Excited Hydrogen Atoms Research Articles

Coulomb excitation of hydrogen atoms by vortex ion beams

Abstract Coulomb excitation of hydrogen atoms by vortex protons is theoretically investigated within the framework of the non--relativistic first--Born approximation and the density matrix approach. Special attention is paid to the magnetic sublevel population of excited atoms and, consequently, to the angular distribution of the fluorescence radiation. We argue that both these properties are sensitive to the projection of the orbital angular momentum (OAM), carried by the projectile ions. In order to illustrate the OAM--effect, detailed calculations have been performed for the $1s \to 2p$ excitation and the subsequent $2p \to 1s$ radiative decay of a hydrogen target, interacting with incident Laguerre--Gaussian vortex protons. The calculation results suggest that Coulomb excitation can be employed for the diagnostics of vortex ion beam at accelerator and storage ring facilities.

Recent advances in relativistic excitation of atomic hydrogen

Within the framework of the relativistic regime ( E k > 2700 eV) and the first Born approximation, we investigate the inelastic excitation of atomic hydrogen by electron impact from 1 s to 2 p state. The incoming and outgoing electrons are described by free Dirac spinors, while the hydrogen atom target is described by the exact solutions of Dirac equation with a central potential. A detailed study has been devoted to the non-relativistic regime as well as the relativistic regime of the process, and the results obtained show the differences between the two regimes. These differences have revealed that the differential cross section (DCS) is influenced by the parameters of the 1 s and 2 p states, along with significant effects on diffusion during small pulse transfers. The relativistic effects are responsible for the significant changes in the DCS as a function of diffusion angles which depend on the chosen geometry.

Dipolar focusing in laser-assisted positronium formation

We consider positronium formation in collisions of positrons with excited hydrogen atoms H(n) in an infrared laser field theoretically. This process is assisted by the dipolar focusing effect: a positron moving in a superposition of a laser field and the dipolar field can approach the atomic target even if its trajectory starts with a very large impact parameter, leading to a significant enhancement of the Ps formation cross section. The classical trajectory Monte Carlo method, which is justified for n⩾3 , allows efficient calculation of this enhancement. A similar effect can occur in collisions of positrons with other atoms in excited states, which can lead to improvements in the efficiency of positronium formation.

An experimental platform for investigation of the Zeeman effect in strong magnetic fields.

An experimental platform is developed for the investigation of the Zeeman effect in strong magnetic fields. Mega-Gauss magnetic fields are generated by a 1 MA Zebra pulsed power machine using metal rod loads. A gas jet or CH oil on the load is the source of hydrogen. Excited hydrogen atoms are backlit by black body radiation from the rod load. Hydrogen absorption spectra are recorded with a grating spectrometer and intensified gated CCD camera. The experimental platform enables the observation of the quadratic Zeeman effect in hydrogen gas jets using the spectral shift of the central line in the Zeeman triplet. Other gases can be studied using the gas jet method.

Computational Analysis of Excited State Intramolecular Hydrogen Atom Transfer and Microsolvation Studies on 8-Acetyl-7-hydroxy-4-methylcoumarin using DFT and TDDFT

The computational calculations were carried out on 8-acetyl-7-hydroxy-4-methyl coumarin (AHM) and its water complex AHM+(H2O)4- [AHMH] at ground and excited states by employing density functional theory (DFT)/specific state time-dependent density functional theory (SS-TDDFT). In AHM and AHMH molecules, there is an intramolecular hydrogen bond between hydroxyl group and acetyl group along with inter-molecular hydrogen bonds in the hydrated molecule. The computational studies of molecular structural parameters, molecular electrostatic potential, natural bond orbital (NBO) analysis, the molecular orbital’s and UV-Vis spectra of both the molecules under polar solvents were explored by B3LYP/cc-pVDZ/PCM/EFP1 method. The intramolecular hydrogen atom transpired between hydroxyl to acetyl group in AHM/AHMH molecules from S0→S1 state but not in S0→S3/S0→S2 states even though the S3/S2 states have significant oscillation strengths. This indicates that intramolecular chage transfer (ICT) occurs within the molecules and it confirmed using potential energy surface (PES) scan studies.

Role of excitation in the electron rotational asymmetry in circularly polarized laser fields

We comparatively study electron rotational asymmetries in photoionization of excited hydrogen atoms and photodetachment of the fluorine anions in single circularly polarized laser fields by numerically solving the three-dimensional time-dependent Schrödinger equation. We first show that single-photon ionization (or detachment) is more favorable as the electron initially corotates with respect to the direction of the laser field. More interestingly, we systematically demonstrate that the electron rotational asymmetry of the excited hydrogen atom is completely contrary to that of the fluorine anion in the multiphoton ionization (or detachment) regime, which can be attributed to the role played by excited bound states by analyzing the total excitation probability in the excited hydrogen atom. Furthermore, we also confirm that the electron rotational asymmetry of and degenerate orbitals in the excited hydrogen atom changes when the ionization mechanism switches from three-photon to over-the-barrier.

A Monte Carlo simulation study of L-band emission upon gamma radiolysis of water

Several studies have confirmed visible light and ultraviolet emission during water molecule radiolysis; however, radiofrequency emission has been scarcely investigated. This simulation study has revealed that the gamma radiolysis of water creates excited hydrogen atoms which emit radio recombination L-band (1 GHz–2 GHz) radio waves of sufficient strength that a radio-imaging device can detect. The physical and physicochemical stages of radiolysis of water have been modeled via application of Monte Carlo simulation techniques (utilizing Geant4) up to 1 ps from the onset of gamma photon interaction with water molecules. Subsequently an L-band emission analysis up to a few ms duration was accomplished to obtain and understand RF power emission characteristics as a function of gamma radiation dose rate. The generated L-band 1.4 GHz RF power is 19 dBm per one Gy/h of absorbed dose rate. The generated RF power varies linearly with absorbed dose rate up to 3.38 Gy/h, beyond which non-linearity manifests due to line broadening. Unlike conventional gamma radiation detectors, the proposed RF-based gamma radiation detection model indicates that the detection sensitivity of the RF detector does not deteriorate rapidly with an increase in altitude above radiation source at ground. Consequently, source-detector standoff distances are increased significantly at least up to two orders of magnitude higher than the current gamma radiation detection scenario. Preliminary experiments were conducted using an experimentally simulated signal, an RF transmitter and an RF receiver along with suitable data acquisition unit. This was performed to determine the sensitivity of the RF receiver, which would enable laboratory scale testing with safe gamma sources of dose rate less than 0.1 mGy/h.

Two-color laser scattering for diagnostics of hydrogen plasma

A two-color laser scattering (2CLS) method is proposed to measure electron and neutral densities, as well as electron and ion temperatures in hydrogen plasma. 2CLS uses two probe wavelengths to identify the Rayleigh scattering and Thomson scattering contributions coming from neutrals and electrons, respectively. Laser scattering signals were simulated for various conditions of a hydrogen plasma at thermodynamic equilibrium applying the available and calculated cross-sections for Rayleigh scattering by ground-sate and excited hydrogen atoms at probe wavelengths of 355 nm and 532 nm. The developed 2CLS method was eventually applied to study the laser-induced plasma in hydrogen at near atmospheric pressure. Temporally and spatially resolved electron and ion temperatures and densities of electrons and hydrogen atoms (ground-state and excited) were determined.

A Photochemical Macrocyclization Route to Asymmetric Strained [3.2] Paracyclophanes.

The intricate frameworks of paracyclophanes are an important target for synthesis since they are found in various chiral auxiliaries, solar cells, high-performance plastics, pharmaceuticals, and molecular machines. Whereas numerous methods exist for the preparation of symmetric paracyclophanes, protocols for the efficient synthesis of strained asymmetric scaffolds are limited. Here we report a remarkably simple photochemical route to strained [3.2]paracyclophanes starting from readily available educts. By way of NMR and X-ray analyses, we discovered that UV-irradiation of an aromatic carboxylic ester tethered to a toluene moiety leads to the intramolecular formation of a new C-C bond, with loss of an alcohol. A systematic evaluation of the reaction conditions and substituents, as well as radical starter and triplet quenching experiments, point to a reaction mechanism involving an excited triplet state and hydrogen atom transfer. The new method proved to be robust and versatile enabling the synthesis of a range of cyclophanes with different substitutions, including an unusual diastereoisomer with two planar chiral centers, and thus proved to be a valuable addition to the synthetic toolbox.

Experiments on the Electron Impact Excitation of Hydrogen Molecules Indicate the Presence of the Second Flavor of Hydrogen Atoms

In one of our previous papers, we performed a comparative analysis of the experimental and theoretical cross-sections for the excitation of atomic hydrogen by electrons. We found that the theoretical ratio of the cross-section σ2s of the excitation of the state 2s to the cross-section σ2p of the excitation of the state 2p was systematically higher than the corresponding experimental ratio by about 20% (far beyond the experimental error margins). We showed that this discrepancy can be due to the presence of the Second Flavor of Hydrogen Atoms (SFHA) in the experimental gas and that the share of the SFHA in the mixture, required for removing this discrepancy, was about the same as the share of the usual hydrogen atoms. The theory behind the SFHA was based on the standard quantum mechanics—on the second solution of the Dirac equation for hydrogen atoms—and on the experimental fact that the charge distribution inside the proton has the peak at the center of the proton; the term “flavor” was used by the analogy with flavors of quarks. In the present paper, we used the same guiding principles, as employed in that previous study, for the comparative analysis of the experimental and theoretical cross-sections for the excitation of molecular hydrogen by electrons. We found that presumably the most sophisticated calculations, using the convergent close-coupling method involving 491 states, very significantly underestimate the corresponding experimental cross-sections for the two lowest stable triplet states. We showed that if in some hydrogen molecules one or both atoms would be the SFHA, then the above very significant discrepancy could be eliminated. We estimated that it would take such unusual hydrogen molecules to be represented in the experimental gas by the share of about 0.26. This is just by about 40% smaller than the share 0.45 of the SFHA deduced in our previous analysis of the experiment on the electron impact excitation of hydrogen atoms (rather than hydrogen molecules). It should be emphasized that from the theoretical point of view, the share of the unusual hydrogen molecules in any experimental gas and the share of the unusual hydrogen atoms (SFHA) in any experimental gas should not be expected to coincide (it would be the comparison of “apples to oranges”, rather than “apples to apples”). In addition, given the roughness of the above estimates, we can state that the results of the present paper reinforce the main conclusion of our previous papers of the very significant share of the SFHA in the experimental hydrogen gases. Thus, the experiments on the electron impact excitation of hydrogen molecules are the fourth type of atomic experiments that proved the existence of the SFHA.

Visible Light‐Mediated Dearomative Hydrogen Atom Abstraction/ Cyclization Cascade of Indoles

AbstractThe photochemical synthesis of yet unknown 2‐oxospiro[azetidine‐3,3′‐indolines] (17 examples, 80–95 % yield), 2,4‐dioxospiro[azetidine‐3,3′‐indolines] (eight examples, 87–97 % yield), and 1‐oxo‐1,3‐dihydrospiro[indene‐2,3′‐indolines] (17 examples, 85–97 % yield) is described. Starting from readily accessible 3‐substituted indoles, a dearomatization of the indole core was accomplished upon irradiation at λ=420 nm in the presence of thioxanthen‐9‐one (10 mol%) as the sensitizer. Based on mechanistic evidence (triplet energy determination, deuteration experiments, by‐product analysis) it is proposed that the reaction proceeds by energy transfer via a 1,4‐ or 1,5‐diradical intermediate. The latter intermediates are formed by excited state hydrogen atom transfer from suitable alkyl groups within the C3 substituent to the indole C2 carbon atom. Subsequent ring closure proceeds with pronounced diastereoselectivity to generate a 4‐ or 5‐membered spirocyclic dearomatized product with several options for further functionalization.

Visible Light-Mediated Dearomative Hydrogen Atom Abstraction/ Cyclization Cascade of Indoles.

The photochemical synthesis of yet unknown 2‐oxospiro[azetidine‐3,3′‐indolines] (17 examples, 80–95 % yield), 2,4‐dioxospiro[azetidine‐3,3′‐indolines] (eight examples, 87–97 % yield), and 1‐oxo‐1,3‐dihydrospiro[indene‐2,3′‐indolines] (17 examples, 85–97 % yield) is described. Starting from readily accessible 3‐substituted indoles, a dearomatization of the indole core was accomplished upon irradiation at λ=420 nm in the presence of thioxanthen‐9‐one (10 mol%) as the sensitizer. Based on mechanistic evidence (triplet energy determination, deuteration experiments, by‐product analysis) it is proposed that the reaction proceeds by energy transfer via a 1,4‐ or 1,5‐diradical intermediate. The latter intermediates are formed by excited state hydrogen atom transfer from suitable alkyl groups within the C3 substituent to the indole C2 carbon atom. Subsequent ring closure proceeds with pronounced diastereoselectivity to generate a 4‐ or 5‐membered spirocyclic dearomatized product with several options for further functionalization.

Circular Rydberg States of Lithium Atoms or Lithium-like Ions in a High-Frequency Laser Field: The Break of the Algebraic Symmetry

There have been publications presenting studies of relatively simpleatomic systems in a high-frequency laser field. The systems studied under sucha field were highly excited (Rydberg) hydrogen atoms and hydrogen-like ions, aswell as highly-excited helium atoms and helium-like ions. These studies wereperformed classically, as appropriate for Rydberg states and revealed celestialanalogies, various kinds of the precession of the electron orbit and a shift ofthe energy. In the current paper we analyze a lithium atom or a lithium-likeion, one of the three electrons being in a Rydberg state, subjected to a laserfield of high frequency. We used the generalized method of effectivepotentials. We showed that for a relatively small distance of the outerelectron from the nucleus, the system has higher than geometrical symmetry. Foran arbitrary separation of the outer electron from the nucleus, we dealt withthe broken algebraic symmetry reduced to the geometrical (axial) symmetry. Wedescribe analytically the following two outcomes for circular Rydberg states.One of the outcomes is the precession of the plane of the orbit of the highlyexcited electron – the precession whose frequency we calculate analytically. Itleads to the following modification of the radiation spectrum: The appearanceof satellites at the distances from the unperturbed frequency of the spectralline equal to multiples of the precession frequency. The other outcome is anenergy shift of the Rydberg electron. Its dependence on the nuclear charge wasfound to be a non-monotonic function. This is a counterintuitive result.

High Speed Ashing of Ion Implanted Photoresist by Microwave Excited Water Vapor Plasma with Powered Substrate

This study examined a substrate bias voltage introduced into a microwave excited water vapor plasma to enable fast ashing of photoresist implanted with high-dose ions without occurring popping. The ashing rate at the center of the ashing distribution for the photoresist implanted with boron with an implantation dose of 1×1016 atoms/cm2 was estimated as 1.0 μm/min. Results of plasma emission diagnostics suggest that rf bias application effects generated a plasma rich in excited hydrogen and oxygen atom near the substrate surface. This plasma is presumed to assist the removal of the hardened layer of ion implanted photoresist and to affect hydrogen gas addition. A self-bias voltage during bias application was sufficiently low, approximately －30 V, at a peak-to-peak voltage of 1 kV at 1 MHz.

Theoretical Insights into the Intermolecular Hydrogen Bond Effect on ESIHT Process in 2′-Hydroxychalcone: A Combined DFT/TDDFT Study

Present computational study lighting up the ground and excited states properties of 2′-hydroxychalcone (2′-HC) and 2′-HC + (H2O)2-[2′-HCH] molecules by employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Furthermore, micro-solvation, hydrogen bond dynamics and natural charge analysis studies have been done for both molecules at ground/excited states by using effective fragment potential (EFP1)/natural bond orbital (NBO) methods at 6-31G(d,p)/B3LYP level. The excited state intra-molecular hydrogen atom transfer (ESIHT) mechanism of 2′-HC was investigated via potential energy scans (PES). No hydrogen atom transfer is observed in the S3 state of 2′-HC and S1/S3 states of 2′-HCH. The optimized molecular structures, molecular orbital’s and electrostatic potential maps were depicted along with UV-Vis absorption spectra. Good consistency between experimental and computational absorption wavelength for a 2′-HC molecule with methanol as solvent.

Desorption of excited H* atoms from free clusters Ar/CH4 and solid Ar doped with CH4

Desorption of excited hydrogen atoms was detected from both solid Ar doped with CH4 and free nanoclusters Ar/CH4 at irradiation with an electron beam. It was monitored by an emission of the Lyman-a line. Measurements of cathodoluminescence (CL) spectra in the VUV range were performed within the CH4 concentration limits 0.1–10% in the solid matrix. The CL of free clusters with an average size of 1200 atoms per cluster was detected from pure Ar cluster jet and from Ar clusters doped with 0.1% CH4. The mechanisms of desorption of electronically excited H* atoms from solids and clusters are proposed on the basis of an analysis of energy transfer pathways with the final stage of relaxation — population of the n = 3 state of hydrogen atoms.

Excited state hydrogen atom transfer pathways in 2,7-diazaindole – S1-3 (S = H2O and NH3) clusters

The photoinduced tautomerization reactions via hydrogen atom transfer in the excited electronic state (ESHT) have been computationally investigated in 2,7-diazaindole (27DAI) – (H2O)1-3 and 27DAI – (NH3)1-3 isolated clusters to understand the role of various solvent wires. Two competing ESHT reaction pathways originating from the N(1)–H group to the neighbouring N(7) (R(1H-Sn-7H)) and N(2) (R(1H-Sn-2H)) atoms were rigorously examined for each system. Both one- and two-dimensional potential energy surfaces have been calculated in the excited state to investigate the pathways. The R(1H-Sn-7H) was found to be the dominant route with reaction barriers ranging from 26–40 kJmol−1 for water clusters, and 14–26 kJmol−1 for ammonia clusters. The barrier heights for ammonia clusters were found to be nearly half of the that observed for the water systems. The lengthening of the solvent chain up to two molecules resulted in a drastic decrease in the barrier heights for R(1H-Sn-7H). The barriers of the competing reaction channel R(1H-Sn-2H) were found to be significantly higher (31–127 kJmol−1) but were observed to be decreasing with the lengthening of the solvent wire as in the R(1H-Sn-7H) pathway. In both the reactions, the angle strain present in the transition state structures was dependent upon the solvent chain's length and was most likely the governing factor for the barrier heights in each solvent cluster. The results have also affirmed that the ammonia molecule is a better candidate for hydrogen transfer than water because of its higher gas-phase basicity. The results delineated from this investigation can pave the way to unravel the excited-state hydrogen atom transfer pathways in novel N–H bearing molecules.

Epitaxial growth of 3C-SiC film by microwave plasma chemical vapor deposition in H2-CH4-SiH4 mixtures: Optical emission spectroscopy study

Microwave (MW) plasma in silane-hydrogen and silane-hydrogen-methane mixtures is used effectively for chemical vapor deposition of Si, SiC, diamond, and SiC-diamond composite films; however, the properties of such plasma at pressures of the order of 100 Torr remain largely unexplored. Here we characterize the MW plasma (2.45 GHz) in SiH4 + H2 and SiH4 + СH4 + H2 mixtures (72 Torr) with silane content ranging from 0% to 5% in the process gas using high-resolution optical emission (OE) spectroscopy. Besides the OE lines of C2 dimer, Balmer series of excited atomic hydrogen (Hα, Hβ, Hγ, Hδ, and Hε), and CH radical, we observed atomic Si lines at 263, 288, and 391 nm and a relatively weak SiH emission. Gas temperature Tg of ≈3160 K is assessed from the rotational structure of the C2 dimer (Δν = 0, λ = 516.5 nm) emission band, and the absorbed microwave power density (MWPD) in the plasma fluctuates in the narrow range between 36 and 43 W/cm3 with a slight tendency to decrease with silane addition. The MWPD, intensity ratio Hα/Hβ of hydrogen Balmer series lines (related to excitation temperature Texc), and Si lines’ intensities in OE spectra as functions of SiH4 concentration in H2 and H2 + CH4 mixtures all show an extremum or a kink in slope near a special point at ≈0.5% SiH4. Finally, we produced a silicon carbide film of cubic polytype 3C-SiC on a (111) oriented Si substrate, which was characterized with Raman spectroscopy and x-ray diffraction, and its monocrystalline structure was confirmed.

A novel hydrogenic spectroscopic technique for inferring the role of plasma–molecule interaction on power and particle balance during detached conditions

Detachment, an important mechanism for reducing target heat deposition, is achieved through reductions in power, particle and momentum; which are induced through plasma–atom and plasma–molecule interactions. Experimental research in how those reactions precisely contribute to detachment is limited. Both plasma–atom as well as plasma–molecule interactions can result in excited hydrogen atoms which emit atomic line emission. In this work, we investigate a new Balmer Spectroscopy technique for Plasma–Molecule Interaction—BaSPMI. This first disentangles the Balmer line emission from the various plasma–atom and plasma–molecule interactions and secondly quantifies their contributions to particle (ionisation and recombination) and power balance (radiative power losses). Its performance is verified using synthetic diagnostic techniques of both attached and detached TCV and MAST-U SOLPS-ITER simulations. We find that H 2 plasma chemistry involving and/or H − can substantially elevate the Hα emission during detachment, which we show is an important precursor for Molecular Activated Recombination. An example illustration analysis of the full BaSPMI technique shows that the hydrogenic line series, even Lyα as well as the medium-n Balmer lines, can be significantly influenced by plasma–molecule interactions by tens ofpercent. That has important implications for using atomic hydrogen spectroscopy for diagnosing divertor plasmas.

DFT / TDDFT insights into excited state intra-molecular hydrogen atom transfer mechanism in Liqcoumarin: an EFP1 study.

The theoretical calculations were carried out on Liqcoumarin (LC) and its water complex LC+(H20)4-[LCH] at the ground (S0) / first excited states (S1) by employing density functional theory (DFT) / state specific time-dependent density functional theory (SS-TDDFT). In LC and LCH, there is an intra-molecular hydrogen bond between hydroxyl group and acetyl group along with four inter-molecular hydrogen bonds in the hydrated molecule. The computational studies of molecular structural parameters, electrostatic potential, NBO analysis, molecular orbital's, and UV-Vis spectra of both molecules under polar solvents were explored by B3LYP / 6-31G (d,p) / PCM / EFP1 method. The intra-molecular hydrogen atom transpires between hydroxyl to acetyl group in both pure andhydrated molecules confirm the ICT process, and it affirms through potential energy surface (PES) scans.