TEA CO 2 laser-induced isotopically selective dissociation of SF 6 in a cold shock wave

Article abstract of DOI:10.1016/S0009-2614(00)00520-0

When a pulsed gas dynamically cooled supersonic molecular flow interacts with solid surface a cold shock wave is formed in front of it, non-equilibrium conditions in which may be 'reverse' to those in the incident (unperturbed) flow. Isotopically selective infrared multiphoton dissociation of SF₆ in the cold shock wave was studied. Anomalously a large increase (more than one order of magnitude) of the yield of products was found, as compared with the case of excitation of SF₆ in unperturbed flow, without essential decrease of the selectivity of process.

Full text of DOI:10.1016/S0009-2614(00)00520-0

16 June 2000
Ž
.
Chemical Physics Letters 323 2000 345–350
www.elsevier.nlrlocatercplett
TEA CO2 laser-induced isotopically selective dissociation
of SF6 in a cold shock wave
b
G.N. Makarov a,), A.N. Petin
a
Institute of Spectroscopy, Russian Academy of Sciences, 142190, Troitzk, Moscow region, Russian Federation
b
Troitzk Institute of InnoÕation and Thermonuclear Research, 142190, Troitzk, Moscow region, Russian Federation
Received 10 April 2000
Abstract
When a pulsed gas dynamically cooled supersonic molecular flow interacts with solid surface a cold shock wave is
Ž
.
formed in front of it, non-equilibrium conditions in which may be ‘reverse’ to those in the incident unperturbed flow.
Isotopically selective infrared multiphoton dissociation of SF6 in the cold shock wave was studied. Anomalously a large
Ž
.
increase more than one order of magnitude of the yield of products was found, as compared with the case of excitation of
SF6 in unperturbed flow, without essential decrease of the selectivity of process. q 2000 Elsevier Science B.V. All rights
reserved.
1. Introduction
In this Letter we present the first experimental
results of studies into the isotopically selective IR
Ž
The use of gas dynamically cooled molecular jets
multiphoton dissociation of molecules using the ex-
w x
.
and flows 1 in studies of laser-induced processes
ample of SF6 in the non-equilibrium conditions of
Ž
.
including isotopically selective infrared IR multi-
the cold shock wave formed under interaction of a
pulsed gas dynamically cooled supersonic molecular
flow with solid surface. An anomalously large in-
crease )10–20 times of the yield of products was
observed compared with that in an unperturbed
w
x
photon dissociation of molecules 2–4 is well known
2,5,6 . Due to strong cooling of the gas, the absorp-
w
x
Ž .
tion bands of molecules are strongly narrowed. This
results in an essential increase of the selectivity of
excitation and dissociation of molecules. However,
in gas dynamically cooled molecular jets and flows,
the efficiency of photochemical processes is very
low. Because of low temperature and small concen-
tration of molecules in jets and flows, the rates of
chemical reactions, including those resulting in the
formation of desired products, are rather small.
Ž
.
flow without essential decrease of the selectivity of
process.
2. Experimental
The experimental set-up is schematically shown
in Fig. 1. The basic features of the pulsed molecular
beam apparatus used in the present experiments have
been previously described 6–8 . Here we will give
only a brief description.
)
w
x
Corresponding author. Fax: q7-095-334-0886; e-mail:
g.makarov@isan.troitsk.ru
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 00 00520-0

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)
346
G.N. MakaroÕ, A.N. PetinrChemical Physics Letters 323 2000 345–350
experimental conditions was in the range ;0.2–5
mm.
The molecules were excited near the surface at a
distance D xs1.5–3 mm from it. The laser beam
was directed perpendicular to the flow direction and
was focused into the interaction region by a cylindri-
cal lens with a 12 cm focal length. The axis of the
lens was parallel to the surface. The cross-section of
the laser beam in the focal region of the lens was
about 0.2=8.5 mm2.
The HF) fluorescence detection technique was
used in the experiments. The HF) fluorescence l
Ž
.
(2.5 mm accompanies the SF6 dissociation in the
presence of H2 or CH4 13 , and its intensity corre-
w
x
w
x
lates well with the dissociation yield of SF6 5,14 .
Fig. 1. Schematic view of the experimental setup. A section in the
The vibrationally excited HF) molecules are formed
xz-plane is shown. The laser beam is directed along the y-axis.
Ž
in the reaction of fluorine atoms product of SF6
)
.
dissociation with hydrogen or methane. The HF
fluorescence was measured by non-cooled IR detec-
tor based on a PbS crystal with 1=1 cm2 active
element. The yield of SF4 product and the enrich-
ment factor in it in the 34 S isotope were also mea-
sured. The enrichment factor in SF4 was determined
as:
The pulsed nozzle used in the experiments was
w x
identical to one described in Ref. 9 . The nozzle
outlet diameter was 0.75 mm and the opening time
Ž
.
of the nozzle was about 100 ms FWHM . The
pressure of the gas in the nozzle could be varied
from ca. 0.1 to 3 atm. The exit of the nozzle outlet
had a conical shape with an angle of 608 and a cone
length of 15 mm. The molecular flow was formed
with the help of two thin metallic strips mounted to
the exit cone. The vacuum chamber, in which the
prod
34
K
s
34 SF4 r 32SF4 Pß
1
Ž .
w34
x w32
x
where SF4 r SF4 is the ratio of concentrations
of 34 SF4 and 32 SF4 molecules in the product and
ßs34 Sr 32 S(0.044 is the ratio of concentrations of
34 S and 32 S in the natural SF6 mixture. The concen-
trations of 34 SF4 and 32 SF4 molecules in the product
molecular flow was formed, was pumped down to
y6
Ž
.
1–2 =10
Torr by diffusion pump. The number
of molecules flowing from the nozzle in one pulse
depended on the pressure above the nozzle, and in
the present experiments it varied from about 1016 up
to 1017 moleculesrpulse. The mean flow velocity
were measured on the IR spectra of the n6 vibration
of SF4 728 cm for 32SF4 15 taken with the IR
spectrophotometer ‘Specord 75-IR’. The isotope shift
in the n6 vibration of SF4 for 32 SF4 and 34 SF4 is
y1
Ž
w x.
w
x
was measured using the time-of-flight method 7,8 .
It was equal to 420"20 mrs.
about 12.3 cmy1 16 .
w
x
At a distance of ;50–150 mm from the nozzle,
a solid surface was placed perpendicular to the flow
3. Results and discussion
Ž
velocity direction the plates of the KBr or CaF2
.
crystals were used as a surface . As a consequence of
the interaction between the pulsed supersonic molec-
ular flow with a surface, a shock wave was formed
Fig. 2 shows the dependencies of the HF) fluo-
rescence intensity on the time delay td between the
nozzle pulse and the CO2 laser radiation pulse in the
case of excitation of SF6 in the mixture with CH4
w
x
in front of it 10–12 with essentially inhomoge-
neous, non-stationary and non-equilibrium condi-
tions. The characteristic dimension of the front of the
shock wave, which is of the order of the mean free
Ž
Ž
Ž
.
SF6rCH4 s10r1 in unperturbed molecular flow
.
curve 1 and in the flow interacting with surface
curve 2 . The total pressure of the gas above the
w
x
.
path of the molecules 10,11 , under the present

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)
G.N. MakaroÕ, A.N. PetinrChemical Physics Letters 323 2000 345–350
347
only radiation dissociation of molecules occurs,
whereas collisional dissociation of vibrationally
highly excited molecules, which usually gives an
essential rise to the total dissociation yield of
w
x
molecules 2,3 , does not take place. In the flow
interacting with a surface due to formation of a
shock wave, where the concentration of molecules
increases sharply, the collisional dissociation of
highly excited molecules coming to the shock wave
region is greatly enhanced, giving rise to the HF)
fluorescence signal.
Fig. 3 presents the dependencies of the HF)
fluorescence intensity on the CO2 laser pulse energy
fluence in the case of excitation of SF6 in a mixture
Fig. 2. The dependencies of the HF)-fluorescence intensity on the
time delay between the nozzle pulse and CO2 laser pulse in the
Ž
.
with CH4 SF6rCH4 s1r1 in unperturbed flow
Ž
case of excitation of SF6 in the mixture with CH4 SF6 rCH4 s
Ž
. Ž
curve 1 there was no surface on the way of the
flow , in the flow incident on the surface curve 2 ,
.
Ž
.
10r1 in unperturbed flow curve 1 and in the flow interacting
.
Ž
.
Ž
.
with surface curve 2 . For details see the text.
Ž
.
and in the shock wave curve 3 . The total pressure
of the gas in the nozzle was 2.4 atm. The distance
Ž
.
nozzle was 1 atm. The distance from the nozzle to
the surface was 51 mm, and D xs2.5 mm. The
excitation of molecules was carried out by the
from the nozzle to the surface for curves 2 and 3
was 51 mm, D xs2.5 mm. The delay times corre-
sponded to the maxima in the TOF spectra of
Ž
.
Ž
10P 16 laser line, which is in resonance with the n3
SF6rCH4 flow td s240 ms for curves 1 and 2, and
vibration of 32 SF6 (948 cm
y1
)
Ž
w x.
17 .
.
td s310 ms for curve 3 . One can see that the HF
One can see in the case of the excitation of SF6 in
the flow interacting with surface the maximum value
of the HF) fluorescence intensity is about one order
of magnitude larger as compared with that in an
unperturbed flow. The sharp front of the shock wave
appears in the region where the molecules are ex-
fluorescence intensity in the flow incident on the
surface is about 4 times greater and in the shock
wave more than 30 times greater than in unperturbed
flow. The difference is even larger at Fav (3–4
Jrcm2, indicating increase of collisional dissociation
Ž
.
cited at a distance D xs2.5 mm from the surface
at td (370 ms. At smaller distances D x from the
surface to the excitation zone the peak of the HF)
fluorescence intensity was observed at smaller delay
times, and its intensity was greater. For example, at
D xs1.6 mm, the front of the shock wave was
observed at td (330 ms and the intensity of HF)
fluorescence in the shock wave was about 20 times
greater than in an unperturbed flow.
Note, in the flow interacting with a surface, the
HF) fluorescence intensity was considerably greater
as compared with that in an unperturbed flow at all
delay times, even if the molecules were excited not
in the shock wave but rather in the incident flow
Fig. 3. The dependencies of the HF)-fluorescence intensity on the
CO2 laser energy fluence in the case of excitation of SF6 in the
Ž
.
e.g., at td (350 ms for the case of Fig. 2 . This is
connected with the following. In an unperturbed
Ž
.
Ž
mixture with CH4 SF6 rCH4 s1r1 in unperturbed flow curve
flow, the concentration of molecules is rather low
.
Ž
.
1 , in the flow incident on the surface curve 2 , and in the shock
15
N(10 cmy3 . Because of a deficit of collisions,
Ž
.
Ž
.
wave curve 3 . For details see the text.

(
)
348
G.N. MakaroÕ, A.N. PetinrChemical Physics Letters 323 2000 345–350
of molecules in the shock wave at low-energy flu-
ences.
We also carried out experiments on the direct
measurement of the yield of SF4 product under
excitation of SF6 in unperturbed flow and in the flow
interacting with surface. The method of measurement
of product yield in the pulsed gas dynamic flow is
w
x
described in Ref. 6,18 . In these measurements, we
used a pure SF6 flow. The SF4 yield in an unper-
turbed flow has been measured at td s260 ms and in
the flow interacting with a surface at td s260 ms
and td s370 ms. These delay times correspond to
Ž
.
maxima in TOF spectra of molecules see Fig. 2 .
The distance from the nozzle to the surface was 52
mm and D xs2.5 mm. The SF6 pressure above the
nozzle was 1.25 atm. The excitation energy fluence
was Fav (12 Jrcm2. It was found that in the case of
excitation of molecules in the flow incident on the
)
Ž
.
Fig. 4. The frequency dependencies the spectra of the HF -fluo-
rescence intensity in the case of excitation of SF6 in the mixture
with H2 SF6 rH2 s1r1 in unperturbed flow curve 1 , in the
flow incident on the surface curve 2 , and in the shock wave
curve 3 . The spectra are normalized in maxima. The intensities
of the spectra in maxima are related as: I1 rI2 rI3 s1:5.2:19.5.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
surface at td s260 ms the SF4 yield was 2.5 times
For further details, see the text.
Ž
.
greater, and in the shock wave at td s370 ms
about 12 times greater than in unperturbed flow.
indicating comparable selectivities of SF6 dissocia-
tion in unperturbed flow and in the shock wave.
When measuring the enrichment factor in SF4 in
the 34 S isotope the SF6 molecules were excited at the
Note that similar results were also obtained by us
with CF3I molecules. The C2 F6 product yield in the
shock wave was found to be about 16 times greater
than in unperturbed flow.
frequency of 929 cmy1 10P 36 CO2 laser line ,
Ž
Ž
.
.
The selectivity of the process was studied by
measuring the frequency dependencies of the HF)
fluorescence intensity in the case of excitation of SF6
in the flow interacting with surface and in unper-
turbed flow, and the enrichment factor in SF4 in the
which is in resonance with the n3 vibration of the
34 SF6 19 . The results of study of the yield of SF4
product and the enrichment factor in it in the 34 S
isotope under excitation of SF6 in unperturbed flow
and in the flow interacting with surface are summa-
rized in Table 1. In the case of excitation of molecules
w
x
34 S isotope. Fig. 4 shows the frequency dependen-
)
Ž
.
cies the spectra of the HF fluorescence intensity
in the case of excitation of SF6 in the mixture with
in an unperturbed flow at energy fluence Fav (10
prod
Jrcm2, the enrichment factor was equal to K
s
34
Ž
.
Ž
.
H2 SF6rH2 s1r1 in unperturbed flow curve 1 ,
17"5, and in the case of excitation of SF6 in the
prod
Ž
.
in the flow incident on the surface curve 2 , and in
shock wave it was equal to K
s14"3. Thus,
34
Ž
.
the shock wave curve 3 . The total pressure of the
gas above the nozzle was 2.4 atm. The exciting
energy fluence was Fav (5 Jrcm2. The distance
from the nozzle to the surface was 51 mm, D xs2.5
mm. The spectra are normalized in maxima. The
HF) fluorescence intensities in maxima of these
spectra are related as I1rI2rI3 s1:5.2:19.5. One can
see that, although the width of the HF) fluorescence
intensity spectrum in the shock wave is much broader
than in unperturbed flow, the ratios of intensities of
the HF) fluorescence in maxima and in long wave-
the yield of products in the shock wave more than an
order of magnitude exceeds that in an unperturbed
flow, while the selectivity is only insignificantly
smaller.
The increase of the yield of products in the shock
Ž .
wave is connected with the increase of i the gas
Ž .
Ž .
density, ii the rate of chemical reactions, and iii
Ž .
the dissociation yield of molecules due to a more
effective excitation of them in the shock wave, and
Ž .
b collisional dissociation of molecules excited be-
low the dissociation limit. Such molecules cannot
dissociate in an unperturbed flow because of the
deficit of collisions.
length wings of the spectra near the 34 SF6n3 absorp-
x.
Ž
tion band, (930.5 cmy1 19 do not differ strongly
w

(
)
G.N. MakaroÕ, A.N. PetinrChemical Physics Letters 323 2000 345–350
349
Table 1
The results of study of the yield of SF4 product and the enrichment factor in it in the 34 S isotope in the case of excitation of SF6 in
unpertubed flow, in the flow incident on the surface and in the shock wave
SF6 pressure
CO2 laser Energy SF4 product yield
Enrichment factor in SF4
above the nozzle line
fluence
Unperturbed flow Incident flow Shock wave Unperturbed flow Incident flow Shock wave
2
prod
prod
prod
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
atm
Jrcm
arb. un.
arb. un.
arb. un.
K34
K34
K34
Ž
.
.
1.25
1.25
10P 16
12
10
1"0.2
2.5"0.5
12"3
Ž
10P 36
17"5
15"3
14"3
Ž
.
.
The maximum increase of the gas density in the
ture in the incident flow T2,vib (T1,vib , whereas
2,tr )T1,tr and T2,rot )T1,rot
The heating of the gas in the shock wave at the
w
x
shock wave is determined by the relation 10–12 :
r rr s gq1 r gy1
Ž .
T
2
Ž
. Ž
.
2
1
w
x
expense of stagnation can be estimated 12 : DTs
yo2r2cp, where yo is the flow velocity and cp is the
heat capacity of the gas. Taking corresponding val-
where r1 and r2 are the gas densities in the incident
flow and in the shock wave accordingly, and gs
cprcÕ is the ratio of heat capacities. For SF6g(1.1
Ž
w x.
ues for SF6 yo (420 mrs, cp (665 Jrkg K 20
w
x
20 , therefore r2rr1 (21.
we will obtain DT (130K. If the translational
Note that the maximum density of the gas in the
temperature of SF6 in the incident flow is T1,tr (40
Ž
excitation region at a distance D xs2.5 mm from
w x
K 8 , then in the shock wave it is T2,tr (170 K. The
rotational temperature is probably close to the trans-
.
Ž
the surface was not reached at smaller values of
D x the HF) fluorescence intensity was greater , and
.
lational one, and the vibrational temperature T2,vib
w x
(
the ratio r2rr1 in the excitation region was less
than the limit value. Therefore, the increase of the
SF4 yield in the shock wave was connected not only
with the increase of gas density, but also with other
factors considered above.
A rather high selectivity in the shock wave is a
consequence of that the vibrational temperature as
well as the rotational one of molecules in the flow
interacting with surface are still rather low.
T
1,vib (150 K 8 . Therefore, if in forming of selec-
tivity the vibrational temperature is a dominant fac-
tor, then the selectivity of dissociation of molecules
in the shock wave should not differ strongly from
that in unperturbed flow, what we observed in the
present experiments. Note also, the decrease of the
selectivity in the shock wave due to heating of the
gas in some cases can be compensated by its in-
crease connected with the increase of the concentra-
In the gas dynamically cooled molecular flow, the
following non-equilibrium conditions are usually re-
w
x
tion of irradiated molecules 23 .
w x
alized 1 :
Ž
T1,tr (T1,rot (T1,vib T1,tr , T1,rot and T1,vib are the
4. Conclusions
translational, the rotational and the vibrational tem-
peratures of the molecules, respectively. In the shock
wave formed under interaction of the pulsed gas
dynamically cooled molecular flow with the surface,
due to the difference in the translational, rotational
The isotopically selective IR multiphoton dissoci-
ation of SF6 molecules in the non-equilibrium condi-
tions of the cold shock wave formed under interac-
tion of a pulsed gas dynamically cooled molecular
flow with solid surface was studied for the first time.
The HF) fluorescence detection method was used to
obtain the laser frequence and energy fluence depen-
dencies of the dissociation yield of molecules in the
shock wave and in unperturbed flow. The SF4 prod-
uct yield and the enrichment factor in SF4 in the 34 S
isotope were also studied in the case of excitation of
w
x
and vibrational relaxation rates 21 the ‘reverse’
non-equilibrium conditions may be realized: T2,tr 0
T
2,rot 0T2,vib. In addition, because of a rather slow
Ž
vibrational to translational relaxation rate for
w
x.
SF6 pt V – T (150 ms Torr 22 the vibrational tem-
perature of molecules in the shock wave in the case
of utilization of a pulsed rarefied gas flow may
practically not differ from the vibrational tempera-

(
)
350
G.N. MakaroÕ, A.N. PetinrChemical Physics Letters 323 2000 345–350
Ž
.
Ž
.
Ogurok, Kvantovaja Elektron. 25 1998 545 in Russian ;
SF6 in the shock wave and in unperturbed flow. The
obtained results show that the efficiency of isotopi-
cally selective IR multiphoton dissociation of
molecules in the pulsed gas dynamic flow can be
essentially increased at the expense of formation of a
shock wave under interaction of molecular flow with
solid surface.
Ž
.
Soviet J. Quantum Electron. 28 1998 530.
V.M. Apatin, L.M. Dorozhkin, G.N. Makarov, L.M.
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vysokotemperaturnykh gidrodinamicheskikh javlenii Physics
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.
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Acknowledgements
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x
Ž
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.
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The authors are grateful to V.N. Lokhman and
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was partly supported by the Russian Fund for Funda-
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Ž
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