Incident Angle Θi Research Articles

Thermally tunable nonreciprocal radiation in lithography-free vanadium dioxide-dielectric-Weyl semimetal stack

Nonreciprocal thermal radiation (NTR) holds great promise in revolutionizing energy harvesting and conversion process, has become a hot topic of various research fields ranging from renewable energy, molecular sensing, to thermal circuitry and camouflage. Despite intensive effort into enhance the nonreciprocal response, static manipulation of NTR has limited the potential application, dynamic control of thermal radiation is highly demanded. Here, by using a multilayered stack comprised of vanadium dioxide (VO2) thin film, germanium (Ge) spacer layer, Weyl semimetal (WSM) layer terminated by molybdenum (Mo) substrate, we propose a thermally tunable nonreciprocal emitter. The effects of the incident angle (θi), azimuthal angle (ϕi), and the thickness of each layer for the VO2/Ge/WSM stack on the characteristics of thermal radiation have been discussed thoroughly. Dominant peaks and dips of nonreciprocity have been obtained with an incident angle of θi=60° and azimuthal angle of ϕi=0°. A switching effect between the near-unity nonreciprocity (”ON”) and zero nonreciprocity (”OFF”) states are induced when the temperature increases from 20 °C to 68 °C. It is shown that the magnitude and sign of the nonreciprocity can be managed simultaneously for transverse electric (s-) and transverse magnetic (p-) polarizations. The proposed structure shows good near-unity nonreciprocity stability in a wide range of θi and ϕi. This structurally lithography-free nonreciprocal thermal emitter not only provides the ability to tune nonreciprocal radiative properties, but also leads to the possible development of power scavenging and conversion devices.

Reflection imaging with a helium zone plate microscope

Neutral helium atom microscopy is a novel microscopy technique which offers strictly surface-sensitive, non-destructive imaging. Several experiments have been published in recent years where images are obtained by scanning a helium beam spot across a surface and recording the variation in scattered intensity at a fixed total scattering angle θSD and fixed incident angle θi relative to the overall surface normal. These experiments used a spot obtained by collimating the beam (referred to as helium pinhole microscopy). Alternatively, a beam spot can be created by focusing the beam with an atom optical element. However up till now imaging with a focused helium beam has only been demonstrated in transmission (using a zone plate). Here we present the first reflection images obtained with a focused helium beam (also using a zone plate). Images are obtained with a spot size (FWHM) down to 4.7μm±0.5μm, and we demonstrate focusing down to a spot size of about 1μm. Furthermore, we present experiments measuring the scattering distribution from a focused helium beam spot. The experiments are done by varying the incoming beam angle θi while keeping the beam-detector angle θSD and the point where the beam spot hits the surface fixed - in essence, a microscopy scale realization of a standard helium atom scattering experiment. Our experiments are done using an electron bombardment detector with adjustable signal accumulation, developed particularly for helium microscopy.

Angular and impact energy dependence of intensity ratio of Kα,β x-rays to underneath bremsstrahlung radiation emitted from 15 to 25 keV electrons incident on a pure thick Cu (Z=29) target

We present new results on angular and impact energy dependence of the relative intensity ratios of Kα and Kβ characteristic (CS) x-ray lines to their respective underneath bremsstrahlung (BS) radiation produced from 15-25 keV electrons incident on a thick polycrystalline pure target of Cu (Z = 29). The angular distributions of relative intensity ratios were carried out by measuring the radiations emitted at incidence angle θi that varied from 15° to 75° in reflection geometry mode of the target. The x-rays emerging from the target surface were detected by an energy dispersive Si PIN photodiode detector. The magnitude of relative intensity ratio of Kα/BS is found to be larger than that of Kβ/BS by a factor of about 5.11, 5.24 and 5.49 for 15 keV, 20 keV and 25 keV impact energy, respectively measured, for example, at θi = 15°. However, the intensity ratio of a given characteristic line to its underneath BS increases on average by a factor of about 2.2 in varying the impact energy from 15 keV to 25 keV measured at different angles. The experimental results on angular and impact energy dependence are compared with the corresponding results obtained from Monte-Carlo simulations using the PENELOPE code. The overall agreement between experiment and theory is found to be satisfactory within the uncertainty of measurements.

Measurement of local temperature around the impact points of fast ions under grazing incidence

Gold and platinum nanoparticles of few-nm size were deposited on amorphous silicon nitride (a-SiN) films. These samples were irradiated with 380 MeV Au ions at grazing incident angles (θi = 2°–5°) to a fluence of ∼1 × 1010 ions/cm2. The irradiated samples were observed using transmission electron microscopy (TEM). Ion tracks were clearly observed as long bright lines. Nanoparticles were found to be desorbed from long and narrow regions along the ion tracks. The surface temperature at the thermal spike produced by the ion impact was evaluated from the observed nanoparticle desorption. The observed temperature distribution is qualitatively explained by a one-dimensional two temperature model (1D-TTM) although there are some discrepancies which may be attributed to the surface effects which are not taken into account in 1D-TTM.

Image authentication by assessing manipulations using illumination

With the ever increasing use of digital media, image tampering has become imperative. This spurs the need to identify such tampering for authentication and jurisdiction. The main idea of this paper is an assessment of the possible light source direction from the image. This technique uses the inconsistencies in the light source direction to detect the image forgery. Initially, in the preprocessing step on input image, surface normals are calculated using surface texture profile. RED band is mainly used for obtaining surface texture information and, further, surface normal calculations are done. With estimated illumination profile and normals, the incident angle θi is computed for various chosen image patches. The θi angle is the estimated angle from image object to light source direction. The inconsistency in θi values is used as an evidence of tampering. The proposed technique is tested on different known fake images and is found capable of identifying manipulated objects in an image. This technique works for homogenous illuminated surfaces and has better forgery detection accuracy. Additionally, our technique also diminishes human intervention for forgery detection. The performance of proposed forgery detection technique is examined using CASIA1 image database to give users a feel of the performance.

Light transfer through windows with external condensation

This study investigates systematically light transfer through windows supporting cap-shaped droplets on their external face. The presence of such droplets may have negative effects on the conversion efficiency of solar cells, distorts image quality of lenses, or hinders visibility through windows and windshields. Here, the directional-hemispherical transmittance was predicted by the Monte Carlo ray-tracing method. The droplets were monodisperse or polydisperse randomly distributed on the outside face of optically smooth windows. For nonabsorbing droplets, the diameter and size distribution did not have a significant effect on the window directional-hemispherical transmittance. The latter was nearly independent of contact angle for incident angle θi ≤ 30°. However, the directional-hemispherical transmittance decreased monotonously with increasing incident angle and droplet contact angle for contact angle θc ≤ 70° to reach a minimum at a contact angle θc,min beyond which it increased with increasing contact angle before reaching a plateau at large contact angles. This was attributed to total internal reflection at the back window/air and droplet/air interfaces. For absorbing droplets, the normal-hemispherical transmittance decreased significantly with increasing droplet contact angle, mean diameter, polydispersity, and projected surface area coverage due to strong absorption within the droplets. Moreover, the normal-hemispherical transmittance decreased with increasing contact angle for θc < 90° and remained constant and independent of the droplets’ absorption index, mean diameter, and contact angle for θc ≥ 90°. Finally, Analytical expressions for the upper and lower bounds of the normal-hemispherical transmittance as a function of droplet contact angle, optical properties, and projected surface area coverage were derived.

Investigating the use of the dual-polarized and large incident angle of SAR data for mapping the fluvial and aeolian deposits

Mapping the spatial distributions of the fluvial deposits in terms of particles size as well as imaging the near-surface features along the non-vegetated aeolian sand-sheets, provides valuable geological information. Thus this work aims at investigating the contribution of the dual-polarization SAR data in classifying and mapping the surface sediments as well as investigating the effect of the radar incident-angle on improving the images of the hidden features under the desert sand cover. For mapping the fluvial deposits, the covariance matrix ([C2]) using four dual-polarized ALOS/PALSAR-1 scenes cover the Wadi El Matulla, East Qena, Egypt were generated. This [C2] matrix was used to generate a supervised classification map with three main classes (gravel, gravel/sand and sand). The polarimetric scattering response, spectral reflectance and temperatures brightness of these 3 classes were extracted. However for the aeolian deposits investigation, two Radarsat-1 and three full-polarimetric ALOS/PALSAR-1 images, which cover the northwestern sandy part of Sinai, Egypt were calibrated, filtered, geocoded and ingested in a GIS database to image the near-surface features. The fluvial mapping results show that the values of the radar backscattered coefficient (σ°) and the degree of randomness of the obtained three classes are increasing respectively by increasing their grain size. Moreover, the large incident angle (θi = 39.7) of the Radarsat-1 image has revealed a meandering buried stream under the sand sheet of the northwestern part of Sinai. Such buried stream does not appear in the other optical, SRTM and SAR dataset. The main reason is the enhanced contrast between the low backscattered return from the revealed meandering stream and the surroundings as a result of the increased backscattering intensity, which is related to the relatively large incident angle along the undulated surface of the study area. All archaeological observations support the existence of paleo-fresh water lagoon at the northwestern corner of the study area, which might have been the discharge lagoon of the revealed hidden stream.

Improved absorption efficiency of silicon (Si) solar cells through Resonant Waveguide Gratings (RWGs) – A hybrid design of RWG and Si solar cell

This research work focuses on the improved performance of a Si solar cell by increasing solar absorption. The silicon solar cell is designed with a hybrid photonic structure of Resonant Waveguide Gratings (RWGs) to enhance absorption efficiency. The Waveguide Gratings have been designed in Fourier Model Method (FMM) using regression algorithm. The subsequent hybrid design of RWG with Si solar cell has been carried out using Finite Difference Time Domain (FDTD) design tool. The grating corrugated layer is designed in a fused silica (SiO2) material and coated with a high index amorphous TiO2 material. One of the possible realizable grating designed parameters are: grating period d=316nm, refractive index of TiO2, nt=2.385, refractive index of fused silica ns=1.450, duty cycle=65%, structure line width w=205nm, resonance wavelength λr=632.8nm (Helium-Neon laser), angle of incidence θi=18°, and grating grooved height hg=120nm, whereas the designed parameters for Si solar cell are: refractive index of Si N=3.882+i(0.019) at resonance wavelength λr=632.8nm, cell thickness T=500nm. The employment of optimized grating parameters in simulations leads to efficient hybrid device performance.

Photon acceleration in plasma wake wave

The photon acceleration effect in a laser wake field is investigated based on photon Hamiltonian dynamics. A test laser pulse is injected into a plasma wave at an incident angle θi, which could slow down the photon velocity along the propagating direction of the wake wave so as to increase the acceleration distance for the photons. The photon trapping condition is analyzed in detail, and the maximum frequency shift of the trapped photon is obtained. The acceleration gradient and dephasing length are emphatically studied. The compression of the test laser pulse is examined and used to interpret the acceleration process. The limit of finite transverse width of the wake wave on photon acceleration is also discussed.

Broadband and omnidirectional highly-transparent coverglasses coated with biomimetic moth-eye nanopatterned polymer films for solar photovoltaic system applications

Highly-transparent coverglasses coated with broadband and omnidirectional antireflective (AR) moth-eye nanopatterned ultra-violet (UV) curable polymer, i.e., Norland Optical Adhesive (NOA) 63, films are reported. The moth-eye nanopatterns on the NOA63 polymer films/glasses are transferred from the silicon molds with conical nanogratings (NGs) using poly-dimethylsiloxane stamps by a soft imprint lithography technique. For the NOA63 NGs/glasses, surface wetting behaviors, UV radiation/thermal effects, and optical properties, together with theoretical analyses using rigorous coupled-wave analysis calculations, are investigated. For a period of 380nm, the NOA63 NGs/glass exhibits a hydrophobic surface with a water contact angle of ~112°. Furthermore, it increases the transmittance of the bare glass over a wide range of wavelengths and incident light angles, showing the higher solar weighted transmittance (TSW) of ~93.2% at an incident angle (θi) of 0° and the larger average TSW value of ~83.6% at θi=10–70° than those (i.e., TSW~90.1% at θi=0° and average TSW value of ~81.5% at θi=10–70°) of the bare glass in the wavelength range of 350–1800nm. In addition, it shows a relatively good stability under intense UV radiation and high temperature. By employing the 380nm-period NG patterned NOA63 films into an AR coating of coverglasses in encapsulated III-V GaInP/GaInAs/Ge triple-junction solar photovoltaic (PV) sub-receiver modules, an enhanced power conversion efficiency (PCE) of 31.33% is obtained mainly due to an increased short-circuit current density (JSC) of 14.68mA/cm2 compared to reference PV modules with the bare coverglass (i.e., PCE=30.12% and JSC=14.07mA/cm2) at θi=0°. For incident angle-dependent PV module characteristics, it also has more outstanding solar energy conversion properties in a wide θi range of 10–70°.

Absorption characteristics of single-layer ceramics under oblique incident microwave irradiation

The absorption characteristics of single-layer ceramics under oblique incident microwave irradiation were investigated by evaluating the effect of microwave incident angle (θi) on reflection losses (RL) for both transverse electric (TE) and transverse magnetic (TM) polarizations. The materials investigated were a high-loss SiC layer and a low-loss Al2O3 layer with impedance matching thicknesses of 0.0054 and 0.923m, respectively. The reflection losses of the ceramic layers over the incident angle ranging from 0° to 90° were determined using equations derived from transmission line theory. From the RL patterns obtained, SiC exhibits a much smaller minimum reflection loss (RLmin) at a higher θi for TM polarization compared with the equivalent for TE polarization. This difference is believed to be a result of the effect of Brewster׳s angle. For Al2O3 at room temperature, there is a negligible difference of RLmin between TE and TM polarizations due to the low dielectric loss of Al2O3. When temperature increases to 1379°C, the Al2O3RLmin values for TE polarization show frequent fluctuations in the entire temperature range, indicating the difficulty in predicting absorption in low-loss materials for TE polarization during microwave heating. For TM polarization, reflection loss of Al2O3 varies monotonically with θi as the temperature is increased that is shown by a decreasing RLmin value when the corresponding incident angle increases. These results indicate that TM-polarized microwaves help reduce reflection losses of both high- and low-loss ceramics. Power absorption in ceramics throughout the heating process can be improved by modifying the angle of incidence and polarization type of microwaves.

Efficiency improvement of III–V GaAs solar cells using biomimetic TiO2 subwavelength structures with wide-angle and broadband antireflection properties

The power conversion efficiency (PCE) improvement of III–V gallium arsenide (GaAs) single-junction (SJ) solar cells with titanium dioxide (TiO2) subwavelength structures (SWSs) as antireflection coatings (ARCs) is reported. TiO2 SWSs are fabricated by dry etching using dotted gold nanopatterns prepared with glancing angle deposition and thermal dewetting processes. Their optical reflection properties are studied in the wavelength range of 350–900nm at incident light angles (θi) of 3–80°, together with the theoretical prediction by a rigorous coupled-wave analysis. For SWSs on the 50nm-thick TiO2 layer/GaAs substrate, a lower solar weighted reflectance (SWR) of ~6.2% is obtained compared to that (i.e., SWR ~9.5%) of the TiO2 single-layer ARC (SLARC). By employing the TiO2 SWSs into the III–V GaAs SJ solar cell, the increased short circuit current density (Jsc) of 24.82mA/cm2 is achieved, thus leading to an improved PCE of 19.66% at θi=0° (i.e., Jsc=18.38 and 23.65mA/cm2 and PCE=14.74 and 18.98% for the solar cells with bare surface and TiO2 SLARC, respectively). For incident angle-dependent device characteristics, the solar cell with the TiO2 SWSs also exhibits superior performance over a wide θi range of 20–80°. Additionally, after the encapsulation, its Jsc and PCE values are slightly increased to 26.15mA/cm2 and 20.73%, respectively. For the stability, there are no significant variations in Jsc and PCE after about 50 days.

Numerical Studies of Sea Surface Scattering With the GMRES-RP Method

In this paper, we apply the right preconditioned generalized minimal residue (GMRES-RP) method for the analysis of electromagnetic scattering from 1-D ocean surfaces. We first verify that the GMRES-RP method has the desirable features of high accuracy, robustness, and efficiency, and, hence, is well suited to such analysis. We then carry out this analysis systematically by examining the effects of wind speed, incidence angle, ocean spectrum, and the degree of inclusion of the gravity and intermediate waves. With a large surface length of 16384 λ and a noise floor pushed down below -70 dB, we have found more refined structures than suggested in the literature: At near-normal incidence angle (θi=20°), both VV and HH results remain sensitive to wind speed. Across the incidence angle range, the sensitivity is higher at small wind speed than at medium to high wind speed. Plant argued that the contribution from intermediate waves separately, not as a part of the large wave spectrum, becomes increasingly important with increasing wind speed and, accordingly, a multiscale model is desired. Our numerical analysis validates this viewpoint by showing that a normalized bistatic scattering coefficient value as large as 4 dB at certain angles for H-pol at L band can be attributed to the intermediate wave and the tilt modulation provided by large waves. The viewpoint is further validated by a comparison of the two-scale model and three-scale model against numerical results where the two-scale model shows appreciable discrepancy in the forward scattering angles whereas the three scale model shows very good agreement with MoM, hence manifesting the necessity of advancing multiscale models beyond the conventional two-scale model. This finding also implies the need for numerical studies to use adequately large surface length, at least to cover the intermediate waves, even for below intermediate incidence cases.

Chemical Dynamics Simulations of High Energy Xenon Atom Collisions with the {0001} Surface of Hexagonal Ice

Simulation results are presented for Xe atoms colliding with the {0001} surface of hexagonal ice (Ih) with incident energies EI of 3.88, 4.56, 5.71, and 6.50 eV and incident angles θI of 0, 25, 45, and 65°. The TIP4P model was used for ice and ab initio calculations were performed to determine an accurate Xe/ice potential. Three types of events were observed; that is penetration below the ice surface and then desorption, penetration with Xe remaining in ice for times greater than 6 ps trajectory, and direct scattering without surface penetration. Surface penetration is most probable for normal (θI = 0°) collisions and direct scattering becomes important for θI = 45° and 65°. Penetration into ice becomes deeper as EI is increased. For θI = 0 and 25°, all of the Xe atoms penetrate the surface and there is no direct scattering. The probability that the Xe atoms remain trapped below the surface increases as EI is increased and is more than 70% for θI = 0° and EI = 6.50 eV. For θI of 0 and 25° the trapped Xe atoms have a thermal energy of ∼25 meV at 6 ps and are close to being thermalized. For θI of 0 and 25° the average translational energy of the scattered Xe-atoms ⟨EF⟩ is highest when θF is very close to normal and then gradually decreases for higher values of θF. For θI of 45 and 65°, ⟨EF⟩ is less than 250 meV for θF varying from 0 to 40°, but for larger θF the value of ⟨EF⟩ rapidly increases to ∼1/3 to 1/2 of the collision energy. The probability of the subsurface Xe desorbing is greatest between 0 and 3 ps, with as much as 65% of the desorption occurring within a 1 ps interval of this time frame. Desorption is greatly diminished at longer times consistent with Xe becoming more thermalized. Simulation results using the TIP3P model for ice are similar to those above for the TIP4P model, with the caveat that trapping below the ice surface is more pronounced for the TIP3P model. The simulation results are in overall quite good agreement with experiment.

In-Plane and Out-of-Plane Diffraction of H2 from Ru(001)

H(2) diffraction from the Ru(001) surface has been measured for incident energies E(i) = 78-150 meV and incident angles Θ(i) = 22.1-64.1°. In-plane and out-of-plane angular distributions were measured for incidence along [110] and [100] directions. Out-of-plane diffraction channels were found to be predominant for the explored experimental conditions regardless of the incidence direction. An analysis of diffraction intensities reveals that diffraction out of the scattering plane is enhanced for high incidence angles. Diffractive transitions with wavevector change in the surface plane and transversal to the incidence direction ΔK(⊥) were observed to be favored among the out-of-plane diffractive transitions. These features could be reproduced by model calculations of diffraction intensities performed using a three-dimensional soft potential. This suggests that a kinematic effect is responsible for the large out-of-plane intensities observed in experiment, more than any other features of the six-dimensional H(2)-surface interaction potential.

Low energy Ar+ bombardment of GaN surfaces: A statistical study of ion reflection and sputtering

Statistical molecular dynamics simulations are performed to analyze the sputtering of w-GaN (wurtzite) and z-GaN (zinc blende) surfaces under 100 eV Ar+ ion bombardment. Ion reflection and physical sputtering mechanisms are investigated as a function of the ion impact angle and the crystalline nature of samples. The probability of ion reflection is lower for the w-GaN phase and increases with the angle of incidence θi. As θi becomes more glancing, the reflected ions become more energetic and their angular distribution tends to narrow. The sputtering yields of w-GaN and z-GaN surfaces are maximum for θi=45°. For near-normal incidence, the probability of sputtering is smaller for the w-GaN phase, suggesting that the atomic arrangement in the pristine state modifies the characteristics of the momentum transfer occurring between the ion and the surface atoms during the collision cascade. Atomic nitrogen sputters preferentially and represents 87% to 100% of sputtered species due to its lower mass. These statistical results differ from the predictions of continuous ion bombardment simulations since the surfaces are not allowed to evolve self-consistently during the gathering of impact statistics.

Chemical Dynamics Simulations of Energy Transfer in Collisions of Protonated Peptide−Ions with a Perfluorinated Alkylthiol Self-Assembled Monolayer Surface

Classical trajectory simulations are performed to study energy transfer in collisions of protonated diglycine, gly2-H+, and dialanine, ala2-H+, ions with a fluorinated octanethiol self-assembled monolayer (F-SAM) surface for collision energies Ei in the range of 5−70 eV and incident angles θi of 0 and 45° with respect to the surface normal. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface. The simulations show the distribution of energy transfer to the peptide−ion’s internal degrees of freedom, ΔEint, to the surface, ΔEsurf, and in peptide−ion translation, Ef, are very similar for gly2-H+, and ala2-H+. The average percentage energy transferred to ΔEsurf and Ef increases and decreases, respectively, with an increase in Ei, while the average percentage energy transfer to ΔEint is nearly independent of Ei. Changing θi from 0 to 45° decreases and increases the percentage of energy transfer to ΔEsurf and Ef, respectively, but has little change in the transfer to ΔEint. Average percentage energy transfer to the surface is found to approximately depend on Ei according to exp(−b/Ei). Comparisons with previous simulations show that peptide-H+ collisions with the EA F-SAM model transfer approximately a factor of 2 more energy to ΔEint than do collisions with the hydrogenated SAM, that is, H-SAM. Replacing the mass of the F atoms by that of a H atom in the simulations, without changing the potential, shows that the different ΔEint energy transfer efficiencies for the F-SAM and H-SAM surfaces is a mass effect. The simulations for ala2-H+ colliding with the EA F-SAM surface give P(ΔEint) distributions in good agreement with previous experiments and an average transfer to ΔEint of 15% as compared with the experimental value of 21%. The UA F-SAM model gives energy transfer efficiencies in qualitative agreement with those of the EA model, but there are important quantitative differences.

Exploring the Energetic Deposition of Pentacene on Pentacene through Molecular Dynamics Simulations

We use molecular dynamics simulations to explore in detail the factors affecting the behavior of an incident pentacene molecule colliding with the pentacene (010) step edge. We examine the effect of molecular orientation in conjunction with incident kinetic energy (Ei) and angle (θi) of the incoming molecule and proximity of the collision of the incoming molecule to the step edge in determining the relative contributions of fundamental mechanisms such as insertion, ejection, and interlayer transport. We find that to maximize 2D growth through experimentally controllable parameters, Ei and θi, the molecular orientation within the beam must be controlled. For θi = 0°, when the momentum of the incident molecule is aligned along the long axis of the molecule, downward interlayer transport of the incident molecule is dominant for all hyperthermal energies. When the molecular orientation is random, higher energies must be employed to achieve the same degree of downward interlayer transport. For nonzero values of θi, the mechanism for 2D growth moves away from direct insertions, thus reducing the bias on molecular orientation.

Surface modification of an amorphous Si thin film crystallized by a linearly polarized Nd:YAG pulse laser beam

Amorphous Si films of 60 and 10nm thick on glass substrates were irradiated by a linearly polarized Nd:YAG pulse laser with the wavelength λ=532nm at the incident angle θi=0. The surface of the irradiated 60-nm-thick film had both periodic ridges perpendicular to the electric field vector E and aperiodic ridges roughly parallel to E, where the spatial period of the periodic ridges was almost λ. From the continuous 10-nm-thick film, the separate rectangular Si islands were formed with a periodic distance of λ, with the edges parallel or perpendicular to E. When θi was increased from normal incidence of the s-polarized beam for a 60-nm-thick film, the aperiodic ridges were reduced while the periodic ridges were still formed. For a 10-nm-thick film, the Si stripes were formed perpendicular to E, using the s-polarized beam at θi=12°. In order to investigate the mechanisms of the surface modifications of, in particular, aperiodic ridges, islands, and stripes, we improved the previous theoretical model of the periodic distribution of the beam energy density (periodic E-D) generated by irradiation of the linearly polarized laser beam, taking account of the multireflection effect in the Si film which is semitransparent for λ. Further, the calculated E-D was corrected with respect to the thermal diffusion in the irradiated Si film. The calculation results show that the two-dimensional E-D consists of a constant or a dc term and a sinusoidal or an ac term which contains various spatial periods. The multireflection effect strongly influences the amplitude and phase of every ac term, which means that the amplitude and phase depend on the film thickness. The thermal diffusion during the heating of the irradiated film greatly reduces the amplitudes of the ac terms with periods below the thermal diffusion length. The theoretical calculation showed that, by increasing θi, the temperature distribution in the irradiated Si film was changed from two-dimensional toward one-dimensional, which can explain the above experimental results reasonably.

Dynamics of Energy Transfer in Collisions of O(3P) Atoms with a 1-Decanethiol Self-Assembled Monolayer Surface

Chemical dynamics simulations are reported of energy transfer in collisions of O(3P) atoms with a 300 K 1-decanethiol self-assembled monolayer (H-SAM) surface. The simulations are performed with a nonreactive potential energy surface, developed from PMP2/aug-cc-pVTZ calculations of the O(3P) + H-SAM intermolecular potential, and the simulation results represent the energy transfer dynamics in the absence of O(3P) reaction. Collisions energies E(i) of 0.12, 2.30, 11.2, 75.0, and 120.5 kcal/mol and incident angles theta(i) of 15, 30, 45, 60, and 75 degrees were considered in the study (theta(i) = 0 degrees is the surface normal). The translational energy distribution of the scattered O(3P) atoms, P(E(f)), may be deconvoluted into Boltzmann and non-Boltzmann components, with the former fraction identified as f(B). The trajectories are also analyzed in terms of three types; that is, direct scattering from and physisorption on the top of the H-SAM and penetration of the H-SAM. There are three energy regimes in the scattering dynamics. For the low E(i) values of 0.12 and 2.30 kcal/mol, physisorption is important and both f(B) and the average final translational energy of the scattered O(3P) atom, E(f), are nearly independent of the incident angle. The dynamics is much different for hyperthermal energies of 75.0 and 120.5 kcal/mol, where penetration of the surface is important. For hyperthermal collisions, the penetration probability decreases as theta(i) is increased, with a significant transition between theta(i) of 60 and 75 degrees . Hyperthermal penetration occurs upon initial surface impact and is more probable if the impinging O(3P) atom may move down a channel between the chains. For E(i) = 120.5 kcal/mol, 90% of the trajectories penetrate at theta(i) = 15 degrees , while only 3% penetrate at theta(i) = 75 degrees. For the former theta(i), the energy transfer to the surface is efficient with E(f) = 4.04 kcal/mol, but for the latter theta(i), E(f) = 85.3 kcal/mol! Particularly interesting penetrating trajectories are those in which O(3P) is trapped in the H-SAM for times exceeding 60 ps, linger near the Au substrate, and strike the Au substrate and scatter directly. For E(i) = 11.2 kcal/mol, there is a transition between the scattering dynamics for the low and hyperthermal collision energies. Additional detail in the energy transfer dynamics is obtained from the final polar and azimuthal angles, the residence time on/in the H-SAM, the minimum height with respect to the Au substrate, and the number of inner turning points in the O-atom's velocity. Calculated values of E(f) vs the final polar angle, theta(f), are in qualitative agreement with experiment. The O(3P) + H-SAM nonreactive energy transfer dynamics, for E(i) of 11.2 kcal/mol and lower, are very similar to previously reported Ne + H-SAM simulations.