Laser-excited States Research Articles

Coherent Vibrations Promote Charge-Transfer across a Graphene-Based Interface.

Discerning the impact of the coherent motion of the nuclei on the timing and efficiency of charge transfer at the donor-acceptor interface is essential for designing performance-enhanced optoelectronic devices. Here, we employ an experimental approach using photocurrent detection in coherent multidimensional spectroscopy to excite a donor aromatic macrocycle and collect the charge transferred to a 2D acceptor layer. For this purpose, we prepared a cobalt phthalocyanine-graphene (CoPc-Gr) interface. Unlike blends, the well-ordered architecture achieved through the physical separation of the two layers allows us to unambiguously collect the electrical signal from graphene alone and associate it with a microscopic understanding of the whole process. The CoPc-Gr interface exhibits an ultrafast electron-transfer signal, stemming from an interlayer mechanism. Remarkably, the signal presents an oscillating time evolution modulated by coherent vibrations originating from the laser-excited CoPc states. By performing Fourier analysis on the beatings and correlating it with the Raman features, along with a comprehensive first-principles characterization of the vibrational coupling in the CoPc excited states, we successfully identify both the orbitals and molecular vibrations that promote the charge transfer at the interface.

Full-scale ab initio simulations of laser-driven atomistic dynamics

The coupling of excited states and ionic dynamics is the basic and challenging point for the materials response at extreme conditions. In the laboratory, the intense laser produces transient nature and complexity with highly nonequilibrium states, making it extremely difficult and interesting for both experimental measurements and theoretical methods. With the inclusion of laser-excited states, we extend an ab initio method into the direct simulations of whole laser-driven microscopic dynamics from solid to liquid. We construct the framework of combining the electron-temperature-dependent deep neural-network potential energy surface with a hybrid atomistic-continuum approach, controlling non-adiabatic energy exchange and atomistic dynamics, which enables consistent interpretation of experimental data. By large-scale ab initio simulations, we demonstrate that the nonthermal effects introduced by hot electrons play a dominant role in modulating the lattice dynamics, thermodynamic pathway, and structural transformation. We highlight that the present work provides a path to realistic computational studies of laser-driven processes, thus bridging the gap between experiments and simulations.

State-resolved ultrafast charge and spin dynamics in [Co/Pd] multilayers

We use transient absorption spectroscopy with circularly polarized x rays to detect laser-excited hole states below the Fermi level and compare their dynamics with that of unoccupied states above the Fermi level in ferromagnetic [Co/Pd] multilayers. While below the Fermi level, an instantaneous and significantly stronger demagnetization is observed, above the Fermi level, the demagnetization is delayed by 35 ± 10 fs. This provides a direct visualization of how ultrafast demagnetization proceeds via initial spin-flip scattering of laser-excited holes to the subsequent formation of spin waves.

Zero-range-potential model for strong-field molecular processes: Dynamic polarizability and photodetachment cross section

We develop a general theoretical framework for the analytical description of the interaction of a model diatomic molecular system with an intense, arbitrarily polarized monochromatic laser field. The model molecule comprises an electron in the field of two zero-range potentials separated by the internuclear distance. This model has an exact analytical solution within the theoretical framework of the quasistationary quasienergy (Floquet) approach. In addition to the development of this general framework, we also present a detailed analysis of the weak-field limit, within which we obtain both the frequency-dependent polarizability and the angle-resolved photodetachment cross section for the model system. These fundamental properties are analyzed for both homonuclear and heteronuclear molecular systems in a linearly polarized laser field, for both ground and excited electronic states, and for arbitrary orientation of the molecular axis relative to the polarization vector of the laser field. The analytical expressions for the polarizability and angle-resolved photoelectron spectra exhibit characteristic double-slit interference patterns, allowing one to study their dependence on the parameters of the problem beyond the level of the Born approximation.

High-Yield Synthesis of Cubic and Hexagonal Boron Nitride Nanoparticles by Laser Chemical Vapor Decomposition of Borazine

We report a new method for the synthesis of boron nitride nanostructures (nBN) using laser chemical vapor decomposition (LCVD). Borazine was used as precursor and excited with two simultaneous radiations, the fundamental and second YAG laser harmonics. If only one of the two radiations is employed, no reaction takes place. Abundant BN powder is obtained after one hour of laser radiation. The BN yield obtained with the LCVD technique is about 83% by weight. The BN material was characterized using scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. They all indicate that the BN powder consists of a mixture of hexagonal and cubic BN nanostructures. No other BN phases or stoichiometries were found. The size of the resulting BN nanostructures is in the range of 20–100 nm and their B : N composition is 1 : 1. A simplified mechanism involving laser-excited states followed by photoinduced removal of hydrogen is proposed to understand the synthesis of BN nanopowder by LCVD of borazine.

Nonlinear polarization response of an atomic gas medium in the field of a high-intensity femtosecond laser pulse

Polarization response that appears in silver vapors in the field of a high-intensity femtosecond Ti:sapphire laser has been studied by the direct numerical integration of the time-dependent Schrodinger equation. The regions of applicability have been determined for perturbation theory and the power series expansion of the polarization in the field. The contribution of free electrons to the response at the frequency of the interacting field has been calculated, which is due to the photoionization process and limits the Kerr effect. An important contribution of laser-excited atomic states to nonlinear atomic responses of neutral atoms has been demonstrated.

Spectroscopy of Cs2, RbCs, and Rb2 in solid H4e

We present comparative experimental and theoretical studies of the absorption and fluorescence spectra of the alkali-metal dimer molecules Cs(2) and RbCs immersed in a solid helium matrix, thereby extending our recent observations of Rb(2) in solid (4)He. The laser-excited molecular states are mostly quenched by the interaction with the He matrix. The quenching efficiently populates the second lowest excited state of the molecule, i.e., (1) (3)Π((u)) that is metastable in the homonuclear dimers. Molecular excitation and emission bands are modeled by calculating Franck-Condon factors that give a reasonable agreement with the experimental findings.

Photoionization of laser-excited caesium atoms above the 4d ionization threshold

The photoionization of the ground state and 6p laser-excited caesium atoms was studied above their 4d ionization thresholds. The 4d photoelectron spectrum of 6p laser-excited atoms shows a stronger excitation of satellites upon ionization than its ground state counterpart. The relative intensities of satellite and main photolines show a slow variation with the incoming photon energy for both the ground state and the 6p laser-excited states. An assignment of the excited state spectra, supported by recently published ground state photoionization spectra and calculations, is given and a preliminary analysis of the 4d Auger spectrum of laser-excited atoms is also presented.

Emission spectrum and relaxation kinetics of SO 2 induced by 266 nm laser

Laser induced fluorescence (LIF) emission spectrum of SO 2 in the range of 270.0–470.0 nm has been obtained with the quadruple harmonic output (266 nm) of a pulsed Nd:YAG laser as excitation source. The spectrum is composed of a continuous envelope in the short wavelength side, while it shows the character of banded structure superimposed on a continuous one in the long wavelength region. Fluorescence emission from the hybrid states of A 1A 2 + B 1B 1 and X 1A 1 + B 1B 1 forms the continuous envelope and phosphorescence emission from the triplet state a 3B 1 forms the banded progression. It is also found that direct emission from laser excited states is very weak. The primary portion of the emission is from the energy levels populated by collision relaxation or collision induced intersystem crossing process. The harmonic frequencies and inharmonic coefficients of the symmetric stretching vibration and the bending vibration of X 1A 1 state are derived from the ascription of the phosphorescence progression.

Quantitative nanoparticle structures from electron crystallography data

We describe the quantitative refinement of nanoparticle structures from gold nanoparticles probed by ultrafast electron crystallography (UEC). We establish the equivalence between the modified radial distribution function employed in UEC and the atomic pair distribution function (PDF) used in x-ray and neutron powder diffraction analysis. By leveraging PDF refinement techniques, we demonstrate that UEC data are of sufficient quality to differentiate between cuboctahedral, decahedral and icosahedral nanoparticle models. Furthermore, we identify the signatures of systematic errors that may occur during data reduction and show that atomic positions refined from UEC are robust to these errors. This work serves as a foundation for reliable quantitative structural analysis of time-resolved laser-excited nanoparticle states.

Electron-correlation-induced interchange of lifetime broadenings of5s−1multiplet states in atomic Cs

The lifetime broadenings of the most intense $5s$ photolines of Cs were studied. The aim of this study was to understand the origin of the remarkable differences in the lifetime widths of different $5{s}^{\ensuremath{-}1}$ multiplet states. The $5s$ photoelectron spectra of the $6s\ensuremath{\rightarrow}6{p}_{1/2,3/2}$ laser-excited states are presented in the binding energy region of 30--34 eV showing the main $5s$ photolines of the ground state and laser-excited states as well as the $5s5{p}^{6}6p$ shakeup satellites. The lifetime widths of the states were determined. The Hartree-Fock calculations were carried out to predict the $5s$ photoelectron spectrum. The lifetime broadenings of the $5s$ photolines were found to be due to the transition rates of the subsequent Coster-Kronig transitions to the $5{s}^{2}5{p}^{5}$ states. It was found that a straightforward single-configuration explanation cannot be given for the lifetime differences. Instead, electron correlation plays an essential role and the lifetime broadenings were found to be predicted well only when configuration interaction between the $5{s}^{\ensuremath{-}1}$ and $5{p}^{\ensuremath{-}2}5d$ configurations is taking into account.

Spectroscopy ofRb2dimers in solidHe4

We present experimental and theoretical studies of the absorption, emission and photodissociation spectra of Rb$_{2}$ molecules in solid helium. We have identified 11 absorption bands of Rb$_{2}$. All laser-excited molecular states are quenched by the interaction with the He matrix. The quenching results in efficient population of a metastable (1)$^{3}\Pi_{u}$ state, which emits fluorescence at 1042 nm. In order to explain the fluorescence at the forbidden transition and its time dependence we propose a new molecular exciplex Rb$_{2}(^{3}\Pi_{u})$He$_{2}$. We have also found evidence for the formation of diatomic bubble states following photodissociation of Rb$_{2}$.

Excited-state photoelectron spectroscopy of excitons inC60and photopolymerizedC60films

Laser-excited states in films of pristine ${\mathrm{C}}_{60}$ and photopolymerized ${\mathrm{C}}_{60}$ $(pp\ensuremath{-}{\mathrm{C}}_{60})$ prepared in ultrahigh vacuum have been characterized in situ with pump-probe photoelectron spectroscopy using both synchrotron radiation and picosecond laser sources. Photoelectron spectra of singlet ${(S}_{1})$ and triplet ${(T}_{1})$ excitons overlap because of vibrational broadening in the photoemission final state. The spectra have been individually isolated in $pp\ensuremath{-}{\mathrm{C}}_{60}$ with time-resolved methods and are split by 0.33 eV. Signals from pristine ${\mathrm{C}}_{60}$ are weaker but are spectroscopically similar. The origin for exciton transitions for both ${\mathrm{C}}_{60}$ and $pp\ensuremath{-}{\mathrm{C}}_{60}$ is found to be properly located near the maximum of the highest occupied molecular orbital. Increasing excitation density favors ${T}_{1}$ production over ${S}_{1}$ such that, at high exciton concentrations, ${T}_{1}$ states predominate, even at times much shorter than the unimolecular intersystem crossing time, measured to be \ensuremath{\sim}2.5 ns for $pp\ensuremath{-}{\mathrm{C}}_{60}$ at 81 K. A weaker photoemission band located \ensuremath{\sim}0.5 eV above ${S}_{1}$ is also observed and is attributed to charge carriers and/or charge-transfer excitons.

Strong temperature dependence of laser-enhanced charge transfer in collisions of sodium clusters with sodium atoms

We have performed a combined experimental and theoretical study of the charge transfer for 5-keV ${\mathrm{Na}}_{n}^{+}$ cluster-ion collisions with Na atoms in their ground and laser-excited states. The theoretical analysis of the laser-enhanced charge-transfer probability, based on a self-consistent treatment of both charge-transfer and collision-induced fragmentation, shows a crucial dependence on the energy content of the ${\mathrm{Na}}_{n}^{+}$ clusters before the collision. Quantitative information on this energy is obtained by comparing experimental data with theoretical results.

Picosecond investigation of the collisional deactivation of OH A^2Σ^+(v′ = 1, N′ = 4, 12) in an atmospheric-pressure flame

The collisional deactivation of the laser excited states A 2Sigma+(v' = 1, N' = 4, 12) of OH in a flame is studied by measurement of spectrally resolved fluorescence decays in the picosecond time domain. Quenching and depolarization rates, as well as vibrational energy-transfer (VET) and rotational energy-transfer (RET) rates are determined. An empirical model describes the temporal evolution of the quenching and VET rates that emerge from the rotational-state relaxation. Fitting this model to the measured 1-0 and 0-0 fluorescence decays yields the quenching and VET rates of the initially excited rotational state along with those that correspond to a rotationally equilibrated vibronic-state population. VET from the higher rotational state (N' = 12) shows a tendency for resonant transitions to energetic close-lying levels. RET is investigated by analysis of the temporal evolution of the 1-1 emission band. The observed RET is well described by the energy-corrected sudden-approximation theory in conjunction with a power-gap law.

Pump-probe anisotropies of Fenna-Matthews-Olson protein trimers from Chlorobium tepidum: a diagnostic for exciton localization?

Exciton calculations on symmetric and asymmetric Fenna-Matthews-Olson (FMO) trimers, combined with absorption difference anisotropy measurements on FMO trimers from the green bacterium Chlorobium tepidum, suggest that real samples exhibit sufficient diagonal energy disorder so that their laser-excited exciton states are noticeably localized. Our observed anisotropies are clearly inconsistent with 21-pigment exciton simulations based on a threefold-symmetric FMO protein. They are more consistent with a 7-pigment model that assumes that the laser-prepared states are localized within a subunit of the trimer. Differential diagonal energy shifts of 50 cm(-1) between symmetry-related pigments in different subunits are large enough to cause sharp localization in the stationary states; these shifts are commensurate with the approximately 95 cm(-1) inhomogeneous linewidth of the lowest exciton levels. Experimental anisotropies (and by implication steady-state linear and circular dichroism) likely arise from statistical averaging over states with widely contrasting values of these observables, in consequence of their sensitivity to diagonal energy disorder.

Charge asymmetry in collisional depopulation of Na 36s Rydberg atoms

Impact of low-velocity charged particles on laser-excited 36s Rydberg states of Na in externally applied static electric fields up to 28 V cm -1 drives transitions into the neighbouring Stark energy levels, and the final-state distributions may be inferred by state-selective field ionization (SFI). We have bombarded a Na(36s) target with three positive projectile ions (Ar+, K+ and Na+) and two negative projectile ions (S- and Cl-) at identical velocities equal to the Bohr orbital velocity of the electron in the initial Rydberg state. The resulting final-state distributions of the sodium Rydberg atom differ significantly between positive and negative ion impact at intermediate values of the field, but differences among ions of the same charge are much smaller. We suggest an interference mechanism as the cause of this charge asymmetry.

Enhanced electron attachment to superexcited states of nitric oxide

We report the observation of enhanced electron attachment to laser-excited superexcited states of nitric oxide. These states were produced in multiphoton excitation processes via (i) real and virtual states, and (ii) only virtual states. The attaching electrons were produced in situ via concomitant multiphoton ionization. Several orders-of-magnitude enhancement in the electron-attachment rate constant was observed for the superexcited states compared to the ground electronic state.

Pure state Ne*(*)[| J, M⪢]-preparation in a magnetic field: Polarization effects in ionizing collisions

A compact device (lengh 175 mm) has been built for the energy resolved detection of low energy (1–5 eV) Penning electrons with a 2π solid angle collection efficiency, based on the principle of adiabatic parallelisation of electron motion in a diverging magnetic field. A retarding field analysis is then used as a high pass filter to discriminate between Penning electrons released in collisions of rare gas atoms in metastable and shortlived, laser excited states. The overall detection efficiency is 0.13. The Zeeman-splitting of the atomic levels in the scattering center (maximum B = 222 G) allows the preparation of single magnetic substates | J, M⪢ B . By rotating the detector in the collision plane, well defined | J, M⪢ g states can be produced with respect to the relative velocity g, the quantization axis relevant for the collisions. The system has been tested by measuring the collision energy dependence of polarized-atom cross sections J Q | M| for the Ne* [ 3P 2]-Ar and Ne** [ 3D 3]-Ar systems. For the Ne* [ 3P 2] metastable atoms we find 2 Q 0/ 2 Q 2 = 1.55 ± 0.06 and 1.05 ± 0.06 in the thermal and superthermal energy range, respectively, which should be compared to 1.30 of Bregel et al. at thermal energies. For the Ne** [ 3D 3] state we find 3 Q 0,1/ 3 Q 2,3 = 1.65 ± 0.06 and 1.00 ± 0.10 for the same energy ranges.

Absolute cross sections for photoionization of laser-excited BaI states measured on a thermionic diode.

We describe a method that allows the measurement of absolute photoionization cross sections with a simple experimental setup. In a barium oven designed as a thermionic diode, ions are produced by use of a two-step pulsed-laser excitation and ionization combined with optogalvanic detection. The variation of the photoionization cross section of the excited BaI 6s6p /sup 1/P/sup 0//sub 1/ state is measured versus wavelength. At threshold the cross section is (5 +- 2) x 10/sup -20/ m/sup 2/. In the experiment strong evidence for the existence of Ba/sub 2//sup +/ dimers was found.