The measurement of gain on the 1.315 μm transition of atomic iodine in a subsonic flow of chemically generated NCl(a 1Δ )

Article abstract of DOI:10.1016/S0009-2614(98)01292-5

Gain is measured on the electronic I(²P_3/2)-I^*(²P_1/2) transition of atomic iodine at 1.315 μm when hydrazoic acid HN₃ is injected into a flow of iodine and chlorine atoms. The inversion was generated in a transverse subsonic flow device that produced electronically excited I^*(²P_1/2) atoms from the efficient energy transfer reaction between NCl(a ¹Δ) metastable and ground state I(²P_3/2) atoms. The population inversion was directly observed using a 1.315 μm tunable diode laser that scanned the entire line shape of the (3,4) hyperfine transition of iodine.

Full text of DOI:10.1016/S0009-2614(98)01292-5

2
2 January 1999
Chemical Physics Letters 299 Ž1999. 583–588
The measurement of gain on the 1.315 mm transition of atomic
1
ž
/
iodine in a subsonic flow of chemically generated NCl a D
)
John M. Herbelin, Thomas L. Henshaw , Brent D. Rafferty, Brian T. Anderson,
Ralph F. Tate, Timothy J. Madden, Gerald C. Manke II, Gordon D. Hager
Air Force Research Laboratory, Kirtland AFB, 3550 Aberdeen AÕenue SE, Kirtland AFB, NM 87117-5776, USA
Received 25 September 1998; in final form 10 November 1998
Abstract
Gain is measured on the electronic IŽ P .–I Ž P . transition of atomic iodine at 1.315 mm when hydrazoic acid HN3
2
) 2
3
r2
1r2
is injected into a flow of iodine and chlorine atoms. The inversion was generated in a transverse subsonic flow device that
)
2
1
produced electronically excited I Ž P . atoms from the efficient energy transfer reaction between NClŽa D. metastable
1
r2
2
and ground state IŽ P . atoms. The population inversion was directly observed using a 1.315 mm tunable diode laser that
scanned the entire line shape of the Ž3,4. hyperfine transition of iodine. q 1999 Elsevier Science B.V. All rights reserved.
3
r2
1
. Introduction
efficient transfer of electronic energy between
NClŽa D. and atomic iodine,
1
Metastable atoms and diatomic molecules have
1
2
3
qI) P1r2
2
NCl
Ž
a D
.
qI P
™NCl
Ž
X S
.
/,
Ž .
long been recognized for their importance as energy
carriers in laser media. For example, in the near-in-
ž
3r2
/
ž
1
y11
3
frared COIL chemical laser system w1,2x the O Ža D.
k300 s2=10
cm rs
1
2
)
2
2
metastable pumps the I Ž P .™IŽ P . lasing
1
r2
3r2
increased attention has been directed toward the
demonstration of gain and or lasing on the 1.315 mm
transition of the atomic iodine excited by NClŽa D..
In 1992, evidence of a population inversion in a
transition at 1.315 mm through the resonant energy
1
transfer reaction between O Ža D. and ground state
2
1
2
IŽ P .. Similarly, chemically generated nitrene
3
r2
1
1
metastables such as NClŽa D. and NFŽa D. have
subsonic flow using ICl as a source of iodine w8x was
been under investigation for a number of years as an
energy source to power an electronic transition
chemical laser w3–5x. Following the discovery of the
presented and shortly thereafter near threshold laser
oscillation was observed from the photolysis Ž193
nm. of mixtures containing ClN3 and CH2 I2 w9x.
Unfortunately, due to the near threshold conditions
involved in both of these experiments, the results
could not be quantified and it was felt that a direct
measurement of gain would be the logical next step
in the investigation of this system. In this regard, we
)
Corresponding author. Fax: q1 505 846 4807; E-mail:
henshawt@plk.af.mil
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž98.01292-5

5
84
J.M. Herbelin et al.rChemical Physics Letters 299 (1999) 583–588
present a sensitive and direct measurement of the
ŽPAR, model 1412. mounted on a 1r4 m monochro-
mator ŽJarrell–Ash., a 1r3 m McPherson Spectrom-
eter equipped with an intrinsic Ge detector ŽADC,
model 403L. and the diode laser. Near-infrared emis-
2
population inversion between the IŽ P . and
3
r2
)
2
I Ž P . states of atomic iodine produced by energy
1
r2
1
transfer from NClŽa D..
1
3
1
3
sions, NClŽa D–X S. at 1.08 mm, NFŽa D–X S.
2
2
at 0.874 mm and IŽ P – P . at 1.315 mm, were
1
r2
3r2
2
. Experimental
measured via the 1r3 m McPherson-Ge detector
1
The gain determination reported here was made in
system. Emission stemming from the NClŽb S–
3
a transverse subsonic flow reactor equipped with
sensitive optical diagnostics for measuring chemilu-
minescent emissions and ground state iodine atom
absorption. The approach in this study follows the
general methodology used by earlier investigators
w6,8,10x with a few notable exceptions. By far the
most important is the use of a tunable diode laser
probe system. The device is a continuously tunable
single-mode 1.315 mm laser ŽNew Focus model
X S. transition was dispersed and detected with the
1r4 m monochromatorrOMA arrangement. All
emissions were absolutely calibrated using a black
body source.
The transverse flow facility, shown schematically
in Fig. 2, was designed to allow sequenced injection
of the reactants. Molecular fluorine F Ž20% in He.
2
was passed through a D.C discharge under condi-
tions that dissociated )95% of the F . The conver-
2
6
248. that provides single line lasing with a spectral
sion efficiency was determined previously by a titra-
line width ŽFWHM. of -500 kHz over 30 GHz
tion technique using excess hydrogen iodide w12x.
y1
from 7602.4 to 7603.5 cm . With this apparatus it
Near unit conversion was required in order to avoid
2
)
2
)
2
was possible to scan across the IŽ P .–I Ž P
.
formation of IF, an effective quencher of I Ž P .,
3
r2
1r2
1r2
transition with complete hyperfine resolution and
obtain excellent signal to noise ratios at extremely
small signal levels w11x. The experimental layout for
the system diagnostics is presented in Fig. 1 and
includes an Optical Multi-channel Analyzer, OMA,
that was observed when F was present in the flow
2
stream. It is important to note that at F flow rates
2
above 1.0 mmolrs a significant decline in the degree
of F dissociation occurs thus limiting our F density
2
to conditions described here.
Fig. 1. Layout of the optical diagnostics including an Ž1. Optical Multi-channel Analyzer, OMA, on a 1r4 m Jarrell-Ash Spectrometer, Ž2.
Intrinsic Ge detector mounted on a 1r3 m McPherson spectrometer and Ž3. the scanning diode laser with appropriate monitoring apparatus.
M1–M7, mirrors; BS, beam splitter.

J.M. Herbelin et al.rChemical Physics Letters 299 (1999) 583–588
585
Upon exiting the discharge region, excess deu-
terium chloride, DCl, was injected along with helium
via an axial injector to produce the desired Cl atoms
w13x:
FqDCl™DFqCl, k300 s1=10y11 cm rs .
3
Ž .
2
A sufficient distance of about 10 cm, which trans-
lates to approximately 0.26 ms, was provided to
allow this reaction to run to completion. Through the
first set of transverse wall injectors we introduced
hydrogen iodide, HI, along with a carrier flow of
helium to ensure proper penetration. This produced a
flow of ground state iodine atoms via the reaction
w14x,
2
ClqHI™HClqI P
3r2 /,
ž
y10
3
k300 s1.0=10
cm rs .
3
Ž .
The presence of ground state iodine atoms was read-
ily monitored with the diode laser probe. A typical
absorption spectrum of the Ž3,4. transition is shown
Fig. 2. Schematic representation of the flow device showing Ž1. dc
discharge, Ž2. axial injector ŽDCl., Ž3. first set of transverse wall
injectors ŽHI., Ž4. second set of transverse wall injectors ŽHN3.
and Ž5. flow confinement shrouds.
by the negative curve in Fig. 3.
2
2
Fig. 3. The observed Ž1. absorption and Ž2. gain line shape profiles for the Ž3,4. transition of the IŽ P ™ P . transition at 1.315 mm.
1
r2
3r2
Flow conditions for Ž1. absorption: 16 Torr total pressure, 0.7 mmolrs F , 2.0 mmolrs DCl, 0.03 mmolrs HI, 0 mmolrs HN Ž2. gain: 16
2
3;
Torr total pressure, 0.7 mmolrs F , 2.0 mmolrs DCl, 0.03 mmolrs HI, 3.3 mmolrs HN . Note that the frequency axis is expressed as a
2
3
relative frequency obtained by mapping the absorption spectrum to the 300 MHz spacing of the transmission peaks of the Fabry–Perot
interferometer.

5
86
J.M. Herbelin et al.rChemical Physics Letters 299 (1999) 583–588
When a diluted mixture of HN was introduced
positive direction as shown in Fig. 3. The magnitude
3
via the second set of transverse wall injectors,
NClŽa D. was produced according to the reaction,
of the peak gain, 0.02%rcm, provides an estimate of
1
the fraction of the ground state iodine atoms con-
)
2
verted to the excited I Ž P . state. Accordingly,
ClqHN ™HClqN ,
1r2
3
3
using the following equation for gain,
k300 s1=10y12 cm rs
3
4a
Ž
.
.
gss I) P1r2 –1r2 I P
2
2
5
Ž .
½
ž /
ž
3r2
/
5
1
1
ClqN ™NCl
Ž
a D
.
qN2
Ž
X S ,
.
3
where s is the temperature corrected cross-section
k300 s2=10y11 cm rs
3
4b
y16
2
Ž
for stimulated emission, 1.3=10 r6T in cm ,
and noting that the total iodine concentration in the
system is constant,
which is believed to be around 75% efficient ŽEq.
Ž4a. w15x; Eq. Ž4b. w16x.. The reported rate coeffi-
cients here are for room temperature and as such, it
)
2
2
w x
I s I
P1r2 q I P
/ ž 3r2
Ž .
6
T
ž
/
1
is unlikely that sufficient NClŽa D. would be gener-
)
2
quantitative equations relating the I
P1r2
and
ated from the ClqHN reaction. However, mea-
ž /
3
2
sured reactor flow temperatures under these condi-
tions approach 475 K and a significant enhancement
in the magnitude of this rate constant is expected.
Indeed, preliminary data from temperature dependent
rate measurements of reaction 4a and model calcula-
tions suggest a 6–7-fold increase in the rate coeffi-
cient is observed at this temperature w17x.
I P
ž
density under gain conditions can be
3r2
/
generated 18 . Solving Eqs.
w
x
Ž .
5
and 6 for
Ž .
I) P1r2 and I P
2
2
yields
ž /
ž
3r2
/
2
I P
s2r3wIx y2r3
Ž
grs
.
Ž
Ž
7
8
.
.
ž
3r2
/
T
)
2
I
P
s1r3wIx q2r3
Ž
grs
.
.
ž
1r2
/
T
1
3
y3
Hydrogen azide, HN , was synthesized by the
For an wIx s6.0=10 cm , measured prior to
3
T
reaction of molten stearic acid ŽCH ŽCH . COOH.
introducing HN to the flow system, and the peak
3
2
16
3
)
with sodium azide ŽNaN . under vacuum at 1108C.
gain from Fig. 3, the wI x and wIx densities are
3
1
3
13
y3
A 10:1 molar excess of stearic acid over sodium
4.0=10 and 2.0=10 cm , respectively. Thus
2r3 of the available ground state iodine atoms are
azide was used. The gaseous HN was stored as a
3
)
2
5
% or 10% mixture in helium. Helium diluent
converted to the I Ž P . state, a result that con-
1r2
1
ŽMatheson, 99.995%., F ŽMatheson, 20% in helium.,
DCl ŽCambridge Isotopes, 99%., HI ŽMatheson, 99%.
firms previous measurements that the NClŽa D.q
2
2
IŽ P . reaction system is an efficient process for
3
r2
)
2
were used without further purification.
generating I Ž P . w6,7,16x.
1
r2
The maximum small signal gain Ž g . is ob-
max
2
tained when all the iodine is converted to the P
1
r2
3
. Results and discussion
state. Under these conditions Eq. Ž5. reduces to
gmax ss wIx and the maximum gain expected is
T
Under the flow conditions provided in Table 1,
the previous absorption curve was reversed to the
0.036%rcm. However, scaling the system to higher
flow rates failed to generate total inversion of ground
Table 1
2
)
2
Typical flow reactor conditions for the observed population inversion stemming from the IŽ P .–I Ž P . transition
3
r2
1r2
Reagent species Molar rate Flow Reactor pressure Reagent density Cross-sectional area Flow velocity Flow temperature
y3
2
Žmmolrs.
ŽTorr.
Žcm
.
Žcm .
Žcmrs.
ŽK.
3.0=101
1.3=10
4.0=10
6.6=10
6.3=10
3.1=10
7
5
5
5
3
7
10
3.0=10
4
470
He
F2
DCl
HN3
HI
151.28
0.66
2.00
3.32
0.032
157.29
15.50
1
1
1
1
1
Total

J.M. Herbelin et al.rChemical Physics Letters 299 (1999) 583–588
587
2
)
2
1
2
)
2
3
Fig. 4. The IŽ P .yI Ž P . gain zone resulting from the NClŽa D.qIŽ P .™I Ž P .qNClŽ X S. reaction. The gain profile
3
r2
1r2
3r2
1r2
extends over a reasonably long flow time of nearly 0.5 ms.
state iodine atoms. It is not known at this time if the
and acted as a most convenient indicator of when the
flows were properly optimized for maximum gain. If
the DCl flow was turned off, then the visual emis-
sion would change from bright red–orange to bright
green corresponding to 530 nm, NFŽb S –X S .
emissions in the reaction zone. This emission was
also monitored utilizing the Optical Multi-channel
Analyzer, OMA, and served as an excellent method
for determining when sufficient DCl had been added
to assure complete conversion of the F atoms to Cl
atoms.
)
2
system is limited by the I Ž P . branching fraction
1
r2
or other loss mechanisms. One system limitation is
traceable to the dc discharge efficiency and its fail-
ure to generate large F atom densities without incur-
1
q
3
y
)
2
1
ring significant I Ž P . andror NClŽa D. removal.
1
r2
Fig. 4 presents a zone profile of gain under similar
conditions described in Table 1. The gain was found
to extend well over the field of view from the
terminus of the flow confinement shroud at 0.17 ms
Ž5 cm. to beyond 0.53 ms Ž16 cm. where the flow
begins to break up in this device or possible colli-
sional deactivation of the energy carriers occur. This
long zone length is not surprising since the flow
conditions were chosen to minimize deactivation.
These observations were very reproducible over a
range of conditions.
4. Conclusions
A direct measurement of the population inversion
Under these same flow conditions, the emission
2
)
2
between the IŽ P .yI Ž P . states of iodine
1
3r2
1r2
2
from the excited NClŽb S. state was enhanced, just
1
resulting from the NClŽa D.qIŽ P . reaction has
3
r2
as observed by earlier investigators w19x. This indi-
1
been observed. This represents a significant step in
the development of an all gas phase iodine laser. The
observed gains were substantially lower Žapproxi-
mately a factor of 50. than that of the familiar
chemical oxygen iodine laser ŽCOIL.. To obtain a
cated that the primary source of the NClŽb S. was
via the energy transfer process,
1
qI) P1r2
2
1
2
NCl
Ž
a D
.
/™NClŽ
b S
.
qI P
3r2 /.
ž
ž
Ž .
9
meaningful laser demonstration a significant en-
hancement in the small signal gain is essential. The
current experimental result is limited by number
1
q
3
y
The NClŽb S –X S . emission at 665 nm was
readily observed visually as well as with the OMA

5
88
J.M. Herbelin et al.rChemical Physics Letters 299 (1999) 583–588
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1
) 2
ing whether or not the NClŽa D.rI Ž P . system
1
r2
will scale to appropriate levels for a successful laser
demonstration. If these scaling experiments are
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