Ion Source Conditions Research Articles

Transverse ionization of a seeded molecular beam: Effects of space-charge on mass spectra and velocity measurements

Optimization of the signal of the minor component of a seeded molecular beam results in ion source conditions that allow space-charge trapping. Space-charge trapping of the ions from XeF 6 seeded into rare gases at or below a few mol% has two deleterious effects: the relative intensities of the ionic species are inaccurately represented, and the translational temperature of the molecular beam cannot be accurately determined. Ion residence times are sufficiently long to allow second or third ionization events, yielding large relative intensities for Xe 3+, and Xe 2+. When this situation is corrected, the relative intensities of these latter species are significantly reduced. Ionic species containing more than one xenon atom are found to be fragmentation products from ionization of the van der Waals' dimer Xe 2F 12, and not a result of ion—molecule reactions. Accurate determination of the translational temperature of the molecular beam by the time-of-flight method requires elimination of space-charge trapping of the ions in the ionizer. The translational temperature of an argon molecular beam generated from a 0.0025-cm diameter orifice with one atm pressure was 6 K, in good agreement with other time-of-flight techniques.

Measurement of the distributions of internuclear separations in 3.0 MeV H 2+ and 3.63 MeV HeH + beams

Angular distributions of charged dissociation fragments are measured for 3.0 MeV H 2 + and 3.63 MeV HeH + ions incident on ∼ 160 A ̊ carbon targets. By using the reflection method for a pure Coulomb potential we unfold from these data the distributions of internuclear separations for each molecular-ion species prior to dissociation. These results are insensitive to ion-source conditions. For H 2 + this distribution, while ∼2 times wider than a pure ground vibrational state population, is markedly different from the Franck-Condon distribution that has been previously assumed by other authors with similar rf and duo-plasmatron ion sources. For HeH + the distribution is slightly broader (∼1.5 times) than that expected for a pure ground state population. From the data, we are able to extract the initial vibrational state population in the incident beam.

Identification of C8H10 and C9H12 Alkylbenzene Isomers by Chemical Ionization Mass Spectrometry

The CI mass spectra of C8H10 and C9H12 alkylbenzene possessing four and eight isomers respectively, were evaluated for the the purpose of their isomeric distinction and identification. About 2μg of each isomer in acetone was introduced into a Finnigan 3300E GC/QMS system equipped with an OV-1 or Squalane column. The ion source conditions were CH4 (1 Torr), i-C4H10 (0.5 Torr) at 70°C. The relative standard deviation was 1~10% for thrice repeated measurements and also 1~20% on another day. In CH4 reagent gas, (M-27)+ peak only appeared in ethylbenzene and ethyltoluene, while C3H7+, (M-41)+ and (M-13)+ was only given by propylbenzene. Since there was a considerable difference in the intensity ratio of (M-15)+, (M-1)+, M+ or (M+57)+ to (M+1)+ when using i-C4H10 as reagent gas, distinction of isomers among each alkylbenzene was possible. It was found that the CI mass spectra using CH4 reagent gas were advantageous for grouping C8H10 and C9H12 isomers, whereas those using i-C4H10 reagent gas were suitable in distincting and identifying each isomer. The CI method was superior to EI method concerning this study.

Photodissociation of vibrationally excited Ar2+ formed by associative ionization

The photodissociation cross sections of a beam of Ar2+ formed by associative ionization in a conventional electron impact ion source have been measured over the wavelength range from 265 to 715 nm. The measured cross sections are for transitions from the 2Σu+ ground state to the repulsive 2Σg+ excited state and are dependent on the ion source conditions. At low pressures (0.01 Torr) in the ion source, the cross section is 1×10−17 cm2 and varies little over the entire wavelength range. When pressures are increased to 0.1 Torr, a peak in the cross section at about 300 nm is obtained with a value of 1.7×10−17 cm2. A simplified two state mechanism involving high and low lying vibrational levels of Ar2+ in its ground state is proposed to explain the qualitative effects of pressure on the cross sections. Measurement of the Ar2+ and Ar+ beam currents as a function of repeller voltage showed that collisional dissociation is occurring in the ion source under conditions of relatively high pressure and positive repeller voltage at rates comparable to the Langevin rate. With zero repeller voltage an average collisional stabilization rate coefficient of 8×10−11 cm3 sec−1 molecule−1 is obtained for vibrational relaxation of Ar2+ from ’’high’’ to ’’low’’ lying levels. Using the calculated cross sections of Michels and Wright [J. Chem. Phys. 69, 5151 (1978)] for each vibrational level, a vibrational population is derived from the observed cross section versus wavelength plot. A comparison of the vibrational population derived from the low and high pressure results confirms that collisional relaxation is occurring, but that at 0.1 Torr the distribution in the ion source is still ’’hotter’’ than a 300° K Boltzmann distribution.

Studies in negative ion mass spectrometry. V—A Comparison of positive a positive and negative ion mass spectra of copper(II) complexes of bidentate schiff base ligands

Abstract70 eV positive and negative ion mass spectra of a series of copper(II) Schiff base complexes have been obtained consecutively under the same ion source conditions. The characteristic feature of the negative ion spectra is their extreme simplicity relative to the corresponding positive ion spectra, the only ions present in significant abundance being the molecular anions and ligand ions. The influence of substituents (R) on positive and negative ion fragmentation patterns is discussed. Metastable peaks have been obtained in all cases for the transition [Cu(Ligand)2]− → [Ligand]−.

16O contamination in 4He analysis beams

Abstract The 4 He + beam from a Van de Graaff accelerator is often accompanied by an 16 O + beam of the same energy. If, after acceleration and before magnetic analysis, one electron is stripped from the oxygen ion to form 16 O 2+ , these ions will not be separated from 4 He + during magnetic analysis. The 16 O 2+ fraction in the 4 He + beam was measured by analyzing back-scattering spectra for a thin Au film and the O to He ion charge ratio of 2:1 was confirmed by electrostatic deflection. Ion source conditions and the pressure in the beam lines strongly affect the 16 O 2+ fraction and the dependence of the O 2+ /O + ratio on pressure is found to be in approximate agreement with predicted values. A formula for estimating the 16 O 2+ beam intensity, based on charge exchange data and measurement of the primary 16 O + beam intensity, is N [ O 2+ ] = 13 P ( torr ) l ( cm ) N [ O + ], where P is the total gas pressure, l the length of the beam line between the accelerator and switching magnet and N [O 2+ ] and N [O + ] are the beam intensities of the respective species. The 16 O 2+ contamination of 4 He + beams is easily removed by electrostatic separation.