No abstract

Transparent plates of silica aerogel were studied by infrared spectroscopy. Experiments with D exchange, silanization, and heat treatment showed that dry silica gel, degassed at 875 K, has four absorption bands in the O-H stretching region. These bands are assigned to four different types of surface hydroxyl groups, viz. isolated single and geminal silanols (absorbing at 3749 and 3742 cm'1, respectively), and asymmetric H-bridged vicinal silanol pairs (absorbing at about 3720 and 3500-3700 cm™1). Interfering water vapor proved not to cause the observed spectral features. It is concluded that silica powders (Cabosil, Aerosil), having only one absorption at 3748 cm™1 after vacuum treatment at the same temperature, and silica gel do not have identical surfaces.

rf impedance, dc self-bias, and ion sheath (dark space) thicknesses are measured in an O2 discharge for 7–53 Pa pressure and 50–800 W rf power (13.56 MHz). Special attention is paid to corrections for reactor stray impedances. It is concluded that the discharge can be described as a capacitance (the ion sheath) with both a parallel and a series resistance, the series element being the more important one. Good agreement is found between optical and electrical measurements of the ion sheath thickness. Evidence is presented that the dc potential difference between plasma and ground and rf electrode can be estimated with reasonable accuracy from the dc self-bias and the optical dark space thicknesses. Positive ion acceleration in the ion sheath and electron-neutral collisions in the bulk of the plasma glow account for only part of the total rf power transfer. It is suggested that significant dissipation takes place near the glow-sheath boundary, although a quantitative description cannot be given yet.