Chemical Lasers and Their Applications Casper J. Ultee United Technologies Research Center, East Hartford, CT 06108 Chemical lasers find their origin in the study of the radiation emitted from chemical reactions. In many cases such radiation is of thermal origin. Thus, much of the radiation from combustion processes is due to the black or grey body radiation of d i d (soot1 made up mostly of carbon. The infrared radiation consists of C02 and Hz0 emissions which reflect the temperature of the combustion zone. UItraviolet radiation is weak heause most chemical reactions do not nroduce hieh enoueh temoeratures to " zive rise t o blackbody ultraviolet radiation. In these cases the energy released by the chemical reactions is observed as thermal energy, and the same radiation could be ohsewed by indirectly heating the reaction ~ r o d u c t to s the a.. p ~ r. o ~ r i atemperature. te This is not always the ease; perhaps the best known exception is the blue-green emission observed from the oxidation of phosphorus in air, which gave rise to the term "phosphorescence!' Wehster gives as one definition: "an enduring luminescence without sensible heat" Indeed, the phosphom flame, in spite of its visible radiation, is without heat: a piece of paper held in the flame will not ienite. A more nertinent case is the radiation ohserved from low pressure flames. Polanyi and his coworkers (1-31 have studied the infrared emission from the H + Clz flame at low pressureand found vibrational populntions rhararteristic of 7350 K. simultnneouslv with rotational populations indicating a 500 K temperature.-~natmospheric pressure flame of H2-CI2, on the other hand, has a temperature of about 2500 K with infrared emission characteristic of this temperature. Clearly, therefore, the HCI produced in the low pressure flame has a vibrational population distrihution which is not representative of the gas temperature and many studies have demonstrated conclusively that the product is formed
-
462
Journal of Chemical Education
in an excited vibrational state. At low pressures the vibrationally excited HC1 has a chance to radiate before the vibrational enerw .." is chaneed t o translational enerev ... and averaged over the system. Rotational relaxation is much faster than vibrational relaxation. i . ~ .take5 . fewer collisiams: thus. the rotational levels tend'to be in equilihrium with thd translational gas temperature even at pressures of a few torr. In a one atmosphere flame there are sufficient collisions to relax the vibrationally excited HCI, the mixture reaches thermal equilihrium and the emission is that of HC1 a t the reaction temperature. Polanyi (2),and independently Penner ( 4 ) , suggested in 1961 that the non-equilibrium distrihution resulting from such reactions could be used to "pump" an infrared chemical laser. A laser, chemical or otherwise, requires a mechanism to populate an excited state a t a sufficiently fast rate such that a t some time point there are more molecules in an upper (higher energy) state than in a lower. Under these conditions the number of photons produced by stimulated emission can exceed those absorbed, and optical amplification or gain will result (See Fig. 1).Under equilibrium conditions the ratio of the number of molecules in an upper and lower state are given by the Boltzmann distribution:
where T is the absolute temperature, AE is the energy difference between the states, and the g's are the dengenerates of the states, If we ignore the physical meaning of T and examine the behavior of eqn. 1as a function of T (see Fig. 2). it is clear that for T > 0, a t least for a simple two-level system,
N u< N1, i.e., an inversion is not possihle for finite, positive T's. Only when T is negative can we have an inversion. This has given rise to the term negative temperature. Similarly, application of eqn. (1) to any two vibrational or rotational levels out of an assemhlv of levels of a molecule eives rise to such terms as vibrational and rotational "tem~eratures." These terms are rather unfortunate in that the conventional use of the word temperature implies a condition of thermodynamic equilihrium, whereas in the present case just the opposite is true. When an inversion is achieved by a chemical reaction, the laser is referred to as a chemical laser. Historically, photodissociation lasers and sometimes gasdynamic lasers have been considered chemical lasers. They are not included in the present discussion. A review of these lasers, as well as a more detailed treatment of chemical lasers may he found in reference (5) and the references cited therein. As indicated above, the chemical production of excited state species must be fast enouzh to overcome the losses of excited species, which in the case of infrared molecular lasers, are due mainly to collisons. An examination of these loss processes shows that vihrational-to-translational (V-T) and vihrational-to-rotational (V-R) energy conversion are generally much slower than either vihrational-vibrational (V-V),rotational-rotational (R-R) and rotational-to-translational(R-T) energy transfers. This suggests the possihility of a pseudoequilihrium where the vihrational levels are in equilihrium among themselves due to rapid V-V exchange, but not with the rotational or translational levels because of the much slower V-R and V-T processes. This then leads to the concept of partial inversions characterized hy vihrational and rotational "temperatures." For a diatomic molecule, in the harmonic oscillator-rigid rotator approximation the energy levels are given by:
where w is the rotationless vibrational frequency, B is the rotational constant. and V and J are vihrational and rotational quantum numbers,r~spertively.'I'ransitione with A\' and M = f 1 ~ i v rise e to the fnmiliar R and P brmches in the swctra of het&ogeneous diatomic molecules (see Fig. 3). The condition for gain is given by:
The number of molecules in a vihrational state is given by:
Nv
N
[-w(VkfT:/2)hc]
= -exp
Qv In any given vihrational level the rotational level populations are given by:
under condiIf the whole system is in equilihrium Tv = TE, tions of partial equilihrium Tv and TR can he different. In general, TR will be equal to the translational temperature. Substitution of (4) and (5)in (3) gives:
> exp[-whe(V
+ 1/2)]eap [-J(Jk+i)Bhc]
where the primes indicate the upper level. Taking logarithms and letting V' = V 1 (i.e., V is the lower level, V 1the upper)
+
+
-+w
BJ'(J'
Tv
+ 1) > BJ(J + 1 )
TR
TR
or
For P branch transitions: A J = +1,i.e., S = J A",!
+ 1,and
=spontaneous emission
B!,+ =(induced) absorption
B u s t =induced emission
P,, (Nu
Bu.1 - Ne 0 1 . u )
For amplification Nu > N j Figure 1. Absorption and emission processes
- A 4 (-)
T
I+)
Figure 2. Values of NUIN, = exp(-AEIkT) for A E = 4000 cm-'
Figure 3. Typical vibration-rotation energy levels and spechum fwa heterage~BOUS diatomic molecule.
Volume 59
Number 6
June 1982
463
For R branch transitions: A J = -1, i.e., J' = J
- 1,and
Since J, B, and ware all positive, gain in the P-branch can occur with both Tv and TRpositive. All that is required is that T v > T a hv an amount determined hv the vihrational and r o k i o & constants and the rotational &anturn number. For the R-branch, on the other hand, a negative Tv or total inversion among the vihrational levels is required. Thus, we see that lasing can occur when there is a non-equilibrium between vibrationand rotation, a requirement that is much easier to achieve than a total inversion. In accordance with the above contents. . . moat molecular lasers onerate on the P-branch lines only. Even those with a uaal vil)rational inverriion have output spectra with n ~ t a t ~ o ndistrihutims al characteristic of the gas temperature, an indication that the concept of separate vibrational and rotational manifolds is eenerallv valid. Fiaure 4 shows the calculated gain in H F at one torr fo; the indicated ~ 1,onlv the ratios of the V = 0 and V = 1levels. For N I I N < P-branchesshow gain: the R-branchesshow ah;orption.bnly aain on the R-branch tmnwhen N,IN,, - .> l(T, < 0)is there sitions. Earlier we defined a chemical laser as one in which the inversion is produced by a chemical reaction. Ideally, one would like to react two stable chemicals and ohtain lasing without the need for auxiliary electrical input. In reality this has been achieved only recently in combustion driven HF, HC1, HBr, and CO lasers. Most chemical lasers reauire atomic soecies as a t least one of the reagents, which on; small scale Ean be vroduced easiest bv a vulsed or cw discharee. In other instances photodissociation is used either to prepare the atoms or to cause photodissociation into excited species. The first molecular chemical laser was reported by Pimentel in 1965 (6). I t was obtained from the flashlamp initiated reaction between Hz and Clz. Shortly after this initial work, a large number of pulsed HC1 and particularly H F lasers were reported. The corresponding cw lasers were not reported until 1969 and 1970. A detailed discussion of various pulsed and cw chemical lasers is beyond the scope of this article and has been given elsewhere (5).Here a few examples of different chemical lasers will be discussed with emphasis on the chemistry.
Figure 4. Relativegain and absorption lor HF at 1 tom and indicatedvibrational inversions. Comespandine vibrati-1 "temperaMes"are N. + ,IN,, = 3: -5237 K. I: -mo, 0.8: 25.800'. 0.4: 6280'. 0.2: 3580' RmtioMl '?emperahne": 300
Small Hydrogen Fluoride and Other Hydrogen Hallde Lasers The H F laser is one of the best characterized chemical lasers. A laree number of reactions vroducina H F have been studied in h a i l , forming a convenient hasisfor a discussion of the hvdroeen - - halide lasers. Most HI.' lasers are based on the reaction
-
F + Hz HF + H
464
&
Journal of Chemical Education
E.=
7.6 KCALiMOLE
.--_-
AH = -31 kcallmole
More than 60% of the reaction energy is released as vibrational excitation of the HF.Ficure 5 shows the energetics. Figure 6 shows product distributions ohtained by several workers (7-9). Most of the H F is formed in the V = 2 state; thus, the reaction creates an absolute inversion between V = 2 and V = 1and V = 1and V = 0. Figure 7 shows the distribution for a number of other hvdroeen abstraction reactions with fluorine atoms (8,10). ~ e c e & y , more detailed studies (11-12) of some of these reactions have ~ r o v i d e dmuch information on the fine details of the kineticbrocess. In general, these data are ohtained from svontaneous emission studies of the corresponding reactions a t very low pressure. Since the ground state cannot emit, experimental data for its production are not available. Figure 8 shows a cell used by Polanyi and coworkers (7). Pressures in the 10-5 tom range and cooled walls are used to minimize collisional deactivation of the excited species. The outout snectra of oulsed H F lasers reflect the nroduct distribution results. In thkse lasers a pulsed electric d&charge through a mixture of SF6 or CF4, Hz and He produces F and &
H+HFIV=31
H+HFIV=Zl
>
U Y1
1
AH-31.6,
5 ?
I
:
I
:
I
HIHFIV-11 '---
I
I
\ KO
,
...-T ..--'.*----
HIHFRI-Ol
REACTION COORDINATE Figwe 5. Energy diagram for the F
+ H2
-
HF
+ H re~ctlon.F r m Ultee (5).
h y , Y
1.0
0.8 a8
H-atoms. The F-atoms react with Hz to form excited HF, leading to a short laser pulse. The reaction of H-atoms with SF6 or CF4, the recombination of F to Fz and reaction of H with Fz are all much too slow to contribute. The laser pulse is limited in time by the build-up of ground state H F which is a very efficient deactivator for vibrationally excited HF, and by consumption of the F-atoms. Figure 9 shows typical time evolution of laser oulses on several H F lines (13).Note that the 2-1 lines uccurhrst followed hy t h e 1-0 lines in acwrdance with the product distrihurion. In addition, secondary 2-1 lines can occu; as a result of depletion of the lower level b y the 1-0
'\.*02
a4 0.2 F*".
Oo
1
2
3 V - LEVEL
4
HF LASER OUTPUT
5
Figure 9. Time evolution of HF lasers lines in a small pulsed laser. (Ultee.
!in).
Figvre 7. Relative rates ol HF(V1fwmation
0 F
v O 0
A
F F F F F
+ CH., Jonahn et. al.. (8) + HCI. Jonalhan et. al.. (8) + Her. Jonathan et. al.. (8) + HI. Jonathan et. al.. (8) + H2S.Chang et. al.. ( 1 4 W e e ( a 1 + H,O,, Chang et. al.. ( 1 4 (Ultee. (5)).
Figure 8. Schematic drawing of reaction vessel lor chemilumineswnw measurements. A m i c species are famed in a microwave cavity (A), the molecular reagem is innoducad through BC. Dolied portions are liquid nitrogen cooled. is collected by the gold coated m i r r m at the ends of the vessel and passes out through a sapphire window (Polanyi and Wwdall(7)).
Fioure 11. Small oulsed HF laser. The laser tube is on the rioht hand side between
he radiation
Volume 59
Number 6 June 1982
485
transitions. The 2-1 P(6) transition has V = 1, J = 6 as its lower state. Lasing on the Pl.0 (7) transition which has V = 1J = 6 as upper level, decreases the population of this level thereby increasing the gain in the 2-1 P(6) transition, which results in addition laser output seen in Figure 9. Figures 10 and 11 show typical pulsed H F lasers with different electrode structures. These lasers form a convenient source of infrared radiation at the HF, and when used with deuterium, a t the DF waveleneths. Table 1shows the commonlv observed wavelengths from small lasers of this type. Single line operation is achieved readily by using a grating as one of the cavity mirrors. The range can he expaned to include higher levels by the use of HI as the hydrogen source. However, HI is extremely detrimental to vacuum pumps, and some arrangement for trapping i t becomes necessary. The same reactions have heen used in small scale cw lasers. The problem here is one of getting a fast reaction in the optical region and removing the "used" H F to avoid absorption and relaxation losses. The F-atoms again can he obtained by discharging a stream of SF6 with diluent, channeling the discharged gas into a low rectangular channel (see Fig. 12) to increase its velocitv. . H7.(or . Dd . is mixed into the stream in the optical cavity region. A convenient and well-characterized laser of this type is the one described by Hinchen (14,151 and shown schematically in Figure 12. When operated with a short optical cavity and grating this laser will provide single line, single mode (i.e., a single cavity frequency) lasing withashort term frequency stability of 1part in 108. For long term stability the laser can he stabilized on the Lamb dip (See Fig. 13). The laser typically operates with SF8:2.4 mmole/sec, He:5.8 mmolelsec. and 0 . ~ 0 . 5mmolelsec throueh the discharee and Hz:O.X mmole/sec;njerted into the opti&il region. hes small amount of 01 minimizes sulfur deoaAtion. Ootimizario~iof individual lines can he made by v&ing these flows. Typical
-
operating pressures in the cavity are 6-20 torr. Typical total power is 1to 3 w. In general, higher power can be obtained by increasing the size and the flow velocity through the cavity, although at some point scaling the discharge becomes difficult and the combustion-driven lasers (see below) become more convenient. In the above lasers. the laser Dower is a t hest proportional to the number of fluorine a1om.s prnduced by the discharge or flashlamp. In the rase of HF it is also oossihle tooperate u laser on thdchain reaction between Hz and Fz. AH = -31 kcallmole (9) AH = -98 kcallmole (10)
F+H2-HF*+H, H+Fz-HFa+H
Both steps in the chain are exothermic and produce excited HF. Although the mechanisms of the chain reaction are not completely understood, i t is clear from experimental ohservations that chain branching does take place and that only a very small initial i.'-atom concrntratim is required to initiate the reaction. Reactions like HF' (\' >_ 4 ) + Fz 21: t lW and H2* FZ H F H F have been suggested (16) as chainbranching steps. for a laser, Hz and Fz,with a small . Tv~ically .. amount of O2 are premixed and then ignited with an electrical pulse, a photoflash or electron beam pulse. The 0 2 serves to inhibit spontaneous ignition of the Fz-Hz mixture. These lasers have operated at total pressure in excess of 1.5 atm, with output energies of 4.2 kdlpulse (130 Jil) (17). Electrical efficiencies of 875% (18) have been observed. The electrical efficiencies are based on the laser ontout and the electrical power input for the spark, flush lamp, or e-hcam used to initiate the H7-F, reaction Iexolosion~.'l'he chemical eflirienries. i.e., the fraction of total chekical energy obtained as radiation,
+
-
+ +
Figure 12. Electric discharger laser (Hinchsn ( 14))
-
Table 1. HF Lines Observed wlth Small HF Lasers (F H, HF H Reaction)
+
Transilion"
+
Wavelength' (microns) 1-0 2-1
1
1
1
1
1
1
FREuEIE". a3 U h
"
1
J
a"
Figure 13. Tuning cutve wilh Lamb dip of laser output on the P2., line.
HYDROGEN PRESSIRE (TORR) 300 TORRF2 / 90 TORR O2 'Vacuum, submct 0.001 to obtain wavelenglh in air.
'me number in brackets designates!he bwer level J value, ie.. 1 4 (PS) is abanrition from V = l . J = 4 t o V=O.J=5. 466
Journal of Chemical Education
Figure 14. Chemical and electrical efficiencies as a function of hydrogen pressure with 300 torr of F2and 90 ton of D2(Gerber and Panerson (17)).
-
-
are eenerdv.onlv a few oercent. The low chemical efficiencies are disappointing and are apparently due t o high collisional relaxation rates of the higher vibrational states of H F produced by the H Fz reaction. Figure 14 shows typical operating parameters for alarge, e-beam pumped H F chain laser (17). Since the H F2 H F F is exothermic to the extent of 98 kcal/mole excitation of H F to the V = 10 level is possible. Results ot product distributions studies indicate a maximum rate 01 formation intu u = 6 with appreciable rates into I' = 1 throughu = 8(19,20). Figure 15showstheoutput sequence of a low pressure (50 torr) laser. Apparently the rrlnxation lossesofstates with V > 6 are t w larye to nllow las~ny.Anuther less likely possibility is that the reaction H F (V > 4) + Fz 2F HF(u = 0)depletes the higher levels. No cw H F chain reaction lasers with output on higher V-levels have been reported. In general HCI and HBr lasers are similar to the non-chain H F lasers. Since the C1+ Hz HC1+ H and Br Hz HBr H reactions are endothermic, they do not lead to vibrationally excited products. Generally the reactions H Clz, CI HI, and H Br2 provide the excitation. Typical observed HC1 and HBr wavelengths are shown in Tables 2 and 3. The simple reactions discussed above have been most popular in driving chemical hydrogen halide lasers. There are a number of m i t e different reactions which, although not important for k g h power lasers, are quite interesting from the viewoint of chemical kinetics. Pimentel and co-workers (22) have observed lasing from elimination reactions such as:
+
-
+
+
-
+
-
+
+
+
+
CH3
-
+
-
+ CFB
CH3CF3*
-
HF'
+ CHCFz
Lin and co-workers (23-25) observed lasing from O-insertion reactions such as: In most of these cases, even though the exothermicities are well over 100 kcallmole, lasing is observed only from the V = 2 and V = 1 states. Apparently the reaction energy is distributed among both products. Detailed studies (26) have also been made of photoelimination reactions of isomeric chloroethylenes such as:
From results obtained with isomeric chloroethylenes, it is clear that both a a - and a@-elimination can occur. The photochemistry of the chloroethylenes is complex. Many reaction channels are possible. A detailed discussion is beyond the scope of this presentation. Much depends on the intermediate state: if its lifetime is long compared to vibration times (-10-13 sec), the energy can he distributed into the many vibrational modes and the final partition in the reaction products will approach a statistical distribution. If the lifetime is short, the distribution will be non-statistical and reflect the geometry and dynamics of the photoexcitation. Berry in his detailed study (26) concludes that the latter is most likely the case. From these results, it follows that in the addition of HC1 to chloroacetylene,the products will depend on the vibrational excitation of the HCI reagent, thus making it possible to control the stereochemistry. Hydrogen . . Halide Combustion Lasers Another important development in high power hydrogen halide lasers is that of comhustion driven cw lasers, requiring no external enerev inout. Meinzer (26-28) has r e ~ o r t e d comhustion d r i v e ; ~ F ; ~ ~ and l , HBr lasers and their deuterium analoeues. For a DF laser a mixture of H? and FI (in excess of stoichiometric) and diluent is fed into a comhustor. As a result of the HYFI reaction, the tem~eraturein the comhustor is high enough to dissociate the excess Fz into F atoms. The conditions are particularly favorable for F atoms because of the low dissociation energy and the high heat of reaction. The diluent is added to control the temperature independently of the mixture ratio. The reaction mixture from the combustor consisting mainly of F, HF, and diluent is passed through a supersonic nozzle to cool the gases to room temperature or below (See Fig. 16). The fluorine atom recombination is too slow to follow the ex~ansion.At the nozzle exit plane Dz is injected, leading to the reaction F Dz DF* D. Generally, there is insufficient Fz for the D Fzreaction to contribute significantly. The optical cavity is located close to the nozzle exit plane. Since the original work, complex
+
+
+
-
Table 2. HCI Laser Lines Wavelengthb
PEAK POWER
4
5-
-
Transition'
1-0
2-1
3-2
4-3
P(4) P(5) P(8) P(7) P(8) P(9) p(10) p(11) P(12) P(13) P(14)
3.573 3.603 3.634 3.666 3.700 3.734 3.771 3.808 3.847 3.888 3.930
3.707 3.738 3.771 3.805 3.840 3.877 3.915 3.955 3.996 4.039 4.083
3.851 3.885 3.918 3.954 3.991 4.029 4.070 4.111 4.155
4.005 4.040 4.076 4.1 13
4
4-3
b
-
-
Labelingas in Table 1. wavelenpths are for ~ ~ 1 3ln6 many . cares HCP linesere also observed: their w a v e
langms are -0.002 microns higher.
Table 3. HBr Laser Lines Wavelengthb
TIME AFTER FLASH INITIATION l/lSECI Figure 15. Time resolved specbascopy of Vle observed laser transitions of H2/F2/He = 0.5/1/40 mixture. Total pressure 50 ton, output coupling 10% (Suchard (2lN
~ranritinn.
1-n
2- 1
3-2
4-3
p(4) P(5) P(6) P(7) P(8) P(9)
4.017 4.047 4.078 4.111 4.144 4.179
4.165 4.197 4.230 4.263 4.299 4.335
4.325 4.358 4.393 4.428 4.465 4.504
4.533 4.569 4.607 4.648
-
-
Labeling as in Table 1 b wavelengfho are f
a HE.",
la HBr" they are 4 . 0 0 1 higher.
Volume 59
Number 6 June 1982
467
nozzles have been developed and multikilowatt outputs have been obtained. The use of Dz in the cavity, giving a DF laser, slows down the relaxation and avoids absorption by H F produced in the combustor. If an H F laser is desired, the Hz and Dz are interchanged. The concept has been extended to HCI and HBr lasers. In these cases the chemistry becomes more complex. In the case of HC1 hoth Fz and Clz are introduced into the comhustor where they form a mixture of CIF and F atoms (see Fig. 17). In practice a small amount of Hz is also added to provide easier ignition and better temperature control. As in the H F laser, this mixture is expanded and Hz is added in the optical cavity region. Vibrationally excited HF and HCl are then produced by the chain reactions: F+H2-HF1+H H+CIF-HF*+CI HCL* F
-
+
Although the detailed kinetics of these reaction are poorly known, the device does lase on hoth H F and HCI. Figure 18 shows the output power of such a laser as a function of the optical cavity position from the nozzle exit plane. The HC1 lasing persists much longer than the H F lasing because of the lower relaxation rates. Different reactions have been used on a smaller scale for hoth cw H F and HCI lasers. Cool and coworkers (29) used the reaction NO Fz NOF F a t low pressures to generate F-atoms. Arnold et al. (30) used the reaction sequence:
-
+
Figwe 16. Schematic of combustion chemical laser.
+
for a m a l l scale HCI laser. In the rase of HHr, Meinzer (28) develooed a combustion HF-HBr laser t ~ iniecting y both Hr* and H; into the cavity region using the sequence: F+Hf-HF*+H H+Br2-HBr* + B r
These lasers become chemically quite complex, and their operation requires a careful balance of flow rates, pressures, mixing, and temperature in order to promote the desired reaetions while a t the same time minimizing the effects of parasitic side reactions and relaxation. The job is made more difficult by the lack of kinetic data on these reactions, which makes computer modeling of little predictive value.
TEWWATLRE T - DEO K
Figure 17. Composition of reacted F.: CI2 = 2 1 mixture at 1 atm as a function of temperature (Meinzer (27)).
Chemical CO Lasers
Another chemical laser of major importance is the CO laser. Most CO lasers reported are based on the reaction: CS
+0
-
CO*
+S
AH = -80
+ 5 KcaVmole
The CS radical can he produced thermally from CSz or hy the reaction: CS2+o-Cs+so
The product distribution has been determined by several authors (3135) and is given in Figure 19. There has been some indication that reactions such as: CS+02-CO+SO CS~+0--C0+S2
contribute to excited CO formation to a minor extent. As a result of the large number of CO states excited and the small rotational soacine. the outout of the CO laser consists of manv lines (see ~ i20j:~An.important difference between CO and H F is the much slower vibrational relaxation of CO (1.9 X lo5 cm3/mole versus 1X 1012cm3/mole for HF). Because of the slow V-T relaxation, V-V exchange plays an important part in restructuring the CO-vihrational distribution. As a result of the slow relaxation, laser pulses are longer in pulsed CO 468
Journal of Chemical Education
Figure 18. Power distribution of a HCI-HF combustion laser as a function of the optical axis position. The distance is measured from me nozzle exit plane. Primary flows (Combustor): He = 0.10. F, = 0.0585, Ci, = 0.027, HI = 0.016. Secondary flaw: HI = 0.3 all in moleslsec. Combustor pressure 293 twr. cavity pressure 3.0 torr (Meinzer (27)).
0
2
4
8
Figure 19. Relative rates of h0 Se reactions. (Ultee (5)).
8 10 12 V -LEVEL
+ CS
-
Cqv)
14
11) 18 2 0
+ Sand 0 + CSe
+
CO
+
lasers, and in cw flow lasers, the lasing zones are stretched out. In addition, the V-V processes tend to lead t o similar distribution in chemical and electrically pumped CO lasers. A typically small cw CO laser is shown in Figure 21 (37). Oxygen is discharged to produce 0-atoms which are mixed with CSz. Typical flow rates are Oz 1.2 molelmin, He:1.7 molelmin, CSz:0.03 moleslmin with a mixing zone pressure of 6 torr. Lasing starts just downstream from the mixing zone and persists for 5-6 cm (lOW sec). As in the H F laser, the CO laser requires electrical input to make atoms. In this case, the dissociation reauires more enerav and as a result 0-atoms are difficult to produce in a c o m h t i o n process. I t is possible, however. to obtain lasing directly from the CS2-02 reaction flame zone at low pressure. Although the roomtemperature reaction rates for 0-atoms formation are too slow to carry the chain, a t higher temperature conditions they are more favorable and by a careful balance of temperature, concentration, and flow rates, lasing can be obtained from the CS2-02 flame. Figure 22 shows an operating CSz-02 flame laser. The burner is horizontal and rests on the small jacks just below the bright region. Lasing occurs just above the burner. The visible glow is due to the SO Oz continuum. The laser output (10's of watts) is not spectacular, and the efficiency is low. Although the chemical CO laser appears attractive from the standpoint of its relatively benign reagents, lack of a goad chemical source of 0-atoms have limited its development as a chemical laser.
+
Chemical Iodine L a s e r s
In addition to the lasers discussed, a number of other chemical vibrational lasers have been reported such as OH, various metal oxides, and HCN. None of these have been studied in detail, and little is known about their chemistry. There is another chemical laser system that is of considerable interest. An early laser, sometimes referred to as chemical, was the photodissociation iodine laser: WAVELENGTH. A-p
Figure 20. Spectral disbibution of supersonic CO laser &put (Boedekw. et al., (36)).
CAVITY REGION 50.8 x m.5 r 1.3 CM
WDIECHARGE90CM-1
Recently, a completely chemical scheme for excitation of I(2P~/z)has been reported (38-39). In this laser, the I-atom is excited by energy transfer from Oz ('A). This excited state of Oz is produced in many oxidations. One reasonahly convenient and efficient way is by reaction of Clz with H202 in the presence of NaOH giving an overall reaction of: C12+ H202+ 2NaOH
-
02
+ 2NaC1+ 2H20
The reaction is complex and most likely involves ionic intermediates such as OC1-. The OzlA yield is surprisingly high 0-25 KV Oe
o-m
1
mn
I
KBREWSTER WINDOW DIECHARGE TU
H.
TO PUMP
TRANSITION SECTION 1.3~ m.5 FLOW
CHANNEL
Figure 21. CO mixing laser (Ultee and Bonczyk (37)).
Figure 22. CSZ-O2flame laser. The burner is horizontal just below the bright region. The blue glow is due to the SO f 0, continuum.
Volume 59
Number 6
June 1982
469
(>30%),and a 100-W cw laser has been reported. This is the only example of a chemical electronic transition laser. In principle, such lasers could extend the wavelengths of chemical lasers into the visible and ultraviolet range, a tempting prospect indeed. In spite of considerable effort, no visible or UV chemical lasers have been reported. Considering the required conditions on the chemical rates, the radiative and collision lifetimes of the upper states, perhaps this is not surprising. Applications
The unique applications of chemical lasers depend on the narrow widths and high intensities of the laser lines. Laser line widths are extremely narrow compared to the pressurebroadened or Doppler width of absorption line profiles. With a single line laser operating at the line center, one can determine directlv and easilv the absorntion coefficient a t line center for the lasing gas, without the usual concerns of spectrometer broadening, slit functions, etc. Figure 23 (40-41) gives a typical example for a DF line. At low pressure the line width is determined hv the Dovvler effect. i.e.. the temverature, and is independkt of p r e k r e . ~ h e ' a b s & ~ t i oani line center is nronortional to area under the absorvtion curve and thus to the density. At higher pressures the line is broadened bv collisions, and since the collisional broadening. is.provor. tional to density, and since the area under the curve remains ~roportionalto the number density, the absorption a t line center becomes constant. Thus, from the low pressure section of the data one obtains the true absorption coefficient a t line center from which the Einstein A value can he calculated. If the laser can he tuned over the line profile, the line width, line shifts, and perturbations can be observed directly (42). Once these data are available, the lasers can be used for concentration determinations with high precision and sensitivity. Using the sensitivity and selectivity of absorption measurements with a tunable laser. i t has been vossible to make a detailed study of the kinetics of energy-exchange between individual vibrational and rotational levels of H F and DF in a simple but elegant experiment. A sample of H F at low pressure is pumped with a pulsed (40 nsec) single line H F laser. The laser pulse populates a single V = 1,J level. By monitoring the fluorescence from the V = 1manifold (all J levels) the overall vibrational relaxation rate is measured. However, much more detailed information can be obtained by monitoring individual rotational level populations. Although in principle we could monitor fluorescence from individual rotational levels, the intensities are generally too low. Instead, a tunable cw H F laser is used to determine the level populations by absorption. By monitoring the pumped level as well as neighboring J levels (43), the kinetics of rotational energy transfer can be measured in great detail. I t is also possible t o obtain information on kinetic energy exchange during collisions. The initial laser pulse, being a t line center excites H F molecules with zero velocity relative to light di-
rection. This velocity is maintained and the result is an excited state population with a narrow velocity spread. If the probe laser, which is collinear with the exciting pulse, is operated a t line center it will detect these molecules. If it is operated offline center, it will detect molecules only after they have chaneed their velocities in collisions. i.e.. there will be a time dela;(see Fig. 24). Measurements of this type have allowed the estimation of rates for translational energy transfer. Figure 25 summarizes the results of these types of measurement. The rotational relaxation rates are very ravid for low J values (small energies) and indeed are faster than translational energy transfer rates. However, for large J values the times approach or even exceed vibrational relaxation times. The translational "rate constant" is determined from the lifetime of the nouulation a t line center. The observed line widths reflect t i e total rate of collisions which shorten the lifetimes of the levels involved as the transition. Finallv. studies are now underway to determine more detail about ihe vibrational relaxation mechanisms, i.e., is there V-R transfer to high J levels, and if so, to which levels and a t what rates? Similar studies under way for a number of other molecules, including polyatomics, are providing a detailed picture of energy flow in molecules (44). Another area that has benefited greatly from chemical laser development is chemical kinetics. The usual Arrhenius form of the deuendence of chemical reaction rates on temuerature implies an energy barrier to a chemical transformation. Little was known, however, on the relative effectiveness of translational, rotational, or vibrational energy in surmounting this barrier. With the availability of lasers i t is now possible to selectively excite a single energy mode and by working at low pressure, to study its influence on the reaction rate and in some cases on the energy distribution in the products. The data for endothermic reactions of atoms with diatomic molecules indicates that vibrational energy is more important
Pump trkquency Figure 24. Velocity chenges in HF.
Linewidth
1
10-21 101
100
(01
102
I
105
RESSLRE IT-,
Figure 23.Absorption coefficient at line center vs pressure v = 1-0. P(9)line. (Bonczyk (44).
470
Journal of Chemical Education
- 3o l
Translational O
1
s
7 RotationalW
9
13
Rotational level, J Figure 25. Vibrational, m t a t i o ~and l translatiOM1relaxation rates and linewidth far HF. (Hinchen (43)).
than translational; while for exothermic reactions the opp6sitc is true. A detailed discussion of the interaction of reagent energy with the potential energy surfarp may he found in the studies of I'olanvi and co workers ( 4 5 ) . The wurk of Znreand co-workers (46)-is an example of some recent experimental the H F (DF) studies. A pulsed H F laser (5mJ) is used to pump either coliinear or perpendicular to the exciting laser beam. The H F is reacted with a Sr beam; the produced SrF is detected by laser-induced fluorescence using a tunable dye laser (see Fig. 26). As the dye laser is tuned, its radiation is absorbed and remitted as fluorescence. The fluorescence intensity reflects the absorption spectrum, and with a knowledge of the Franck-Condon factors the population distributions can be calculated. In a study of the reaction of Sr with HF and DF, the Sr reacts only with H F in the ground state but not with DF. The energetics for these reactions are shown in Figure 27. Excitation of H F to V = 1gave a large increase in the reaction rate; the excitation was su7ficient to overcome the endothermicity and activation energy barrier. In the case of DF, excitation did not lead to a reaction sueeestine that there was insufficient energy to overcome the activation barrier. Similar investigations with 10.6 micron (COz) excitation have extended studies of the type to many polyatomic molecules. In this case the situation is comolex. and the theoretical background is generally lacking s; that the interpretation of the experiments is not always clear cut.
@
chamber
F:;::h Light
baffle
~
-"
-
To the laser community chemical lasers have been somewhat of a disappointment, in that a high efficiency "Bunsen burner laser" to replace the discharge lasers never materialized. The search for efficient uv and visible chemical lasers so far has failed to turn up a near-term candidate. On the positive side, however, chemical lasers have had a large impact in physical chemistry. The search for new lasers had lead to detailed studies of product distributions of reactions and has rreatlv stimulated the interest in both ex~erimentaland rheor&ical chemical reaction dynamics. similarly, the desire for uv and visible chemical lasers has provided an impetus for many spectroscopic studies. Finaliy, the availability of chemical lasers has opened up the areas of state-to-state chemistry and has led to many investigations of intra- and intermolecular energy transfer. Literature (1) (2) (3) (4) (5)
Polsnyi. J. C..Ploc. Roy. Soc. (Canads). 540.25 (1960). Polanyi,J.C.. J. Chem.Phys.,34.347 (1961). Char-, P. E. and Polanyi, J. C., Disc. Forodoy Soc., 33,107 (1962). Penner, S. S.,J Quant. Spectrmc. Rodiafiue Tronslar. 1.163 (1961). Ulteo,C. J.:'Chemicslsnd GasdwamicLasers"in"Lsser Handbmkvol. 3,"Stitch. M. L., (Edilor). NorthHolland PublishiiCo.,Amsterdam. NewYork.Oxford. 1979.
&
Delayed Swingin mirror Chart recorder
Signal integrator Delay b o x nr
ti"4I
laser
+
HF reaction Figure 26. S c h e m a t i c view of experimental set-up for Ca. Sr studies. The swing mirror permits excitation of HF by different light paths. one anti-parallel the other perpendicular t o t h e probe laser b e a m (Karny and Zare
(45)).
Flgure 27. Energy diagram for reaction of Sr wim HF and DF.
Lin,M. C., J. Phys. Chem.,75.3612(1971), 76,142511972). Lin. M. C., lnt. J. Chem. Kin., 5,173 (1973). Lin,M. L a n d Brus,L.E., J. Chem.Phys.,54.5423 (1971). Mcinzer,R. A,. J. Chem. Kin.. 2,335 (1970). Meimer, R. A,. Hall, R. J.. and Dobbs. G.M.. "Comparison of Inferhalogen Bsaod C.W. HCIandDCI ChainReaction Ls~ers,"GasFlovandChemiealLaaers,Wendt, J.F., (Editor), Hemisphere Publishing Corporation, New York, 1978, (281 Meinzer, R. A. and Dobbs, G. M. Combustion Oliven HBr Laser. CLEOSnCF Ssn Diego. CA. February (1980). (29) Cool,T. H.and Stephen,R. R.,Appl.Phy8.Letf., 16,55(1970). (30) h o l d . S. J.,Foster, K. D.,Smlling,O. %and Suart, R. D., Appl. P h p . Letl., 30,637
(23) (24) (25) (26) (27)
-
". .om L"".
(6) Kssper, J. V. V. end G. C. Pimentel, Phys. Rev. Letters, 14,352 (1965). (7) Palanyi, J.C.and Wwda1l.K. B.. J. Chem. Phys.,57.1574(1972). (8) Jonathan,N., MeU'~uSmifh.C.M.0kada.S.. Sister, D. M..andhlin,O., MoL P h y ~ . 77 C. 1.
