Single Ionization Spectra Research Articles

Spectral Broadening of a Single Er3+ Ion in a Si Nanotransistor

Single rare-earth ions in solids show great potential for quantum applications, including single photon emission, quantum computing, and high-precision sensing. However, homogeneous linewidths observed for single rare-earth ions are orders of magnitude larger than the sub-kilohertz linewidths observed for ensembles in bulk crystals. The spectral broadening creates a significant challenge for achieving entanglement generation and qubit operation with single rare-earth ions, so it is critical to investigate the broadening mechanisms. We report a spectral broadening study on a single Er$^{3+}$ ion in a Si nano-transistor. The Er-induced photoionisation rate is found to be an appropriate quantity to represent the optical transition probability for spectroscopic studies, and the single ion spectra display a Lorentzian lineshape at all optical powers in use. Spectral broadening is observed at relatively high optical powers and is caused by spectral diffusion on a fast time scale.

Track Structure Components: Characterizing Energy Deposited in Spherical Cells from Direct and Peripheral HZE Ion Hits.

To understand the biological effects of radiation, it is important to determine how ionizing radiation deposits energy in micrometric targets. The energy deposited in a target located in an irradiated tissue is a function of several factors such as the radiation type and the irradiated volume size. We simulated the energy deposited by energetic ions in spherical targets of 1, 2, 4, and 8 µm radii encompassed in irradiated parallelepiped volumes of various sizes using the stochastic radiation track structure code Relativistic Ion Tracks (RITRACKS). Because cells are usually part of a tissue when they are irradiated, electrons originating from radiation tracks in neighboring volumes also contribute to energy deposition in the target. To account for this contribution, we used periodic boundary conditions in the simulations. We found that the single-ion spectra of energy deposition in targets comprises two components: the direct ion hits to the targets, which is identical in all irradiation conditions, and the contribution of hits from electrons from neighboring volumes, which depends on the irradiated volume. We also calculated an analytical expression of the indirect hit contributions using the local effect model, which showed results similar to those obtained with RITRACKS.

Single-ion properties of theSeff=12XY antiferromagnetic pyrochloresNaA′Co2F7(A′=Ca2+, Sr2+)

The antiferromagnetic pyrochlore material NaCaCo$_2$F$_7$ is a thermal spin liquid over a broad temperature range (~ 140 K down to $T_F = 2.4 $K), in which magnetic correlations between Co$^{2+}$ dipole moments explore a continuous manifold of antiferromagnetic XY states. The thermal spin liquid is interrupted by spin freezing at a temperature that is ~ 2 % of the mean field interaction strength, leading to short range static XY clusters with distinctive relaxation dynamics. Here we report the low energy inelastic neutron scattering response from the related compound NaSrCo$_2$F$_7$, confirming that it hosts the same static and dynamic spin correlations as NaCaCo$_2$F$_7$. We then present the single-ion levels of Co$^{2+}$ in these materials as measured by inelastic neutron scattering. An intermediate spin orbit coupling model applied to an ensemble of trigonally distorted octahedral crystal fields accounts for the observed transitions. The single-ion ground state of Co$^{2+}$ is a Kramers doublet with a strongly XY-like $g$-tensor ($g_{xy}/g_{z}$ ~ 3). The local disorder inherent from the mixed pyrochlore $A$ sites (Na$^{+}$/Ca$^{2+}$ and Na$^{+}$/Sr$^{2+}$) is evident in these measurements as exaggerated broadening of some of the levels. A simple model that reproduces the salient features of the single-ion spectrum produces approximately 8.4 % and 4.1 % variation in the $z$ and $xy$ components of the $g$-tensor, respectively. This study confirms that an $S_{eff} =1/2$ model with XY antiferromagnetic exchange and weak exchange disorder serves as a basic starting point in understanding the low temperature magnetic behavior of these strongly frustrated magnets.

Electron paramagnetic resonance of Cr3+ ions in ABO3 (A = Sc, Lu, In) diamagnetic crystals

A magnetic resonance method is applied to the investigation of a number of isostructural diamagnetic compounds ABO3 (A = Sc, Lu, In) with small additions of Cr3+ ions (S = 3/2) sufficient to observe single-ion spectra. It is shown that the resonance spectra for isolated Cr3+ ions can be described to a good accuracy by the ordinary axial spin Hamiltonian for 3d ions in octahedral oxygen environment. The parameters of the spin Hamiltonian are determined. It is established that Cr3+ ions in these crystals are characterized by easy-axis-type anisotropy.

The electronic states of TeH(+): a theoretical contribution.

This work reports the first theoretical characterization of a manifold of electronic states of the as yet experimentally unknown monotellurium monohydride cation, TeH(+). Both Λ + S and Ω representations were described showing the twelve states correlating with the three lowest (Λ + S) dissociation channels, and the twenty five states associated with the five lowest Ω channels. The X (3)Σ(-) state is split into X1 0(+) and X2 1 separated by 1049 cm(-1); they are followed by the states a 2 (a (1)Δ) and b 0(+) (b (1)Σ(+)) higher in energy by 8554 and 17 383 cm(-1), respectively. These states can accommodate several vibrational energy levels. The potential energy curves of the Ω states arising from the bound A (3)Π, the weakly bound (1)Π, and the repulsive (5)Σ(-) states have a complex structure as shown by the very close avoided crossings just above ∼30 000 cm(-1). In particular, a double minima potential results for the state A1 2 that in principle could be probed experimentally through the A1 2-X2 1 system transitions. The states A2 1, b 0(+), and A4 0(+) offer possible routes to experimental investigations involving the ground state X1 0(+). Higher energy states are very dense and mostly repulsive. The high-level of the electronic structure calculations, by providing a global view of the electronic states and reliable spectroscopic parameters, is expected to further guide and motivate experimental studies on this species. Additional discussions on dipole and transition dipole moments, transition probabilities, radiative lifetimes, and a simulation of the single ionization spectrum complement the characterization of this system.

Signatures of the non-Maxwellianκ-distributions in optically thin line spectra

We investigate the possibility of diagnosing the degree of departure from the Maxwellian distribution using single-ion spectra originating in astrophysical plasmas in collisional ionization equilibrium. New atomic data for excitation of Fe IX-XIII are integrated under the assumption of a kappa-distribution of electron energies. Diagnostic methods using lines of a single ion formed at any wavelength are explored. Such methods minimize uncertainties from the ionization and recombination rates, as well as the possible presence of non-equilibrium ionization. Approximations to the collision strengths are also investigated. The calculated intensities of most of the Fe IX-XIII EUV lines show consistent behaviour with kappa at constant temperature. Intensities of these lines decrease with kappa, with the vast majority of ratios of strong lines showing little or no sensitivity to kappa. Several of the line ratios, especially involving temperature-sensitive lines, show a sensitivity to kappa that is of the order of several tens of per cent, or, in the case of Fe IX, up to a factor of two. Forbidden lines in the near-ultraviolet, visible, or infrared parts of the spectrum are an exception, with smaller intensity changes or even a reverse behaviour with kappa. The most conspicuous example is the Fe X 6378A red line, whose intensity incerases with kappa. This line is a potentially strong indicator of departures from the Maxwellian distribution. We find it possible to perform density diagnostics independently of kappa, with many Fe XI-XIII line ratios showing strong density-sensitivity and negligible sensitivity to kappa and temperature. We also tested different averaging of the collision strengths. It is found that averaging over 0.01 interval in log(E/Ryd) is sufficient to produce accurate distribution-averaged collision strengths at temperatures of the ion formation in ionization equilibrium.

Ab initio MRCI + Q study on potential energy curves and spectroscopic parameters of low-lying electronic states of CS+

Carbon monosulfide molecular ion (CS+), which plays an important role in various research fields, has long been attracting much interest. Because of the unstable and transient nature of CS+, its electronic states have not been well investigated. In this paper, the electronic states of CS+ are studied by employing the internally contracted multireference configuration interaction method, and taking into account relativistic effects (scalar plus spin—orbit coupling). The spin—orbit coupling effects are considered via the state-interacting method with the full Breit—Pauli Hamiltonian. The potential energy curves of 18 Λ—S states correlated with the two lowest dissociation limits of CS+ molecular ion are calculated, and those of 10 lowest Ω states generated from the 6 lowest Λ—S states are also worked out. The spectroscopic constants of the bound states are evaluated, and they are in good agreement with available experimental results and theoretical values. With the aid of analysis of Λ—S composition of Ω states at different bond lengths, the avoided crossing phenomena in the electronic states of CS+ are illuminated. Finally, the single ionization spectra of CS (X1Σ+) populating the CS+(X2Σ+1/2, A2Π3/2, A2Π1/2, and B2Σ+1/2) states are simulated. The vertical ionization potentials for X2Σ+1/2, A2Π3/2, A2Π1/2, and B2Σ+1/2 states are calculated to be 11.257, 12.787, 12.827, and 15.860 eV, respectively, which are accurate compared with previous experimental results, within an error margin of 0.08 eV∼0.2 eV.

Interference effects in single ionization spectra of N2 and O2 molecular targets by proton impact

Single ionization of N2 and O2 molecules by proton impact is analysed. Interference patterns appear in the angular distribution of electrons. For double differential cross sections, fingerprints of coherent electron emission are also observed for the most internal molecular orbitals σg1s and σ*u1s of these molecular targets.

Spin–orbit effects, electronic decay and breakdown phenomena in the photoelectron spectra of iodomethane

In this work we discuss the theoretical photo-double ionization spectrum of iodomethane which was obtained by the newly developed relativistic two-particle propagator and compare it to experimental data. For a correct interpretation of the spectral features in the outer valence region, spin–orbit effects play a major role and induce new features to the spectra. Additionally, wide-energy range single and double ionization spectra of iodomethane were calculated revealing possible electronic decay reactions after high-energy primary ionization. In the low energy region of the single ionization spectrum pronounced breakdown was observed resulting in a highly correlated manifold which was characterized in detail via population analysis.

Theoretical calculations on the NaS and NaS+ radicals: electronic structure, spectroscopy and spin-orbit couplings

We used an ab initio methodology for the computation of the potential energy curves of the lowest electronic states of NaS and NaS+ diatomics. Using these highly correlated wavefunctions, we calculated their spin-orbit couplings. The X2Π and the A2Σ+ electronic states of NaS are found to possess deep potential wells. The upper bound electronic states have either shallow potential wells located in the molecular region or potentials of ∼ 1 eV depth located for large NaS distances. For NaS+, our calculations reveal the existence of shallow potentials for all states correlating to the four lowest dissociation limits. Using our potentials, we calculated an accurate set of spectroscopic constants for NaS and NaS+. Most of the data relative to the electronic excited states represent predictions. The spin-orbit induced predissociation of NaS(A2Σ+ is discussed. Finally, our cationic potentials and the NaS(X2Π) potential were used for the prediction of the single ionization spectrum of NaS.

Gas-Phase Protein Inner-Shell Spectroscopy by Coupling an Ion Trap with a Soft X-ray Beamline.

C, N, and O near-edge ion yield spectroscopy of 8+ selected electrosprayed cations of cytochrome c protein (12 kDa) has been performed by coupling a linear quadrupole ion trap with a soft X-ray beamline. The photoactivation tandem mass spectra were recorded as a function of the photon energy. Photoionization of the precursor, accompanied by CO2 loss, is the dominant relaxation process, showing high photoion stability following direct or resonant photoionization. The partial ion yields extracted from recorded mass spectra show significantly different behaviors for single and double ionization channels, which can be qualitatively explained by different Auger decay mechanisms. However, the single ionization spectra reveal characteristic structures when compared to existing near-edge X-ray absorption fine structure (NEXAFS) spectra from thin films of peptides and proteins. Therefore, the present experiment opens up new avenues for near-edge X-ray spectroscopy of macromolecules in the gas phase, overcoming the radiation damage issue or the environmental effects as due to the surface, intermolecular interactions, and solvent.

Theoretical investigations of the MgO+ cation: spectroscopy, spin–orbit coupling and single ionization spectrum

We used ab initio methodology for the computation of the potential energy curves of the doublet and quartet electronic states of the MgO+ cation correlating with the four lowest dissociation limits. Using these highly correlated wavefunctions, we calculated their spin–orbit coupling and transition moments. We deduced an accurate set of spectroscopic constants for this cation. Most of the data relative to the electronic excited states of this cation are predictive in nature. Moreover, our potentials, transition moments and spin–orbit coupling evolutions are incorporated into Fermi golden rule calculations to deduce the natural lifetimes of the lowest MgO+(12Σ+) ro-vibrational levels. Finally, our potentials were used for the prediction of the single ionization spectrum of MgO.

Lack of evidence for a singlet crystal-field ground state in the magnetic pyrochlore Tb2Ti2O7

We present high-resolution inelastic neutron scattering data on the candidate spin liquid Tb${}_{2}$Ti${}_{2}$O${}_{7}$. We find that there is no evidence for a zero-field splitting of the ground-state doublet within the $0.2$ K resolution of the instrument. This result contrasts with a pair of recent works claiming that the spin-liquid behavior of Tb${}_{2}$Ti${}_{2}$O${}_{7}$ can be attributed to a 2 K split singlet-singlet single-ion spectrum at low energies. We also reconsider the entropy argument presented by Chapuis et al. [Phys. Rev. B 82, 100420 (2010)] as further evidence of a singlet-singlet crystal-field spectrum. We argue that estimates of the low-temperature residual entropy drawn from specific heat measurements are a poor guide to the single-ion spectrum without a proper understanding of the nature of the correlations in the spin-liquid regime.

Theoretical spectroscopy and metastability of BeS and its cation

Multiconfiguration self-consistent field and multiconfiguration reference interaction including the Davidson’s correction techniques were employed to calculate the potential energy curves (PECs) of the BeS/BeS + electronic states correlating to the 4/5 lowest dissociation limits. After nuclear motion treatment, we deduced reliable spectroscopic data for the neutral and cationic bound states. For BeS, the transition moments and spin-orbit couplings were also evaluated and used later with the PECs to deduce the rovibronic transition probabilities and the radiative lifetimes in the low-lying states, and to investigate the unimolecular decomposition processes of BeS (X 1Σ +, A 1Π, 3Σ + and B 1Σ +) leading to Be( 1S g) + S( 3P g). The prominent mechanism is a spin-orbit induced predissociation via the repulsive BeS(1 3Σ −) state. Finally, we give the single ionization spectrum of BeS (X 1Σ +) populating the BeS + (X 2Π, 1 2Σ −, 1 2Σ +, 1 2Δ, 2 2Σ +, 2 2Π and 3 2Π) electronic states. The adiabatic ionisation energy of BeS is estimated to be ∼9.15 eV.

Detection of single-ion spectra by Coulomb-crystal heating

The coupled motion of ions in a radiofrequency trap has been used to connect the frequency- dependent laser-induced heating of a sympathetically cooled spectroscopy ion with changes in the fluorescence of a laser-cooled control ion. This technique, sympathetic heating spectroscopy, is demonstrated using two isotopes of calcium. In the experiment, a few scattered photons from the spectroscopy ion are transformed into a large deviation from the steady-state fluorescence of the control ion. This allows us to detect an optical transition where the number of scattered photons is below our fluorescence detection limit. Possible applications of the technique to molecular ion spectroscopy are briefly discussed.

X-Ray Two-Photon Photoelectron Spectroscopy: A Theoretical Study of Inner-Shell Spectra of the Organic Para-Aminophenol Molecule

The inner-shell single and double ionization spectra of the organic molecule para-aminophenol are calculated using many-body Green's function methods. The inner-shell double ionization spectrum displays more pronounced sensitivity to the chemical environment and to electronic many-body effects than does the inner-shell single ionization spectrum. A kinetic model is employed to determine the probability of inner-shell double hole formation in para-aminophenol exposed to an intense, 1 fs x-ray pulse. The resulting photoelectron spectrum at a photon energy of 1 keV is calculated. This work suggests that x-ray two-photon photoelectron spectroscopy using x-ray free-electron lasers will provide access to electronic-structure information not currently available.

Theoretical study of the spectroscopy and the metastability of the NS+ cation

The potential energy curves of the NS+ electronic states are computed at the aug-cc-pV5Z/CASSCF/MRCI level of theory. Using these highly correlated wavefunctions, we evaluated their mutual spin–orbit coupling terms and the transition moment evolutions. Then, we deduced an accurate set of spectroscopic constants and investigated the spin–orbit induced predissociation of the lowest electronic excited states of this cation. In particular, we identify a new well-bound quintet state, namely the NS+ (15Π). The NS+(15Π) v′+ ⩽ 5 levels decay radiatively to populate the NS+(1 5Σ+) state after emission of visible light, whereas the upper rovibrational levels undergo rapid spin–orbit induced predissociation processes forming S+(4S) and N(4S) fragments via the repulsive 17Σ+ state. The potential energy curve of NS (X2Π) is also computed and used together with those of NS+ electronic states for the prediction of the single ionization spectrum of NS.

Spectroscopy and metastability of BeO+

The potential energy curve of the ground electronic state of BeO and those of the lowest electronic states of the BeO+ cation are computed using the CASSCF/MRCI methods and a large basis set. For the cation, the spin–orbit coupling and the transition momentum integrals are also evaluated. These data are used later to deduce an accurate set of spectroscopic constants and to investigate the spin–orbit-induced predissociation of the lowest electronic excited states of BeO+. Our calculations show that the high-rovibrational levels of the BeO+ (12Σ+) electronic state exhibit rapid predissociation processes forming Be+(2S) + O(3P). Our curves are also used for predicting the single ionization spectrum of BeO.

Narrow linewidth light source for an ultraviolet optical frequency standard

We report an ultra-narrow linewidth light source applicable for a frequency standard in the ultraviolet. The laser is a Nd:YAG laser that emits at 946 nm with 300-mW output power. It is locked to a high-finesse cavity. The minimum Allan deviation is 1.3×10-14 for an integration time of 1 s, which corresponds to a laser linewidth of 2.8 Hz. The cavity drift is measured by a frequency comb and a single-ion spectrum for different time scales. In order to investigate broadening mechanisms due to the fiber transport and doubling systems, the laser light is frequency doubled with two independent systems and compared. The measured minimum beat-note between the two laser fields is less than 1 Hz. By carrying out a high-resolution scan on a trapped single indium ion, we observe a linewidth of 260 Hz on the ion clock transition. Possible reasons for the broadening effects are discussed.

Photoion-Yield Study of thens–mp(n=3, 4, and 5) Autoionizing Resonances in Ca, Sr, and Ba Atoms

An experimental study has been carried out for photoabsorption and subsequent decay of Ca, Sr, and Ba atoms in n s – m p ( m > n , n =3, 4, and 5, respectively) autoionization resonance regions. We used a photoionization mass spectroscopy method with ionic charge state discrimination. Monochromatized synchrotron radiation was used as the light source. For all the atomic species, the first member of the n s -1 m p resonance series was observed to a reasonable extent, whereas the higher lying members are obscure. These n s -1 m p resonances appear as broad and shallow window-type resonances in their double ionization spectra but as peak-type resonances in the single ionization spectra. The results of the n s – m p resonance excitation are discussed on the basis of the MCDF calculation for the resonance states and also on the basis of Fano's profile parameter q , the resonance width Γ, and the resonance energy E r , which are determined by a curve-fitting analysis.