We address theoretically and numerically the possibility of observing ellipticity in high-order harmonic generation (HHG) from aligned molecules driven by linearly polarized fields-a subject of controversy in the recent literature with significant implications. To that end we develop a numerical method for solution of the electronic dynamics and extend a recently developed theory of HHG from aligned molecules. Our numerical results are in good agreement with recent experimental data. The theory explains analytically several observations of polarization experiments. We note the conditions under which ellipticity can be observed and the information content of elliptically polarized harmonics regarding the molecular system.
Photoluminescent porous silicon has been produced in a variety of etchants using laser-assisted etching of silicon without an applied potential. Porous silicon (por-Si) produced from etchants containing K + , Cs + , or Rb + can have cubic crystallites of hexafluorosilicate on top of and embedded in the porous silicon film. These are formed during etching from the metal ion and the etch product, SiF 6 2-, via heterogeneous nucleation and growth. The photoluminescence of the hexafluorosilicate-coated por-Si on excitation with UV light is blue-shifted (590-610 nm), as compared to porous silicon that shows red photoluminescence (∼640 nm), even though the pure hexafluorosilicates exhibit no photoluminescence of their own.
We explore the information content of the polarization of high-order harmonics emitted from aligned molecules driven by a linearly polarized field. The study builds upon our previous work [Ramakrishna et al., Phys. Rev. A 81, 021802(R) (2010)], which illustrated that the phase of the continuum electronic wave function, and hence the underlying molecular potential, is responsible, at least in part, for the ellipticity observed in harmonic spectra. We use a simple model potential and systematically vary the potential parameters to investigate the sense in which, and the degree to which, the shape of the molecular potential is imprinted onto the polarization of the emitted harmonics. Strong ellipticity is observed over a wide range of potential parameters, suggesting that the emission of elliptically polarized harmonics is a general phenomenon, yet qualitatively determined by the molecular properties. The sensitivity of the ellipticity to the model parameters invites the use of ellipticity measurements as a probe of the continuum wave function and the underlying molecular potential.
Laser assisted etching of n-type silicon, without an applied potential, to form porous silicon has been studied in a variety of etchants. The range of pore diameters correlates with the ratio of the activities of HF and 2 HF − . Where the activity ratio 2 HF − : HF is low, small (<10 nm) pores are formed, while where 2 HF − : HF is high only larger pores (20-100 nm) are observed. The dependence of pore morphology on etchant composition demonstrates the importance of specific etch chemistry during pore formation. The composition of the film is also affected by the etchant. In particular hexafluorosilicates can deposit during porous silicon formation in solutions containing K + , Rb + and Cs + . The presence of hexafluorosilicates strongly affects the photoluminescence from the layers. We ascribe the strong, blue-shifted photoluminescence to emission from states at the hexafluorosilicate/Si interface.
We point to the ability of noncontact measurements of electron transport via self-assembled monolayers to provide chemical, A-resolved information about the underlying molecule. A conceptual framework is presented to model a current flow of soft electrons through a molecular monolayer to a substrate and explore the information content of this and other emerging noncontact measurements. A numerical scheme is developed where advantage is taken of the split-operator formalism to propagate the incident electronic wave function over a suitable periodic potential energy surface representing the self-assembled monolayer. The (experimentally observable) potential difference introduced by the transmitted electrons is extracted from the time-averaged electron density using the Poisson equation of classical electrostatics.
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