A pulsed electron gun for ultrafast electron diffraction at surfaces.

The construction of a pulsed electron gun for ultrafast reflection high-energy electron diffraction experiments at surfaces is reported. Special emphasis is placed on the characterization of the electron source: a photocathode, consisting of a 10 nm thin Au film deposited onto a sapphire substrate. Electron pulses are generated by the illumination of the film with ultraviolet laser pulses of femtosecond duration. The photoelectrons are emitted homogeneously across the photocathode with an energy distribution of 0.1 eV width. After leaving the Au film, the electrons are accelerated to kinetic energies of up to 15 keV. Focusing is accomplished by an electrostatic lens. The temporal resolution of the experiment is determined by the probing time of the electrons traveling across the surface which is about 30 ps. However, the duration of the electron pulses can be reduced to less than 6 ps.

Water soluble vitamin requirements in home parenteral nutrition patients.

Ascorbic acid, thiamin, niacin, pyridoxine, and folic acid status was evaluated in eight physiologically stable home parenteral nutrition patients. Six of these subjects received these vitamins as a twice weekly bolus and were studied over a period of 6 days. All vitamin levels were normal except for blood thiamin which was low, 72 h after each bolus. Since transketolase levels remained normal, this fall in blood thiamin probably had no functional significance. The urine excretion pattern of niacin and pyridoxine indicated normal metabolism and retention of these vitamins. Two patients, who required only parenteral fluid and electrolytes to remain weight stable, received none of these vitamins parenterally, but also maintained adequate vitamin status. These results suggest that in long term home parenteral nutrition patients these five vitamins can safely be given twice weekly, rather than daily, and that short bowel patients who maintain their weight without intravenous calories and protein also assimilate adequate amounts of these proximally absorbed water soluble vitamins from their diet.