Electron-Impact Excitation of Gas-Phase Sulfur

Abstract

Herein we present results from a study of optical emission in the range 270–550 nm that was stimulated in collisions of low-energy electrons (2–50 eV) with gas-phase S. Experimental. Excitation of S was studied using an optical method with an MDR-2 high-aperture diffraction monochromator in the range 270–550 nm on an automated apparatus. The apparatus and procedures for measuring and calibrating the beam energy of exciting electrons were described in detail [7]. The beam diameter was 1.5 mm for electrons emitted by an oxide cathode. The beam was formed by a four-electrode electron gun, passed through a gas-fi lled cell 12 mm high and 10 mm in diameter, and was detected by a deep Faraday cup. The energy of electrons in the beam (full width at half-height of the distribution maximum of differentiated voltammetric characteristics) was ~0.5 eV at 20 ��A. Gas-phase S was fed into the collision cell from a separate resistance-heated reservoir loaded with S (~5 g) through a thin-walled tube 4 mm in diameter and 60 mm long. This enabled the temperatures of the collision cell, electron beam, and Faraday cup, which had their own independent resistance heaters, to be maintained 20–30 K above the temperature of the S reservoir. Thus, condensation of gas-phase S on the components of the aforementioned electron-optical system was avoided. The S reservoir temperature for all measurements was maintained within 335 ± 2 K. The vacuum chamber was evacuated using an oil pump at 500 L/s. The residual gas pressure in the vacuum chamber during the measurements was <10 –6 Torr. Emission resulting from collisions of electrons and gas-phase S passed through quartz windows of the collision cell and vacuum chamber, was focused using a dual-lens condenser onto the input slit of the MDR-2 diffraction monochromator (grating 1200 lines/mm, reverse dispersion 2 nm/mm), and was detected by an FEU-106 photoelectron multiplier. Single FEU photoelectron pulses were pre-amplifi ed, shaped by a broad-band amplifi er-discriminator, fed through an interface card into a pulse counter, and stored in a PC. The signal at each measurement point was accumulated for 10–60 s of exposure depending on the intensity of the emitted spectral lines or molecular bands in order to ensure that the measurement accuracy was better than 5–10% at the emission maximum.