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Chemical Lasers.

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bond is formed during the reaction. The synthesis
should provide a route to substituted 2-amino 1alcohols, since 2-oxazolines are readily hydrolyzed
in acidic mediaC781. A synthesis of 2-amino l-alcohols that allows wide scope for variation of the substituents on the carbon is of great interest, since many
substances of this class are biologically active. In this
connection, mention should be made of the 4-ethoxycarbonyl-2-oxazolines (cf. Table 4, last three
lines), which can be obtained from ethyl u-isocyanoacetate or-propionate and aldehydes in ethanol with
sodium cyanide as catalyst. Their hydrolysis in
aqueous ethanol with a little triethylamine affords
ethyl esters of N-formylserine and derivatives substituted i n the a- or ?-position [731.
3.5. Oxazoles from a-Metalated Isocyanides and
Acylating Agents
H
I
\-I
R'
RZ
(69)
R' = COzCzH,, R2 = C,H5
= H, R2 = C,H5
(c), R' = CH=C(CH,),,
((I),
N+c.o
( h j , R'
0
R2 = -<!-\Yc",)2
of 50 % from l-lithio-3,3-dimethylallyl isocyanide
(51e) and 9,P-dimethylglycidic acid ethyl ester [641.
Isocyanornethyllithium ( 5 l a ) does not appear to
react uniformly with esters of carboxylic acids [641;
systematic studies will be required in order t o
establish which acylating reagents are most suitable
for each of the compounds ( 5 1 ) .
On addition of benzoyl chloride to a suspension of
ethyl lithioisocyanoacetate (59a) in tetrahydrofuran at -70°C followed by acidic working up, 4ethoxycarbonyl-5-phenyloxazole (69) can be isolated in a yield of about 80%[741. This synthesis
probably proceeds via the P-keto isocyanide (68),
which enolizes and cyclizes during working up [801.
5-Phenyloxazole (69) is obtained by reaction of isocyanomethyllithium (51a) with N-rnethylbenzanilide
and addition of methanol before working up 1641.
Similarly, the oxazole (69) can be obtained in a yield
I am grateful to Dr. W. Fabian, Dr. D. Walter, Dr. H.
Schafer, Dr. K. Fellenberger, Dr. M . Rizk, Dr. F.
Gerhart, Dip1.-Chem. D. Hoppe, and R. Schroder for
their enthusiastic cooperation. We thank the Deutsche
Forschungsgemeinschaft, the Fonds der Chemischen
Industrie, and the Badische Anilin- & Soda-Fabrik
for their support of these investigations. Thanks are
also due to Dr. A. Niirrenbach and Dr. W . A. Boll,
Badische Anilin- & Soda-Fabrik, for the use of unpublished results.
I801 For the acid-catalyzed cyclization of 9-keto isocyanides,
cf. 1. Hagedorn, Angew. Chem. 73, 26 (1961).
Received: October 23, 1969
[A 784 IE]
German version: Angew. Chem. 82, 795 (1970)
Translated by Express Translation Service, London
Chemical Lasers[**I
By K. L. KompaI*]
Chemical lasers are based on the new principle of obtaining energy in the form of coherent radiation from chemical reactions, Since the discovery of the $rst laser of this
type in 1965, laser emission has been investigated in a considerable number of fast gasphase reactions. In physical chemistry such lasers provide a diagnostic tool for the
detailed investigation of the energy distribution in elementary reactions. The barrier to
technological use has probably also been overcome by the recent continuous operation
of a chemical hydrogen fluoride laser. An attempt is made in the present article to trace
the development of this still youthful jield and to demonstrate the possibilities and
limitations of the generation of energy with chemical lasers.
1. Introduction
Besides the well-known methods of energy production
by chemical means, such as are used for heating.
-,
lighting, and driving machinery, a new principle of
been
production Of energy has
developed. This is the total or Partial liberation of
Angew. Chem. internat. Edit. / Vol. 9 (1970)/ No. I0
the heat of a chemical reaction as stimulated emission
of one or more reaction products. Reaction systems
of this nature are known as chemical lasers, since the
1'
.1 _Dr. K . L. Komua
Institut fur Anorganische Chemie der Universitat
8 Miinchen 2, Meiserstrasse 1 (Germany)
[**I Based on a lecture given at the General Meeting of the
G D c h , September 15 t o 20, Hamburg 1969.
773
excitation energy of the molecules, which appears as
laser emission, stems from a chemical reaction. The
laser is therefore chemically pumped. This principle
can best be illustrated by the following specific
example.
ground and excited states are interchanged ( I becomes greater than ZO), the absorption coefficient E
must become positive, and is then better named
amplification coefficient. The amplification process
is complementary to the absorption process.
In the exothermic reaction of a fluorine atom with a
hydrogen molecule to form hydrogen fluoride
A preliminary estimation may serve to show the
energy output that can be expected from such
chemical laser amplifiers. If a reaction volume of 10 1
can be filled in a sufficiently short time with a total
pressure of 150 torr of a 1 : 7 mixture of fluorine
atoms and hydrogen molecules [cf. eq. (l)],corresponding to 45 mmole of each reactant, and if half of
the hydrogen fluoride molecules (22.5 mmole) formed
emit a vibrational quantum CorresFonding to
m 1 2 kcal/mole or m 50 kWs/mole, the radiation
pulse emitted will have an energy of >lo00 Ws,
which exceeds the pulsed energy output of any of the
high power laser systems described so farr*I. However, it will be necessary to modify this estimate
repeatedly in the course of the following discussion.
A chemical laser can be constructed at relatively low
experimental cost, since it merely involves a chemiluminescence experiment in a suitable optical resonator. Various methods have been used for the
production of the initial concentration of a reactive
species, like, for instance, fluorine atoms in the above
example. Before proceeding to the description of experimental details, let us consider the reaction types
and some of the reaction systems in which chemical
laser emission has been observed up to now.
(activation energy E A - 1.7 kcal, reaction enthalpy
A H = -31.6 kcal/mole), 33.3 kcal/mole are liberated.
This energy can be distributed over the translational
degrees of freedom (Etrans.)
of HF and H and can
lead to the excitation of rotation (E,,t.) and vibration (Evib.) of the hydrogen fluoride molecules.
Electronic excitation of the reaction products requires much more energy, and is not possible here.
The total energy of the hydrogen fluoride is thus
Provided that the system is in thermal equilibrium,
the concentration of vibrationally excited molecules
is given by the Boltzmann equation:
( N O = concentration of unexcited molecules, k =
Boltzmann const., T = absolute temperature). Based
on the vibrational excitation of hydrogen fluoride,
with A E (energy for the excitation of the first vibrational state) -12 kcal o r ~ 4 0 0 cm-1,
0
the concentration Nexcited found for room temperature, i.e.
with kT m200 cm-1, is negligible. However, the
Boltzmann equilibrium with the environment may
be considerably disturbed during o r shortly after a
fast chemical reaction in the gas phase, and certain
molecular states may be overpopulated under these
conditions of disequilibrium. The population densities of excited states may then be inverted with respect to the numbers of the Boltzmann distribution.
If one still tries to express such an inversion of
population by means of a Boltzmann relation, a
negative temperature value appears in the formula
given. Since equilibrium is reached more rapidly for
translation and rotation than for vibrational excitation, population inversions are most readily found
among vibrational states. A reaction system with an
overpopulation of excited molecular states constitutes a molecular amplifier, as can be easily shown.
According to Beer-Lambert’s law, the decrease in the
intensity of a photon beam during passage through an
absorbing medium is described by
-d _I
I
(I
-
-&cf
or I
=
I , . exp ( - E c I )
- - intensity after passing through the medium,
I0 == incident intensity, 1 = thickness of absorbing
layer, c = concentration). If the populations of the
774
2. Reaction Principles
Table 1 shows the types of reaction that can lead to
chemical lasers. The range extends from the photodissociation type through a number of bimolecular
exchange reactions to elimination and photoelimination. All the chemical lasers described so far in the
literature are listed in Table 2. The photodissociation
lasers, which can only be regarded with reservation
as chemical lasers, have been arranged in a separate
chronological list. This list includes the first example
in the production of excited iodine atoms by flash
photolysis of alkyl iodides, preferably trifluoromethyl
iodide. The resulting iodine laser, whose emission
occurs at 1.315pm, has a n optical amplification of
more than 100 dB/m [**I, which is greater than that of
any other known gas laser. Lasers of this type operate
pulsewise, since the reaction mixture must be
renewed after each “shot”. Continuous operation is
in principle possible in a flow system with a continuous throughput of the reaction material. The iodine
atom and the bromine atom lasers are the only examples in Table 2 in which electronically excited reaction products appear. In all other cases known at
present, with the exception of a hydrogen fluoride
[*I In conventional high power lasers ruby or neodymium
glass is mostly used as amplifying material.
[**I dB/m = decibels per meter length of laser tube; decibel =
10 x log (output power/input power) of a n amplifier.
Angew. Chem. internat. Edit. / VoI. 9 (1970)
/ No. 10
Table I. Types OF reactions in which chemical laser emission has been
observed
Refs
Photodissociation
hv
+ I*
CFll
Photoeliniination
Abstraction
Chain reaction
(3-Elimination
CHiCHF + HFt
CI 1 H I 4 HClt
F I- H Z + H F t
H i F2 + H F t
CFiCHi + H F t
Pump system [a]
Hz(Dz)
[19, 20. 261
I1 171
[lo, 1 1 , 131
114, 16, 251
i I
’- H
I F
.’
CF~CHI
“81
List of chemical lasers
Table 2.
cs2 -;-
CF3
4- CzHz
’
hv
Clz
4
0 2
Hr(D2, R H ) t MFn
Hz(D2) I R F
Clr -C HI
co !- O z ( t C O 2 )
Clz -t- H l ( ~ iCOz)
C H ; I i CF;I
Hz(RH) -k Fz
HN,(-i-COz)
Hz(D2) i- Fz( vCOz)
CH31 -t N2FI
Method [bl
Emittrns
molecule
First
described
hv
Irv
HCI(DC1)
co
hv
PE
hv, PE
HF(DF)
HF(DF)[cl
HCI
hv
coz
coz
1965
1966
1967
1967
1967
1967
1968
1968
1969
I969
1969
I969
/I v
hv, PE
hv
HF
HF
coz
co2
hV
HF
h.J
T
hv
NO
CN
Br
H F(HCI)
Refs
Photodissociation Lasers
CFsI(R1)
NOCl
(CNh
1Br
CHZCHf(CI)
!I
v
IIV
lIV
[a] Abbreviations under pump system: R
-=
1964
1966
1966
I969
I969
[ 19,20,26]
121,221
[231
~241
[ I , 171
nlkyl group, frequentlyCH3;
( tCO2) = carbon dioxide as a n additive is pumped indirectly;
MFn
i--
UFg, SbF5, FzO, XeFJ, WFg, MoF6. XeF2 IF5.
[b] Abbreviations under method: hv - flash photolysis; PE - pulse discharge; n o entry ~:
spontaneous reaction.
[c]
.
Rotational eniission of H F also observed.
~~
laser based on pure rotational excitation, vibrationalrotational emission is observed in tt.e infrared region
of the spectrum. The first really chemical laser as
opposed to photodissociation was discovered in 1965
by Kasper and Pimentel in the flash-photolytically
initiated hydrogen/chlorine explosion (Table 2 ) .
Vibrationally excited hydrogen chloride is formed
here in the reaction of a hydrogen atom with molecular chlorine [eq. (4)]. The hydrogen atom$ required
are formed i n the preceding reaction step (3):
..
~
[ l ] G. C. Pimentel, 2nd. Conf. on Chemical and Molecular
Lasers, St. Louis!Mo. (USA), May 1969.
[2] J . V . V. Kasper and G . C. Pimentel, Physic. Rev. Letters 14,
352 (1965).
[3] P . H . Corneil and G . C . Pimentel, J. chem. Physics 49, 1379
(1968).
141 M . A . Pollack, Appl. Physics Letters 8 , 237 (1966).
[ 5 ] K . L . Kompa and C . C . Pimentel, J . chem. Physics 47, 857
(1967); K . L . Kompa, J . H . Parker, and G . C. Pimentel, J. chem.
Physics 49,4257 (1968); K . L . Kompa, P . Gensel, and J . Wanner,
Chem. Physics Letters (Amsterdam) 3, 210 (1969); P . Gensel,
K . L. Kompa, J . Wanner, Chem. Physics Letters (Amsterdam),
in press.
[6] J . H . Parker and G . C. Pimentel, J. chem. Physics 48, 5273
(1968); 51, 91 (1969).
[7] R. W . T . Gross, N . Cohen, and T . A . Jacobs, J . chem.
Physics 48, 3821 (1968).
[8] G. M . Chumak, G. G. Dolgev-Saveluev, and V . .4. Polakov,
Symposium on Chemical Lasers, Moscow, Sept. 1969, JETP
Letters, in press.
[9] T . F. Deutsch, Appl. Physics Letters 10, 234 (1967); I Z , 18
(1967).
[lo] J . R. Aireg, IEEE (Institute of Electrical and Electronic
Engineers, New York) J . Quant. Electr. Q E-3, 208 (1967).
52 (1968).
[ l l ] C . B. Moore, I E E E J . Quant. Electr. Q €4,
[12] I . Wieder, Physics Letters 24 A , 759 (1967).
[13] H . L . Chen, J . C. Stephensor, and C . B. Moore, Chem.
Physics Letters 2, 593 (1968).
1141 N . G . Basov, L. V . Koulakov, E . P . Markin, A . I . Nikirin,
and A . N . Oraevsky, J E T P Letters 9 , 375 (1969).
[151 N . G. Basov, V . V . Gromov, E . L . Koshelev, E. P . Markin,
and A . N . Oraevsky, J E T P Letters 10, 2 (1969).
Angew. Chem. internal. Edit.
I Vol. 9
(1970)
1 No. 10
CI t H2
+
HCI t H
AH
H
+ HCI + CI
4 H
CIz
-
I 1 kcdl
(3)
-45 kcal
(4)
:
The nearly thermoneutral reaction (3) yields only
“cold”, non-vibrationally excited hydrogen chloride.
In the formation of hydrogen fluoride [eq. (l)], on
the other hand, even the first step of the chain reaction is exothermic, and it was this, among other
reasons, that led to the discovery of hydrogen
fluoride laser emission in 1967 by Kon7pa and P i m m ?el. Similar considerations also led to the development of a more effective hydrogen chloride laser,
which is pumped by the reaction of chlorine atoms
with hydrogen iodide molecuIes (Table 2). The
carbon monoxide laser from the oxidation of carbon
disulfide has so far received comparatively little
attention. Molecular fluorine did not seem very
suitable as a source of fluorine atoms for a laser
according to eq. (l), owing to its problems of handling and to its low photochemical yield of fluorine atoms. On the other hand, a large number of
volatile inorganic fluorides constitute photochemical
fluorine atom sources with high absorption coefficients for the photolysis light; they present n o difficulty in handling, even i n mixtures with hydrogen,
and d o not attack the laser apparatus. The various
fluorides exhibit characteristic differences in the
emission spectra of the corresponding laser system.
Among the many hydrogen fluoride lasers, those in
which excited hydrogen fluoride molecules are formed
1161 T . A . Cool and R . R . Stephews, J. chem. Physics 51, 5175
(1969); Appl. Physics Letters. i n press; cf. Laser Focus,
December 1969.
[17] M . J . Berry and G. C . Pinlentel, 2nd. Conf. on Chemical
and Molecular Lasers, St. LouisiMo. (USA), May 1969.
[ l 8 ] M . J . Berry and G . C . Pimentef, J. chern. Physics 4Y, 5190
(1968).
[19] J . V . V . Kasper and G. C. Pimentel, Appl. Physics Letters
5 , 231 (1964); J . V . V . Kasper, J . H . Parker, and G. C . Pimentel,
J. chem. Physics 43, 1827 (1965).
[20] M . A . Pollack, Appl. Physics Letters 8, 36 (1966).
[21] M . A . Pollack, Appl. Physics Letters 9, 94 (1966).
[22] C. R. Giuliano and L . D . Hrss, J. appl. Physics 38, 4451
(1967).
[23] M . A . Pollack, Appl. Physics Letters 9, 230 (1966).
[24] C . R. Giuliano and L . D. Hess, 2nd. Conf. on Chemical
and Molecular Lasers, St. Louis/Mo. (USA), May 1969.
[25] J . F . Spinnler and P . A . Kirtle, 2nd. Conf. on Chemical
and Molecular Lasers, St. LouisiMo. (USA), May 1969.
[26] C . M . Ferrar, Appl. Physics Letters 12, 381 (1968).
[27] T . D . Padrick and G. C . Pimeritel, J. chem. Physics, in
press.
775
by photoelimination from fluoroalkenes are a special
case. It may be supposed that conclusions concerning
the stereochemical course of the elimination could
be drawn from the differences in the laser emission
observed with cis and with trans alkenes. Laser emission is also observed on spontaneous decomposition
of molecules formed by combination of free alkyl
and fluoroalkyl radicals. Pure rotational laser emission
has occasionally been observed to accompany the
vibrational-rotational emission of hydrogen fluoride
under certain conditions (cf. Table 1).
The excited vibrational states of hydrogen halide
molecules are subject to extremely effective deactivation processes, which always compete with the
stimulated emission. It may therefore be advantageous to transfer the excitation energy initially present
in the reaction products to added molecules of an
admixture that is capable of better energy storage.
A number of carbon dioxide lasers can be pumped
indirectly in this way. The situation is particularly
favorable in the transfer of energy from hydrogen
chloride o r from deuterium fluoride to carbon dioxide, which is a near-resonance process. This principle of laser-pumping may be called chemosensitization. Finally, we should also mention a carbon dioxide laser that can be pumped by the oxidation of
carbon compounds (Table 2). The group of indirectly
chemically pumped lasers can undoubtedly be expanded further, and it is unlikely that carbon dioxide will remain the only example.
The construction principles of this apparatus are
shown in Figure 1. The eight spark gaps are supplied
by eight high-voltage pulse capacitors [*I (1 pF/20
kV each, Emax =: 200 Ws). The discharge gaps are
connected as in a Marx generator, and can therefore
be triggered synchronously. Figure l a shows the
lucite spark chamber with the eight pairs of electrodes
and the absorption tube, which is made of fused
quartz with a high optical transparency, along the
axis of the cylinder. Photolysis light sources of this
type are characterized by good accessibility of the
vacuum UV (below 2000 A) and high reliability, as
well as by the short duration of the flash. However,
xenon flash tubes will retain their place for studies in
which a high light yield and a good optical coupling
between the photolysis lamp and the absorption cell
are of primary importance. The light characteristic
of such lamps can be greatly improved, with a
simultaneous increase in lifetime, by effective preionization. The energy from the laser oscillator can
be coupled out for instance by a partly reflecting
take-off plate placed in the beam path as shown in
Figure 1. A beam splitter then cuts off part of the
signal for recording of the total pulse shape, while
the main part of the emission is spectrally analyzed
in an infrared monochromator. The individual lines
in the emission signal are then recorded by a second
detector placed behind the monochromator exit slit.
tP
3. Experimental Arrangements
The experimental problems that arise in the operation of chemical lasers will be illustrated for the example of the flash-initiated hydrogen fluoride laser.
“Classical” flash photolysis, the successful development of which earned the Nobel Prize for R. G. W .
Norrish and G . Porter in 1967[281, uses a pulse capacitor discharge through a xenon-filled quartz tube
as the light source. Flash durations of between 10
and 50 ps can be achieved with discharge energies of
a few thousand Ws. The half-life of reaction (1) for
initial concentrations >1 torr, however, is considerably less than 1 ps. Shorter flashes are therefore
desirable to achieve the full possible radiation
density, i.e. to make full use of the possibilities offered
by the high reaction rate for the fast build-up of the
hydrogen fluoride concentration. An arrangement
that has proved suitable for this purpose is described
b y Welge et al.c291, in which a series of spark gaps
(arranged in a nitrogen atmosphere along the absorption tube) is used instead of closed xenon flash tubes
for the generation of light [5,301.
I&
+@-Fig. 1.
Block diagram of a chemical laser apparatus.
a) Vacuum-tight spark chamber with eight spark gaps, b) 20 kV mains
power supply, c) 30 kV trigger unit, d) laser tube with sapphire windows
attached at the Brewster angle, e) t o the vacuum line, f ) spherical
resonator mirrors, g) deflection plate, h) beam splitter, i) phototransistor
for flash control, k) infrared monochromator, I ) double-beam oscillograph, m) infrared detectors.
[ 2 8 ] R . G . W . Norrish, Angew. Chem. 80, 868 (1968); G. Porter,
Angew. Chem. 80, 882 (1968).
[29] K . H . Welge, J . Wanner, F. Stuhl, and A . Heindrichs,
Rev. sci. Instruments 38, 1728 (1967).
Fig. la. Polymethyl methacrylate spark chamber with eight pairs of
electrodes and Suprasil @ laser tube at the axis of the vessel.
[30] K . L . Kompa and J . Wanner, unpublished results.
[*I Hirotronic Ltd., Basingstoke/Hants. (England)
776
Angew. Chem. internat.
Edit. Vol. 9 (1970)/ No. 10
The total signal (or alternatively the flash profile)
and the single-line emission are displayed on a
double-beam oscilloscope and photographed.
4. Build-Up of the Laser Emission
The build-up of the laser emission is illustrated
in Figure 2. The spark chamber with the eight
spark gaps, the laser tube with the end windows at
Brewster’s angle, and the resonator mirrors are
shown schematically. The photons entering the tube
from the sparks first produce reactive particles,
e.g. chlorine atoms in accordance with eq. (2) or
fluorine atoms. After a certain number of collisions
of the fluorine atoms with hydrogen molecules,
vibrationally excited hydrogen fluoride is formed.
Some of the molecules formed emit spontaneously
in various directions in space and form the noise
background of the oscillator. The noise components
that lie in the axis of the tube undergo feedback by
reflection at the mirror surfaces, are amplified, and
cause the laser to start oscillating. In the arrangement shown in Figure 2, part of the power is coupled
rn
Fig. 2.
I
L
W O f i 61
______________
-1
Build-up of the laser ernisston (lor explanations see text)
o u t by the semitransparent plane mirror shown on
the left. The emission signal of a laser oscillator of
this type can be further amplified by passage through
other tube volumes that contain excited molecules
produced in the same manner. Two additional experimental aspects deserve mention in this connection.
These are the production of giant pulses and the
tuning of resonators with the aid of a grating instead of a resonator mirror. The buildup of the laser
emission (see Figure 2) can be divided into three
steps: 1) the primary photolysis; 2) the production
of energy by the chemical reaction; 3) the output of
energy by stimulated emission, which occurs under
the conditions described as soon as a certain threshold value is exceeded. A limited period of storage of
energy in the molecuIes may be inserted between
steps 2 and 3. T o achieve this, the resonator is switched on (at a predetermined time after the photolysis
flash) only when the energy is far in excess of the
threshold value. Controlled switching of the resonator
quality (“q-switching”) leads to emission of the
stored excitation energy in a very short time (a few
nanoseconds) and hence to very much higher peak
Angew. Chem. internat. Edit. J VoI. 9 (1970)/ No. 10
power outputs 1311. Variation of the switch-on time
also yields information on the time dependence of
the storage, and so allows conclusions to be drawn
regarding the relaxation and deactivation processes
in chemical laser systems 1321. Chemical lasers, like
other molecular lasers, emit not only single lines but
a spectrum that is more o r less rich in lines. However,
if a dispersing element is inserted, e.g. a prism or a
grating in t h e resonator, the resonator condition can
be satisfied only for a limited range of wavelengths,
and hence usually only for one emission line. The
resonator can then be tuned over the entire emission
spectrum by grating rotation. The usefulness of such
line selection in physical-chemical studies becomes
apparent in the use of chemical lasers as spectroscopic light sources. On the other hand, the possibility of studying isolated single transitions allows the
investigation of coupling phenomena and questions
of communication between the molecular energy
states involved in the emission 130’.
5. Pulsed Operation of Chemical Lasers, Energy
Yield
The discussion so far has been confined to t h e pulsed
operation of chemical lasers. I n every case the pumping reaction must be started by an external auxiliary
energy source that provides an igniting pulse. The
oscillogram in Figure 3 shows the initiating photolysis flash of an arrangement as shown in Figure 1,
as well as the laser emission signal produced by it.
It can be seen that the flash has a half-peak duration
of 1.2 ps, whereas the laser signal lasts up to 8 ps.
The laser was produced here by photolysis of a mixture of iodine pentafluoride as a source of fluorine
atoms and hydrogen. Figure 4 shows a comparison
of two single-line signals, which were not recorded
directly in this case, but were measured with an RC
integrator for better comparability of the line intensities. The change in the shape of the total emission
signal as compared with Figure 3 is explained by the
Fig. 3. Time profiles of the photolysis flash (upper trace) and of the
hydrogen fluoride laser emission (lower trace): 30th illumination of a
mixture of iodine pentaiiuoride (13 tom) and hydrogen (13 torr). time
scale 1 us/division.
1311 Cf. e.g. W . Kleen and R. Midler, Laser, Springer Verlag,
Berlin-Heidelberg-New York 1969, p. 161 ff.
[ 3 2 ] P. Gensel and K. L . Kompa, unpublished results.
777
A chain reaction for the formation of hydrogen
fluoride may proceed as follows 114,331,
Chain branching occurs here in reaction (7), where
vibrationally excited hydrogen fluoride from the
preceding step (6) causes fluorine molecules to
dissociate in collisions. It can be seen that reaction
(7) competes w i t h the equally possible stimulated
emission of the hydrogen fluoride molecules formed
i n (6). Chain branching therefore occurs only at
certain pressures and under certain reaction conditions. Lasers having reaction (6) in addition to reaction (1) as a pumping step, which therefore have a
chain length ‘ 1 , have been found not o n l y in the
hydrogen/fluorine system, but also in the flash photolysis of iodine pentafluoride/hydrogen mixtures 151.
However, so far there have been no reports on accurate quantum yield determinations and energy
conversion data.
Fig. 4. Measurement of laser emission signals by a gold-doped germanium detector in a n integrating circuit: upper trace in both pictures:
single-line emission signal (detector Au :G e with R C integrator), lower
trace: total emission (derector InSb).
fact that tungsten hexafluoride was used as the source
of fluorine atoms in this case.
If the emission in a photolytically initiated laser is
pumped by only one reaction step, e.g. reaction (l),
the radiation output of the laser is limited by the
available amount of photolysis light, provided that
not more than one photon can be emitted per hydrogen fluoride molecule. The quantum yield of the conversion of flash light into laser radiation cannot
exceed 1. The yield can be increased in two ways. On
excitation of sufficiently high vibrational states,
several photons can be emitted from each hydrogen
fluoride molecule formed, e.g. by an emission cascade
via several vibrational states. Moreover, the use of
chain reactions enables a large number of hydrogen
fluoride molecules to be obtained for each fluorine
atom that reacts initially. In particular, branched,
self-accelerating chain reactions 1331 give quantum
yields that can be as high a s l o 5 for the formation of
hydrogen fluoride from hydrogen and molecular
fluorine, depending on the reaction conditions 1141.
A very small trigger pulse is then sufficient for the
production of a high hydrogen fluoride concentration, and this is perhaps the greatest technological
stimulus for the development of chemical lasers. The
“ideal” chemical laser is thus largely o r even totally
independent of external energy sources. Lasers
having these properties have very recently come
within the bounds of technical feasibility 15,161.
1331 N . N . Sernenov, Some Problems of Chemical Klnetics and
Reactivity, Vol. 11, Pergamon Press, Oxford 1959; cf. also
K . J . Laidkr, Chemical Kinetics, McGraw-Hill Book Comp.,
New York, Toronto-London 1950, p. 180ff.
778
6. Continuous Wave Operation
In addition to the pulsed mode described, continuous
operation of a chemical laser is possible for instance
in a self-propagating chain reaction. Figure 5 shows
JL
NO ---F2 + He
Fig. 5 . Continuous operation of a hydrogen fluoride laser (after I161).
a ) Teflon reaction capillary, b) Pyrex glass tube, c ) gas injectors, d) to
cold t r a p and pump, e) highly reflecting resonator mirror, f) plane mirror
(0.5 % transmission), g) chopper. h) bolometer, i) tuned amplifier.
the experimental arrangement used in this case. The
initial concentration of fluorine atoms is produced
here by the reaction:
N O + - Fz
+ NOF-t F
(8)
The maximum output of this laser, which is still a
very small device operating with a fluorine throughput of 390pmole/s, was reported to be 0.23 W.
Though this laser can be operated both as a hydrogen fluoride and as a deuterium fluoride laser, the
energy transfer to carbon dioxide is advantageous.
The main reason for this is that the energy transfer
largely eliminates the strong self-deactivation of the
Angew. Chem. internat. Edit.
1 Vol. 9 (1970) 1 No.
10
excited hydrogen halides[*]. Another approach to the
problem of self-quenching in hydrogen fluoride
lasers is the use of gas-dynamic principles in addition
to chemical pumping. Such a device has yielded the
notable energy conversion of 12%1**1 with a cwpower output of nearly 500 W[35J. Figure 5a illustrates its principles of operation. First nitrogen
100% Mirror
t o pump
(v
3 ) . The rotational states that subdivide each
vibrational state are not shown in the diagram. The
observed laser emission lines correspond to transitions between vibrational-rotational states, the
vibrational quantum number Y always changing by 1.
An important advantage of the hydrogen fluoride
molecule over other diatomic molecules containing
heavy atoms and over compounds with three o r more
atoms is the relatively small number of energy states
that can be occupied. Because of this, no excessive
energy “dilution” occurs. Figure 6 illustrates rate
--:
i
H * HF (v=Z]
.Reaction
zone
-Resonator
I/
Hiinjector
s
1
Fig. 5a.
‘mirror
with
coupling hole
laser beam
Chemical and gas-dynamic hydrogen fluoride laser.
molecules are heated to 2300 “K in a plasma heater.
By subsequent mixing with sulfur hexafluoride
fluorine atoms are produced thermally. The atomic
fluorine flow is 0.03 moles/s. This mixture which
has been cooled by rapid expansion behind a Lava1
nozzle to nearly room temperature is brought
together with molecular hydrogen in a special injection system. The reaction zone of the mixture that is
still moving with supersonic velocity is placed in an
optical resonator, whose active length is only 1 7 cm.
The number of experimental cw-laser set-ups of this
type is considerably increasing a t present.
m
Reaction coordinate
4
Fig. 6. Relative energies of the vibrational levels of hydrogen fluoride
HZ + H
H F (further
that can be populated by the reaction F
explanations in text).
+
+
constants k , that differ fromt heoverall rate constants
of the reactions in that they give the specific rates of
formation of hydrogen fluoride for definite vibrational states. The differences in the constants k , then
determine the differences in the populations of t h e
various states:
7. Physico-Chemical Conditions
The choice of hydrogen fluoride for the system used
as an example in the present discussion will now be
explained. Figure 6 shows some energy profiles corresponding to reaction (1) for formation of various
vibrational states of hydrogen fluoride. Since the
energy associated with the excitation of a vibrational
quantum is approximately 1 2 kcal/mole, the highest
excited state that can be reached with the heat of reaction of this process is the third vibrational level
[*I N o t e added in proof: A CO2 laser operating in the arrangement shown in Fig. 5 has now been reported t o have a conversion
efficiency of 4% (T.A . Cool and R . R . Stevens, Appl. Phys. Lett.
16, 55 (1970)).
[**I One should note for comparison that the carbon dioxide
laser which is the most efficiently pumped gas laser so far has
7 5-20’1.6 energy conversion.
[34] J . C. Polanyi and D . C. Tardy, J. chem. Physics, in press;
for a description of the method of investigation cf. K . G.
AnlauJ P . J . Kuntz, D . H . Maylotte, P . D . Pacey, and J . C.
Polanyi, Discuss. Faraday SOC.44, 1 8 3 (1967).
1351 D . J . Spencer, H . Mirels, T . A . Jacobs, and R . W. F. Gross,
Appl. Phys. Lett. 16, 235 (1970).
Angew. Chem. internat.
Edit. J VoI. 9 (1970)1 No. 10
The constants k, can be derived from the characteristics of the laser emission, which contain the
excess populations of certain excitation states and
hence the differences in population as the principal
parameters [1,6,17,181.
The determination of these
constants is of great importance to the understanding
of elementary chemical reactions, and therefore
appears as an important goal of chemical laser
studies in physical chemistry. Investigations of this
nature have given a value of 5.5 for the population
ratio N , = 2 / N v = 1 = kv = 2 / k v = 1161. Other determinations carried out by the alternative method
of observing spontaneous infrared chemiluminescence 1341 gave k , = 2/kv = 1 = 3.5 and k, = 3 / k v = 2 =
0.47. Moreover, the part of the total energy available
in reaction (1) that leads to vibrational excitation,
E v / E t o t a l , is found to be 57%, while only 6 % appears as rotational energy and 37 % as translational
energy. These values show an effective conversion of
the heat of reaction into vibrational energy, and also
779
indicate a strong preference for the formation of
hydrogen fluoride with) = 2. Accordingly transitions
from v -- 2 to v = 1 also occur preferentially in lasers
pumped by reaction (1). This emission appears in the
infrared region between 2.7 and 2.9pm. In laser
systems in which the additional reaction (6) contributes to the pumping, higher vibrational states up to
v = 6 are excited. The emission at about 3 pm corresponds to a large number of vibrational-rotational
lines, but n o detailed rate constants have been
published so far 15,141. This description of the parameters of chemical laser emission has been confined
to hydrogen fluoride lasers as the area that has been
most extensively studied. In view of the intense
activity in this field, however, similar results may be
expected in the near future for other, new chemical
lasers.
8. Possible Applications
There is considerable scope for the use of these lasers
in various fields. In chemistry, sources of strong
monochromatic infrared radiation offer the longterm prospect of use as a “selective Bunsen burner”
with the object of carrying out chemical reactions
with a controlled supply of energy in certain vibrational degrees of freedom of a reaction system.
Chemical lasers are also already being used as
spectroscopic light sources for IR fluorescence
studies [13J. An obvious point of technological
interest is the possible freedom from dependence on
external energy sources. Despite great hopes, it is not
yet clear whether the possibly very high conversion
efficiency of these lasers would make their use in
high energy applications (e.g. for plasma production)
seem interesting. No chemical laser with a pulse
energy of more than 1 Ws has yet been described,
but pulse energies of the order of lo3 Ws, which
correspond to the value estimated at the beginning
of this article, have been predicted for laboratory
systems.
Financial support of our work in this field by the
Stiftung Volkswagenwerk, the Deutsche Forschungsgemeinschaft, and the NATO Office of Scientific
Research is gratefully acknowledged. I also wish to
thank Dip1.-Phys. Jochen Wanner and Dipl.-Chem.
Peter Gensel for their independent and enthusiastic
cooperation.
Received: January 28, 1970 [B 2886 IE]
German version: Chemie-In&!.-Techn. 42, 573 (1970)
Translated by Express Translation Service, London
Developments in Inorganic Arc Plasma Chemistry
By U. Landt [*I
For some 10 to 15 years, plasma chemistry has been in a state of rapid development on
the research side, whereas only a few reactions are at present carried out industrially in
a plasma jet. After a short description of the properties of thermal plasmas, the technique of production and stabilization of arc plasmas and the various types of arc plasma
generators are discussed. The course of chemical reactions at plasma temperatures is
then considered, with particular reference to kinetic and thermodynamic aspects.
A special part of the review is devoted to a detailed description of the inorganic reactions
that have been carried out in the plasma jet (preparation e.g. of compounds containing
nitrogen, C-N and C-F compounds, ozone, nitrides, cyanides, carbides, metal oxides,
and metals from metal oxides and from halides).
1. Development and Present Position of
Plasma Chemistry
A plasma is nowadays generally taken to be a partly
or completely ionized gas that is externally electrically
neutral, conducts electricity, and has a higher internal
energy than an un-ionized gas. The energy required
for the production and maintenance of a plasma can
[*I Dr. U. Landt
Knapsack AG
5033 Knapsack bei Koln (Germany)
780
be supplied as heat or as electrical or mechanical
energy. The most important methods for the supply
of electrical energy are arc and glow discharges and
electrodeless discharges (high frequency and microwave discharges). Only the arc plasma and its use
will be discussed here.
The term “plasma chemistry” is at present used only
reluctantly in the literature. Since chemical bonds d o
not exist at the high temperatures characteristic of
most types of plasma, the term is really self-contradictory. Nevertheless, it is shorter than any of the
Angew. Chem. internat. Edit. 1 Vol. 9 (1970)
1 No. 10
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