Wire Emitter Research Articles

On the formation of doubly charged ions in field ionization

AbstractThe observation of doubly charged ions in field ionization with blade or wire emitters does not in itself indicate that high excitation energies (≥ 17 eV) are accessible through gas phase ionization with such emitters.

The energy dispersion effect in field ionization/field desorption quadrupole mass spectrometers

A method is described whereby field ionization/field desorption quadrupole mass spectrometers can be used for energy analysis and appearance potential determination of field ions. With this method no special instrumental modifications are necessary. When employed for measurements of ionization potentials of some rare gases and some organic molecules an accuracy of better than +or-0.2 eV could be achieved when Pt tips were used as field anodes. The accuracy of measurement decreased only slightly when field ionization was performed at high temperature activated tungsten wire emitters. The same method of measurement also could be applied successfully to ions generated by field desorption.

A new field ion emitter for field ion sources of quadrupole mass spectrometers

When compared with the wire emitters usually employed, a fivefold increase in sensitivity of the quadrupole mass spectrometer for the field ionization of gases is obtained.

Activation of field desorption emitters

Successful field desorption mass spectrometry requires a reliable means for wire emitter activation, i.e. growing semi-conducting micro-needles on the emitter surface. We have had difficulty in initiating needle growth and have overcome this problem by developing a simple process for controlled preroughening provides a large number of nucleation sites which have sufficient field strength to initiate needle growth. In addition to this technique a few observations concerning the use and performance of the emitters are discussed.

Sondenmessungen in einem durch Kontaktionisation erzeugten Cäsium‐Plasma

AbstractUsing a hot wire (emitter) in the axis of a cylindrical glass vessel containing cesium vapour a plasma was formed by contact ionization. The plasma diffuses to the walls where it recombines. Plasma density, electron temperature (respectively the energy distribution function of the electrons), and the space potential were measured by probes in different distances from the emitter. Perturbations of the plasma by the current in the directly heated emitter are eliminated by using a pulsed current and a sampling method for the probe measurements. At vapour pressures exceeding 10−3Torr the plasma follows the equilibrium theory. At low pressures when an ion sheath at the emitter is present nonmaxwellian electron distribution functions occur. The perturbations of the plasma by the probe are thoroughly discussed.

A combined field ionisation-electron impact ion source for high molecular weight samples of low volatility.

A new combined field ionisation/electron impact ion source is described which can be used in connection with a direct inlet probe. The distance between the electron beam and the wire emitter is only one mm, sot hat the vapour of the sample reaches both ionizing zones. The vapour passes the ion source as a quasi molecular beam. Results obtained with this source will be given.

Feldionen-Massenspektren organischer Moleküle

The field ion (FI) mass spectra of aliphatic alcohols from methanol to n-decanol are discussed. The influence of the ion emitter geometry and metal was investigated. Tips as well as thin wires were used as FI emitters. The wires consisted of platinum only, whereas platinum, gold and tungsten were used as emitter tip metals. The difference of the FI mass spectra with tip and wire emitters are mainly due to the difference in field strengths, which are usually lower with wires. The differences of the spectra with platinum and gold tips are correlated with differences in adsorption properties of these metals. The main ions in the FI mass spectra of alcohols besides the molecular ions M are: 2M + 1, M + 1, M — 1, M — 2, CnH2n+1 and (CH2)nOH with varying n-values. The mechanisms of formation of these ions are discussed.

Photochemical Studies by Means of a Field Ion Mass Spectrometer

A mass spectrometric investigation was carried out on the direct photolyses of propene, 1-butene, and hydrazine at 1849 A with a field ion source in a flow system. Comparisons were made with Pt tip and wire emitters. It was found that, without illumination, mass spectra obtained with the wire were accompanied by a number of fragment peaks amounting to almost 1%. Since these peaks interfere with those produced photochemically, the tip emitter was used mostly for the photochemical studies although it gave 100 times less current and was less stable. The photochemical products formed at a gas pressure of 10 μ by a low pressure mercury lamp were detected after approximately 10 m sec. The three main peaks observed in the propene photolysis were at masses 27, 28, and 56, indicating the processes: C3H6+hv→C2H3+CH3, C3H6+hv→C2H4+CH2, CH2+C3H6→C4H8. The photolysis of 1-butene gave four main peaks at masses 40, 41, 42, and 70, suggesting steps, C4H8+hv→C3H4+ (H+CH3) or CH4, C4H8+hv→C3H5+CH3, C4H8+hv→C3H6+CH2, CH2+C4H8C5H10. The only peak found with the photolysis of hydrazine was at mass 17, indicating the step, N2H4+hv→NH3+NH. The possibility of forming these products by secondary processes is discussed.

Thermionic Work Function of Thin-Oxide-Coated Aluminum Electrodes in Vacuum and in Cesium Vapor

The thermionic work function of aluminum electrodes with anodized aluminum oxide coatings of 30 and 2000 Å was determined for each electrode from the contact potential difference of each electrode with a well outgassed and aged polycrystalline tungsten wire emitter. In vacuum, the thermionic work function of the 30-Å coated electrode was 4.3±0.1 eV, relative to a polycrystalline tungsten work function of 4.6 eV and a tungsten emission constant of 120 A/cm2· °K2. In cesium vapor at reservoir temperatures up to 70°C, the 30-Å coated electrode had a minimum observed work function of 1.4±0.1 eV relative to the same tungsten reference electrode. The resistance of the 2000-Å film coated collector electrode was too large to allow accurate determination of the work function of this electrode. In both vacuum and cesium vapor, however, there was an indication that the work function of the 2000-Å coated electrode was a few tenths of an electron volt lower than that for the 30-Å coated electrode. It is suggested that similar but perhaps less stable oxide-coated electrodes could occur naturally on the collector of a thermionic energy converter due to oxidation of the collector surface or buildup on the collector surface of outgassing products from the emitter. Auxillary experiments were performed on the secondary-electron emission from these oxide-coated electrodes. No lingering secondary electron emission current corresponding to a Malter effect was observed. The true secondary yield from the 2000-Å coated electrode was 3 in vacuum and 7 in cesium vapor for a primary electron energy of 150 eV. The secondary yield for the 30-Å coated electrode was similar in cesium vapor but was not studied in vacuum.