Optogalvanic spectrometry
Vítězslav Otruba


Fundamentals of the method
•The optogalvanic effect utilizes a combination of atom excitation by resonance radiation and
collision ionization by plasma (flame) particles to selectively ionize the determined elements.
Ionization is measured using the generated ions and thus indirectly the absorption of radiation.
•The first experimental observation was made by Penning (r.1928) during irradiation of the neon
discharge with another neon lamp.
2010
prof. Otruba
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Multiphoton Ionization
2010
prof. Otruba
3
•Ionization in MPI is achieved by intense non-selective radiation of very high intensity.
Absorption of a series of photons that excite an atom (molecule) into virtual energy states leads
to ionization.

Resonance Ionization Spectroscopy
2010
prof. Otruba
4
•RIS uses stepped excitation by resonant radiation followed by ionization. Usually requires two to
three tunable lasers.
•The method is highly selective (the resulting selectivity is the product of the selectivity of
excitation to individual stages).
•Achieved selectivity of 1022 (Cs in Ar), isotopic ratios up to1013 – 1018 (1 pg in 1t, 1 ag 14C)

Optogalvanic Effect
2010
prof. Otruba
5
•It uses a combination of resonant laser radiation with collision excitation with particles with
high kinetic energy:
oKinetic energy of particles at high temperature (thermal movement in flame, plasma)
oKinetic energy of charged particles accelerated by electric field (discharges, especially under
reduced pressure)
•It is a certain variant of atomic fluorescence, where there is a high probability of  deexcitation
by collisions
•Does not require optical detection equipment
•It detects all ions as opposed to the tiny number of photons detected during fluorescence.

Applications
•Laser keying measurement
•Calibration of wavelengths (e.g. tunable lasers)
•Spectroscopy of states with long life
•Doppler-free spectroscopy
•Spectroscopy of radicals
•Trace analysis
oin flame
oin electrotermal atomizer
oin hollow cathode
•
2010
prof. Otruba
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Laser-Enhanced Ionization Spectrometry
In Flames
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prof. Otruba
7
High voltage on electrodes - 1000 V, burner isolated from apparatus,
connected to the preamplifier input. An analytical signal is taken from the torch (anode).

LEI measurement system
2010
prof. Otruba
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Boxcar integrator
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prof. Otruba
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time
tcycle
tgate

Basic integrator modes
•In the second case,  in the time of the pulse waveform scanning is performed. It is also possible
to set an arbitrarily wide time window for the selected time resolution of the pulse waveform.
2010
prof. Otruba
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•The integrator can be used as a gateway that transmits periodic signals comparable to pulse
duration Δt. The gate opening length can be freely set, including the determination of the sample
pulse offset against the measured pulse.

LEI - flame
2010
prof. Otruba
11
•Dynamic range of concentrations •The linear concentration range is 4 - 5 orders of magnitude
•Application in practice to about 20 elements

2010
prof. Otruba
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LEI – flame - LOD

LEI – flame - selectivity
2010
prof. Otruba
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•The spectral resolution is determined by the absorption profile of the spectral line and the
properties of the measuring radiation.
•Using a sodium dye laser, 589.0 nm was R≅60 000
•When using a commercial broadband laser it was R≅8700 – see fig.

LEI – absorption of non-resonance transitions
2010
prof. Otruba
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LEI-flame-molecular spectra
2010
prof. Otruba
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LEI - flame
•LEI differences from other flame methods:
•It is possible to use non-resonant lines with good sensitivity. E.g. for Li, the 2p level
occupation in the flame (Boltzmann) is only 2.10-4 of baseline, but the LOD is only 12 times worse.
•Possibility to use two-photon transitions with good sensitivity
•Low sensitivity for elements with high ionization potential. For elements with IP> 9 - 10 eV it
would be necessary to work in the vacuum UV region of the spectrum.
•Interference of determination (reduced sensitivity) by easily ionizable matrix elements
2010
prof. Otruba
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OGE electrothermal atomization
2010
prof. Otruba
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OGE in gas discharges
2010
prof. Otruba
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Applications:
Calibration  of lasers wavelength using discharge tubes, a hollow cathode without using a
complicated optical apparatus (line cathode material and filler gas).
High energy levels can also be excited in discharges, which can be measured by OGE.
Sufficient concentration of atoms even low volatile materials.
Possibilities of Doppler-free spectrometry of atoms and molecules with resolution up to 100 MHz.
Isotope analysis.

2010
prof. Otruba
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Experimental configuration: The OGE cell inside the cavity has Brewster windows to reduce losses.
The C12 laser incident on the OGE cell provides a “C12 signal” that is used for normalization of
the C14 signal. The shutter inside the laser cavity is for modulating the 14CO2 laser. M1: High
reflective mirror & grating, M2: 85% reflective output coupler, M3: Gold plate mirror, PS: Pressure
Sensor, FC: Flow Controller, RGA: Residual Gas Analyzer, DAQ: Data Acquisition Board
Intracavity Optogalvanic Spectroscopy, Ultra-sensitiveAnalytical Technique for 14C Analysis Murnick
et al.Anal Chem. 2008 July 1; 80(13): 4820–4824

Experimental results
•The OGE signal in response to a laser modulated at 63 Hz. The sample is 5% CO2 in N2 at
•10-11 14C enrichment
•Resonance curve for intracavity optogalvanic effect. The solid line is a best fit Voigt Profile,
•The width, 48 MHz is expected for 14CO2 in the 5 mbar discharge at 385°C
2010
prof. Otruba
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Photoionization - atom detection
2010
prof. Otruba
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