Glow Discharge Optical Emission Spectroscopy: General Overview & Selective excitation/ionization mechanisms

by Dr Sohail Mushtaq (FoC Research Fellow)

Abstract:
Glow discharge (GD) plasma is an ionized gas primarily utilized as an excitation/ ionization source for direct solid sample analysis. The sample to be analysed forms the cathode of a discharge in a noble gas, normally pure argon; the elements present are sputtered from the cathode, excited and ionized and then detected either by optical emission spectroscopy (OES) or by mass spectrometry (MS). Nowadays GD-OES is widely used for performing surface, interface analysis, and depth profiling of thin films. The scope of GD-OES has further expanded to the direct analysis of ultra-thin films (less than 10 nm thick). Although the basic processes of excitation and ionization occurring in GD are well understood, particularly in argon, there are little data available on the relative importance of different excitation processes for particular levels, e.g. Asymmetric charge transfer. Therefore, a major concern in analytical results is the possible error that can arise due to the presence of trace molecular gases such as oxygen, hydrogen and nitrogen. For good analytical practice, it is important to know how these traces of molecular gases affect the ionization and excitation processes, and hence the accuracy of results.

In low pressure GD such as the Grimm-type source (the most commonly used source for OES), the relative intensities of lines in the analyte spectrum depend primarily on the plasma gas. The probability of selective excitation of certain analyte spectral lines by asymmetric charge transfer involving carrier gas ions is increased if excited levels are close to resonance. Traces of light gaseous elements, present either as constituents in the sample such as oxide, hydrides, nitrides or occluded within the sample, can be involved in selective excitation or affect selective excitation by the plasma gas. Prof. Steers et. al. reported for the first time that trace hydrogen ions in an argon glow discharge can produce ACT and that asymmetric charge transfer involving hydrogen ions (H-ACT) is a very important selective mechanism for certain Fe II and Ti II spectral lines.

In the current work, asymmetric charge transfer involving oxygen ions (O-ACT) has been observed in spectra recorded with the Imperial College high resolution vacuum UV Fourier transform spectrometer (FTS). In addition, lines excited by Ar-ACT are relatively weaker with the addition of oxygen. Results from our recently published mass spectrometry measurements, show that significant quenching of the argon ion population occurs with the progressive addition of oxygen to the glow discharge.

Keywords:
Glow discharges, excitation processes, analysis of surface layers

Short Biography:
Sohail Mushtaq received his M.Sc degree in 2005 and his M.Phil degree with major subjects Plasma Physics and Atomic & Molecular Spectroscopy in 2007, both from the Government College University (GCU) Lahore. His M.Phil research work was on "Optimization of discharge conditions for plasma nitriding by optical emission spectroscopy". During his M.Phil he was selected for scholarship for ICSC-WORLD Laboratory in Lausanne, Switzerland and Salam Chair, Physics Department, GCU, Lahore. After M.Phil degree he was selected as Marie Curie Early Stage Researcher in Imperial College London. His PhD research work was on the "Effects of traces of molecular gases in analytical glow discharge: glow discharge optical emission spectrometry and time of flight mass spectrometry studies".

For his PhD research work he has carried out the first multi-line study for oxygen as an impurity in glow discharge spectroscopy (GDS). The results of studies using Fourier Transform Optical Emission Spectroscopy (FT-OES) and Time of Flight Mass Spectrometry (ToF-MS) are reported to investigate the effects of controlled addition of oxygen (0-1 % v/v) on observed spectra from a Grimm-type glow discharge, generated in argon plasma with various pure samples (i.e. iron, titanium, copper and gold) and calamine (an oxide layer sample). He has successfully his thesis in June and has been appointed as Postdoctoral Research Fellow in the Faculty of Computing and working with Prof. Edward Steers.