CHEMILUMINESCENT EMISSION IN GASEOUS REACTIONS for the award of a postdoctoral fellowship. This work was supported by grants from the National Research Council of Canada, the United States Air Force Cam-
3721 bridge Research Laboratories, and the Petroleum Research Fund (administered by the American Chemical Society).
Chemiluminescent Emission in Gaseous Reactions at Low Concentrations
Iby John Emerson, Robert Reeves, and Paul Harteck IChemistry Department, Mason Laboratory, Rensselaer Polytechnic Institute, Troy, New York 18181 (Received M a y 7 , 1968)
‘The advances of electronic technology applied to photochemical systems make it possible to follow radical and atom reactions emitting only a few hundred quanta per second. This technique will make it possible to invcstigate many reactions by this experimental approach. Exceptionally long-lived low-lev$ luminescence is observed from gases after being irradiated for a few seconds to 1 min with the mercury 2537-A line or discharged by a Tesla coil. These emissions in the visible and ultraviolet regions with a half-life in the order of hours were investigated for gases such as COz or SO2in the pressure region of a few hundred microns. The decay appears to be close to second order. Evacuated or argon filled cells observed as blanks after similar excitation gave no significant luminescence after a few minutes. Initial emission was presumably due to phosphorescence of the glass. ‘The emissions are apparently due to very long-lived radicals or atoms which are consumed partially over chemiluminescent reactions. Concentrations of these radicals and atoms may be less than 1010 per cubic centimeter (or square centimeter of surface area). In addition, reactions may be followed where even only a minute fraction results in chemiluminescence or reactions with small light emissions which go so slowly that over only geological ages would they be completed.
Introduction Chemiluminescence and afterglows from atom and radical reactions have been observed for many years. Readily visible afterglows are observed in the 1-torr range in fast-flow systems.’ Rayleigh observed the decay of the nitrogen visible afterglow in a closed system over the period of about 1 hr.2 By quantitatively measuring such afterglows many reactions have been studied, rate coefficients determined, and mechanisms evaluated. I n recent years, there has been a general improvement in the quality of electronics and fast rise time photomultipliers to allow essentially low-level photon measurements to be made. Scintillation spectrophotometers are used to measure photons generated simultaneously by @-particledecay of radioactive substance. Although such equipment must include coincidence to measure the p activity, the noncoincidence mode of operation is generally readily used. Using sealed vessels of the necessary size (about 20 cc), lowlevel luminosity could be observed from various gases after appropriate excitation or from direct chemical reaction. I n this work we have made, for example, preliminary observation on emissions which extend in some cases over many hours after excitation for only 10 sec using a Tesla coil discharge. Using a mixture of carbon monoxide and ozone, strong emission can be
observed due to the carbon monoxide-ozone reaction where the oxygen atoms are generated in thermal equilibrium with the ozone3 over many days.
Experimental Section Cylindrical quartz or Pyrex vessels of about 25 mm diameter and 20 cm3 volume were evacuated to less than torr, then heated and exposed to a discharge generated by a Tesla coil in order to degas the cell walls. The cells then were filled with the desired gas or gases and sealed. Two methods of inducing chemiluminescence were used: plasma inside the cell generated a Tesla discharge or exposure to a low-pressure (Hanovia) mercury quartz lampe4 The cells were placed in either a Beckman LS-200B1 Nuclear Chicago, or Packard Tri-Garb liquid scintillation spectrometer, and the relative luminosity or afterglow emission was observed as a function of time, using the noncoincidence singles mode of operation. I n this way, a limit of detection was determined by a background counting (1) See, for example, Discussions Faraday SOC.,37,26 (1964). (2) Lord Rayleigh, Proc. Roy. SOC.,A151, 567 (1935); A176, 1
(1940). (3) P.Harteck and 8.Dondes, J. Chem. Phys., 26, 1734 (1957). (4) Hanovia Chemical and Mfg. Co., Newark, N. J., Model 30600.
Volume 78, Number 11 October 1968
J. EMERSON, R. REEVES,AND P. HARTECK
3722 rate equivalent to about a total of 300 photons per second. The discussion of the operation of a liquid scintillator can be found in the manuals of each manufacturer or el~ewhere.~ Essentially a high-efficiency, fast-response photomultiplier such as the RCA 8575 is used with high-speed amplification resulting in pulse signals, and the output is equivalent to photons per second when efficiencies are included. By designing the experiment around this standard type equipment which is frequently available in nuclear laboratories, low-level emissions studies can be readily made. The gases used here were Matheson Research Grade except for the ozone which was made in a Welsbach ozonizer and purified by desorption from silica gel. The ozone-carbon monoxide experiments were made in %ern3Pyrex vials. The purified ozone was added to the evacuated vial and then frozen with liquid nitrogen. Sufficient CO was then added and the vial was sealed. This small vial was inserted inside a standard 20-cm3 counting vessel to which water could be added to control the temperature of the vial itself. I n each run observation of luminescence was over a period of generally 1 to several hr with the counting time for each point being 0.1 to 1 min. Since the counting rate was high in the noncoincidence mode, statistical errors were less than 1%.
TIME M I N U T E S
Figure 1. Emmission from oxygen vessel-after Tesla coil, 10 sec.
-
IO6
15011 C o p
Results Emissions were observed as a function of time fo,r Pyrex and quartz vessels exposed to the mercury 2537-A line for a period 1 to 10 min or by discharging the gas inside for about 10 sec using a Tesla coil. The emission intensity observed was considerably higher than background or blank sample vials after equivalent treatment except, for example, Pyrex cells exposed to the mercury lamp phosphoresced for 2-3 hr. Luminescence was observed from oxygen after excitation with the Tesla coil as shown in Figure 1. The counting rate per second (cps) is given as a function of time.’ The level of intensity was similar for many gases, although argon gave no observable emission as might be expected. At lower pressures, much less than 100 p, the luminosity was more intense and persistent. The luminosity from nitrogen at a pressure of 300 p was observed a t a higher counting rate than that, from oxygen even a t 100 p . SO2 was also considerably higher at 900 p as can be seen by comparison with the results given in Figure 2. Typical counting rates observed from ceils filled with COZ and exposed to the mercury 2537-A line (uv) are also given in Figure 2. Excitation of the sample by a low current beam of 1-MeV electrons was considered, but initial experiments showed that the Pyrex glass not only darkened slightly after a total exposure of several 0.5-J pulses of 0.5-psec duration, but the glass itself was phosphorescing so The Journal of Physical Chemistry
IO minutes
IO’ 0
-
TESLA COIL loseconds *27
RUN
5
4
lo
0
Figure 2.
I
I
25
50
I
I
75 100 TIME MINUTES
I
125
I
150
5
Emission from quartz vessels.
strongly that the counting scalers were essentially flooded for several days. Emission was also observed from a mixture of carbon monoxide with ozone in a 2-cm3Pyrex vessel filled with 13 torr of ozone and 23 torr of carbon monoxide. The rate of photon emission was relatively constant over (5) W. J. Price, “Nuclear Radiation Detection,” McGraw-Hill Book Co., Inc., New York, N. Y., 1964, Chapter 7. (6) J. B. Birks, “The Theory and Practice of Scintillation Counting,” The Macmillan Co., New York, N. Y., 1964. (7) Background for irradiation of blank vessels was two orders of magnitude less than irradiated samples. Thus we discarded the background due to phosphorescence of the glass.
CHEMILUMINESCEPU’T EMISSION IN GASEOUS REACTIONS long periods of time-the decrease probably corresponding to a decreaae in the concentration of ozone due t o decomposition on. the walls of the vessel. I n this case no excitation by a Tesla coil or mercury lamp was used. The emission apparently resulted mainly from the reaction of carbon monoxide with the 0 atoms. The atoms could be expected to be in thermal equilibrium with the ozone present. This equilibrium is known to be very temperature dependent and the counting rate was observed to increase rapidly with temperature. This dependence was equivalent t o slightly more than 20 kcal of activation. The energy of dissociation of ozone into 0 atoms is about 24 kcal, in good agreement with this observation. However, with samples filled with pure ozone, no appreciable counts above background were observed. Some typical experimental data are presented in Table I. The results were highly reproducible, showing that reactions have a real meaning. Table I : Experimental Data Pressure, Gas mixture
co -k os -k Hz
+ Oa
mm 0 2
Oaa 0 2
so2
coz Blank (Pyrex)“ Blank (quartz) Blank (qua,rtz) Background a
Type of excitation
25
...
20
...
5
0.1 0.5 0.5 .
t
.
...
Tesla Tesla Tesla Tesla Tesla
UV
Counting rate at start, cp8 142,436 4,943 380 62,820 122,800 498,700 170 Background Background 300-330
Background subtracted.
Discussion The purpose of this work is to bring attention to the possibility of measuring low-level light emissions from gas samples uaing equipment now generally available with the intent to provide further information to the general field of reaction kinetics involving gases, luminous read ions, and possibly shed additional light on the role that wall reactions may play. Except for the possibly unique case of the nitrogen atom afterglow, most emissions from reactions terminate in a closed system after a second or so, and it was anticipated that only a flow cell would in general yield any observable emission. First-order decay schemes are usually anticipated, but here emissions were observed in almost all cases with the half-life increasing inversely with time equivalent to a second-order decay scheme. I n the same gas the decay rate of the emission was found to increase approximately with the square root of the pressure, although this has been studied only over a limitcd range.
3723 The kinetic scheme equivalent to -dA/dt = kA2, where I (cps) is proportional to A, would fit virtually all the results obtained for the samples exposed to the mercury line or the Tesla coil. It is not, however, within the scope of this paper to evaluate in detail the mechanism operating in each case. Considerably more work needs to be done on the specific gas studied and especially any effect of the wall. I n the case of SO2, for example, it is well known that the kinetics are very complicated and many reactions occur simultaneously. I n each case the spectral region of emission can be characterized by use of light filters. Addition of glass or quartz wool was made in a few cases, but no major effect was observed. Some metal surfaces are known, however, to have a major effect on emission from gases containing oxygen atoms.8,Q Although this type of study will need extensive characterization to produce useful applicable results, it appears to give information in a time region not readily obtainable with atom and radical reactions. Possibilities of a much more direct applica,tion are the studies of mixtures of ozone with other gaseous reactants where the ozone acts as a source of 0 atoms via thermal decomposition
08
+ M 2 + A4 + 0 0 2
(1)
The reaction of CO with 0 atoms yields emission
via GO
+0
--f
coz + hv
(2)
Here the role of the third body is not apparent. Assuming a rate for this reaction equal to 3 x (particles/cc)-’ sec-l, lo and calculating the steadystate oxygen atom concentration assuming the ozone and oxygen molecules to be equivalent,l’ the photons produced via reaction 2 are in the right order of magnitude compared to the counting rate (assuming a photomultiplier efficiency overall of about 5% (20 hv/sec = 1 cps)). The temperature dependence of this emission lends support to this kind of mechanism because the heat of dissociation for ozone is 23.5 kcal/mol. The capability of measuring coincidence leads to the speculation if any of the observed luminescence could be in coincidence; i t might be either by two-photon d e ~ a y ’ ~ or , ’ ~rapid successively emitted photons in a (8) G. Mannella and P. Harteck, J . Chem. Phys., 3 4 , 2177 (1961). (9) P. Harteck and R. R. Reeves, Jr., Discussions Faraday Soc., 37, 82 (1964). (10) B.H. Mahan and R. B. Solo, J. Chem. Phys., 37,2669 (1962). (11) The oxygen atom concentration is about 108 particles/cc as calculated from the values in the tables in B. Lewis and G. von Elbe, “Combustion, Flames, and Explosions of Gases,’’ Academic Press, New York, N. Y.,1961,p 682. This is in good agreement with the light emission data when efficiencies are included. (12) M. Lipeles, R. Novick, and N. Tolk, Phys. Rev. Letters, 15, 690 (1965). (13) R. C. Elton, L. J. Palumbo, and H. R. Grien, ibid., 20, 783 (1968).
Volume 78, Number 11
October 1968
J. EMERSON, R. REEVES,AND P. HARTECK
3724 cas~ade.~43~5 At present the limitations of the efficiency of the photomultipliers and the geometry preclude a high percentage of coincidence counts being observed even if all photon emission would be a true two-photon process. I n the case of SOz emission it is known that the characteristic SO 0 reaction produces a continuum which goes through a maximum, and the characteristic intensity curve us. energy or wavelength is not unlike the curvelBfor p emission where an antineutrino’’ is simultaneously emitted. Counting rates can be made in a noncoincidence mode or in a coincidence mode where the two photons observed must be within a gate time of the circuit of less than 100 nsec for commercial scintillation spectrophotometers. Random emission results in accidental coincidences being observed proportional to the square of the number of random observed pulses. I n high count rates the accidental coincidences can be very-large; however, the ratio of accidental coincidence to any real coincidence will decrease with d e creasing intensity and obviously the coincidence signals become proportional to the intensity of the measured
+
The Journal
of
Physical Chemistry
noncoincidence a t low intensity levels. No such coincidence emission was observed here, although the gating times were rather long and the total intensity was rather high to have a sensitive test of such coincidence. From the relative number of accidental coincidences observed compared to the noncoincidence counts, one can readily obtain a measure of the gate time effective within the electronic circuit.
Acknowledgment. The authors wish to thank Mr. R. Waldron and Mr. 14, Riozsi for their technical assistance in performing some of the experiments. We also thank Mr. N. I. Sax, New York State Health Radiological Laboratories, for his cooperation in the use of counting equipment. This work was carried out under a research grant from the National Aeronautics and Space Administration. (14) G. H.Nussbaum and F. M. Pipkin, ~ h y sRev. . Letters, 19, 1089 (1967)* (16) R.C.Kaul, J . Opt. SOC.Am., 56,1262 (1966). (16) C. S. Wu, “Beta and Gamma-ray Spectroscopy,” K. Siegbahn, Ed., North-Holland Publishing- Co., Amsterdam, 1965, p 1365. (17) E.J. Konopinskjand M. E. Rose, ref 16, P 1237.
