Near Edge X-Ray-Absorption Fine-Structure Determination of Alkyl-Chain Orientation: Breakdown of the "Building-Block" Scheme

Abstract

ERRATA Near Edge X-Ray-Absorption Fine-Structure Determination of Alkyl-Chain Orientation: Breakdown of the "Building-Block" Scheme [Phys. Rev. Lett. 67, 851 (1991)1 G. Hahner, M. Kinzler, Ch. W611, M. Grunze, M. K. Scheller, and L. S. CederbaumIn the Letter we reported NEXAFS (near edge x-ray-absorption fine-structure) measurements for different thin films containing long alkyl chains, the analysis of which indicated that the so-called "building-block" scheme commonly used to interpret NEXAFS data for larger molecules fails to describe the angular dependence of the C-C resonance at 293.4 eV. We also presented results of ab initio SCF-RHF calculations which supported our experimental conclusions. In the meanwhile, however, we discovered an error in the plotting procedure for the transition dipole moments (TDM) as obtained from the ab initio calculations. When correctly evaluated the calculations indicate the presence of two resonances with significant TDM's at the energy where experimentally the C-C resonance is observed. This behavior is not predicted by the building-block scheme and has not been observed experimentally, as the C-C resonance half-width («2.8 eV) is larger than the splitting ( -0.7 eV). From their magnitude and orientation (parallel and perpendicular to the chain axis, respectively, with both of them are in the C-C-C plane) an intensity ratio of 2.2:1 for the C-C resonance at normal incidence versus grazing incidence is obtained, which does not differ significantly from the ratio predicted by the building-block scheme (2:1).The experimental analysis reported in our Letter, however, is not affected by this error, and the conclusion, namely, that there is a breakdown of the building-block scheme, is still considered correct. Note that a recent report [1] on high-resolution NEXAFS measurements on thin layers of hexatriacontane [CF^CFh^CHs] on Cu substrates presents virtually the same experimental angular dependence of the C-C resonance.The discrepancy between the (correctly plotted) SCF-RHF calculations and our experimental data for the orientation of the C-C resonance transition dipole moments is partially related to general problems with the theoretical description of virtual orbitals with energies considerably above (> 3 eV) the absorption-step edge. We would also like to mention that the calculated TDM's show an interesting strong dependency on the site of the core hole. With regard to a better understanding of the theoretical aspects this observation should be further pursued.We would like to thank Dr. P. Bagus (IBM Research Division) for very helpful discussions on SCF-RHF calculations on alkyl chains and for providing us with results of his own calculations. We have also profited from discussions with Dr. J. Stohr (IBM Research Division).[1] T. Ohta, K. Seki, T. Yokoyama, I. Morisada, and K. Edamatsu, Phys. Scr. 41, 150 (1990).

Self-assembled monolayers of perfluoroterphenyl-substituted alkanethiols: specific characteristics and odd–even effects

“…The assignments of the weak resonances 2 0 , 3 0 , and 3 00 are elusive. The characteristic R* resonance of the aliphatic linker at B287.7 eV [87][88][89][90] is presumably weak and overlaps with the C 1s C-Fp* feature, so that no information on this linker can be obtained from the NEXAFS data. Along with the above analysis of the NEXAFS spectra, the linear dichroism effects in the target films were monitored.…”

Orientation and order in microcontact-printed, self-assembled monolayers of alkanethiols on gold investigated with near edge x-ray absorption fine structure spectroscopy

“…͑1͒ that the signal of randomly oriented transition dipole moments cannot be distinguished from the signal of dipole moments with a specific tilt angle of 54.7°͑''magic angle''͒, both being invariant with . 24,25 In principle, the tilt angle of a molecular orbital can be found by determining the intensity of the corresponding resonance as a function of the angle of incidence . The difference in the lateral force between both regions can be estimated on the basis of the images together with single line scans ͑not shown͒, the images indicating the localization of the differently prepared regions.…”

Calibrated NEXAFS spectra of common conjugated polymers

“…[1][2][3][4] In NEXAFS, photoabsorption creates excited states that can be described by electronic configurations in which a core electron is promoted to an energy level (orbital or band) that is empty in the ground state. 4,8,9 It is thus difficult to predict the NEXAFS spectra of conjugated polymers on the basis of their functional groups and backbone composition. In addition, due to a close relationship between molecular bonding and the energy and spatial distribution of the empty electronic levels, unique spectroscopic fingerprints can be assigned to specific chemical moieties.…”