J . Phys. Chem. 1990, 94, 2399-2404
2399
Photochemlcal Decomposition of RuO, George L. Zimmerman,* Sylvia J. Riviello,t Todd A. Glauser, Department of Chemistry, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010
and Jack G. Kay Department of Chemistry, Drexel University, Philadelphia, Pennsylvania I9104 (Received: May 23, 1989; In Final Form: August 31, 1989)
For the first time, the photochemical decomposition of gaseous RuO, as a function of wavelength has been studied. Two types of studies were employed: (a) irradiation with a constant-intensity mercury arc, isolating lines with filters, and (b) flash photolysis and kinetic absorption spectroscopy using a xenon flash lamp and liquid solution filters. The steady mercury arc irradiation gave quantum yields as a function of wavelength, and the flash photolysis experiments gave spectra of previously unreported products. Photochemical reactions of RuO, have been determined for three spectral regions: (I) 440-370, (11) 370-320, and (111) 320-240 nm. For (I) the product is a solid, thin film of Ru03 deposited on the cell wall, and the quantum yield is 0.05; for (11) the product is a solid Ru02 aerosol with submicron-sized particles, and the quantum yields are 1.O-1.2; for (111) both Ru03 and Ru02 are formed simultaneously in the forms described above, and in addition, on a microsecond time scale, absorption spectra of gaseous RuO and Ru are observed. These results are interpreted in terms of two thresholds for predissociation or, more likely, vibrationally hot molecule dissociation, one at -370 nm for dissociation to O2 and Ru02 and another at -320 nm for breaking a single Ru-O bond. The observed threshold energies agree well with thermodynamic estimates. The production of RuO and Ru species is attributed to secondary dark reactions involving 0 atoms.
I. Introduction R u 0 4 is a volatile solid which melts at 25.4 O C 1 + and which is thermodynamically unstable with respect to Ru02(g) and 02(g) with = -104 f 6 kJ/mol.'V2 It has been known qualitatively for at least a ~ e n t u r y lthat, ~ ~ , at ~ room temperature, gaseous RuOl decomposes slowly (weeks) in the dark and somewhat faster in the daylight. However, except for a few such qualitative observations, neither the thermal nor the photochemical decomposition of R u 0 4 has been studied at all, previously. Since the molecular structure and the vibrational and rotational spectroscopic parameters of the R u 0 4 molecule are ~ e l l - k n o w n ~ ~ and since the overlapping electronic absorption bands extend from 440 nm to well into the vacuum-UV region, it is of interest to study the photochemical behavior quantitatively. For this quite simple molecule, not only can the excitation energy be varied over a wide range but the photolysis can be studied in a low-pressure gas or gas mixture or in an aqueous or nonaqueous solution. The present work concentrates on understanding the basic photochemical processes in the gas phase. Unfortunately, the electronic absorption spectrum of the gas at low p r e s s ~ r e ~has . ' ~only ~ ~unresolvable broad diffuse features (see Figure 1) which preclude any careful spectroscopic analysis. Nevertheless, in the region between 200 and 440 nm, one can identify three (or possibly four) electronic bands,'* for which the estimated short-wavenumber limit (origin), wavenumber for band maximum, long-wavenumber limit, and oscillator strength (this last from ref 4) are listed in Table I. Vacuum-UV4 and He I photoelectron spectral"" also have been reported. For Ru1604, each of the bands A and B shows two diffuse vibrational progressions with a spacing of about 760 cm-' probably corresponding to the upper electronic state vI(A1)vibration; for the ground state, v I = 882 cm-I. Starting in 195218 a number of theoretical treatments of molecules isoelectronic with MnO, and including RuO, and OsO, have appeared (e.g., refs 4 and 18-21), and there is reasonable agreement (see ref 1 ) with the experimental ionization energies and upon the order of the molecular orbitals. The transitions for A and B are most probably explained in terms of electron promotions, t, -,2e and 3t2 2e, and both have A, ground and T, and T2 excited states. Both A and B bands have been interpreted as allowed transitions to the T2 states4~l2 despite the somewhat
-
*To whom correspond1ence should be addressed. Current address: GE Astrospace Division, 230 Goddard Blvd , King of Prussia, PA 19406
0022-3654/90/2094-2399$02.50/0
TABLE I band A B C
wavenumber, cm-' lower limit maximum upper limit 235 280 440 *5
260 325 510 f2
300 380 580 f5
osc strength 0.020 0.044 0.018
-
small oscillator strengths. For band C, three possibilities have been proposed: Itl 4t2, 2al -,2e, and l e -,2e. The less certain interpretation of band C and the probability of a weak vibronically allowed band lying between B and C are discussed in some detail in ref 4. N o emission spectrum has been observed for RuO,. We have searched for visible fluorescence on excitation of the A band (see Experimental Procedure) and estimated a rough upper limit of for the fluorescence yield using a 10-100-ns time range. This limit combined with the oscillator strength of the band predicts an average lifetime of the excited singlet state of less than (1) Sneddon, E. A.; Sneddon, K. R. In Topics in Inorganic and General Chemistry; Monograph 19; Elsevier: New York, 1984. (2) Rard, J. A. Chem. Reu. 1985, 85, 1. (3) DeBray, H.; Joly, A. C.R . Hebd. Seances Acad. Sci. 1888, 106, 328. (4) Roebber, J. L.; Wiener, R. N.; Russell, C. A. J . Chem. Phys. 1974, 60, 3166. (5) Nikol'skii, A. B. Russ. J . Inorg. Chem. (Engl. Transl.) 1963,8, 541. (6) Nikol'skii, A. B. Russ.J. Inorg. Chem. (Engl. Transl.) 1965, 10, 152. (7) Schafer, L.; Seip, H. M. Acta Chem. Scand. 1967, 21, 737. (8) McDowell, R. S.; Asprey, L. B.; Hoskins, L. C. J . Chem. Phys. 1972, 56, 5712. (9) Levin, I. W.; Abramowitz, S. J . Chem. Phys. 1969, 50, 4860. (10) Connick, R. E.; Hurley, C. R. J . Am. Chem. SOC.1952, 74, 5012. (11) Ortner, M. H. J . Chem. Phys. 1961, 34, 556. (12) Dunn, T. Private communication. See also: Wells, E. J.; Jordan, A. D.; Alderice, D. S.; Ross, 1. G. Ausr. J. Chem. 1967, 20, 2315. (13) Foster, S.; Felps, S.;Johnson, L. W.; Larson, D. B.; McGlynn, S. P. J . Am. Chem. SOC.1973, 95, 898. (14) Diemann, E.; Muller, A. Chem. Phys. Lett. 1973, 19, 538. (15) Foster, S.; Felps, S.; Cusachs, L. C.; McGlynn, S. P. J . Am. Chem. SOC.1973, 95, 5521. (16) Burroughs, P.; Evans, S.; Hammett, A,; Orchard, A. F.; Richards, N. V. J. Chem. SOC.,Faraday Trans. 2 1974, 70, 1895. (17) Evans, S.;Hammett, A,; Orchard, A. F. J. Am. Chem. SOC.1974,96, 6221. (18) Wolfsberg, M.; Helmholz, L. J . Chem. Phys. 1952, 20, 837. (19) Ballhausen, C. J.; Liehr, A. D. J . Mol. Spectrosc. 1958, 2, 342. (20) Viste, A,; Gray, H. Inorg. Chem. 1952, 3, 837. (21) Rauk, A,; Ziegler, T.; Ellis, D. E. Theor. Chim. Acta 1974, 34, 49.
0 1990 American Chemical Society
2400 The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 TABLE 11: Thermodynamic Dataa AHro(SS)
O(g)
RWg)
RuO,(g)
RuOAs) RuOdg) R’J04(k?)C
249.1 640 f 4 (650 f 13)b 376 f 4 (372 f 42) 140 f 4 (-119) -301 f 5 (-307.2 f 7.8) -58 f 4 (-70.8 f 6.6) -188 f 4 (-192.7 f 4.0)
AGro(ss) 0 0 23 1.7 594 f 5 (602 f 16) 343 f 4 (339 f 45) 128 f 4 (- 1 IO) -248 f 4 (-253.1 f 8.2) -46 f 5 (-55.8 f 9.1) -144 f 4 (148.5 f 4.1)
Zimmerman et al.
205.1 28.5 (28.6 1) 161.0 186.4 ( I 86.4) 241 f 4 (- 242) 275 f 3 (265 f 5) 54 f 3 (52.2 f 8.7) 287 f 8 (285.8 f 8.4) 289.1 f 1.3 (290.6 f .6)
40.5 33.3
-498.3 -640 f 4 (-650 f 13) 0 0
25.3 30.5
-518
42.9
- 1000
64.9 f 5 87.9 f 5 92.0 f 3
-1446
118.6 f .5
-1828 f 4
“All values are in kJ at 298.2 K except So,which is in J/K; ss represents standard states of the elements. bValues in parentheses are from ref 2. First three values are from ref 6.
11
’
between 0 OC and the melting point was measured by Nikol’ski? and is given by the relation In P (Torr) = -6590/T 24.57
+
Our attempts to check this sublimation pressure are described in the experimental section (vide infra) where we conclude that the above data of Nikol’skii are preferable. 11. Experimental Procedure
Preparation of RuO,. RuO, was prepared according to either (a) Berkowitz and Rylander25or (b) Connick and Hurley,Io in both cases by oxidizing commercial ruthenium chloride hydrate 00 obtained from Alpha Chemical Co. and Englehard Industries. I I I I 280 290 300 310 320 330 340 This latter material is of high purity with respect to metallic W a v e l e n g t h lnml impurities but is a mixture of chlorides and oxychlorides of both Figure 1. Absorption spectrum of RuO,(g) for band B with maximum Ru(II1) and Ru(IV) containing varying amounts of water. An at 306 nm (3.26 X IO4 cm-’). See also refs I O and 12. acidic aqueous solution is oxidized to R u 0 4 by Br03-2s or can be converted to a solution of ruthenium(1V) sulfate by precipitating 1 ps and suggests predissociation or internal conversion to the the hydrated oxides with NaOH, washing and centrifuging, and ground state as the principal source of the broadening seen in the redissolving in dilute H2S04. The sulfate solution is then oxidized absorption spectrum. Because of a lack of agreement on the molar to R u 0 4 by Mn0[.l0 In our procedure, the RuO, is distilled in extinction coefficient (e) of gaseous Ru04, we have redetermined a stream of He and condensed as a solid at 0 OC. The excess water the value at 306.4 nm (vide infra). is removed, and the R u 0 4 is dried by passing through a column The thermodynamic properties of R u 0 4 in various states and containing anhydrous Mg(C10,)Z and then condensed in a trap of solid R u 0 2 are known with fair accuracy.’*z For gaseous RuO, at -80 O C , evacuated, sealed off, and stored at -80 OC. The RuO,, and Ru03, RardZ has made estimates of AHozsSof forpermanganate oxidation, though more lengthy, avoids the posmation and based on extrapolations from high-temperature sibility of Br2 being formed and contaminating the Ru04. After experiments22and statistical mechanical calculations. We also preparation, all handling of R u 0 4 was carried out by using have made independent estimates of the same quantities based high-vacuum techniques and, to prevent photolysis, in red light. on some additional knowledge of structures and vibrational frequencies obtained from low-temperature matrix s t u d i e ~and ~ ~ , ~ ~ Determination of Absolute Absorbance and Sublimation P r e s s ~ r e . ~RuO, ~ . ~ was ~ distilled, with some fractionation, into using the same thermochemical data22plus some empirical esampules which could be sealed off, weighed before and after filling, timates based on data for molecules with similar structures. Our and later broken inside of an evacuated vessel of known volume values and Rard’s are shown in Table I1 and agree well. Assuming (- 1 L) and having an extension ending in a 1-cm path length that the thermochemical dataZ2are correct, the uncertainties in optical absorption cell made from precision bore, Suprasil quartz our calculated values arise mainly from uncertainties in bending square tubing, obtained from Englehard-Amersil. The UV-visible vibrational frequencies, ground-state identifications, and, to a lesser absorption spectrum was taken on a Model 14 Cary spectroextent, bond lengths and extrapolation of AH‘S from 1500 to 298 photometer. Six independent determinations were made; the result K. The AH values for 1500 K are obtained from temperature is that the maximum of band B is found to be 2660 f 40 L/(mol dependence of high-temperature equilibrium constants by using cm). The sublimation pressure was directly measured between Knudsen effusion cells and monitoring the effusate mass spec0 and 25 O C with a Wallace and Tiernan stainless steel bourdon trometrically. Although absolute equilibrium constants may be gauge, which had just been recalibrated by the company and also inaccurate for mass spectrometric reasons, the AH values which had been pretreated with RuO,. The R u 0 4 was repeatedly depend only on ratios of ks appear to be more reliable. Bond fractionated until reproducible pressure readings (to 10.02 Torr) energies thus obtained are consistent with the average bond energy were obtained. To check possible decomposition, the zero reading obtained from gaseous RuO, data alone. The sublimation pressure was repeatedly checked by cooling the sample container with liquid nitrogen. Our values were 8-16% higher than Nikol’skiis found. I
I
(22) Norman, J H , Staley, H G , Bell, W E Adu Chem Ser 1968, No
-. in1 -
72
(23) Kay, J. G.; Green, D. W.; Duca, K.; Zimmerman, G. L. J. Mol. Spectrosc. 1989, 138, 49. (24) Green, D.W.; Kay, J. G.; Zimmerman, G. L.; Balko, B. J . Mol. Spec!rosc. 1989, 138, 62.
(25) Berkowitz, L. M.; Rylander, P. N. J . Am. Chem. SOC.1958,80,6683. (26) Thompson, A. M. Ph.D. Thesis, Bryn Mawr College, 1978. (27) Thompson, A. M.; Goswami, P. C.; Zimmerman, G. L. J . Phys. Chem. 1979, 83, 314.
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2401
Photochemical Decomposition of R u 0 4 TABLE 111 A, nm
1 2 3 4 5 6 7 8 9 IO
438.5 405.0 365.0 365.0" 365.0b 365.0b 365.0b 313.0 313.0" 253.7
added substances
Ar (10 Torr) CC14 ( I O Torr) H20(liquid soh)
Light intensity attenuated by a factor of 30.
Io,einstein/min 3.61 x 10-7 2.63 x 10-7 6.23 x (6.27 X 5.37 x 5.49 x 6.71 x 1.61 x (2.07 X 6.39 x
10-7 10-7)/30 10-7 10-7 10-7 10-7 10-7)/30 10-7
--
R u 0 3 t o.502 O.7RUO3 t O.3RUO2 R u 0 2 + O2 R u 0 2 + O2
--
R u 0 2 + O2 R u 0 2 + O2 0.2Ru0, + 0.8Ru02 + 0 . 9 0 2
overall decomDosition reaction 0.048 f 0.005 0.048 f 0.005 1.28 f 0.10 1.12 f 0.21 1.05 0.73 0.14 1.43 f 0.12 1.11 i 0.14 1.15 f 0.16
RuO, RuO, Ru04 Ru04 C C
c
Ru04 Ru04 Ru04
+
+ 0.6502
Single experiment. Not determined, but presumably R u 0 2 only.
We prefer the vapor pressure data of Nikol'skii since the extinction coefficient values calculated from Nikol'skii's data agree with our independent determination described above. Search f o r Fluorescence. Samples of RuO, sealed in a conventional fluorescence cell of IO-cm path length and at pressures between 1 and 5 Torr were irradiated by a commercial tunable dye laser (Molectron DL200) pumped by a variable-frequency pulsed N 2 laser (Molectron UV-12) and detected by using a variable time delay gated integrator (PAR, Model 165) of 1050-11s width and a photomultiplier. The possible emitted radiation was first dispersed with a McPherson (Model 216, 0.3 m) monochromator in order to obtain a spectrum. The dye laser was tuned to a wavelength of 406 nm, which falls near the low-energy end of band A, and the entire remaining part of the visible range was scanned. To make a rcugh estimate of the fluorescence yield, the fluorescence spectra of a number of ethanol solutions of varying concentrations (down to lo-' M) of the dye Coumarin 6B were recorded. The dye absorbs over a wavelength range similar to that of band A and has a known fluorescence yield. Since no fluorescence was observed for Ru04, one could find the concentration of the dye where detection failed and estimate an upper limit for the RuO, fluorescence yield of approximately Quantum Yields. RuO, samples at 6-8 Torr pressure with no other gas present, except for two runs with added argon or carbon tetrachloride gas, were irradiated with a conventional mediumpressure quartz mercury arc lamp with appropriate collimating lenses and filters to isolate the prominent mercury lines. The wavelengths used are shown in Table 111. The disappearance of RuO, was monitored, after each irradiation, for successive time intervals, by measuring the absorbance with a Cary Model 14 spectrophotometer at 306.4 nm, the wavelength of maximum absorbance for band B; the measured absorbance was corrected at each point for the changing base line, the changes being caused by the deposition of solid RuOl or R u 0 3 on the walls of the absorption cell as the photochemical reaction proceeded. A run was stopped after five or six time intervals and when about 10% of the RuO, had decomposed. The whole of band B was recorded each time. The cell was made from precision bore square Suprasil quartz tubing with I-cm inside dimensions and sealed to a Pyrex tube for evacuating and filling. The amount of RuO, in the cell was determined by its known vapor pressure in a storage vessel at a fixed temperature and by its absorbance; the RuO, was condensed at liquid N 2 temperature in the cell, and the cell sealed off at a pressure of approximately IO" Torr. A small annular cup, ring-sealed to the Pyrex stem, could be filled with liquid N2 while the cell was in the spectrophotometer, thereby condensing the RuO, as a solid and allowing a measurement of the base line after each irradiation. When the liquid N2was removed and rmm temperature reestablished, the spectrum of band B was rerun and was, in all cases, within the uncertainty of the instrument, the same as before condensing. Thus, no detectable errors due to lack of homogeneity were found for the Ru04-02 gas mixtures or with added inert gases. Also attached to the cell was a Pyrex break-seal which, at the end of a run, could be broken to determine the pressure of the O2 gas formed, thus giving the stoichiometry of the net reaction. The radiation intensity was measured with an azobenzene a c t i n ~ m e t e r ~ ~using - ~ O an identical absorption cell with magnetic
stirring. The actinometer and RuO, cell were alternately irradiated and monitored to check the constancy of the light source. Also, the possibility of thermal decomposition occurring along with the photochemistry was periodically checked and found to be negligible over the time range of the experiments. Flash Heating and Photolysis. The apparatus and experimental procedure for flash heating and kinetic absorption spectroscopy were employed, with modifications, as has been previously de~ c r i b e d . ~ The ' , ~ ~spectroscopic light source was a small, xenonfilled flash lamp (EG&G FX-100) operated at 100 J per flash. The optical path included two quartz lenses mounted outside the ends of the reaction cell. The spectroscopic light source was at the focus of the first lens so that light from it traveled through the interior of the cell and, after exiting from the other end, was focused by the second lens onto the entrance slit of the 1.5-m Wadsworth spectrograph. A cylindrical cover lens was used on the entrance slit, and a Hartman slide, with a fishtail opening, was used to limit the height of the 40-pm slit. For calibration, Hg emission lines were superimposed on the edge of each spectrum. A Jarrel-Ash step filter was used to calibrate the response of the photographic emulsion. Spectra were recorded on Kodak Tri-X film, chosen for this purpose because of its balance between high speed and minimum grain size. For flash photolysis, the same spectroscopic apparatus was employed except for the substitution of a straight, xenon-filled flash tube (EG&G FX-47, or equivalent) in place of the helical lamp generally employed as the preparative flash lamp for flash heating. In the photolysis experiments, the cell, which was 54 cm in length, was placed alongside the preparative flash tube inside a polished aluminum reflector. The reflector was in the shape of a double elliptical cylinder with the cell centered along one focal axis and the flash tube centered along the other. For experiments in which the preparative flash was filtered, the cell was manufactured from fused silica with an outer fused silica jacket which could be filled with the desired filter solution. For photolysis, the cells were filled with 0.02-1 Torr of R u 0 4 alone or with 10 Torr of argon added. After filling, the concentration of RuO, was checked by measuring the optical absorption across a diameter of the cylindrical cell (3-cm path). Various thicknesses of Pyrex or solutions33in the quartz filter cell were used to vary the irradiating wavelength range. For flash heating, 0.1-0.2 g of solid R u 0 2 was spread on an open cylindrical Pyrex liner inside a quartz cell and 10 Torr of Ar was added. Under vacuum or inert gas, R u 0 2 decomposes to the elements under flash heating conditions but does not melt; this behavior does not necessarily imply the absence of transient gaseous oxide species. Both first- and second-order spectra were taken by using appropriate filters to isolate the desired order. Reciprocal linear (28) Riviello, S. J . Ph.D. Thesis, Bryn Mawr College, 1985. (29) Glauser, T. A. Masters Thesis, Bryn Mawr College, 1987. (30) Zimmerman, G. L.; Paik, U.-J.; Chow, L.-Y. J . Am. Chem. SOC. 1958, 80, 3528. (31) Diebner, R. L.; Kay, J. G. J . Chem. Phys. 1969, 5 1 , 3547. (32) Hopkins, M. W.; Antal, M. J.; Kay, J. G . J . Appl. P d y m . Sci. 1984, 29, 2163 (33) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York,
1966.
2402 The Journal of Physical Chemistry, Vol. 94, NO. 6,I990
r
o!o
Zimmerman et al.
I
0!5
110
1‘5
2!0
Time [mini
Figure 2. Plot of RuO,(g) absorbance, A , as a function of time of irradiation.
dispersion in the first order was approximately 10.5 A/”. For each experiment, a spectrum (“before”) was taken of the cell contents without the preparative flash, then a kinetic spectrum (“during”) was obtained with the spectroscopic flash timed to coincide with or come somewhat after the maximum intensity of the preparative flash, and the final spectrum (“after”) was obtained of the cell contents less than a minute after the preparative flash. The spectra were measured with a manually operated Gaertner optical comparator. Densitometer traces were obtained with a Joyce-Loebl automatic recording densitometer. Wavelength calibrations were based on observed emission lines in the reference Hg spectra as well as absorption lines of ruthenium observed internally as described below. Chemical purity of the materials could readily be ascertained because of the high sensitivity of absorption spectroscopy for detecting trace elemental impurities in the optical path. In these experiments, the only elemental spectra observed in absorption were from ruthenium with sodium appearing occasionally. Very broad emission lines from silicon were observed and characterized as originating from the sides of the flash tubes, indicating some deterioration of the quartz as a result of interactions with the hot discharge plasma during each flash. Because of the light sensitivity of ruthenium tetraoxide, care had to be taken to protect the cells from light until the flash lamp was triggered. 111. Results Quantum Yields. A plot of RuO, absorbance, A , vs time for a sample run is shown in Figure 2. Although the point-by-point correction for the base line gave reproducible results for the RuO, absorbance, the deposition of lower oxides on the wall and thus the changing base line absorbance were not as reproducible from one run to another; also, only the sum of the absorbances due to deposition on both front and back windows could be measured. For these reasons the initial slope, (dA/dt),,,, was calculated from the data and the quantum yield 4, in turn, calculated from the equation
where u and I are the volume and optical path length for the cell, t is the molar extinction coefficient, A, is the absorbance at zero time, and Io is the light intensity in einstein/min (see Table 111). The initial slope was the slope at t = 0 of the least-squares quadratic function fit to the data points. The initial slopes were not sensitive to whether 4, 5, or 6 points were used in the calculation. Table I l l shows the quantum yields and reaction products for the various wavelengths and intensities. Except when noted, the results and uncertainties given are for at least five duplicate runs.
300
400 500 Wavelength lnml
$00
I
100
I
Figure 3. Absorption spectrum of thin film of Ru03(s) deposited on quartz (Suprasil).
The reaction stoichiometries are calculated from total moles of R u 0 4 decomposed and total moles of 0, produced. In addition, for 3, 4, 8, and 9 the product (RuO,) was produced mainly as a black aerosol which in several minutes settled to the bottom of the cell. This settling could be seen as a drift in the base line at 550 nm (where there is no RuO, absorption) right after stopping irradiation. No absorption spectra were measured until after settling was complete. The order of magnitude “particle diameter”, estimated by using Stokes’ law, is 200-600 nm. For 1 and 2, no aerosol was seen, but a brownish smooth continuous deposit built up gradually on all four cell walls as the photolysis progressed. The deposit, which had a very broad band absorption spectrum (shown in Figure 3), was identified (as described in section 11) as a thin layer of solid Ru03. Confirmation of this came from the absorption spectrum of the aqueous solution formed when, after a run, the remaining RuO, was removed and 2 mL of 1 M NaOH was injected into the cell. The spectrum of the resulting solution agreed well with the published spectrumla of aqueous alkaline Ru04,- (ruthenate ion), R u 0 3 being the anhydride of ruthenic acid. In 2 and 10, the same thin-film absorption spectrum was seen and, in addition, black aerosol particles in 10. It seems clear (Table 111) that in 10 and in 2 both RuO, and RuO, were generated simultaneously. Flash Photolysis and Flash Heating. In each of the flash photolysis experiments, all of the RuO, was decomposed, as should have been the case given the total power of the flash and its energy distribution and as confirmed by the disappearance of the RuO, absorption spectrum which could be seen before the main flash. Most of the absorbed light lies in the wavelength region for which solid RuO, is the product. RuO, formed mostly as an aerosol which settles out in minutes and partly as a direct deposit on the walls. Only in the presence of added argon and when wavelengths shorter than 310 & 10 nm were allowed (the filters did not have sharp cutoffs, hence the uncertainty) were any kinetic absorption spectra (of transient species) seen. Carriers of two of the spectra were easily identified as gaseous Ru atoms and RuO molecules (not previously reported in absorption). About 500 Ru lines and their corresponding lower and upper states have been identified in these absorption ~pectra.’~.’~ (34) Meggers, W. F.; Corliss, C. H.; Scribner, B. F. NBS Monogr. ( U S ) 1961, No. 32 (Part 1). ( 3 5 ) Corliss, C. H.; Bozman, W. R.NBS Monogr. (U.S.) 1962, No. 53.
Photochemical Decomposition of RuO, From the distribution of lower states combined with oscillator strength data,j5 one can obtain a rough electronic temperature estimate of about 3000 K for the Ru atoms. For RuO, we observed band systems lying between 490 and 6 I O nm, the band heads of which correspond to those observed in emission and reported by Scullman and T h e l i ~ ~ . ’Based ~ on the Scullman and Thelin interpretation, some of our observed absorption bands are hot bands arising from the first excited vibrational state (approximately 1200 cm-I). The resolution in our RuO spectra is not yet sufficient to allow a more detailed analysis, but we plan to do further experiments toward this end in the near future. In addition to the Ru and RuO spectra, there was seen a broad continuous absorption ranging from 270 to 240 nm and perhaps to shorter wavelengths. (The spectroscopic source intensity was rapidly decreasing with shorter wavelengths, and the filter used to isolate the second order was starting to cut off below 240 nm.) This continuous absorption is presumably due to gaseous R u 0 2 and/or RuO,, but one cannot make an identification from these observations alone. For two of the flash heating experiments, one can see absorption spectra of RuO molecules and Ru atoms as in flash photolysis, but much weaker.
IV. Discussion The appearance of the RuO, electronic absorption bands and the lack of observable fluorescence suggest either predissociation or rapid internal radiationless conversion to a vibrationally excited ground state. In addition, the lowering of the quantum yield by added inert gases and in aqueous solutions suggests the latter case which we shall assume henceforth. Assuming a collision diameter of 0.6 nm for R u 0 4 gives, for a pressure of 10 Torr, a mean time between collisions of about s. In all cases, if the excitation energy exceeds the energy for some mode of dissociation, we see that within s the dissociation occurs giving a quantum yield, 4 (for R u 0 4 disappearance), close to I . It is convenient, however, to consider three cases: I, X = 440-367 nm; 11, X = 367-313 nm; and 111, X = 313-e250 nm. Figure 4 is an energy level diagram showing the various dissociation levels and their uncertainties calculated from the data in Table 11. The three wavelength subdivisions were chosen to correspond to the dissociation thresholds shown and are seen to have no significant correspondence with the absorption bands A and B of Table I. The uncertainties in the calculated thermodynamic quantities are shown. The three cases will be discussed in turn. Case I . There is no photodissociation energetically allowed. However, we find that RuO, is formed photochemically with = 0.048. Consider the case for X = 438.5 nm for which the product is entirely Ru03(s). The only energetically possible elementary process seems to be 2RuO,(g) 2Ru0, 02(g)
-
+
where the Ru0, might be either gaseous (AGO298 = +197, 1 w 0 2 9 8 = +264 kJ/mol) or solid. for the production of solid R u 0 , and O2is Although possibly negative, this reaction cannot be observed in the dark and must depend, in this photochemical case, on one of the Ru04’s having approximately 293 kJ of excess vibrational energy. One can thus imagine a collision of one “hot” and one “cold” R u 0 4 to give a transition state for which there might be a small probability of forming an 0-0 bond and leaving two Ru03’s. The excess vibrational energy available to such a complex slightly exceeds its dissociation energy for the above reaction. It seems more likely, however, that the above process takes place in a surface film. Since the pressure of R u 0 4 used is not far from the equilibrium vapor pressure of the solid, it seems reasonable to suppose something like a monolayer adsorbed and that either one of the molecules in the adsorbed layer absorbs the photon or that a hot molecule collides with the surface. As soon as a monolayer (36) Scullman, R.; Thelin, B. J . Mol. Spectrosc. 1975, 56, 64
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2403 42
-
40
RuO
-
38-
T RuO, + R u 0 2 + 0
36
+ Ru t 0
253.7nm
-
1 “t
313.0nm
w
L
30
28
26
24
22
1tt-
365.0 nm
+ R u O ~t 0 2
4050nm
435.8 nm
Figure 4. Energy level diagram showing thresholds for ruthenium oxide reactions. Theerror bars hdicate the average reaction AH uncertainty arising from the estimated uncertainties given in Table 11.
of R u 0 3 is formed, additional molecules can be formed as part of the R u 0 , solid film. It is energetically not possible to form 0 atoms in this wavelength region under any circumstances. There is some evidence to support a wall catalysis mechanism. In the quantum yield experiments, the deposit of R u 0 , was 2-3 times as thick on the windows perpendicular to the light beam as on the parallel windows which received correspondingly less light from the not perfectly collimated light beam. In the search for fluorescence, the laser beam (405 nm, not spread out) was converging slowly on entering the cell and came to a sharp focus approximately 1 cm beyond the exit window; the path length in the cell was 7.0 cm. Solid RuO, deposited almost entirely (and rapidly) only on the glass surface where the beam traversed the window. On the exit window the dark deposit (of millimeter dimensions) showed clearly the transverse mode structure of the beam, like a fingerprint. Case ZI. It is interesting to note that with X = 365 nm, with the photon energy slightly above the predicted threshold for a singly excited vibrational dissociation forming an O2 bond and leaving a bent RuOz (0 = 149O for the ground state2,), the quantum yield becomes 1 or slightly greater. At the same time, the final product becomes solid RuO,, as expected. So, indeed, it seems clear that the primary photochemical step is the dissociation of R u 0 4 into gaseous RuO, and 0, with a quantum yield of 1 or near 1. It is, however, difficult to find an explanation for the quantum yields having the values greater than 1 in Table 111 and the apparent increase of 4 with the increase in light intensity. One is forced to postulate some thermochemical interaction between remaining RuO, molecules and product species, Le., RuOz or its dimer, or a more highly polymerized species. In the flash experiments for which the filter cutoff is >320 nm, no kinetic absorption spectrum, continuous or discrete, is seen. By the time the spectroscopic flash is triggered relative to the triggering of the main photochemical flash (-2-10 ps), it is fairly likely that the polymerization is well underway making spectroscopic de-
J. Phys. Chem. 1990, 94, 2404-241 3
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tection of the gaseous monomer difficult. If one postulates the reaction RuO,(g) + RuO2(g) 2RuOj(g) which is thermodynamically favored but, especially for X = 365 nm, kinetically improbable, then somehow the RuO3 produced must decompose to Ru02(.s) + '/202(g) to match the observed final product, RuO,. No evidence for RuO, is found in this region, however. We have observed that thermal decomposition of RuO,(g) at room temperature appears to be catalyzed by deposited Ru02(s) and, perhaps, in the far from equilibrium conditions during the irradiation, RuO,(g) decomposition is rapidly catalyzed by the very small, developing R u 0 2 clusters. For the continuous irradiation with X = 313 nm, although slightly above the threshold shown in Figure 4 (318 nm), there is no evidence for the 0 atoms or RuO, production, as found for shorter wavelengths. This result is not inconsistent, especially bearing in mind the uncertainty in the estimated threshold. Case 111. For the flash photolysis radiation with X
