Complementary vibrational and reflectance anisotropy spectroscopic determination of molecular azimuthal orientation
Introduction
Reflectance anisotropy spectroscopy (RAS) was developed for in situ, real-time monitoring of semiconductor molecular-beam epitaxy (MBE) and metalorganic vapour phase epitaxy (MOVPE) growth processes [1], [2], [3], [4], [5]. The measured quantity is the reflectance anisotropy (RA) which is inherently surface sensitive for cubic systems. The anisotropy can be either ‘intrinsic’ or ‘extrinsic’ [6]; by ‘intrinsic’, we mean anisotropy derived from the clean surface related to surface states or resonances. ‘Extrinsic’ effects include adsorbate driven reconstructions producing new surface states or accessible intramolecular electronic transitions. This paper exploits the latter ‘extrinsic’ cause to determine the azimuthal orientation of a chemisorbed molecule.
In this paper, we demonstrate the complementary use of RAS and vibrational spectroscopies to detect molecular reorientation and azimuthal alignment. We studied the adsorption on Cu(110) of 9-anthracene carboxylic acid (9-AA), a polycyclic aromatic molecule which has intramolecular electronic transitions in the RAS optical range (1.5–5.5 eV), to explore the potential to monitor molecular orientation in organic film growth; for example, in synthesis of non-linear optical materials. This work complements a previous study [7] which focussed on the feasibility of RAS to determine azimuthal orientation, given the upright geometry of the carboxylate species (9-AC) on the oxygen pre-dosed Cu(110) surface. Here, we give vibrational assignments of the normal modes. On clean and p(2×1)O/Cu(110) surfaces, most 9-AC species have their aromatic plane perpendicular to the surface, but there is more azimuthal alignment to [11̄0] on the p(2×1)O surface than on the clean surface.
Section snippets
Experimental
9-Anthracene carboxylic acid was prepared as described previously [7]. This molecule adsorbs dissociatively to form the carboxylate (9-AC) at room temperature. The Cu(110) was cleaned by a sputtering (Ar+ +500 V) and annealing (∼800 K) cycles [7]. The RAS spectrometer has been described previously [1], [7]. RAIRS experiments (FT-IR Mattson Galaxy 6021) were performed in a separate UHV chamber [8]. HREELS measurements were made with a VSW HIB 1000 double pass spectrometer [9]. The resolution was
RAS and LEED
The real RA spectrum of the clean Cu(110) surface (Fig.1a) shows a band at 2.1 eV which has been assigned predominantly to a surface state transition at Ȳ for light polarised along [001] [13]. A range of adsorbates quench this surface state [13], [14], [15] similarly to the behaviour shown in the inset of Fig. 1 at 2.1 eV. A sequence of RA spectra acquired while dosing the clean surface is shown in Fig. 1, leading to the saturation coverage spectrum (Fig. 1b) and the LEED pattern shown. The RA
Summary
Whereas RAS clearly revealed anisotropies assigned to molecular azimuthal alignment for adsorption on the clean and p(2×1)O/Cu(110) surface, additional information from vibrational spectroscopy was required to determine the polar (tilt) angle. The rotation of the polycyclic ring away from the high symmetry direction is evident in HREELS from contributions of b2 modes in both high symmetry directions. In conclusion, the information from RAS and RAIRS are particularly complementary when
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