INSTITUTE OF PHYSICS PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGY
Plasma Sources Sci. Technol. 13 (2004) 237–244 PII: S0963-0252(04)76086-7
An ambient air RF low-pressure pulsed
discharge as an OH source for LIF
calibration
G Dilecce, P F Ambrico and S De Benedictis
Istituto di Metodologie Inorganiche e Plasmi, Sez. di Bari, CNR c/o Dipartimento di Chimica
Universit` a di Bari, via Orabona 4, 70126, Bari, Italy
E-mail: g.dilecce@area.ba.cnr.it
Received 7 July 2003
Published 12 March 2004
Online at stacks.iop.org/PSST/13/237 (DOI: 10.1088/0963-0252/13/2/007)
Abstract
A reference OH source for on-line, real-time calibration of OH laser-induced
fluorescence (LIF) measurements, based on a pulsed N
2
–H
2
O RF discharge,
is described. Its operation principles are demonstrated together with its
stability with respect to variations of discharge conditions and laser energy.
Such stability is the basis for operation of the source under less controlled
conditions. The correctness of the working principle is then demonstrated
with a simple ambient air gas feed that greatly simplifies the source
apparatus. This is an important requisite for operation in field instruments.
The absolute OH density can be recovered with an error of about ±20%.
1. Introduction
In this paper, we present a low-pressure, low-power pulsed RF
discharge operating at an ambient air gas feed, which can be
used as a reference source for on-line, real-time calibration of
OH laser-induced fluorescence (LIF) measurements. Such an
OH source can obviously be used every time a calibration
of OH LIF is required, but its elective and probably more
interesting possible use is as an on-line calibration device
in field instruments for OH measurement in the troposphere.
The detection of OH is in fact a difficult issue in atmospheric
research [1], due to the low OH concentration found in the
troposphere (1 × 10
6
cm
−3
[2–4]). LIF has been employed to
this end [5,6] in an arrangement called FAGE (fluorescence
assay by gas expansion), in which the atmospheric pressure
sample is expanded through a nozzle down to a pressure of
the order of 1Torr, and both LIF excitation and detection are
made on the (0, 0) band of the OH(A–X) transition. This
experimental arrangement suffers from the not easily solvable
problem of its calibration and, in particular, of the availability
of a simple and robust calibration method available in field
and airborne instruments. We propose our calibrated source
as a step towards the realization of ‘inexpensive, light-weight
instruments to enable routine OH measurements to be made
continuously at many locations’ [1].
In particular, we present the operation principle of the
source and its implementation by a laboratory prototype, in
which the LIF scheme for the simultaneous detection of OH
and NO described in [7] is a fundamental step. We then
discuss some tests demonstrating how well the source works
and, finally, give some guidelines for the construction of a
compact version of this device.
2. Principle of operation
The ‘source’ discharge we have investigated is a pulsed,
low-pressure (few tenths of Torr), low-power (around 1W
average power) RF discharge. The operation principle
is based on the observation that in the discharge under
investigation the OH(A
2
+
) and NO(A
2
+
) states have the
same excitation precursor, N
2
(A
3
+
u
), through the energy
transfer reactions [8]:
N
2
(A
3
+
u
) + OH(X
2
) → N
2
(X
1
+
) + OH(A
2
+
);
rate const = k
OH
(1)
N
2
(A
3
+
u
) + NO(X
2
) → N
2
(X
1
+
) + NO(A
2
+
);
rate const = k
NO
(2)
Therefore the OH density can be quantitatively correlated to
the NO density through the intensity ratio of OH(A
2
+
–X
2
)
and NO(A
2
+
–X
2
) emissions, the 3064 Å system and the
γ system, respectively. The OH density can then be related to
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