Resonance Lifetime Research Articles

Passivation of nanocrystalline silicon photovoltaic materials employing a negative substrate bias

Hydrogenated nanocrystalline silicon (nc-Si:H) shows great promise in the application of third-generation thin film photovoltaic cells. However, the mixed-phase structure of nc-Si:H leads to many defects existing in this important solar energy material. Here we present a new way to passivate nc-Si:H films by tuning the negative substrate bias in plasma-enhanced chemical vapor deposition. Microstructures of the nc-Si:H films prepared under a negative bias from 0 to −300 V have been characterized using Raman, x-ray diffraction, transmission electron microscope, and optical transmission techniques. A novel passivation effect on nc-Si:H films has been identified by the volume fraction of voids in nc-Si:H, together with the electrical properties obtained by electron spin resonance and effective minority lifetime measurements. The mechanism of the passivation effect has been demonstrated by infrared spectroscopy, which illustrates that the high-energy H atoms and ions accelerated by an appropriate bias of −180 V can form more hydrides along the grain boundaries and effectively prevent oxygen incursions forming further Si–O/Si interface dangling bonds in the nc-Si:H films. The detrimental influence of a bias over −180 V on the film quality due to the strong ion bombardment of species with excessively high energy has also been observed directly from the surface morphology by atomic force microscopy.

The vibrational dependence of dissociative recombination: Cross sections for ${\rm N}_2^ +$N2+

Theoretical ab initio calculations are reported of the cross sections for dissociative recombination of the lowest four excited vibrational levels of N2(+) at electron energies from 0.001 to 1.0 eV. Rydberg vibrational levels contributing to the cross section structures are identified as are dissociative channels contributing more than 10(-16) cm(2) to the total cross sections. In contrast to the prior study of v = 0 (S. L. Guberman, J. Chem. Phys. 137, 074309 (2012)), which showed 2(3)Πu to be the dominant dissociative channel, 4(3)Πu is dominant for v = 1. Both 2 and 4(3)Πu are major routes for dissociative recombination from v = 2-4. Other routes including 2(3)Σu(+), 3(3)Πu, 2(1)Πu, 2(3)Πg, 2(1)Σg(+), 1(1)Δg, and b('1)Σu(+) are significant in narrow energy ranges. The results show that minor dissociative routes, included here for N2(+), must be included in theoretical studies of other molecular ions (including the simplest ions H2(+) and H3(+)) if cross section agreement is to be found with future high resolution dissociative recombination experiments. The calculated predissociation lifetimes of the Rydberg resonances are used in a detailed comparison to two prior storage ring experiments in order to determine if the prior assumption of isotropic atomic angular distributions at "zero" electron energy is justified. The prior experimental assumption of comparable cross sections for v = 0-3 is shown to be the case at "zero" but not at nonzero electron energies. Circumstances are identified in which indirect recombination may be visualized as a firefly effect.

Line-profile analysis of excitation spectroscopy in the even 4p5(2P1/2)nl′ [K′] J (l′ = 1,3) autoionizing resonances of Kr

The even-parity autoionizing resonance series 4p5 np′ [3/2]1,2, [1/2]1, and 4p5 nf′ [5/2]3 of krytpon have been investigated by laser excitation from the two metastable states 4p55s [3/2]2 and 4p55s′ [1/2]0 in the photon energy region of 29000–40000 cm−1 at experimental bandwidth of ∼0.1 cm−1. The excitation spectra of the even-parity autoionizing resonance series, most of which are experimentally studied for the first time in this work, show typical asymmetric line shapes. Complementary information on level energies, quantum defects, line profile indices and resonance widths, resonance lifetimes and reduced widths of the autoionizing resonances are derived by Fano-type line-shape analyses of the experimental results. Results from this work indicate that the line profile index (q) and the resonance width (Γ) are approximately proportional to the effective principal quantum number (n*); the line separation of the 4p5 np′ autoionizing resonances is also in good agreement with theoretical model.

Stark Ionization of Atoms and Molecules within Density Functional Resonance Theory

We show that the energetics and lifetimes of resonances of finite systems under an external electric field can be captured by Kohn–Sham density functional theory (DFT) within the formalism of uniform complex scaling. Properties of resonances are calculated self-consistently in terms of complex densities, potentials, and wave functions using adapted versions of the known algorithms from DFT. We illustrate this new formalism by calculating ionization rates using the complex-scaled local density approximation and exact exchange. We consider a variety of atoms (H, He, Li, and Be) as well as the H2 molecule. Extensions are briefly discussed.

Line-profile Analysis of Excitation Spectroscopy in Even 5p5(2P1/2)nl′ [K′]J (l′=1, 3) Autoionizing Resonances of Xe

The even-parity autoionizing resonance series 5p5np′ [3/2]1, [1/2]1, and 5p5nf′5/2]3 of xenon have been investigated, excited from the two metastable states 5p56s [3/2]2 and 5p56s′ [1/2]0 in the photon energy range of 28000–42000 cm−1 with experimental bandwidth of ̃0.1 cm−1. The excitation spectra of the even-parity autoionizing resonance series show typical asymmetric line shapes. New level energies, quantum defects, line profile indices and resonance widths, resonance lifetimes and reduced widths of the autoionizing resonances are derived by a Fano-type line-shape analysis. The line profile index and the resonance width are shown to be approximately proportional to the effective principal quantum number. The line separation of the 5p5np′ autoionizing resonances is discussed.

129Xe nuclear resonance scattering on solid Xe and 129Xe clathrate hydrate

Nuclear inelastic and nuclear forward-scattering experiments utilizing the 129Xe Mössbauer resonance were performed on solid Xe and enriched 129Xe clathrate hydrate. The lifetime and energy of the nuclear resonance were determined to be and , respectively. Nuclear inelastic scattering spectra could be obtained with an instrumental resolution of . Despite low Lamb-Mössbauer factors, Xe specific densities of phonon states were derived for solid Xe and Xe clathrate hydrate. Their reliability was investigated using 1-phonon terms obtained by fixing Lamb-Mössbauer factor values within the applied Fourier-Log decomposition. ABINIT calculations of the density of phonon states in solid Xe supplement the interpretation of the experimental data for fcc Xe and good agreement with published densities of phonon states could be achieved below . Results for the Xe clathrate hydrate essentially confirm the rattling nature of the Xe guests and indicate that Xe modes do not contribute to lattice dynamics above .

Kalb–Ramond field localization on the Bloch brane

This work deals with new results on the Kalb-Ramond (KR) field localization in braneworld models. We consider a five-dimensional warped spacetime with an embedded 4D thick brane which is generated by two real scalar fields coupled with gravity (the so called Bloch brane). We find a KR field zero mode localized with the inclusion of the dilaton coupling. Analyzing the massive spectrum, we detected a series of resonant modes that arise from the solutions of the Schr\"odinger-like equation for KR field. The effects of the brane thickness and of the dilaton coupling over the resonance structures are determined. Such analysis is extended to the resonance lifetimes of the massive modes, allowing a better understanding on the localization mechanism of the model.

Green's function approach to the lifetimes of image potential resonances at metal surfaces

We present a theoretical study of the image potential resonances (IPRs) at metal surfaces. We develop the Green's functions approach allowing us to calculate binding energies ${E}_{n}$ and lifetimes ${\ensuremath{\tau}}_{n}$ of IPRs with high quantum numbers $n$ (up to 10 in this work). A systematic study is performed at the $\overline{\ensuremath{\Gamma}}$ point for the close-packed metal surfaces: Cu(111), Ag(111), Au(111), Al(001), Al(111), Be(0001), Mg(0001), Na(110), Li(110), and also at the $\overline{\mathrm{Y}}$ point on Cu(110). The calculated lifetimes of IPRs on close-packed surfaces demonstrate the scaling law ${\ensuremath{\tau}}_{n}\ensuremath{\propto}{n}^{3}$. Our results are in agreement with available experimental data. We show that at the $\overline{\mathrm{Y}}$ point on Cu(110) each quantum number $n$ corresponds to a pair of IPRs ${n}^{+}$ and ${n}^{\ensuremath{-}}$, where the energy difference ${E}_{n+}\ensuremath{-}{E}_{n\ensuremath{-}}$ is proportional to ${n}^{\ensuremath{-}3}$. The lifetimes ${\ensuremath{\tau}}_{n+}$ and ${\ensuremath{\tau}}_{n\ensuremath{-}}$ differ significantly, however, they both obey the scaling law ${\ensuremath{\tau}}_{n\ifmmode\pm\else\textpm\fi{}}\ensuremath{\propto}{n}^{3}$. Since the electrons trapped in the long-lived IPRs are strongly localized on the vacuum side, we argue that the inelastic electron-electron and electron-phonon scattering have a small contribution to the decay rate of these IPRs. The latter is dominated by the resonant electron transfer into the bulk.

Monolayer Graphene Platform for the Study of DNA Damage by Low-Energy Electron Irradiation

Novel materials and devices that probe the dynamics and stability of biomolecules under nonequilibrium conditions are necessary to advance our fundamental understanding of processes such as radiation-induced carcinogenesis. Development of effective radiotherapy strategies also relies upon the ability to control low-energy electron-induced DNA breakage in vitro. Here, we report the use of a sensitive chemical-vapor-deposited graphene platform for controlled and enhanced sequence-dependent low-energy electron-induced DNA damage studies. The use of p-doped graphene on Au thin films enhances DNA breakage due to phosphate-mediated parallel adsorption geometries, direct ballistic electron transfer to dissociative sugar–phosphate shape resonances, and image-potential-induced changes in the resonance lifetimes and energy widths. Graphene adsorbed on Au thin films also provides enhanced electric fields for surface-enhanced Raman spectroscopy (SERS). The combination of these effects allows direct, rapid assessment of ≤1 eV electron-induced DNA damage as a function of base sequence without separation and amplification steps.

Comprehensive Study of Altered Amorphous Structure in Functionalized Polypropylenes Exhibiting High Tensile Strength

Polypropylene (PP) and its derivatives, poly(5-hexen-1-ol-co-propylene) (PPOH) and poly(1-hexene-co-propylene) (PPH), were comprehensively investigated by means of wide-angle X-ray diffraction, dynamic mechanical analysis (DMA), 1H pulsed nuclear magnetic resonance (NMR), infrared (IR), and positron annihilation lifetime (PAL) techniques. The deduced nanoscopic amorphous structure was discussed in comparison with the mechanical properties such as tensile strength. Introducing polar hexenol and nonpolar hexene comonomers into PP chains resulted in a significant reduction of the crystallinity for both the derivatives, which failed to explain the anomalous higher tensile strength of PPOH. The long decay T2 observation from NMR indicated a lower chain mobility in the amorphous region of PPOH compared to PPH, consistent with the DMA analysis, providing a higher glass transition temperature for the former polymer. The PAL and IR results signified reduced free-volume size along with hydrogen bond formation in the amorphous region of PPOH, illustrating an essential role of the nanoscopic amorphous structure in the superior mechanical strength of PPOH compared to the other polymers.

Photonic crystals and optical mode engineering for thin film photovoltaics

In this paper, we present the design, analysis, and experimental results on the integration of 2D photonic crystals in thin film photovoltaic solar cells based on hydrogenated amorphous silicon. We introduce an analytical approach based on time domain coupled mode theory to investigate the impact of the photon lifetime and anisotropy of the optical resonances on the absorption efficiency. Specific design rules are derived from this analysis. We also show that, due to the specific properties of the photonic crystal resonances, the angular acceptance of such solar cells is particularly high. Rigorous Coupled Wave Analysis simulations show that the absorption in the a-Si:H active layers, integrated from 300 to 750 nm, is only decreased from 65.7% to 60% while the incidence angle is increased from 0 to 55°. Experimental results confirm the stability of the incident light absorption in the patterned stack, for angles of incidence up to 50°.

Coherent control of a resonance lifetime with laser pulses

Quantum control of the lifetime of the Br2(B,v′=35)–Ne ground van der Waals resonance is investigated theoretically by creating coherent superpositions of v′-1 resonances overlapping with the v′ resonance. This control scheme exploits the quantum interference occurring between the overlapping resonances, which is controlled by varying the width of the laser pulse creating the superposition state. For v′=35 the v′ resonance overlaps more strongly and with a higher number of v′-1 resonances than in the previously studied v′=27 case. It is found that as a consequence of this stronger overlapping regime, the effects of control over the resonance lifetime become more intense.

Energies and Lifetimes of Temporary Anion States of Chloromethanes by Stabilized Koopmans’ Theorem in Long-Range Corrected Density Functional Theory

To investigate the resonance energies and lifetimes of temporary anion states of chloromethanes, long-range corrected density functional theory is adopted in this article. Their values are determined by calculating the density of resonance states via the stabilized Koopmans' theorem. The characteristics of these resonance orbitals are also analyzed. By comparing with experimental values and previous theoretical calculations, our method can yield not only conformable results but also more complete information on the resonance states.

Multiplet resonance lifetimes in resonant inelastic x-ray scattering involving shallow core levels

Resonant inelastic x-ray scattering (RIXS) spectra of model copper- and nickel-based transition metal oxides are measured over a wide range of energies near the M edge ($h\ensuremath{\nu}=60$--80 eV) to better understand the properties of resonant scattering involving shallow core levels. Standard multiplet RIXS calculations are found to deviate significantly from the observed spectra. However, by incorporating the self-consistently calculated decay lifetime for each intermediate resonance state within a given resonance edge, we obtain dramatically improved agreement between data and theory. Our results suggest that these textured lifetime corrections can enable a quantitative correspondence between first-principles predictions and RIXS data on model multiplet systems. This accurate model is also used to analyze resonant elastic scattering, which displays the elastic Fano effect and provides a rough upper bound for the core hole shakeup response time.

STM-switching of organic molecules on semiconductor surfaces: an above threshold density matrix model for 1,5 cyclooctadiene on Si(100)

The scanning tunnelling microscope (STM)-induced switching of a single cyclooctadiene molecule between two stable conformations chemisorbed on a Si(100) surface is investigated using an above threshold model including a neutral ground state and an ionic excited state potential. Switching was recently achieved experimentally with an STM operated at cryogenic temperatures (Nacci et al 2008 Phys. Rev. B 77 121405(R)) and rationalized by a below threshold model using just a single potential energy surface (Nacci et al 2009 Nano Lett. 9 2997). In the present paper, we show that experimental key findings on the inelastic electron tunnelling (IET) switching can also be rationalized using an above threshold density matrix model, which includes, in addition to the neutral ground state potential, an anionic or cationic excited potential. We use one and two-dimensional potential energy surfaces. Furthermore, the influence of two key parameters of the density matrix description, namely the electronic lifetime of the ionic resonance and the vibrational lifetimes, on the ground state potential are discussed.

Strong Enhancement of the Lifetime of a Resonance State by Using a Combination of Two Laser Pulses

The lifetime of a resonance state overlapping with a second resonance is shown to be strongly enhanced (by a factor of 3) by combining two pump pulses to simultaneously excite the two resonances. Such an enhancement is produced by interference effects occurring between the two coherently excited overlapping resonances. A high degree of control on the intensity of interference, and thus on the resonance lifetime enhancement, is found to be achieved by varying the delay time between the pulses and their relative intensities.

Calculating the Lifetimes of Metastable States with Complex Density Functional Theory.

Among other applications, complex absorbing potentials (CAPs) have proven to be useful tools in the theory of metastable states. They facilitate the conversion of unbound states of a finite lifetime into normalized bound states with a complex energy. Adding CAPs to a conventional Hamiltonian turns it into a non-Hermitian operator. Recently, we introduced a complex density functional theory (CODFT) that extends the Kohn-Sham method to the realm of non-Hermitian systems. Here, we combine CAPs with CODFT and present the first application of CODFT to metastable systems. In particular, we consider the negative ions of the beryllium atom and the nitrogen molecule. Using conventional exchange-correlation functionals as functionals of a complex density, the resonance positions and the resonance lifetimes are obtained, and they are in line with the findings of other studies.

Rovibrational dynamics of the strontium molecule in the ${\rm A}\sideset{^1}{_{u}^{+}}{\Sigma}$AΣu+1, c3Πu, and ${\rm a}\sideset{^3}{_{u}^{+}}{\Sigma}$aΣu+3 manifold from state-of-the-art ab initio calculations

State-of-the-art ab initio techniques have been applied to compute the potential energy curves for the electronic states in the ${\rm A}\sideset{^1}{_{u}^{+}}{\Sigma}$AΣu+1, c3Πu, and ${\rm a}\sideset{^3}{_{u}^{+}}{\Sigma}$aΣu+3 manifold of the strontium dimer, the spin-orbit and nonadiabatic coupling matrix elements between the states in the manifold, and the electric transition dipole moment from the ground ${\rm X}\sideset{^1}{_{g}^{+}}{\Sigma}$XΣg+1 to the nonrelativistic and relativistic states in the A+c+a manifold. The potential energy curves and transition moments were obtained with the linear response (equation of motion) coupled cluster method limited to single, double, and linear triple excitations for the potentials and limited to single and double excitations for the transition moments. The spin-orbit and nonadiabatic coupling matrix elements were computed with the multireference configuration interaction method limited to single and double excitations. Our results for the nonrelativistic and relativistic (spin-orbit coupled) potentials deviate substantially from recent ab initio calculations. The potential energy curve for the spectroscopically active (1)$0_u^+$0u+ state is in quantitative agreement with the empirical potential fitted to high-resolution Fourier transform spectra [A. Stein, H. Knöckel, and E. Tiemann, Eur. Phys. J. D 64, 227 (2011)]10.1140/epjd/e2011-20229-6. The computed ab initio points were fitted to physically sound analytical expressions, and used in converged coupled channel calculations of the rovibrational energy levels in the A+c+a manifold and line strengths for the ${\rm A}\sideset{^1}{_{u}^{+}}{\Sigma}\leftarrow {\rm X}\sideset{^1}{_{g}^{+}}{\Sigma}$AΣu+1←XΣg+1 transitions. Positions and lifetimes of quasi-bound Feshbach resonances lying above the 1S0 + 3P1 dissociation limit were also obtained. Our results reproduce (semi)quantitatively the experimental data observed thus far. Predictions for on-going and future experiments are also reported.

On the relationships between guest molecular dynamics and free volume in a series of small molecular and polymer glass-formers

The combined local dynamics and free volume investigations in a series of fourteen glass-formers by means of electron spin resonance (ESR) and positron annihilation lifetime spectroscopy (PALS) are presented. The reorientation of the spin probe TEMPO is related to the annihilation of the ortho-positronium (o-Ps). It was found that the slow to fast transition at T50G is characterized by the almost constant o-Ps lifetime, τ3(T50G)=2.17±0.15ns. Then, this transition is connected with the occurrence of the mean free volume hole fluctuation of Vh(T50G)=114±15Å3 nearly independent of the chemical and topological character of the matrix constituents as well as of the type and extent of intermolecular interactions.

Resonance energies, lifetimes and complex energy potential curves from standard wave-packet calculations

We show here for a simple model system that the wavepacket dynamics in the interaction region can be described by a superposition of the non-Hermitian exponential divergent eigenfunctions of the physical Hamiltonian. We demonstrate how it is possible to obtain the complex eigenvalues and also the corresponding resonance eigenfunctions from the propagation of the wavepacket within the framework of the standard formalism of quantum mechanics. The general results demonstrated here for a simple model can lead to two different types of computational applications: (i) for systems where one can obtain the resonance energies and lifetimes as well as their corresponding eigenfunctions it is possible to study the evolution of the physical properties solely based on the initially populated resonance states without the need to propagate the wavepacket; (ii) for molecular systems where it is quite difficult to solve the non-Hermitian time-independent Schrödinger equation and obtain molecular resonance energies and functions. For this type of problem, the methods presented here enable one to evaluate the topology of complex potential energy surfaces from the wavepacket propagation and facilitate the study of the nuclear dynamics of ionizing molecular systems.