The antiparallel and parallel beta sheets are two of the most abundant secondary structures found in proteins. Although various spectroscopic methods have been used to distinguish these two different structures, the linear spectroscopic measurements could not provide incisive information for distinguishing an antiparallel beta sheet from a parallel beta sheet. After carrying out quantum-chemistry calculations and model simulations, we show that the polarization-controlled two-dimensional (2D) IR photon echo spectroscopy can be of critical use in distinguishing these two different beta sheets. Particularly, the ratio between the diagonal peak and the cross peak is found to be strongly dependent on the quasi-2D array of the amide I local-mode transition dipole vectors. The relative intensities of the cross peaks in the 2D difference spectrum of an antiparallel beta sheet are significantly larger than those of the diagonal peaks, whereas the cross-peak amplitudes in the 2D difference spectrum of a parallel beta sheet are much weaker than the main diagonal-peak amplitudes. A detailed discussion on the origin of the diagonal- and cross-peak intensity distributions of both the antiparallel and parallel beta sheets is presented by examining vibrational exciton delocalization, relative angles between two different normal-mode transition dipoles, and natures of the cross peaks in the 2D difference spectrum.

The effects of a bismuth surfactant layer on the molecular beam epitaxy of GaAs and InxGa1−xAs layers on GaAs (001) were studied. The InxGa1−xAs surface reconstruction changed from arsenic stabilized 2×4 to bismuth stabilized 1×3 for high enough bismuth fluxes and low enough substrate temperatures. Maintaining a bismuth stabilized surface during InxGa1−xAs growth resulted in a larger number of reflection high-energy electron diffraction (RHEED) oscillations. RHEED patterns were also streakier after InxGa1−xAs growth with Bi. Roughness measurements using atomic force microscopy showed reduced root mean square roughness with Bi, e.g., from 3.8 to 2.8 nm, for 4 nm thick In0.3Ga0.7As layers. Simulations of x-ray diffraction results from 10 period In0.5Ga0.5As/GaAs superlattices showed that Bi reduced interface roughness from 1.1 to 0.5 nm and reduced interfacial broadening from 2.8 to 2.1 nm. The latter was attributed to reduced In segregation. InxGa1−xAs/GaAs (x=0.2–0.4) multiple quantum wells grown with Bi exhibited photoluminescence peaks that were more intense than those grown without Bi.