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J . Phys. Chem. 1990, 94, 5650-5652
The Spatial Distributlon and the Electronic State of Iodine Atoms Produced from the Reaction Products of Metal Clusters with Methyl Iodide Li Song and M. A. El-Sayed* Department of Chemistry and Biochemistry. University of California, Los Angeles, Los Angeles, California 90024- 1569 (Received: April 12, 1990)
The iodine atoms in 2P3 and *P,,, states produced from the laser photodissociation of the reaction products of AI, (or V,) with CH31are detected 6y selective 2+ 1 polarized photoionization one-dimensionaltime-of-flight mass spectrometric methods and compared with those produced from CH31 under the same conditions. The ground-state iodine atoms produced from the products of the reaction of aluminum or vanadium clusters with CH31are not found to be formed along a unique direction. This might suggest that the majority of the iodine atoms detected are produced either from the reaction or else from the photodissociationof metal iodide clusters, which is slower than the cluster rotation, or from an isotropicallyabsorbing oscillator in the metal iodide clusters. It is also found that the branching ratio of I*/I is essentially zero, compared to approximately 3: 1 observed in the photodissociation of CHJ. This is discussed in terms of the mechanism of the production of the iodine atoms and the energetics of the dissociation process.
Introduction The photodissociation of CHJ has been extensively studied in recent One-dimensional time-of-flight photofragment spectroscopy2-8 has been used to study the line shape of the fragment ion signal as a function of the laser polarization. The iodine atoms produced in either spin-orbit state I(2P3,2)or I*(2Pl,2) are selectively ionized by resonance-enhanced multiple photon ionization (REMPI). The polarization direction of the transition leading to rapid dissociation has been determined)” from the mass peak shapes as a function of the angle between the polarization of the exciting laser and the extraction field directions. In this method, the extraction field could be either p u l ~ e dor~ ~constant.s,6*8 ,~ I n this work, one-dimensional spatial distributions and the electronic state of the iodine atoms produced from the reaction products of CH31 with aluminum clusters (AI,) or vanadium clusters (V,) are studied. The results are compared with those for the iodine atoms produced from the photodissociation of CH31. The method of one-dimensional mass spectrometry is used for analyzing the polarization of the photodissociation. It was found that the reaction products produce only the ground-state I(2P3,2) atoms, and no excited-state I*(2P,12)atoms. The shapes of the mass peaks observed for the J(2P3/2) suggest that it is produced along the laser polarization axis from CH31, but is randomly distributed from the reaction products. This last result suggests either that the absorbing oscillator in the cluster iodides does not ( I ) Szaflarski, D. M.; El-Sayed, M. A. J . Pbys. Chem. 1988, 92, 2234. (2) Hall, G. E.; Sivakumar, N.; Ogorzalek, R.; Chawla, G.; Haerri, H.-P.; Houston, P. L.; Burak, I.; Hepbur, J. W. Faraday Discuss. Cbem. Soc. 1986. 82, 13. (3) (a) LOO,0. R.; Hall, G. E.; Haerri, H.-P.; Houston, P. L. J . Pbys. Cbem. 1988,92,5. (b) Chandler, D. W.: Houston, P. L. J . Chem. fbys. 1987, 87. 1445. (4) Szaflarski, D. M.; van den Berg, R.; El-Sayed, M. A. J . Pbys. Chem. 1989, 93, 6700. ( 5 ) Curtiss, T.J.; Gandhi, S. R.; Berstein, R . B. Phys. Rev. Lett. 1987. 59, 2951. (6) Penn, S. M.; Hayden, C . C.; Carlson, K. C.; Crim, F. F. J . Cbem. Pbys. 1988.89, 2909. (7) Black, J . F.; Powis, 1. J . Cbem. Phys. 1988, 89, 3986. (8) Black, J . F.; Powis, 1. Chem. f b y s . 1988, 125, 375. (9) Hunter, T. F.; Kristjansson, K. S. Cbem. Pbys. Left. 1978, 58, 291. (IO) Baugchum, S.L.; Leone, S. R. J . Cbem. f b y s . 1980, 72, 6531. ( I 1 ) Godwin, F. G.; Paterson, C.; Gorry, P. A. Mol. fbys. 1987, 61, 827. (12) Gedanken, A.; Rowe, M. D. Chem. Phys. Lett. 1975, 34, 39. (13) Riley, S. J.; Wilson, K. R. Discuss. Faraday Soc. 1972, 53, 132. (14) Minton, R. K.; Felder, P.; Brudzynski, R. J.; Lee, Y . T. J . Cbem. f b y s . 1984.81, 1759. (15) Knee, J. L.; Khnudkar, L. R.; Zewail, A. H . J , Chem. Pbys. 1985, 83, 1996. (16) Donaldson, D. J.; Vaida, V.; Naaman, R. J . Chem. Phys. 1987, 87, 2522
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have a preferred direction (this could be due to a transition from 5pa to a linear combination state of an iodine atom and a few niobium atoms instead of a 5pn-a* transition along the Nb-I bond), or that the dissociation is slower than the cluster rotation time, or else the majority of the iodine atoms ionized are free iodine atoms resulting from the thermal reaction between CH31and the metal clusters.
Experimental Section The experimental apparatus used in this study has been described elsewhere.” The dissociation light was either the fourth harmonic output of a Quanta-Ray DCR-2A Nd:YAG laser (266 nm) or the frequency-doubled output from a Quanta-Ray pulsed dye laser (PDL-1) using Rhodamine 640 dye pumped by the second harmonic output of the Quanta-Ray DCR-2A Nd:YAG laser (532 nm). Formation of AI, and V, clusters and their reaction with 10% CH31 seeded in He with a backing pressure of 5 psig was achieved in a similar way as that described for niobium clusters in a previous publication.” The bare cluster distributions and their product distributions were characterized by using 193-nm (6.4-eV) laser light as the ionization source. When the anisotropy of the 1’ mass signal was studied, the dissociation laser was polarized either parallel or perpendicular to the axis of the time-of-flight mass spectrometer. In the experiment reported here, a single laser pulse of the same color serves to both dissociate the species of interest and to ionize the iodine atoms formed from the dissociation. The voltages on the repeller plate and the extraction plate were lower than those used in ref 17. Typically 1530 V were provided to the repeller plate and 1500 V to the extraction plate. These parameters were very important and were optimized during the experiment such that a maximum separation and decent intensities of the two components of the iodine atom from the photodissociation process were obtained. If the mass spectrometer was operated under “normal” conditions (3200 V on the repeller plate and 3000 V on the extraction plate), the doublet of the iodine mass signal could not be resolved. The polarization of the laser was selected as follows. The output light of the dye laser has a vertical polarization. The frequency-doubled output of the dye laser has a horizontal polarization. It was used without further polarization separation and selection. In order to obtain vertically polarized light, a half-wave plate was used to rotate the polarization and a nonlinear polarizer crystal was used to spatially separate the resulting beams (vertically polarized and horizontally polarized). The vertically polarized light beam was guided and focused into the mass spectrometer. ( 1 7 ) St. Pierre, R. J.; Chronister, E. L.; El-Sayed. M. A. J . Phys. Cbem. 1987, 91. 5228.
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The Journal of Physical Chemistry, Vol. 94. No. 15, 1990 5651
Letters (a) Parallel, Me1
(b) Perpendicular, Me1
the photodissociation of A1,I and AI,OI or due to the formation of ground-state iodine atoms in the flow reactor from the following reaction: AI,
(c) Parallel, Me1 + AIx
(d)
Perpendicular, Mei + AIx
I ' Li----------J
--.-
~
28S
Time-of-flight (ps) 30S
(e) Parallel
28.5
(I*), Me1
Time-of-flight (ps)
28
(f)
30.5
28.5
30
Parallel (I*), Me1 + AIx
30.5
Time-of-flight (1s) Time-of-flight (ps) Figure 1. (a) A 2+1 laser photoionization mass spectrum of I(2P3/2) produced from the photodissociation of CHJ at 304.7 nm. The polarization of the laser light is parallel to the time-of-flightaxis of the mass spectrometer. (b) Similar to (a), but the polarization of the laser light is perpendicular to the time-of-flightaxis of the mass spectrometer. (c) Similar to (a), but with AI, clusters present in the flow reactor. (d) Similar to (c), but the polarization of the laser is perpendicular to the time-of-flightaxis of the mass spectrometer. (e) A 2+1 photoionization mass spectrum of I*(2P1,2)at 304.0-nm laser radiation with only CHJ in the flow reactor. The polarization of the laser is parallel to the flight axis of the mass spectrometer. (f) Similar to (e), but with AI, present in the flow reactor. There was no enhancement observed for I*(2P1,2) mass signal after AI, was introduced into the reactor.
Results and Discussion The bare cluster distributions of AI, and V, clusters and their reaction products with CH31were characterized by using 193-nm (6.4-eV) laser light as the ionization source. Typically, A!, clusters with x I35 and V, clusters with x I20 were observed. After the reaction with methyl iodide in the flow reactor, very strong AlzI and much weaker A1,I and A1,OI for x 1 3 were observed for aluminum clusters whereas V,I and V,OI were observed for all of the vanadium clusters. No AI,CH3 or VxCH3was observed. Figure l a and Figure l b show the mass spectra of I+ produced from CH,I when the polarization of the laser light was parallel and perpendicular to the time-of-flight axis, respectively. The dissociation and ionization were achieved by tuning the doubled dye laser frequency to 304.7 nm, which is in two-photon resonance with the I(zP3/z)absorption. From these spectra, one can see the separation of the two components of the ground-state iodine atoms when the polarization of the laser light is parallel to the timeof-flight axis, but only one peak when the polarization of the laser light is perpendicular to the time-of-flight axis. Although the separation is not complete in the "parallel" configuration, the polarization of the dissociation of methyl iodide can be confirmed from this experiment to be dominated by a parallel transition, as was reported by the other group^.^^,^ Figure I C shows the results of the I+ mass signal produced by 2+1 MPI of the I(2P3/z)observed when both CH31 and AI, were present in the flow reactor. The laser light used was at 304.7nm and polarized parallel to the flight axis. The center peak in between the doublet peaks was either due to the contributions from
+ CH,I
-
Al,CH,
+I
(1)
Although AI,CH3+ was never observed in the photoionization mass spectrum, its absence could be due to its higher ionization potential or its photochemical instability to the ionization photons. Thus reaction 1 cannot be ruled out. If the center peak is from the iodine atoms formed from the photodissociation of the metal cluster iodides, it suggests either that the absorbing oscillator is polarized perpendicular to the dissociation axis, or that the dissociation is slower than the rotation of the metal cluster iodides, or that it results from dissociations of the metal cluster iodides that have unpolarized absorption spectra. In order to examine whether or not the dissociation results from a perpendicular transition, the polarization of the laser light was rotated by 90'. If the observed center peak in Figure I C is due to iodine atoms from perpendicular absorption of the cluster iodides, then the contribution from the dissociation of AI,I and A1,OI should be observed as shoulders to the peak of the I+ signal in a "perpendicular" configuration. Figure Id shows the result obtained with the polarization of the laser perpendicular to the time-of-flight axis when both CH31 and Al, were present in the flow reactor. No shoulders were observed in Figure Id. Due to fluctuations in the mass peak intensity of I+, any increase in the mass signal intensity near the center of the peak is difficult to observe in Figure Id compared to that in Figure IC. Thus if the observed center peak in Figure I C is due to the photodissociation of the reaction products (cluster iodides), it is very likely due to an isotropically polarized dissociation or an isotropically polarized absorption, but not a perpendicularly polarized absorption. A similar experiment was also performed by replacing aluminum with vanadium. The results obtained from the photodissociation of the vanadium cluster products were very similar to those obtained from the photodissociation of the aluminum cluster products. Figure l e shows the iodine atom signal obtained from the photodissociationof methyl iodide under conditions similar to those used in Figure la. The difference is that the dissociation and ionization laser radiation was tuned to the resonance absorption of the excited iodine atom I*(zP1/2)at 304.0 nm. Figure If shows the result after Al, was formed in the molecular beam and introduced into the flow reactor. Comparing both figures, there seems to be no formation of I*(2P,/2). If the observed I(2P3?) atom is mainly from the dissociation of iodide clusters, then this means that the branching ratio of I*/I produced from the photodissociation of A1,I clusters is much smaller than that of CHJ near this photodissociation wavelength. A similar experiment with the vanadium cluster system also showed no contribution to the mass signal of the excited-state iodine atoms. In another experiment, the laser wavelength was changed to 266 nm. This wavelength is nearly in resonance with the ground-state iodine atoms. The polarization of the laser light used was parallel to the time-of-flight axis. From this study, an isotropic ground-state iodine signal was also observed when A1 clusters were introduced into the reactor, although the contribution was not as sharply centered as observed earlier. Also, in a two-color experiment (266-nm dissociation, 304.7- or 304.0-nm ionization), an increase in only the I(2P3,2) but not the I*(2P1/2)signal was observed when the Al, clusters were added to CH31. It was shown that the photodissociation of CH31 from its The condissociative A state is a parallel tribution from the reaction of Al, and V, with CH31 is found to be isotropically distributed with respect to the polarization of the laser. Only ground-state iodine atoms were observed. The excess iodine atom signal observed when metal clusters are reacted with (18) Dzvonik, M.; Yang, S.; Bersohn, R. J . Chem. Phys. 1974,61,4408. (19) Barry, M. D.; Gorry, P . A. Mol. Phys. 1984, 52, 461. (20) van Veen, G. N. A.; Baller, T.; DeVries, A. E.; Van Veen. N. J. A. Chem. Phys. 1984, 87, 405.
J . Phys. Chem. 1990,94, 5652-5654
5652
C H J could be due to (a) iodine atoms produced directly from the reaction of AI, or V, with CH31; (b) the photodissociation of metal cluster iodides, which is slow compared to their rotation times; or (c) an isotropic absorbing oscillator for the photodissociating cluster iodides. If indeed the iodine atom results from the iodide clusters, the absence of I*(2P1,2)from the dissociation of metal cluster iodides could be explained by the energetics of the dissociation process. According to the bond dissociation energies available,2’ the energetics of the dissociations of CH31and AI-I are AH = 53.0 kcal/mol (2.3 eV/mol) CH3-I CH3 I
AI-I
-
+
AI
+I
AH = 88.0 kcal/mol (3.8 eV/mol)
The energy difference between the excited-state iodine I*(2PI/z) and the ground-state iodine I(2P312)is 0.94 eV.22 Therefore, the longest wavelength accessible to the I*(2Pl/2)state from the dissociation of methyl iodide is about 381 nm while that from the dissociation of A H is about 261 nm. At all the wavelengths used in this experiment, the one-photon energy is sufficient to form I*(2P,,2) from methyl iodide but not from AI-I. However, the (21) Darwent, D. deB. Bond Dissociation Energies in Simple Molecules; National Bureau of Standards: Washington, DC, 1970. (22) Gaydon, A. G. Dissociation Energies and Spectra of Diatomic Molecules; Chapman and Hall: London, 1968.
binding energy of I to AI, could be very different from that of AI-I. Furthermore, no bond energy was found available for VI, which is expected to have lower bond dissociation energy due to the fact that most of the main-group metals have higher metaliodine bond energies than those of the transition metals.2’ It is possible that, even if the laser photon energy is such that it could produce an I*(2PI 2), the metal clusters in the photodissociating cluster iodides courd very well quench the excess electronic energy of the I*(2P1,2)iodine atoms to be produced, in particular if the dissociation of the cluster iodides is slower than that of CH31. In summary, the photochemistry of the reaction products of AI, and V, with CHJ is found to produce isotropic ground-state iodine atoms only. This isotropy of the ground-state iodine signal could be due to the ionization of ground-state iodine atoms produced from the thermal-chemical reaction in the flow reactor between C H J and AI, or V,, or due to iodine atoms produced from photodissociation of the metal cluster iodides that is slower than their rotation or else from the photodissociation of isotropically absorbing metal cluster iodides. The absence of the formation of the excited-state I*(2P12) atoms could be either due to the energetics involved during the dissociation process or due to the statistical nature of the slow photodissociation of the metal cluster iodides.
Acknowledgment. This work is supported by the National Science Foundation. L.S. thanks Hyun-Jin Hwang for assistance.
Spectroscopy and Laser Action of Rhodamlne 6G Doped Alumlnosllicate Xerogels John M. McKiernan, Stacey A. Yamanaka, Bruce Dunn, and Jeffrey I. Zink* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024 (Receiued: April 13, 1990)
Rhodamine 6G (R6G) doped aluminosilicate glass synthesized by the sol-gel method exhibits laser action. Transparent 5 mm X 5 mm X 10 mm monoliths were used as cast in a simple laser cavity. This new material was pumped at rates of up to 25 Hz and was still active after as many as 40000 pump pulses. Luminescence and free-running laser spectra are measured. The dependence of the R6G doped aluminosilicate dye laser output on the number of pump pulses and the pump pulse energy is discussed.
’
Introduction The sol-gel technique for the synthesis of glassesI4 has made possible the microencapsulation of organic and inorganic molecules in an inorganic oxide matrix. In sol-gel processing, the sol (metal precursor solution) first forms a gel as hydrolysis, condensation, and polymerization proceed and then a xerogel (dried gel) as the solvents and hydrolysis byproducts are removed by evaporation. The xerogel is a rigid, transparent, and porous matrix. By starting with different metal alcoholates, a wide range of glass compositions can be synthesi~ed.~f’ A variety of dopants have been used to probe the matrix7-I2 or to create materials with novel optical
properties. One important application is the use of the technique to create solid-state laser host materials. A solid-state laser host material has many advantages. Isolation of the active species should result in improved stability and increased efficiency. The mechanical stability could be exploited to make new laser devices. P ~ l y m e r s , ’ ~ silicate gels,14 xerogels,15 and ORMOSILSI6 (organically modified silicates) have previously been used in this manner. Here we report the use of an aluminosilicate xerogel as a host matrix for Rhodamine 6G (R6G). The new material exhibits laser
( I ) Mackenzie, J. D.; Ulrich, D. R., Eds. Proceedings o f t h e Third International Conference on Ultrastructure Processing; Wiley: New York, 1988. (2) Sakka, S.; Kanichi, K. J . Non-Cryst. Solids 1980, 42, 403. (3) Dislich, H. J . Non-Cryst. Solids 1983, 57, 371. (4) Klein, L. C. Annu. Rev. Mater. Sci. 1985, I S , 227. ( 5 ) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1989, 1. (6) Pouxviel. J. C.; Boilot, J. P.; Lecomte, A,; Dauger, A. J . Phys. (Les Ulis, Fr.) 1987, 48, 921. (7) McKiernan, J.; Pouxviel, J.-C.; Dunn, B.; Zink, J. I. J . Phys. Cbem.
(12) Preston, D.; Pouxviel, J. C.; Novinson, T.; Kaska, W. C.; Dunn, B.; Zink. J. I. J . Phys. Chem. 1990, 94, 4167. (13) Gromov, D.A.; Dyumaev, K. M.; Manenkov, A. A.; Maslyukov, A. P.;Matyushin, G. A.; Nechitailo, V.S.; Prokhorov, A. M. J . Opt. Soc. Am. B 1985, 2, 1028. (14) Dunn, B.; Knobbe, E.;McKiernan, J.; Pouxviel, J. C.; Zink, J. I. In Better Ceramics Through Chemistry IIk Brinker, C . J., Clark, D.E.,Ulrich, D. R., Eds.; MRS Symp. Proc. Vol. 121; Materials Research Society: Pittsburgh, 1988; pp 331-342. (15) Salin, F.; Le Saux, G.; Georges, P.; Brun, A.; Bagnall, C.; Zarzycki, J. Opt. Lett. 1989, 14, 785. (16) Knobbe, E. T.; Dunn, B. D.; Fuqua, P. D.; Nishida, F. Appl. Opt., in Dress. (17) Kobayashi, Y.;Kurokawa, Y.; Imai, Y.; Muto, S. J . Non-Cryst. Solids 1988, 105, 198.
1989, 93, 2129. (8) Kaufman, V . ; Avnir, D. Langmuir 1986, 2, 717. (9) Avnir, D.; Levy, D.; Reisfeld, R. J . Phys. Chem. 1984, 88, 5956. (IO) Avnir, D.; Kaufman, V. R.; Reisfeld, R. J. Non-Cryst. Solids 1985, 74, 395. ( I I ) Levy, D.; Avnir, D. J. Phys. Cbem. 1988, 92, 4743.
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