EGAS41 - Swansea University

41 st EGAS CP 93 Gdańsk 2009
Detection of fluorine with cavity ring-down spectroscopy
T. Stacewicz 1,∗ , E. Bulska 2 , A. Ruszczyska 2
1 Institute of Experimental Physics, University of Warsaw, ul. Hoza 69, 00-681 Warsaw,
Poland
2 Chemistry Department, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland
∗ Corresponding author: tadstac@fuw.edu.pl
Fluorine compounds play very important role for living organism and are of great importance
for agriculture and industry. However a quick development of these activities leads
to excessive emission of this element and its deposition to soil, water and atmosphere.
There is a small difference among doses which are desirable for living organism and which
are harmful. Therefore there is a need of trace determination of this element. Good
opportunities are provided by contemporary laser spectroscopy techniques.
Our approach to the trace detection of fluorine consists in atomic spectroscopy in
plasma which is doped by the investigated matter. The plasma dissociates the matter
and produces free atoms. Due to collisions with electrons some atoms are excited to
metastable levels which are detected using the absorption spectroscopy at 685.603 nm
line corresponding to 3s 4 P 5/2 − 3p 4 D o 7/2
transition. High sensitivities might be expected
due large oscillator strength of this transition (0.36) and due to application one of the
most efficient method of absorption measurement – Cavity Ring Down Spectroscopy.
The experiment was performed in a tube ended with two mirrors of high reflectivity.
Time of the radiation imprisonment in such optical resonator reached about 1.5 µs. In
central part of the tube the RF cavity was installed. It was supplied by 150 W generator
providing the plasma column of 20 cm.
The investigated mixture was composed of helium at a pressure of several Torr with
addition of evaporated XeF 2 or SF 6 . The cavity was illuminated with of single mode
radiation from cw diode laser working at 685.603 nm. We used off-axis illumination of
the resonator. Due to that the beam is reflected by the mirrors in this way that the
light spots do not overlap. It provides opportunity to avoid sharp resonances that usually
occurs in the optical cavity in the case of on-axis illumination. The laser beam was
amplitude modulated by AOM with frequency of 50 KHz.
The light leaving the cavity through the front mirror was registered by the photomultiplier
protected by the interference filter against noises occurring due to plasma luminescence.
Digital oscilloscope and lock – in amplifier measured the radiation decay time and
phase shift of the signal which occur due to light imprisonment. Using them the cavity
Q-factor and the atomic concentration of absorbing fluorine were determined. We stated
that such system provides opportunity to detect fluorine at the level of about 100 µg/kg.
Acknowledgment
This work was partially supported by Polish Ministry of Science and Higher Education, research
grant N202 085 31/0548.
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