Langmuir 1989,5, 286-288
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effect diminishes by further addition of the ionic surfactant; this seems to correspond to the counterion condensation threshold. As noted above, the cut-off distances should not be taken too literally. Even within Bjerrum's model, this distance is only defined as a minimum of a painvise potential function. Thus the above analysis only points out a possible origin of the cloud point increase
phenomenon discussed. Further investigation is necessary in order to provide evidence of the change of micellar interactions around the micelle composition for which counterions start to condensate on the mixed micelle. Registry No. C U ( D S ) ~7016-47-9; , Triton X-100, 9002-93-1; Brij 35, 9002-92-0.
Notes Resonant Nonlinear Surface Spectroscopy: Range and Limitations Frank W. Gordon, Stephanie A. Cresswell, and Jack K. Steehler* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received August 2, 1988. I n Final Form: September 30, 1988
Introduction As has been amply demonstrated in recent years, x@) spectroscopy is a unique tool for probing surface structures with monolayer selectivity.' Signal generation has been observed from both the surface of the bulk substrate and from molecular adsorbates. Recently the emphases in this area have been on finding detailed explanations for observed signals2" (e.g., symmetry properties of signals from single-crystal surfaces) and defining unique applications for this surface probe. Applications emphasized by different groups include time-resolved monitoring of surface processes6 and selective monitoring of single species in mixture environment^.'-'^ The widest range of application of x@) spectroscopy results from the selective monitoring of adsorbates. Surface systems of practical importance including lubricants, corrosion inhibitors, surfactants, and electrochemical electrodes are potential users of this methodology. The initial demonstration efforts in this area have concentrated on the use of dye molecules as surface ad~orbates,'J'-'~ primarily for the convenient spectral resonances which they provide. A notable exception was the work of Van Wyck et al.,14where a ni..mber of nondye molecules were considered. In that work it was noted that a number of these molecules did not give observable x ( ~signals ) at the low laser intensities required to prevent loss of the adsorbate layer. This paper will report data for a range of molecular species on both the experimentally convenient fused silica surface and on metal surfaces, which are chemically more interesting. The nondye molecules chosen for these studies included systems of interest to electrochemical and corrosion inhibitor applications. The first question being asked is whether noncentrosymmetric surface layers are forming for all sample species. In general it will be seen that such layers do form for almost all cases. The second question is whether the intensities of adsorbate specific *Author to whom correspondence should be addressed. Current address: Department of Chemistry, Roanoke College, Salem, VA 24153.
0743-7463/89/2405-0286$01.50/0
signals are high enough to be conveniently useful in practical analysis of mixed surface systems. This desired condition will be seen to exist for only a subset of cases, including resonant experiments with molecules possessing strong optical transitions. If the selectivity of optical resonances is not required and high laser intensities can be tolerated, nonresonant experiments provide useful signals for a wider range of molecules.
Experimental Section Our experimental system has been described previously." Briefly, the system includes two dye lasers pumped by the same XeCl excimer laser, appropriate optics to generate and collect the s u m of the two laser frequencies, a 0.22-m double monochromator, gated detection, and photon-counting software. Laser wavelengths varied from 425 nm t o 1.06 pm, depending on the experiment. Nonresonant experiments were performed with 840-nmor 1.06-pm light (using a DCR 11NdYAG laser), where neither the incident laser frequencies nor the sum (or second harmonic) frequency was resonant with sample energy levels. Doubly resonant experiments were performed for rhodamine 590, nile blue 690, and cresyl violet 670. All other resonant experimenta were singly resonant, generally at the sum frequency, and sometimes used second harmonic generation from a single incident laser. The 1-in.-diameter metal disks were polished with alumina pastes down to 0.05 pm. All samples were spin coated onto the substrate from MeOH solutions M). Fused silica samples were used in a (typically 1 X transmission geometry while metal surface samples were used in a 45O reflection geometry. Molecules of electrochemical interest were also studied directly in solution environments with Ag, Cu, and Pt electrodes. Survey results are presented here with in-depth discussion of solution results to be reported separately. Similarly,
(1) Richmond, G. L.; Rojhantalab, H. M.; Robinson, J. M.; Shannon, V. L. J. Opt. SOC.Am. B 1987,4,228. (2)Shannon, V. L.; Koos, D. A.: Richmond, G. L. J.Phvs. Chem. 1987, 91, 5548. (3)Epperlein, D.; Dick, B.; Marowsky, G.; Reider, G. A. Appl. Phys. B 1987,44,5. (4)Sipe, J . E.; Moss, D. J.; van Driel, H. M. Phys. Rev. B 1987,35, 1129. (5)Guyot-Sionnest, P.;Chen, W.; Shen, Y. R. Phys. Reu. B 1986,33, 8254. (6) Shannon, V. L.; Koos, D. A.; Robinson, J. M.; Richmond, G. L. Chem. Phys. Lett. 1987,142,323. (7) Cresswell, S. A.; Steehler, J. K. Appl. Spectrosc. 1987,41,1444. (8)Harris, A. L.; Chidsey, C. E. D.; Levinos, N. J.; Loiacono, D. N. Chem. Phys. Lett. 1987,141,350. (9)Zhu, X. D.; Suhr, H.; Shen, Y. R. Phys. Rev. B 1987,35,3047. (10)Guyot-Sionnest,P.;Hsiung, H.; Shen, Y. R. Phys. Reu. Lett. 1986, 57,2963. (11)Cresswell,S. A.;Steehler, J. K. Appl. Sepctrosc. 1987,41,1329. (12)Marowsky,G.; Gierulski,A.; Dick, B. Opt. Commun. 1985,52,339. (13)Nguyen, D. C.;Muenchausen, R. E.; Keller, R. A.; Nogar, N. S. Opt. Commun. 1986,60,111. (14)Van Wyck, N. E.; Koenig, E. W.; Byers, J. D.; Hetherington, W. M . 111. Chem. Phys. Lett. 1985,122,153.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 1, 1989 287
Notes Table 1. Signal Levels for Multimonolayer Resonant Experiments on Fused Silica' molecule coumarin 440 coumarin 460 coumarin 480 coumarin 500 coumarin 540A rhodamine 590 sulforhodamine 640 cresyl violet 670 nile blue 690 oxazine 725 LDS 698 1-naphthylacetonitrile 1-nitronaphthalene 2-methoxynaphthalene 2-naphthaldehyde 1,4-naphthoquinone 1,2-naphthoquinone hydroquinone benzotriazole
low
-
0 0
+ + ++ ++ + + 0 0 0 0
+ + + 0
power medium -
high
-
++ ++ -
-
+++ ++ + + + ++ +0
'For all tables: - indicates no data available, 0 indicates no signal observed, + indicates signal < 0.1 photon/pulse, ++ indicates 0.1 photon/pulse < signal < 0.5 photon/pulse, and +++ indicates signal > 0.5 photon/pulse. Detection limit is -0.003 photon/pulse. For Table I: low power is 30 (mJ/cm2)/ pulse.
Table 11. Signal Levels for Multimonolayer Nonresonant ExDeriments on Fused Silica' molecule PBBO BPBD 365 coumarin 500 coumarin 540A rhodamine 560 rhodamine 590 rhodamine 610 kiton red 620 rhodamine 640 sulforhodamine 640 DCM nile blue 690 oxazine 720 oxazine 725 LDS 722 3,3'-dihexyloxacarbocyanineiodide isoquinoline pyridine 2,g-lutidine aniline 2-hydroxypyridine hydroquinone 1,4-naphthoquinone 1,2-naphthoquinone 2-mercaptobenzothiazole stearic acid
low
power medium
high 0
+++ +++ +++ +++ +++ +++ +++ +++ +++ +++ +++ +++ +++ 0 0 0 0 0
+++ +++ +++ -
'Low power is lo00 (mJ/cm2)/pulse. surface coverage dependencies, which differ for different surfaces and for resonant versus nonresonant cases, will be published separately. The multimonolayer coverages indicated in the tables yield essentially the same signal levels as a single-monolayer sample.
Results and Discussion The data obtained are presented in Tables I-IV. Compounds in these tables are grouped to best show chemical similarities, as follows. All dye molecules are listed first, ordered by increasing absorption wavelengths,
Table 111. Signal Levels for Multimonolayer Resonant Experiments on Various Metals' molecule rhodamine 590 rhodamine 590 nile blue 690 nile blue 690 nile blue 690 nile blue 690 nile blue 690 3,3'-dihexyloxacarbocyanine iodide 1-naphthylacetonitrile 2-methoxynaphthalene 2-naphthaldehyde 1,4-naphthoquinone 1,2-naphthoquinone 1,2-naphthoquinone 1,2-naphthoquinone 1,2-naphthoquinone hydroquinone pyridine isoquinoline 2,6-lutidine aniline
power low medium
metal cu Ag Ai3 CU Pt Au A1 Au Ag, Au, Cu, Pt, A1 Ag, Au, Cu, Pt, A1 Ag, Au, Cu, Pt, A1 Pt, Ag Pt cu Au Ag
Pi, Ag Ag, Au, Cu, Pt, A1 Ag, Au, Cu, Pt, A1 Ag, Au, Cu, Pt, A1 Ag, Au, Cu, Pt, A1
'Low power is
