22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Windowless VUV spectroscopy of atmospheric pressure plasmas
J. Benedikt, V. Layes, S. Schneider, S. Große-Kreul, G. Willems, M. Hefny, S. Hübner and A. von Keudell
Research Department Plasma with Complex interactions, Ruhr-University Bochum, Germany
Abstract: Measuring plasma-emitted VUV photons can provide useful information about
excitation processes in the plasma and about the presence of atomic and molecular species.
In this contribution, the windowless VUV spectroscopy will be introduced in details and the
VUV spectra of variety of atmospheric plasmas will be presented. These measurements will
be compared to densities of O, N and other species as measured by mass spectrometry.
Keywords: VUV spectroscopy, atmospheric plasma
1. Introduction
Atmospheric pressure non-equilibrium plasmas (APPs)
are effective source of large densities of reactive radicals,
metastables and ions and also high fluxes of photons.
Among those are also highly energetic vacuum UV
(VUV) photons with wavelengths below the cutting edge
of LiF or MgF 2 window materials. The resulting high
reactivity of these APPs can be used in many surface
treatment applications such as activation of polymer
surfaces, treatment of living tissues (decontamination,
acceleration of wound healing) or in deposition of thin
films or nanostructured materials. To gain insights into
the effects of the different plasma components, a
microscale Atmospheric Pressure Plasma Jet (µ-APPJ,
Fig. 1) operated in helium with a small (up to 1%)
admixture of a molecular gas (e.g. O 2 ) can be used.
With this plasma source, the treated substrates are not
directly immersed in plasma but they are exposed to the
effluent, which main reactive parts are plasma generated
reactive species and emitted photons. This source has
been described and studied extensively in the past by
several groups and quantitative measurements of O, N, O 3
and O 2 (a1∆ g ) densities and qualitative information about
ions in the effluent are available [1-5]. Most of the inplasma-generated ions and electrons recombine in the
effluent and only some in photochemical reactions
induced water cluster ions has been measured by MS near
the substrate.[6]
2. Windowless VUV Spectroscopy
Many excited species in APPs are emitting photons in
the vacuum ultraviolet (VUV) wavelength region with
wavelength below 200 nm. This photons cannot propagate
through air, but since many of the APP sources are
operated in argon or helium, they can propagate through
the gas channel formed by the noble gas and can therefore
reach the surface even at the distance of several tens of
mm from the jet. It is, therefore, important to investigate
at least qualitatively the VUV spectra of APPs. To
achieve that a windowless VUV spectrometer has been
designed for this measurements and is schematically
shown in Fig. 2.
Fig. 1. Scheme of the µ-APPJ remote plasma source,
where in the Helium/O 2 plasma generated species are
transported to the substrate by the gas flow.
Fig. 2. Scheme of the windowless VUV spectrograph.
O-17-1
1
[8] in surface wave tubular discharge at atmospheric
pressure operated with Ne and Ar gases.
He/Ar
0.05
Intensity [a.u.]
The principle of coupling the plasma emitted VUV and
UV photons into the He-filled VUV spectrograph without
necessity of having any window between the plasma and
the spectrograph function as follows: all the potentially
absorbing species in the plasma (such as molecules
introduced into the plasma as precursor gas) are steered
by the additional helium flow into the side and cannot
enter the spectrograph. Additionally, controlled
atmosphere around the jet is used to prevent air to diffuse
into the light path. Fig. 3 shows the measured spectrum
of the µ-APPJ operated with helium gas in the 50 to 140
nm wavelength range. Next to the few atomic oxygen
lines, resulting from the excitation of the gas impurities,
the clear emission of the 1st and 2nd helium excimer
continuum is visible.
Ar
0.04
He/O2
He/H2
0.03
0.02
He
0.01
0.00
50
60
70
80
90 100 110 120 130 140 150 160 170 180
Wavelength [nm]
0.025
continuum
Intensity [a.u.]
He plasma
1st He*2 exciner
0.020
Fig. 4. Exemplary VUV and UV spectra of µ-APPJ with
He, Ar, He/0.07%O 2 , He/0.14%H 2 and He/0.14%Ar gas
mixtures.
*
2
nd
2 He exciner
2nd order of the
1st He*2 exciner
continuum
continuum
O
0.015
O
0.010
O
0.005
0.000
50
60
70
80
90
100
110
120
130
140
Wavelength [nm]
Fig. 3. VUV spectrum of the µ-APPJ operated in the 5.0
grade helium gas.
These photons can propagate through the helium gas
without being absorbed, since the first photon absorption
by ground state helium is possible to the (1s)(2p) level
corresponding to the wavelength of 58.44 nm only. The
spectrum in Fig. 2 is very similar to the spectrum reported
for a micro-hollow cathode discharge in helium [7].
The windowless VUV spectrograph have been used to
record emission spectra of several gases admixed at
concentrations below 1% into the helium in the µ -APPJ.
Three examples of the plasma with O 2 , N 2 and Ar are
compared to the spectrum of helium gas only from Fig. 3
in Fig. 4.
It should be noted that the spectrometer grating is
optimized for 130 nm and its efficiency decreases both at
shorter and longer wavelengths. Worth to mention is
especially the spectrum of the He/0.14%Ar mixture,
where the He excimer emission is fully replaced by
atomic lines of argon. The plasma is maintained by argon
ion atoms, since it has much lower ionization and
excitation energy than He, without formation of Ar 2 + ions
and argon excimers. This mechanism was nicely
demonstrated and explained by Castaños-Martínez et al.
2
The VUV spectra, and their dependence on the variety
of plasma parameters, can be used as an additional
information for the analysis of the plasma-chemical
processes in APPs. Special feature of this high energy
VUV photons is that they can photo-ionize or dissociate
species at larger distances from the jet, as was already
suggested in the past [9]. These in photochemistry
generated species should be considered for example in the
treatment of biologically relevant substrates. We have
reported recently the possible effect of protonated water
cluster ions, which formation was initialized through
photoionization, on bacteria [6].
3. Summary
Atmospheric pressure plasmas operated with noble
gases can generate vacuum UV photons, which are then
transported through the noble gas to substantial distances.
The windowless vacuum UV emission spectroscopy was
introduced and applied to atmospheric pressure plasmas
measuring photons down to the wavelength of 58 nm.
These measurements provide useful information about
possible reaction mechanisms in the discharge (excimers
or atomic species), about excitation processes in the
plasma and about the presence of atomic and molecular
species. The parameter dependencies for helium and
argon containing plasmas with small admixture of
molecular gases will be presented and compared to
available mass spectrometry data.
4. Acknowledgments
This work has been funded by the German Research
Foundation (DFG), project package
PAK 728
„PlasmaDecon“ and research unit FOR 1123 „Physics of
Microplasmas“, and by 7FP Marie Curie Initial Training
Network RAPID „Reactive Atmospheric Plasma
Processing–Education Network“.
O-17-1
5. References
[1] Murakami T, Niemi K, Gans T, O’Connell D,
Graham W G, 2013 Plasma Sources Sci. Technol. 22
015003
[2] Benedikt J, Hecimovic A, Ellerweg D, von Keudell
A 2012 J. Phys. D: Appl. Phys. 45 403001
[3] Benedikt J, Ellerweg D, von Keudell A 2009 Rev.
Sci. Instrum. 80 055107
[4] Ellerweg D, von Keudell A and Benedikt J 2012
Plasma Sources Sci. Technol. 21 034019
[5] Ellerweg D, Benedikt J, Knake N, Schulz-von der
Gathen V and von Keudell A 2010 New J. Phys. 12
013021
[6] Schneider S, Lackmann J-W, Ellerweg D, Denis B,
Narberhaus F, Bandow J E, Benedikt J 2012 Plasma
Process. Polym. 9 561
[7] Kurunczi P, Lopez J, Shah H, Becker K 2001 Int. J.
Mass Spectr. 205 277
[8] Castaños-Martínez E, Moisan M, Kabouzi Y 2009 J.
Phys. D: Appl. Phys. 42 012003
[9] Reuter S, Niemi K, Schulz-von der Gathen V, Döbele
H F 2009 Plasma Sources Sci. Technol. 18 015006
O-17-1
3