms suggest significant multiple-bounce scattering in addition to single-bounce scattering and trapping desorption. An empirical model of the atom-surface interaction dynamics is used in a Monte Carlo simulation of topography evolution during neutral beam etching of Si. Model predictions of profile phenomena are validated by experiments. [S0031-9007(96)

Previous findings of time-of-day differences in athletic performance could be confounded by diurnal fluctuations in environmental and behavioral "masking" factors (e.g., sleep, ambient temperature, and energy intake). The purpose of this study was to examine whether there is a circadian rhythm in swim performance that is independent of these masking factors. Experienced swimmers (n = 25) were assessed for 50-55 consecutive hours in the laboratory. The swimmers followed a 3-h "ultra-short" sleep-wake cycle, involving 1 h of sleep in darkness and 2 h of wakefulness in dim light, that was repeated throughout the observation. The protocol distributes behavioral and environmental masking factors equally across the 24-h period. Each swimmer was scheduled to perform six maximal-effort 200-m swim trials that were distributed equally across eight times of day (n = 147 trials). Each trial was separated by 9 h. A cosine fit of intra-aural temperature data established the time of the lowest body temperature (Tmin). Swim performances were z-transformed and compared across the eight times of day and across twelve 2-h intervals relative to Tmin. Analysis of covariance, controlling for trial number, revealed a significant (P < 0.001) pattern in swim performance relative to environmental and circadian times of day. Performance peaked 5-7 h before Tmin (approximately 2300) and was worst from 1 h before to 1 h after Tmin (approximately 0500). Mean swim performance was 169.5 s; circadian variation from peak to worst performance was 5.8 s. These data suggest a circadian rhythm in athletic performance independent of environmental and behavioral masking effects.

Chlorine nitrate photolysis has been investigated with the use of a molecular beam technique. Excitation at both 248 and 193 nanometers led to photodissociation by two pathways, CIONO(2) --> CIO + NO()2 and CIONO(2) --> Cl + NO3, with comparable yields. This experiment provides a direct measurement of the CIO product channel and consequently raises the possibility of an analogous channel in CIO dimer photolysis. Photodissociation of the CIO dimer is a critical step in the catalytic cycle that is presumed to dominate polar stratospheric ozone destruction. A substantial yield of CIO would reduce the efficiency of this cycle.