OH detection by absorption of frequency-doubled diode laser radiation at 308 nm

Article abstract of DOI:10.1016/S0009-2614(00)00105-6

Radiation at 308 nm has been obtained by frequency doubling the output of a commercial diode laser cooled to 165 K. A single pass through a crystal of LiIO₃ converted 1 mW of 616 nm radiation to 50 pW of UV, and this was used to detect the OH radical in absorption in a flow tube. Possible extensions of the method for detection of OH in the atmosphere are discussed.

Full text of DOI:10.1016/S0009-2614(00)00105-6

10 March 2000
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Chemical Physics Letters 319 2000 125–130
www.elsevier.nlrlocatercplett
OH detection by absorption of frequency-doubled diode laser
radiation at 308 nm
Hugh R. Barry, Ben Bakowski, Laura Corner, Tim Freegarde, Oliver T.W. Hawkins,
)
Gus Hancock , Robert M.J. Jacobs, Robert Peverall, Grant A.D. Ritchie
Physical and Theoretical Chemistry Laboratory, Oxford UniÕersity, South Parks Road, Oxford OX1 3QZ, UK
Received 7 January 2000
Abstract
Radiation at 308 nm has been obtained by frequency doubling the output of a commercial diode laser cooled to 165 K. A
single pass through a crystal of LiIO3 converted 1 mW of 616 nm radiation to 50 pW of UV, and this was used to detect the
OH radical in absorption in a flow tube. Possible extensions of the method for detection of OH in the atmosphere are
discussed. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction
lasers however are bulky and expensive, and the lack
of a suitable alternative 308 nm source has meant
that only a small number of atmospheric OH detec-
tion systems operate world wide.
Absorption strategies involving the injection of
narrow band radiation into resonant cavities have
been shown to yield ultrasensitive detection limits. In
1996 Hall showed that when absorption in a 60 cm
long resonant cavity was measured by frequency
Detection of the OH radical is of crucial impor-
tance in both fundamental and applied gas-phase
studies. The highly reactive species plays an essen-
tial role in both atmospheric 1 and combustion 2
chemistry, and it has become one of the most com-
monly observed products of quantum state resolved
experiments designed to measure detailed reaction
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w x
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dynamics 3 . Observation normally relies on either
Ž
.
modulation FM techniques, a fractional absorption
Ž
w x
.
absorption or laser-induced fluorescence LIF on the
of 1 part in 1010 could be detected in 1 s with unity
2
2
q
A S –X P transition in the near UV 4 , a region
where frequency-doubled tunable dye lasers can be
operated efficiently in both pulsed and cw modes.
Both types of dye laser have been used near 308 nm
in the important and demanding application of atmo-
spheric detection of OH at concentrations of the
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x
SrN ratio 12 and this limit has recently been
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x
extended by a factor of 50 13 . Increased effective
pathlength within the cavity is the key to the in-
creased sensitivity, in a similar but improved way to
that gained by cavity ring-down spectroscopy, al-
ready applied with pulsed lasers to OH detection
order of 106 cmy3 in the troposphere 5–11 . Such
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x
10
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x
14–18 . For tropospheric OH the 1 in 10 criterion
would translate to measurement of a concentration of
104 cmy3 in a 60 cm path, and thus in principle the
technique would offer sufficient sensitivity for it to
)
Corresponding author. Fax: q44-1865-275410; e-mail:
gus.hancock@chemistry.ox.ac.uk
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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.
PII: S0009-2614 00 00105-6

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126
H.R. Barry et al.rChemical Physics Letters 319 2000 125–130
be used in atmospheric observations. A simple and
compact cw UV source which could be frequency
modulated and injected into a cavity would be an
advantage for such applications. Here we describe
the production of coherent 308 nm radiation by
frequency doubling the output of a cooled semicon-
ductor diode laser, and its use in the detection of OH
by single-pass absorption in a flow tube.
single pass through an 8 mm long crystal of LiIO3,
separated from the fundamental by reflection from
dichroic mirrors, and passed into a flow cell of
length 1 m. The radiation was chopped at 2 kHz, and
the signals before and after the flow tube measured
with photomultipliers and processed with lock-in
detection. OH radicals were produced from either the
Ž
reaction of H atoms from a microwave discharge in
.
H2rAr with NO2 , or from a discharge in a HerH2O
mixture, both at total pressures between 2 and 10
Torr. Fig. 1 shows the experimental arrangement. At
a given temperature, ramping the diode laser current
allowed its fundamental output wavelength to be
varied by typically 0.04 nm before the laser hopped
to a different longitudinal mode, causing a jump in
wavelength. The output was monitored with both a
2. Experimental
The short-wavelength limit of commercial diode
lasers operating at room temperature with the excep-
tion of the recently available GaN lasers around 400
Ž
w
x.
Ž
nm 19 is near 635 nm for an AlGaInP diode
.
Ž .
structure . Cooling a diode laser leads to an increase
spectrum analyser TecOptics V4523 and a
Ž
.
in the band gap and thus operation at a shorter
wavemeter Burleigh WA1000 to ensure mode hops
were absent and to determine the relationship be-
tween laser current and wavelength.
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x
Ž
wavelength 20 . A 635 nm laser Hitachi HL 6314
MG, output 5 mW at room temperature was re-
.
moved from its hermetically sealed can and mounted
on a brass block equipped with a resistance heater
and platinum resistance thermometer. The block was
separated from a liquid N2 filled Dewar by a Delrin
spacer, and variation of the power to the heater
resulted in a temperature which could be con-
trolled with a precision of "1 mK over the range
290–77 K. The output was frequency doubled by a
3. Results
Fig. 2 shows the variation of diode laser’s output
wavelength with temperature. The expected decrease
of wavelength with temperature is observed, with
mode hops clearly apparent, and it can be seen that
Fig. 1. Experimental arrangement for production of 308 nm radiation and detection of OH in a flow system. PMTsphotomultiplier tube;
DCsdichroic reflector, BSsbeamsplitter.

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H.R. Barry et al.rChemical Physics Letters 319 2000 125–130
127
Fig. 2. Temperature tuning curve of the diode laser. The current was varied in this experiment from 55 mA at 290 K to 12 mA at the lowest
temperature studied, 153 K. Mode hops can be seen as jumps in the wavelength at specific temperatures, and these restricted the tuning
range of the laser as explained in the text.
the required wavelength of 616 nm is reached at a
temperature of 165 K. The laser current threshold
showed a monotonic decrease with decreasing tem-
perature, attributable to a reduction in the leakage of
thermally excited electrons out of the heterojunction
nm, a conversion efficiency about an order of magni-
tude lower than that calculated for an optimally
focused Gaussian beam in the crystal. Our UV re-
flection loss at the crystal output face was estimated
to be 30%, and the remaining difference between
measured and optimal second-harmonic generation
efficiencies may be caused by a combination of
absorption losses and poor beam quality. Signal lev-
els however were ample for OH detection. The
radical was identified spectroscopically by absorp-
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x
region as the temperature is lowered 21,22 . Values
ranged from 20 mA at 290 K to 9.5 mA at 153 K. At
Ž
616 nm the laser slope efficiency d Poutrd Pin, where
Pout is the laser output and Pin the electrical power
.
applied was linear over the output range 0–1 mW at
Ž .
Ž .
Ž .
a value of 0.24. A maximum output value of 2.5 mW
was achieved. Although a continuous wavelength
tuning range near 616 nm could not be obtained
tion on the P11 1 , Q11
3
and P21
3
lines of the
2
2
q
Ž
.
A S –X P 0, 0 band. Here the spectroscopic no-
Ž Ž ..
Ž .
Ž .
tation is D JFŽu.FŽl. N l , where F u and F l give
Ž
because of mode hops the laser was operated with-
the relation between the angular momentum quantum
.
Ž
out grating feedback control , the available range
numbers J and N F1 for JsNq1r2, F2 for
.
Ž .
Ž .
was sufficient for the observation of isolated OH
features following frequency doubling.
Several materials have been evaluated for fre-
quency-doubling diode laser radiation near 635 nm,
and although LiIO3 appears to have the highest
JsNy1r2 for upper u and lower l levels. Fig.
3a shows the photomultiplier signals before and after
absorption with a single scan of the diode laser
output between 308.153 and 308.164 nm, produced
by a 2.3 mA linear increase in current applied to the
diode laser over a period of 20 s, with the output
passed through the doubling crystal but without si-
multaneous angle tuning. Marked on the figure are
the positions expected for absorption at line centre of
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x
second-harmonic conversion efficiency 23 , its use
in this wavelength region has been questioned be-
cause of its UV absorption between 300 and 350 nm
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x
caused by colour centre impurities 23,24 . In the
present experiments an output power of 1 mW at 616
nm yielded ;50 pW of doubled radiation at 308
Ž .
3
Ž .
the Q11
transition and its P21 3 satellite, and,
when the data were corrected for background varia-

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128
H.R. Barry et al.rChemical Physics Letters 319 2000 125–130
Ž .
Fig. 3. Absorption spectrum of the OH radical near 308.16 nm. The upper trace, a , shows a single scan of the diode laser producing
Ž
.
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.
frequency doubled output between 308.153 and 308.164 nm. Traces before dashed line and after solid line passing through the flow tube
Ž . Ž . Ž .
show absorption features identified as the Q11
3
and P21 transitions in OH. The lower trace, b , shows the background correc-
3
Ž .
ted absorption on the P11 1 feature of OH at 308.1665 nm, with the solid line a simulation for absorption by a room temperature sample at
10 Torr total pressure.
tion the peak heights were in the ratio expected from
the relative line strengths of the two transitions from
the F1 Ns3 lower level. Fig. 3b shows an average
of four scans at a slightly higher wavelength indicat-
nm, with the solid line showing a simulation of the
Voigt lineshape expected for a 300 K sample at a
w
x
total pressure of 10 Torr He 25 . From the absorp-
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x
tion cross-section data 25 the product of total OH
concentration and path length is calculated to be
Ž .
ing absorption on the P11 1 transition at 308.1665

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H.R. Barry et al.rChemical Physics Letters 319 2000 125–130
129
2=1014 cmy2 , from which we estimate an OH
concentration averaged along the flow tube of 2=
1012 cmy3. The major source of noise on the data
shown in Fig. 3 originates from the microwave
discharge. Scattered photons were two orders of
magnitude higher in intensity than the 308 nm radia-
tion formed from the laser, and electrical noise con-
tributed to the varying baseline.
absorption at 308 nm should be adequate for the
observation of tropospheric OH, there are major
Ž
problems of overlapping absorptions from SO2 ,
.
w x
H2CO and naphthalene near the OH transitions 8 .
Furthermore, the enhanced 308 nm radiation levels
in a resonant cavity can lead to OH formation through
Ž1
.
reactions of O D produced from the photolysis of
w x
ozone 8 . If these problems can be overcome, the
use of this relatively straightforward way of generat-
ing 308 nm radiation from a compact laser system
should lead to the development of a light, portable
easily electrically controlled OH monitor for atmo-
spheric applications. Absorption methods have al-
ready been used in measurements of the nascent
lineshapes of products formed by photodissociation
4. Discussion
Although diode lasers have been used in the past
Ž
to produce 308 nm radiation by mixing the 835 nm
output of a diode laser with 488 nm Arq radiation in
w
x
w x
33 and chemical reactions 34 , and we anticipate
w
x.
a crystal of BBO 26 , we are not aware of previous
reports of formation of this wavelength by straight-
forward frequency doubling. For operation as a sen-
sitive detection scheme in absorption we note that an
output larger than the present 50 pW level is needed,
but this will be able to be attained in a resonant
doubling cavity such as has been extensively used
with LiIO3 as the non-linear medium at longer wave-
that a 308 nm source will aid in the extension of
these studies to the OH radical.
5. Conclusions
Radiation at 308 nm has been produced from
frequency doubling the output of a commercial diode
laser cooled to 165 K, and has been used to detect
the OH radical in absorption. The method of generat-
ing coherent light at this wavelength from a simple
and compact device may find use in a detection
method for the radical in the atmosphere.
w
x
lengths 27–30 . Doubling efficiencies depend upon
both wavelength and input powers, and we give
examples of generation of 44 mW at 410 nm from
w
x
600 mW of Ti:sapphire radiation 27 and 18 mW at
w
x
370 nm from 10 mW of diode laser radiation 28 .
Although we have used a commercial diode laser in
these experiments, we are aware of research devices
which operate at or close to room temperature at the
required wavelength 22,31,32 , albeit with relatively
high threshold currents. Furthermore, with the recent
availability of diode lasers near 400 nm 19 , genera-
tion of 308 nm radiation by mixing the outputs of
two diode lasers in a non-linear crystal becomes a
possibility. For FM detection within a resonant cav-
ity we shall need to produce modulation in the UV.
FM of the diode laser fundamental output can be
carried out directly by current modulation, and this
can be efficiently extended into the frequency-dou-
bled region by modulation such that both the carrier
and sidebands are resonant with the doubling cavity:
we have demonstrated this by modulating a 680 nm
diode laser at 1.3 GHz, the free spectral range of the
doubling cavity containing the LiIO3 crystal. We
note that although the sensitivity of resonant cavity
Acknowledgements
w
x
We are grateful to the NERC and EPSRC who
have supported this work under the URGENT and
Analytical Chemistry Initiatives.
w
x
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