7422 Cdy
-
J. Phys. Chem. 1991, 95,1422-1426 s a d
+ (fl2)H2!
A is the fractional C6H6 coverage corresponding to flat C6H6 admolecules, which are thermally converted into tilted C6H6. By heating the Pd( 110) surface (saturated with benzene) to 400 K, we find the ~ ( 4 x 2 structure ) is well developed and the Pd( 1 10) surface is mostly covered with tilted benzene; the fractional coverage of tilted benzene is -0.25. As compared with the low-exposure region, the thermal decomposition temperature is shifted higher (400-650 K). It appears that the ethynyl species
does not exist as a stable intermediate. These results are attributed to the site-blocking effect of benzene admolecules themselves. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Areas (Research Program "Surface as a New Material") from the Ministry of Education, Science and Culture, and by a Grant-in-Aid from the Murata Science Foundation. Registry No. Pd, 7440-05-3.
Second Harmonic Generation Studies of Silver Metal Liquldilke Films R. Bavli: D. Yogev,* S. Efrima, Department of Chemistry, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva. Israel 84105
and G. Berkovic* Department of Structural Chemistry, Weizmann Institute of Science, Rehovot. Israel 76100 (Received: January 8, 1991; In Final Form: March 27, 1991)
Silver metal liquidlike films (MELLFs) exhibit second harmonic generation signals (SHG) that are clearly different from those produced by smooth silver films or those given by electrochemically roughened silver surfaces. These differences are consistent with the interfacial colloidal model proposed for the MELLFs. The polarization and enhancement of the SHG signals are rationalized quantitatively in a model consisting of noninteracting silver particles. The results show that the particles are very nearly spherical. Trends in the results can be possibly explained by small eccentricities that vary slightly in the various MELLF preparations. We also find the volume fraction of the silver cores at the uppermost layer of the film to be 6-21%. The time dependence of the SHG revealed a curing process, on the time scale of several hours, that is attributed to diffusional rearrangements of the silver particles within the film.
Introduction Silver metal liquidlike films (MELLFs) are thin films of a silver colloid located in the interface of two immiscible liquids (water and an organic liquid).'" These systems have been characterized with respect to many of their properties, composition, and structure, but the full, intimate details of the structure and the prevalent interactions associated with the film were not yet brought to light. It seems that one single set of experiments is not sufficient for the complete understanding of these systems, and only a cross correlation between many different measurements will give the full picture. Previous investigations addressed various aspects of the relevant questions and measured some of the properties of the MELLFs. Following the exploratory study' a detailed Raman spectroscopy investigation2indicated that the MELLF is indeed a colloid that is stabilized, in part, by the adsorption of anionic species on the colloidal particles. The continuous phase was identified as the organic liquid. By means of Moire reflectometry,'" densitometry and optical microscopy, SEM, and TEM: the size of the basic particles was inferred (the smallest being of 2-5 nm in diameter), but the exact shapes were not determined. The films were found to be 3-4 pm thick, with an average silver content of 1%. The silver was found to be distributed throughout the film in an inhomogeneous manner. Electron and X-ray diffraction5showed that the silver was crystalline FCC with the usual silver metal spacing. The identity of the other components of the film, such as organic acid derivatives and surfactants, was established.
-
'
Prmnt address: Department of Chemistry, University of Califomia Santa Barbara, Santa Barbara, CA 93106. *Present address: Department of Chemistry, Syracuse University, SyraCUSC, N Y 13244-4100. To whom correspondence should be directed.
OO22-3654/9 1 /2O95-1422SO2.5O/O
The role of surfactants in forming and stabilizing the MELLFs was also investigated: It was found that the surfactant probably enshrouds the metal particles by adsorption, by micellization, or by forming an internet within which the colloid resides. The measurement of the viscoelastic properties' of MELLFs indicated Coulombic interactions between the particles. Here we report on the SHG effect of silver MELLFs. SHG from ensembles of nanometer size particles may be of interest by its own merit, but we are mostly aiming at obtaining a more detailed structural picture of the upper reaches of the MELLFs, at the waterjfilm interface. We are interested in the particle shapes and the particle density. Also the time dependence of the SHG signals was followed to monitor processes occurring within the film in its formation stage. SHG was chosen for this purpose as it is specifically sensitive to the discontinuity at the boundary between centrosymmetric or isotro ic phases and has been widely used to study silver surfaces!Is and it is also known that rough silver surfaces exhibit ~
~
~~~~~~
(I) Yogev, D.; Efrima, S.J . fhys. Chcm. 1988, 92, 5754. (2) Yogev, D.; Efrima, S. J. fhys. Chrm. 1988, 92,5761. (3)Yogev, D.; Efrima, S.; Kafri, 0. Opt. Len. 1988, 13,934. (4) Yogev, D.; Deutsch, M.; Efrima, S.J . fhys. Chem. 1989, 93,4174. (5) Yogev, D.; Shtutina, S.; Efrima, S.J . fhys. Chcm. 1990, 91, 752. (6) Yogev, D.; Efrima, S.Lungmuir 1991, 7, 267. (7) Yogev, D.;Kuo, C. H.; Neuman, R. D.; Efrima, S.J . Chem. Phys. 1989,91,3222. (8) Gordon, K. C.; McGarvey, J. J.; Taylor, K. P. J . fhys. Chcm. 1989, 93,6814. (9) Bloembergen, N.;Chang, R. K.; Jha, S.S.;Lee,C. H. fhys. Rw. 1968, 174, 813. (IO) Brown, F.; Matsuoka, M. fhys. Reo. 1969, 185, 985. ( I 1) Boyd. G. T.; Rasing, Th.;kite. J. R. R.; Shen, Y. R. fhys. Rw. 1984, E30, 5 19.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 1423
SHG of Silver Metal Liquidlike Films TABLE I: SHC Signal Intensities from Mature Silver MELLFsO acid surf. org G, SD GI, I 1 d 6.1 2.1 4.0 2 1 d 12.8 2.9 16.2 3 4 5
1 2 2
6
2
7 8 9
3 3 3
10
silver film
+ + ++ + +
d d
3.9 5.5 2.6
C
3.0
d d
5.9 5.9
C
2.9
C
1.o
SD1.2 0.2 1.4 1.9
Gpp/Gsp
1.51 0.79 1.50 1.16
1.1
2.9
1.9 1.4 1.8 1.6
3.8 2.7 4.8
0.6 3.5
0.93
2.3
0.9
1.9 2.0
5.8 3.9
1.1 1.5
2.69 0.99 0.71 >20.00
