J . Phys. Chem. 1990, 94, 4913-4920 rate constants would be most helpful here. One important factor neglected thus far is the magnitude of geometry change required in the 0 - X coordinate along the lowest energy paths to reach the surface intersection seam. The change in equilibrium bond length from OX to OX- is +O. 1 1 8, for N O and +O. 13 8, for 02.Perhaps less potential energy is required to stretch OX into configurations suitable for electron acceptance for N O than for 02.Finally, as N 2 0has a special electronic problem along often noted by paths toward N2 + 0. The 'E+ground state of N 2 0 correlates with excited O('D) + N2('Zg+). This may contribute to the generally slow 0 atom abstraction rates with N 2 0 .
Summary Abstraction of 0 atoms from NO, 02,and N 2 0 by the ground-state transition-metal atoms Sc, Ti, and V is inefficient for all nine M OX combinations at 300 K. In all nine cases, the activation energy is small, below 2-4 kcal mol-'. The pattern
+
4913
of rate constants suggests that electron transfer from the metal atom to the oxidant may be the key mechanistic step. Measurement of the temperature dependence of the rate constants would provide an important experimental test 6f this suggested OX mechanism. Rate constants for excited-state M*(d"'s) reactions and electronic branching ratios into various M O product states would also be highly informative. Multiconfiguration ab initio quantum chemical calculations could test the electrontransfer model suggested here. Chemically accurate calculations on triatomic Sc OX systems are now feasible.
+
+
Acknowledgment. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation (Grant CHE-8703076) for support of this research. Registry No. Sc, 7440-20-2; Ti, 7440-32-6; V, 7440-62-2; NO, 10102-43-9; 0 2 , 7782-44-7; N20, 10024-97-2.
Time-Resolved Surface Second Harmonic Generation: A Test of the Method and I t s Application to Picosecond Isomerization in Adsorbates Stephen R. Meecht and Keitaro Yoshihara* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: September 25, 1989; In Final Form: December 27, 1989)
The application of time-resolved surface second harmonic generation (TRSSHG) as a surface-specific probe of picosecond reaction kinetics has been studied. The method was tested by comparing the measured dynamics with those obtained from the time-resolved fluorescence (TRF) technique. The agreement between the two methods was good. It is concluded that the TRSSHG method is a useful probe of picosecond surface chemistry. Small differences between fluorescence and second harmonic data are explained by a consideration of the different natures of the two experiments. The surface-specific method is essentially an analogue of transient absorption spectroscopy, except that induced changes in hyperpolarizability are measured. The combined results show that large-scale intramolecular motions can occur on a picosecond time scale in molecules adsorbed on solid surfaces. The rates of the isomerization reactions are contrasted with those measured in solution. It is concluded that the details of the adsorbate-substrate interaction, especially the effect on ground-state structure, determines the rate of isomerization.
I. Introduction Ultrafast laser spectroscopy has contributed greatly to the understanding of reaction dynamics in the vapor and condensed phases.' Given the importance of surface chemistry and catalysis it is clearly necessary to extend these methods to the study of surface reaction kinetics. Some recent progress is reported in ref 1 (Part VI, Chapter 8). The majority of subnanosecond timeresolved studies of surfaces have been made with the time-resolved fluorescence (TRF) techniques2 The method has high sensitivity and picosecond time resolution, though its applicability is obviously limited to fluorescent molecules. More recently picosecond transient absorption methods have been e m p l ~ y e d . ~However, both of these techniques suffer from the drawback that they are not surface selective. Therefore, they are, in general, not useful when the adsorbate is a minority species. The greatest advances in the understanding of the fundamental aspects of molecule surface interactions have been made by the application of the wide range of "surface science technique^".^ Recent progress has been made in time-resolved applications of some of these techniques,5 but the intrinsic time resolution seems to be limited to microseconds. Further, such methods usually rely on electron or mass spectrometric detection, so vacuum conditions are required. 'Permanent address: Department of Chemistry, Heriot-Watt University, Edinburgh EH 14 4AS, U.K.
0022-3654/90/2094-49l3$02.50/0
Clearly, important new information could be obtained if surface-specific all-optical methods with picosecond time resolution were applied to the study of surface reaction kinetics. Three methods seem potentially useful. Surface-enhanced Raman scattering (SERS) certainly fulfills the condition of being surface specific6 However, the method is limited to a few rough metal surfaces. Picosecond time-resolved measurements have not been reported. Recently Miller and co-workers developed a surface restricted version of picosecond transient grating scattering.' This method gave detailed information about some semiconductor interfacial phenomena and seems to have a wide range of potential applications. The most common of all the surface-specific optical methods is surface second harmonic generation (SSHG). In the dipole ( I ) See the series: Ultrafast Phenomena; Springer Series in Chemical Phvsics
(2) Thomas, J . K. J . Phys. Chem. 1987, 91, 267. ( 3 ) Wilkinson, F. J . Chem. Soc., Faraday Tram. 2 1986,82,2073. Ikeda, N.; Imagi, K.; Masuhara, H.; Nakashima, N.; Yoshihara, K. Chem. Phys. Leu.1987, 140,281. Heilwell, E. J.; Casassa, M.; Cavanagh, R.; Stephenson, J C. J . Chem. Phys. 1985, 82, 5216. (4) Somorjai, G.Chemisrry in Two Dimensions; Cornell: Ithaca, NY, 1981. ( 5 ) Ho, W . J . Phys. Chem. 1987, 91, 166. (6) Moscovits, M. Reu. Mod. Phys. 1985, 57, 783. ( 7 ) Kasinski, J.; Gomez-Jahn, L.; Faran, K.; Gracewski, S.; Miller. R. J . D. J . Chem. Phys. 1989, 90, 1253.
0 1990 American Chemical Society
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approximation S H G is forbidden in media with inversion symmetry. Since inversion symmetry is absent at an interface, SSHG is allowed and is surface specific.s The method has been applied extensively in steady state to study adsorption p h e n ~ m e n a ad,~ sorbate geometry and orientation,I0 surface symmetry,” and others.* Chemical reactions and equilibria have also been studied.l2.l3 Recently the spectroscopic potential of the method has been extended by the development of the infrared plus visible sum frequency analogue of second harmonic generation.14 Time-resolved surface second harmonic generation (TRSSHG) has been used in several studies of picosecond surface dynamics. The original report by Shank and c o - ~ o r k e r s ’monitored ~.~~ the surface melting of a Si( 1 1 1) crystal. More recently the method has been applied to picosecond studies of adsorbate dynamics. First reactions at a liquid-air interface were studied.” These measurements were extended to the solid-air interface.’* Phot o d e s ~ r p t i o n ’ ~and . ’ ~ energy transferZohave also been studied. Quite recently the sum frequency analogue was used to study vibrational dynamics in Langmuir-Blodgett films and adsorbates.21 I n this paper the recent observation of the picosecond isomerization dynamics of an adsorbate on a solid surface18 is extended to other systems, such as malachite green (MG). The results are contrasted with those reported for the solution phase, and discussed in terms of the adsorbate-substrate interactions. Importantly, for the solid-air interface a direct comparison of TRSSHG data with results from T R F is possible. This permits a direct experimental test of the accuracy of the dynamics measured in the TRSSHG method. This was required as high, possibly distorting, laser energy densities are employed. A consideration of the kinetics which are measured in the TRSSHG and linear methods permits some insight into the nature of the TRSSHG experiment. It is concluded that the method is essentially a surface-specific analogue of transient absorption. The important differences are (i j the transient molecular hyperpolarizability, rather than absorption, is measured and (ii) the polarization dependence is somewhat more complex. 11. Experimental Section An actively and passively mode-locked Nd:YAG laser operating at a 2-Hz repetition rate was used to generate 1064-nm pulses.22 A single pulse was selected from the pulse train and amplified in a chain of three amplifiers to an energy of 30 mJ. The pulse width was 25 ps. The pulses were converted to 532 nm in a KDP crystal, divided 50:50 at a beam splitter, routed through a normal pump and probe optical dela) arrangement, and recombined at the sample. The geometry for the pump and probe experiment was as (8)Shen, Y. R.Annu. Rev. Mater. Sci. 1986,16, 69 and references therein. (9)Tom, H. W . K.; Mate, C . M.; Zhu, X. D.; Crowell, J. E.; Heinz, T. F.;Somorjai, G.; Shen, Y. R. Phys. Reu. Lett. 1984,52, 348. ( I O ) Heinz, T. F.; Tom, H. W. K.; Shen, Y . R. Phys. Rev. 1983,A28, 1883. Kemnitz, K.; Battacharyya, K.; Hicks, J . M.; Pinto, G. R.; Eisenthal, K. B. Chem. Phys. Lett. 1986,131, 285. ( 1 1) Tom, H. W. K.; Heinz, T. F.; Shen, Y. R. Phys. Reu. Lett. 1983,51, 1983. Shannon, V . L.;Koos, D. A.; Richmond, G . L. J . Chem. Phys. 1987. 87, 1440. (12) Battacharyya, K.; Sitzmann, E. V.; Eisenthal, K. B. J . Chem. Phys. 1987. ~ . .87.. 1442. , (13) Battacharyya, K.; Castro, A.: Sitzmann, E. V.; Eisenthal, K . B. J . Chem. Phys. 1988,89, 3376. (14) Harris, A.; Chidsey, C.: Levinos, N. J.; Loiacono, D. N. Chem. Phys. Lett. 1987,141, 350. Hunt, J. H.; Guyot-Sionnest, P.; Shen, Y. R. Chem. Phys. Left. 1987,133, 355. (15) Shank, C. V.; Yen, R.; Hirlimann, C. Phys. Reu. Lett. 1983,51,900. (16)Tom, H. W.K.: Aumiller, G. D.: Brito-Cruz. C. Phys. Rev. Lett. 1983,60, 1438. ( 1 7 ) Sitzmann, E. V.; Eisenthal, K. B. J . Phys. Chem. 1988,92, 4579. (18)Meech, S. R.; Yoshihara, K. Chem. Phys. Lett. 1989. 154, 20. (19)Arjavalingham, G.; Heinz, T.F.; Glownia, J. H. Ultrafasf Phenomena V; Fleming. G., Siegmann. A,, Eds.; Springer Series in Chemical Physics: Berlin, 1986;Vol. 46, p 370. (20)Sitzmann, E. V.; Eisenthal. K. B. J . Chem. Phvs. 1989. 90. 2831. (21) Harris, A. L.; Levinos, N. J. J . Chem. Phys. 1989,90, 3878. Harris, A . I n Time Resolved Vibrational Spectroscopy IV; Princeton, June 1989. (22)Takagi, Y.;Sumitani, M.; Nakashima, N.; O’Connor, D. V . ; Yoshihara, K. Appl. Phys. Lerf. 1983.42. 489.
MG
’
Et
ClO,
DBI
PY2
CH,
CH3 DEI
Figure I. Chemical structure of molecules studied: malachite green oxalate (MG); 2-@-(dimethylamino)styryl)- I-ethylbenzothiazolyl iodide
(DBI); I-ethyl-4-(4-(p-(dimethylamino)phenyl)-l,3-butadienyl)pyridinium perchlorate (PY2); 3,3’-dimethyl-9-ethylthiacarbocyanine iodide (DEI).
previously shown.la The pump beam was weakly focused (1.1 mm diameter) onto the sample at 0’ angle of incidence. This geometry eliminated pump induced SSHG.23 The attenuated probe beam was focused to a spot size of 0.7 mm and was incident at 45’. The different probe beam polarizations employed are given in the text. The pump beam was not plane polarized due to depolarization in the optical path. The pump beam perturbed the sample at t = 0, and the time dependence of the relaxation was monitored by the surface second harmonic intensity due to the time delayed probe beam. The SSH signal was detected in reflection. The 266-nm signal was separated from the 532-nm fundamental by U V band-pass filters and an interference filter. It was detected by a solar blind photomultiplier tube (PMT) via a polariser. The anode signal from the PMT was amplified and recorded with a boxcar averager. Ten percent of the probe beam was split off and focused into a KDP crystal. The resultant 266-nm reference signal was detected by a fast diode and routed to a second boxcar averager. The averaged outputs from the two boxcars were ratioed in an analog processor, which output the square root of the result. Typically 10 or 30 shots were averaged. The output signal was measured as a function of the pump-probe time delay, which was controlled by a computer linked with the boxcar reference signal. The time resolution of the experiment was determined by placing the surface of a KDP crystal at the point of overlap of the beams and recording the autocorrelation in reflection. The measured autocorrelation function had a width at half-height of 30 ps and did not exhibit intensity in the “wings”. This measurement was also used to establish the position of time zero, but the slightly different geometries used in the autocorrelation and TRSSHG experiments introduced an uncertainty of f 3 ps. The time-resolved fluorescence (TRF) measurements were made using a single photon counting apparatus which has been described in detail elsewhere.24 (23)Mizrahi, V.;Sipe, J. J. Opr. SOC.Am. B 1988,5 , 660. (24)Kemnitz, K.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1988, 92. 39 1 5 ,
Surface Second Harmonic Generation
The Journal of Physical Chemistry, Vol, 94, No. 12, 1990 4915
Samples were prepared on 50 mm diameter silica disks of X/4 flatness. Before monolayer deposition the disks were cleaned by washing in nitric acid followed by sonication with a basic soap solution and finally with distilled water. Deposition was by the dipping method, which has been described in some detail.25 Briefly, the disk is lowered into a solution of the dye in propanol and then slowly (ca. 0.5 mm/s) withdrawn. As the solvent slides off the surface the dye was adsorbed. The coverage can be controlled by varying the concentration of the solution. The samples were mounted such that they could be rotated (translated) in (along) the plane of the disk. In this way it was possible to arrange for each pump-probe pair to sample a fresh portion of the surface. This meant that a homogeneous layer was required over a large area, which in practice requires rigorously clean substrates. All solvents were of spectroscopic grade. The dyes were PY2 ( 1 -ethyl-4-(4-(p-(dimethylamino)phenyl)-1,3-butadienyl)pyridinium perchlorate), DBI (2-(p-(dimethylamino)styryl)-lethyl-benzothiazolyl iodide), MG (malachite green), and DME (3,3’-dimethyl-9-ethylthiacarbocyanineiodide). The structures are shown in Figure I . 111. Results and Discussion A . Steady-State Measurements. ( i ) Absorption Spectra. In Figure 2 the absorption spectra of the dyes in the adsorbed state are compared with the spectra in free ethanol solution. Coverages (legend to Figure 2) obtained by the dipping technique described above were calculated assuming emax in the adsorbed state was the same as that in solution. Since the spectra have been perturbed by adsorption this assumption could lead to systematic errors in the coverage calculations. For all the SSHG experiments described below the coverages of Figure 2 were employed (f20%). Reducing the coverage by 50% gave qualitatively the same results but the signal to noise ratio was much degraded, the signal being quadratic in the adsorbate number density (Na),so quantitative data were not obtained.I8 For PY2 and DBI the adsorbed-state spectra are substantially blue-shifted relative to ethanol solution (35 f 5 nm and 50 f 5 nm, respectively). Conversely MG exhibits a I O f 5 nm red shift. The MG spectrum is substantially broadened on adsorption; this effect is less marked for PY2 and is essentially absent in DBI. In all cases the adsorbed state exhibits a longer tail to the red than the solution spectra; this will minimize the role of monomer to monomer excitation transport on the surface, which could affect the T R measurements, especially TRF. Over the limited range of 0.2-0.7 monolayer the spectra were independent of coverage. This suggests that dipole-dipole interactions, which are a prominent feature of rhodamine d y e ~ , 2 ~are , ~ ’not important in the cases considered here. It is tentatively proposed that adsorption is primarily due to the dimethylamino group, common to all the adsorbates. Support for this proposal comes from the null observation that identical techniques did not result in any measurable (by absorption or SSHG) adsorption of the dye DME. This dye has benzothiazolyl rings, like DBI, but lacks the NMe, group. Further support for this assignment comes from the observation that cresyl violet and pyronin B, which have the amino functional group, are readily adsorbed on silica.** Silica presents a heterogeneous substrate surface, but the most likely sites for adsorption are the free O H groups. These may be present on silica itself or arise from adsorbed water molecules.29 There is evidence for a strong interaction between amino groups and silica surface sites in the observation of the acidification of (25) Garoff, S.; Stephens, R.; Hanson, C. S.;Sorenson, E. K. Opt. Commun. 1982, 41, 257. (26) Petersen, E. S.; Harris, C. B. J . Chem. Phys. 1989, 91, 2683. (27) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1986, 90,5094. (28) Dilazzaro, P.; Mataloni. P.; De Martin, F. Chem. Phys. Leu. 1985, 114, 103. Anfrinud. P. A.; Cracknel, R . L.;Struve, W. S. J . Phys. Chem. 1984, 88, 5873. (29) Hair, M.: Herl. W . J. Phys. Chem. 1969, 73, 4269.
.
/ _ 0 / ’
\
‘.-
450 490 530 570 610 650 690 730
WAVELENGTWnm
WAVELENGTHhm
WAVELENGTHhm Figure 2. Absorption spectra of the dyes in the adsorbed state (solid lines) and in free ethanol solution (dashed). A sloping background due to the substrate, measured separately, has been subtracted from the spectra. (a) Malachite green adsorbed from a 1.5 X IO4 M solution in propanol, onto a flat (X/4) quartz plate. A coverage of 0.35 of a monolayer was calculated assuming emax = 8.1 X IO4 M cm-’ and Nu,,,, = IOl4 the number density of adsorbates for a full monolayer. (b) PY2, adsorption from a 2 X IO4 M solution, 0.25 monolayer for emax = 4 X IO4 M cm-I. (c) DBI,adsorption from a 2.5 X IO4 M solution, 0.4 monolayer for ,e, = 5.5 X IO4 M cm-I.
aminopyrene on a d s ~ r p t i o n . The ~ ~ effect is evidently less extreme in the present case. However, the large blue shift of the PY2 and DBI spectra could arise from a strong specific interaction of the N R 2 group with the surface hydroxyl groups, similar to that observed for (dimethy1amino)naphthalene derivatives in aqueous s~lution.~’The aqueous solutions indeed exhibit blue shifts relative to alcohol solution: 15 nm for DBI and 50 nm for PY2. To understand the features of the MG spectrum shown in Figure 2a the solvent effects on the free molecule spectrum were studied. Relative to ethanol solution the MG spectrum is redshifted and broadened in toluene, red-shifted in tetrahydrofuran, and blue-shifted in water. These data may be compared with Yip and co-workers’ detailed study of the solvent effect on the electronic spectra of crystal violet, the symmetrically trisubstituted analogue of MG.32 For MG the peak wavelength is more strongly dependent on the solvent polarity. This may reflect an increased (30) Hite, P.; Krasnansky, R.;Thomas, J. K. J . Phys. Chem. 1986, 90, 5795. (31) Meech, S. R.; OConnor, D.;Phillips, D.; Lee, A . G . J . Chem. Soc., Faraday Trans. 2 1983, 79, 1563. (32) Korppi-Tommola, J.; Kolehmainen, E.; Salo, E.; Yip, R. W . Chem. Phys. Leu. 1984, 104, 373. Korppi-Tommola, J.; Yip, R. W. Can. J . Chem. 1981, 59, 191.
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importance for the intramolecular charge-transfer states, resulting from the less symmetric structure of MG. For crystal violet in toluene the spectrum splits into two well-resolved bands. This was not observed for MG, though the spectrum does broaden. Insofar as the spectrum of adsorbed MG resembles any of the solution spectra. it is most similar to that in toluene. This is a surprising result, as there is adequate evidence that silica provides a binding site characterized by a high polarity. For example, the spectral shifts of rhodamine B2’ and the distribution of intensity among the vibronic bands of ~ y r e n both e ~ ~suggest an environment with a “polarity” close to that of water. Thus, the surface dielectric constant cannot easily be made to account for the observed MG spectrum. Apparently factors other than simple polarity arguments are needed to account for the spectrum of adsorbed MG. Two effects may be important. The splitting of the crystal violet spectrum in toluene was ascribed to the formation of an ion pair state.32 Such a mechanism could only be important in the present case if the surface is unable to stabilize the separated ions. A second important effect, which could be quite general when dealing with nonrigid adsorbates, is a structural perturbation on adsorption. It has been suggested that both a twisting of the phenyl rings34 or of the dimethylamino could lead to changes in the electronic spectra of triphenylmethane dyes. The existence of such structural changes to MG on adsorption would not be surprising; they have been observed for Ru(bpy)32f adsorbed on clays3*and bianthryl on vycor glass.36 The question arises as to whether or not the spectrum of Figure 2a can be assigned to two distinct ground-state isomers.34 It was observed (section IIIC) that the decay dynamics on the blue edge (532 nm, by TRSSHG) were in reasonable agreement with those on the red edge (626 nm, by TRF). This suggests that the two bands have a common ground state. I t should be noted that this does not imply that only a single isomer exists on the surface (indeed the T R F data (below) suggest a range of sites) but merely that the effect is not so dramatic as to lead to two distinct absorption bands. Thus, a model which is consistent with Figure 2a involves a perturbed ground state due to the interaction with the surface and. possibly? with an adsorbed counterion. The perturbation results in an enhanced S2 So transition strength relative to the solution spectrum. Distortions to the structure are evidently a more important influence on the spectrum than any general “surface polarity” effect. Such a model could be tested by accurate quantum chemical calculations. Further studies on the spectra and dynamics of adsorbed triphenylmethane dyes are in progress. Summarizing, the changes in absorption spectra on going from the solution to the adsorbed state can be understood on the basis of changes induced in the electronic and/or geometrical structure of the adsorbates. The dominant site for adsorbate-substrate interaction is probably the dimethylamino group and the free surface O H groups. The importance of these effects in understanding the isomerization dynamics of adsorbates will be discussed be Io w . (ii) SSHG Intensify. In principle, information concerning the adsorbate geometry can be obtained from the dependence of the SSHG efficiency on the incident polarisation.I0 In fact for the systems studied here, with resonant or multiply resonant fields and high coverage, the results should be interpreted with some caution. In particular local field effects could operate to distort the data.37 However, the ratio of the second harmonic intensities generated by s- and p-polarized incident radiation, r(0) = Is(2w)/Ip(2w),was approximately 0.5 for both PY2 and DBI. The similarity of r(0) for both of these rodlike adsorbates supports the
-
(33) Kaiyanasundram, K . ; Thomas, J. K. J . Am. Chem. Soc. 1977, 99, 2039. (34) Lewis, G . N.: Magel, T. T.; Lipkin. D. J . Am. Chem. SOC.1942,64, 1774. (35) Abdo, S . ; Canesson, P.; Cruz, M.; Fripat, J.; Van Damme. H. J . Phys. Chem. 1981, 85, 797. (36) Nakashima, N.; Phillips, D. Chem. Phys. Leu. 1983, 97, 337. (37) Bagchi, A.; Barrera, R. G.; Fuchs, R . Phys. Rei.. B 1982. 25, 7086. Y e , P.;Shen. Y R Phys. Rer. 1983. 28, 4288
Meech and Yoshihara
0.4~
1 y’ ~
0.2
P
f/
> 10
20
30
40
50
60
INTENSITY/mJcm.z
Figure 3. Dependence of the square root of the generated second harmonic intensity, E ( 2 w ) , on the incident energy density, I ( w ) .
idea that a common group interacts with the substrate. In the case of MG, which has a disklike structure, r(0) = 3 was observed. Both results suggest that the adsorbates are projecting out from the surface.Io Measurements at lower coverages and with a single resonant field are required for quantitative conclusions. For all the TRSSHG experiments described below the probe polarization which gave the maximum intensity was employed. Other aspects of the steady-state S S H G intensity deserve consideration. For the T R measurements it is clearly important that there exist a well-defined relationship between the probe intensity, I ( w ) , and the SSHG intensity, I ( 2 w ) . One e x p e c t ~ ~ , ~ ~ f(2w)
0:
P(w)
In Figure 3 E(2w) for DBI is plotted as a function of I ( w ) (E(2w) = [ ~ ( Z W ) ] ” ~The ) . plot shows the expected linear behavior up to 15 mJ/cm2, but at higher powers the signal saturates. This behavior is a result of exploiting a resonance at the fundamental frequency to enhance the SSHG signal. At the higher powers a significant fraction of the ground-state population will be promoted to SI during the pulse. This will in effect result in the removal of the resonant states and consequently an anomalously low SSHG efficiency. The relative efficiency of SSHG for the adsorbates used in this study was MG > PY2 = DBI. An important factor in determining SSHG efficiency is the extent of the charge-transfer character of the first absorption band.38 The high efficiency observed for MG supports the idea of significant charge transfer in this asymmetric triphenylmethane dye. In all experiments, less than 12 mJ/cm2 was used for the probe intensity, and for MG powers as low as 3 mJ/cm2 could be used, well within the linear region of Figure 3. Additional steady-state measurements were made concerning the efficiency of photodesorption, but these will be described in section HIE. B. Background to Time-Resolved Measurements. The SSHG signal intensity may be expressed as9%*) where C i s a constant which contains the angle of incidence, and f ( w ) and 1(2w) are the intensities at the fundamental and second harmonic frequencies. x:i)r is the effective second-order surface nonlinear susceptibility, which is in general a function of the polarization of the fields and the experimental eometry; these are fixed in the present experiments. Further, &if is dependent on the orientation of the adsorbate, which has an important bearing on the comparison with the T R fluorescence data (section IIIC). However, for the purpose of this section it is convenient to make the approximation (dropping subscripts) x ( ~=) Nad2),which neglects local field effects; N , is the number density of the adsorbate and is its effective hyperpolarizability. This approximation permits a focus on the populations, which are of primary interest in kinetics. As has been described above, in the TRSSHG experiment the monolayer is perturbed at t = 0 by the pump pulse, and the return (38) tilman. A. J Phys Chem. 1988, 92, 2384
The Journal of Physical Chemistry, Vol. 94, No. 12, I990 4917
Surface Second Harmonic Generation
for t > 0, where f l is the population in state 1 at t = 0 (for t < 0, E(2w) 0: NaaJ2)). When as is likely to be the 100 mJ/cm2 the efficiency was permanently reduced. For I > 300 mJ/cm2 a single pulse resulted in a decrease of E(2w) by up to 80%. At this intensity laser ablation is observed, and Figure 7a reveals its multiphoton character. The absence of any visible absorbing species remaining on the surface suggests that the dye is ejected intact. The result of a time-resolved study for DBI is shown in Figure 7b. Similar results have been obtained for rhodamine 6G and PY2 layers. The signal is bleached during the laser pulse. In an experiment where both incident fields are resonant with the SI So transition no conclusions regarding desorption dynamics can be drawn from the time course of the bleaching. However, the fact that the signal does not recover at all in the 300 ps demonstrates that desorption competes effectively with ground-state recovery. This suggests desorption on a time scale of
