Resonant Electronic Transition Research Articles

Effects of electronic-vibrational resonance on the absorption and two-dimensional rephasing spectra of the Fenna–Matthews–Olson complex

The absorption and two-dimensional rephasing spectra of the Fenna–Matthews–Olson (FMO) complex at 77 K have been simulated using the hierarchical equations of motion method. Three cases of vibrational coupling have been studied in the FMO complex model. In the first case, no specific vibrational mode is coupled. In the second and third cases, low-energy and high-energy electronic-vibrational resonance dimers are considered in the model, respectively. It has been observed that the vibrational mode significantly weakens the intensity of the absorption peak of resonant high-energy electronic transitions. We have also confirmed that long-lived quantum beatings originate from vibration, while short-lived electronic coherence still persists. Furthermore, although the electronic resonance mode speeds up the rephasing dynamics, it is not directly proportional to the coupling strength. The low-energy electronic-vibrational resonance dimer exhibits a greater speed-up of rephasing dynamics.

Strong-Field Extreme-Ultraviolet Dressing of Atomic Double Excitation.

We report on the experimental observation of a strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phase shifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way of performing nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.

Dinitrogen Coupling to a Terpyridine-Molybdenum Chromophore Is Switched on by Fermi Resonance

Summary The traditional view of a chemical change is inherently local and classical, and such a change relies on a mix of thermodynamic and kinetic parameters to control reactivity. Often, the thermodynamic stability of chemical bonds necessitates significant energy input for activation. One fundamental question is potentially transformative: can quantum mechanics enable selective bond activation? A possible approach involves strategic input of energy to reaction-specific vibrational levels. Toward this goal, our work describes the coupling of vibrational motions in a terpyridine-molybdenum complex hosting a nonreactive substrate—dinitrogen. Ultrafast coherence spectroscopies revealed a Fermi-resonance coupling mechanism connecting in-plane breathing motion of the light-harvesting terpyridines with the stretching motion of the spatially disparate dinitrogen bridge. Notably, the coupling is significantly enhanced in the photoexcited state. This Fermi resonance indicates an energy conduit that drives the two motions in sync and thereby amplifies vibrational energy exchange. Achieving selective bond activation by bridging vibrations could present a quantum-inspired design principle in synthetic chemistry.

Raman excitation profiles of hybrid systems constituted by single‐layer graphene and free base phthalocyanine: Manifestations of two mechanisms of graphene‐enhanced Raman scattering

The ability of single‐layer graphene (SLG) to enhance Raman scattering of planar aromatic molecules denoted graphene‐enhanced Raman scattering (GERS) is currently the subject of focused interest. We report on manifestations of two mechanisms of GERS in Raman spectra of glass/SLG/free base phthalocyanine (H2Pc) monolayer (ML) hybrid systems: (i) photoinduced charge transfer from SLG Fermi level to LUMO of H2Pc excited at onset of the near IR region, and (ii) modification of resonance Raman scattering of H2Pc in the visible region by SLG–H2Pc interaction resulting into delocalization of the electronic transition over the benzene rings of H2Pc. Glass/SLG/H2Pc hybrid systems with either a bilayer or a monolayer of H2Pc molecules and a graphite/H2Pc (ML) reference system were prepared by a spectrally controlled adsorption–desorption of H2Pc from solution, followed by Raman mapping of samples at excitation wavelengths in the 532–830 nm range, construction of excitation profiles for H2Pc Raman bands of the glass/SLG/H2Pc samples and determination of GERS enhancement factors for the glass/SLG/H2Pc (ML) sample versus the graphite/H2Pc (ML) reference sample (3–24 at 633 nm and 3–19 at 647 nm excitations). Selectivity of the excitation profiles and of the GERS enhancement factors with respect to localization of the vibrational modes within the H2Pc molecule demonstrates involvement of a different resonant electronic transition in each of the two mechanisms of GERS. Copyright © 2017 John Wiley & Sons, Ltd.

Ab initio calculation of the G peak intensity of graphene: Laser-energy and Fermi-energy dependence and importance of quantum interference effects

We present the results of a diagrammatic, fully ab initio calculation of the $G$ peak intensity of graphene. The flexibility and generality of our approach enables us to go beyond the previous analytical calculations in the low-energy regime. We study the laser and Fermi energy dependence of the $G$ peak intensity and analyze the contributions from resonant and non-resonant electronic transitions. In particular, we explicitly demonstrate the importance of quantum interference and non-resonant states for the $G$ peak process. Our method of analysis and computational concept is completely general and can easily be applied to study other materials as well.

The nonmonotonous shift of quantum plasmon resonance and plasmon-enhanced photocatalytic activity of gold nanoparticles.

The surface plasmon resonance (SPR) of metal nanoparticles exhibits quantum behaviors as the size decreases owing to the transitions of quantized conduction electrons, but most studies are limited to the monotonous SPR blue-shift caused by off-resonant transitions. Here, we demonstrate the nonmonotonous SPR red-shift caused by resonant electron transitions and photocatalytic activity enhanced by the quantum plasmon resonance of colloidal gold nanoparticles. A maximal SPR wavelength and the largest photocatalytic activity are observed in the quantum regime for the first time for the gold nanoparticles with a diameter of 3.6 nm. Theoretical analysis based on a quantum-corrected model reveals the evolution of SPR with quantized electron transitions and well explains the nonmonotonous size-dependencies of the SPR wavelength and absorption efficiency. These findings have profound implications for the understanding of the quantum nature of the SPR of metal nanoparticles and their applications in areas ranging from photophysics to photochemistry.

Superradiance by molecular nitrogen ions in filaments

Results of the experimental study of population inversion in the resonant electronic transition \({B^3}{\pi _g} - {A^3}\sum _u^ + \) of nitrogen ions by optical pumping of atmospheric air and pure nitrogen by a femtosecond laser pulse at a wavelength of 950 nm are presented. It is shown that the inversion results from selective population of the \(N_2^ + \left( {{B^2}\sum _u^ + ,v' = 0} \right)\) excited state during multiphoton excitation of the autoionization state of the nitrogen molecule with an energy of 18.7 eV. Seed photons for superradiance at transitions of molecular nitrogen ions are photons of the axial supercontinuum that occurs in a filament at the corresponding wavelengths. The superradiance mode is implemented at the wavelength λ = 358.4 nm referred to the transition of the CN molecule.

Collision‐independent detection of molecular two‐photon excitation by time‐resolved parametric four‐wave mixing

We report collision‐independent detection of gas‐phase nitric oxide (NO) via a two‐pulse, femtosecond (fs) time‐resolved parametric four‐wave‐mixing (PFWM) optical‐probe scheme. This approach exploits the broadband two‐photon excitation of rovibrational manifolds associated with resonant electronic molecular transitions, allowing species‐specific detection. The observed time‐domain fs‐PFWM spectral signature associated with this broadband electronic two‐photon excitation process is shown to exhibit negligible dependence on colliding‐partner concentrations at short (<10 ps) time delays, even in the presence of species that strongly quench excited‐state fluorescence. Furthermore, by employing a hybrid fs/ps PFWM scheme, the broadband two‐photon absorption spectrum of the molecular species of interest is demonstrated to be resolved in the wavenumber domain without need to scan the probe‐pulse delay. Copyright © 2015 John Wiley & Sons, Ltd.

Electronic State-Resolved Electron-Phonon Coupling in an Organic Charge Transfer Material from Broadband Quantum Beat Spectroscopy.

The coupling of electron and lattice phonon motion plays a fundamental role in the properties of functional organic charge-transfer materials. In this Letter we extend the use of ultrafast vibrational quantum beat spectroscopy to directly elucidate electron-phonon coupling in an organic charge-transfer material. As a case study, we compare the oscillatory components of the transient reflection (TR) of a broadband probe pulse from single crystals of quinhydrone, a 1:1 cocrystal of hydroquinone and p-benzoquinone, after exciting nonresonant impulsive stimulated Raman scattering and resonant electronic transitions using ultrafast pulses. Spontaneous resonance Raman spectra confirm the assignment of these oscillations as coherent lattice phonon excitations. Fourier transforms of the vibrational quantum beats in our broadband TR measurements allow construction of spectra that we show report the ability of these phonons to directly modulate the electronic structure of quinhydrone. These results demonstrate how coherent ultrafast processes can characterize the complex interplay of charge transfer and lattice motion in materials of fundamental relevance to chemistry, materials sciences, and condensed matter physics.

K-Shell Excitation and Ionization of a Gas-Phase Protein: Interplay between Electronic Structure and Protein Folding

Understanding the correlation between proteins’ tertiary and electronic structures is a great challenge, which could possibly lead to a more efficient prediction of protein functions in living organisms. Here, we report an experimental study of the interplay between electronic and tertiary protein structure, by probing resonant core excitation and ionization over a number of charge-state selected precursors of electrically charged proteins. The dependence of the core ionization energies on the protein charge state shows that the ionization of a protonated protein is strongly correlated to its tertiary structure, which influences its effective Coulomb field. On the other hand, the electronic core-to-valence shell transition energies are not markedly affected by the unfolding of the protein, from compact to totally elongated structures, suggesting that frontier protein orbitals remain strongly localized. Nevertheless, the unfolding of a protein seems to influence the cross section ratio between different resonant electronic transitions.

Characterizing morphology in organic systems with resonant soft X-ray scattering

Resonant soft X-ray scattering (R-SoXS) has proven to be a highly useful technique for studying the morphology of soft matter thin films due to the large intrinsic contrast between organic materials and the anisotropic nature of the resonant electronic state transitions from which the contrast originates. This allows R-SoXS users to measure spatial composition correlations from crystalline and amorphous phases in heterogeneous organic samples, infer relative domain purity, and determine average local molecular ordering correlations. R-SoXS has been used to study the morphology of organic photovoltaics, organic thin film transistors, biological systems, and block copolymer engineering applications. The mesoscopic morphological information compliments molecular packing information determined with hard X-rays, so that complex structure–property relationships can be elucidated over a large range of length scales. Extensions of R-SoXS have also emerged that make use of more advanced sample setups or different experimental geometries than normal transmission, such as θ–2θ reflectivity or grazing incidence.

Direct visual evidence for chemical mechanism of SERRS of pyrazine adsorbed on Ag nanoparticle via charge transfer

In this paper, the chemical enhancement of surface-enhanced resonance Raman scattering (SERRS) of pyrazine adsorbed on Ag nanoparticles through charge transfer was experimentally and theoretically investigated. Based on the calculations by density functional theory (DFT) and time-dependent DFT (TD-DFT), we theoretically analyzed the absorption spectra and SERS spectrum of the S-complex of pyrazine–Ag20. The charge transfer in the process of resonant electronic transitions between adsorbed molecule and metal cluster can be visualized by the method of charge difference density. It is a direct evidence for the chemical enhancement mechanism of SERRS of pyrazine molecule adsorbed on Ag nanoparticle via charge transfer between molecule and metal. Additionally, the intracluster charge redistribution was also considered as an evidence for the electromagnetic enhancement. By comparing the experimental and theoretical results, it was demonstrated that the SERRS of the pyrazine molecule absorbed on silver clusters in different incident wavelength regions is dominated by different enhancement mechanisms via the chemical and electromagnetic enhancements.

Excited and ground state vibrational dynamics revealed by two-dimensional electronic spectroscopy

Broadband two-dimensional electronic spectroscopy (2DES) can assist in understanding complex electronic and vibrational signatures. In this paper, we use 2DES to examine the electronic structure and dynamics of a long chain cyanine dye (1,1-diethyl-4,4-dicarbocyanine iodide, or DDCI-4), a system with a vibrational progression. Using broadband pulses that span the resonant electronic transition, we measure two-dimensional spectra that show a characteristic six peak pattern from coherently excited ground and excited state vibrational modes. We model these features using a spectral density formalism and the vibronic features are assigned to Feynman pathways. We also examine the dynamics of a particular set of peaks demonstrating anticorrelated peak motion, a signature of oscillatory wavepacket dynamics on the ground and excited states. These dynamics, in concert with the general structure of vibronic two-dimensional spectra, can be used to distinguish between pure electronic and vibrational quantum coherences.

Photoemission band mapping with a tunable femtosecond source using nonequilibrium absorption resonances

The authors show that the tunability of a femtosecond optical parametric amplifier combined with its high-repetition rate and short pulses provide a powerful tool for an alternate approach to conventional nonresonant band mapping by two-photon photoemission (2PPE). The authors demonstrate this 2PPE mapping via use of two model systems, i.e., the pair of sp surface and image states on flat Cu(111) and vicinal Cu(775) surfaces, over a photon energy range of 3.9–4.6 eV by making use of direct resonant band-to-band electronic transitions. Since the experimental excitation of the Cu image state from the surface state is comparable in time to the electron-electron equilibration time, the authors measure sharp resonant features in the electron energy distributions. In this approach, the authors track these resonant electronic transitions using 2PPE by varying the photon energy so as to achieve resonant excitation at each value of photoelectron emission angle over a large wavelength range on both types of surfaces. In addition, the authors explore the range of photon energies and optical intensities which may be used for this approach. The repetition rate of this laser was sufficient to yield a good signal-to-noise ratio while maintaining pump pulse intensities at levels that were low enough to prevent significant photon-induced space-charge broadening and electron-kinetic-energy shifting, even for photon energies close to the work function of the sample.

Dominant phonon wavevectors of the 2D Raman mode of graphene

We investigate the dominant phonon wavevectors q* and the associated dominant phonon-assisted electronic transitions implied by the 2D Raman mode of graphene by combining ab initio calculations with a full two-dimensional integration over the graphene Brillouin zone. We find that q* are highly anisotropic and rotate with the polarizer:analyzer condition, providing access to the entire angular extent around K. The resonant electronic transitions do not lie along the line and can be transformed from being apparently “inner” to “outer” with the addition of a reciprocal lattice vector, showing that both are equivalent. We thus invalidate the notion of “inner” and “outer” processes completely.

Theoretical study on contribution of charge transfer effect to surface-enhanced Raman scattering spectra of pyridine adsorbed on Ag n ( n = 2–8) clusters

We investigate surface-enhanced Raman scattering (SERS) spectra of pyridine–Ag n ( n = 2–8) complexes by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In simulated normal Raman scattering (NRS) spectra, profiles of pyridine–Ag n ( n = 2–8) complexes are analogical with that of isolated pyridine. Nevertheless, calculated pre-SERS spectra are strongly dependent on electronic transition states of new complexes. Wavelengths at 335 nm, 394.8 nm, 316.9 nm and 342.6 nm, which are nearly resonant with pure charge transfer excitation states, are adopted as incident light when simulating pre-SERS spectra for pyridine–Ag n ( n = 2–8) complexes, respectively. We obtain enhancement factors from 10 3 to 10 5 in pre-SERS spectra compared with corresponding NRS spectra. The obvious increase in Raman intensities mainly result from charge transfer resonance Raman enhancement. A charge difference densities (CDDs) methodology is adopted in describing chemical enhancement mechanism. This methodology aims at visualizing charge transfer from Ag n ( n = 2–8) clusters to pyridine on resonant electronic transition, which is one of the most direct evidences for chemical enhancement mechanism.

DFT study of chemical mechanism of pre-SERS spectra in Pyrazine–metal complex and metal–Pyrazine–metal junction

The chemical mechanism of Normal Raman Scattering (NRS) and pre-surface enhanced Raman scattering (pre-SERS) spectra for Pyrazine–Ag 2 complex, Ag 2–Pyrazine–Ag 2 junction and Ag 2–Pyrazine–Au 2 junction were investigated with density functional theory (DFT) and charge difference densities (CDDs) for the first time. The NRS intensities of the above three structures enhanced obviously relative to isolated Pyrazine and the enhancement mechanism was confirmed to be static chemical enhancement. The pre-SERS intensities of the above three structures enhanced evidently compared to corresponding NRS intensities, and the enhancement mechanism was confirmed to charge transfer (CT) resonance Raman enhancement. The largest enhanced orders of NRS and pre-SERS intensities among the three structures were up to 10 3 and 10 5, respectively. Compared the intensity of pre-SERS with corresponding intensity of NRS spectra, the enhancement effect of Pyrazine–Ag 2 complex was larger than the others. Intramolecular and intermolecular CT on resonant electronic transition were described by CDDs.

Influence of the Resonant Electronic Transition on the Intensity of the Raman Radial Breathing Mode of Single Walled Carbon Nanotubes during Electrochemical Charging

In situ Raman pectroelectrochemistry has been used to probe the E 11 S and E 22 S electronic transitions of the semiconducting single walled carbon nanotube tube (6,5). These two transitions were investigated through the intensities of the radial breathing mode by using 1.16 and 2.18 eV laser excitation energies, respectively. The bleaching behavior of the (6,5) tube is similar for both laser excitation energies if the magnitude of the applied electrode potential is smaller than about ±0.5 V. At potentials outside the ±0.5 V window, the intensity/ potential profile is steeper for the 1.16 eV laser excitation. But a significant bleaching of the Raman signal is observed also for the 2.18 eV laser excitation energy even at mild potentials. This behavior shows that the filling of the E 11 S electronic state has a strong impact also on the E 22 S electronic transition. Hence, we suggest that a broadening of the resonance profile of E 22 S is a major reason for the change of the resonance enhancement of the Raman spectra before the Fermi level reaches the energy of E 22 S transition.

Direct visual evidence for chemical mechanisms of SERRS via charge transfer in Au20–pyrazine–Au20 junction

AbstractThe essence of the chemical mechanism for surface‐enhanced resonance Raman scattering (SERRS) is the charge transfer (CT) between the metal and the molecule at the resonant electronic transition, which results in the mode‐selective enhancement in the SERRS spectrum. The site‐orientated CT can directly interpret the mode‐selective chemical enhancement in SERRS. However, it is a great challenge to intutively visualize the orientation and site of the CT. In this paper, for the pyrazine–Au2 complex, a three‐dimensional (3D) cubic representation is built to provide direct visual evidence for chemical mechanisms of SERRS via CT from the Au2 cluster to pyrazine at the resonant electronic transition. The relationship between the mode‐selective enhancements in SERRS and the site‐orientated CT was clearly revealed. The intracluster excitation (analog of plasmon excitation in large naonoparticles) was also visualized by the 3D cubic presentation, which provided the direct evidence of local electromagnetic field enhancement of SERRS. To study the quantum size effect and the coupling effect of the nanoparticles, the photoexcitation mechanisms of the Au20–pyrazine complex and the Au20–pyrazine–Au20 junction were also investigated. The tunneling charge transfer from one Au20 cluster to another Au20 cluster outside the pyrazine in Au20–pyrazine–Au20 junction was also revealed visually. The calculated normalized extinction spectra of Au nanoparticles using the generalized Mie theory reveal that the resonance peak is red‐shifted due to the coupling between particles. Copyright © 2009 John Wiley & Sons, Ltd.

Direct Visualization of the Chemical Mechanism in SERRS of 4‐Aminothiophenol/Metal Complexes and Metal/4‐Aminothiophenol/Metal Junctions

Direct visualization of photoinduced tunneling charge transfer (TCT) in an Au(5)/para-aminothiophenol (PATP)/Ag(6) junction in which Au and Ag clusters form the first and second layer, respectively, is provided by the charge difference density (see picture; green and red stand for holes and electrons, respectively). We theoretically investigate the mechanism of chemical enhancement of surface-enhanced resonance Raman scattering (SERRS) of para-aminothiophenol (PATP)/metal complexes and metal/PATP/metal junctions. The method of charge difference density is used to visualize intracluster excitation and charge transfer (CT) between PATP and metal during the process of resonant electronic transitions. It is found that the selective enhancement of the b(2) mode in SERRS spectra result not only from Albrecht's A term (the Frank-Condon term), but also from the Herzberg-Teller term (Albrecht's B mechanism) via resonant CT. For the metal/PATP/metal junctions, the calculated results reveal that the Raman spectrum is of SERRS nature and the nontotally symmetric b(2) mode is strongly enhanced at the incident wavelength of 1064 nm when Au and Ag nanoparticles are the first and second layer, respectively, and the dominant enhancement mechanism is the Herzberg-Teller term in chemical enhancement via tunneling charge transfer (intervalence electron transfer from the Ag cluster to the Au cluster). When the first and second layers were inverted (i.e. the Ag and Au nanoparticles are the first and second layers, respectively), the Raman spectrum at an incident wavelength of 1064 nm is due to normal Raman scattering, and the nontotally symmetric b(2) mode is not strongly enhanced. Our theoretical results not only support the experimental findings, but also provide a clear physical interpretation.