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Infrared (IR) photodetection is of major scientific and technical interest since virtually all molecules exhibit characteristic vibrational modes in the mid-infrared region of the spectrum, giving rise to molecular spectroscopy and chemical imaging in this wavelength range. High-resolution IR spectroscopies, such as Fourier Transform IR spectroscopy, typically require large, bulky optical measurement systems and expensive photodetector components. Here, we present a high-responsivity photodetector for the mid-IR spectral region which operates at room temperature. Fabricated from silicon and aluminum, the photodetection mechanism is based on free carrier absorption, giving rise to a photoresponse rivalling commercially available cooled IR photodetectors. We demonstrate that infrared spectra of molecules deposited on this detector can be obtained by a direct electrical read-out. This work could pave the way for simple, fully integrated chemical sensors and other applications, such as chemical imaging, which would benefit from the combination of mid-IR detection, room-temperature operation, and ultracompact portability.Infrared (IR) photodetection is of major scientific and technical interest since virtually all molecules exhibit characteristic vibrational modes in the mid-infrared region of the spectrum, giving rise to molecular spectroscopy and chemical imaging in this wavelength range. High-resolution IR spectroscopies, such as Fourier Transform IR spectroscopy, typically require large, bulky optical measurement systems and expensive photodetector components. Here, we present a high-responsivity photodetector for the mid-IR spectral region which operates at room temperature. Fabricated from silicon and aluminum, the photodetection mechanism is based on free carrier absorption, giving rise to a photoresponse rivalling commercially available cooled IR photodetectors. We demonstrate that infrared spectra of molecules deposited on this detector can be obtained by a direct electrical read-out. This work could pave the way for simple, fully integrated chemical sensors and other applications, such as chemical imaging, which...

By combining a first-principles periodic density functional theory calculation of adsorbate resonance widths with a many-body dynamical theory of charge transfer, we assess charge-transfer rates for ions scattering off metal surfaces. This goes beyond previous approaches, which have been limited to modeling the surfaces with either static potentials or finite clusters. Here we consider Li(+) scattering from an Al(001) surface. We show how the Li 2s orbital hybridizes with metal valence bands, near the surface, increasing the width of the 2s energy level. This in turn affects the charge-transfer rates between the ion and the metal surface. Our predictions for Li(+)-Al(001) scattering yield the correct angular dependence of the fraction of neutral Li atoms formed when compared to experiment.

We consider a one dimensional fermionic gas confined on a circle of finite length. At finite temperatures, in the absence of any background potential, one can show that a constant density of particles is a solution to the Hartree equation, regardless of the type of interparticle interaction. Moreover, at finite temperatures and small coupling constants, our previous analysis shows that this is the only solution of the Hartree equations. We show in this paper that, at zero temperature, there is another solution, which has an asymptotic expansion: n(x)=n0(1+ct.cos 2kFx)+o(λ), where λ is the coupling constant. Moreover, this solution is stable while the trivial solution become unstable at zero temperature.

We consider the Hartree approximation at finite temperature. We give a justification of this approximation by using the methods of functional integration. For finite temperatures, a fixed point approach to solving the Hartree problem is proposed. For a class of two-body interactions and background potentials that includes the Coulomb interaction, we prove that the Hartree equation has a unique solution provided the coupling constant is small. We also prove a similar result for local density theory.

We investigate the thermodynamic limit of the Hartree approximation for periodic background potentials and short range two-body interactions. We prove that, for any finite volume, the Hartree problem has a unique solution among the periodic densities of particles provided the coupling constant is smaller than a certain value. This value is independent of volume. We also prove that these solutions converge as the thermodynamic limit is considered.

Wavelet systems can be used as bases in quantum mechanical applications where localization and scale are both important. General quadrature formulas are developed for accurate evaluation of integrals involving compact support wavelet families, and their use is demonstrated in examples of spectral analysis and integrals over anharmonic potentials. In contrast to usual expectations for these uniformly spaced basis functions, it is shown that nonuniform spacings of sample points are readily allowed. Adaptive wavelet quadrature schemes are also presented for the purpose of meeting specific accuracy criteria without excessive oversampling.

Scattered Ne+ and recoiled S− ion fractions resulting from 4 keV Ne+ and 4 keV Kr+ impingement, respectively, on both the Cd- and S-terminated surfaces of CdS{0001} have been measured. The absolute values of these ion fractions as well as their dependence on surface structure and electron density have been determined. Using a density functional approach, a clear correlation has been demonstrated between these Ne+ and S− ion fractions and the lateral variation of the electrostatic potential along the outgoing trajectories of the scattered and recoiled atoms. The observed anisotropy in the ion fractions is a result of the variations in surface to atom electron transfer rates due to tunneling barriers introduced by the electrostatic potentials. Both the Ne+ and S− ion fractions are higher on the Cd-terminated surface than on the S-terminated surface and their azimuthal patterns are different due to the spatial modulation of the electron tunneling rates on the surface caused by the electrostatic barriers. The...

Dissociative electron attachment to the brominated methanes CH2Br2, CHClBr2, and CCl3Br is investigated by measuring the velocity and angular distributions of negative ions produced through electron transfer in collisions with velocity selected K(np) Rydberg atoms. The data are analyzed with the aid of a Monte Carlo collision code that models the detailed kinematics of the reactions. Measurements with CH2Br2 show that essentially all the excess energy of reaction appears in translation, indicating that the electron is captured directly into an antibonding orbital. Data for CHClBr2 show that reaction proceeds by two channels: in the first, essentially all the excess energy appears in translation; in the second, limited conversion of the excess energy to internal motions occurs, pointing to a CHClBr2−* intermediate lifetime of ∼0.1 to 1 ps. In the case of CCl3Br, electron transfer leads to the formation of both Br− and Cl− ions. Br− production is again associated with two channels: in one, essentially all t...

We report on the transmission of F+ and F− ions through ultrathin films of condensed water at 20 K, and compare the experimental results with theoretical calculations. The F+ and F− ions are produced by ESD (electron stimulated desorption) of a PF3 monolayer adsorbed on a Ru(0001) surface (PF3/Ru(0001) surface). We find two surprising results: (a) the off-normal F+ signal is attenuated to ∼1% by only ∼2.5 monolayer (ML) of H2O, while a much thicker layer, ∼10 ML of H2O, is necessary for equivalent attenuation of the F− ion emission, and (b) 1 ML of H2O increases the emission of F− ions and causes a dramatic change in the ion angular distribution. The striking changes in the angular distribution of F− ions transmitted through condensed H2O films indicate that elastic scattering is an important process in determining the attenuation of F− by H2O. No direct evidence for any kind of ion–molecule chemical reaction or collision induced dissociation reaction has been found. The strong attenuation of F+ without s...

Ammonia chemisorption on a small and positively charged gallium arsenide cluster in the gas phase has been investigated using density‐functional methods. The reactivity at various cluster sites are characterized in terms of binding energy and charge transfer from the adsorbate to the cluster with a strong correlation shown between the latter two quantities. The calculations show a strong dependence of the reactivity of the cluster on its charge state. Ionizing the bare cluster to a cationic charge state enables multiple chemisorption of ammonia, which is much less favored by a neutral cluster and can be prohibited on a negatively charged cluster.

It is well known that the chemical reactivity of small gallium–arsenide clusters can be sensitive to cluster size and geometrical structure. We have performed ab initio calculations probing the interaction of NH3 with different sites of a free Ga5As5 cluster. Our results show that bonding tends to be strongest at gallium sites. Adsorption of NH3 is found to induce strong relaxation of the cluster global geometry. The cluster sites are compared with sites on the (111) bulk surface of gallium arsenide, and the possibility for exploring the similarity between cluster reactivity and surface reactivity is discussed.

The electronic structure of recently discovered hollow‐cage molecule Ti8C12 has been calculated and compared to similarly shaped hypothetical molecules M8X12 (M=metal and X=B, C, or N). The cohesion of the cage is related to the σ‐bonding between M–X arising from d–sp hybridization. It is also shown that the cage can become electronically closed shell with all the valence electrons paired when the carbon is replaced by boron or nitrogen, e.g., in the formula of Sc8B12.

Spectral experiments on mass‐selected negative cluster ions of gold and silver were performed in the wavelength range near the threshold for one‐photon photodetachment of the extra electron. The Au−6 cluster ion displayed a uniquely well resolved spectrum consisting of a progression in a single vibrational mode. Details of this threshold photodetachment spectrum and the associated photoelectron energy distribution suggest an explanation based on autodetachment from totally symmetric vibrational levels of very weakly bound excited electronic state (bound by image charge forces) of the Au−6 cluster in the form of a planar, six‐fold symmetric, gold ring.

The coupling of optical excitation and electron transport through a sodium atom in a plasmonic dimer junction is investigated using time-dependent density functional theory. The optical absorption and dynamic conductance is determined as a function of gap size. Surface plasmons are found to couple to atomic-scale transport through several different channels including dipolar, multipolar, and charge transfer plasmon modes. These findings provide insight into subnanoscale couplings of plasmons and atoms, a subject of general interest in plasmonics and molecular electronics.

We investigate the conditions for perfect transmission of terahertz radiation in periodic two-dimensional plasmonic system of square holes. Changing the period in the direction perpendicular to the incident light polarization reveals the optimum areal coverage for perfect transmission at a specific period. The simulated near-zone energy flow distributions show that all incident light is accumulated on the apertures, under the condition that the period is shorter than the resonant wavelength, acting as static local capacitors restricted to a wavelength-confined region. Our finding can be exploited for designing cost-effective terahertz filters and may be applicable to terahertz spectroscopy requiring strong local field enhancement.

Using the plasmon hybridization method the authors examine light-induced coupling between the localized plasmons of a metallic nanosphere and the propagating plasmons of an infinite metallic wire. The plasmon resonances of the coupled system are found to be shifted in frequency relative to the plasmonic structure of the isolated nanoparticles. The magnitude of the shifts depends on the polarization of the incident light and the system geometry. In the limit of a thin wire, it is shown that the nanosphere can serve as an efficient nanoantenna which can couple incident electromagnetic radiation into low-energy propagating wire plasmons.

We extend the plasmon hybridization method to investigate the plasmon modes of metallic nanoshell dimers. The formalism is also generalized to include the effects of dielectric backgrounds. It is shown that the presence of dielectrics shifts the plasmon resonances of the individual nanoparticles to lower energies and screens their interaction in the dimer configuration. The net result is a redshift of dimer energies compared to the system without dielectrics and a weaker dependence of the dimer plasmon energies on dimer separation. We calculate the plasmon energies and optical absorption of nanoshell dimers as a function of dimer separation. The results are in excellent agreement with the results of finite difference time domain simulations.

We present a simple method to incorporate electrostatic forces into the extended Huckel scheme, thereby extending its range of application to ionic systems. A series of applications to alkali halide molecules and clusters shows the importance of including the Madelung energy in such calculations. The amount of charge transfer, the position of the valence levels, and the band gap are found to depend on the inclusion of the electrostatic potential. The calculated bulk and surface electronic structure compares well with experimental findings.

The binding of multiple deuterium (D) atoms to vacancies in iron (Fe) is investigated both experimentally, by use of ion‐beam techniques, and theoretically, by use of the effective‐medium scheme. The experimental D‐release stages are accounted for by trapping at vacancies with a binding enthalpy that depends on occupancy. It is found that for 1–2 D in a vacancy, the trap strength is 0.63 eV, whereas the binding enthalpy for 3–6 D in a vacancy is 0.43 eV. These results are in good agreement with predictions from the effective‐medium theory.