CONTINUUM EMISSION FROM XENON IN
THE
VAPORPHASE
3129
Continuum Emission from Xenon in the Vapor Phase Induced by Absorption of 14743-A Radiation by L. Wayne Sieck Radiation Chemistry Sectwn, National Bureau of Standards, Washington, D. C. 80884 (Received December 14, 1967)
When xenon is exposed to a source of its own lowest energy resonance radiation (1470 A) in the vapor phase, a symmetric continuum is observed in emission which exhibits a maximum at approximately 1715 A. The addition of excess krypton to low pressures of xenon increases the continuum intensity substantially, and the "quenching" effects of other additives which are transparent to the resonance radiation are also reported. mm in diameter), was of the general type described Introduction previously4 and providoed a flux of approximately 10l6 The origin and characteristics of the vacuum ultraquanta/sec at 1470 A. A second lithium fluoride violet continua observed in emission when the heavier window (8 mm in diameter) was also attached to the rare gases are exposed to electron bombardment at photolysis vessel a t the point of connection to the enelevated pressures have been the subject of continuing trance-slit housing of the monochromator. study in several laboratories.'V2 These luminescence Volatile impurities were removed from xenon (Airco features have been ascribed to radiative transitions in assayed reagent) by outgassing a t - 196" and trap-toexcited rare gas molecules generated by a molecular trap distillation in vacuo. Filling of the photolysis ion-electron combination (Az+ e 4 Az*) and mechavessel was carried out by volatilization from a resernisms of the type A* nA --+ A2* (n - 1)A, where voir of previously condensed xenon which was slowly A* is a metastable rare gas atom formed directly by warmed from liquid nitrogen temperatures. Methaneelectron impact or following collisions of atoms excited d4 was purified by gas chromatography. Airco assayed to resonance levels. However, since the usual opticalreagent grade H2, N2, and krypton were used without selection rules for electronic excitation are not applicfurther purification, except for passage through a cold able in electron impact, the direct or indirect role of trap maintained at -196" prior to usage. A cold excited resonance levels in generating this luminesfinger attached to the photolysis vessel was also maincence is difficult to establish unambiguously. I n the -78" in all experiments. tained at present study, selective excitation of a resonance level was achieved by exposipg xenon to a source of its resoResults nance radiation (1470 A, 8.4 eV). This method, which Absorption of Incident 1470-A Radiation. The has not been reported previously in a luminescence 1470-A light source was operated at two different presstudy, uniquely defines the starting material, and any sures during the course of these experiments. For resulting continua can be traced to the subsequent convenience, the operating pressures will subsequently interactions of atoms initially excited to the ~ P ~ ( ~ P , , , ) ~ S be distinguished by the following notation: lamp I [11,2]10level. (70 p ) and lamp I1 (700 F ) . Figure 1 indicates the transmitted 1470-A intensity as a function of xenon Experimental Section pressure in the photolysis vessel. The transmission The agparatus consisted of three components: (1) curve for lamp I may be clearly resolved into two coma 1470-A xenon lamp, (2) a spherical photolysis ponents. The rapid decrease a t low cell pressures, vessel, and (3) a vacuum uv mono~hromator.~All indicated by curve A (lamp I), appears to be a comcomponents were coaxially aligned such that the outposite associated with the absorption of true resonance put of the lamp was directed through the vessel (the radiation and slightly shifted fringe components. The total path length was 26 cm) directly onto the entrance other distinct component, curve B (lamp I), which exslit of the monochromator (the slit width was 200-500 hibits essentially the same slope as the transmission p ) . Transmitted resonance radiation as well as an emission situated in the wavelength region 1100-5500 was monitored by operating the monochromator as a (1) P. G.Wilkinson, Can. J . Phys., 45, 1716 (1967). (2) R.Turner, Phys. Rev., 158, 121 (1967). scanning spectrograph. The amplified signal from the (3) For a description of the monochromator, see H. Okabe, J . Chem. photomultiplier was displayed directly on a chart rePhys., 47, 101 (1967). corder. The xenon microwave-powered discharge lamp, (4) See, for example, J. R. McNesby and H. Okabe, Advan. Photowhich was fitted with a lithium fluoride window (12 chem., 3 , 6 (1964).
+
+
+
d
Volume 78, Number 9 September 1968
3130
L. WAYNESIECK '.OF1
0.5
I
I
I
1
I
I
I
I
I
4
1
x(a) 20M I
1900 l
l
le00 l
l
1700 l
l
l
1400
IS00
1600 l
l
l
l
i
h t I 1
I
2
I
3
I
4
I
I
5 6 7 8 PlXEN0N)IN m m Hp
L
I
I
9
i
O
1
Figure 1. Typical transmission curves for 1470-A radiation through xenon for the two lamps. Data plotted us. xenon pressure, where ZTOdenotes the transmitted intensity obtained when the cell was evacuated. Curves A and B were obtained for lamp I (70 p ) , while curves C and D were obtained for lamp I1 (700 p ) . Curve C reflects transmission properties when the discharge region is close to the window when compared with curve D.
curves indicated for lamp 11, reflects absorption of radiation exhibiting significant wavelength shifts due to Holtsmark broadening. The fact that absorption is much more efficient at low lamp pressures is not surprising, since less reversal of true resonance radiation and less line broadening would be anticipated under these conditions. At a constant lamp pressure, the intercept obtained following extrapolation of the absorption curve due to fringe radiation (curves B-D) to zero cell pressure was found to depend somewhat on the path length between the discharge region and the window of the lamp. The net effect of decreasing this distance was to increase the apparent absorption coefficient of xenon. Luminescence from Xenon; Production of Continua. As the pressure of xenon was increased in the photolysis vessel, a continuum appeared in emissionwhich exhibited a maximum at approximately 1715 25.. Figure 2 contains a reproduction of a typical photomultiplier tracing (B) of this system obtained a t a pressure of 150 mm with lamp 11. For comparison, the intensity of the 1470-25. radiation transmitted through the evacuated vessel is also includod (A). Increasing the instrument resolution to 2.5 A (half-width for an atomic line) The Journal of Physical Chemistry
Figure 2. Typical photomultiplier tracing of the continuum emission from xenon. Tracing A indicates the signal obtained with the cell evacuated. Tracing B indicates the signal obtained when the vessel wm filled with xenon a t a pressure of 150 mm using lamp 11. (Note the change of the scale during the scan.)
failed to reveal any structure, and no other emission features were detected in the wavelength region 14505500 25. at any pressure. The general contour of the continuum and the position of the maximum was found to be independent of the pressure of both xenon in the cell and in the 1470-25.lamps. The shoulder at approximately 1500 8 (curve B) is transmitted fringe radiation from the lamp which was not being absorbed in the cell. As expected, the shoulder was relatively more intense when the xenon lamp was operated a t 700 p, since broadening of atomic resonance radiation was more pronounced. The intensity of this virtual component of the continuum decreased exponentially with cell pressure, and the apparent maximum was also shifted to longer wavelengths. The wavelength shift and the concurrent decrease in intensity was due to absorption of this incident fringe radiation by xenon dimers of the type reported by McLennan, et al., as early as 1933.5 The effect of cell pressure on the continuum intensity is indicated in Figure 3 for lamps I and 11. The increase in intensity was found to be second order below -40 torr (see the insert in Figure 3). As the pressure was increased further, the intensity increased more or less linearly and approached a high-pressure asymptote. It was also found that extrapolation of the linear segment of an intensity us. pressure plot did not pass through the origin but gave a slight positive inter(6) J. C . McLennan and R. Turnball, PTOC.Roy. Soc., A139, 683
(1933).
CONTINUUM EMISSION FROM XENON IN
400
44
P I KRYPTON) 800 1200
THE
3131
VAPORPHASE
k;
1630
1
20
30
40
PIXENONI l N m m Hg
3 I
0
4
I
t
p P 0
40
I IBO
I
I
I
I
I
I
I
I
120 160 200 240 PIXENONI I N m m Hg
I
I I I 280 320
1
Figwe 3. Effect of xenon pressure and added krypton on the continuum intensity: curve A, effect of xenon pressure using lamp IT; curve B, intensity obtained with lamp I ; curve C, intensity obtained when krypton was added to a constant pressure ( 120 mm) of xenon using lamp 11; curve D, intensity observed when krypton was added to 50 mm of xenon using lamp 11; curve E, intensity observed when krypton was added to 80 mm of xenon using lamp I. All intensities were normalized to a constant value for the incident flux of a 1470-A radiation. The insert is the log-log plot of typical data obtained in the low-pressure region for pure xenon: curve A, lamp 11; curves B, lamp I. Dotted line drawn with a slope of 2.0. Intensities are integrated values taken over the entire continuum range.
cept on the P axis, which varied depending on the lamp used and the discharge conditions. This result was not surprising, in view of the transmission curves shown in Figure 1. A.t P > 150 torr, the emission level was relatively constant over a wide range, although a gradual decrease was observed a t pressures greater than 400-500 torr. The luminescence efficiency, when defined as the number of counted photons a t 1715 A divided by the number of 1470-A photons transmitted through the evacuated cell, was approximately a factor of 2 higher when lamp I1 was used. E$ect of Additives on the Continuum Intensity. The effect of added CD,, Nz, and Hzon the intensity of the continuum was determined a t a constant xenon pressure of 150 torr using lamp 11. These data are shown in Figure 4. Also included are data obtained with added NZa t xenon pressures of 50 and 400 torr. The additives (with the except'ionof CD4, which exhibited an extinction coefficient approximately 0.1 atm-' cm-' a t 1470 A)
I
0.005
I 0.01
I
I
0.02
I
I
0.03
I
P(ADDITIVEl/PlXENON1
Figure 4. Effect of added Nz, Hz, and CDI on the continuum intensity at constant pressures of xenon. Data plotted as I l l 0 os. additive concentration, where I l l 0 is the intensity observed in the presence of the additive divided by the intensity observed from pure xenon at that pressure of xenon. Intensities are integrated values taken over the entire continuum range.
were found>o be transparent in the wavelength region 5500-1450 A, and no new emission features were found in any of the mixtures investigated. The continuum intensities obtained for Xe-CD4 mixtures were found to be dependent upon the time lapse between the introduction of CD4 and the measurement of the luminescence intensity, This effect can be attributed to modification of the over-all mechanism by accumulated decomposition products of CD,. A variation in slit width did not affect the observed percentage decrease in the continuum emission in any mixture, as long as the comparison was made with pure xenon a t the same setting. When excess krypton was added to a constazt low pressure of xenon, the intensity of the 1715-A continuum increased substantially. Typical data obtained with lamps are given in Figure 3. When lamp I1 was used, the luminescence intensity obtained when 600-900 torr was added to 120 torr of xenon (curve C) was essentially equal to that measured when the vessel was charged with 300-400 torr of pure xenon (curve A). More krypton was required to reach this maximum value when the partial pressure of xenon was maintained at 50 mm (curve D). One set of data taken with lamp I is indicated by curve E in Figure 3. In this case the limiting high-pressure intensity was substantially higher than the maximum level obtained from the photolysis of pure xenon. The contour of the Volume 78, Number 9 September 1968
L. WAYNESIECK
3132 1715-A continuum was insensitive to introduction of krypton in all experiments, and no new continua were detected in these mixtures at any wavelength in the region 1400-5500 8. When a considerable excess was introduced, however, the shoulder on the continuum (which, in the absence of krypton, was simply transmitted fringe radiation from the lamp) appeared to increase substantially and a new feature with the same contour but degraded to shorter wavelengths was detected at approximately 1450 A. This continuous system, which was a true emission feature, is attributed to Lorentzian broadening of trapped xenon resonance radiation rather than the formahion of bound upper states of XeKr*. As mentioned earlier, no new emission features were detected in mixtures of xenon with N2, H2, and CD,. It is, however, very difficult to establish conclusively whether or not the presence of 1-3 mol % of these additives modifies the bulk absorption characteristics of xenon a t pressures greater than 30 torr. Typical photomultiplier tracings obtained from X0e-N2 mixtures in the wavelength region 1450-1700 A are given in Figure 5. The curve indicated by the notation "vacuum" indicates the contour of the iransmitted fringe radiation associated with the 1470-A source obtained with the cell evacuated. When xenon was admitted a t a pressure of 150 torr, curve B (containing component A) was obtained. The effect of adding increasing amounts of N2 to 150 torr of xenon is indicated by curves C-E. All of these curves share a common, superimposable portion indicated by A. The dotted line, which was not obtained experimentally, gives an approximate indication of the intensity distribution which one might expect to obtain when the continuum is completely quenched (infinite dilution with N2). It follows that xenon was itself absorbing that portion of the monitored signal indicated by the shaded area a t a pressure of 150 torr and was, in fact, actually absorbing a small fraction of its own continuum radiation in the wavelength region 1500-1600 A. However, in all of the xenon-additive mixtures investigated, that portion of the monitored signal indicated by curve A was exactly superimposable, indicating that the presence of N2, H2, and CD4 in relatively small amounts did not affect the bulk absorption characteristics of xenon.
Discussion E$ect of Pressure. I n order to discuss the data, the following general formalization must be considered
+ hv (1470 A) +Xe(*PI) Xe(aPl) +Xe('So) + hv Xe(3PI) + IzXe('S0) +Xez* + (n - l)Xe(lSo) Xe(lS0)
(1) (2) (3)
Processes 1 and 2 reflect absorption and emission of resonance radiation without stipulating whether or not The Journal of Phyaical Chemistry
I , /&O
'
/6b
'
S /O '
'
/&O
,i$O
WAYELEHGTH (A) Figure 5. Typical photomultiplier tracings obtained for xenon and xenon-nitrogen mixtures. The xenon pressure waa 150 torr. All curves were obtained at a constant, incident flux of 1470-A radiation, with the resolution equivalent to a 1.5-A half-width for an atomic line.
the emitted photon exhibits a wavelength (energy) shift due to Holtsmark broadening at higher pressures. Reaction 3 denotes an over-all sequence(s) for formation of Xe2*, which may eventually emit the 1715-A continuum. Unfortunately, any photolysis experiment suffers from the consequences of Beer's law, namely, that initial absorption of the incident photons is localized in the window region as the pressure of the absorbing gas is increased. This situation, as well as resonance diffusion away from the sampling volume, will always tend to obscure the significance of an observed pressure dependence when sampling takes place from a small portion of the vessel. Consequently, it is extremely difficult to relate unambiguously the observed secondorder behavior observed a t lower pressures to any fundamental step in reaction 3. Only in those cases where the spatial distribution of excited species is relatively uniform (as in high-energy electron impact) will such data give any insight into the mechanism. The fact that the normalized continuum intensity in the range 200400 torr was substantially lower when lamp I was used as the excitation source is not surprising. This effect is directly related to the transmission curves shown in Figure 1. At any given pressure, absorption of the incident radiation was relatively more efficient with lamp I. Consequently, the initial concentration of Xe(3P1) near the lamp window was higher and the probability for a wall reaction resulting in deexcitation of any of the precursors for Xe2* (or Xez*
CARBON MONOXIDE OXIDATIONWITH
AN
OXYGENTRACER
itself) was enhanced. The absorption characteristics of the system when the high-pressure lamp was used were such that the spatial distribution of intermediates was more homogeneous and energy loss a t the window was reduced. Deexcitation of precursors for Xez* may also have occurred via removal (at the window) of the photon emitted in process 2. The present data are not sufficient to define the exact mechanism(s). Eflect of Additives. It has already been established that the addition of excess krypton increased the continuum intensity to essentially the same level as was obtained from pure xenon a t higher pressures (lamp 11). The same behavior was observed with lamp I, but the intensity was considerably greater than that obtained from pure xenon (see Figure 3). This behavior (lamp I) is related to the wall effect just discussed. A higher intensity was obtained with added krypton relative to pure xenon, because the measurements with the mixture were taken a t lower pressures of xenon where the distance of average penetration of the incident light beam was greater. This condition tends to reduce wall effects, and the continuum intensity (curve E, Figure 3)
3133
increased over the limiting high-pressure value (curve B, Figure 3). The quenching data may be treated rigorously, since the kinetic analysis may be carried out without consideration of wall effects and complications due to radiation diffusion. A steady-state approximation incorporating excited X e atoms and a quenching step which is first order in the additive yields a family of quenching curves (depending on the value of n in reaction 3) which may be compared with the data obtained with added Nz. Assuming that the additive does not react with Xez*, the only consistent solution yields a value nearly equal to 2. This result may be interpreted in several ways. It may be taken as evidence for production of metastable 3Pz atoms, direct threebody conversion of Xe(3PI) to Xez*, two concurrent mechanisms which exhibit a different pressure dependence, etc. Turner2 has recently concluded that the rate of production of a molecular emission in krypton can be explained by two mechanisms, each of which involves metastable atoms. As indicated above, the results of the quenching analysis can neither confirm nor deny the analogous situation for this system.
Carbon Monoxide Oxidation with an Oxygen Tracer over a Vanadium Pentoxide Catalyst
by Kozo Hirota, Yoshiya Kera, and Shousuke Teratani Department of Chemistry, Faculty of Science, Osaka UnCversity, Toyonaka, Osaka, Japan
(Received January #, 1968)
The oxidation reaction of carbon monoxide with gaseous oxygen on a powdered VZO~ catalyst was studied over the temperature range from 345 to 410°, using heavy oxygen l80(about 3 atom %) &s the tracer. When the lattice oxygen of the catalyst was partially substituted by concentrated heavy oxygen before the experiment, the percentage of 1 8 0 in the produced carbon dioxide changed gradually during the oxidation, in accordance with the 1 8 0 concentration in the catalyst, even though the percentage of "0 in oxygen and in carbon monoxide was practically invariant. When a mixture of oxygen and carbon dioxide, both containing about 60 atom % of 1 8 0 , was brought into contact with the catalyst at 370°,only the oxygen in carbon dioxide was found to be exchangeable with the lattice oxygen. The exchange rate of carbon dioxide was increased by the presence of carbon monoxide. The oxidation of carbon monoxide on vanadium pentoxide was explained by the oxidation-reduction mechanism. The relative rate of each elementary step and the surface intermediates during the reaction are discussed, and a detailed reaction scheme is proposed.
Introduction Vanadium pentoxide, supported or unsupported, has been used extensively for the catalytic oxidation of hydrocarbons or sulfur dioxide. Most of the mechanisms of these oxidation reactions have been discussed assuming a regeneration process of oxygen in the
catalysts, i.e. , by a oxidation-reduction mechanism. Basing their conclusions on the kinetic study of (1) (a) C. E. Senseman and 0.A. Nelson, Ind. Eng. Chem., 15, 521 (1923); (b) J. M. Weiss, C. R. Downs, and R. M. Burns, ibid., 15, 965 (1923); ( 0 ) B. Neumann, Z . Elektrochem., 41, 589, 821 (1935); (d) G.L.Simard, J. F. Steger, R. J. Arnot, and L. A. Siegel, I d . Eng. Chem., 47,1424(1965).
Volume 78, Number 9 September 1968
