One-photon Process Research Articles

An Orbital Basis Set for Double Photoionization of Atoms and Molecules.

The ab initio theoretical treatment of one-photon double photoionization processes has been limited to atoms and diatomic molecules by the challenges posed by large grid-based representations of the double ionized continuum wave function. To provide a path for extensions to polyatomics, an energy-adapted orbital basis approach is demonstrated that reduces the dimensions of such representations and simultaneously allows larger time steps in time-dependent computational descriptions of double ionization. Additionally, an algorithm that exploits the diagonal nature of the two-electron integrals in the grid basis and dramatically accelerates the transformation between grid and orbital representations is presented. Excellent agreement between the present results and benchmark theoretical calculations is found for H- and Be atoms, as well as the hydrogen molecule, including for the triply differential cross sections that relate the angular distribution and energy sharing of all of the particles in the molecular frame.

Surface Enhanced Nonlinear Raman Processes for Advanced Vibrational Probing.

Surface enhanced Raman scattering (SERS) is not restricted to the well-known one-photon excited spontaneous Raman process that gives information on molecular composition, structure, and interaction through vibrational probing with high sensitivity. The enhancement mainly originates in high local fields, specifically those provided by localized surface plasmon resonances of metal nanostructures. High local fields can particularly support nonlinear Raman scattering, as it depends on the fields to higher powers. By revealing plasmon-molecule interactions, nonlinear Raman processes provide a very sensitive access to the properties of metal nanomaterials and their interfaces with molecules and other materials. This Perspective discusses plasmon-enhanced spontaneous and coherent nonlinear Raman scattering with the aim of identifying advantages that lead to an advanced vibrational characterization of such systems. The discussion will highlight the aspects of vibrational information that can be gained based on specific advantages of different incoherent and coherent Raman scattering and their surface enhancement. While the incoherent process of surface enhanced hyper Raman scattering (SEHRS) gives highly selective and spectral information complementary to SERS, the incoherent process of surface enhanced pumped anti-Stokes Raman scattering (SEPARS) can help to infer effective nonresonant SERS cross sections and allows to see "hot" vibrational transitions. Surface enhanced coherent anti-Stokes Raman scattering (SECARS) and surface enhanced stimulated Raman scattering (SESRS) combine the advantages of high local fields and coherence, which gives rise to high detection sensitivity and offers possibilities to explore molecule-plasmon interactions for a comprehensive characterization of composite and hybrid structures in materials research, catalysis, and nanobiophotonics.

Photodesorption efficiency of OH radical on the ice surface in the wavelength range from ultraviolet to visible

The photodesorption efficiencies of the hydroxyl (OH) radical from the water ice surface were measured in the range of 310–700 nm for the first time. Although isolated H2O molecules and OH radicals do not absorb visible photons, the photodesorption of OH interacting with the ice surface was found to occur by a one-photon process in the entire visible range. The photodesorption efficiency strongly depended on the wavelength, and the analyses of photodesorption cross sections indicated that only strongly bound OH radicals were desorbed at longer wavelengths, whereas both weakly and strongly bound OH radicals were desorbed at shorter wavelengths.

QED corrections to the correlated relativistic energy: One-photon processes.

This work is a collection of initial calculations and formal considerations within the Salpeter-Sucher exact equal-time relativistic quantum electrodynamics framework. The calculations are carried out as preparation for the computation of pair, retardation, and radiative corrections to the relativistic energy of correlated two-spin-1/2-fermion systems. In this work, particular attention is paid to the retardation and the "one-loop" self-energy corrections, which are known to be among the largest corrections to the correlated relativistic energy. The theoretical development is supplemented with identifying formal connections to the non-relativistic quantum electrodynamics framework, which is based on a correlated but non-relativistic reference, as well as to the "1/Z approach," which is built on a relativistic but independent-particle zeroth order. The two complementary directions currently provide the theoretical framework for light atomic-molecular precision spectroscopy and heavy-atom phenomena. The present theoretical efforts pave the way for relativistic QED corrections to (explicitly) correlated relativistic computations.

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene derivative

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene (fDAE) derivative were investigated by time-resolved absorption and fluorescence spectroscopies. Prescreening with quantum chemical calculation predicted that a derivative with methylthienyl groups (mt-fDAE) in the closed-ring isomer has a two-photon absorption cross-section larger than 1000 GM, which was experimentally verified by Z-scan measurements and excitation power dependence in transient absorption. Comparison of transient absorption spectra under one-photon and simultaneous two-photon excitation conditions revealed that the closed-ring isomer of mt-fDAE populated into higher excited states deactivates following three pathways on a timescale of ca. 200 fs: (i) the cycloreversion reaction more efficient than that by the one-photon process, (ii) internal conversion into the S1 state, and (iii) relaxation into a lower state (S1’ state) different from the S1 state. Time-resolved fluorescence measurements demonstrated that this S1’ state is relaxed to the S1 state with the large emission probability. These findings obtained in the present work contribute to extension of the ON–OFF switching capability of fDAE to the biological window and application to super-resolution fluorescence imaging in a two-photon manner.Graphical

Tuning the linear optical properties and two-photon absorption on the cinnamylideneacetophenone derivatives: The effect of methoxy group by a experimental and theoretical point of view

The present work studied the effect of the methoxy group position in two cinnamylideneacetophenone (CA) derivatives on both one-photon (1PA) and two-photon absorption (2PA) processes. Our outcomes exhibit a dependence on the methoxy group position spectrally and in magnitude for both. A decrease in the lower energy 1PA band of 0.23eV was observed when the methoxy group was moved to the more conjugated part of the cinnamaldehyde molecule. The opposite was also observed for the next higher absorption band. The Quantum Chemical Calculations (QCCs), besides a good agreement between the experimental and simulated 1PA spectra, except for the magnitude of the high energy absorption band, showed that the effect behind the observed experimental red-shift of the lower absorption band is caused by the higher charge transfer when the methoxy group is bounded to the longer conjugated part of the cinnamaldehyde derivative. For experimental 2PA spectra, the higher energy band is strongly affected by the presence of the methoxy group, pointing out an increase of about 56 % in the values of the two-photon absorption cross-section (2PACS). Furthermore, the phenomenological Sum-Over-Essential States (SOS) approach and QCCs were performed for further clarification in the 2PACS experimental results. Hence, our findings demonstrate that changing the methoxy position on opposite sides of the CA may open up a novel insight for designing new organic molecules, particularly when seeking to enhance the values of 2PACS significantly of CA derivatives.

The Norbornadiene/Quadricyclane Pair as Molecular Solar Thermal Energy Storage System: Surface Science Investigations.

For the transition to renewable energy sources, novel energy storage materials are more important than ever. This review addresses so-called molecular solar thermal (MOST) systems, which appear very promising since they combine light harvesting and energy storing in one-photon one-molecule processes. The focus is on norbornadiene (NBD), a particularly interesting candidate, which is converted to the strained valence isomer quadricyclane (QC) upon irradiation. The stored energy can be released on demand. The energy-releasing cycloreversion from QC to NBD can be initiated by a thermal, catalytic, or electrochemical trigger. The reversibility of the energy storage and release cycles determines the general practicality of a MOST system. In the search for derivatives, which enable large-scale applications, fundamental surface science studies help to assess the feasibility of potential substituted NBD/QC couples. We include investigations under well-defined ultra-high vacuum (UHV) conditions as well as experiments in liquid phase. Next to the influence of the catalytically active surfaces on the isomerization between the two valence isomers, information on adsorption geometries, thermal stability limits, and reaction pathways of the respective molecules are discussed. Moreover, laboratory-scaled test devices demonstrate the proof of concept in various areas of application.

Se-substituted pentamethine cyanine for anticancer photodynamic therapy mediated using the hot band absorption process

Photodynamic therapy (PDT) is a promising cancer treatment modality owing to its high spatiotemporal selectivity and noninvasive nature. However, conventional photosensitizers (PSs) used in PDT are responsive only to visible light, which makes them unsuitable for tissue penetration. In this study, we propose a PS based on hot band absorption (HBA), which can be triggered by anti-Stokes light at 808 nm via a one-photon process. The introduction of selenium (Se) into pentamethine cyanine (Secy5) not only facilitates intersystem crossing for reactive oxygen species (ROS) production but also enhances HBA efficiency, thereby prolonging the excitation wavelength. In addition, Secy5 demonstrates excellent biocompatibility, unlike its I-substituted counterpart (Icy5), and produces not only 1O2 but also O2•−, making it a desirable candidate for treating hypoxic solid tumors. According to the results of in vivo and in vitro experiments, Secy5 can efficiently inhibit cancer cell growth via anti-Stokes activation processes, thereby providing a novel approach to design anti-Stokes excitation PSs for anticancer treatment.

Upconversion photoluminescence of monolayer WSe2 with biaxial strain tuning.

Mechanical strain can be used to tune the optical properties of monolayer transition metal dichalcogenides (1L-TMDs). Here, upconversion photoluminescence (UPL) from 1L-WSe2 flakes is tuned with biaxial strain induced by cruciform bending and indentation method. It is found that the peak position of UPL is redshifted by around 24 nm as the applied biaxial strain increases from 0% to 0.51%. At the same time, the UPL intensity increases exponentially for the upconversion energy difference that lies within a broad range between -157 meV to -37 meV. The observed linear and sublinear power dependence of UPL emission in 1L-WSe2 with and without biaxial strain at three different excitation wavelengths of 784 nm, 800 nm, and 820 nm indicates the multiphonon-assisted one-photon upconversion emission process. The results of strain-dependent UPL emission from 1L-TMDs pave a unique path to the advances in photon upconversion applications and optoelectronic devices.

Stark effect and orbital hybridization of moiré interlayer excitons in the MoSe2/WSe2 heterobilayer.

In this paper, we undertake a theoretical investigation into the effects of both in-plane and out-of-plane static electric fields on moiré interlayer excitons (IXs) within a WSe2/MoSe2 heterobilayer. We thoroughly analyze a wide range of properties pertaining to the IXs, including the binding energy, Stark shift, orbital hybridization, photoluminescence (PL) spectra, and radiative lifetime. Various factors influencing IX behavior, such as the dielectric environment, spacing separation, and moiré trap effects, are examined in detail. Our results demonstrate that the in-plane electric field leads to energy splitting between states with non-zero angular momentum, such as the 2p± dark states. Consequently, we analyze IX orbital hybridization, including hybrid Rydberg states like 1s, 2p±, and 2s. In contrast, we show that an out-of-plane electric field induced by a double-gate setup causes a quadratic Stark effect on the center of mass (COM) eigenenergies, leading to energy splitting of degenerate states and resulting in orbital hybridization of COM eigenvectors. Additionally, we demonstrate that a parallel electric field brightens the 2p± dark state through a one-photon PL process, due to the hybridization phenomena between s- and p-type Rydberg states. In short, our investigation is in great agreement with previous research and can assist experimenters in designing novel optoelectronic applications, such as on-chip electro-optic modulators and TeraHertz devices.

Theoretical study on photoinduced charge transfer in one-photon and two-photon absorption of branching oligofluorenes

Oligofluorenes have been identified as very promising two-photon absorption (TPA) materials and present great application potential for the fabrication of nonlinear optical devices, but the TPA mechanism and corresponding electron excitation properties have not been studied. Here, the photoinduced charge transfer characteristics of V-shaped and Y-shaped branching oligofluorenes that consist of two and three fluorene units in each branch during one-photon absorption (OPA) and TPA processes are analyzed theoretically using the density functional theory and visualization sum-over-states model. The calculated results show that the OPA intensity and TPA cross-section are significantly enhanced by increasing the branch length or changing the structure from V-shaped to Y-shaped. The long-distance charge transfer only occurs on the second transition of TPA at high excited states. Compared to Y-shaped molecules, V-shaped structures exhibit a stronger cooperative effect among the different branches.

Multifunctional Characteristics of BCTH:0.5% Sm3+ Ceramics Prepared via Hydrothermal Method and Powder Injection Molding

Briefly, 0.005-mol Sm3+-doped (Ba0.85Ca0.15)(Ti0.9Hf0.1)O3 ([(Ba0.85Ca0.15)0.995Sm0.005](Ti0.9Hf0.1)O3, BCTH:0.005Sm3+) lead-free ceramics were prepared via hydrothermal method and powder injection molding using paraffin and oleic acid as binders, and the effects of preparation method and sintering conditions on microstructure, dielectric behavior and optical properties were investigated. XRD Rietveld refinement reveals the coexistence of orthogonal, rhombohedral and tetragonal phases, in which the crystal structure and phase fraction are influenced greatly by sintering temperature and holding time. The ceramics present enhanced relaxor behavior and frequency dispersion phenomenon as compared with those prepared by the solid-state sintering method, and the diffusive index γ value is within 1.421–1.673. The transition mechanism and luminescence performance of BCTH:0.005 Sm3+ were analyzed by Blasse formula, photoluminescence spectrum and fluorescence lifetimes, where emission peaks show slight blueshift, fluorescence decay lifetime becomes shorter, electric multipole interaction dominates the energy transfer mechanism, and the down-conversion luminescence is one-photon absorption process. The CIE chromaticity color coordinate (0.4746, 0.5048), correlated color temperature 3134 K and color purity 93.58% are achieved, which reveals that the BCTH:0.005 Sm3+ ceramics express high quality yellow emission rather than orange-red light of the hydrothermal method synthesized nano-powder, and have potential application in optical field.

Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots

Semiconductor quantum dots have emerged as an especially promising platform for the generation of polarization-entangled photon pairs. However, it was demonstrated recently that the two-photon excitation scheme employed in state-of-the-art experiments limits the achievable degree of entanglement by introducing which-path information. In this work, the combined impact of two-photon excitation and longitudinal acoustic phonons on photon pairs emitted by strongly-confining quantum dots is investigated. It is found that phonons further reduce the achievable degree of entanglement even in the limit of vanishing temperature due to phonon-induced pure dephasing and phonon-assisted one-photon processes, which increase the reexcitation probability. In addition, the degree of entanglement, as measured by the concurrence, decreases with rising temperature and/or pulse duration, even if the excitonic fine-structure splitting is absent and when higher electronic states are out of reach. Furthermore, in the case of finite fine-structure splittings, phonons enlarge the discrepancy in concurrence for different laser polarizations.

Reaction dynamics of molecules in highly electronically excited states attained by multiphoton and multiple excitation methods

Abstract Multiphoton absorption and multiple excitation can lead to the formation of highly electronically excited states of molecules. We have been applying these excitation methods to explore specific photochemical reactions, which are rather difficult to attain by normal one-photon absorption processes. In the present review, we will introduce several examples of these photochemical responses specific to highly excited state in the condensed phase, such as two-photon-gated cycloreversion, one-color control of both reactions in photochromic systems and rapid capture of an electron ejected from the higher excited state leading to rapid generation of charge-separated states at the high energy level with a lifetime much longer than microseconds.

Structural, morphological, and non-linear optical properties of thermally evaporated neodymium doped zinc oxide thin films

The thermal evaporation procedure has been used in the current work to synthesize various compositions of neodymium-doped zinc oxide thin films with varied thicknesses on glass substrates. The Films have a hexagonal wurtzite structure, according to x-ray Diffraction (XRD) study, and the structure has not changed after doping of Neodymium. Raman spectroscopy revealed peaks at 100.49, 331, 434,574, and 581 cm−1 that supported the ZnO phase, and the intensity of the peaks reduced as the number of dopants rose. FTIR analysis verified that Nd had completely dispersed in ZnO. The calculations of the bandgap (Eg) and absorption coefficient (α) are done using absorption spectra. The Z-Scan method has been employed to assess nonlinear optical characteristics for all Nd-doped ZnO thin films. The samples show the change from saturable absorption (SA) to reverse saturable absorption (RSA), the process of focusing to self-defocusing, and vice versa. This is due to the fact that the two-photon process is more potent than the one-photon process. The acquired value of the third-order optical non-linear susceptibility χ (3) is of the order of 10−5 to 10 −6 (e.s.u.) that makes the samples suitable for use in various photonic applications like using as an optical limiter and/or optical switch due to higher values of optical non-linear parameters.

Influence of nitro group on solvatochromism, nonlinear optical properties of 3-morpholinobenzanthrone: Experimental and theoretical study

We report the synthesis of 3-morpholino-9-nitrobenzanthrone (NMB) dye. By using FT-IR, 1H NMR, and X-ray crystallography, we characterized the molecular structure of the synthesized dye. The structural features of the dye were compared to those of an identical dye without nitro substituents. The push pull kind of benzanthrone dye (NMB) was examined for solvent-dependent photophysical properties. NMB exhibits a large Stokes shift. The polarity plots indicate that hydrogen-bonding interactions are influential in most of the solvents. Detailed structural analysis as well as one-photon absorption and emission process was computationally investigated for both the molecules. NMB exhibits significant two photon absorption (TPA) activity at various electron transitions ranging from visible to NIR regions. Further, Nonlinear absorption coefficient was calculated for the dye samples from the open aperture Z-scan data. The nonlinear absorption coefficient values were obtained for the dye samples for input laser pulse energies ranging from 5 to 20 µJ. The results decipher that these dye samples are prospective contenders for the fabrication of optical limiter devices. Nitro substituents at the active site enhance charge transfer, thereby enhancing the NLO response.

Electrocatalytic Energy Release of Norbornadiene-Based Molecular Solar Thermal Systems: Tuning the Electrochemical Stability by Molecular Design.

Molecular solar thermal (MOST) systems, such as the norbornadiene/quadricyclane (NBD/QC) couple, combine solar energy conversion, storage, and release in a simple one-photon one-molecule process. Triggering the energy release electrochemically enables high control of the process, high selectivity, and reversibility. In this work, the influence of the molecular design of the MOST couple on the electrochemically triggered back-conversion reaction was addressed for the first time. The MOST systems phenyl-ethyl ester-NBD/QC (NBD1/QC1) and p-methoxyphenyl-ethyl ester-NBD/QC (NBD2/QC2) were investigated by in-situ photoelectrochemical infrared spectroscopy, voltammetry, and density functional theory modelling. For QC1, partial decomposition (40 %) was observed upon back-conversion and along with a voltammetric peak at 0.6 Vfc , which was assigned primarily to decomposition. The back-conversion of QC2, however, occurred without detectable side products, and the corresponding peak at 0.45 Vfc was weaker by a factor of 10. It was concluded that the electrochemical stability of a NBD/QC couple is easy tunable by simple structural changes. Furthermore, the charge input and, therefore, the current for the electrochemically triggered energy release is very low, which ensures a high overall efficiency of the MOST system.

Electrochemically Triggered Energy Release from an Azothiophene-Based Molecular Solar Thermal System.

Molecular solar thermal (MOST) systems combine solar energy conversion, storage, and release in simple one-photon one-molecule processes. Here, we address the electrochemically triggered energy release from an azothiophene-based MOST system by photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) and density functional theory (DFT). Specifically, the electrochemically triggered back-reaction from the energy rich (Z)-3-cyanophenylazothiophene to its energy lean (E)-isomer using highly oriented pyrolytic graphite (HOPG) as the working electrode was studied. Theory predicts that two reaction channels are accessible, an oxidative one (hole-catalyzed) and a reductive one (electron-catalyzed). Experimentally it was found that the photo-isomer decomposes during hole-catalyzed energy release. Electrochemically triggered back-conversion was possible, however, through the electron-catalyzed reaction channel. The reaction rate could be tuned by the electrode potential within two orders of magnitude. It was shown that the MOST system withstands 100 conversion cycles without detectable decomposition of the photoswitch. After 100 cycles, the photochemical conversion was still quantitative and the electrochemically triggered back-reaction reached 94 % of the original conversion level.

Study on the mechanism of blue light generation by ultra-broadband absorption in Rb–He mixture

In the ultra-broadband range, by continuously tuning the wavelength of the pump laser to excite the rubidium-helium (Rb–He) mixture, an obvious blue fluorescence signal can always be detected. Through the experimental study of the difference between resonant excitation and non-resonant excitation to produce blue fluorescence, it is verified that Rb–He excimer plays an important role in ultra-broadband absorption, and five mechanisms for producing blue fluorescence are summarized: (1) excimer + one-photon resonant excitation process from 5P; (2) two-photon resonant excitation process from 5S; (3) one-photon resonant excitation process from 5S + energy pooling process; (4) excimer + one-photon resonant excitation process from 4D; (5) excimer + energy pooling process. The influence of ionization on the generation of blue fluorescence is discussed. Under several typical pump wavelengths (based on different mechanisms of generating 420 nm fluorescence), the variation of 420 nm fluorescence with the Rb vapor atom concentration, the characteristics of 420 nm collimated light, and its competition with the fluorescence were discussed. The ultra-broadband absorption properties of the alkali metal-noble gas system can be applied to the detection of infrared signals.

Two-photon annihilation of positrons with K -shell electrons of H-like ions

The two-photon annihilation of a positron with an electron bound in the $1s$ state of a H-like ion is calculated within the fully relativistic QED framework. The interaction with the nucleus is treated nonperturbatively, thus allowing the calculations to be carried out for the annihilation with strongly bound inner shells of heavy ions. Infrared divergences, appearing when one of the emitted photons approaches the low-frequency limit, are accurately eliminated from final expressions. The total cross section of the two-photon and one-photon annihilation processes are compared for a wide range of collision energies and nuclear charge numbers. It is demonstrated that the two-photon annihilation channel dominates over the one-photon channel for the low- and medium-$Z$ ions, whereas for the high-$Z$ ions the situation reverses.