1. Plasma diagnostics using spectroscopic techniques
Timo Gans
York Plasma Institute

2. 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

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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. Typical OES set-up

11. 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

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

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

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

15. 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

16. 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

17. 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)

18. 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

19. 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

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

21. 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

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

23. 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

24. 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

25. 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?

26. 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

27. Actinometry & Limitations
Direct excitation:
Dissociative excitation:

28. Influence of the EEDF

29. Time & space dependence of the EEDF

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

31. Thank you!
York Plasma Institute