Metastable Excited State Research Articles

The Improved Self-assembled Monolayer of Octadecyltrichlorosilane as Positive Resist for Patterning Silicon Surface by Metastable Helium Atom Beam Lithography

We utilized a beam of helium atoms in a metastable excited state to expose an improved ultrathin self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) acted as positive resist grown directly on silicon wafer substrate. The metastable helium beam incident on the samples was patterned with a mask, which was placed in gentle contact with the substrate surface. A three-step wet-chemical etching process was successfully developed to transfer mask patterns into the underlying silicon substrate. Only positive patterns were fabricated on silicon surface by using the improved positive resist and controlling of the exposure dosage and the etching time. The sizes of positive patterns on silicon were successfully decreased from micrometer scale to 100 nm.

State-dependent lattices for quantum computing with alkaline-earth-metal atoms

Recent experimental progress with Alkaline-Earth atoms has opened the door to quantum computing schemes in which qubits are encoded in long-lived nuclear spin states, and the metastable electronic states of these species are used for manipulation and readout of the qubits. Here we discuss a variant of these schemes, in which gate operations are performed in nuclear-spin-dependent optical lattices, formed by near-resonant coupling to the metastable excited state. This provides an alternative to a previous scheme [Phys. Rev. Lett. 101, 170504 (2008)], which involved independent lattices for different electronic states. As in the previous case, we show how existing ideas for quantum computing with Alkali atoms such as entanglement via controlled collisions can be freed from important technical restrictions. We also provide additional details on the use of collisional losses from metastable states to perform gate operations via a lossy blockade mechanism.

Spin-Polarized Transient Electron Trapping in Phosphorus-Doped Silicon

Experimental evidence of electron spin precession during travel through the phosphorus-doped Si channel of an all-electrical device simultaneously indicates two distinct processes: (i) short time scales (≈50 ps) due to purely conduction-band transport from injector to detector and (ii) long time scales (≈1 ns) originating from delays associated with capture or reemission in shallow impurity traps. The origin of this phenomenon, examined via temperature, voltage, and electron density dependence measurements, is established by means of a comparison to a numerical model and is shown to reveal the participation of metastable excited states in the phosphorus-impurity spectrum. This work therefore demonstrates the potential to make the study of macroscopic spin transport relevant to the quantum regime of individual spin interactions with impurities as envisioned for quantum information applications.

Observation of exclusively He-induced H emission in cooled laser plasma

An experiment was performed for the observation of H emission induced in a cooled laser-induced atmospheric pressure gas plasma of He atoms in their metastable excited state. The strong H emission detected clearly established, to the exclusion of other well known major excitation processes, the exclusive contribution of the He-induced excitation (HIE) mechanism. The process is suggested to take place by means of energy transfer from the excited He atoms to the H atoms via Penning collision induced ionization involving electron exchange. The result further shows that this mechanism may also work for elements other than H and thereby strongly suggests the use of ambient He gas to broaden and complement the applications of standard laser-induced breakdown spectroscopy.

Photogenerated cumulenic structure of adamantyl endcapped linear carbon chains: An experimental and computational investigation based on infrared spectroscopy

The infrared (IR) spectrum of an adamantyl endcapped α, ω-polyyne (the hexayne, Ad-C(12)-Ad) is investigated both experimentally and computationally. A new IR band is observed upon UV photoexcitation of the compound (embedded in a poly methyl methacrylate matrix at 78 K), thus, revealing the existence of new photogenerated molecular structure trapped at low temperature. Complete reversibility is found, thus, demonstrating that the photoexcitation is responsible for the generation of metastable excited states of the molecule. Density functional theory and time dependent density functional theory calculations indicate that these metastable states result from the forbidden singlet (S(1)) or triplet (T(1)) excited states, and geometry optimizations of the polyyne trapped in either S(1) and/or T(1) states demonstrate that the carbon chain takes on a cumulenic structure. Comparison of the experimental and the computed IR spectra for the molecule trapped in the forbidden states confirms that the new IR features are clear markers of cumulenic species. The temperature and time dependent behavior of the new IR band is analyzed, while the experimentally determined value of the activation energy highlights the low stability of these molecular structures.

Thermometry in noble gas dielectric barrier discharges at atmospheric pressure using optical emission spectroscopy

We measure the temperature, T, of dielectric barrier discharges (DBD), in noble gases using optical emission spectroscopy (OES), by analysing rotational bands in the emission spectra of the first negative system (FNS) of N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> . This has the advantage that rotational structure can be fully resolved even with a spectrograph of average performance, and that the rotational temperature, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rot</sub> (~ T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gas</sub> ) can then be determined from a conventional Boltzmann plot. Ionization of N2 occurs mainly via Penning transfer from metastable excited states of He (ca. 20 eV) or Ne (ca. 16.6 eV). Using two glass-walled DBD chambers of very different volumes (0.1 and 20 liters), we have studied atmospheric-pressure discharges in flowing helium (He) or neon (Ne) containing traces of nitrogen. Discharges were excited by audio-frequency (10 kHz) high voltage (HV) using a needle as the HV electrode and a dielectric (alumina)-covered planar grounded counter-electrode. OE spectra were acquired with a 0.5 m focal length spectrograph, coupled to an intensified charge coupled device (ICCD) detector. Using the (0-0) R-branch of the FNS N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> (B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> - X <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) bands near a wavelength of 391.4 nm, we have measured axial (inter-electrode) distributions of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rot</sub> for the two different reactor volumes in both He and Ne. T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rot</sub> values were found to be highest at the needle electrode, of about 450 K and 740 K for He and Ne, respectively; in He, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rot</sub> dropped to a minimum of about 405 K at the mid-gap position in the small chamber, and ~ 360 K near the planar electrode in the large chamber. We conclude that temperatures in noble gas discharges depend critically on thermal conductivities of the particular gases (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">He</sub> = 1.9; K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ne</sub> = 0.6, both in mW.cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and on other experimental factors that influence heat transfer.

Light Propagation and Paired Superradiance in Coherent Medium

The problem of light propagation of frequency corresponding to half of the energy difference between a metastable excited state and the ground state of atoms is examined, and solved for coherent medium by analytic means. We demonstrate that the non-linear system of Maxwell-Bloch equation for the effective model of the $\Lambda-$type three levels is integrable in the mathematical sense. Analytic solutions thus obtained describe pulse splitting accompanied by compression, indicating a kind of non-linear instability of propagating pulses. The instability is eventually terminated by coherent two photon emission (called paired superradiance or PSR in short). These results are displayed by numerical outputs for visual understanding, as well. It is further shown that the integrable system allows a new class of soliton solutions. Solitons, implying the phenomenon of seff-induced transparancy at non-resonant frequencies, are stable against PSR. One of our goals of the present work is construction of a calculable theoretical framework for PSR rates associated with a trigger pulse propagation, which is achieved by combining analytic results with perturbative methods. PSR photon spectrum and its rate $\propto$(target number density)$^2$, along with their time structure, are clarified this way. These results may open a new path for interesting technological applications such as quantum entanglement and for solving the remaining problems of the still mysterious neutrino. Some basic strategy for realistic experiments of PSR detection and soliton production is also outlined.

Electron scattering with metastable H*2(c3Πu) molecules: ionization and other total cross sections

In this paper, we report total cross sections for electron-induced ionization and excitations from both the excited metastable state H*2(c3Пu) and the electronic ground state of H2. Total cross sections are reported for incident energies from the ionization threshold to 2000 eV using the ‘complex scattering potential-–ionization contribution’ method and the relative importance of various scattering channels, including super-elastic scattering and ionization, are identified. There appear to be no previous data on the ionization cross sections for the metastable target H*2(c3Пu). This work has potential applications in the fields of plasma physics and planetary atmospheres.

Nonlinear absorption microspectroscopy of single perylene nanocrystals with a multichannel double lock-in amplifier

Nonlinear confocal absorption microspectroscopy of single nanocrystals at low temperatures was introduced in the study of perylene nanocrystals. By the stationary pump-probe method, single nanocrystals were photoexcited into metastable excited states, and difference absorption spectra due to excited-state absorption and ground-state depletion were simultaneously recorded using a multichannel lock-in amplifier. It was revealed that the free Frenkel exciton band for single perylene nanocrystals is broader in width than that for the bulk crystal at low temperatures, suggesting that the exciton band is inhomogeneously broadened even for single perylene nanocrystals.

Gauge fields for ultracold atoms in optical superlattices

We present a scheme that produces a strong U(1)-like gauge field on cold atoms confined in a two-dimensional square optical lattice. Our proposal relies on two essential features, a long-lived metastable excited state that exists for alkaline-earth or ytterbium atoms and an optical superlattice. As in the proposal by Jaksch and Zoller (2003 New J. Phys.5 56), laser-assisted tunneling between adjacent sites creates an effective magnetic field. In the tight-binding approximation, atomic motion is described by the Harper Hamiltonian, with a flux across each lattice plaquette that can realistically take any value between 0 and π. We show how one can take advantage of the superlattice to ensure that each plaquette acquires the same phase, thus simulating a uniform magnetic field. We discuss the observable consequences of the artificial gauge field on non-interacting bosonic and fermionic gases. We also outline how the scheme can be generalized to non-Abelian gauge fields.

Genetically Encoded Singlet Oxygen Generator (SOG) Requiring No Exogenous Cofactors

Controlled local photogeneration of singlet oxygen (1O2, the metastable excited state of O2) is useful for generating electron-microscopic contrast, rapidly inactivating proteins of interest, reporting protein proximities over tens of nanometers, and ablating cells by photodynamic damage. The best previous genetically targetable SOG was the biarsenical dye ReAsH bound to tetracysteine motifs, but this system has modest quantum efficiency (0.024), requires antidotes to prevent toxicity, is limited by background staining, and is difficult to extend to intact multicellular organisms. We now report that Arabidopsis phototropin, a blue light photoreceptor containing flavin mononucleotide (FMN) as its chromophore, can be engineered into a small (106-residue) SOG (“miniSOG”), which absorbs maximally at 448 and 473 nm with extinction coefficients of 16,700 and 13,600 M-1cm-1 respectively. Quantum yields for fluorescence and 1O2 generation are 0.30 and 0.47. MiniSOG binds endogenous FMN very tightly (dissociation constant ∼ 10-10 M), so bacteria and mammalian cells upregulate their total FMN to keep miniSOG saturated, without any obvious toxicity in the absence of illumination. Although the green fluorescence of miniSOG is weak and bleachable, it shows that fusions of miniSOG to a variety of proteins in mammalian cells appear to localize correctly, even inside organelles when appropriate. After fixation, illumination of miniSOG to generate 1O2 efficiently polymerizes diaminobenzidine into an osmiophilic deposit, enabling correlative electron microscopy. In an initial biological application, electron microscopy shows that a cell-adhesion molecule, SynCAM1, fused to miniSOG, predominantly localizes to the presynaptic side of cortical neuron synapses. This compact SOG relying only on ubiquitous endogenous FMN will greatly expand the utility of imaging and ablation techniques based on 1O2.

Optical absorption and emission characteristics of Pr 3+-doped RTP glasses

Judd–Ofelt (J–O) parameters are calculated for Pr 3+ions doped alkaline earth potassium titanium phosphate (RTP, where R=Mg, Ca, Sr) glasses. The best fit for the oscillator strengths has been obtained by omitting the 3H 4→ 3P 2 hypersensitive transition. In all the three glasses, the J–O parameters follow the same trend as Ω 2< Ω 4> Ω 6. These J–O intensity parameters are then used to compute the radiative properties such as the radiative transition probabilities ( A R ), branching ratios ( β R ) and radiative lifetimes ( τ R ) for the observed fluorescence bands. The fluorescence spectra obtained upon 445 nm excitation exhibited an intense emission band centered at 484 nm ( 3P 0→ 3H 4), four medium intense bands at 525 nm ( 3P 1→ 3H 5), 598 nm ( 1D 2→ 3H 4), 608 nm ( 3P 0→ 3H 6) and 641 nm ( 3P 0→ 3F 2) and one weak band at 669 nm ( 3P 1→ 3F 3). The experimental branching ratios ( β exp ) obtained from steady state fluorescence are compared with the calculated values. The decay curves measured from the 3P 0 excited metastable state are found to be single exponential in all the glasses. The gain bandwidths ( σ e ×Δ λ P ) and optical gain ( σ e × τ m) parameters suggest that the RTP glasses could be used for laser active materials to emit intense blue emission at 484 nm.

Comprehensive analysis of the effect of atomic and molecular metastable state excitation on air plasma composition behind strong shock waves

A detailed analysis of the processes of charged and neutral species formation in N2–O2–Ar system behind strong shock waves is conducted on the basis of an extended thermally non-equilibrium kinetic model with careful allowance of reactions with electronically excited atoms and molecules and vibration–electron–chemistry coupling. The model is validated against experimental data such as measurements of electron number density behind the strong shock wave front, temporal profile of O2 vibrational temperature, temporal evolution of NO(γ) radiation intensity in the post-shock region and values of time instants at which maximum quantities of radiation intensity of N2(1+) and bands are achieved. It is demonstrated that the model without the allowance of processes involving electronically excited atoms and molecules overestimates the concentrations of both charged and neutral species. For electron and NO concentrations the overestimation can come up to a factor of 2. The computations also indicate the existence of a strong vibration–electron–chemistry coupling in the air plasma produced by the strong shock wave. Only the model, which takes into account such an interaction, makes it possible to properly predict the variation of gas dynamic parameters and species concentrations in the post-shock region.

Doubly excited P, D and F unnatural parity states of hydrogen negative ion using correlated wavefunctions

We have investigated the doubly excited 1,3Pe, 1,3D° and 1,3Fe states of the hydrogen negative ion using highly accurate correlated wavefunctions. The doubly excited 2p2 3Pe metastable bound state (so-called second bound state) energy of H− is obtained using the Ritz variational principle. The upper bound of the energy of the 2p2 3Pe bound states is −0.125 355 451 242 au, which is comparable with the available best results −0.125 355 451 24 (Bylicki M and Bednarz E 2003 Phys. Rev. A 67 022503). We employ the complex-coordinate rotation method to extract resonance parameters. The resonance parameters (both resonance position and width) of H− for 1,3Pe, 1,3D° below the n = 3, 4, 5, and 6 thresholds and 1,3Fe states below the n = 4, 5, and 6 thresholds are reported. We have also employed the stabilization method to extract resonance parameters for the 1,3Pe states. In addition to Feshbach resonances lying below the n = 3, 4, 5, 6 hydrogen thresholds, we have calculated one 3D° shape resonances lying above the H (n = 3) threshold. Resonance parameters obtained from present work are comparable with the reported results using CI-type basis functions and hyperspherical potential curves. The 1Pe resonance state, and some of 1,3Pe, 1,3D°, 1,3Fe resonance states are reported for the first time using the explicitly correlated exponential wavefunctions.

Re-entrance phase and excited metastable electronic spin states in one-dimensional spin crossover compounds explained by atom-phonon coupling model

An unusual behavior of the disappearance and the reappearance of the hysteresis loop of the spin crossover compounds under external pressure was observed experimentally by Garcia et al. [Hyperfine Interact. 139/140, 543 (2002)] and modeled within the framework of the “atom-phonon coupling” model. In this contribution we explain the evolution of the hysteresis width versus the applied pressure. Using the same model, we also show the existence of stable as well as metastable and unstable states at very low temperatures. These excited metastable electronic spin states at low temperature can be associated to the “out of equilibrium” states in the light-induced electronic spin state trapping.

Intensity distributions of enhanced H emission from laser-induced low-pressure He plasma and a suggested He-assisted excitation mechanism

An experimental study was conducted on the spatial distributions of hydrogen emission intensities from low-pressure plasmas generated by laser ablation of zircaloy-4 and black stone targets in nitrogen and helium ambient gases. In addition to confirming the previously observed intensity enhancement effect in ambient helium gas, the hydrogen and helium emission intensities measured along the plasma expansion direction revealed remarkable extended spatial distributions featuring unexpected maxima near the far end of the plasma where the available shock-wave generated thermal excitation energy should have been significantly reduced. This “anomalous” feature necessarily implied the presence of an additional excitation process beside the well known shock-wave excitation process which is responsible for the plasma emission of heavy atoms in low-pressure ambient gas. Further analysis of the data led to a suggested physical mechanism explaining the possible contribution of a helium metastable excited state to the unusual phenomenon observed in this experiment.

Search for low-energy induced depletion of 178Hfm2 at the SPring-8 synchrotron

Electromagnetic transitions within nuclei reflect specific aspects of nuclear structure. This is particularly true for metastable excited states, or isomers, like 178Hfm2 (T1/2=31 years, excitation energy 2446 keV). The interaction of external radiation with isomers can be used to study atomic and nuclear properties and, perhaps, to induce a release of the stored energy. Some experiments indicated that low-energy photons near the L3 edge (9.561 keV) of hafnium could cause this to occur for 178Hfm2, but the lack of a viable physical model and null experiments by other groups have left these claims in doubt. The present work describes a new experiment to examine this process by closely duplicating the irradiation conditions in positive studies, but using a more advanced multi-detector γ array. No support for an induced depletion of 178Hfm2 by low-energy photons was obtained, with an upper limit for the integral cross section that is eight orders-of-magnitude below the reported value.

Structure and time-resolved phosphorescence spectra of crystalline and glassy 2-bromobenzophenone

Microsecond-level time-resolved photoluminescence spectra of 2-bromobenzophenone are measured on crystalline (at 1.6, 52, and 300K) and glassy (at 1.6 and 95K) samples. For each of these physical situations the characteristic decay times are determined for the 0–0 band of monomer emission from metastable (blue 0–0 band) and global (red 0–0 band) excited states of the 2-benzophenone molecule in the crystal and also for the bimolecular excimer emission. The time dependence of the intensity of the red 0–0 band at 52K in the crystal is found to be nonmonotonic, a fact that provides yet another beautiful demonstration of the presence of two excited states and is indicative of thermally activated nonradiative transitions from the metastable excited state to the global. The corresponding kinetics is well described by a system of equations for joint radiative decay. To gain a better understanding of the nature of the anomalies of the phosphorescence, the crystal and molecular structures of 2-benzophenone at 100 and 200K are determined by the single-crystal x-ray diffraction method. Within the temperature interval 100–300K the crystal structure of 2-benzophenone remains monoclinic, with symmetry space group P21∕a (Z=4). The coefficient of linear thermal expansion is anisotropic and of a magnitude (10−5−10−4K−1) typical for organic molecular crystals. The conformation parameters of the molecule vary relatively strongly with temperature; the behavior of the C–Br and C=O bonds is most interesting: they become shorter with increasing temperature, possibly indicating a weakening of the weak hydrogen bonds of the peripheral oxidizer atoms with the acceptors of the neighboring molecules.

The role of He in enhancing the intensity and lifetime of H and D emissions from laser-induced atmospheric-pressure plasma

A series of measurements have been performed on the time dependences of the intensities of helium, hydrogen, and deuterium emission lines from the corresponding laser-induced helium plasma at atmospheric pressure for two different He flow rates. The prolonged Hα and Hβ emissions along with their constant intensity ratio over a relatively extended period indicate the need to provide an alternative excitation mechanism other than the well-known thermal excitation process in a hot plasma. This additional excitation mechanism is also related to the metastable excited state of a He atom as indicated by the similar characteristics of the observed time dependence of the emission intensities. The enhanced intensity and lifetime of He emission at a high He flow rate was explained in terms of the collision-induced increase in the number of He atoms excited to above the 2 S10 metastable state, which was also responsible for the delayed excitation of H and D atoms via an energy transfer mechanism involving a Penning-like chemi-ionization process. Finally, the benefits of He-assisted delayed excitation of H and D atoms and the aforementioned enhanced intensity and lifetime at a high He flow rate were demonstrated by the achievement of clearly resolved Hα and Dα emission lines.

One-photon two-electron transitions for metastable helium

We have obtained very accurate excitation energies for helium corresponding to the transitions 2p23Pe ↦nln’l’ 3Do\((n, n' \geqslant 3)\) using highly correlated Hylleraas-type wave functions. The doubly excited 2p23Pe metastable bound state energy is obtained using Ritz variational principle. The upper bound of 2p23Pe energy is –0.7105001556783 a.u., which is close to the best results –0.71050015567833 [R. Hilger et al., Chem. Phys. Lett. 262, 400 (1996)]. We employ complex-coordinate rotation method to obtain 3Do resonances. The 3Do resonance energies and widths along with one-photon two-electron transition energies are reported.