811
CHEMICAL LASERSPRODUCED FROM O( ‘D) ATOMREACTIONS
Chemical Lasers Produced from O(1D) Atom Reactions. Elimination Lasers from the Q(lD)
111. HCl
+ CH,C14-, ( n = 1, 2, and 3) Reactions1
by M. C. Lin Physical Chemistry Branch, Naval Research Laboratory, Washington, D . C. 20390
(Received September 83, 1971)
Publication costs assisted by the Naval Research Laboratory
Chemical HC1 laser emissions were observed in the four-centered unimolecular elimination of HC1 from vibrationally excited Cl&OH, CLHCOH, and ClHzCOH molecules in an optical cavity. The vibrationally excited methanol molecule, which possesses a t least 130 kcal/mol of internal energy, was generated by insertion of an O(lD) atom into a C-H bond of a chloromethane molecule. The O(lD) atom was produced from the flashphotolytic decomposition of 03. The vibration-rotation transitions of the laser emission were identified, and the initial populations ratios of the highest gain transitions have been estimated.
Introduction We have recently reported the observation of H F laser emissions produced from the four-centered elimination reaction of chemically activated a-fluoromethanols. The vibrationally excited methanol was generated by insertion of an O(lD) atom into a C-H bond of a fluoromethane molecule. We have also observed HC1 laser emissions in a similar study of the chloromethane analog. This is, to our knowledge, the first HC1 elimination laser produced in a chemically activated system. Photochemical HC1 elimination lasers, however, have been observed previously in the vacuum uv flash photolysis of chloroethylenes by Berry and Pimentel.3 Experimental and Results Section In the present work, the mixtures of 0 3 and CH,C14-, (n = 1, 2, and 3) were flash-photolyzed, in the presence of a diluent such as He or SFe, in an optical cavity. The optical cavity and the detecting system have been described previously.* HCl laser emissions were detected in all three chloromethane systems. The total emission traces are shown in Figure 1; in all cases, a total pressure of 30 Torr with SFe as a diluent and a constant energy of 1.5 kJ were employed. Figure 2 shows the total laser intensity as a function of the total pressure of the 03-CH2C12 mixture, with either He or SFBas a diluent. The latter was found to be more efficient in enhancing the laser output, thanks probably to its small cross section for deactivation of the O(lD) atom5 and its higher efficiency in lowering the rotational temperature of the lasing medium. At higher pressures ( i e . , P > 40 Torr), however, the SF6 mixture shows a rapid decrease in the laser intensity; this may be due to the deactivation of HCI(v) by SFa. Helium, on the other hand, exhibits a slower deactivation rate. Similar results were observed for the CHC13 and CH3Cl systems.
The laser transitions were identified with a 50-cm Model 305 SRIP03 monochromator, using 250-p slits, which provided a resolution of about h0.5 cm-l in the spectral region of interest. The observed emissions were predominantly due to H3%l, despite the high isotope enrichment of H3’Cl. Only Pzl(7) of the H3’Cl lines was detected in the highest gain CHzClz system. On account of the lower gain of the H3’Cl line, it has a longer appearance time and weaker peak power than the H35Clcounterpart, as shown in Figure 3. Ozone was again found to be essential to the laser action. No laser oscillation mas detected when chloromethane-SF6 mixtures were flashed in the absence of O3 under the same conditions. Mass spectrometric analysis of the condensable (at 77°K) fractions of the flashed samples indicate the presence of CiZCO,HClCO, and HzCO, together with large amounts of COz, in the CHC13, CHzCl2, and CH3C1 systems, respectively. Large quantities of CO were detected in the noncondensable fractions of all three flashed samples.
Discussion Similar to the fluoromethane systems (except CHF3),2 the presence of CO and COz indicate that the vibrationally hot formaldehyde molecules formed in the following insertion-elimination reactions O(lD)
+ CHC13
O(lD)
+ CHzClz-% CHClzOHt+HClt + HCICOt
+
CC130Ht ----f HClt ClzCOt AHlo = -155 kcal/mol AHzO S -145 kcal/mol
(1) This work is partially supported by the Advanced Research Projects Agency under ARPA Order 660. (2) M. C. Lin, J . P h y s . Chem., 75, 3642 (1971). (3) M. J. Berry and G. C. Pimentel, J . Chem. Phys., 51, 2274 (1969); ibid., 53, 3453 (1970). (4) L. E. Brus and M.C. Lin, J . P h y s . Chem., 75, 2546 (1971). (5) K. F. Preston and R . J. CvetanoviO, J . Chem. Phys., 45, 2888 (1966).
T h e Journal of Physical Chemistry, Vol. 76, No.6 , 1978
812
O(lD)
M. C. LIN
+ CH3Cl3,CH2CIOHt+HCI’+
HzCOt
AHaO = -134 kcal/mol
may readily undergo further decomposition or elimination reactions, such as
-% Clz + CO -% C1 + ClCO HCICOt -% HC1 + CO C12COt
+ HCO -% Hz + CO +H + HCO 5b
--f
HpCO’
C1
6b
=
26 kcal/mol
AH4b0 =
79 kcal/mol
AH4,’
AH5,’ ES -7 kcal/mol AHjbo ES 79 kcal/mol
AHBB’= 1.3 kcal/mol AHsb’ = 87 kcal/mol
For both ClpCO and HpCO, the molecular elimination reactions are probably less likely, although these processes have been shown to occur in the photodecomposition reaction~.~J The energetics of the above reactions were calculated from the heats of formation of various species recommended in Benson’s book.* The heat of formation of HClCO, AHrozss(HClCO) = -40.7 kcal/mol, is the
P (TORR) Figure 2. The effect of total pressure on the laser intensity of the CHZCI~ system: 0,:CH2C12:He(SF6) = 1:1.5:20; flash energy = 1.5 kJ. 2 psec
I
I
0.1 v l d i v
t
2 vsec I I
H
’ 11 n . / 0.2
vldiv
4Figure 3. Emission traces of the Pt1(7)transition from the 03-CHzC12 flashes: upper traces, HW1; lower trace, H36CI.
1 v/div
L
0.5 v / d i v
-t-
Figure 1. The total emission traces: (a) CHC13mixture (03:CHC13:SF~= 1:3:20); (b) CHzClzmixture (Os:CH2C12:SFe = 1:1.5:20); (c) CHaCl mixture ( 0 3 : CHsCl :SF6 = 1:1 : 20. Solid curves, laser emissions; dotted curve, flash lamp output at 240 nm. The Journal of Physical Chemistry, Vol. 76, N o . 6,1972
average of two values estimated by assuming D(HC0Cl) = 78.6 and D(C1CO-H) = 87.0 kcal/mol; both estimates agree within 4 kcal/mol. The energetics of the laser pumping reactions 1-3 are similar to those of the CH,F4-, analog.2 The presence of CO and C02 in the flashed CHCl, sample, in contrast to the CHFs system, is consistent with the fact that ClzCO decomposes more readily than F2CO; the first C-F bond dissociation energy of FzCO is known to be about 137 kcal/ m 0 1 . ~A ~ ~more thorough kinetic study of these reactions using a conventional photolytic setup is certainly warranted, since neither the kinetics nor the mechanism of this type of insertion-elimination reaction has been investigated before. (6) H. Okabe, A. H. Laufer, and J. J. Ball, J. Chem. Phys., 5 5 , 373 (1971). (7) J. G. Calvert and J. N. Pitts, Jr., “Photochemistry,” Wiley, New York, N. Y., 1966. (8) S. W. Benson, “Thermochemical Kinetics,” Wiley, New York, N. Y . ,1968. (9) H. Henrici, M. C. Lin, and S. H. Bauer, J . Chem. Phys., 52, 5834 (1970).
CHEMICAL LASERSPRODUCED FROM O(lD) ATOM REACTIONS
Table I : Observed HCl Vibration-Rotation Transitions and Their Appearance Times in the CHCla, CHZC12, and CH&1 Systems" Transition
AV =
U , om-*
CHCls
CHzClz
CHsCl
2 +1
P(5) P(6) P(7) P(8) P(9) P(10)
2675. O l b 2651.82 2628.13 2604 09 2579.42 2554.34
3.9 3.5 4.0 4.6 5.2 6.2
I
AV = 1 -+ 0
P(7) P(8) P(9) POO)
2727.78c 2703.01 2677.73 2651.97
5. 2d 4.8 5.6 6.6
4.4 5.2 6.2 7.0
Flash energy = 1.5 kJ, Ptota1= 30 Torr and Os/CH,CL,/SFe 1:3: 20, 1: 1.5: 20, and 1:1:20 for n = 1, 2, and 3, respectively. bVacuum wavelengths observed by T. F. Deutsch, IEEE J. Quantum Electron., QE3, 419 (1967). c Vacuum wavelengths observed by D. H. Rank, B. S. Rao, and T. A. Wiggins, J. Mol. Spectrosc., 17, 122 (1965). Appearance time of the individual laser pulse in microseconds. 4
=
The results given in Table I allow us to estimate the initial relative population of HC1 for the transition that has the highest gain. If we assume that the deactivation of HC1 is negligible and that the rotational temper-
813
ature of HC1 is about 300"K, then the observation that Plo(8) reaches threshold first in the CHCla flashes, as shown in Table I, implies that N1/No = 0.50 f 0.04 for the HC1 molecule produced in reaction 1 according to our gain calculations. Similar calculations lead to N2/N1 = 0.76 k 0.10 for the HC1 molecule formed in reaction 2 and N1/No = 0.61 f 0.05 in reaction 3. The higher laser output observed for reaction 2 may be attributed to a higher HC1 concentration resulting from the successive elimination reactions 2 and 5a. A similar result was also observed for the CH2F2system.2
Conclusion It is shown that the population ratios of the highest gain transitions in these three systems are all less than unity. This observation further supports the conclusion that the fraction of reaction energy that is distributed into the HX product of a unimolecular elimination reaction is usually relatively smal1,2*3*10-2in contrast to the exothermic bimolecular abstraction reacHY -+ HXt Y or H X2 -+ HXt tion, X
+
+
+
+
x*12,1a
(10) P. N. Clough, J. C. Polanyi, and R. T. Taguchi, Can. J . Chem., 48,2919 (1970). (11) T. D. Padrick and G. C. Pimentel, J. Chem. Phys., 54, 720 (1971). (12) H. W. Chang, D. W. Setser, and M. J. Perona, J . Phys. Chem., 75, 2070 (1971). (13) P. D. Pacey and J. C. Polanyi, A p p l . Opt., 10, 1725 (1971), and the references cited therein.
The Journal of Physical Chemistry, Vol. 76,No. 6, 197.2
