Ground State Of Ion Research Articles

Twisted intramolecular charge transfer and its contribution to the NLO activity of Diglycine Picrate: A vibrational spectroscopic study

Single crystals of Diglycine Picrate (DGLP) were grown by slow evaporation technique and the vibrational spectral analysis is carried out using FT Raman and FT-IR spectroscopy, supported by Density Functional Theoretical (DFT) computations to derive equilibrium geometry, vibrational wavenumbers and first hyperpolarizability. The vibrational spectra confirm the existence of NH3+ in DGLP. The influence of Twisted Intramolecular Charge Transfer (TICT) caused by the strong ionic ground state hydrogen bonding between charged species making DGLP crystal to have the non-centrosymmetric structure has been discussed. The Natural Bond Orbital (NBO) analysis confirms the occurrence of strong intermolecular NH⋯O hydrogen bond. The HOMO–LUMO energy gap and the first order hyperpolarizability were calculated and it supports the nonlinear optical activity of the Diglycine Picrate crystal.

Entanglement Entropies in the Ground States of Helium-Like Atoms

We examine the entanglement in the ground states of helium and helium-like ions using an original Hylleraas expansion. The von Neumann and linear entropies of the reduced density matrix are accurately computed by performing the Schmidt decomposition of the S singlet spatial wavefunctions. The results presented are more accurate than currently available in published literature.

Electronic state spectroscopy of diiodomethane (CH₂I₂): experimental and computational studies in the 30,000-95,000 cm⁻¹ region.

The electronic absorption spectrum of diiodomethane in the 30,000-95,000 cm(-1) region is investigated using synchrotron radiation; the spectrum in the 50,000-66,500 cm(-1) region is reported for the first time. The absorption bands in the 30,000-50,000 cm(-1) region are attributed to valence transitions, while the vacuum ultraviolet (VUV) spectrum (50,000-95,000 cm(-1)) is dominated by several Rydberg series converging to the first four ionization potentials of CH2I2 at 9.46, 9.76, 10.21, and 10.56 eV corresponding to the removal of an electron from the outermost 3b2, 2b1, 1a2, and 4a1 non-bonding orbitals, respectively. Rydberg series of ns, np, and nd type converging to each of the four ionization potentials are assigned based on a quantum defect analysis. Time dependent density functional theory calculations of excited states support the analysis and help in interpretation of the Rydberg and valence nature of observed transitions. Density functional theory calculations of the neutral and ionic ground state geometries and vibrational frequencies are used to assign the observed vibronic structure. Vibronic features accompanying the Rydberg series are mainly due to excitation of the C-I symmetric stretch (ν3) and CH2 wag (ν8) modes, with smaller contributions from the C-H symmetric stretch (ν1). UV absorption bands are assigned to low lying valence states 1(1)B2, 1(1)B1, 2(1)A1, 3(1)A1, 2(1)B1, and 2(1)B2 and the unusually high underlying intensity in parts of the VUV spectrum is attributed to valence states with high oscillator strength. This is the first report of a comprehensive Rydberg series and vibronic analysis of the VUV absorption spectrum of CH2I2 in the 50,000-85,000 cm(-1) region. The VUV absorption spectrum of CD2I2 which serves to verify and consolidate spectral assignments is also reported here for the first time.

Relativistic ground state of a hydrogenlike molecular ion

In this preliminary Brief Report we explore a method for examining relativistic effects in one-electron molecules that avoids the evaluation of multicenter integrals. Here the Dirac equation for a simplified version of the ${{\mathrm{H}}_{2}}^{+}$ ion is solved exactly for the ground-state Schr\"odinger and Dirac energy levels. The model consists of representing the electron-proton Coulomb interaction as a projection operator onto a linear subspace of Hilbert space.

Radiative capture of twisted electrons by bare ions

Recent advances in the production of twisted electron beams with a subnanometer spot size offer unique opportunities to explore the role of orbital angular momentum in basic atomic processes. In the present work, we address one of these processes: radiative recombination of twisted electrons with bare ions. On the basis of the density matrix formalism and the non-relativistic Schrödinger theory, analytical expressions are derived for the angular distribution and the linear polarization of photons emitted due to the capture of twisted electrons into the ground state of (hydrogen-like) ions. We show that these angular and polarization distributions are sensitive to both the transverse momentum and the topological charge of the electron beam. To observe in particular the value of this charge, we propose an experiment that makes use of the coherent superposition of two twisted beams.

Electronic states of tetrahydrofurfuryl alcohol (THFA) as studied by VUV spectroscopy and ab initio calculations.

The electronic spectroscopy of isolated tetrahydrofurfuryl alcohol (THFA) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 5.0-10.8 eV energy-range, with absolute cross-section measurements derived. The He(I) photoelectron spectrum was also collected to quantify ionization energies in the 9-16 eV spectral region. These experiments are supported by the first high-level ab initio calculations performed on the excited states of the neutral molecule and on the ground and excited state of the positive ion. The good agreement between the theoretical results and the measurements allows us to quantify for the first time the electronic-state spectroscopy of THFA. The present work also considers the question of the lowest energy conformers of the molecule and its population distribution at room temperature.

Direct and indirect processes in electron-impact ionization of 1s2 1S0 ground-state and 1s2s 3S1 metastable He-like N5+ ions

Absolute cross sections for electron-impact ionization of two-electron N5+ ions in the 1s2 1S0 ground state and in the 1s2s 3S1 metastable state have been measured. Spectroscopic-quality electron-energy scans were obtained revealing the details of indirect ionization mechanisms which contribute to net single ionization of the metastable, K-shell excited parent N5+(1s2s) ion.

Low-temperature specific heat of uranium germanides

We report measurements of the specific heat down to the lowest temperature of 2K for the paramagnetic binaries U5Ge4 (Ti5Ga4-type) and UGe (ThIn-type) as well as for the ferromagnetic binaries U3Ge5−x (x=0.2) and UGe2−x (x=0.3) (with TC=94 and 47K) having defect crystal structures of the AlB2- and ThSi2-type, respectively. The obtained data were compared to those of other uranium germanides which have been earlier studied: UGe2 (ZrGa2) and UGe3 (Cu3Au). Among all these germanides, only UGe exhibits enhanced electronic specific heat coefficient, γ(0), equal to 137mJ/molUK2. This value can be compared to that derived for the most known spin fluctuator, UAl2 (143mJ/molUK2). The other uranium germanides have less enhanced γ(0) values (27–65mJ/molUK2). The lowest value of about 20mJ/molUK2 was reported earlier for the typical temperature independent paramagnet UGe3. For the ferromagnetic new phase UGe2−x the inferred magnetic entropy, Sm, reaches at the Curie temperature, TC, a value of Rln2 which corresponds to a doublet ground state of the uranium ion in this deficit digermanide.

Ionization cross sections and rate coefficients for the ground state of AsI-like ions

Detailed level-by-level calculations of cross sections and rate coefficients for electron impact direct ionization and excitation-autoionization of AsI-like ions in the 3d 104s 24p 3 (4S3/2) ground state have been performed. The cross sections are presented in the energy range near the threshold for the five ions Mo9+, Xe21+, Pr26+, Dy33+ and W41+. The rate coefficients are given for all the ions from Sr5+ to U59+ in the AsI sequence at the seven electron temperatures (k T e = 0.1E I ,0.3E I ,0.5E I ,0.7E I ,E I ,2E I and 10E I , where E I is the first ionization energy). The calculations include the contribution of direct ionization (DI) calculated using the Lotz formula approximation and the contributions of excitation-autoionization (EA) computed in the framework of the Distorted Wave (DW) approximation for the 4s − nl, 3d − nl and 3p − nl resonant inner-shell excitations. The ionization enhancement due to the EA channels is presented as a function of Z along the AsI isoelectronic sequence. The present results show the great importance of the EA processes; an ionization enhancement factor of up to 6.5 is predicted for instance for Dy33+ (Z = 66) at electron temperature of coronal equilibrium maximum abundance.

Theoretical study of the HeN2+ 2 dication

The structure and stability of the helio nitrogen molecular dication, HeN2+ 2, is investigated by means of standard quantum chemical methods based on both single-reference and multi-reference formalisms. Sequences of correlation consistent basis sets are employed to establish convergence with respect to the size and the quality of the single-particle basis set. MRCI and CASPT2 calculations are reported for the HeN2+ 2 ground-state ion which is found to be metastable. The barrier for the transition state of the reaction HeN2+ 2→ N+ …NHe+ → N+ + NHe+ is calculated to be 59.5 kcals mol−1. The structure of the transition state NHeN2+ was also determined from calculations using the MRCI and CASPT2 methods.

Nuclear-size self-energy and vacuum-polarization corrections to the bound-electron g factor

The finite nuclear-size effect on the leading bound-electron g factor and the one-loop QED corrections to the bound-electron g factor is investigated for the ground state of hydrogen-like ions. The calculation is performed to all orders in the nuclear binding strength parameter Zα (where Z is the nuclear charge and α is the fine structure constant) and for the Fermi model of the nuclear charge distribution. In the result, theoretical predictions for the isotope shift of the 1s bound-electron g factor are obtained, which can be used for the determination of the difference of nuclear charge radii from experimental values of the bound-electron g factors for different isotopes.

Multichannel contributions in the nonsequential double ionization of CO2

The intermediate molecular states in the double ionization of molecules by laser-driven electron recollision are encoded into the angular-dependent signature of the process. We identify through this signature the specific ionic states participating in the double-ionization mechanism. Using CO${}_{2}$ as an example we employ a 1350-nm strong field at $2\ifmmode\times\else\texttimes\fi{}{10}^{14}$ W cm${}^{\ensuremath{-}2}$ to ensure that nonsequential double ionization (NSDI) is the exclusive mechanism in CO${}_{2}^{2+}$ formation. Under these conditions we obtain two main results. First, the shape of the recolliding electron wave packet is inferred from measurements of the ellipticity dependence of the signal, and second, the inelastic recollision cross section in the molecular frame is extracted from the variation of the signal as a function of the angle between strong-field probe polarization and molecular axis. These results provide an angularly resolved electron-ion collision measurement in CO${}_{2}$ and demonstrate contributions to NSDI from both ground and first excited ionic states.

The stereodynamics of the Penning ionization of water by metastable neon atoms

The stereodynamics of the Penning ionization of water molecules by collision with metastable neon atoms, occurring in the thermal energy range, is of great relevance for the understanding of fundamental aspects of the physical chemistry of water. This process has been studied by analyzing the energy spectrum of the emitted electrons previously obtained in our laboratory in a crossed beam experiment [B. G. Brunetti, P. Candori, D. Cappelletti, S. Falcinelli, F. Pirani, D. Stranges, and F. Vecchiocattivi, Chem. Phys. Lett. 539-540, 19 (2012)]. For the spectrum analysis, a novel semiclassical method is proposed, that assumes ionization events as mostly occurring in the vicinities of the collision turning points. The potential energy driving the system in the relevant configurations of the entrance and exit channels, used in the spectrum simulation, has been formulated by the use of a semiempirical method. The analysis puts clearly in evidence how different approaches of the metastable atom to the water molecule lead to ions in different electronic states. In particular, it provides the angular acceptance cones where the selectivity of the process leading to the specific formation of each one of the two energetically possible ionic product states of H2O(+) emerges. It is shown how the ground state ion is formed when neon metastable atoms approach water mainly perpendicularly to the molecular plane, while the first excited electronic state is formed when the approach occurs preferentially along the C2v axis, on the oxygen side. An explanation is proposed for the observed vibrational excitation of the product ions.

Non linear optical analyses of hexamine: Phenol cocrystals based on hydrogen bonding: A comparative study

Fourier-Transform Infrared (FT-IR) spectroscopy supported by Raman spectroscopy has been employed to explain the conventional and unconventional hydrogen bonding present in the 4,4′thiodiphenol: hexamine and 4,4′sulfonyldiphenol: hexamine cocrystals. The possible internal and external vibrational modes are predicted through factor group analysis. Influence of intra molecular charge transfer (ICT) interaction caused by the strong ionic ground state hydrogen bonding between charged species, giving rise to a non centro symmetric structure which is a criterion for second harmonic generation (SHG) efficiency has been discussed. Intense low wave number hydrogen bond vibrations in Raman which arise due to electron–phonon coupling are analyzed. Optical quality of adducts is identified through UV–Vis analysis. The second harmonic generation efficiency of both adducts is determined by Kurtz–Perry method.

Vibronic valence and Rydberg transitions in geminal chloro-fluoro-ethene (1,1-C2H2FCl): a spectroscopic and quantum chemical investigation

The vacuum ultraviolet photoabsorption spectrum of 1,1-C2H2FCl has been examined in detail between 5 and 15 eV photon energy by using synchrotron radiation dispersed by three different monochromators. Quantum chemical calculations are performed to help in the analysis of the valence/Rydberg transition region centred at 7.05 eV including the 3a″(π)→π* and the 3a″(π)→3s Rydberg transitions. Interactions between states involving transitions to the 3s, 4d and σ* orbitals are identified. A vibrational analysis is proposed for the structures belonging to these transitions. For the π(3a″)→π* transition, one vibrational progression is observed with ω3 = 1410 ± 50 cm−1 and its lowest excitation energy is determined at about 6.398 ± 0.003 eV. The π(3a″)→3s Rydberg transition is characterised by a single progression with ω3 = 1410 ± 80 cm−1 likely starting at about 6.45 eV. These vibrations are ascribed to the C=C stretching motion. The abundant structure observed in the spectrum between 7.8 and 10.5 eV has been analysed in terms of vibronic transitions to ns (δ = 0.97), np (δ = 0.63 and 0.40) and nd (δ = 0.13 and −0.11) Rydberg states, which belong to series converging to the 1,1-C2H2FCl+() ionic ground state. The analysis of the vibrational structure of the individual Rydberg states has been attempted leading to average values of the wave numbers ω3 = 1420 ± 20 cm−1, ω7 = 720 ± 50 cm−1 and ω9 = 390 ± 50 cm−1. Between 10.5 and 12.5 eV, nine other Rydberg states converging to the 1,1-C2H2FCl+() first excited state were analysed by the same way. The vibrational structure of these Rydberg states results from the excitation of one vibrational normal mode ν7 with an average value of ω7 = 520 ± 20 cm−1, which is assigned to the C–Cl stretching vibration as inferred from quantum chemical calculations.

On the large interelectronic distance behavior of the correlation factor for explicitly correlated wave functions

In currently most popular explicitly correlated electronic structure theories, the dependence of the wave function on the interelectronic distance rij is built via the correlation factor f(r(ij)). While the short-distance behavior of this factor is well understood, little is known about the form of f(r(ij)) at large r(ij). In this work, we investigate the optimal form of f(r12) on the example of the helium atom and helium-like ions and several well-motivated models of the wave function. Using the Rayleigh-Ritz variational principle, we derive a differential equation for f(r12) and solve it using numerical propagation or analytic asymptotic expansion techniques. We found that for every model under consideration, f(r12) behaves at large r(ij) as r12(ρ)e(Br12) and obtained simple analytic expressions for the system dependent values of ρ and B. For the ground state of the helium-like ions, the value of B is positive, so that f(r12) diverges as r12 tends to infinity. The numerical propagation confirms this result. When the Hartree-Fock orbitals, multiplied by the correlation factor, are expanded in terms of Slater functions r(n)e(-βr), n = 0,...,N, the numerical propagation reveals a minimum in f(r12) with depth increasing with N. For the lowest triplet state, B is negative. Employing our analytical findings, we propose a new "range-separated" form of the correlation factor with the short- and long-range r12 regimes approximated by appropriate asymptotic formulas connected by a switching function. Exemplary calculations show that this new form of f(r12) performs somewhat better than the correlation factors used thus far in the standard R12 or F12 theories.

Tuning the emission colour by manipulating terbium-terbium interactions: Terbium doped aluminum nitride as an example system

Terbium-terbium interactions in terbium doped semiconductors and insulators may lead to the so-called cross-relaxation process, which increases the D54 (green) emission of the terbium ions at the cost of the D53 (blue) luminescence intensity. This effect can generally be reduced by increasing the distance between an excited ion and the nearest ion in the ground state. A straightforward measure is to use a specimen with a decreased terbium concentration. The alternative is to increase the intensity of the excitation (either by photons or electrons) and thereby to reduce the population of terbium ions in the ground state. This paper works this process out with the example of AlN:Tb on the basis of a model and respective experimental results. As will be seen, stronger excitation causes in essence more Tb ions to be excited, thus less ions in the ground state which increases the distance between an excited and the nearest ground state ions. This hinders energy transfer between the terbium ions and thus counteracts the cross-relaxation process. The advantage of changing the excitation intensity lies in the possibility to deliberately shift the apparent colour of the Tb luminescence from a single specimen between green and blue.

Coulomb explosion of diatomic molecules in intense XUV fields mapped by partial covariance

Single-shot time-of-flight spectra for Coulomb explosion of N2 and I2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N4+–N5+ channel for nitrogen and up to the I8+–I9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers.

Rotamers of 3,4-difluorophenol studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

Abstract We applied the resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibrational spectra of 3,4-difluorophenol in the first electronically excited and the ionic ground states. The band origins of the S1 ← S0 electronic transition of the cis and trans rotamers are found to be 35,486 ± 2 and 35,704 ± 2 cm−1 and the adiabatic ionization energies are 70,016 ± 5 and 70,203 ± 5 cm−1, respectively. The distinct spectral features mainly result from the in-plane ring deformation and substituent-sensitive bending vibrations. Spectral analysis suggests that the molecular geometry and vibrational coordinates of the cation in the D0 state resemble those of the neutral species in the S1 state for both cis and trans rotamers.

Rydberg states of chloroform studied by VUV photoabsorption spectroscopy

The VUV photoabsorption spectra of CHCl3 and CDCl3 in the energy region 6.2–11.8eV (50,000–95,000cm−1) have been investigated using synchrotron radiation from the Indus-1 source. Rydberg series converging to the first four ionization limits at 11.48, 11.91, 12.01 and 12.85eV corresponding to excitation from the 1a2, 4a1, 4e, 3e, orbitals of CHCl3 respectively are identified and analyzed. Quantum defect values are observed to be consistent with excitation from the chlorine lone pair orbitals. Vibrational progressions observed in the region of 72,500–76,500cm−1 have been reassigned to ν3 and combination modes of ν3+ν6 belonging to the 1a2→4p transition in contrast to earlier studies where they were assigned to a ν3 progression superimposed on the 3e→4p Rydberg transition. The assignments are further confirmed based on isotopic substitution studies on CDCl3 whose VUV photoabsorption spectrum is reported here for the first time. The frequencies of the ν3 and ν6 modes in the 4p Rydberg state of CHCl3 (CDCl3) are proposed to be ~454 (409)cm−1 and~130 (129)cm−1 respectively based on the vibronic analysis. DFT calculations of neutral and ionic ground state vibrational frequencies support the vibronic analysis. Experimental spectrum is found to be in good agreement with that predicted by TDDFT calculations. This work presents a consolidated analysis of the VUV photoabsorption spectrum of chloroform.