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J. Phys. Chem. 1992, 96, 1160-1 164
GaAs colloidal particles in the quantum size regime have been produced by reacting G a ( a ~ a c )and ~ ((CH3)3Si)3A~ in the tri-
ethylene glycol dimethyl ether at reflux temperature (216 OC). The resulting colloid contains GaAs particles that have a size distribution ranging from 20 to 80 A. This has been confirmed through experiments involving TEM, electron diffraction, ultrafiltration, optical and photoluminescence spectroscopy, and transient hole-burning spectroscopy. TEM and electron diffraction results indicate that the GaAs particles range from 20 to 80 A. Absorption spectra show an absorption onset at 600 nm which corresponds to a particle size of about 65 A. Absorption, photoluminesxnce, and holeburning spectroscopy experiments of filtrates passed through 700, 100, and 15 A pore size ult<crs show that the colloids do not contain GaAs particles s d e r than about 15 A or larger than about 100 A. Unlike the previously reported synthesis,lO*llthe present preparation does not produce molecular species that obscure the optical properties of the GaAs particles over the wavelength range 600-470 nm. Below 470 A, the absorption rises to a peak at 440 A; the present work cannot establish whether this peak is due to GaAs particles (40 A in diameter) or to molecular species. Hole-burning experiments show higher energy photobleaching of the GaAs nanocrystals beginning at 475 nm, in addition to the usual hole-burning peak centered at the pump wavelength (532 nm); the high-energy photobleaching may be caused by twephoton absorption. The rise time and decay of photobleaching in the GaAs nanocrystals is slow (hundreds of p) and is not understood. Further work is in progress to understand the optical properties of colloidal GaAs nanocrystals.
(14) Peyghambarian, N.; Fluegel, B.;Huylin, D.; Migus, A,; Joffre, M.; Antonetti, A,; Koch,S. W.; Lindberg, M. IEEE J. Quant. Electronics 1989, 25,2517. Roussingnol, P.; &card, D.; Flytzanis,C.; Neumth, N. Phys. Rev. Letts. 1989, 62, 312. Alivisatos, A. P.; Hams, A. L.; Levinos, N . J.; Steigerwald, M. L.; Bms, L. E. J . Chem. Phys. 1988,89, 4001.
Acknowledgment. This work was funded by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. H.U. was supported by the U.S.-Japan Cooperative Program on Photoconversion and Photosynthesis.
usual photobleaching process that is well established for small semiconductor parti~1es.l~The time dependence of the photobleaching for the GaAs nanocrystals is unexpectedly long. The photobleaching protxss takes from 100 to 200 p to develop fully and then decays slowly (seeFigure 4). Further work is in progress to understand the details of the hole-burning phenomena in our GaAs colloids. Finally, the emission spectrum in Figure 6 also supports the presence of GaAs nanocrystals in the 700 and 100 A filtrates but not in the 15 A filtrate. Under excitation at 530 nm, the former shows appreciable PL that begins at 720 nm while the latter shows much weaker emission that resembles the blank solution of Ga( a c a ~ ) ~The . onset of emission for the GaAs nanocrystals is red shifted from the absorption edge at 600 nm. This is expected if traps and defects are present in the GaAs. However, the fact that PL emission is observed at all in the untreated GaAs colloid is of interest since surface states usually quench PL in GaAs nanocrystals,5 where the ratio of surface atoms to bulk atoms is very high. Our results therefore indicate that the nonradiative surface states are passivated to some degree by the use of triglyme and G a ( a c a ~for ) ~ the synthesis. The details of thii surface chemistry and passivation are not known. Future work will also address this issue.
Inelastic Electron Tunneling In ACAI,OS-Au Junctions Exposed to Thlocyanate Ion: Sensitive Thiocyanate Detection K.W.Hipps* and Ursula Manv Department of Chemistry, Washington State University, Pullman, Washington 99164-4630 (Received:August 22, 1991)
Inelastic electron tunneling spectra are reported for A1-Al2O3-Au junctions externally doped with KSCN from methanol solution. Spectra of the thiocyanate ion can be observed over a wide range of concentration, 10-'-lW2 M. Because the tunnel diodes are completely fabricated prior to the doping process, and because the doping process is simple and only requires a few minutes, these devices may serve as solid-state chemical sensors. The spectra observed for externally doped junctions are essentially the same as those for junctions doped with KSCN during device fabrication. While most of the bands are easily assigned, some questions remain about the assignment of the band@)near 350 cm-'. Scanning tunneling micrcxscopy images of A1-Al2O3-Au junctions (prepared on mica) before and after infusion are also presented.
Introduction Inelastic electron tunneling spectroscopy (IETS) is now two decades old.' It is an all electronic (photon-free) technique that provides vibrati~nal*-~ and electronic2.@ spectra of microscopic (1) Lambe, J.; Jaklevic, R. C. Phys. Rev.1968,165,821.
( 2 ) Hansma, P. K. Tunneling Spectroscopy; Plenum Press: New York,
1982.
(3) Hansma, P. K. Phys. Rep. C 1977, 30, 145. (4) Weinberg, W. H. Vib. Spectra Struct. 1982, 11, 1.
(5) Yaw, J. T., Madey, T. E., Eds. Mbrational Spectroscopy of Molecules
on Surfaces; Plenum Press: New York, 1987. (6) Hipps, K.W.; Mazur, U. J. Am. Chem. Soc. 1987, 109, 3861.
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quantities of material (S1014molecules). The object of study, a metal-insulator-metal sandwich, can be made small enough to be integrated with other electronic circuitry on a single chip. In fact, tunneling spectroscopy is the only broad-band spectroscopic technique wherein the sample and the entire spectrometer might be placed in a single integrated circuit. Thus, it would seem natural that IETSbe applied to sensor development. There have been, however, several barriers to its application in analysis. The first and most obvious problem relates to how these devices arc (7) Hipps, K. W.; Mazur, U. Sur/. Sci. 1989, 207, 385. (8) Hipps, K. W.; Mazur, U. J. Phys. Chem. 1987, 91, 5218. (9) Hipps, K. W. J . Phys. Chem. 1989,93, 5958.
Q 1992 American Chemical Society
Inelastic Electron Tunneling in A1-Al2O3-Au Junctions conventionally constructed. Usually, the material of interest is added during device fabrication-after insulator growth and before top metal deposition. This procedure is completely impractical for sensor applications. The second major problem is what we like to call the SkT barrier. The spectral line width in IETS is very dependent on sample temperature during the measurement process. For conventional devices, the full width is about 5kT. In the vibrational region of the spectrum, therefore, significant loss in selectivity results if the sample is measured at temperatures higher than about 30 K. While it is technologically and conceptually possible to develop a sensor system including a refrigerator to cycle the device from room temperature or beyond to about 10 K for the purposes of spectral measurement, the added cost and size is discouraging. In recent years, it has begun to appear that the fabrication problem can be solved and that the 5kT barrier is surmountable. Consider the 5kT problem first. In the vibrational region of the tunneling spectrum, the application of deconvolution methoddo can significantly improve selectivity at elevated temperatures if the IETS line shape is known. Recently, we experimentally verified the theoretically predicted line shape in the temperature region from 4 to 77 K." Thus, the door is open for the application of deconvolution to reduce thermal smearing of tunneling spectra. An alternative solution involves changing the bandwidth of the tunneling experiment. If focus is placed on electronic transitions located in the near-IR region, many of which have intrinsic widths of lo00 cm-' or more, the 5kT barrier has little significance at room temperature.' The fabrication problem can also be solved. In 1977, Jaklevic and Gaerttner first reported external doping of A1-Al2O3-Pb tunnel diodes.'* These authors demonstrated that small organic molecules in the presence of water vapor would penetrate the Pb top metal of the completed junction. These vapor-infused diodes gave tunneling spectra comparable to those obtained from conthey showed ventionally doped devices. In subsequent paper~,Z'~J~ premade A1-A1203-Pb junctions could be infused with various species from either water vapor or aqueous solution. In addition, they demonstrated hydrogenation of Al-A1203-Pd devices.15 A few other group reproduced Jaklevic's Al-A1203-Pb results and were able to demonstrate that water vapor played a significant role in the Pb infusion process.1ti18 The picture that emerges is that water vapor etches the grain boundaries and thereby provides a path for entry of the add-species into the M-I-M' device. Unfortunately, the very process that allows Pb to be infused makes it a very poor choice for a sensor electrode. Pb films 300 nm thick are converted to oxy-hydroxides over a period of days in wet air. A chemically stable, but infusible, top electrode is needed. In some of Jaklevic's work, he mentions that Sn- and Au-topped junctions can be infused, but he never presents spectra. Very recently, the variations of resistance and capacitance with time of Au-topped junctions exposed to humid air were reported.lg These reports suggest that IETS may be obtainable from postfabrication-doped (infused) Al-A1203-Au junctions. Since gold is nearly an ideal material in terms of its thermal and environmental stability, we felt that infusible Au-topped junctions might serve as solid-state sensors. Our first study of these junctions is the subject of this paper. Conceptually, one has the situation depicted in Figure 1. We wanted to answer several fundamental questions. Can small
'
(IO) Blass, W. E.; Halsey, G. W. Deconvolution of Absorption Spectra; Academic Press: New York, 198 1. (11) Hipp. K. W.; Peter, S. L.J. Phys. Chcm. 1989, 93, 5717. (12) Jaklevic, R. C.; Gaerttner, M. R. Appl. Phys. Lett. 1977, 30, 646. (13) Jaklevic, R. C.; Gaertner, M. R. Appl. Surf.Sci. 1978,1,479. (14) Jaklevic, R. C. Appl. Surf.Sci. 1980, 4, 174. (15) Gaerttner, M. R.; Jaklevic, R. C. Surf.Sci. 1979, 517. (16) Nelson, W. J.; Walmsley, D. G.; Bell, J. M. Thin Solid Films 1981. 79, 229. (17) Heiras, J. L.; Adler, J. G. Appl. Surf.Sci. 1982, 10, 42. (18) Mallik, R. R.; Ritchard, R. G.; Oxley, D. P.; Horley, C. C.; Comyn, J. Thin Solid Films 1984, 112, 193. (19) Bellingham. J. R.; Adkins, C. J.; Phillips, W. A. Thin Solid Films 1991, 198, 85.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1161 K+ SCN-
Figure 1. Cartoon of the infusion of KSCN from solution into an AlAI2O3-Au tunnel diode.
inorganic ions (thiocyanate here) be infused into gold-covered devices under anhydrous conditions? If they can, what range of concentration can be used? Is the species formed by infustion the same as that resulting from conventional fabrication methods, and what is that species? We will address all of these questions in what follows.
Experimental Section All devices were fabricated in a diffusion-pumped vacuum system. The pump was fitted with a liquid nitrogen trap, and a system base pressure of 3 X lO-' Torr could be obtain in 1 h. This is a 14-in. bell jar system having removable stainless steel shields and a substrate positioner, allowing three different materials to be deposited on three different substrates without breaking the vacuum. The aluminum used was electronics grade wire clips and was deposited from high-purity tungsten wire filaments. Gold was deposited from an alumina-coated basket. We found that extended (- 1 month) use of these baskets produced contaminated f h and unreliable results. Frequent replacement of these sources is advised. The substrates used were Corning glass slides that had been cleaned in a nitric acid and hydrogen peroxide bath. The KSCN was Baker's reagent grade, and the methanol was removed from a freshly opened bottle of anhydrous (absolute) methanol. Tunnel diodes were prepared four to a substrate in the following manner. First, about 100 nm of aluminum was deposited in the form of a long 1 mm wide strip. This deposition was performed at pressures less than 5 X lo-' Torr. The AI was oxidized for 4-6 min in a 100-mTorr oxygen plasma formed by a 400-V ac discharge. The bell jar was then opened to air. At this point, the preparation of the infused and conventionally doped junctions diverged. In the case of the conventionallyprepared devices, the substrate was removed from the vacuum system, a few drops of KSCN in MeOH solution was applied to the surface, and the substrate was spun at high speed to remove exms material. The substrate was then returned to the vacuum system and four gold strips (40 nm thick by 1 mm wide) were deposited. The postfabrication-doped (infused) junctions were prepared by immediately reevacuating the bell jar and depositing thin gold strips (1 mm wide by 10 nm thick) across the oxidized aluminum electrode. The Au deposition was carried out at relatively high pressure (between 3 X lod and 5 X Torr) and with a rate of 0.02 nm/s. Basically, we started the deposition as soon as turnover to diffusion pumping started. These devices were found to have a four terminal resistance of between 5 and 100 Cl at the time they were removed from the chamber. Allowing them to sit in air for several days generally resulted in an increase in resistance to several hundred ohms. Infusion with thiocyanate was carried out as follows. A few drops of a freshly prepared solution of KSCN in methanol were used to wet the surface of the AI-A1203-Au devices. The solution remained in contact with the device for exactly 5 min. Infusion was terminated by spinning the substrate at high speed to remove residual solution. A few drop of pure absolute methanol were applied to the device surface, and the substrate was again spun dry. This washing procedure was used throughout. Electrical contact was made to completed 1m"m junctions by the use of indium solder. The devica were then immersed in liquid helium, and tunneling spectra were measured. The spectrometer used has been described elsewhere.20*21 The tunneling intensities
1162 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Hipps and Mazur
I I
1\
AI -A1203- KSCN -Au
(new)
AI-A120,-KSCN-Au
(oged)
I I , I
1000
0
2000
Energy (cm-') I
1
I
0
1000
2000
I
Energy (cm-') Figure 2. Comparison of the tunneling spectra obtained from conventionally prepared tunnel diodes. In the Pb-topped junction, a 1 g/L aqueous solution of KSCN was used. A 0.1 g/L solution of KSCN in MeOH was used to dope the oxide prior to Au (40nm) depition. A
Figure 3. Comparison of the tunneling spectra obtained from conventionally prepared gold-topped tunnel diodes. For the upper trace, a 0.1 g/L methanolic solution of KSCN was used and the spectra was measured immediately after junction preparation. For the lower trace, 0.3 g/L solution of KSCN in MeOH was used. In this case, the junction sat in air for 50 h prior to measurement. A modulation voltage of 2.5 mV and a gold thickness of 40 nm were employed in both cases.
modulation voltage of 2 mV was employed in both cases.
reported are proportional to (d2V/drZ) taken at fiied phase and constant modulation voltage. The lock-in phase was set 90° out from the null setting obtained at an applied bias voltage of 39 mV (the strongest spectral band). The accuracy of the band positions depends on the modulation voltage (resolution) used. At high modulation voltage, the band maximum is shifted somewhat to higher energy. The half-width at l/e height of the observed peaks (in cm-') is given approximately by I' = [(2.3n2 (7.1V , ) 2 ti2]ll2, where Tis in Kelvin, V,, is the modulation voltage in mV, and 6 is the intrinsic half-width at l/e height for the transition." Since all our data were taken at 4.2 K, J? = [9.7 + (7.1 V a 2+ 62]1/2. Thus, for a very sharp line, the modulation width is the dominant factor when V,, > 1.5 mV. All modulation amplitudes will be reported as rms values. Scanning tunneling microscopy images were obtained of the Al-A1203-Au devices prior to and following infusion. For these experiments, the preparation procedure was identical to that used for spectral studies, except that a mica substrate was used. The instrument used has been described elsewhere.23
+
k
+
Results and Discussion We first consider conventionally prepartdgold-topped junctions. In Figure 2, we constrast the results obtained from P b and Au-topped devices. The most striking difference is the band near 321 cm-I having a shoulder near 360 cm-'. This strongest band in the Au-topped junction spectrum of KSCN is absent in KSCN-free devices and in Pbtopped KSCN-doped junctions. This band (and its intensity) is similar to bands near 306 and 350 cm-' seen in cyanide-containing gold-topped diodes.22 Isotopic substitution of those cyanide species proved that these low-frequency bands were due to cyanide motions. The 305-m-l band was assigned to Au-CN stretching in a gold(1) dicyanide-like species. As in the present case, the 300-cm-'region band was the strongest band in the spectrum. In S-bonded gold(II1) thiocyanates, the Au-SCN stretch is expected near 300 cm-'." In nitrogen-bonded tetrahedral and octahedral thiocyanate com* the plexes, the M-NCS stretch also lies near 300 ~ m - ' . ~Thus, pair of bands near 320 cm-' in the KSCN-doped gold-topped junctions are almost certainly due to a gold thiocyanate complex (20) (21) (22) 2305. (23) (24)
I
Hipps, K. W. Rev. Sci. I " m . 1987, 58, 265. Hipps, K. W.; Mazur, U. Rev. Sei. Intrum. 1988, 59, 1903. Mazur, U.; Williams, S. D.; Hipps, K. W. J . Phys. Chem. 1981,85,
Hipps, K. W.; Fried,G.; Fried, D.R w . Sci. Instrum. 1990.61, 1869. Adam. D. M. Metal-Ligand and Related Vibrations; Edward Arnold London, 1967.
AI-AIZO3-Au
/1
day/
KSCN
AI-AI~OJ-KSCN-AU
0
1000
2000
3000
4000
Energy (cm-') Figure 4. Tunneling spectra obtained from conventionally doped and
from infused Au-topped junctions. The infused junction stood in air for 24 h prior to doping with 0.1 g/L KSCN in methanol. The lower
spectrum was obtained from a conventionallyprepared device doped with the same solution.
of some kind. On the basis of this obsewation, the SCN- species in Al-A1203-Au tunnel diodes conventionally doped with KSCN is complexed with the gold top layer. The bands near 480,737, and 2100 cm-' can be assigned to the SCN- bend, S-C stretch, and C-N stretch, respecti~ely.~~ The broad band peaking near 950 cm-' is the alumina A 1 4 motion. The Au-topped junctions are very hardy. Unlike Pb-topped devices that seldom survive rewarming in air to room temperature after measurement, the Au-topped devim can be cycled to helium and back several times. Figure 3 shows the tunneling spectra obtained from two different conventionally doped gold-topped tunnel diodes. We obtained the upper trace when the freshly prepared junction was immediately immersed in helium. The lower spectnun was taken from a different junction that had been cooled to 4.2 K, warmed to room temperature, allowed to stand in air for 50 h, and then recooled to 4.2 K for measurement. The signal-to-noise ratio of the aged device is essentially the same as that of the fresh one. Figure 4 contrasts the spectra obtained from A1-A1203-Au junctions doped with 0.1 g/L KSCN in MeOH by conventional and postfabrication techniques. The infused junction (upper trace) shows essentially the same spectral features as the junction wherein KSCN was adsorbed prior to gold deposition. The only difference (25) Mazur, U.;Hipps, K. W. J . Phys. Chem. 1979,83, 2773.
Inelastic Electron Tunneling in A1-Al2O3-Au Junctions
. AI-Al203-A~
O.Olg/L
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1163
I
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KSCN in MeOH
.
335
5E-5 M; 2.5mV
$
.
2
3
c
Y
0
1000
2000
;
3000
Energy (cm-')
4000
Figure 5. Tunneling spectra obtained from the same infusion doped device before (a) and after (b) cycling through room temperature. The junction was allowed to stand in air for about 18 h prior to measuring trace b. A modulation voltage of 3 mV was used for both spectra.
infused with KSCN
1000
150
950
550
Energy (cm-')
W
Figan 7. Effect of concentration on the low-frequency region of the tunneling spectrum. The apparent downshift of all bands in the lower trace is due to the reduced modulation voltage employed in that spectrum.
AI-AI~O~-AU( 1 Onm)
Energy (cm-')
:
5E-6 M; 1.5mV
2000 .
Figure 6. A1-Al2O3-Au junctions infused with varying concentrations of KSCN in methanol: (a) lW, (b) 5 X lW5; (c) IW5; (d) 5 X lod, (e) lod; (f) 5 X lo-' M. All spectra were obtained at 4.2 K with 2.5-mV
on m i c a
Al-AlO-(lOnm)Au
[Id a y
old1
Figure 8. STM image of an A1-Al2O3-Au device fabricated on mica.
modulation.
to be an increase in the CH stretching band (near 2900 cm-') in the infused junction. Note also that the AI-A1203-Au diode used for the infusion experiment was allowed to stand in air for 24 h prior to addition of KSCN. We obtained similar results when AI-Al203-A~ diodes sat in air for several days prior to infusion with KSCN. Thermal cycling of SCN--infused junctions does not significantly degrade their tunneling spectra, either. As shown in Figure 5, cycling the junction through room temperature between measurements has little effect on the resulting spectra. The role of solution concentration in determining the spectra of infused gold-topped junctions is depicted in Figure 6. These spectra were obtained from junctions exposad to KSCN in methanol solutions with ranges over 3 orders of magnitude in concentration. Even at the lo-' M level, the intense Au-SCN band can still be observed. Thus, these AI-A1203-Au junctions can function as sensitive detectors for thiocyanate. As evidenced by Figure 6,they are qualitative rather than quantitative sensors. Above about 5 X lod M,all the available sites for SCN-' are saturated and the intensity of its spectrum is little changed with increasing concentration. Below 5 X 1od M,the SCN- spectral bands drop in intensity with decreasing concentration, but the usable working range is probably only about 1 or 2 orders of magnitude; i.e. 5 X lv-1 X lO-' M. Another point worth noting about Figure 6 is the variation in Al-0 band intensity. These junctions were made Over a period of weeks. During that time, the humidity in the air changed considerably and unpredictably. Because our fabrication procedure involved exposing the freshly made oxide to air prior to Au deposition, changing atmospheric conditions affected the thickness of the oxide. Thus, the A1-0 stems
20nm
Al-AlO-(IOnm)
Au
on
Mica
[KSCNI
Figure 9. STM image of an A1-Al2O3-Au device fabricated on mica, infused with KSCN in MeOH, and washed with methanol.
band intensity cannot be taken as a "reference" value. There is still much to be learned about the nature of the SCN--gold interaction. As evidenced by Figure 7, the shape of the hand in the 33O-cm-I region changes with concentration. The upper two spectra were taken with 2.5-mV modulation from junctions infused with KSCN concentrations differing by a factor of 10. The lowest spectrum in Figure 7 was obtained from the same junction as the middle trace but with much smaller modulation voltage. The shoulder near 360 cm'l clearly seen in the bottom trace appears with varying intensity in both the conventionally doped and infused junctions. We are currently working to identify the origin of this shoulder. Figures 8 and 9 show typical scanning tunneling microscope images obtained from A1-Al2O3-Au devices before and after
1164
J. Phys. Chem. 1992,96, 1164-1173
infusion with 0.05 g/L KSCN in methanol. The corrugation in these pictures is about twice the mean thickness of the gold layer. Thus, some of the deepest valleys may, and probably do, reach the underlying oxide layer. The width of these penetrations, however, is probably quite narrow since we see no evidence of the increased noise expected as the tunneling tip traverses the alumina layer. Further, there is no evidence that infusion from methanol increases the roughness of the surface in a substantial way. This picture is unlike that for lead, as developed by Nelson and coworkers.16 Their TEM images show large openings (of the order of 20 nm wide and hundreds of nm long) in their 15 nm thick lead films that substantially increase in size with exposure to water vapor. Our pictures of the gold surface are also consistent with the fact that these films are only slightly permeable to formic acid in the vapor phase. The observed infusion may be, in part, due to a specific chemical reaction between gold and thiocyanate. We are currently exploring this possibility.
Conclusiom We show for the first time inelastic electron tunneling spectra of gold-topped postfabricationdoped tunnel diodes. We find that the thiocyanate ion readily infuses into these Al-Al,OrAu junctions without the presence of water. These infused devlces
give a strong and distinct thiocyanate spectrum over more than 4 orders of magnitude variation in KSCN concentration; the band near 330 cm-I is still easily identified from junctions infused with lo-' mol/L KSCN in methanol. Comparison of tunneling spectra obtained by conventional doping of KSCN in Au- and Pb-topped junctions indicates that there is a thiocyanate-gold interaction resulting in the formation of a metal complex of some kind. This complex has an especially strong vibrational band(s) near 330 cm-'that signals the presence of SCN- over a wide concentration range. Tunneling spectra taken from infused and conventionally doped Au-topped junctions are essentially the same, indicating that the gold-thiocyanate reaction is not driven by the thermal energy of the gold as it is deposited. We tentatively conclude that gold-topped tunnel junctions may have especially high spectral sensitivity for thiocyanate because of this complex formation. STM images of the junctions before and after infusion suggest that the gold layer studied here is a much denser structure than the infusible lead layers studied previously.
Acknowledgment. This work was supported by the Environmental Protection Agency under Grant R-816329-01-0. Their assistance is gratefully acknowledged.
Ionlratlon-Locrs Stimulated Raman Spectroscopy of Jet-Cooled Hydrogen-Bonded Complexes C o n t a w Phends Gregory V. Hartland,+ Bryan F. Heason, Vincent A. Venturo, and Peter M. Felker*J Department of Chemistry and Biochemistry, University of California. Los Angeles, California 90024- 1569 (Received: August 30, 1991)
Stimulated Raman spectra of a variety of hydrogen-bonded, one-to-one complexes of phenol and substituted phenols are presented and discussed. The Raman spectra were recorded with ionization-lossstimulated Raman spectroscopy on supersonic molecular beam samples. Complexation-induced vibrational frequency shifts and vibrational line widths are reported for both the protondonating moieties (the phenols) and the proton-accepting moieties in the complexes. The vibrational frequency shifts are discussed in terms of the strength of the hydrogen bond and the usefulness of different vibrational modes as indicators of hydrogen bonding. particular attention is paid to the phenolic CO and OH stretch fundamentals. The line-width measurements suggest that vibrational modes closely associated with the hydrogen bond (Le., the phenolic CO and OH stretches) relax more efficiently than other modes in the complexes. Even so, the lifetimes of the excited OH stretches are at least a few picaseconds for several of the complexes. Evidence for mode selective dynamics in the OH stretch region of the substituted phenol-water complexes is also presented and discussed.
I. Introduction Hydrogen-bonded complexes involving phenol have been the subject of numerous molecular beam spectroscopy experiments.'+ The interest in phenol H bonding has several motivations. First, phenol is the simplest aryl alcohol. Its H-bonding interactions can serve as prototypes for the interactions of larger species. Second, the molecule undergoes a large increase in acidity upon electronic excitation into its SImanifold. This increase allows, under some circumstances, for the Occurrence of excited-stated proton transfer to proton-accepting solvent species.6.' A detailed understanding of such processes requires information conceming the solvation of phenol by H-bonding solvents. Third, the study of the H-bonding properties of phenol (and derivatives) can shed light on the analogous properties of tyrosine residues in proteins.l0 Thus, important information relevant to intraprotein and protein-solvent interactions may be obtained by studying the details of phenol H bonding. Fourth, H bonding involving phenol and phenol derivatives has been studied extensively in the condensed Current address: Departmentof Chemistry, University of Pennsylvania, Philadelphia. PA 19104-6323. 'NSF Presidential Young Investigator 1987-92.
0022-3654/92/2096-1164$03.00/0
phase." The potential exists, therefore, for making connections between results from these condensed-phase studies and results (1) (a) Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1982,86, 1768. (b) A h , H.; Mikami, N.; Ito, M.; Udagawa, Y. J . Phys. Chem. 1982,86,2567. (c) Abe, H.; Mikami, N.; Ito, M.; Udapwa, Y. Chem. Phys. Lett. 1982,93, 217. (d) Oikawa, A.; A h , H.; Mikami, N.; Ito, M. J. Phys. Chem. 1983,87, 5083. (e) Gonohe, N.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1985, 89,3642. (f) Ito, M.J. Mol. Srmr. 1988,177, 173. (g) Ebata, T.; Furukawa, M.; Suzuki, T.; Ito, M. J . opt. Soc. Am. B 1990, 7, 1890. (2) (a) Fuke, K.; Kaya, K. Chem. Phys. Lett. 1982, 91, 311. (b) Fuke, K.; Kaya, K. Chem. Phys. Lett. 1983,9497. (c) Fuke, K.; Yoshiuchi, H.; Kaya, K.; Achiba, Y.; Sato, K.; Kimura, K. Chem. Phys. Lett. 1984,108, 179. (3) Sur, A.; Johnson, P. M. J . Phys. Chem. 1986,84, 1206. (4) Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Chem. Phys. 1987,87, 115.
(5) (a) Lipert, R. J.; Bermudez, G.; Colson, S.D. J . Phys. Chem. 1988, 92,3801. (b) Lipcrt, R. J.; Colson, S.D. J. Chem. Phys. 1989,89,4579. (c) Lipert, R. J.; Colson, S.D. Chem. Phys. Lett. 1989, 161, 303. (6) Solgadi, D.; Jouvet, C.; Tramer, A. J . Phys. Chem. 1988, 92, 3313. (7) Steadman, J.; Syage, J. A. J. Chem. Phys. 1990, 92,4630. (8) Stanley, R. J.; Castleman, A. W. J . Chem. Phys. 1991, 94, 7744. (9) Reiser, G.; Dopfer, 0.;Lindncr, R.; Henri, G.; Mllller-Detlefs, K.; Schlag, E. W.; Colson, S. D. Chem. Phys. Lett. 1991, 181, 1. (10) Takeuchi, H.; Watanabe, N.; Satoh, Y.; Harada, I. J. Romon Specrrosc. 1989, 20, 233.
0 1992 American Chemical Society
