Subsurface Absorption Research Articles

Near-surface climate and surface energy budget of Larsen C ice shelf, Antarctic Peninsula

Abstract. Data collected by two automatic weather stations (AWS) on the Larsen C ice shelf, Antarctica, between 22 January 2009 and 1 February 2011 are analyzed and used as input for a model that computes the surface energy budget (SEB), which includes melt energy. The two AWSs are separated by about 70 km in the north–south direction, and both the near-surface meteorology and the SEB show similarities, although small differences in all components (most notably the melt flux) can be seen. The impact of subsurface absorption of shortwave radiation on melt and snow temperature is significant, and discussed. In winter, longwave cooling of the surface is entirely compensated by a downward turbulent transport of sensible heat. In summer, the positive net radiative flux is compensated by melt, and quite frequently by upward turbulent diffusion of heat and moisture, leading to sublimation and weak convection over the ice shelf. The month of November 2010 is highlighted, when strong westerly flow over the Antarctic Peninsula led to a dry and warm föhn wind over the ice shelf, resulting in warm and sunny conditions. Under these conditions the increase in shortwave and sensible heat fluxes is larger than the decrease of net longwave and latent heat fluxes, providing energy for significant melt.

First-Principles Assessment of H2S and H2O Reaction Mechanisms and the Subsequent Hydrogen Absorption on the CeO2(111) Surface

The main goal of this study is to assess the resistance of ceria against hydrogen penetration into its bulk, in the context of its application as a protective surface coating against hydrogen embrittlement in metals. We evaluate the reaction mechanisms between the H2S and H2O molecules and the CeO2(111) surface and their kinetic descriptors, using first principles based calculations in the density functional theory framework. Our approach is validated by performing an extensive comparison with the available experimental data. We predict that hydrogen penetration into CeO2(111) is a surface-absorption-limited process with a high-energy barrier (1.67 eV) and endothermicity (1.50 eV), followed by a significantly lower bulk dissolution energy and diffusion barrier (0.67 and 0.52 eV, respectively). We find that the presence of surface vacancies and higher coverages affects significantly the energetics of H2S/H2O adsorption, dissociation, and hydrogen subsurface absorption, facilitating most of these processes and degrading the protectiveness of ceria against hydrogen penetration. The reasons behind these effects are discussed. Overall we expect ceria to hinder the hydrogen incorporation significantly due to the effectively large energy barrier against subsurface absorption, provided vacancy formation is suppressed.

Reconstruction of sectional images in frequency-domain based photoacoustic imaging

Photoacoustic (PA) imaging is based upon the generation of an ultrasound pulse arising from subsurface tissue absorption due to pulsed laser excitation, and measurement of its surface time-of-arrival. Expensive and bulky pulsed lasers with high peak fluence powers may provide shortcomings for applications of PA imaging in medicine and biology. These limitations may be overcome with the frequency-domain PA measurements, which employ modulated rather than pulsed light to generate the acoustic wave. In this contribution, we model the single modulation frequency based PA pressures on the measurement plane through the diffraction approximation and then employ a convolution approach to reconstruct the sectional image slices. The results demonstrate that the proposed method with appropriate data post-processing is capable of recovering sectional images while suppressing the defocused noise resulting from the other sections.

Quantum nuclear effects on the location of hydrogen above and below the palladium (100) surface

We report ab initio path integral molecular dynamics simulations of hydrogen and deuterium adsorbed on and absorbed in the Pd(100) surface at 100 K. Significant quantum nuclear effects are found by comparing with conventional ab initio molecular dynamics simulations with classical nuclei. For on-surface adsorption, hydrogen resides higher above the surface when quantum nuclear effects are included, an effect which brings the computed height into better agreement with experimental measurements. For sub-surface absorption, the classical and quantum simulations differ in an even more significant manner: the classically stable subsurface tetrahedral position is unstable when quantum nuclear effects are accounted for. This study provides insight that aids in the interpretation of experimental results and, more generally, underscores that despite the computational cost ab initio path integral molecular dynamics simulations of surface and subsurface adsorption are now feasible.

Electronic damping of anharmonic adsorbate vibrations at metallic surfaces

The nonadiabatic coupling of an adsorbate close to a metallic surface leads to electronic damping of adsorbate vibrations and line broadening in vibrational spectroscopy. Here, a perturbative treatment of the electronic contribution to the lifetime broadening serves as a building block for a new approach, in which anharmonic vibrational transition rates are calculated from a position-dependent coupling function. Different models for the coupling function will be tested, all related to embedding theory. The first two are models based on a scattering approach with (i) a jellium-type and (ii) a density functional theory based embedding density, respectively. In a third variant a further refined model is used for the embedding density, and a semiempirical approach is taken in which a scaling factor is chosen to match harmonic, single-site, first-principles transition rates, obtained from periodic density functional theory. For the example of hydrogen atoms on (adsorption) and below (subsurface absorption) a Pd(111) surface, lifetimes of and transition rates between vibrational levels are computed. The transition rates emerging from different models serve as input for the selective subsurface adsorption of hydrogen in palladium starting from an adsorption site, by using sequences of infrared laser pulses in a laser distillation scheme.

Novel laser printing technique for miniaturized biosensors preparation

A new laser backward transfer technique for printing transparent liquids which avoids the drawbacks associated with the preparation of the liquids in thin film form has been developed. The principle of operation consists in strongly focusing a short laser pulse underneath the free surface of the liquid. Subsurface absorption of the laser radiation results in the generation of a bubble and the propulsion of liquid away the surface, leading to material deposition on a substrate facing that surface. The substrate, being traversed by the laser beam, must be transparent to the laser radiation. It has been demonstrated that there exists a focusing depth range which allows the deposition of well-defined, uniform microdroplets on the substrate. In addition, it has been shown that very high degrees of reproducibility and resolution can be achieved. Finally, the preparation of microarrays of different biomolecules (proteins and DNA) has revealed that the technique is feasible for miniaturized biosensors production.

Selective subsurface absorption of hydrogen in palladium using laser distillation

A theoretical model for the selective subsurface absorption of atomic hydrogen in a Pd(111) surface by infrared (IR) laser pulses is presented. The dynamics of the adsorbate is studied within the reduced density matrix approach. Energy and phase relaxation of the hydrogen atom are treated using the semigroup formalism. The vibrational excitation leading to subsurface absorption is performed using rationally designed pulses as well as IR laser pulses optimized on-the-fly. It is shown that dissipation can be used as a tool to transfer population to an otherwise inaccessible state via a mechanism known as "laser distillation." We demonstrate that when the reaction path is generalized from a reduced one-dimensional to full three-dimensional treatment of the system, the laser control strategy can prove very different.

Quantitative photoacoustic measurement of tissue optical absorption spectrum aided by an optical contrast agent

In photoacoustic imaging, the intensity of photoacoustic signal induced by optical absorption in biological tissue is proportional to light energy deposition, which is the product of the absorption coefficient and the local light fluence. Because tissue optical properties are highly dependent on the wavelength, the spectrum of the local light fluence at a target tissue beneath the sample surface is different than the spectrum of the incident light fluence. Therefore, quantifying the tissue optical absorption spectrum by using a photoacoustic technique is not feasible without the knowledge of the local light fluence. In this work, a highly accurate photoacoustic measurement of the subsurface tissue optical absorption spectrum has been achieved for the first time by introducing an extrinsic optical contrast agent with known optical properties. From the photoacoustic measurements with and without the contrast agent, a quantified measurement of the chromophore absorption spectrum can be realized in a strongly scattering medium. Experiments on micro-flow vessels containing fresh canine blood buried in phantoms and chicken breast tissues were carried out in a wavelength range from 680 nm to 950 nm. Spectroscopic photoacoustic measurements of both oxygenated and deoxygenated blood specimens presented an improved match with the references when employing this technique.

Coverage-dependent absorption of atomic hydrogen into the sub-surface of Cu(1 1 1) studied by density-functional-theory calculations

With density-functional-theory calculations, we have studied coverage-dependent absorption of H atoms into the sub-surface below a face-centered-cubic (fcc) hollow site of Cu(1 1 1). Both frozen and relaxed surface lattices were considered when the atomic H migrated from the surface to the sub-surface. The potential energy curve for the absorbing H shows that the surface site is in general favored over the sub-surface site, and this trend varies little with the H coverage (0.11–0.67 ML). If the hexagonal-close-packed (hcp) hollow sites immediately vicinal to the absorbing H are pre-adsorbed with other H atoms, the surface adsorption potential is greatly increased, because of the repulsive H–H interaction, to a value near, or even greater than, the sub-surface absorption potential; when two or three H atoms (on the hcp sites) are beside the absorbing H, the energy barrier for the sub-surface absorption is decreased, whereas that for diffusion from the sub-surface to the surface is enhanced. These results indicate that, on an H-saturated Cu(1 1 1) surface (0.67 ML), the sub-surface sites below the fcc sites with two or three neighboring H atoms can trap the sub-surface H.

Absorption-sensitive diffuse reflection imaging of concealed powders using a terahertz quantum cascade laser

We report diffuse reflection imaging in air of concealed powdered samples using a terahertz quantum cascade laser. The sensitivity of the detection scheme to sub-surface absorption within samples is confirmed using fully-characterized powdered admixtures of polystyrene and polymethyl methacrylate (PMMA). Measurements of the backscattering intensity from these samples are then used in conjunction with Kubelka-Munk scattering theory, as well as several models based on the quasi-crystalline approximation, to extract the absorption coefficient of PMMA. Our research demonstrates the feasibility of high-resolution frequency-domain terahertz imaging for the detection and identification of concealed powders in a reflection geometry.

Potassium intercalation in graphite: A van der Waals density-functional study

Potassium intercalation in graphite is investigated by first-principles theory. The bonding in the potassium-graphite compound is reasonably well accounted for by traditional semilocal density functional theory (DFT) calculations. However, to investigate the intercalate formation energy from pure potassium atoms and graphite requires use of a description of the graphite interlayer binding and thus a consistent account of the nonlocal dispersive interactions. This is included seamlessly with ordinary DFT by a van der Waals density functional (vdW-DF) approach [Phys. Rev. Lett. 92, 246401 (2004)]. The use of the vdW-DF is found to stabilize the graphite crystal, with crystal parameters in fair agreement with experiments. For graphite and potassium-intercalated graphite structural parameters such as binding separation, layer binding energy, formation energy, and bulk modulus are reported. Also the adsorption and sub-surface potassium absorption energies are reported. The vdW-DF description, compared with the traditional semilocal approach, is found to weakly soften the elastic response.

Room temperature SiO2∕SiO2 covalent bonding

Room temperature covalent bonds between bonded silicon oxide layers can be realized by forming surface and subsurface absorption layers followed by terminating outmost bonding surfaces with desired bonding groups prior to bonding. For example, by introducing fluorine into bonding oxide layers and NH2 groups onto surfaces of bonding oxide layers before bonding, bonding energy equivalent to silicon fracture energy (2500mJ∕m2) has been realized at room temperature after storage in air. Fluorine incorporation causes Si–O–Si ring breaking leading to fluorinated oxide formation with lower density, thus facilitating a higher diffusion rate of polymerization by-products and enhanced moisture absorptivity. Results indicate that by-products of the polymerization reaction between NH2 groups on mating surfaces appear to be more easily diffused and dispersed away from the bonding interface by the low density fluorinated oxide than are polymerization by-products of OH groups. This enhanced by-product removal results in covalent bonding at room temperature.

Preferential sub-surface occupation of atomic hydrogen on Cu(1 1 1)

We demonstrate sub-surface absorption of hydrogen on Cu(1 1 1) to be energetically favoured, even in the limit of zero coverage and without surface reconstruction. Helium atom scattering measurements indicate desorption of hydrogen from the surface at a lower temperature, than that required to remove all absorbed hydrogen. The stablest bonding site, which is invisible to the helium beam, is therefore not a surface site. Our experimental observations of the behaviour of Cu(1 1 1) differ from previous calculations and contrast with other systems, where population of subsurface sites generally occurs only after population of surface states, or after significant surface reconstruction.

An inversion method for parabolic equations based on quasireversibility

This paper is concerned with a new method to solve a linearized inverse problem for one-dimensional parabolic equations. The inverse problem seeks to recover the subsurface absorption coefficient function based on the measurements obtained at the boundary. The method considers a temporal interval during which time dependent measurements are provided. It linearizes the working equation around the system response for a background medium. It is then possible to relate the inverse problem of interest to an ill-posed boundary value problem for a differential-integral equation, whose solution is obtained by the method of quasireversibility. This approach leads to an iterative method. A number of numerical results are presented which indicate that a close estimate of the unknown function can be obtained based on the boundary measurements only.

Interaction of atomic hydrogen with metal surfaces

The interaction of atomic hydrogen (deuterium) with clean and precovered nickel and aluminum single-crystal surfaces has been investigated. The kinetics has been studied by thermal desorption spectroscopy and direct monitoring of the reaction products with a multiplexed mass spectrometer. Dynamical studies in terms of the rotational and vibrational state distribution of the reaction products have been performed using REMPI spectroscopy. The initial sticking coefficient of atomic hydrogen is close to unity on nickel and about 0.6 on aluminum. Subsurface absorption can be observed on nickel but not on aluminum single-crystal surfaces. Atomic hydrogen (deuterium) interacts with pre-adsorbed deuterium (hydrogen) in form of Eley-Rideal reactions, hotatom reactions and collision-induced desorption. The internal states of the reaction products (rotation, vibration) are highly excited.

The influence of molecular rotation on the direct subsurface absorption of H2 on Pd(111)

Within the generalized gradient approximation (GGA) of density functional theory (DFT) we have calculated a three-dimensional (3D) potential energy surface (PES) including an angular degree of freedom for a H2 molecule interacting with a Pd(111) surface. There is an entrance channel barrier (≈0.09 eV) to both dissociative chemisorption and direct subsurface absorption, but after this barrier is crossed direct subsurface absorption can proceed almost without a barrier. 3D quantum mechanical wave packet calculations incorporating the rotation of H2 in a plane perpendicular to the surface show a large part of the hydrogen going directly subsurface even at low incident kinetic energies. The wave packet calculations also show that in the low energy regime rotation inhibits direct subsurface absorption at low j0 and promotes it at high j0.

Dynamics and kinetics of subsurface absorption and desorption for the system hydrogen (deuterium)–Ni(111)

We have investigated the desorption kinetics of subsurface hydrogen (deuterium) from Ni(111) with angle resolved thermal desorption spectroscopy. For the first time we directly compare the desorption from subsurface states populated either by dosing with hydrogen atoms or by implantation of molecular hydrogen ions. For the various dosing modes subsurface desorption proceeds at clearly different temperatures. The angular distribution of the desorption probability for all subsurface states can be described by a cos n Θ function with n<1, whereas desorption from the surface states is strongly forward focused ( n=4.5). The initial sticking coefficient of H (D) on the clean Ni(111) surface is 1.0 and independent of the angle of incidence. The “initial absorption” coefficient for atomic hydrogen at the H-saturated surface is about 0.1. Interaction of atomic hydrogen with the H-saturated surface takes place in form of abstraction, adsorption and absorption simultaneously.

The dynamics of H absorption in and adsorption on Cu(111)

We have studied theoretically the dynamics of H adsorption on and absorption in Cu(111) using a classical molecular dynamics approach. A key ingredient of this study is plane-wave and pseudopotential calculations of potential energy curves for the major high symmetry sites. These calculations are based on density functional theory and the generalized gradient approximation. The extracted chemisorption bond parameters from the energy curves are in good agreement with available experimental data. We find that the calculated energy barriers for absorption of an adsorbed atom are lowered dramatically by relaxations of the Cu atoms; these barriers are so low that, even in the rigid surface lattice situation, the absorption of an incident H atom is non-activated for impacts close to the so-called fcc and hcp hollow sites. The model interaction potential that we have used in the dynamics calculations is determined from the calculated potential energy curves and its form is taken from a semi-empirical effective medium theory for binary compounds. The main results of the dynamics calculations are: the relaxations and thermal fluctuations of the Cu atom do not affect the absorption of H in the surface; the energy transfer to the phonons is rather inefficient so the H atom has to make a large number of collisions with the surface atoms before it sticks either in the surface adsorption well or in the subsurface absorption well; a simple estimation shows that the energy transfer to electron-hole pairs can be as efficient as the energy transfer to phonons; our results are consistent with experiments, which indicate that subsurface sites can be populated by an incident atomic H beam and show that the scattering probability is small.

Acoustic Imaging and Subsurface Absorption Structure of Sunspots

Recently, Chang et al. (1997, hereafter Paper I) developed a new method, based on time-distance curves (Duvall et al. 1993), to construct a three-dimensional acoustic intensity image of the solar interior. Here we report results from another data set.Solar p-mode waves, which are continually generated and dissipated stochastically by the turbulent convection, are scattered and absorbed by local inhomogeneities. If we appropriately add p-mode waves observed at the surface, based on the knowledge of how the waves propagate (time-distance curves), such that acoustic signals emanating from a particular point at a particular time are collected in phase, we can reconstruct the p-mode amplitude at the target point at the target time. The scheme of adding signals from a target point in phase plays the same role as a lens in optics. Thus we call it the “computational acoustic lens” (Paper I). This imaging technique can be used to construct acoustic amplitudes at any point in the solar interior based on the time-distance curve between that depth and the surface, which can be computed from a standard solar model based on the ray theory. The time series of constructed a plitudes provides information of both the intensity and the phase of the wavetrain. The intensity can be used to study p-mode absorption regions, and the phase can be used to probe other properties of local inhomogeneities, such as flow and change in wave speed.

The influence of surface motion on the direct subsurface absorption of H2 on Pd(111)

Based on density functional theory (DFT) within the generalized gradient approximation (GGA) we have calculated a three-dimensional (3D) potential energy surface (PES) for H2+Pd(111), depending on two hydrogen molecular degrees of freedom and one palladium surface degree of freedom. The PES is then used in 3D quantum mechanical wave packet calculations to investigate the effect of the surface motion on the direct subsurface absorption. We also compare the full 3D calculations to calculations where the surface motion is included through the sudden approximation. The calculations show a large downward shift of the onset energy for direct subsurface absorption upon inclusion of palladium surface motion (from 0.74 eV to 0.40 eV for H2). The use of the sudden approximation works well at collision energies greater than 1.3 eV (for H2), but leads to a significant underestimation of the direct subsurface absorption probability at lower energies.