High-lying States Research Articles

Spontaneous excitation of an accelerated hydrogen atom coupled with electromagnetic vacuum fluctuations

We consider a multilevel hydrogen atom in interaction with the quantum electromagnetic field and separately calculate the contributions of the vacuum fluctuation and radiation reaction to the rate of change of the mean atomic energy of the atom for uniform acceleration. It is found that the acceleration disturbs the vacuum fluctuations in such a way that the delicate balance between the contributions of vacuum fluctuation and radiation reaction that exists for inertial atoms is broken, so that the transitions to higher-lying states from ground state are possible even in vacuum. In contrast to the case of an atom interacting with a scalar field, the contributions of both electromagnetic vacuum fluctuations and radiation reaction to the spontaneous emission rate are affected by the acceleration, and furthermore the contribution of the vacuum fluctuations contains a nonthermal acceleration-dependent correction, which is possibly observable.

Excited-State Dynamics of Carotenoids in Light-Harvesting Complexes. 1. Exploring the Relationship between the S1 and S* States

Dispersed transient absorption spectra collected at variable excitation intensities in combination with time-resolved signals were used to explore the underlying connectivity of the electronic excited-state manifold of the carotenoid rhodopin glucoside in the light-harvesting 2 complex isolated from Rhodopseudomonas acidophila. We find that the S state, which was recently identified as an excited state in carotenoids bound in bacterial light-harvesting complexes, exhibits a different response to the increase of excitation intensity than the S(1) state, which suggests that the models used so far to describe the excited states of carotenoids are incomplete. We propose two new models that can describe both the time-resolved and the intensity-dependent data; the first postulates that S(1) and S* are not populated in parallel after the decay of the initially excited S(2) state but instead result from the excitation of distinct ground-state subpopulations. The second model introduces a resonantly enhanced light-induced transition during excitation, which promotes population to higher-lying excited states that favors the formation of S* over S(1). Multiwavelength target analysis of the time-resolved and excitation-intensity dependence measurements were used to characterize the involved states and their responses. We show that both proposed models adequately fit the measured data, although it is not possible to determine which model is most apt. The physical origins and implications of both models are explored.

Experimental and quantum-chemical studies on the three-particle fragmentation of neutral triatomic hydrogen

Dissociation of well-defined H{sub 3} Rydberg states into three ground state hydrogen atoms reveals characteristic correlation patterns in the center-of-mass motion of the three fragments. We present an extensive experimental dataset of momentum correlation maps for all lower Rydberg states of H{sub 3} and D{sub 3}. In particular the states with principal quantum number n=2 feature simple correlation patterns with regular occurence of mutual affinities. Energetically higher-lying states typically show more complex patterns which are unique for each state. Quantum-chemical calculations on adiabatic potential energy surfaces of H{sub 3} Rydberg states are presented to illuminate the likely origin of these differences. We discuss the likely dissociation mechanisms and paths which are responsible for the observed continuum correlation.

Electron energy loss spectra from polycrystalline Cr and Cr 2O 3 before and after surface reduction by Ar + bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr 2O 3 before and after surface reduction by 2 keV Ar + bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr 2O 3 exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar + sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr 2+ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr 2O 3 surface exhibit features characteristic of both Cr 0 and Cr 2O 3, indicating that Ar + sputtering reduces Cr 2O 3. The fact that neither the surface- nor the bulk-plasmon features of Cr 0 can be observed in the ELS spectra obtained from sputtered Cr 2O 3 while the loss features due to Cr 0 interband transitions are clearly present indicates that Cr 0 atoms form small clusters lacking a bulk metallic nature during Ar + bombardment of Cr 2O 3.

Energetic stability, equilibrium geometry, and the electronic properties ofCa∕Si(111)surfaces

We have performed an ab initio theoretical investigation of the energetic stability, equilibrium atomic geometry, and scanning tunneling microscope (STM) images of the $\mathrm{Ca}∕\mathrm{Si}(111)$ surface. We have considered the $(3\ifmmode\times\else\texttimes\fi{}1)$, $(3\ifmmode\times\else\texttimes\fi{}2)$, and $(2\ifmmode\times\else\texttimes\fi{}1)$ structural models for Ca coverage of $1∕3$ ML, $1∕6$ ML, and $1∕2$ ML, respectively. Our total energy results indicate that the $(3\ifmmode\times\else\texttimes\fi{}1)$ phase is not expected to occur, even for Ca-rich conditions. In the $(3\ifmmode\times\else\texttimes\fi{}2)$ phase the Ca adatoms lie on the $T4$ sites along the surface trench separated by Si honeycomb chains. Similarly, in the $(2\ifmmode\times\else\texttimes\fi{}1)$ phase the Ca adatoms are adsorbed on the $T4$ sites of the surface trench separated by Si zigzag chains. The equilibrium geometries and electronic charge transfers between Ca adatoms and Si(111) surface for the $(3\ifmmode\times\else\texttimes\fi{}2)$ and $(2\ifmmode\times\else\texttimes\fi{}1)$ phases have been detailed. Our simulated STM images indicate that the higher-lying occupied states are located on the topmost Si atoms along the chains, while the empty states lie on the Ca adatoms, forming bright spots and stripes on the $(3\ifmmode\times\else\texttimes\fi{}2)$ and $(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces. For occupied states, the simulated STM images confirm the experimentally verified $2\ifmmode\times\else\texttimes\fi{}$ modulation along the honeycomb chains in the $\mathrm{Ca}∕\mathrm{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surface, induced by attractive interactions between Ca adatoms and the nearest-neighbor Si atoms. Finally we have calculated the electronic band structures for the energetically stable $(3\ifmmode\times\else\texttimes\fi{}2)$ and $(2\ifmmode\times\else\texttimes\fi{}1)$ phases of the $\mathrm{Ca}∕\mathrm{Si}(111)$ surface, and compared (in detail) with the recent experimental findings.

26Al g, m + p resonances studied via proton decays of 27Si

We used the Al(He,t)Si*(p)Al reaction at the Wright Nuclear Structure Laboratory to probe resonances of Al + p and Al + p for excitation energies of astrophysical interest. We used a beam of 25-MeV He ions produced by the ESTU Tandem at WNSL, and a target of 125 g/cm Al. Ejected tritons were detected at 0◦ in the gas ionization detector at the focal plane of the Enge Split-pole Spectrometer. Decay protons were detected in two position-sensitive silicon-strip detectors, located in the target chamber very close to the target, spanning an angular range of 55 − 125◦. We detected protons from the decay of states in Si as little as 600 keV above the proton threshold, populating the ground and first (metastable) and second excited states of Al. The excitation energies of states in Si that are resonances for Al + p and Al + p were determined. Work is ongoing to determine branching ratios for these decays, as well as resonance strengths, reaction rates, and consequences for stellar nucleosynthesis. The observation of Al in the galaxy [1] provided the first direct evidence for ongoing stellar nucleosynthesis. The gamma ray from the decay of Al is an observable that can be used to test astrophysical models of such nucleosynthesis sites as novae. These models use nuclear reaction rates as input. In order for the predictions made by the models to be reliable, the nuclear physics input also needs to be reliable: relevant reaction rates must be known to at least 20%. The Al(p,γ)Si reaction is important to the total amount of Al present in a star, since it is the only destruction mechanism of Al that competes with beta decay, thus preventing emission of the signature 1.8-MeV gamma ray. The rate is currently known to only about a factor of four. In addition to proton capture on the ground state, there could be proton captures on the low-energy metastable first excited state of Al. (Because this state has a very different spin from the ground state (0+ vs 5+), there is no direct communication between the two states, although at very high temperatures they can communicate via higher-lying states of intermediate spin; the states must therefore be treated as separate entities in most calculations.) No experimental information currently exists on Al(p,γ)Si. Any resonant contributions to this reaction rate could have a significant effect on the amount of Al synthesized in high-temperature astrophysical events.

Ultrafast Intrachain Photoexcitation of Polymeric Semiconductors

We report on excited state dynamics in isolated poly(9,9-dioctylfluorene) chains obtained by embedding the polymer in an inert plastic matrix. Early events (<300 fs) of intrachain photophysics are detected by pump-probe spectroscopy using tunable UV 25-fs pump pulses and sub-10-fs visible probe pulses. We show that higher-lying optical states, reached by multiphoton transitions, give rise to on-chain charge separation on the ultrafast time scale. The intrachain charge pair decays geminately within 500 fs to the lowest singlet state. Characteristic time scales for internal conversion and intramolecular vibrational redistribution are also determined.

Synthesis of Hydroxy and Methoxy Perylene Quinones, Their Spectroscopic and Computational Characterization, and Their Antiviral Activity

Hydroxy and methoxy perylene quinones are synthesized in an attempt to isolate the essential spectroscopic and biological features of light-induced antiviral agents such as hypericin and hypocrellin. Unlike their naturally occurring counterparts, these synthetic quinones bear the carbonyl, hydroxyl, and methoxy groups in the "bay region." The hydroxy and methoxy compounds have rich absorption spectra with broad features in the visible (approximately 450-800 nm) and relatively more intense and narrow features at wavelengths < or = 350 nm. High-level ab initio quantum mechanical calculations assign the features in the absorption spectra to electronic transitions from S0 to S2 and to higher-lying electronic states. The calculations indicate that in the ground state the trans dihydroxy isomer is 12.5 kcal/mol lower in energy than the cis dihydroxy isomer and is thus the only species present. The lowest-energy trans methoxy ground state isomer and the lowest-energy cis methoxy ground state isomer are found to be degenerate. An additional cis methoxy isomer 6.3 kcal/mol higher in energy than the global minimum is assumed to contribute to the spectrum and is also considered. Finally, the synthetic compounds exhibit similar light-induced antiviral activity to each other, but significantly less than that of hypericin.

Synthesis of Hydroxy and Methoxy Perylene Quinones, Their Spectroscopic and Computational Characterization, and Their Antiviral Activity¶

Abstract Hydroxy and methoxy perylene quinones are synthesized in an attempt to isolate the essential spectroscopic and biological features of light-induced antiviral agents such as hypericin and hypocrellin. Unlike their naturally occurring counterparts, these synthetic quinones bear the carbonyl, hydroxyl, and methoxy groups in the “bay region.” The hydroxy and methoxy compounds have rich absorption spectra with broad features in the visible (∼450–800 nm) and relatively more intense and narrow features at wavelengths ≤350 nm. High-level ab initio quantum mechanical calculations assign the features in the absorption spectra to electronic transitions from S0 to S2 and to higher-lying electronic states. The calculations indicate that in the ground state the trans dihydroxy isomer is 12.5 kcal/mol lower in energy than the cis dihydroxy isomer and is thus the only species present. The lowest-energy trans methoxy ground state isomer and the lowest-energy cis methoxy ground state isomer are found to be degener...

Transition Probabilities of Some High Lying States of Cd II

Experimental transition probabilities for 38 transitions with origin of the 4d106p, 7p, 8p, 4f, 5f and 4d95s5p electron configurations of Cd II have been measured. Relative values of the spectral line intensities have been determined in a laser-produced plasma. The experiment was carried out with pure cadmium target as well as with several Zn-Cd alloys, where the Cd content was lower than 10%. Theoretical lifetime values of Cd II excited states have been obtained in this work using Hartree-Fock wavefunctions and multiconfiguration mixing in an intermediate coupling in order to place the experimental data of branching ratio in an absolute scale. The Boltzmann plot and plasma temperature in LTE conditions is also used for obtaining the absolute transition probabilities. Comparison of the present experimental and theoretical results with the previously published data has been made.

Variational vibrational calculations using high-order anharmonic force fields†

A simple variational procedure, termed DOPI for discrete variable representation—Hamiltonian in orthogonal coordinates—direct product basis–iterative diagonalization, is described and applied to compute low-lying vibrational band origins (VBOs) of the triatomic systems H2O, CO2, and N2O, employing published empirical and theoretical sextic force fields. While in these cases no difficulties arise when quartic potentials are used, the limited range of applicability of 6th-order potentials presents difficulties for the variational determination of VBOs, in particular for the higher-lying bending states. For H2O, transformation of quadratic and quartic force fields from simple bond stretching to Simons–Parr–Finlan (SPF) coordinates results in computed VBOs deviating less from experiment. This, however, does not hold for the VBOs computed from the transformed sextic force fields where the two representations provide highly similar results. While use of empirical quartic and sextic force fields result in a much better reproduction of experimental VBOs than that of ab initio force fields, especially at higher (fifth- and sixth-) order the empirical force constants, obtained through different refinement procedures, do not correspond to the associated derivatives of the potential energy surface (PES). Rotational constants characterizing low-lying vibrational states have been evaluated as expectation values using inertia tensor formulas in the Eckart and principal axis frames. Only the Eckart axes should be used for these computations and they yield accurate vibrationally averaged rotational constants.

Two-color resonance-enhanced multiphoton ionization study of the lowest Rydberg p state of bis(eta6-benzene)chromium and its deuterated derivatives.

Two-color resonance-enhanced multiphoton ionization (REMPI) spectra of jet-cooled (eta(6)-C(6)H(6))(2)Cr(1), (eta(6)-C(6)D(6))(2)Cr(2), and (eta(6)-C(6)D(6))(eta(6)-C(6)D(5)H)Cr(3) have been measured with use of the 3d(z)2-->R4p(x,y) Rydberg transition as the first step of the electronic excitation. The 0(0) (0) Rydberg component shifts by 59 and 54 cm(-1) to red when one goes from 1 to 2 and 3, respectively. Surprisingly, the REMPI spectra of 1-3 show very rich vibronic structures revealing both totally symmetric vibrations and degenerate vibrational modes. Presence of intense peaks corresponding to the e(2g) modes in the spectra of 1 and 2 is indicative of Jahn-Teller coupling in the R4p(x,y) Rydberg state. Additional REMPI resonances appear on going from 1 and 2 to 3 as a result of the symmetry reduction. The vibronic components in the spectra of 1-3 were assigned on the basis of the selection rules and comparison with the vibrational frequencies of the 1 and 2 ground-state molecules. The frequencies of over 10 normal vibrations have been determined for the gas-phase 1-3 Rydberg-state molecules from the REMPI experiment. The wavenumber corresponding to the lowest-energy mode (the ring torsion vibration) appears to be 40 cm(-1) in 1 and 35 cm(-1) in the deuterated complexes. The REMPI peaks are homogeneously broadened. The lower lifetime limits for the upper-state components increase on going from the vibrationless level to higher-lying vibronic states and on going from 1 to the deuterated derivatives.

Study of light proton-rich nuclei by complete kinematics measurements

Light proton-rich bound and unbound nuclei were produced by means of stripping reactions of secondary beams of 20Mg and 18Ne. The decays of the unbound nuclei 18,19Na have been measured by detecting their decay products 17,18Ne and one proton and by performing an invariant-mass reconstruction. For 18Na, the present work is the first measurement of its decay. As the decay scheme of this nucleus could not be determined, two possible scenarios are proposed and discussed. In addition, the decay of excited states in 17Ne via two-proton emission was observed. The proton-proton angular distribution is isotropic for the first two-proton-emitting states, whereas higher-lying states seem to decay by a correlated two-proton emission, consistent with a 2He emission pattern for part of the decay strength.

Radiative lifetimes of Rydberg 6p n d J =2 states of Pb I by multichannel quantum defect theory

Energy levels of the odd-parity 6pnd J=2 Rydberg states of Pb I are analysed by the multichannel quantum defect theory (MQDT) in the frame of a five-channel three-limit calculation model. With optimal MQDT parameters, channel admixture coefficients are obtained and used to calculate the theoretical lifetimes of the levels by comparing to the previously measured lifetimes. The predicted lifetimes for higher-lying Rydberg states are given and discussed. These predicted lifetimes are very different from those obtained by the four-channel two-limit model previously used, which means that introduction of the additional interacting channel is important for studying the 6pnd J=2 Rydberg states of Pb I.

Room-temperature photoluminescence and electroluminescence properties of sputter-grown gallium nitride doped with europium

We have studied the room-temperature photoluminescence (PL) and electroluminescence (EL) properties of europium (Eu)-doped gallium nitride (GaN) thin films grown by radio frequency planar magnetron cosputter deposition. X-ray photoelectron core level spectra collected for Eu 4d revealed that the Eu ions in the GaN host were mainly in the trivalent state. A series of PL peaks were observed in the region of 530–700 nm, with the most intense peak at 614 nm. They were assigned to the radiative transitions between the 4f6 energy levels of the Eu3+ ion, specifically DJ5→7FJ′ (J=0,1;J′=1,2,3,4). The maximum PL intensity was obtained at a Eu concentration of ∼1.8 at. %. Electroluminescent devices were fabricated with an Al electrode, Eu-doped GaN light-emitting layer, Al2O3–TiO2 dielectric layer over an indium-tin-oxide electrode on a Corning 7059 glass substrate. The EL emission spectra from the fabricated devices were almost identical to the PL spectra. Higher electron energies (higher applied voltages) were needed to excite Eu3+ ground-state electrons into the higher-lying D15 state, which is consistent with the EL excitation mechanism being direct impact by hot electrons.

Influence of the coupling between the normal and lateral motions on surface states of a semi-infinite superlattice with a cap layer

Using an effect-barrier height method, we investigate theoretically the influence of the coupling between normal and lateral motions of an electron on the surface electron states in a semi-infinite Al 0.3Ga 0.7As/GaAs superlattice with a cap layer consisting of ternary mixed crystal Al x Ga 1− x As within the framework of effective-mass theory. Previous works on the surface states in semi-infinite superlattice paid much less attention to the coupling effect. However, our study showed that the coupling effect cannot be neglected when the difference of the effective mass of the electron between the well and barrier layers exists. It is found that the existence and features of the surface electron states, especially the higher-lying surface electron states, as well as the energy positions and widths of the miniband and minigaps are sensitively dependent on the transverse wave number. A brief analysis of these results is given.

Density functional calculations of the vibronic structure of electronic absorption spectra.

Calculations of the vibronic structure in electronic spectra of large organic molecules based on density functional methods are presented. The geometries of the excited states are obtained from time-dependent density functional (TDDFT) calculations employing the B3LYP hybrid functional. The vibrational functions and transition dipole moment derivatives are calculated within the harmonic approximation by finite difference of analytical gradients and the transition dipole moment, respectively. Normal mode mixing is taken into account by the Duschinsky transformation. The vibronic structure of strongly dipole-allowed transitions is calculated within the Franck-Condon approximation. Weakly dipole-allowed and dipole-forbidden transitions are treated within the Franck-Condon-Herzberg-Teller and Herzberg-Teller approximation, respectively. The absorption spectra of several organic pi systems (anthracene, pentacene, pyrene, octatetraene, styrene, azulene, phenoxyl) are calculated and compared with experimental data. For dipole-allowed transitions in general a very good agreement between theory and experiment is obtained. This indicates the good quality of the optimized geometries and harmonic force fields. Larger errors are found for the weakly dipole-allowed S0 --> S1 transition of pyrene which can tentatively be assigned to TDDFT errors for the relative energies of excited states close to the target state. The weak bands of azulene and phenoxyl are very well described within the Franck-Condon approximation which can be explained by the large energy gap (>1.2 eV) to higher-lying excited states leading to small vibronic couplings. Once corrections are made for the errors in the theoretical 0-0 transition energies, the TDDFT approach to calculate vibronic structure seems to outperform both widely used ab initio methods based on configuration interaction singles or complete active space self-consistent field wave functions and semiempirical treatments regarding accuracy, applicability, and computational effort. Together with the parallel computer implementations employed, the present approach appears to be a valuable tool for a quantitative description and detailed understanding of electronic excitation processes in large molecules.

Exciton migration dynamics in a dendritic molecule: quantum master equation approach using ab initio molecular orbital configuration interaction method.

We investigate the exciton migration dynamics in a dendritic molecular model composed of pi-conjugation linear-leg units (acetylenes and diacetylene) and a benzene ring (branching point) using the quantum master equation approach with the ab initio molecular orbital (MO) configuration interaction (CI) method. The efficient migration of exciton from short-length linear legs (acetylenes) to long-length linear leg (diacetylene) via a benzene ring is observed. As predicted in previous studies, the exciton (electron and hole) distributions are relatively well localized in each generation segmented by the meta-branching point (meta-substituted benzene ring) though the electron and hole distributions are delocalized and are somewhat spatially different from each other within each generation. It is found that the excitons localized in the generation composed of short linear legs occupy in higher-lying exciton states, while those in the generation composed of long linear legs do in lower-lying ones. These features suggest the decoupling of pi-conjugation at the meta-branching point. On the other hand, the relaxation effect between exciton states is found to be caused by the exciton-phonon coupling, in which the existence of common configurations (electron-hole pairs) in CI wave functions between adjacent exciton states (having primary distributions on short and long linear-leg regions, respectively) is important for the relaxation between their exciton states. This feature indicates the importance of partial penetration of pi-conjugation through the meta-substituted benzene ring in excited states for such exciton migration.

Theoretical analysis of peroxynitrous acid: characterization of its elusive biradicaloid (HO...ONO) singlet states.

Various high levels of theory (DFT, QCISD, BD(TQ), and CASSCF) have been applied to the characterization of two higher-lying biradicaloid singlet states of peroxynitrous acid. A singlet minimum (cis-2) was located that had an elongated O-O distance of 2.17 A and was only 14.4 kcal/mol [UB3LYP/6-311+G(3df,2p)] higher in energy than its cis-peroxynitrous acid ground-state precursor. A trans metastable higher-lying singlet (trans-2) was 12.8 kcal/mol higher in energy than ground-state HO-ONO. Complete active space calculations [CAS(12,10)/6-311+G(d,p)] predicted the optimized geometries of these cis and trans metastable singlets to be quite close to those obtained with the DFT method. Geometry optimization of both cis- and trans-2 within the COSMO solvent model suggest that both exist as energy minima in polar media with elongated O-O distances of 2.14 and 2.09 A. Both cis- and trans-2 exist as hydrogen-bonded complexes with several water molecules. These collective data suggest that solvated forms of cis-2.3H2O and trans-2.3H2O represent the elusive higher-lying biradicaloid minima that have been previously advocated (J. Am. Chem. Soc. 1996, 118, 3125) as the metastable forms of peroxynitrous acid (HOONO*).

Real-time observation of vibrational coherence persisting after internal conversion and vibrational relaxation in cyanine dye molecules

Vibrational coherence after internal conversion from the higher-lying excited singlet state to the first excited singlet state (S n →S 1) and vibrational relaxation in cyanine dye molecules in solution are real-time observed with two-color pump–probe experiments with 20-fs time resolution. The intensity of oscillating signal due to 300-cm −1 molecular vibration increases with time constant of about 1 ps. This phenomenon shows that the vibrational coherence preserves even after a large energy loss, 1.55 eV, by internal conversion and vibrational relaxation.