208
J. Phys. Chem. C 2009, 113, 208–212
Surface Raman Spectroscopy of trans-Stilbene on Ag/Ge(111): Surface-Induced Effects Li-Wei Chou,† Ya-Rong Lee,‡ Ching-Ming Wei,‡ Jyh-Chiang Jiang,*,† Jiing-Chyuan Lin,*,‡,§ and Juen-Kai Wang*,‡,# Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei, Taiwan, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, Department of Chemistry, National Taiwan Normal UniVersity, Taipei, Taiwan, and Center for Condensed Matter Sciences, National Taiwan UniVersity, Taipei, Taiwan ReceiVed: August 4, 2008; ReVised Manuscript ReceiVed: October 24, 2008
The vibrational features of self-organized submonolayer trans-stilbene on the Ag/Ge(111)-(3 × 3)R30° surface have been investigated by Raman spectroscopy in ultrahigh vacuum at 100 K. A red shift in the CdC stretching mode of the olefinic group at 1625 cm-1 and enhanced Raman activity of the peak at 1568 cm-1 were observed, as compared with those corresponding to a multilayer trans-stilbene. Electronic interaction between trans-stilbene and the underneath surface and Raman spectra were calculated based on density functional theory. The calculated Raman spectra are in good agreement with the experimental results, supporting the calculated molecular geometry of the adsorbed trans-stilbene, which exhibits a predominant bond elongation in the olefinic group and a torsional angle between the phenyl ring and the olefinic plane. Analysis of the partial density of states shows that the lowest unoccupied molecular orbital is broadened and lowered to cross the surface Fermi level by the interaction with the surface, facilitating surface charge transfer and thus destabilizing the CdC double bond. 1. Introduction Stilbene is a prototype molecule for an important reaction class that involves internal rotation around a carbon-carbon double bond upon UV illuminationstrans-cis photoisomerization. Its reaction dynamics have been heavily studied in both gas and solution phase.1-6 Recently, we have made use of scanning tunneling microscopy to directly examine the photoisomerization reaction of stilbene molecules adsorbed on the Ag/Ge(111)-(3 × 3)R30° (abbreviated as Ag/Ge(111)-3) surface.7 The observed pairwise photoisomerization process has revealed its synergetic nature under the influence of the surface. A question then emerges: how do the adsorbed stilbene molecules prepare for this photoisomerization reaction? It is known that the one-bond-flip mechanism of trans-cis photoisomerization is associated with the conformation-relevant energy states prior to the UV photoexcitation.1,8,9 The molecular planarity and the olefinic bond strength of stilbene molecules, revealed by vibrational spectroscopy, have been considered as critical information for understanding and inferring the photoisomerization dynamics in gas and solution phases.1,10 The vibrational characteristics of adsorbed stilbene molecules therefore can hold the key to answer the question above, because any slight alternation of molecular structure and bonding strength upon adsorption can be clearly revealed in the corresponding change in the vibrational signatures. Vibrational modes of adsorbed molecules have been popularly investigated with high-resolution electron energy loss spectroscopy (HREELS).11 For example, Tautz and co-workers have applied this technique to investigate the vibrational properties * Corresponding author. E-mail: J.-K.W.,
[email protected]; J.C.L.,
[email protected]; J.-C.J.,
[email protected]. † National Taiwan University of Science and Technology. ‡ Academia Sinica. § National Taiwan Normal University. # National Taiwan University.
of 3,4,9,10-perylene-tetracarboxylic dianhydride adsorbed on Ag surfaces12,13 and observed shifting up to ∼50 cm-1, which indicates the change of the force constant within the molecule induced by the interaction with the metal. HREELS has also recently been used to investigate the trans-cis isomerization reaction of tetra-tert-butylazobenzene adsorbed on the Au(111) surface.14 The isomerized products were, however, not wellresolved in the observed vibrational spectra due to the limited spectral resolution of HREELS. In comparison, surface optical vibrational spectroscopy (infrared absorption, Raman scattering, and sum-frequency generation) probes the adsorbed molecule species15 and their dynamics16 on surfaces with high precision because of the fingerprint-typed signature and the wellunderstood spectral profile provided by vibrational spectra. Owing to the simplicity of the experimental setup of Raman spectroscopy, surface Raman spectroscopy can become a general technique to investigate surface molecular adsorbates. The change of the vibrational frequency and the Raman cross section extracted from the Raman spectra of the molecular absorbate reflects the molecular conformation alternation due to the interaction between the molecule and the surface which cannot be monitored by other means. Moreover, these vibrational characteristics provide a stringent test of high-precision calculations. In this study, we acquired vibrational frequencies of transstilbene (TSB) adsorbed on the Ag/Ge(111)-3 surface with Raman spectroscopy and verified the calculated results with density functional theory (DFT). The calculation then served as the basis for an in-depth understanding of the molecule-surface interaction. In this report, the experimental setup to prepare the sample and to perform Raman spectroscopy is presented first. It is followed by a brief description of the methods used to calculate the electronic structures and the Raman polarizability of free TSB and adsorbed TSB on the Ag/Ge(111)-3 surface. The experimental and the calculated results are presented subse-
10.1021/jp806936d CCC: $40.75 2009 American Chemical Society Published on Web 12/11/2008
Spectroscopy of trans-Stilbene on Ag/Ge(111)
J. Phys. Chem. C, Vol. 113, No. 1, 2009 209
Figure 1. Schematic layout of surface Raman spectroscopy. L1 and L2, spherical lenses; LPF, long-pass filter; LC1 and LC2, cylindrical lenses; P, polarizer; λ/2, wave plate; LF, laser line filter.
quently. We then discuss the electronic interaction between TSB and the Ag/Ge(111)-3 surface based on the two calculated results and present its implication from Raman shifts of TSB arising from the surface interaction.
Figure 2. Experimental Raman spectra of 20 and 0.75 ML TSB on the Ag/Ge(111)-3 surface. Characteristic peaks were obtained by curve fitting with Lorentzian profiles (solid curves) and marked numerically.
2. Experimental and Computational Methods Surface Raman spectroscopy was performed in a homemade ultrahigh-vacuum (UHV) chamber (
