2080
J. Phys. Cbem. 1980, 84, 2089-2090
solute concentrations and that reports of radiolytic dosimetry should include an adequate description of the reference system being used. Since the rate constants for reactions 2 and 4 are both w l O I O M-’ the yields for production of (SCN)z-and Fe(CN)t- at a given concentration should be very similar and the extinction cloefficient for (S@N)2-can be determined from the known extinction coefficient of ferricyanide (1027 M-’ cm-l at 420 nm) and the measured absorbance ratio. If one takes the yields to be equal, a value of 7580 f 60 M-l cm-l is obtained for the extinction coefficient of (SCN), at 472 nm. We note that, in contrast to previous determinations which were based on assumed yields and absolute dosimetry, this present value does not involve any explicit knowledge of the absolute yields. Because of the importance of this system to pulse radiolysis studies we are currently attempting to refine these measurements as much as possible. A more detailed account of the experimental methods involved in the dosimetry, along with a critical evaluation of possible systematic errors, will be submitted for publication in the near future.
TABLE I
P,a torr
PH,F
hydrocarbon 1-hexene 1-hexene 1-hexene 1-hexene 1-hexene cy clohexene n-hexane cyclohexane
50.0 49.6 50.7 53.5 82.1 46.8 52.7 47.5
87.0 101.7 95.9 131.3
torr
100.7 107.0 159.5
laser pulse ener- cell gy, temp, “cobJ K webs’’ 5.0 5.0 4.7 5.0 4.8 4.5 4.9 4.5
353 353 355 357 353 358 353 358
yes yes yes yes yes no yes no
a Pressures measured at room temperature. At the cell temperature pressures increase t o values about 1.3 times room-temperature values.
laser might supply energy both to drive the reaction and to create the metal catalyst in situ. A classic, organic textbook reaction, the catalytic hydrogenation of an alkene, was chosen. However, under the experimental conditions described below, catalytic hydrogenation does not occur, but a rather surprising and unexpected result is observed. References and Notes Specifically, “cobwebs” of molybdenum metal are made The research descrlbnd herein was supported by the Office of Basic when COzTEA laser pulses irradiate a mixture of 1-hexene Energy Sclerices of the Department of Energy. This is Document or n-hexane and molybdenum hexacarbonyl. No. NDRL-2115 from the Notre Dame Radiation Laboratory. A Lumonics Model 203 COz TEA laser operating in a R. H. Schuler, A. Hartzell, and B. Behar, to be submitted for publication. T. I. Balkas, J. F. Fenidler, and R. H. Schuler, J. Phys. Cbem., 74, multimode-fixed wavelength (10.6 pm) configuration is 4497 (1970). used as a pulsed laser source. The system produces a R. H. Schuler and L. K. Patterson, Cbem. phys. Lett., 27, 369 (1974). maximum of 15 J/pulse distributed in a 3-cm square K. Bhatia and R. H. Schuler, J . Phys. Chem., 78, 2335 (1974); G. W. Klein and R. H. Schuler, Radiat. Phys. Chem., 11, 167 (1978). output. Aperturing prior to the focusing lens (12.7 cm fl) H. A. Schwarz, private communication. reduces the energy content to 4-5 J in a 1.5-cm diameter J. P. Chauvet, R. Viory, E. J. Land, and R. Santus (C.R. Acad. Sci. circular output. The pulse length is 170 ns (fwhm). The Paris, Ser. D , 288 1423 (1979)) have, for example, recently quoted a value of 41,180, Le., 7100 X 5.8. pulse energy is measured with a Scientech laser power/ G.E. Adams, J. W. Boag, and B. D. Michael, Trans. Faraday SOC., energy meter, and the pulse length with a Rofin photon 61, 1974 (1965). See1 also E. M. Fielden and N. W. Holm, “Manual drag detector and storage oscilloscope. The laser pulse is on Radiation Dosimetry”, N. W. Holm and R. J. Berry, Ed., Marcel Dekker, New York, 1970, p 261ff for a general discussion of pulse focused through a NaCl window into the glass reaction cell. radiolytlc dosimetry. The reaction cell has been described previ~usly.~ The J. H. Baxendale, P. H, T. Bevan, and D. A Scott, Trans. Faraday molybdenum carbonyl (10-30 mg) is loaded into the reSOC.,64, 2389 (1968). action cell, and the cell is evacuated to torr. The While the oxidation of fcmocyankle could have been compared directly with Fricke dosimetry the signal/noise is relatively poor in each case cell is backfiied with reactant gases, the pressures of which and detailed measurernents require considerable slgnal averaging. are measured with a MKS Baratron capacitance monomI t Is experimentally much more convenient to use the thiocyanate eter. The cell is closed off and heated in a laboratory oven dosimeter as Intermediate reference. J. Rabani and D. Meyerstein. J. Phys. Chem., 72, 1599 (1968). for several hours to establish a vapor p r e s ~ u r e of ~ -about ~ G. Hughes and C. Miills, Discuss. Faraday Soc., 36, 223 (1963). 40 torr for the carbonyl. The cell is removed from the L. K. Pattersonand J. Lille, Int. J. Rad&. phys. Cbem., 6, 129 (1974). oven, and one laser pulse is focused into the center of the R. H. Schuler and A. 0. Allen, J. Chem. Pbys., 24, 56 (1956). Farhataziz and A. B. Ross, Natl. Stand. Ref. Data Ser., Natl. Bur. cell. Stand., NO.59 (1977). See entries No. 3.25 and 3.54 for a summary Table I summarizes the reactant gases, experimental of available rate information for reaction of OH with SCN- and parameters used, and whether or not molybdenum Fe(CN),&. “cobwebs” form. Note that hydrogen is not a requirement Radiation Laboratory and Robert H. Schuler’ for “cobweb” formation but that a straight-chained hyDepartment of Cbemistry Larry K. Patterson drocarbon is. Figure 1 shows typical cobwebs after one University of Notre Dame Eberhard Janata laser pulse in a hexene-hydrogen-carbonyl mixture. The Notre Dame, Indiana 46556 webs grow in a time frame of seconds and tens of seconds Received: March 17, 1980 after the laser pulse, in contrast to deposits3 formed from metal carbonyl only, which form faster than the unaided eye can resolve. The cyclic hydrocarbon-hydrogencarbonyl mixtures yield film deposits similar to those formed in carbonyl only e~periments.~ It is significant that, Growth of Molybdenum “Cobwebs” Followlng Pulsed for the pulse energy and gas pressures given in Table I, Laser Irradiation of Molybdenum Carbonyl and the “cobwebs” are observed only for LIDB in the presence of the long straight-chained hydrocarbons. In the hexStraight-Chained Hydrocarbons ene-H2 experiments post-laser-irradiated gases were anaSir: The formation of ultrafine metal particles via C02 lyzed with a Nicolet FT-IR spectrometer. No hexane was TEA laser-induced dielectric breakdown (LIDB) of gaseous detected. Hexene and residual molybdenum carbonyl were species has been reported by Ronn1p2and this re~earcher.~ the only gases detected. IR spectra of the cobwebs show In an attempt to capitalize on the very high surface area no organic constituent. Decomposition induced in reaction and dispersion of metal particles created by this method, cells by elevated oven temperatures does not result in we sought a simple gas-phase reaction where the pulsed “cobweb” structures. 0022-3654/80/2084-2089$0 1.OO/O
0 1980 American Chemical Society
2090
h e Journal 01 Fhysical Chemistry, Vol. 84, No. 16, 1980 ’-
“Cobwebs”of molybdenum grown following C02 TEA laser irradiation of 1-hexene, hydrogen, and molybdenum hexacarbonyl. Figure 1.
None of the organic “reactants” have absorption bands a t 10.6 pm. If one assumes therefore that the energy deposited in the system hy the focused laser pulse induces carbonyl decomposition, then the unusual structure and long growth times result from complex collisional interaction between molybdenum atoms, gas-phase molecules present, and the reaction cell wall. It would appear that the differences in the resulting molybdenum growth morphology are somehow related to the hydrocarbon structure. It should he emphasized that these observations result from a very limited number of experiments. Further experiments characterizing the formation of cobwebs as a function of gas pressures, hydrocarbon length, metal carbonyl, laser puke energies, &d reaction cell&e are clearly
Addnions and Corrections
needed if any mechanistic understanding is to he gained. The role of “wall effects” and cell cooling rate have not been investigated. Ronn and associates’s2 have systematically studied the effects of numerous variables on both the laser-induced decomposition and homogeneous nucleation resulting in particulate formation. It is clear from that work that the pressure regime plays a dominant role in this process. The unusual “wehlike” morphology seen in Figure 1is remarkably similar to that foundsJ in the growth of ferromagnetic (Co, Fe, Ni) materials by the gas evaporation technique. No attempts have been made to examine with TEM the size or crystal habit of the individual particles making up the web chains. The author notes that violent reactions have occasionally occurred when (1)the laser window “pops off‘ exposing hot hydrocarbon-H, to oxygen and (2) when particlecoated reaction cells are irradiated with additional laser pulses. References a n d Notes (1) A. M. Ronn, Chem. Fhys. Len., 42, 202 (1976). (2) S. T. Lin and A. M. Ronn, Chem. Fhys. Len.. 56. 414 (1978). (3) C. W. Draper. Met. Trans. 111. 349 (1980). (4) J. J. lander and L. ti. Germer. Trans. A I M . 175, 648 (1948).
( 5 ) F. A. Canon and G. Wikinson. “Advanced lnwgank Chernisby. 3rd ed. Interscience, New York. 1972. pp 683-684. (6) T. thyashi. T. Ohno, S. Yatsuya. and R. W&. Jpn. J. App. Fhys.. 16, 705 (1977). (7) A. R. Tholen. Acta Metall., 20. 1765 (1979)
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ADDITIONS AND CORRECTIONS 1979, Volume 83 Milan RandiE and Charles L. Wilkins*: Graph Theoretical Ordering of Structures as a Basis for Systematic Searches for Regularities in Molecular Data. Page 1529. In Table I, column 1,4.4-dimethyloctane should be 4b-dimethyloctane and 2-methyl-2-ethyheptane should he 4-methyl-3-ethylheptane. In column 2, 2,2,5-6-tetramethylhexane should he 2,2,4,5-tetramethylhexane. The following entry should he included: 3,3,4,4-tetramethylhexane 9 14 15 6 1.
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