Two-photon laser induced fluorescence spectroscopy: a powerful diagnostic
tool to monitor ground-state atom properties in a plasma environment.
S. Mazouffre, R. Engeln, P. Vankan, and D.C. Schram.
Department of Applied Physics, Eindhoven University of Technology, The Netherlands.
1 Introduction
Radicals like H, N, O, F, play a major role in plasma aided chemistry due to their high reactivity.
Moreover, in plasma processing, the buffer or carrier gas is currently composed of Ar, He, Xe
atoms. Being able to locally measure the density, the velocity, and the temperature of a particular
atomic species is therefore of relevance to get a better understanding of both the dynamics and the
kinetics of a given plasma environment. For the species-selective spatially resolved detection of
ground state atoms (the most populated state in a low temperature plasma) the very sensitive
Laser Induced Fluorescence technique can be used. However due to the large energy spacings
involved, monitoring of ground state atoms requires the use of highly energetic photons of which
generation is experimentally demanding [1]. Problems connected with single-photon excitation
can be avoided with the application of a multi-photon excitation scheme. In this contribution, the
Two-photon Absorption Laser Induced Fluorescence (TALIF) spectroscopy [2,3] is presented as a
diagnostic tool to monitor ground state atom properties in a plasma medium.
2 TALIF basic principles and 2-photon selection rules
The hydrogen atom energy diagram, see Fig. 1, may serve as an example to explain the basic
principles of a 2-photon excitation process. In order to probe ground state H atom, the latter can
be excited to the n=2 level using a Lyman-α photon. However, the need of coherent tuneable
VUV radiation combined with the detection of the fluorescence yield at the excitation wavelength
makes the experiment cumbersome and difficult. One may think about excitation to the n=3 level
and subsequent detection of the fluorescence light at the Balmer–α line, but this would
necessitate a Lyman-β photon. A possible way of avoiding the problem of generation of extreme
UV photons is to use a 2-photon excitation scheme, as depicted in Fig. 1.
ion
13.6
12.1
3s2S
3p2P
3d2D
656 nm
Ha
10.2
energy (eV)
2s2S
2p2P
205.14 nm
0
1s2S
l=0
l=1
Figure 1: Simplified energy diagram of the hydrogen atom. A possible
2-photon excitation scheme is presented. Two 205 nm photons are
used to excite H from the ground-state to the n=3 level. The
fluorescence radiation is detected at the Balmer-α line. Because of
specific optical selection rules the 3p state is not directly accessible
using a 2-photon excitation. However, the 3p state can be populated
due to l-mixing, induced by the laser beam electric field for instance.
The absorption of a third UV photon (from the n=3 level) leads to the
formation of H+, thus modifying the relation between the fluorescence
yield and the laser beam energy. The detection of the created charge
particle may be used to measure the H atom density.
l=2
A 2-photon excitation scheme has to obey specific electric dipole selection rules: for H it is
simply ∆l = 0, ±2 (l is the angular momentum), but it becomes more complex for multi electron
atoms [4]. Therefore “forbidden” transitions become accessible. Note that l-mixing effects [5,6]
can disturb the excitation process (transfer of population between l-states).
3 Experimental arrangement
3.1 UV photon generation
The generation of coherent tuneable UV radiation is based on non-linear optical effects in solids
or gases [7]. Photons with λ>195 nm can be produced with birefringent crystals, whereas photons
with λ<195 nm can be produced either via third harmonic generation in gases or via stimulated
Raman scattering [1]. Here we describe an experimental setup capable of producing a UV laser
beam with 200 nm<λ<210 nm (such photons are used to excite H, N, Kr), see Fig. 2.
A tuneable dye laser is pumped with the
second harmonic (532 nm) output beam
tuneable dye-laser
20 Hz Nd:YAG laser
615 nm
532
nm
(s-rho 640)
(injection seeded)
of an injection seeded pulsed Nd:YAG
laser (operating at 20 Hz). The dye laser
100 mJ
500 mJ
delivers radiation around 615 nm
(sulforhodamine 640) with a pulse
harmonic frequency
harmonic
generation
energy of 100 mJ. The dye laser output
separation
is frequency doubled using a KDP
KDP
BBO
crystal. Upon exiting the KDP crystal
polarizer
the polarization of the generated blue
beam is rotated to coincide with the
205 nm
polarization of the red beam. Both
dielectric
I2 cell
collinear laser beams are input to a BBO
2 mJ
mirror
(frequency calibration)
crystal where sum-frequency process
leads to the generation of a UV laser
Figure 2: Scheme of a TALIF setup (λ >195 nm). The ouput
beam around 205 nm with an energy per
beam of a tuneable dye-laser is converted into a UV laser beam
pulse up to 2 mJ. Both crystal are
after (indirect) third harmonic generation in crytals with
nonlinear optical properties. The frequency of the laser beam is
electronically angle tuned by means of a
calibrated using the absorption spectrum of molecular iodine.
feed back system to provide a constant
output energy when scanning the dye laser. A set of high reflectivity dielectric mirrors centered at
205 nm is used to remove the residual blue and red laser beams. The UV laser beam is
horizontally polarized with a measured bandwidth of 0.25 cm-1. Note that the energy fluctuation
(pulse to pulse) can be high (up to 50%) and the measured spectrum has to be corrected from
these instabilities. The fundamental dye laser frequency is calibrated by simultaneous recording of
the absorption spectrum of molecular iodine.
3.2 Fluorescence radiation detection
An example of a possible way to detect the fluorescence radiation is presented. The UV laser
beam is often tightly focussed (MgF2 lens) to obtain a good spatial resolution. The excited state
fluorescence yield originating from the focus is imaged perpendicular to the laser probe beam by
means of two plano-convex lenses onto an adjustable slitmask, which define the spatial
resolution, located in front of a detector (gated PMT for instance). The continuous background
light emitted by the plasma can be strongly reduced using a narrow bandwidth interference filter
or using a monochromator. The signal from the detector can be fed directly to the input of an
oscilloscope or it can be automatically read using a charge integrator coupled with an ADC. In the
later case, the fluorescence signal is not resolved in time.
4 Quantities to be measured and parasitic effects
From a spectral scan over the two-photon transition, see Fig. 3, several local parameters can be
determined. The spectral profile is a direct measure of the atom velocity distribution function. A
non gaussian shape indicates a departure from thermodynamic equilibrium. The local density is
obtained from the integrated intensity. In the case of a maxwellian distribution function, the local
temperature is derived from the Doppler width of the profile (taking into account the laser beam
spectral profile), and in addition the component of the drift velocity in the direction of the laser
beam can be determined from the absolute Doppler shift of the center frequency of the peak.
Doppler free two-photon absorption spectroscopy with two counter propagating beams enables
the measurement of the local electric field strength in a plasma using the Stark effect [8]. Note
that it also has important applications in precision spectroscopy [9].
∆ν
Fluorescence signal (a.u.)
500
400
300
200
100
0
97490
FW HM
ν theo
97492
97494
97496
97498
97500
Figure 3: Example of a measured spectral profile of the
1→3 two-photon transition in atomic hydrogen
(expanding thermal argon-hydrogen plasma). The
fluorescence is detected at the Balmer-α line. The
profile is gaussian meaning that H atoms are locally in
thermodynamic equilibrium. The area of the peak is a
direct measure of the density (after calibration). The
temperature is deduced from the Doppler broadening of
the line (after deconvolution with the laser spectral
profile) and the velocity (in the direction of the laser
beam) is determined from the absolute Doppler shift of
the line.
-1
F requency (cm )
The density profiles measured via laser induced fluorescence are relative. To obtain absolute
atomic number densities, the LIF set-up has to be calibrated using a well-defined concentration of
the species under investigation. In the case of atomic radicals, the needed concentration has to be
measured. This is accomplished for instance via a titration reaction in a flow tube reactor using
NO2 for H and NO for N [3]. Another way to calibrate the fluorescence yield is to use a twophoton excitation scheme in a noble gas (Kr for H and N [10], Xe for O [11]).
Information about the life time of the excited state and about collisional deexcitation (quenching)
as well as trapping of resonant photons can be obtained from the time resolved fluorescence
profile [5,10].
One should pay careful attention to the numerous parasitic effects which can strongly disturbed
the measured signal: reabsorption of resonant photons [5] (connected to the optical thickness of
the plasma), collisional deexcitation effects [10] (important at high pressure) which lead to an
apparent lower density, multi-photon ionisation [2,12] (often used as a diagnostic tool in
molecular beam experiments for instance), laser induced dissociation of parent molecules [13],
Amplified Spontaneous Emission generation (which can be used as a diagnostic technique [3]), lmixing processes [5,6], and saturation broadening via AC Stark effect.
5 Examples of applications
In order to illustrate the usefulness of the TALIF technique, two examples are given in Fig. 4.
10
8000
22
20 Pa
7000
H
sound
-3
-1
N density (m )
Velocity (ms )
6000
10
21
5000
4000
3000
2000
1000
0
0
10
20
30
40
50
Axial position (cm)
60
70
80
1
10
100
Axial position (mm )
Figure 4: N atom density profile in the afterglow of a nitrogen microwave discharge (left); H atom drift velocity
profile along the jet centerline of a hydrogen plasma expansion (right). Also shown is the local speed of sound
calculated from the measured H atom parallel temperature.
6 Conclusion
We attempted to provide the reader a rapid overview of the TALIF spectroscopy technique
applied to the monitoring of of ground-state atom properties in a low temperature plasma
environment. More accurate description of this diagnostic method as well as a more complete
review of possible applications can be found in literature [3].
References
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