J. Phys. Chem. 1984, 88, 2451-2455
245 1
be due to slightly different rates of reaction at the beginning and end of the data collection period. Diffusion-controlled behavior for Pt and for the forward reaction on n-GaAs indicate that the assumptions made in ref 2 are valid for these systems. It is also safe to assume that thermal convection is not a problem. The fact that the reverse reaction on n-GaAs is not diffusion controlled is a consequence of the electrode kinetics and not thermal convection. Figure 8 shows the spectrum for a n-GaAs electrode which has become passivated (at the spot where the laser is focussed). That is, the rate at which TTF'. is being generated photoelectrochemically is greatly diminished. This is clear from the relative intensities of the 1427-cm-' band due to TTP. and the acetonitrile band at 1370 cm-'. The overall quality of the spectrum is inferior to that of an unpassivated electrode suggesting a change in the optical properties (i.e., reflectivity) of the electrode surface. Such a change is often apparent by simply visually noting the intensity of the Raleigh scatter from the surface of the S C electrode. Also, a large background has grown into the spectrum upon passivation. This background does not change with excitation wavelength and is therefore not fluorescence. Moving the laser beam to a new spot on the electrode surface cause the background (and passivation) to disappear temporarily. This implies that the background is due to a passivating film on the electrode. Often, Raman scattering from bulk phonons of the S C electrode will decrease or completely disappear upon passivation or photocorrosion of the SC electrode.2g Thus, RRSE can supply information about passivation and photocorrosion processes. Following spectral changes described above as a function of time by use of multichannel detection should provide important information about photocorrosion which could conceivably aid in development of more stable PECs.
the study of a photoelectrochemical system. It is possible to not only gain kinetic information about a SC-electrode reaction but also to monitor changes in the SC electrode surface. The potential of RRSE for the study of photoelectrochemical systems has been clearly demonstrated. The superior specificity of the technique is indispensible for sorting out the various processes which can occur simultaneously at a photoelectrode. Although RRSE with visible lasers requires colored reaction products which may be undersirable for a truly viable PEC, RRSE is still quite applicable to the study of certain model PECs. Future work to establish UV or near-IR RRSE may overcome this limitation. As will be reported in a future paper,29RRSE can be applied to the study of S C electrodes which have been pretreated in an attempt to improve reaction kinetics, prevent photocorrosion, and decrease e-h+ pair recombination at the surface. In addition, experiments in which reaction products are monitored at the surface of silver modified GaAs electrodes (Ag/GaAs) will be described. Ag overlayers on n-GaAs have only a minimal effect on photoelectrochemistry while allowing detection and molecular characterization of adsorbed species by surface-enhanced resonance Raman s p e c t r o ~ c o p y . ~ ~
Conclusion It has been shown that RRSE can be successfully applied to
(37) R. P. Van Duyne and J. P. Haushalter, J . Phys. Chem., 87,2999 (1983).
Acknowledgment. We thank Brian K. Johnson for computer programs enabling the acquisition of RRI transients and for general assistance. The support of this research by the Office of Naval Research (Contract No. N00014-79-C-0369) is gratefully acknowledged. We also thank Brian E. Miller and John Roper for their assistance in obtaining the OMA data. Registry No. GaAs, 1303-00-0;Pt, 7440-06-4; TTF, 31366-25-3; TTF'., 35079-56-2.
Fluorescence of Rhodamine B on Semiconductor and Insulator Surfaces: Dependence of the Quantum Yield on Surface Coverage Y. Liang, P. F. Moy, J. A. Poole, and A. M. Ponte Goncalves* Department of Chemistry, Temple University, Philadelphia, Pennsylvania 191 22 (Received: October 24, 1983; In Final Form: February 24, 1984)
The fluorescence quantum yield of rhodamine B deposited on semiconductor (indium oxide) and insulator (glass) surfaces was measured as a function of surface coverage. At high coverage the quantum yield is low on both types of surface, but somewhat lower on the semiconductor. As the surface coverage approaches zero the quantum yield increases by more than one order of magnitude on glass; the increase is much smaller on the semiconductor. The results are discussed in terms of injection and energy transfer quenching.
Introduction Adsorbed dyes may be used to sensitize large bandgap n-type semiconductors to visible and thus make them potentially interesting as electrodes in photoelectrochemical cells for solar energy conversion.3 Sensitization may occur if the dye excited state is above the conduction band of the semiconductor, in which case the dye injects an electron into the semiconductor and be(1) H. Gerischer and F. Willig, Top. Current Chem., 61, 31 (1976). (2)H. Gerischer, M.T. Spitler, and F. Willig in 'Proceedings of the Third Symposium on Electrode Processes 1979",S . Bruckenstein, Ed., The Electrochemical Society, Princeton, NJ, 1980,p 1 1 5. (3) A. Heller, Acc. Chem. Res., 14, 154 (1981).
0022-3654/84/2088-2451$01.50/0
comes oxidized. The overall conversion efficiency, q5 = [(number of electrons detected)/(number of photons absorbed)], is governed by the following: (1) competition between the desired injection of an electron into the semiconductor and any other processes which also shorten the lifetime of the dye excited state; (2) competition between the successful escape of the electron into the bulk of the semiconductor and undesirable processes such as trapping at surface states and recombination with the oxidized dye. Thus: 4= where 4i is the injection efficiency and & is the escape efficiency.* In an attempt to examine the processes which determine q5i, we have recently measured the fluorescence lifetime, 7,of dyes deposited on semiconductor and insulator surfaces. For rhodamine B, 7 = 55 ps on indium oxide and 7 = 46 ps on glass,4 0 1984 American Chemical Society
2452
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 I
I
I
I
I
I
I
t
N
2
I
X (\I
0
460
500
540
580
0.2
620
I
0.41
0
X (nm)
A
‘
A
A
A
Figure 1. Absorption spectra of rhodamine B deposited on glass (a) and on indium oxide thin films (b). The dye was deposited from a lo4 M
solution. Two stacked surfaces were used for each spectrum. in contrast to the 2.7-ns lifetime determined in dilute ethanol solution.5 This means that placing the dye on either surface shortens 7 by almost two orders of magnitude. Because our experiments were carried out at high coverage of the surface by dye molecules, we proposed4that 7 was shortened on both surfaces by energy transfer followed by trapping at nonfluorescent sites (quenching). Ideally, we would have checked the validity of this view by measuring 7 as a function of the surface coverage, 8, since energy transfer becomes less efficient as the average intermolecular distance increases. Unfortunately, because of the relatively low sensitivity of the streak camera detection used in the lifetime measurements, experiments could not be carried out at much lower 0. In order to circumvent this problem, we measured the fluorescence quantum yield of rhodamine B as a function of its optical density (as a measure of 8) on indium oxide and on glass, and the results are reported here.
Experimental Section Tin-doped indium oxide thin films (resistance < 250 ohm/ square) coated on glass substrates were obtained from Optical Coating Laboratories, Inc. The glass surfaces used in our experiments were those of the same substrates, after the semiconductor coating had been stripped off with concentrated nitric acid. Before coating with dye all surfaces were cleaned repeatedly in an ultrasonic cleaner, first with ethanol and then with distilled water. Rhodamine B from Eastman Kodak was first recrystallized several times from ethanol and then dissolved in distilled water to make solutions with concentrations between and 10” M. The dye was placed on the surfaces by adsorption from solutions of various concentrations, after which the surfaces were dried by dragging a strip of lens tissue over them. Although a number of variations of this method of adsorbing the dye were tried as well, the resulting 0 was invariably inhomogeneous. This required that optical density and fluorescence intensity measurements be made at the same point on the surface. All experiments were performed with the samples open to the air. The fluorescence quantum yield measurements were performed on a homemade dual-beam spectrophotometer,6 modified so that optical density and fluorescence intensity could be monitored simultaneously from the same point on the sample. The exciting beam was focused to a diameter less than 0.1 mm in order to decrease the effect of inhomogeneities in 8. The fluorescence was detected by a red-sensitive photomultiplier tube (Hamamatsu R928), which was protected from the exciting light (typically at 550 nm for fluorescence measurements) by a long-pass color filter (Schott OG 570). At low 8 special care had to be taken in determining the optical density of the dye because of the background contribution from (4) Y. Liang, A. M. Ponte Goncalves, and D. K. Negus, J . Phys. Chem., 87, 1 (1983). ( 5 ) M. J. Snare, F. E. Treloar, K. P. Ghiggino, and P. J. Thistlethwaite, J . Photochem., 18, 335 (1982).
( 6 ) Y. Liang and A. M. Ponte Goncalves, unpublished.
0
2
4
6
0 P o s i t i o n (rnrn)
Figure 2. Optical density of rhodamine B deposited on a single indium oxide thin film, measured at various positions on the surface. The dye was deposited from a lod5M solution.
A
I \
>r
c .-
I
540
I
I
580
I
I
620
I
I
660
I
I
700
X(nm) Figure 3. Uncorrected fluorescence spectra of rhodamine B deposited on glass (a) and on indium oxide (b). The dye was deposited from a M solution.
the substrate. The dye optical density was obtained by taking the difference between the total optical densities measured at 550 and at 610 nm (where the dye absorption is negligible). The sample was mounted on a micrometer-driven translation stage, against two orthogonal reference straight edges. This arrangement allowed reproducible positioning of the sample, so that it was possible to reliably map the optical density of any point on the surface before and after coating with dye. A portion of the surface was always left uncoated to serve as fixed additional background reference. Absorption and fluorescence spectra were obtained with a Hewlett-Packard 8450 spectrophotometer and a Perkin-Elmer MPF-2A spectrofluorimeter.
Results Figure 1 shows the absorption spectra of rhodamine B deposited on glass (a) and on indium oxide (b). The two are very similar, although the spectrum is slightly broader and red shifted on indium oxide. The magnitude of the inhomogeneity in 0 may be judged from Figure 2, which shows the optical density of the dye deposited at different points on the same indium oxide surface. (The point-to-point scatter in the optical density of the substrate, mapped both before adsorbing and after removing the dye, was invariably less than the height of the symbols used in the figure.) The optical densities obtained with the large cross-section beam of many standard spectrophotometers would have been of ques-
Rhodamine B Fluorescence
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2453
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i 0
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15
0
O D x IO2
Figure 4. Relative fluorescence quantum yield, &', as a function of the dye optical density for rhodamine B deposited on glass (full symbols) and on indium oxide (open symbols).
tionable validity since a dependence of the fluorescence quantum yield on 6 is suspected. Furthermore, when the optical density and the fluorescence intensity are determined in separate instruments, it is not always easy to ensure that they correspond to the same spot on the surface. Figure 3 compares the fluorescence spectra of rhodamine B deposited on glass (a) and on indium oxide (b). Because the spectra are similar, the spectrally integrated fluorescence intensity (uncorrected) obtained with our experimental arrangement may be used to measure the relative quantum yields in comparisons between the two surfaces. Comparison with solution, in which the fluorescence is somewhat blue shifted, is less accurate; however, the resonably flat spectral response of the R928 photomultiplier in this wavelength range keeps the error relatively small. The fluorescence quantum yields on the surfaces, &, were calibrated by measuring the fluorescence intensity and the optical density of a lod M ethanol solution of rhodamine B contained in a 1-mm-thick optical cell. The cell was mounted in the same manner as the dye-coated surfaces, and correction was made for the reflection losses at the front wall of the cell. All fluorescence quantum yields reported here for the deposited dye are defined relative to the solution quantum yield, df":
4; = lPf/d$
(1)
In addition to the error which results from the differences between surface and solution spectra, ~$4is affected by the different molecular orientations on the surface and in solution. Furthermore, because the optical arrangement of the sample is somewhat different for solution and surface experiments, an additional error is inevitably introduced in 4;. Figure 4 shows 4{ for rhodamine B deposited on glass (closed symbols) and on indium oxide (open symbols), obtained at various surface coverages (dye optical densities). The measurements were made at reduced excitation power (- 1 pW) in order to prevent photochemical degradation of the dye, which becomes significant at much higher powers. The difference between the trends in &' on the insulator and on the semiconductor is clear: As 6 0, &' increases by more than one order of magnitude on glass but by much less on the semiconductor. (Similar results were obtained, for short irradiation times, when the excitation power was increased to -0.1 mW. This rules out nonlinear processes which might arise because of the close molecular packing at high 6.) On glass ${ approaches unity as 0 0, Le., the quantum yield on the surface approaches the solution value. The sharp increase in &' at low 6 is consistent with the expected decrease in the ability of energy transfer quenching to compete with the other excited-state decay channels. In the high 6 limit &' changes very slowly with 6 and has an average value of 0.020, with an estimated uncertainty of 0.005. On the semiconductor the low 6 data are difficult to analyze because of the larger scatter. This is a consequence of the higher background noise found in optical density measurements made on the semiconductor, which is due to the combined effects
-
-
0
IO 20 Time (min) Figure 5. Decay of the fluorescence intensity of rhodamine B deposited on glass (a) and on indium oxide (b), for continued high-power excitation at 550 nm.
of optical inhomogeneities in the semiconductor thin film and of spatial jitter in the exciting beam. Nevertheless, the data may be extrapolated to give a rough estimate of &' 0.05 for 6 0. In the high 6 limit we calculate an average value of 0.012 0.003. Finally, in order to prevent significant photochemical degradation of the dye, experiments had to be performed either at low excitation power or for short irradiation times only. The photochemistry becomes a factor at surprisingly low excitation energies. As an example, Figure 5 shows the irreversible long-term decay of the fluorescence intensity of rhodamine B on glass (a) and on indium oxide (b), with continued irradiation at an excitation power -0.1 mW. (All other experiments were performed at much lower powers, for which no decay of the fluorescence was detected.) On glass the long-term fluorescence decay is slow and very nearly expontential; on the semiconductor the decay is biexponential, with a slow component identical within experimental error with that observed on glass and a fast component roughly 30 times faster. The contribution of the fast component fluctuated between 15% and 35% of the total initial fluorescence intensity, with no detectable dependence on 6.
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Discussion We assume four decay channels for the dye excited state (rate constants in parentheses): fluorescence ( k f ) ,internal conversion (k,,), injection (ki), and energy transfer quenching (k,).' The fluorescence lifetime is then 7
= ( k , + k,
+ ki + kJ-'
(2)
and the fluorescence quantum yield is
df = kf7
(3)
Since all quantum yield measurements were made relative to dilute ethanol solution, we write
&' = k f 7 / ( k r r S )
--
(4)
in which 7s = 2.7 ns and kf"are the solution lifetime5and radiative rate constant. We suppose that kf kf"and use the average high 6 values of #J{ in eq 4 to estimate 7 54 ps on glass and 7 32 ps on indium oxide. These numbers are in relatively good agreement with the lifetimes measured directly, all at high surface coverage: 7 = 46 ps on glass and 7 = 5 5 ps on indium oxide.4 The closeness of the two sets of values suggests that our calibration of the experimental quantum yields with respect to solution is
-
(7) The possibility that a heavy atom effect, caused by the tin-doped indium oxide, might promote intersystem crossing into the dye triplet state was considered by Arden and Fromherz (ref 9). These authors placed a dye monolayer on a LaF3 thin film, which should produce a heavy atom effect comparable to that of indium and tin, but found no evidence of enhanced intersystem crossing.
2454
Liang et al.
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
reasonably accurate. We note that direct measurement gave T unexpectedly shorter on glass than on the semiconductor. This is most likely due to differences in 8 since the dye optical density was not determined in those experiments. The quantum yields now indicate that, for the same surface coverage, T should be shorter on the semiconductor, as required by our model for the decay of the dye excited state. We now assume that k f and ko do not depend appreciably on the nature of the surface. We may then estimate ki by comparing 4; on the semiconductor and on glass at the same 8, Le., the same
k,:
(On the other hand, it may not be reasonable to assume that ko is the same in solution and on the surfaces since ko for rhodamine B decreases significantly with increasing solvent If the average high 8 values of c$[ are used in eq 5 we obtain
kj
-
1.2 x 10'0
s-1
This result is actually independent of the above assumption about kf and ko since at high 8 both are negligible in comparison with kq. For the same reason, the average high 8 value of &' on glass may be used alone, since ki = 0, to estimate kq:
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If we use the above k, value and set k, = 0, we get 4; 0.03 for 8 0, which is compatible with the low B data of Figure 4. This shows a reasonable internal consistency of the data obtained in the two 6' limits. The injection rate constant estimated from our data differs by less than a factor of two from those given by Arden and Fromherzg for electron injection by a cyanine dye into indium oxide and by Nakashima et al.1° for hole injection by rhodamine B into anthracene. Our results are compatible with the familiar model adopted by Gerischer and co-workers2," and 0thers~3'~ to account for low $I values: Injection of electrons into the conduction band of the semiconductor is extremely efficient (4, l), but low-lying surface states trap the injected electrons and prevent their escape into the bulk of the semiconductor. Although the fluorescence quantum yield is insensitive to the eventual fate of the injected electrons, Le., to &, it does provide a check on 4,. The value of k, estimated from our data supports the assumption that 4, 1, at least in the absence of energy transfer. The effect of energy transfer quenching on the injection efficiency is readily estimated. Since at high surface coverage kf and ko are much smaller than k, and k,, energy transfer quenching lowers 4, from 1 to
-
-
-
-k,/(k,
+ k,)
-
0.4
(7)
Therefore, even at high 8, used in most reported photocurrent measurements, 4, should not be too far from unity. The effect of energy transfer quenching is thus not expected to be alone responsible for 4 Spitler and CalvinI3 examined the possibility of an important role of energy transfer quenching but found no dependence of C$on surface coverage. Since they operated at high 8 this is consistent with our results, which indicate that k, does not change much at high 8. Efficient energy transfer quenching of the fluorescence has been reported for rhodamine B deposited on single crystals of condensed aromatic hydrocarbons1° and on ~i1ica.l~
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(8) T. Karstens and K. Kobs, J. Phys. Chem., 84, 1871 (1980). (9) W. Arden and P. Fromherz, J. EIecrrochem. Soc., 127, 370 (1980). (10) N. Nakashima, K. Yoshihara, and F. Willig, J. Chem. Phys., 73, 3553 (1980). (11) M. Spitler, M. Lubke, and H. Gerischer, Ber. Bunsenges. Phys. Chem., 83, 663 (1979). (12) M. Matsumura, Y. Nomura, and H. Tsubomura, Bull. Chem. SOC. Jpn., 52, 1559 (1979). (13) M . T. Spitler and M. Calvin, J . Chem. Phys., 66, 4294 (1977).
Bressel and G e r i s ~ h e rreported '~ very recently that 4 for a cell using a freshly prepared tin oxide electrode is initially -0.8 but that it drops gradually to over a long period of time. Bressel and Gerischer proposed that hydrolysis of the semiconductor surface leads to the formation of an insulating layer which hinders both injection and escape. However, significantly lower k, would require that &' be similar on the semiconductor and on the insulator. This is not supported by our results, particularly at low 8. Although our indium oxide surfaces were stored in air for many months and repeatedly cleaned and immersed in dye solutions, we have found no evidence of insulator-like behavior of the surface.I6 Bressel and Gerischerls suggests that injection may also take place into low-lying states in the insulating layer. In our experiments, however, such injection would not be distinguishable from direct injection into the conduction band of the semiconductor. We have so far implicitly assumed a uniform molecular distribution on the surface. However, a range of degrees of aggregation of the deposited molecules may exist in a given sample. Thus, we might reasonably expect molecules to be mostly isolated at low 0 and mostly in extended aggregates at high 8. The fact that on glass &' approached unity at low 8 indicates a relatively unimportant role of energy transfer quenching and, therefore, no significant aggregation. At high 8, on the other hand, some molecules may remain isolated. However, as we saw earlier, $4 values at both low and high 8 are consistent with a single k,, which suggests that this kind of problem is not severe. We now turn our attention to the long-term fluorescence decays in Figure 5 . The fast component of the decay in Figure 5b must be due to some chemical process specific to the semiconductor, presumably one which involves the oxidized dye.17 Because the slow decay rate is virtually identical with that observed on glass, we suppose first that it arises from molecules which cannot inject. Since the slow component makes a large contribution to the total fluorescence intensity, and we attribute it to molecules which do not inject, we expect then on the semiconductor to be a similarly large fraction of the glass value. The low 8 data in Figure 4 clearly show that this is not the case, which forces us to conclude that the great majority of the molecules deposited on the semiconductor (including those which contribute to the slow component) are capable of injection. This conclusion is strengthened by the fact that the contribution of the fast component does not increase when 8 0, as would be expected of molecules which do not inject. In terms of Gerischer's early model' we then suppose that, although essentially all molecules can inject, rapid recombination of the injected electron with the oxidized dye occurs in the vicinity of surface traps. Recombination suppresses the reaction channel through the oxidized dye and leaves only (as on glass) the chemistry from the dye excited state. Finally, we note that a significant contribution from molecules which do not inject in the vicinity of traps is not incompatible with low 4 since electrons injected under these more favorable circumstances may still be trapped elsewhere near the surface. One difficulty still remains with our proposed interpretation of the long-term decay of the fluorescence on the semiconductor. Although we have reconciled the existence of two very different photochemical decay rates with the conclusion that substantially all molecules are able to inject, we have not accounted for the coincidence (presumably not fortuitous) of the slow decay rates on glass and on the semiconductor. The difficulty arises since, if the slower chemistry takes place in the same dye excited state which injects the electron, access to the injection channel should
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(14) S. Garoff, R. B. Stephens, C. D. Hason, and G. K. Sorenson, Opt. Commun., 41, 257 (1982). (15) B. Bressel and H . Gerischer, Ber. Bunsenges. Phys. Chem., 87, 398 (1983). (16) This is in agreement with a recent private communication from P. Fromherz and W. Arden, who found only slight aging effects on photocurrent efficiency for indium oxide electrodes in air and no changes in aqueous phase. Observation of large aging effects with indium oxide was erroneously attributed to Arden in ref 15. (17) T. Watanabe, T. Takizawa, and K. Honda, Ber. Bunsenges. Phys. Chem., 85, 430 (1981).
J. Phys. Chem. 1984, 88, 2455-2459 make the reaction much less efficient on the semiconductor than on glass. This is resolved if we assume that the photochemistry takes place in unrelaxed excited states and that injection only occurs subsequently, from a relaxed excited state. Interestingly, the dye fluorescence intensity has been found to decrease when an appropriate bias potential is applied to the semiconductor electrode.lsJ9 This has been attributed to an enhancement of the escape of the injected electrons into the bulk of the semiconductor and, therefore, to a decrease in the population of the unoxidized (fluorescent) dye on the surface. The results of Iwasaki et a1.18 are puzzling, however, in view of our observations about the fluorescence of deposited dye molecules. These authors reported 7 = 6.9 ns for sodium fluorescein at the electrode-solution interface, a lifetime which is similar to the solution value.20 This means that the fluorescence monitored in these (18) T. Iwasaki, T. Sawada, H. Kumudu, A. Fujishima, and K. Honda, J . Phys. Chem., 83, 2142 (1979). (19) J. S. Pflug, L. R. Faulkner, and W. R. Seitz, J. Am. Chem. SOC.105, 4890 (1983).
2455
experiments comes from molecules whose excited state can neither inject nor be quenched by energy transfer but which, somehow, respond to the applied bias voltage. In conclusion, we have estimated the injection and energy transfer quenching rate constants for rhodamine B deposited on indium oxide. No indication was found of an insulating layer on the aged indium oxide surfaces used in our experiments. From the long-term fluorescence decays we concluded that most molecules deposited on indium oxide can inject, but that some do so near surface traps at which fast recombination takes place.
Acknowledgment. This research was supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC02-81ER10881. Support by Temple University under a Grant-in-Aid for Research is also acknowledged. Registry No. In203, 1312-43-2; Rhodamine B, 81-88-9. (20) M. M. Martin, Chem. Phys. Left.,35, 105 (1975).
Chemiluminescence and Laser Fluorescence Study of Several Mg(1S,3P0)Oxidation Reactions: On the MgO Dissociation Energy John W. Cox?and Paul J. Dagdigian* Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218 (Received: November 14, 1983)
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Chemiluminescence and laser fluorescence studies have been carried out on the reactions of Mg( 1S,3P0)with NO2, 03,C02, and SG,. For Mg(3F"')+ NO2,continuum emission was observed and ascribed to an E E energy transfer forming excited N02(A2B2).Weak MgO chemiluminescence in the arc bands and B-X system was seen in M ~ ( ~ p + 0 )O3 ; the photon yield was determined to be -0.003%. Ground-state MgO(X'Z+) product was detected by laser fluorescence in the Mg(3Po)+ C02 and SO2reactions and in Mg('S) + NO2and N20. These reactions were used to set lower bounds to the MgO dissociation energy, for which there is a disagreement between theoretical and experimental determinations. The highest lower bound of Doo(MgO) Z 3.1 eV was obtained from the Mg('S) + NO2 reaction. This result is consistent with a recent ab initio value of Doo(MgO) = 2.65 & 0.16 eV if this reaction has a translational energy barrier. Such a barrier is likely in view of the small MgO signals seen in the present experiments.
Introduction Reactions of Mg with simple oxidants have aroused interest in the past few years.'-" From a fundamental viewpoint, new experimental techniques and theoretical advances have allowed detailed study of the dynamics of reactions involving multiple potential energy surfaces. Study of Mg reactions is especially attractive because it is one of the simplest metal atoms for which experimental studies are convenient and a reasonable number of product oxide electronic states are energetically accessible. From a practical point of view, such reactions are of interest because of the possibility of creating chemically large concentrations of electronically excited atoms, as in Mg/N,O/CO flames.' Unfortunately, the MgO dissociation energy has thus far eluded definitive determination. In a review of earlier experimental work, the 1978 supplement to the JANAF tabled8 recommended a value of 3.47 A 0.26 eV. In a very recent reanalysis of all available experimental data, Pedley and M a r ~ h a l lderive '~ a slightly revised value of 3.72 f 0.13 eV. By contrast, a recent extensive ab initio calculationm disagrees with the available experimental results and yields a value of 2.65 h 0.16 eV for the MgO dissociation energy. The wave functions in this calculation are expected to be of high quality. Indeed, a similar calculation2' for the CaO dissociation +Present address: Chemistry Division, Code 61 10, Naval Research Laboratory, Washington, D.C. 20375.
0022-3654/84/2088-2455$01.50/0
energy yields a result in agreement with the accepted experimental va1~e.l~ (1) D. J. Benard, W. D. Slafer, and J. Hecht, J . Chem. Phys., 66, 1012 (1977); D. J. Benard and W. D. Slafer, ibid., 66, 1017 (1977). (2) G. Taieb and H. P. Broida, J . Chem. Phys., 65, 2914 (1976). (3) R. P. Blickensderfer, W. H. Breckenridge; and D. S. Moore, J . Chem. Phys., 63, 3681 (1975). (4) F. Engelke, R. K. Sander, and R. N. Zare, J. Chem. Phys., 65, 1146
, - ~-.,.. 1197h\
(5) R. J. Malins and D. J. Benard, Chem. Phys. Left., 74, 321 (1980). (6) W. H. Breckenridge and W. L. Nikolai, J . Chem. Phys., 73, 2763 (1980); W. H. Breckenridge and H. Umernoto, ibid., 77, 4469 (1982). (7) W. H. Breckenridge and H. Umemoto, J. Chem. Phys., 75, 4153 (1981). (8) W. H. Breckenridge and H. Umemoto, J. Chem. Phys., 75,698 (1981). (9) A. Kowalski and J. Heldt, Chem. Phys. Lett., 54, 240 (1978). (10) A. Kowalski and M. Menzinger, Chem. Phys. Lett., 78,461 (1981); J . Chem. Phys., 78, 5612 (1983). (11) P. J. Dagdigian, J . Chem. Phys., 76, 5375 (1982). (12) J. W. Cox and P. J. Dagdigian, J . Phys. Chem., 86, 3738 (1982). (13) B. Bourguignon, J. Rostas, and G. Taieb, :b, J. Chem. Phys., 77, 2979 (\____,. I 9117) (14) W. H. Breckenridge and H. Umemoto, J. Phys. Chem., 87,476, 1804 (1983). (15) H. H. Michels and R. A. Meinzer, Chem. Phys. Lett., 98, 6 (1983). (16) D. R. Yarkony, J . Chem. Phys. 78, 6763 (1983). (17) N. Adams, W. H. Breckenridge,and J. Simons, Chem. Phys., 56,327 (1981).
0 1984 American Chemical Society
