The Journal of
Physical Chemistry
0 Copyright, 1988, by the American Chemical Society
VOLUME 92, NUMBER 18 SEPTEMBER 8,1988
LETTERS Fluorescence Line Narrowing and Persistent Spectral Hole Burning of Cresyl Violet Adsorbed on Heterogeneous Surfaces Th. Basch6 and C. Brauchle* I n s t i t u t fur Physikalische C h e m i e der Uniuersitat ( R e c e i v e d : May 24, 1988)
Munchen, S o p h i e n s t r a s s e 11, 0-8000 Miinchen 2, FRG
We investigated Cresyl Violet (CV) adsorbed on y-alumina and silica gel with optical high-resolution techniques. Fluoresence excitation spectra indicated that CV is predominantly adsorbed monomeric when low coverages are used. This result was confirmed by fluorescence line narrowing (FLN) which furthermore unambiguously allows the determination of the vibronic transitions of adsorbed CV. Persistent spectral holes burnt into the So-S, transition of adsorbed CV are appreciably broadened when compared to those burnt in an amorphous glass. Supported by the detection,ofthe photoproduct,we suggest a photoinduced H transfer as a possible hole-burning mechanism.
Introduction The optical absorption and emission profiles of dye molecules adsorbed on heterogeneous surfaces are strongly broadened, reflecting the site inhomogeneities of the surface. This resembles the situation in glassy matrices where the inhomogeneous broadening in the optical spectra is due to different sites of the embedded impurity. Persistant spectral hole-burning and linenarrowing techniques have proven to be a powerful tool for eliminating the inhomogeneous broadening.’-* As a result, valuable information about the structure and dynamics of the impurity molecules and their environment can be obtained. As shown in two recent publications, persistent spectral hole burning3 and fluorescence line narrowing (FLN)4are also applicable for optical high-resolution studies of a completely new (1) Friedrich, J.; Haarer, D.Angew. Chem. 1984, 96, 96. (2) Personov, R. I. In Spectroscopy and Excitation Dynamics of Condensed Molecular Systems; Agranovich, V. M., Hochstrasser, R. M., Eds.; North-Holland: Amsterdam, 1983; Chapter 10. (3) Bogner, U.; Schatz, P.; Maier, M. Chem. Phys. Lett. 1985, 119, 335. (4) Deeg, F. W.; Brauchle, C. J . Chem. Phys. 1986, 85, 4201.
class of systems, namely, molecules adsorbed on surfaces. Here the inhomogeneous broadening that prevents detailed spectroscopic information is due to a variety of different adsorption sites on the surface. In this paper we report the first results of our studies of adsorbed dye molecules. The system under investigation was Cresyl Violet (CV) adsorbed on y-alumina and-silica gel. As demonstrated in the following sections, the combined use of hole burning and FLN techniques seems to be a powerful tool for clarifying details in the optical spectra of adsorbed molecules and for studying photon-induced surface phenomena. Also, a comparison with the results for CV in an ethanol glass as obtained by us and other in~estigators5a3~ is given.
Experimental Section y-Alumina (aluminum oxide, Merck) and silica gel (Merck) are white porous powders with a high specific surface area. The (5) (a) Thijssen, H. P. H.; van den Berg, R.; Volker, S. Chem. Phys. Lett. 1985, 120, 503. (b) Volker, S. J. Lumin. 1987, 36, 251.
0022-3654/88/2092-5069$01.50/00 1988 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
5070
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600 550 500nm EX C ITAT ION WAVEL ENG TH Figure 1. Fluorescence excitation spectra of Cresyl Violet embedded in an ethanol glass and adsorbed on y-alumina and silica gel (0 = coverage; T = 1.6 K). 650
surface area of the two adsorbents was determined by using the standard BET method: yielding a value of 110 m2/g for y-alumina and 500 m2/g for silica gel. The average pore sizes as given by the manufacturer are 100 and 60 A, respectively. Cresyl Violet (Lambda-Physik, laser grade) was adsorbed from a methanolic solution with known dye concentration. The filtered substrates were dried at 50 O C under high-vacuum conditions to evaporate the solvent. Finally, the coated powders were sealed off in glass tubes. Assuming a molecular area of 250 A2.for CV, a coverage (0) for different samples with 0.005 > B > 0.001 was estimated with respect to the BET surface area. Fluorescence excitation spectra were recorded with the dispersed (Spex 1402 monochromator) output of a 150-W xenon arc lamp. The fluorescence signal was monitored at wavelengths X > 650 nm with an RCA C3 1034 phototube whose output was connected to an Ortec photon-counting system. Spectral holes were probed in the same manner with a maximum spectral resolution of 0.9 cm-l. For hole burning either a He-Ne laser (line width 1 GHz) or a tunable dye laser (line width 3 GHz) pumped by the second harmonic of a Q-switched Nd:YAG laser was used. Fluorescence spectra were taken with different excitation conditions. The samples were excited either with the output of a Xe lamp followed by a low-resolution monochromator (Spex Minimate) leading to a spectral band-pass of -2 nm or by a He-Ne laser. The spectral resolution in emission was about 3 cm-' . The samples were mounted in a helium bath cryostat in direct contact with liquid helium. By reducing the pressure, we could achieve temperatures down to 1.4 K. The temperature was measured via the vapor pressure and simultaneously by a calibrated carbon resistor. Results and Discussion Figure 1 shows the fluorescence excitation spectra of CV in an ethanol glass and adsorbed on y-alumina and silica gel. First of all, it is seen that the long-wavelength region (S,-S,) of absorption is nearly the same for the three samples with a slight red shift for the adsorbed species. Anfinrud et a].,' who investigated CV adsorbed on quartz, found a strong absorption maximum at 530 nm which they interpreted to originate from the dimeric state ~
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660 650 640 nm WAVELENGTH Figure 2. Fluorescence spectra (7'= 1.6 K) of CV adsorbed on silica gel (a) and y-alumina (b). The samples are excited either with a broad-band light source (Xe arc lamp; spectral band-pass 2 nm) or with a He-Ne laser (line width 1 GHz; P = 20 pW/cm2). 670
of adsorbed CV. Also, for CV/y-alumina and CV/silica gel the absorption maximum is shifted to the short-wavelength side. However, the difference between the maximum and the 0,O absorption band amounts to -600 cm-', which is due to a vibration of CV. This vibration is also distinctly found in the FLN spectra (see below). Thus, this shift of the maximum reflects only the different environment for the adsorbed species due to a change in the Franck-Condon factors. We therefore conclude that the adsorption does not lead to a chemical modification of CV and that it is predominantly adsorbed monomeric. Fluorescence Line Narrowing. As the excitation spectra, the fluorescence spectra ( T = 1.6 K) of adsorbed CV obtained with broad-band excitation (spectral band-pass 2 nm) are strongly inhomogeneously broadened (Figure 2a,b) and the vibronic structure is completely smeared out. The use of a monochromatic = 6328 A) leads-within the laser line laser (He-Ne, A, width-to the selective excitation of a resonantly absorbing ensemble of the inhomogeneously broadened 0,O absorption band. The resulting fluorescence emission lines are drastically narrowed, and the vibronic structure of the SI-& transition shows up in the spectrum (Figure 2a,b). As mentioned above, the He-Ne laser excitation directly populates the 0,O absorption of CV and therefore-based on experimental reasons (scattered laser light)-the resonant fluorescence emission was not detected. During the recording of a spectrum the intensities of the various lines were diminished due to hole burning of the excited chromophores. The FLN spectra of CV/y-alumina and CV/silica gel are almost identical, referring to the relative intensities and spectral position of the fluorescence lines. This suggests a very similar adsorbate-adsorbent interaction which is quite reasonable taking into account that the surfaces of both materials are terminated by surface hydroxyl groups.* These are the predominant adsorption centers for the dye molecules. The site selection permits one to determine the vibronic transitions unambiguously. Here we want to concentrate on the dominant emission line in both spectra. Their distance to the resonant emission amounts to 598 cm-I (CV/y-alumina) and 603
~~~
(6) De Boer, J. H. In Surface Area Determination; Everett, D. M., Ottewill, R. H., Eds.; Butterworths: London, 1970; Chapter 1. ( 7 ) Anfinrud, P.; Crackel, R. L.; Struve, W. S . J Phys Chem. 1984,88, 5873
(8) Boehm, H. P.; Knozinger, H. In Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1983; Vol. 4, Chapter 2.
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The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5071
ads. onY-Alumina e 4 QOOL
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Figure 3. Hole-burning spectra of CV embedded in an ethanol glass and adsorbed on y-alumina and silica gel ( t = laser burning time). Hole width: ethanol, 0.92 cm-I; y-alumina, 2.83 cm-'; silica gel, 4.45 cm-'.
cm-l (CV/silica gel). According to a SERS study of the similar oxazine dye Nile Blue,9 this vibration is assigned to a skeletal deformation of the phenoxazine ring system. The value determined in this study was 593 cm-'. In an FT-IR study we obtained 590 cm-' for CV in KBr. Thus, the FLN results, too, suggest that the adsorbate-surface interactions do not distinctly alter the molecular shape and the vibronic coupling of adsorbed CV. Furthermore, the fluorescence emission excited at 6328 A refers to monomeric species as none of the spectral features points to dimers or higher aggregates. In contrast to laser line excitation, this conclusion cannot be drawn unambiguously with broad-band excitation. As noted in the figure captions, the experiments were performed at 1.6 K. When the samples are warmed up to 40 K, the narrow emission lines disappear due to the strong increase of the homogeneous line width. Hole Burning. With narrow-bandwidth laser excitation it was possible to burn persistent spectral holes into the S,-S, transition of adsorbed CV. The experiments were usually done with a CW He-Ne laser, but also irradiation with a pulsed dye laser (DCM) leads to the formation of spectral holes. With an irradiance of 40 pW/cm2 and a burning time of 15 s we obtained a hole width of 1.35 cm-I (fwhm) at 1.6 K for CV/y-alumina (maximum resolution 0.9 cm-I). Under these experimental conditions no holes could be detected for CV/silica gel with the apparatus used here. With nearly the same radiant exposure Thijssen et aLSafound a much narrower hole width (0.07 cm-' at T = 1.8 K) for CV in an ethanol glass. These authors used a single-frequency Rhodamine 6G dye laser with a bandwidth of a few megahertz. In Figure 3 spectral holes for the three systems mentioned above are presented. With the radiant exposure indicated in Figure 3 also holes for CV/silica gel could easily be detected. Of course, the hole widths obtained under these conditions do not reflect the true homogeneous line width but they allow a comparison of the systems under study. It is clearly seen that the zero phonon holes of the adsorbed species are appreciably broadened while the hole width for CV/ethanol is even smaller than the given value due to the limited monochromator resolution. As seen in Figure 3, the spectral hole for CV/silica gel is additionally broadened when compared to CV/y-alumina. Thus, hole broadening by heating ~~~~~~
(9) Miller, S. K.; Baitzer, A.; Meier, M.; Wokaun, A. J . Chem. Soc., Faraday Trans. 1 1984, 80, 1305.9.
of the surface during laser irradiation is ruled out taking into account the similar radiant exposure used for the three systems. Generally, heating of the surface cannot be neglected when high laser powers are used. The hole-burning mechanism for CV embedded in amorphous matrices seems to depend on the specific host used. In an investigation of CV embedded in a poly(viny1 alcohol) polymer it was claimed that hole burning is nonphotochemical.'O The reasoning was corroborated by hole-filling experiments. In a study of CV embedded in an ethanol glass and PMMA, it was assumed that CV undergoes an intermolecular photochemical reaction in which a charge redistribution should take p l a ~ e . * ~ A, photo~ product 50-400 cm-' higher in energy was reported. No further details were given. For CV adsorbed on y-alumina and silica gel, we assume a photoinduced H transfer from the excited dye to the surface as the most probable hole-burning mechanism. The assumption is strongly supported by the detection of a photoproduct absorption at -550 nm. This value corresponds to the absorption maximum of the deprotonated neutral dye stuff in solution. As mentioned above, the surfaces of the adsorbents are terminated by hydroxyl groups. The latter are thought to have slightly basic properties on y-alumina while those on silica gel show neutral to slight acidic behavior with regard to Brernsted acidity and basicity." Thus, y-alumina should be more easily protonated. The lower holeburning efficiency for silica gel may reflect the differing proton acceptor strength of the two adsorbents. Using the material presented above, it may be convenient to discuss possible causes of the hole broadening of adsorbed species. Referring to two recent publication^,^^'^ hole broadening seems to be a common feature of adsorbed molecules when compared to the same molecules embedded in amorphous hosts. The hole widths of CV embedded in amorphous silica and adsorbed on porous Vycor glass differed by an order of magnitude.I2 Unfortunately, no coverage was given for the coated Vycor glass. The coverage, however, and consequently the average distance of adsorbed molecules, are important parameters for the study of adsorbates. Bogner et aL3 suggested that adsorbate-adsorbate interactions and coupling between dye molecules and aggregates may provide a broadening mechanism for spectral holes. Such broadening may be ruled out for the low coverages studied here, but in order to examine to which degree such broadening effects are involved we are performing hole-burning experiments with higher coverages up to a monolayer. Locher et a1.I2 claimed tentatively that fast spectal diffusion may be of particular importance for hole broadening of adsorbed molecules. Such processes are on a time scale of I 0-8 < t < loo s.I3 A special kind of spectral diffusion, namely, fast proton back-transfer from the surface to the ground state of the neutral dye stuff, may also be responsible for hole broadening of adsorbed cv. Summarizing, at the moment it is not clear whether the increased hole width of adsorbed CV rather reflects the specific microscopic environment of a two-dimensionally disordered adsorbate/substrate system which is quite different from that of an embedded impurity or is due to a spectral diffusion process as mentioned above. Eventually, the measured hole width is a superposition of the two processes. However, the order of magnitude of the latter cannot be estimated qualitatively. Perhaps a theoretical approach applying models used for the glass state14 could provide further insights. Thus, it seems quite reasonable that on this heterogeneous surface the adsorbed chromophores are coupled to a distribution of two-level systems which are built up by the surface hydroxyl group configurations. To clarify the details of ( I O ) Carter, T. P.; Fearey, B. L.; Hayes, J. M.; Small, G. J. Chem. Phys. Lett. 1983, 102, 272. (1 1) Knozinger, H. In The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976; Vol. 111, Chapter 27. (12) Locher, R.; Renn, A.; Wild, U. P. Chem. Phys. Lett. 1987, 138, 405. ( 1 3) Haarer, D. In Persistent Spectral Hole-Burning: Science and Application; Moerner, W. E., Ed.; Springer: Berlin, 1988; Chapter 3. (14) Silbey, R.; Kassner, K. J . Lumin. 1987, 36, 283.
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the proposed mechanism and the hole broadening, we are starting investigations with other oxazine dyes and adsorbents with varying basicities of the surface hydroxyl groups. Preliminary experiments with Oxazine 170 showed similar results as with CV.
Acknowledgment. We thank D r . W. E. Moerner, IBM Almaden Research Center, San JOSE, CA, for helpful discussions
and preliminary hole-burning experiments with adsorbed resorufin. We also thank Prof. H. Knozinger, Institut fur Physikalische Chemie, Univeritat Munchen, for valuable information concerning the surface of the adsorbents used here and Prof. H.-P. Boehm and Mr. A. Guggenberger, Institut fur Anorganische Chemie, Universitat Munchen, for the determination of the BET surface areas.
Dissociative Attachment of Electrons to the A2E+ State of Nitric Oxide C.-T. Kuo, Y. One,+ J. L. Hardwick,* and J . T . Moseley Chemical Physics Institute, University of Oregon, Eugene, Oregon 97403-1274 (Received: June 3, 1988)
Production of 0- from thermal electrons has been identified during the laser excitation of nitric oxide at low pressure. The 0- ion is produced during excitation near 226 nm, corresponding to absorption in the (0,O) band of the A2Z+-X211 band system of NO. An electron gun provides electrons with energy limited to
