Excited Np Research Articles

Long-range interactions of hydrogen atoms in excited states. III. nS−1S interactions for n≥3

The long-range interaction of excited neutral atoms has a number of interesting and surprising properties, such as the prevalence of long-range, oscillatory tails, and the emergence of numerically large can der Waals C_6 coefficients. Furthermore, the energetically quasi-degenerate nP states require special attention and lead to mathematical subtleties. Here, we analyze the interaction of excited hydrogen atoms in nS states (3 <= n <= 12) with ground-state hydrogen atoms, and find that the C_6 coefficients roughly grow with the fourth power of the principal quantum number, and can reach values in excess of 240,000 (in atomic units) for states with n = 12. The nonretarded van der Waals result is relevant to the distance range R << a_0/alpha, where a_0 is the Bohr radius and alpha is the fine-structure constant. The Casimir-Polder range encompasses the interatomic distance range a_0/alpha << R << hbar c/L, where L is the Lamb shift energy. In this range, the contribution of quasi-degenerate excited nP states remains nonretarded and competes with the 1/R^2 and 1/R^4 tails of the pole terms which are generated by lower-lying mP states with 2 <= m <= n-1, due to virtual resonant emission. The dominant pole terms are also analyzed in the Lamb shift range R >> hbar c/L. The familiar 1/R^7 asymptotics from the usual Casimir-Polder theory is found to be completely irrelevant for the analysis of excited-state interactions. The calculations are carried out to high precision using computer algebra in order to handle a large number of terms in intermediate steps of the calculation, for highly excited states.

Topological spin models in Rydberg lattices

We show that resonant dipole–dipole interactions between Rydberg atoms in a triangular lattice can give rise to artificial magnetic fields for spin excitations. We consider the coherent dipole–dipole coupling between np and ns Rydberg states and derive an effective spin-1/2 Hamiltonian for the np excitations. By breaking time-reversal symmetry via external fields, we engineer complex hopping amplitudes for transitions between two rectangular sub-lattices. The phase of these hopping amplitudes depends on the direction of the hop. This gives rise to a staggered, artificial magnetic field which induces non-trivial topological effects. We calculate the single-particle band structure and investigate its Chern numbers as a function of the lattice parameters and the detuning between the two sub-lattices. We identify extended parameter regimes where the Chern number of the lowest band is C=1 or C=2.

Ketyl Radical Formation via Proton-Coupled Electron Transfer in an Aqueous Solution versus Hydrogen Atom Transfer in Isopropanol after Photoexcitation of Aromatic Carbonyl Compounds.

The excited nπ* and ππ* triplets of two benzophenone (BP) and two anthraquinone (AQ) derivatives have been observed in acetonitrile, isopropanol, and mixed aqueous solutions using time-resolved resonance Raman spectroscopic and nanosecond transient absorption experiments. These experimental results, combined with results from density functional theory calculations, reveal the effects of solvent and substituents on the properties, relative energies, and chemical reactivities of the nπ* and ππ* triplets. The triplet nπ* configuration was found to act as the reactive species for a subsequent hydrogen atom transfer reaction to produce a ketyl radical intermediate in the isopropanol solvent, while the triplet ππ* undergoes a proton-coupled electron transfer (PCET) in aqueous solutions to produce a ketyl radical intermediate. This PCET reaction, which occurs via a concerted proton transfer (to the excited carbonyl group) and electron transfer (to the excited phenyl ring), can account for the experimental observation by several different research groups over the past 40 years of the formation of ketyl radicals after photolysis of a number of BP and AQ derivatives in aqueous solutions, although water is considered to be a relatively "inert" hydrogen-donor solvent.

Auger resonant-Raman study at the ArKedge as probe of electronic-state-lifetime interferences

The resonant-Auger decay in argon following Ar 1s - np excitations was studied under resonant-Raman conditions, i.e., with a total instrumental bandwidth much narrower than the natural linewidth of ...

Excitons in motion: universal dependence of the magnetic moment on kinetic energy

AbstractWe have observed remarkable changes in the magnetic properties of excitons as they acquire kinetic energy. In particular, the Zeeman splittings and diamagnetic shifts of excitonic transitions when magnetic fields are applied along the growth direction of (001) wide quantum wells of CdTe, ZnSe, ZnTe and GaAs are found to to have a strong dependence on the translational wavevector Kz. The behaviour of the Zee‐man splittings corresponds to enhancement of the magnetic moments of the excitons. This enhancement is particularly marked when their translational kinetic energy becomes comparable with the exciton Rydberg and can be described by what appears to be a universal function of Kz. A model for the behaviour is outlined which involves motionally‐induced mixing between the 1S hydrogenic exciton ground state and excited nP states. The observations imply that there are significant changes in the structure of the exciton as its translational kinetic energy increases. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Study of the 2p →np (n = 3–6) excitation processes accompanying 1s photoionization of Ne

We measured the relative intensities of satellite lines in the 1s core-level photoemission spectra of Ne, as a function of excitation energy, using high-brilliant soft x-ray undulator radiation. Measured excitation functions for these satellite lines were merged with the previously reported results and discussed in terms of the intra-atomic electron–electron correlations. Some results have suggested that the internal electron scattering might play an important role in characterizing the near-threshold behaviour for the 2p →np excitation process accompanying 1s photoionization. The observed intensity ratios of the satellite lines relative to the main line were analysed with the aid of the analytic Thomas model. The observed near-threshold behaviours were also compared with the simple model calculations which take account of both the sudden creation of an inner-shell vacancy and the Coulomb interaction between the ionized electron and atomic electrons. The comparison has implied that both the amplitude and the shape of the measured excitation functions cannot be satisfactorily explained by the simple model calculations. This fact may call on an improvement in the theoretical treatment in such simple model calculations.

Different mechanisms both lead to the production of the naphthalene–OH adduct in the 355 nm and 266 nm laser flash photolysis of the mixed aqueous solution of naphthalene and nitrous acid

The 355 nm and 266 nm nanosecond laser flash photolysis of the mixed aqueous solution of naphthalene (Np) and nitrous acid (HNO 2) were performed to investigate the photo-initiated microscopic reactions in this binary component system. Following the 355 nm laser flash, HNO 2 was readily photolyzed to NO and OH, and the latter fragment added to the Np ring to form the Np–OH adduct at the rate constant of (1.1 ± 0.1) × 10 10 M −1 s −1. Following the 266 nm laser flash, Np was populated first to its lowest excited singlet state (S 1) and then to its lowest excited triplet state (T 1) via intersystem crossing. Np(S 1) and Np(T 1) both formed the [Np···HNO 2] * exciplex with HNO 2, and the fast dissociation of this exciplex again produced the Np–OH adduct. The second-order quenching rate constants of Np(S 1) and Np(T 1) by HNO 2 were measured to be (6.0 ± 0.2) × 10 9 M −1 s −1 and (4.8 ± 0.2) × 10 9 M −1 s −1, respectively. It was very interesting to conclude that although both the 355 nm and the 266 nm irradiation of the mixed aqueous solution of Np and HNO 2 produced the Np–OH adduct, the mechanisms leading to this species varied essentially for the two cases. This work suggested an unexpected way of releasing OH by HNO 2 other than its direct photolysis, namely via forming exciplex with the excited Np (and possibly other polycyclic aromatic hydrocarbons, PAHs) in the environment.

Electron-Impact Cross Sections for Ground State to np Excitations of Sodium and Potassium.

Cross sections for electron impact excitation of atoms are important for modeling of low temperature plasmas and gases. While there are many experimental and theoretical results for excitation to the first excited states, little information is available for excitation to higher states. We present here calculations of excitations from the ground state to the np levels of sodium (n = 3 through 11) and potassium (n = 4 through 12). We also present a calculation for a transition from the excited sodium level 3p to 3d to show the generality of the method. Scaling formulas developed earlier by Kim [Phys. Rev. A 64, 032713 (2001)] for plane-wave Born cross sections are used. These formulas have been shown to be remarkably accurate yet simple to use. We have used a core polarization potential in a Dirac-Fock wave function code to calculate target atom wave functions and a matching form of the dipole transition operator to calculate oscillator strengths and Born cross sections. The scaled Born results here for excitation to the first excited levels are in very good agreement with experimental and other theoretical data, and the results for excitation to the next few levels are in satisfactory agreement with the limited data available. The present results for excitation to the higher levels are believed to be the only data available.

Cascade effects in the excitation ofnp5(n+1)pstates of krypton and xenon atoms by polarized electrons

The integrated Stokes parameters of the decay photons from the excited np{sup 5}(n+1)p[5/2]{sub 3}, np{sup 5}(n+1)p{sup '}[3/2]{sub 2}, and np{sup 5}(n+1)p{sup '}[3/2]{sub 1} (n=5 and 6) states of Kr and Xe, respectively, are measured from near threshold to 100 eV using incident polarized electrons. The effects of the cascade photons on the Stokes parameters have been investigated using a pulsed beam of polarized electrons. The cascade effects for the final excited states are more significant for the linear polarization P{sub 1} parameter than for the circular polarization P{sub 3} parameter and they do not always have depolarization effects. The integrated state multipoles, determined from the integrated Stokes parameters corrected for cascade effects, are closer to, but not in agreement with, the values from semirelativistic R matrix and relativistic distorted-wave approximation methods.

New Window Resonances in the Potassium 3sPhotoabsorption Spectrum

The photoion spectrum of atomic potassium was measured over the 3s -> np excitation region with the photoion time-of-flight method and monochromatized synchrotron radiation. An unusual spectrum with paired windows structure was found instead of a simple regular Rydberg series. Such subsidiary windows have not been observed in the 3s -> np resonances of Ar, which has a closed outer shell. Based on Dirac-Fock calculations, the dual window structure at 36.7 eV and at 37.4 eV was assigned to the 3s^{-1}3p^{6}4s4p resonance. The line shape can be fitted by Fano's formula and the Fano parameters were obtained.

Photoprocesses in 2,2-diphenyl-5,6-benzo(2 H)chromene

The photocolouration of 2,2-diphenyl-5,6-benzo(2 H)chromene (Chr), upon irradiation of the ring-closed form (Np), and the subsequent relaxation kinetics of one merocyanine isomer into Np were studied in several solvents at different temperatures. Deactivation of the excited Np form leads via the singlet manifold predominantly to the ring-opened cis– trans ( ct) isomer which converts thermally into Np. The activation energy, E ct→ Np ≈77 kJ mol −1 , and the pre-exponential factor, A≈10 12 s −1, are essentially independent of the solvent polarity. The trans– trans ( tt) isomer, which is suggested to be formed photochemically in a competing step, is thermally more stable. The triplet pathway of photocolouration can be induced by energy transfer using several sensitizers with a triplet energy of >200 kJ mol −1. The thermal relaxation time after either direct or naphthalene-sensitized colouration is the same. The mechanisms of ring opening and ring closure are discussed.

Vibronic spectroscopy of jet-cooled indazole: S1↔S spectra and mode assignments

The S0↔S1 electronic transition of 1H-indazole has been investigated by laser-induced fluorescence (LIF) excitation and dispersed fluorescence (DF) spectroscopies in a supersonic free jet expansion. Ground and excited state vibrational modes were assigned based on the observed band frequencies and intensities, ab-initio frequency calculations, and available infrared absorption data. Unexpected “symmetry forbidden” transitions were observed in the DF, which, according to our assignments, would originate from the 271 excited state in-plane fundamental to several ground state out-of-plane fundamentals. Accepting the assignments, this observation would indicate a vibronic coupling between the S1 (or S0) state (A′) and an excited nπ*-type electronic state of A″ symmetry.

Valence p functions for alkali and alkaline-earth atoms

Contracted Gaussian-type function (CGTF) basis sets are reported for valence p orbitals of the six alkali and alkaline-earth atoms Li, Be, Na, Mg, K, and Ca for molecular applications. These sets are constructed by Roothaan–Hartree–Fock calculations for the ns → np excited states of atoms, in which both linear and nonlinear parameters of CGTFs are variationally optimized. The present CGTF sets reproduce well the numerical Hartree–Fock ns → np excitation energies: the largest error is 0.0009 hartrees for Li. New CGTFs are tested with diatomic Li2, Na2, K2, and MH molecules, where M = Li, Be, Na, Mg, K, and Ca, by self-consistent-field (SCF) and multiconfiguration SCF calculations. The resultant spectroscopic constants compare well with those of more elaborate calculations and are sufficiently close to experimental values, supporting the efficiency of the present set for the valence p orbitals.

Phosphorescence Spectra of Two Rotational Isomers of m-Fluorobenzaldehyde

Abstract Phosphorescence spectra of m-fluorobenzaldehyde and its aldehyde deuterated derivative have been observed in the vapor phase at room temperature and in a methylcyclohexane matrix at 4.2 K. Paired band systems were detected for the vapor phase and for each of the two different crystallographic phases, α- and β-phases, and were attributed to two rotational isomers of the aldehyde group. Variable intensity changes between the two paired bands observed in the α-phase crystal are manifestations of the isomeric conversion in the ground state during crystallizations. An energy correlation of the rotational isomers between the lowest excited nπ* singlet and triplet states is given for the vapor phase and for the α-phase methylcyclohexane matrix.

Polarization propagator study of electronic excitation in key heterocyclic molecules I. Pyrrole

The electronic excitation spectrum of pyrrole is studied using a polarization propagator method referred to as the second-order algebraic-diagrammatic construction (ADC(2)), along with a simple model for vibrational excitation accounting for all totally symmetric modes. The method describes the optical absorption profile of pyrrole with an expected accuracy of 0.2 – 0.4 eV for the vertical excitation energies. The vibrational analysis provides for detailed additional spectroscopic information. In the singlet spectrum, besides the ns, np and nd ( n = 3,4) Rydberg excitations, three π-π ∗ valence transitions, V′( 1A 1), V( 1B 2) and V( 1A 1) can clearly be distinguished. No evidence is found for Rydberg-valence interaction near the equilibrium geometry. Substantial vibrational widths and distinct vibrational excitation patterns are predicted for the Rydberg series converging to the first and second ionization thresholds. Some new assignments of major spectral features are proposed. The long-wave absorption maximum in the 5.6 – 6.6. eV region is explained exclusively by the presence of Rydberg transitions, while the most intense absorption in the short-wave band system (7.0 – 8.3 ev) predominantly originates from the V( 1B 2) and V( 1A 1) valence transitions.

Triplet np final state interactions at large momentum transfers

Using the simple relation between the np scattering and bound state wave functions, the spin-triplet contributions to the pp → π +( pn) and backward dp → p( pn) reactions are estimated for low np excitation energies. The good agreement with the pion production data at 400 and 450 MeV over a range of angles shows that spin-singlet final states cannot be more than of the order of 10%. On the other hand it is seen that the spin-singlet states must provide about 30% of the strength in pion production at 1 GeV as well as deuteron-proton backward scattering at 1.6 GeV.

Photochemistry and photoinitiator properties of 2-substituted anthraquinones: 2. Photopolymerization and flash photolysis

The photoinduced polymerization activities of fifteen 2-substituted anthraquinones have been determined in different monomers and prepolymers using real-time Fourier-transform infra-red spectroscopy and photocalorimetry. The relative order in photoinitiator efficiency is found to be highly dependent upon the method used, the nature of the light source, amine co-synergist and monomer being crucial factors. Oxygen quenching in all cases indicates that the triplet state is the active precursor. Without an amine co-synergist, absolute quantum-yield measurements show that anthraquinones with electron-withdrawing groups are more active than those which are electron-donating. However, in the presence of a tertiary amine and under polychromatic illumination, the effects are different. In the latter case amido derivatives with long-wavelength charge-transfer bands and mixed nπ ∗ ππ ∗ triplet states tend to be more reactive. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the triplet state. The relative positions of the lowest excited singlet ππ ∗ and second excited nπ ∗ triplet states play an important role in determining their photoactivities, as shown in paper 1. Semiquinone radical and radical-anion intermediates are observed using microsecond flash photolysis and the data interrelated to their spectroscopic and photopolymerization activities. The haloanthraquinones are shown to undergo an additional mechanism involving dehalogenation.

A close look at Ar photoion spectra around the K edge: non-diagram transitions and double photoionization

Relative abundances of the various ionic charges resulting from photoionization of argon by monochromatized synchrotron radiation at energies between 3150 and 4900 eV have been measured with low statistical error. Despite the fact that already, right at the onset of K excitation (3203.5 eV), almost the full 1s binding energy is deposited in the atom, the relative abundances change smoothly over the region of np excitation up to 6 eV above the limit. While this behaviour, caused by Rydberg shake-off (about 0.5 probability at the 1s to 4p resonance) and by recapture of the photoelectron via post-collision interaction, appears to be understood there is no explanation for a gradual change of relative abundances extending over tens of eV below the lowest K excitation. K+M and K+L single photon double excitations both show up as a smooth rise of the average ionic charge q from the respective threshold to a plateau at considerably higher energy, the overall increase of q being 0.206+or-0.02 and 0.13+or-0.01.

Experimental study of Rydberg states excited from the d shell of atomic bromine and iodine.

Iodine and bromine atoms produced by the laser-induced dissociation of, respectively, molecular iodine and bromine have been photoexcited in the region of the 4d\ensuremath{\rightarrow}np (ng5) and 3d\ensuremath{\rightarrow}np (ng4) excitations. Total ion-yield spectra show numerous lines whose positions and extensions are attributed to open-shell effects. The decay of the 4d\ensuremath{\rightarrow}6p transition in atomic iodine has been investigated. The relaxation of this resonance is dominated by resonant Auger processes accompanied by strong shake processes. The comparison with the decay of the 4d\ensuremath{\rightarrow}5p resonances in atomic iodine and with the 4d\ensuremath{\rightarrow}6p resonances in xenon shows a strong sensitivity of the decay mechanisms of inner-shell resonances to the nature of the orbital into which the core electron has been promoted.

Existence of the second excited triplet state of 1,8-dibromoanthraquinone observed by picosecond laser photolysis in solutions at room temperature

Picosecond laser photolysis of 1,8-dibromoanthraquinone in solutions at room temperature has revealed the existence of a new absorption band which is interpreted to be the absorption of the second excited nπ* triplet [T2(nπ*)] state with a localized charge-transfer character between the bromine and oxygen atoms. The inter-system crossing (ISC) time from the lowest excited nπ* singlet [S1(nπ*)] state to the T2(nπ*) state is ca. 15–20 ps and the internal conversion (IC) time from the T2(nπ*) state to the lowest excited ππ* triplet [T1(ππ*)] state is ca. 70–110 ps. In comparison with the previous result for 1,8-dichloroanthraquinone, it is proposed that the S1(nπ*)→ T2(nπ*) ISC time is affected not only by the internal heavy-atom effect of halogen atom but also by the conformational distortion of molecules, and that both the T2–T1 energy gap and the conformational distortion of molecules affect the T2(nπ*)→ T1(ππ*) IC time.