Filtern
Dokumenttyp
Sprache
- Englisch (8)
Schlagworte
- Coated/Layered Reference Materials (1)
- Doubly charged ions (1)
- GDS (1)
- Glow discharge processes (1)
- Krypton (1)
- Multilayer (1)
- Neon (1)
- Nitrides (1)
- Round Robin (1)
- The charge transfer and ionization process (CTI) (1)
Normally, in analytical GD-MS, the doubly charged metallic ion signals from the sample are several orders of magnitude less than the corresponding singly charged signals. However, we have observed that using a neon plasma, the M++ signals of some elements, which have double ionization energies close to the first ionization energy of neon, are of the same order as the M+ signal. Doubly charged ions may be produced directly in the discharge cell by electron ionization (EI), and also by two electron Penning ionization (TEP), but these processes cannot explain the above effect. In this paper, we suggest that an additional process named as ‘Charge Transfer and Ionization’ (CTI) produces such ions either in their ionic ground state or in an excited state. To confirm that this process is typical of the discharges used in GD-MS and not an artefact of any particular form of cell and ion extraction system, we have carried out comprehensive experimental measurements using three different GD-MS instruments, viz., Nu Astrum, VG9000 and ELEMENT GD and our results provide clear evidence for CTI. This is the first time the process has been identified as an ionization process in analytical GD-MS. CTI must be differentiated from Asymmetric Charge Transfer (ACT), which is a “selective” process and requires a close energy match (e.g. ΔE < 0.5 eV for a strong effect). On the other hand, CTI is “non-selective” in the sense that a close energy match is not required (e.g. a strong effect is observed with ΔE ∼ 2 eV), although the process only occurs for a limited number of elements, depending on the plasma gas used and the total energy required to doubly ionize the metallic atom.
Clear evidence of the effects caused by hydrogen, introduced in different forms in an analytical glow discharge source (GDS) operated with argon, is presented for the different matrix elements copper, stainless steel, titanium, aluminium and silicon. When hydrogen is present even in small quantities in argon, not only are there significant changes in the emission line intensities of most elements (analyte and plasma species), but also new spectral features, such as emission bands of new compounds (hydrides of sputtered sample constituents) and a continuous background in the range ~220440 nm. Moreover, the discharge current (the dependent parameter in our experiments) decreases with increasing hydrogen concentration. Different modes of hydrogen introduction, externally in gaseous form or sputtered as a sample constituent, cause very similar effects in terms of discharge current, line intensity (of analyte and plasma gas) and emission continua, which implies similar changes in the discharge processes in the two cases. The excitation of the hydrogen continuum appears to quench the population of the argon metastables (11.55 and 11.72 eV) and, consequently, other elementary processes in the plasma in which the argon metastables take part, but to a different extent for each analyte.
The influence of hydrogen contained in a sample or otherwise introduced into a glow discharge source (GDS) is here extensively experimentally presented for the case of copper as a sample by means of the addition of small quantities of molecular hydrogen (<1% relative partial pressure) to the argon carrier gas. The progressive addition of molecular hydrogen causes different intensity changes particular to the individual lines of different species such as atomic (Cu I) and ionic (Cu II) copper, and also atomic (Ar I) and ionic (Ar II) argon. Some interesting features of the emission spectrum of hydrogen such as its line, band and even continuum spectrum are observed. It was also found that the depth resolution becomes worse even at very low concentrations of hydrogen.
A Round Robin (RR) exercise on selected coated materials has been carried out with the aim of finding the optimal conditions for the analysis of nitride layers with GD-OES. Such pre-normative work is necessary for the evaluation of parallel development of the production of nitride layers as certified reference materials (CRMs). Two types of samples, TiN layer and VN layer, respectively, with chemical compositions close to stoichiometry and a thickness of ~3 µm, deposited on a steel substrate, have been provided to the RR participants. Additionally, another type of sample, a (100 nm CrNi/100 nm Cu) multilayer (ML) stack deposited on silicon wafer has also been included in the RR. This sample can be used as a CRM for checking GD spectrometer conditions and it has been proved helpful in further development of GDS instrumentation and methodology. The RR exercise has been performed in the frame of the EC Thematic Network on Glow Discharge Spectroscopy for Spectrochemical Analysis (GDS-Net).
In order to gain an insight into the numerous effects caused by the addition of hydrogen to an argon glow discharge, comparison experiments have been made with neon as carrier gas. In some cases the effects are diminished or even disappear. Using various bulk samples, such as copper, steel and titanium, sputtered in a neon glow discharge, the intensities of the analytical lines are affected by the presence of hydrogen in different ways from argon; the dependent parameter (the discharge current in this work) and the sputtering rate vary less than in argon. The crater shape and roughness are also affected and these effects are discussed qualitatively. Probably the most important spectral feature caused by hydrogen in the case of a discharge in argon is the emission of a continuous background. This does not appear in neon under similar discharge conditions and only weakly at high hydrogen concentrations. This supports the suggestion, made in previous work, that an effective quenching process of the argon metastables (11.55 and 11.72 eV) is Penning excitation of the hydrogen molecules, and subsequent decay to a repulsive state with emission of the continuum; in neon the energy match does not occur. It was found with neon, as with argon, that similar features occur as when hydrogen is introduced in different ways into the glow discharge: as a molecular gas contamination or as a constituent of the sample. Glow discharge mass spectrometry (GD-MS) experiments carried out with both argon and neon support the results obtained by optical emission spectrometry (OES) and provide further relevant information.