Population Of Electronic States Research Articles

Integral cross section measurements and product recoil velocity distributions of Xe(2+) + N2 hyperthermal charge-transfer collisions.

Charge exchange from doubly charged rare gas cations to simple diatomics proceeds with a large cross section and results in populations of many vibrational and electronic product states. The charge exchange between Xe(2+) and N2, in particular, is known to create N2 (+) in both the A and B electronic states. In this work, we present integral charge exchange cross section measurements of the Xe(2+) + N2 reaction as well as axial recoil velocity distributions of the Xe(+) and N2 (+) product ions for collision energies between 0.3 and 100 eV in the center-of-mass (COM) frame. Total charge-exchange cross sections decrease from 70 Å(2) to about 40 Å(2) with increasing collision energy through this range. Analysis of the axial velocity distributions indicates that a Xe(2+) - N2 complex exists at low collision energies but is absent by 17.6 eV COM. Analysis of the axial velocity distributions reveals evidence for complexes with lifetimes comparable to the rotational period at low collision energies. The velocity distributions are consistent with quasi-resonant single charge transfer at high collision energies.

Instantaneous ballistic velocity of suspended Brownian nanocrystals measured by upconversion nanothermometry.

Brownian motion is one of the most fascinating phenomena in nature. Its conceptual implications have a profound impact in almost every field of science and even economics, from dissipative processes in thermodynamic systems, gene therapy in biomedical research, artificial motors and galaxy formation to the behaviour of stock prices. However, despite extensive experimental investigations, the basic microscopic knowledge of prototypical systems such as colloidal particles in a fluid is still far from being complete. This is particularly the case for the measurement of the particles' instantaneous velocities, elusive due to the rapid random movements on extremely short timescales. Here, we report the measurement of the instantaneous ballistic velocity of Brownian nanocrystals suspended in both aqueous and organic solvents. To achieve this, we develop a technique based on upconversion nanothermometry. We find that the population of excited electronic states in NaYF4:Yb/Er nanocrystals at thermal equilibrium can be used for temperature mapping of the nanofluid with great thermal sensitivity (1.15% K-1 at 296 K) and a high spatial resolution (<1 μm). A distinct correlation between the heat flux in the nanofluid and the temporal evolution of Er3+ emission allows us to measure the instantaneous velocity of nanocrystals with different sizes and shapes.

Photoassociation dynamics driven by second- and third-order phase-modulated laser fields

We investigate theoretically the photoassociation dynamics of ultracold 85Rb atoms driven by second- and third-order phase-modulated laser fields. The interplay between the second-order and third-order terms of the phase-modulated pulse has an obvious influence on photoassociation dynamics. The different combinations of the second-order and third-order phase coefficients lead to different pulse shapes. Most of the molecular population in the excited electronic state driven only by the third-order phase pulses can be distributed in a single vibrational level. The second-order term of the phase-modulated pulse can change the instantaneous frequency, and therefore the final population is distributed on several resonant vibrational levels, instead of concentrating on a single level. Although the second- and third-order phase-modulated pulse covers more resonant vibrational levels, the total population on the resonant vibrational levels is much smaller than that controlled only by the third-order phase pulse. In particular, the third-order term of the phase-modulated pulse can weaken the ‘multiple interaction’ to some degree.

Interference asymmetry of molecular frame photoelectron angular distributions in bichromatic UV ionization processes

We investigate molecular photoionization by ultrafast bichromatic linearly polarized UV laser pulses at frequencies perpendicular to the internuclear axis R involving π orbital excitation. Results from numerical solutions of time dependent Schrödinger equations for aligned show that molecular frame photoelectron angular distributions (MFPADs) exhibit signatures of asymmetry perpendicular to the molecular symmetry axis, arising from interference of coherent electron wave packets created by respectively one and two-photon absorption. A resonant excitation process between the ground state and the excited state is triggered by the pulse. The asymmetry of MFPADs varies periodically with pulse intensity I0 and duration T, which we attribute to coherent resonant Rabi oscillations in electronic state population. A perturbative model is adopted to qualitatively describe and analyze these effects in both resonant and nonresonant photoionization processes.

Mean field ring polymer molecular dynamics for electronically nonadiabatic reaction rates.

We present a mean field ring polymer molecular dynamics method to calculate the rate of electron transfer (ET) in multi-state, multi-electron condensed-phase processes. Our approach involves calculating a transition state theory (TST) estimate to the rate using an exact path integral in discrete electronic states and continuous Cartesian nuclear coordinates. A dynamic recrossing correction to the TST rate is then obtained from real-time dynamics simulations using mean field ring polymer molecular dynamics. We employ two different reaction coordinates in our simulations and show that, despite the use of mean field dynamics, the use of an accurate dividing surface to compute TST rates allows us to achieve remarkable agreement with Fermi's golden rule rates for nonadiabatic ET in the normal regime of Marcus theory. Further, we show that using a reaction coordinate based on electronic state populations allows us to capture the turnover in rates for ET in the Marcus inverted regime.

Dissipative dynamics at conical intersections: simulations with the hierarchy equations of motion method.

The effect of a dissipative environment on the ultrafast nonadiabatic dynamics at conical intersections is analyzed for a two-state two-mode model chosen to represent the S2(ππ*)-S1(nπ*) conical intersection in pyrazine (the system) which is bilinearly coupled to infinitely many harmonic oscillators in thermal equilibrium (the bath). The system-bath coupling is modeled by the Drude spectral function. The equation of motion for the reduced density matrix of the system is solved numerically exactly with the hierarchy equation of motion method using graphics-processor-unit (GPU) technology. The simulations are valid for arbitrary strength of the system-bath coupling and arbitrary bath memory relaxation time. The present computational studies overcome the limitations of weak system-bath coupling and short memory relaxation time inherent in previous simulations based on multi-level Redfield theory [A. Kühl and W. Domcke, J. Chem. Phys. 2002, 116, 263]. Time evolutions of electronic state populations and time-dependent reduced probability densities of the coupling and tuning modes of the conical intersection have been obtained. It is found that even weak coupling to the bath effectively suppresses the irregular fluctuations of the electronic populations of the isolated two-mode conical intersection. While the population of the upper adiabatic electronic state (S2) is very efficiently quenched by the system-bath coupling, the population of the diabatic ππ* electronic state exhibits long-lived oscillations driven by coherent motion of the tuning mode. Counterintuitively, the coupling to the bath can lead to an enhanced lifetime of the coherence of the tuning mode as a result of effective damping of the highly excited coupling mode, which reduces the strong mode-mode coupling inherent to the conical intersection. The present results extend previous studies of the dissipative dynamics at conical intersections to the nonperturbative regime of system-bath coupling. They pave the way for future first-principles simulations of femtosecond time-resolved four-wave-mixing spectra of chromophores in condensed phases which are nonperturbative in the system dynamics, the system-bath coupling as well as the field-matter coupling.

The Relationship Between the Distribution of Valence Electrons and Electrochemical Properties of Mixtures α-Fe2O3/Al2O3, α-Fe2O3/SiO2 and γ-Fe2O3/SiO2 Before and After Mechanical Activation Synthesis

Using the method of electrochemical analysis the charge capacity of electrochemical cells with cathode material on the bases of simple and mechanically activated mixtures 0,2α-Fe2O3+0,8Al2O3, 0,2α-Fe2O3+0,8SiO2 та 0,2γ-Fe2O30,8SiO2 has been researched. With the help of method of ultra soft X-ray spectroscopy the influence of mechanical activation method on the distribution of valence electrons of these mixtures has been researched. In this research, the relationship between changes in populations of electronic states in the valence band and intercalation properties of studied mixtures has been studied.

Simulating Excited State Dynamics in Systems with Multiple Avoided Crossings Using Mapping Variable Ring Polymer Molecular Dynamics.

Mapping variable ring polymer molecular dynamics (MV-RPMD) is an approximate quantum dynamics method based on imaginary-time path integrals for simulating electronically nonadiabatic photochemical processes. By employing a mapping protocol to transform from a discrete electronic state basis to continuous Cartesian phase-space variables, the method captures electronic state transitions coupled to nuclear motion using only classical MD trajectories. In this work, we extend the applicability of MV-RPMD to simulations of photoinduced excited electronic state dynamics in nonadiabatic systems with multiple avoided crossings. We achieve this by deriving a new electronic state population estimator in the phase space of electronic variables that is exact at equilibrium and numerically accurate in real time. Further, we introduce an efficient constraint protocol to initialize an MV-RPMD simulation to a particular electronic state. We numerically demonstrate the accuracy of this estimator and constraint technique in describing electronic state dynamics from an initial nonequilibrium state in six model systems, three of which describe photodissociation.

Photoexcited states of the harmonic honeycomb iridateγ−Li2IrO3

We report equilibrium and nonequilibrium optical measurements on the recently synthesized "harmonic" honeycomb iridate \gamma-Li$_2$IrO$_3$ (LIO), as well as the layered honeycomb iridate Na$_2$IrO$_3$ (NIO). Using Fourier transform infrared microscopy we performed reflectance measurements on LIO, from which we obtained the optical conductivity below 2 eV. In addition we measured the photoinduced changed in reflectance, \Delta R, as a function of time, t, temperature, T, and probe field polarization in both LIO and NIO. In LIO, \Delta R(t,T) is anisotropic and comprised of three T dependent components. Two of these components are related to the onset of magnetic order and the third is related to a photoinduced population of metastable electronic excited states. In NIO, \Delta R(t,T) has a single T dependent component that is strikingly similar to the electronic excitation component of \Delta R in LIO. Through analysis and comparison of \Delta R(t,T) for two compounds, we extract information on the onset of magnetic correlations at and above the transition temperature in LIO, the bare spin-flip scattering rate in equilibrium, the lifetime of low-lying quasiparticle excitations, and the polarization dependence of optical transitions that are sensitive to magnetic order.

A Symmetrical Quasi-Classical Spin-Mapping Model for the Electronic Degrees of Freedom in Non-Adiabatic Processes.

A recent series of papers has shown that a symmetrical quasi-classical (SQC) windowing procedure applied to the Meyer-Miller (MM) classical vibronic Hamiltonian provides a very good treatment of electronically nonadiabatic processes in a variety of benchmark model systems, including systems that exhibit strong quantum coherence effects and some which other approximate approaches have difficulty in describing correctly. In this paper, a different classical electronic Hamiltonian for the treatment of electronically nonadiabatic processes is proposed (and "quantized" via the SQC windowing approach), which maps the dynamics of F coupled electronic states to a set of F spin-(1)/2 degrees of freedom (DOF), similar to the Fermionic spin model described by Miller and White (J. Chem. Phys. 1986, 84, 5059). It is noted that this spin-mapping (SM) Hamiltonian is an exact Hamiltonian if treated as a quantum mechanical (QM) operator-and thus QM'ly equivalent to the MM Hamiltonian-but that an analytically distinct classical analogue is obtained by replacing the QM spin-operators with their classical counterparts. Due to their analytic differences, a practical comparison is then made between the MM and SM Hamiltonians (when quantized with the SQC technique) by applying the latter to many of the same benchmark test problems successfully treated in our recent work with the SQC/MM model. We find that for every benchmark problem the MM model provides (slightly) better agreement with the correct quantum nonadiabatic transition probabilities than does the new SM model. This is despite the fact that one might expect, a priori, a more natural description of electronic state populations (occupied versus unoccupied) to be provided by DOF with only two states, i.e., spin-(1)/2 DOF, rather than by harmonic oscillator DOF which have an infinite manifold of states (though only two of these are ever occupied).

N2 state population in Titan’s atmosphere

We present a detailed model for the vibrational population of all non pre-dissociating excited electronic states of N2, as well as for the ground and ionic states, in Titan’s atmosphere. Our model includes the detailed energy deposition calculations presented in the past (Lavvas, P. et al. [2011]. Icarus 213(1), 233–251) as well as the more recent developments in the high resolution N2 photo-absorption cross sections that allow us to calculate photo-excitation rates for different vibrational levels of singlet nitrogen states, and provide information for their pre-dissociation yields. In addition, we consider the effect of collisions and chemical reactions in the population of the different states. Our results demonstrate that above 600km altitude, collisional processes are efficient only for a small sub-set of the excited states limited to the A and W(ν=0) triplet states, and to a smaller degree to the a′ singlet state. In addition, we find that a significant population of vibrationally excited ground state N2 survives in Titan’s upper atmosphere. Our calculations demonstrate that this hot N2 population can improve the agreement between models and observations for the emission of the c4′ state that is significantly affected by resonant scattering. Moreover we discuss the potential implications of the vibrationally excited population on the ionospheric densities.

Channeling Vibrational Energy To Probe the Electronic Density of States in Metal Clusters.

We investigate the electronic density of states (DOS) of isolated neutral cobalt clusters by probing the temperature-modulated population of electronic states through UV photoionization. The temperature is controlled via resonant excitation of lattice vibrations using the free-electron laser FELICE, after which the vibrational and electronic systems equilibrate through the electron-phonon coupling, redistributing the population of electronic states. The data are analyzed by surface photoemission theory, modified to incorporate the realistic DOS.

Nonequilibrium optical conductivity in materials with localized electronic states

A wide range of disordered materials contain electronic states that are spatially well localized. In this work, we investigated the electrical response of such systems in non-equilibrium conditions to external electromagnetic field. We obtained the expression for optical conductivity valid for any non-equilibrium state of electronic subsystem. In the case of incoherent non-equilibrium state, this expression contains only the positions of localized electronic states, Fermi's golden rule transition probabilities between the states and the populations of electronic states. The same form of expression is valid both in the case of weak electron-phonon interaction and weak electron-impurity interaction that act as perturbations of electronic Hamiltonian. The derivation was performed by expanding the general expression for AC conductivity in powers of small electron-phonon interaction or electron-impurity interaction parameter. Applications of the expression to two model systems, a simple one- dimensional Gaussian disorder model and the model of a realistic three-dimensional organic polymer material, were presented, as well.

Energy conversion in high enthalpy flows and non-equilibrium plasmas

Recent developments in the study of very high energy non-equilibrium fluid flows are reviewed. These are flows of molecular gases which exhibit substantial degrees of mode disequilibrium, specifically high energy in molecular vibrational and electronic modes, and high electron energies when the gases are weakly ionized. In contrast, the modes of molecular translation and rotation remain at lower energies. Attention is focused on high density, collision-dominated gases. Studies in two systems are presented: A small wind tunnel where an M=5 steady air flow over small models is produced, and a flowing carbon monoxide gas laser, exhibiting very high energy loading of the vibrational quantum states. The development of non-intrusive optical diagnostics to measure vibrational and electronic state populations and rotational/translational mode temperatures in the flows, with high spatial and temporal resolution, is presented. Kinetic modeling and experimental validation studies in these environments are also discussed.

Hysteretic phenomena in a 2DEG in the quantum Hall effect regime, studied in a transport experiment

We investigated experimentally non-equilibrium state of a two-dimensional electron gas (2DEG) in the quantum Hall effect (QHE) regime, studying the hysteresis of magnetoresistance of a 2DEG with a constriction. The large amplitude of the hysteresis enabled us to make the consistent phenomenological description of the hysteresis. We studied the dependence on the magnetic field sweep prehistory (minor loop measurements), recovered the anhysteretic curve, and studied the time dependence of the magnetoresistance. We showed that the hysteresis of magnetoresistance of a 2DEG in the QHE regime has significant phenomenological similarities with the hysteresis of magnetization of ferromagnetic materials, showing multistability, jumps of relaxation, and having the anhysteretic curve. Nevertheless, we revealed the crucial difference, manifested itself in an unusual inverted (anti-coercive) behavior of the magnetoresistance hysteresis. The time relaxation of the hysteresis has fast and slow regimes, similar to that of non-equilibrium magnetization of a 2DEG in QHE regime pointing to their common origin. We studied the dependence of the hysteresis loop area on the lithographic width of the constriction and found the threshold value of width $\sim$1.35 $\mu$m beyond which the hysteresis is not observed. This points to the edge nature of the non-equilibrium currents (NECs) and allows us to determine the width of the NECs area ($\sim$0.5 $\mu$m). We suggest the qualitative picture of the observed hysteresis, based on non-equilibrium redistribution of the electrons among the Landau level states and assuming huge imbalance between the population of bulk and edge electronic states.

Direct spectroscopic observation of closed-shell singlet, open-shell singlet, and triplet p-biphenylyloxenium ion.

The photophysics and photochemistry of p-biphenylyl hydroxylamine hydrochloride was studied using laser flash photolysis ranging from the femtosecond to the microsecond time scale. The singlet excited state of this photoprecursor is formed within 350 fs and partitions into three different transients that are assigned to the p-biphenyloxy radical, the open-shell singlet p-biphenylyloxenium ion, and the triplet p-biphenylyloxenium ion, having lifetimes of 40 μs, 45 ps, and 1.6 ns, respectively, in CH3CN. The open-shell singlet p-biphenylyloxenium ion predominantly undergoes internal conversion to produce the closed-shell singlet p-biphenylyloxenium ion, which has a lifetime of 5-20 ns. The longer-lived radical is unambiguously assigned by nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy, and the assignment of the short-lived singlet and triplet oxenium ion transient absorptions are supported by matching time-dependent density functional theory (TD-DFT) predictions of the absorptions of these species, as well as by product studies that implicate the intermediacy of charged electrophilic intermediates. Product studies from photolysis give p-biphenylol as the major product and a chloride adduct as the major product when NaCl is added as a trap. Thermolysis studies give p-biphenylol as a major product, as well as water, ammonium, and chloro adducts. These studies provide a rare direct look at a discrete oxenium ion intermediate and the first detection of open-shell singlet and triplet configurations of an oxenium ion, as well as providing an intriguing example of the importance of excited state dynamics in governing the electronic state population of reactive intermediates.

The role of the low-lying dark nπ* states in the photophysics of pyrazine: a quantum dynamics study.

The excited state dynamics of pyrazine has attracted considerable attention in the last three decades. It has long been recognized that after UV excitation, the dynamics of the molecule is impacted by strong non-adiabatic effects due to the existence of a conical intersection between the B2u(ππ*) and B3u(nπ*) electronic states. However, a recent study based on trajectory surface hopping dynamics simulations suggested the participation of the Au(nπ*) and B2g(nπ*) low-lying dark electronic states in the ultrafast radiationless decay of the molecule after excitation to the B2u(ππ*) state. The purpose of this work was to pursue the investigation of the role of the Au(nπ*) and B2g(nπ*) states in the photophysics of pyrazine. A linear vibronic coupling model hamiltonian including the four lowest excited electronic states and the sixteen most relevant vibrational degrees of freedom was constructed using high level XMCQDPT2 electronic structure calculations. Wavepacket propagations using the MCTDH method were then performed and used to simulate the absorption spectrum and the electronic state population dynamics of the system. Our results show that the Au(nπ*) state plays an important role in the photophysics of pyrazine.

Modeling of Nitrogen Monoxide Formation and Radiation in Nonequilibrium Hypersonic Flows

The interaction between nitrogen vibrational-translational relaxation with different chemistry models in a hypersonic shock is studied. The vibrational-translational transition models considered include the Larsen–Borgnakke, Schwartz-Slawsky-Herzfeld, and forced harmonic oscillator models; and the nitrogen monoxide formation reaction was studied using the total collision energy chemistry model and reaction cross sections obtained from quasi-classical trajectory simulations. Using direct simulation Monte Carlo solutions that incorporate these models, improved nitrogen monoxide spectral radiation modeling was performed by employing quenching rates from recent experiments. Comparisons of electronic state populations and the corresponding radiative spectra for the different vibrational-translational and chemistry models are presented, and it is shown that the vibronic spectral structure of the nitrogen monoxide radiation could provide a way to test the fidelity of the internal energy transfer and chemistry models.

Equilibrium spectral radiation behind the shock wave front in a CO2-N2 gas mixture

This paper presents calculations of the equilibrium spectral radiation behind shock waves in a CO2-N2 gas mixture, corresponding to the conditions implemented in experiments with NASA Ames Research Center and JAXA Chofu Space Center shock tubes. The studied experimental parameters were selected so that the results obtained for these systems can be cross-validated. The presented data for equilibrium spectral radiation were obtained using two methods for calculating the equilibrium populations of the excited electronic states, based on a hybrid radiative-collisional model (in fact, a model of nonequilibrium radiation) and on the use of the Boltzmann distribution with the calculated equilibrium temperature. The spectral composition of the radiation was also analyzed; a comparison of the calculated and experimental data demonstrated reasonable agreement between them.

Modeling of internal conversion in photoexcited conjugated molecular donors used in organic photovoltaics

Using the Non-Adiabatic Excited States Molecular Dynamics (NA-ESMD) approach, we investigate the ultrafast electronic relaxation in a recently synthesized small molecule donor, p-DTS(PTTh2)2, which belongs to the dithienosilole-pyridylthiadiazole family of chromophores. In combination with the PC70BM acceptor, p-DTS(PTTh2)2 can be used to fabricate high efficiency bulk heterojunction organic solar cells. After photoexcitation to its broad high-energy peak in the 3–4 eV range, associated with multiple excited states, p-DTS(PTTh2)2 undergoes efficient ultrafast internal conversion to its lowest excited state. During this process, about 1–2 eV electronic energy transfers to the vibrational degrees of freedom leading to rapid heating of the molecule. Nevertheless, our simulations do not detect possible bond-breaking or decomposition of the system. This suggests minimal intra-molecular photodamage after photoexcitation to high-energy states in the 3–4 eV region. Calculated radiationless deactivation mainly consists of a sequential mechanism that involves electronic transitions between the current transient state and the corresponding state directly below in energy. Changes in the density of states along the relaxation process lead to pronounced variations and time-dependence of the accumulated populations of the different intermediate electronic excited states. Visualization of the electronic transition density during internal conversion reveals spatial intramolecular delocalization of electronic excitation from the thiophene moieties to the entire chromophore. Finally, our analysis of non-adiabatic coupling vectors suggests characteristic vibrational degrees of freedom coupled to the electronic system during various stages of non-radiative relaxation.