Resonant Electronic Transition Research Articles

Effect of nanotube-nanotube coupling on the radial breathing mode of carbon nanotubes

We study the radial breathing mode (RBM) of resonant single walled carbon nanotubes within individual bundles. As strain is applied, significant debundling of the nanotubes occurs, causing an upshift in the RBM frequency. The RBM frequency is found to change irreversibly with strain, correlating more strongly with the total number of strains and relaxations than with absolute strain, indicating that debundling is the dominant cause of the observed changes. The RBM upshift of semiconducting nanotubes is 70% larger than that of metallic nanotubes. We also observe a large drop in the Raman intensity as the nanotubes are debundled. This drop is not due to a change in the resonant electronic transition energies of the nanotube and indicates that neighboring nanotubes within a bundle play an important role in the resonant nanotube's Raman signal.

Peculiarities of the electron exchange upon collision of a negative hydrogen ion with a cluster of aluminum atoms

The resonant electronic transition upon normal-to-surface collision of a negative hydrogen ion with a cluster of aluminum atoms has been considered. The dependence of the probability of ion survival on the distance to the cluster surface is calculated for different collision velocities. It is shown that, depending on the incident particle velocity, a cluster of atoms can behave as a bulk sample or a nanostructure with energy quantization over two coordinates.

Control of wave packets in Li2 by shaping the pump and probe pulses for a state-selected pump-probe analysis of the ionization continuum

Wave packet signals in Li(2) prepared by shaped pump pulses are also detected with state-selected shaped probe pulses in the ionization continuum. The results show that the final states are discrete Rydberg states instead of continuum states. Final autoionizing states in the continuum are observed and characterized. By selecting specific resonant rovibrational electronic transitions from the superposition states prepared in the wave packets to the final autoionizing states with the pulse shaping system, the modulation depths of the wave packet signals are increased by as much as 5.20+/-0.03 times. Control of the wave packets is also realized by shaping the probe pulses to select specific resonant transitions between the states in the wave packets and the highly excited Rydberg states. The detected amplitude ratio of one specific vibrational quantum beat to one specific rotational quantum beat can be decreased by ten times.

On the control of resonant versus non-resonant electronic transitions in molecular photodissociation

An excitation scheme is regarded where a non-resonant three photon transition populates an excited electronic state leading to molecular dissociation. The absorption of an additional photon initiates a resonant transition to another state where different molecular fragments are built. The control of the relative yield of dissociation products in the two channels is achieved employing modulated laser pulses. The results derived from feedback control combined with a genetic algorithm and optimal control theory are compared, and it is shown that different physical mechanisms can lead to comparable branching ratios.

Optical nonlinearity in PbO–SiO2 glass: Kramers–Kronig analyses

Relationship between nonlinear refractivity and two-photon absorption has been studied for PbO–SiO2 glasses using a nonlinear Kramers–Kronig relation. Nonlinear refractive indices, which are determined with z-scan measurements, are consistent with those which are calculated using the relation from two-photon absorption spectra. This consistency suggests that large intensity-dependent refractivity in this glass system arises from resonant two-photon electronic transitions from oxygen 2p to lead 6p states.

DFT Vibrational Calculations of Rhodamine 6G Adsorbed on Silver: Analysis of Tip-Enhanced Raman Spectroscopy

The tip-enhanced near-field Raman (TERS) bands of Rhodamine 6G (R6G), that we reported earlier [Chem. Phys. Lett. 2001, 335, 369.], are assigned on the basis of density-functional theory (DFT) calculations at the 6-311++G(d,p) level. The Raman and infrared intensities as well as frequencies of the vibrational modes are used for band assignments. These vibrational modes, in combination with characterization of resonant electronic transitions using time-dependent DFT calculations, predict spectral changes in resonant Raman and surface-enhanced resonant Raman scatterings of R6G. Moreover, the TERS spectra of R6G are analyzed in detail, where interactions between the tip and R6G molecules and their enhancement mechanisms are discussed. Finally, we propose a novel Raman spectroscopy technique capable of detecting molecular vibrations at sub-nanometer scale.

Laser-induced resonant electron transitions in a structure with three quantum dots

The effect of a resonance laser pulse on the quantum dynamics of electrons in a system of three semiconductor boxlike quantum dots is theoretically analyzed. It is shown that, in an asymmetric structure, the electron transfer between the two outer dots can be described in terms of a two-level scheme with diagonal transitions. The energies of the operating electron levels and the matrix elements for the corresponding electron transitions are found in the high-barrier approximation. The parameters of the laser pulse corresponding to the highest probability of resonance electron transitions between the ground states of two quantum dots are determined.

Near-infrared resonance Raman excitation profile studies of single-walled carbon nanotube intertube interactions: A direct comparison of bundled and individually dispersedHiPconanotubes

Complete Raman excitation profiles for single-walled carbon nanotube radial breathing modes were obtained for bundled $\mathrm{HiPco}$ carbon nanotube samples in the region from 700 to $985\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ excitation. Results are compared to similar profiles generated from individual carbon nanotubes dispersed in aqueous solution, allowing a direct determination of intertube interaction effects on electronic properties for 12 specific semiconducting nanotube chiralities. Redshifts in the excitation profiles (ranging from 54 to $157\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$) are observed on going from isolated individual to bundled nanotubes. Additionally, bundling is found to broaden the electronic transitions by an average factor of 2.4 compared to individualized nanotube bandwidths. These results compare well with recent theoretical predictions for bundling effects. An investigation of bundling effects on radial breathing mode frequencies for 17 different nanotube chiralities finds no evidence for significant perturbation of these frequencies resulting from intertube interactions. Our results demonstrate that previously reported radial breathing mode frequency shifts are apparent shifts only, resulting from redshifting of the resonant electronic transitions for bundled nanotubes. Bundle inhomogeneity, packing efficiency, orientational disorder, and symmetry reduction are indicated as important factors in determining the degree of intertube interaction.

Probing the molecular basis for solvent-induced dephasing using resonance Raman spectroscopy: betaine-30 in CH 3OH, CH 3OD, and CD 3OD

Absolute Raman cross-sections are reported for fourteen normal modes of betaine-30 dye in CH 3OH, CH 3OD, and CD 3OD solution, at five different excitation frequencies within the visible absorption band. Consistent with previous results from our lab, the Raman intensities of betaine-30 in CD 3OD solution are generally stronger than in CH 3OH solution. For most of the betaine-30 normal modes, however, the resonance Raman cross-sections are similar in CH 3OD and CH 3OH solution. The results suggest that methyl group motion, more so than hydroxyl group motion, is responsible for dephasing of the resonant electronic transition of betaine-30 in methanol.

Resonance Raman Spectra Simulation of the 4,4‘-Bipyridine Anion Radical and N-Protonated Radical

A low-cost computational methodology is utilized to calculate the resonance Raman spectra of the 4,4‘-bipyridine anion radical (44BPY-•) and N-hydro radical (44BPYH•). The resonance Raman intensities are evaluated in the Franck−Condon approximation, by projecting the excited state surface gradient (calculated with the CASSCF method) on the ground state normal modes (determined at ROHF level of theory). TD-DFT and CIS calculations are used to assign the resonant electronic states, estimate the intensity and excitation energy of the related transitions, and identify the molecular orbitals involved in these transitions. These molecular orbitals are then used to construct CASSCF active spaces as small as possible and to compute the excited-state gradients. The results appear to be very satisfactory for 44BPY-• and reasonable for 44BPYH•. They allow identifying the nature of the probed resonant electronic transitions of these transient species and provide a precise picture of their electronic configuration. A comparative analysis of these results and those reported previously for the lowest excited S1 state of 44BPY reveals a clear analogy of the electronic structures of the N-hydro radical and excited S1 state.

Non-Radiative Triggering of Long-Lived Nuclear Isomers

The triggering of long-lived isomeric nuclei by non-radiative excitation to a relatively short-lived mediating state is considered. Coulomb triggering in inelastic scattering of heavy ions, a transfer of triggering energy from resonant electron transitions of atomic shell (NEET) and triggering by capture of a free electron into a bound atomic state (NEEC) are discussed. Cross sections for the above processes of non-radiative triggering are presented and the relative efficiencies of these different triggering mechanisms are discussed. Numerical estimates are presented for the selected isomers.

Surface-enhanced Raman scattering and surface-enhanced resonance Raman scattering excitation profiles of Ag-2,2'-bipyridine surface complexes and of [Ru(bpy)3]2+ on Ag colloidal surfaces: manifestations of the charge-transfer resonance contributions to the overall surface enhancement of Raman scattering.

Excitation profiles of SERS (surface-enhanced Raman scattering) and/or SERRS (surface-enhanced resonance Raman scattering) spectral bands of two forms of a Ag-bpy (bpy = 2,2'-bipyridine) surface complex and of [Ru(bpy)3]2+ on Ag nanoparticle (hydrosol) surfaces were determined from the spectra excited in the 458-600 nm region and are reported together with the FT-SERS spectra of the Ag-bpy surface complex and FT Raman spectra of [Ru(bpy)3] Cl2. Seven of the observed 11 fundamentals as well as their first overtones and combination bands are selectively enhanced in SERS of the Ag-bpy surface complex formed in the Ag colloid/HCl/bpy system. The profiles of these bands show a common maximum at approximately 540 nm. The selectively enhanced bands of the Ag-bpy surface complex have nearly the same wavenumbers as those enhanced in the SERRS and resonance Raman spectra of [Ru(bpy)3]2+ upon excitation close to the 453 nm maximum of its MLCT absorption band. Moreover, the intensity patterns of the bpy vibrations of the two species match both in resonance (541 nm excitation for Ag-bpy, 458 nm for [Ru(bpy)3]2+) and in off-resonance (458 and 1064 nm for Ag-bpy, 1064 nm for [Ru(bpy)3]2+). The distinct band shapes of the excitation profiles of the selectively enhanced vibrational modes of the Ag-bpy surface complex, as well as the observation of overtones and combination bands in the SERS spectra upon excitation into this "band", are interpreted in terms of a charge-transfer resonance contribution to the overall SERS enhancement. In view of the near-coincidence of the vibrational modes coupled to the resonant electronic transition of Ag-bpy with those coupled to the MLCT transition of [Ru(bpy)3]2+, the resonant electronic transition is tentatively assigned to a Ag metal to bpy (pi*) CT transition.

Mechanisms of nuclear excitation in plasmas

This paper discusses several mechanisms that could be responsible for nuclear excitation in a plasma of temperature of order 10--100 eV. Four mechanisms discussed in detail are nuclear excitation by a resonant electronic transition from an excited bound state to a lower-lying bound state, nuclear excitation by electron capture from the continuum, photoexcitation, and inelastic electron scattering. Estimates of the rates for these different processes are presented for the excitation of ${}^{235m}\mathrm{U}$ for which the present experimental data lacks adequate theoretical interpretation.

Structure and bondings in cerium oxysulfide compounds. I—Electronic, infrared and resonance Raman spectra of Ce2.0O2.5S

As a first stage, the electronic (350–900 nm), infrared and Raman (700–10 cm-1) spectra of polycrystalline samples of the oxysulfide compound Ce2.0O2.5S were investigated and tentative vibrational assignments are proposed from a comparison with the simpler Ce2O2S parent compound. Great changes in the vibrational patterns demonstrate that the two oxysulfides are not isostructural and that the oxidized form cannot be described as a Ce3+-containing peroxide or superoxide system. As a second stage, the resonance Raman spectra of Ce2.0O2.5S were recorded within the contour of the first electronic transitions (650–450 nm) on the absorption edge. The Raman excitation profiles maximize at near 490 nm and exhibit large enhancements for most of the vibrational modes, in particular the higher wavenumber νCe–O equatorial stretching modes at 584 and 548 cm-1. This shows that the resonant electronic transition is more likely an excitonic type and involves a large electronic delocalization, particularly over the cerium–oxygen bonds in the equatorial planes. In conclusion, the Ce2.0O2.5S compound does not look like as a mixed-valence sample with distinct Ce3+ and Ce4+ constituent ions, Class II in Day’s notation, as already known in Ce4O4S3; it is better described as a Ce4+-containing broadband semiconductor in which low-energy excitonic-type transitions implying the intermediate 4f levels have to be considered and in which valences do not appear to be firmly trapped. © 1997 John Wiley & Sons, Ltd.

Optical rotation of the second harmonic radiation from retinal in bacteriorhodopsin monomers in Langmuir-Blodgett film: evidence for nonplanar retinal structure

We observed optical rotation of the plane of polarization of the second harmonic (SH) radiation at 532 nm (in resonance with the retinal absorption) generated in reflection geometry in Langmuir-Blodgett film of bacteriorhodopsin (bR). The analysis of the experimental data showed that this effect arises from the nonvanishing contribution of the antisymmetrical part of the hyperpolarizability tensor. This requires that the dipole moment of the resonant electronic transition, the change of the dipole moment upon electronic excitation, and the long axis of the retinal not be coplanar. Such conditions are satisfied only if the retinal has a nonplanar geometry, a conclusion that could lend support to the heterogeneity model of the origin of the biphasic band shape of the linear CD spectrum of the retinal in bR. On the basis of our theoretical analysis, we were able to estimate the angle between the induced dipole moment and the plan that contains the long axis of the chromophore and the transition dipole moment of the retinal absorption.

Origin of optical nonlinearity for PbO, TiO2, K2O, and SiO2 optical glasses

The nonlinear optical properties for the PbO, TiO2, K2O, and SiO2 system have been measured by the Z-scan method. The magnitude and sign of the nonlinear refractive index n2 were determined, as was the negative sign, which indicated a self-defocusing optical nonlinearity. Two optical absorption bands at 540 and 660 nm, respectively, are observed in the optical absorption spectra. The sources of the absorption bands are attributed to the 3d-shell electronic transitions of Ti3+ ions from the ground state to the excited states. The origin of the negative nonlinear refractive index was the contribution of resonant electronic transition processes, which can cancel the positive nonresonant refractive index that mainly resulted from the hyperpolarizabilities of the Pb–O and Ti–O pairs.

Theory of resonance Raman optical activity: the single electronic state limit

The theory of natural Raman optical activity (ROA) is extended to the limit of strong resonance with a single electronic state (SES). It is shown that in the SES limit, the resonance ROA (RROA) bands all have the same sign, and their relative intensities are identical to the corresponding resonance Raman (RR) parent bands. The sign of the RROA bands are predicted to be opposite that of the pure electronic circular dichroism (CD) of the single, resonant electronic transition. The ratio of the RROA to the RR spectrum is the same or one-half the magnitude of the electronic CD anisotropy ratio of the resonant electronic state. Deviation from this simple pattern is indicative of contributions from more than one excited electronic state to the RR and RROA spectra, and hence is a sensitive probe of the electronic origin of ordinary RR spectra.

Resonance Raman spectra of bis (2,4-pentanedithionate) palladium (II) complex

Resonance Raman (RR) spectra of the bis (2,4-pentanedithionate) Pd(II) complex were measured upon excitation into the main band in the visible region of the electronic absorption spectra (λ max=513 nm in benzene,λ max=520 nm in CS 2) and into its short wavelength shoulder (λ max=458 nm in benzene;λ max=469 nm in CS 2). Only three of the eight totally symmetric normal vibrations of the complex are RR active. These modes consist predominantly of the symmetric ▪ stretch and the ring deformation. Totally symmetric Pd-S and ▪ stretching vibrations are inactive. The molecular geometry changes accompanying the resonant electronic transition are thus localized entirely on the ligand. This is consistent with a strongly localized metal-to-ligand charge transfer (MLCT) character of the transitions. The symmetry of the terminal ligand π * orbital involved in the transitions as determined from the RR spectra is a u. The main band is attributed to b 2g(d xz)→a u(π*) and the shoulder to the b 3g(d yz )→a u(π*) MLCT transition.

Time‐of‐flight mass spectrometry of nucleic acids by laser ablation and ionization from a frozen aqueous matrix

AbstractPositive‐ion time‐of‐flight mass spectra of sodium salts of oligodeoxyribonucleic acids (DNA), and a protein, lysozyme, have been obtained using pulsed‐laser ablation and ionization of frozen aqueous solutions from a cooled copper substrate. The efficiency of positive‐ion formation was found to increase when the ablating laser was tuned to a frequency corresponding to resonant electronic transitions in sodium or copper atoms. Molecular ions of double‐stranded oligomeric nucleic acids, with little fragmentation, were observed at masses up to ∼18 500 Da. Single‐stranded nucleic acids could be fragmented by lowering the laser power‐density to reduce the the substrate heating rate. A dimer ion of lysozyme, at mass ∼29 000 Da, was detected at an impact energy on the electron multiplier of only 3.5 keV.

Auger neutralization and electron emission during grazing scattering of fast ions from surfaces

We consider theoretically the potential electron emission (PEE) from a metal surface bombarded by positive ions in the velocity range around the metal Fermi veclocity under grazing-incidence conditions. The angle-resolved spectra of electrons, emitted during Auger neutralization of the impact ions, exhibit a strong anisotropy, thus reflecting the momentum transfer from the projectile to the electron motion. Furthermore, we briefly discuss the influence of the resonant electron transitions on the Auger electron yield. Finally, considering the competing process of kinetic electron emission (KEE), we conclude that, in order to distinguish PEE from KEE, the incidence angle of the projectile above the surface has to be well below the critical angle for surface channeling.