Download Hands-on-training: Introduction to emission spectroscopy

Hands-on-training:
Introduction to emission
spectroscopy
Risto Kronholm
University of Jyväskylä
Department of Physics
Contents of emission spectroscopy
hands-on-training
Introduction to emission spectroscopy
Introduction to experimental setup
Setting up the experimental setup
Optical emission spectrum measurements
Analysis: Identification of the most intensive
peaks
Results: identification of the elements in
plasma
Contents of introduction to emission
spectroscopy
Motivation
Basics of atomic physics
Ionization
Excitation
Spontaneous emission
Spectroscopy
Motivation
Electron cyclotron resonance ion sources are used for
the production of high intensity highly charged ion
beams
To be able to use and make development to the ECR
ion source, the plasma inside the source has to be
studied and understood
Invasive plasma diagnostic methods cannot be used
as they disturb especially the high charge states of
interest.
Due to the spontaneous emission of excited states,
visible light is emitted opening a possibility to study
physics of ECR plasma without disturbing it
Atomic physics
We will concentrate only on atoms and ions
Atoms and ions are composed of a massive nucleus and
electrons bound to the nucleus potential
In quantum mechanical model of atom, electrons can be
described as mathematical wave functions
These are called atomic orbital which are characterized by
unique set of quantum numbers 𝑛𝑛 = 1,2,3, … and 𝑙𝑙 =
0,1,2, … , 𝑛𝑛 − 1 and 𝑚𝑚 = −𝑙𝑙, −𝑙𝑙 + 1, … , 0, … , 𝑙𝑙 − 1, 𝑙𝑙
Each orbital can occupy maximum of two electrons with its
1
own quantum number 𝑠𝑠 = ± (atomic spin orbits)
2
Atomic physics
Atom’s energies are quantized, each
orbital has its specific energy
Allowed energies of atom can be
presented in energy-level diagram
Spectroscopic notation for these states is
𝑛𝑛𝑙𝑙 𝑤𝑤 2𝑆𝑆+1𝐿𝐿𝐿𝐿+𝑆𝑆
1s1 2p 1𝑃𝑃1
Lowest energy state of the atom is stable
and it is called ground state
Other higher energy stationary states are
called excited states
Atomic physics
Electron can jump from a stationary state to another by
emitting or absorbing energy
These electronic transitions are called excitations and deexcitations
If absorbed energy is high enough, electron can be
removed from nucleus potential
Atomic physics
In ECRIS plasma heating is based on energy transfer from
microwaves to electrons via electron cyclotron resonance
Electron energies range from cold electrons with 1 eV
energies up to hot electrons with energies in the order of
100 keV
Collisions between electrons and atoms
Electron impact ionization
• Electron impact
ionization if energy of
electron is higher than
ionization potential
• Ionization rate 𝑅𝑅𝑖𝑖 ∝
𝑛𝑛𝑒𝑒 , 𝑛𝑛𝑛𝑛,𝑖𝑖 , 𝜎𝜎𝑣𝑣𝑒𝑒
From ECRIS
electron energy
distribution function
Excitations
In addition to ionization, electron can also jump to higher
energy stationary state
These reactions are called excitations
Electron impact excitation
• Electron impact
excitation
• Excitation rate 𝑅𝑅𝑒𝑒 ∝
𝑛𝑛𝑒𝑒 , 𝑛𝑛𝑛𝑛,𝑖𝑖 , 𝜎𝜎𝑣𝑣𝑒𝑒
From ECRIS
electron energy
distribution
function
Downward transition
Excited electronic states can decay via radiative transition
to lower energy states
Electromagnetic radiation is emitted in transition
Optically allowed transitions follow the selection rules:
– ∆𝐿𝐿 = 0,1
– ∆𝐽𝐽 = 0,1, but 𝐽𝐽 = 0 to 𝐽𝐽 = 0 is forbidden
– ∆𝑆𝑆 = 0
Electronic states which cannot decay via radiative
transitions are called metastable states
Spontaneous emission
7437 nm
53.7 nm
728 nm
501 nm
2058 nm
58.4 nm
• Due to spontaneous
emission electron in
excited state undergoes
a transition to a lower
energy state
• Photon with wavelength
of 𝜆𝜆 = ℎ𝑐𝑐 ⁄(𝐸𝐸𝑝𝑝 −𝐸𝐸𝑘𝑘 ) will be
emitted
• Intensity of emission is
𝐼𝐼𝜆𝜆 = 𝑛𝑛 𝑝𝑝 𝐴𝐴𝑝𝑝𝑝𝑝
Metastable state
Spontaneous emission
• Emission lines don’t ever
have sharp delta function
profile (∆𝜆𝜆 > 0)
• Profile depends on the
broadening mechanism:
• Natural broadening
• Doppler broadening
• Stark/Zeeman
broadening
• Instrumental broadening
• In our plasma first three are
small compared to
instrumental broadening
Spontaneous emission
In this hands-on-training we will focus only to transitions in
visible part of the whole electromagnetic spectrum
Spontaneous emission
Emitted photons can be
measured and plotted
to wavelength spectrum
Optical emission plasma
spectroscopy
The wavelength
spectrum of light is like a
fingerprint of an element
Light intensity depends on
ion density and electron
energy distribution function
&
Plasma spectroscopy
can be used to identify
particle species in the
plasma
Plasma spectroscopy can give
insight on how different
methods and different ECR
parameters affects on
electrons and high charge
states.
Analysis
Analysis of emission spectrum
Emission lines of different elements are listed
in NIST spectral database
https://www.nist.gov/pml/atomic-spectradatabase
Analysis
Analysis
Analysis