Plasma diagnostics using spectroscopic techniques Timo Gans York Plasma Institute

YPI – Low temperature plasma activities Plasma dynamics & chemical kinetics Advanced plasma diagnostics Special emphasis on optical diagnostics & laser spectroscopy Modelling & numerical simulations Technological exploitations Special emphasis on plasma medicine, plasma etching, plasma deposition

Plasmas & other disciplines Optics Atomic & Molecular Physics Laser Physics Surface Science Electro Dynamics Statistics Numerical Simulations Electrical Engineering Chemistry Bio-medical Sciences

Plasma – Complex Multi-Particle System What is a plasma? ionised gas with variety of particles electrons positive and negative ions neutral particles (atoms, molecules, radicals) excited species dust particles What do we like to measure? densities distribution functions (temperatures) electric and magnetic fields

Challenges & opportunities Multiphase interfaces: Plasma – gas – liquid – surface (solid) Multispecies: Electrons, pos. ions, neg. ions, neutrals, radicals, excited species, photons Multiscale problem – time: Electron dynamics: ps – ns Ion dynamics: 100 ns – μs Plasma chemistry: 100 μs – ms Surface chemistry: s – min Multiscale problem – space: Surface structures: nm – μm Charged particle gradients: μm – m Neutral particle gradients: 10 μm – m

How do we measure plasma quantities? Electrical diagnostics charged particles and fields external current and voltage measurements simple non-intrusive indirect model based global information only probe measurements local information direct reactive environment (gases) intrusive

How do we measure plasma quantities? Mass spectrometry neutral particles and ions energy distribution functions non-intrusive direct complicated in detail external measurement reactive gases

How do we measure plasma quantities? Optical diagnostics in principle all plasma parameters non-intrusive high temporal and spatial resolution Plasma physics Atomic & molecular physics Optical diagnostics

Optical Diagnostics Emission spectroscopy Laser spectroscopy passive simple robust indirect model based data needed Laser spectroscopy direct highly reliable active involving expensive  Combination of passive and active methods

Typical OES set-up

Optical Emission Spectroscopy (OES) line emission which emission lines (qualitative) species absolute intensities (calibration difficult) density of excited species line ratios robust model based analysis (this lecture!) line shapes (high experimental requirements) temperatures, fields, densities temporal variations plasma dynamics continuum radiation spectral distributions absolute intensities

Plasma concepts - CTE Complete thermodynamic equilibrium (CTE) homogeneity unique temperature (Te = Ti = Tgas) black body radiation Maxwell – Boltzmann distribution

Plasma concepts - CTE Maxwell – Boltzmann distribution population distributions Spectroscopy: line intensities and ratios velocity distribution Spectroscopy: line shape, e.g. Doppler effect

Plasma concepts - CTE Main constraints and limitations inhomogeneities Planck Local thermodynamic equilibrium?

Plasma concepts - LTE Local thermodynamic equilibrium (LTE) local parameters collision dominated equilibrium of collisions no equilibrium of radiation requirement example (hydrogen arc) ne = 1016 cm-3, Te  104 K (Ek - Ei)LTE  4 eV Partial LTE

Plasma concepts - PLTE Partial local thermodynamic equilibrium (PLTE) over population of the ground state LTE for excited states constraints and limitations low electron densities Corona model collisional radiative models

Plasma concepts - Corona Corona model model for plasmas with "low" electron densities (ne < 1013 cm-3) applicable to most technological plasmas far from thermodynamic equilibrium most particles are in the ground state electron impact excitation = relaxation by radiation (spontaneous emission)

Plasma concepts - Corona Corona model electron impact excitation = relaxation by radiation (spontaneous emission) ni : population density of state i Aik : spontaneous emission rate nPh,i : photons per unit volume and time

Plasma concepts - Corona n0 : ground state density Ei : electron impact excitation rate of state i, (depending on ne and Te) i : electron impact excitation cross-section of state i f(E): normalised EEDF i : radiative lifetime

Plasma concepts - Corona steady state of excited states aik : branching ratio RF - discharges

Corona: cascade transitions Additional excitation and de-excitation processes applicable to most technological plasmas cascades from higher electronic states one dominating or effective cascade state nc = ? Aci : transition rate from the cascade state c nc : population densities of the cascade state c

Corona: cascade transitions neglecting second order cascades: Determination of Ec is difficult (reabsorption!)

Corona: stepwise excitation excitation out of metastable states long lifetimes of metastable states transport problem plasma wall interaction complex avoid through proper choice of state i with small cross- sections for excitation out of metastable states (small Ei,m) turn into diagnostics of metastable states by comparing with states excited out of metastable levels

Corona: collisional de-excitation collisional de-excitation (quenching) A* + Q  ? especially important at high pressures! kq : quenching coefficient with species Q nq : density of species Q

Corona: collisional de-excitation q : quenching cross-section, Tgas indepedent Tgas : gas temperature <v> : mean velocity  : reduced mass Importance of Tgas Which quenching partners are present? What are the densities?

Which emission line should I analyse? selection rules for emission lines good quality of electron impact excitation cross-sections (same source!) negligible excitation out of metastables small cascade contribution short lifetimes competition with quenching high intensities high temporal resolution known quenching coefficient (better small) no excitation transfer with other species no spectral overlap with other emission

Actinometry & Limitations Direct excitation: Dissociative excitation:

Influence of the EEDF

Time & space dependence of the EEDF

Comparison with laser spectroscopy K NIEMI, et al., Appl. Phys. Lett. 95 (2009) 151504 N Knake, et al., APL, 93 (2008) 131503

Thank you! York Plasma Institute