Light - Matter Interactions

Stein, T. ; Kronik, L. ; Baer, R. Prediction of charge-transfer excitations in coumarin-based dyes using a range-separated functional tuned from first principles. J. Chem. Phys. 2009, 131, 244119–5.Abstract

We study the description of charge-transfer excitations in a series of coumarin-based donor-bridge-acceptor dyes. We show that excellent predictive power for the excitation energies and oscillator strengths in these systems is obtained by using a range-separated hybrid functional within the generalized Kohn–Sham approach to time-dependent density functional theory. Key to this success is a step for tuning the range separation parameter from first principles. We explore different methods for this tuning step, which are variants of a recently suggested approach for charge-transfer excitations [T. Stein et al., J. Am. Chem. Soc. 131, 2818 (2009)]. We assess the quality of prediction by comparing to excitation energies previously published for the same systems using the approximate coupled-cluster singles and doubles (CC2) method.

Stein, T. ; Kronik, L. ; Baer, R. Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theory. J. Am. Chem. Soc. 2009, 131, 2818–2820.Abstract

We show how charge transfer excitations at molecular complexes can be calculated quantitatively using time-dependent density functional theory. Predictive power is obtained from range-separated hybrid functionals using nonempirical tuning of the range-splitting parameter. Excellent performance of this approach is obtained for a series of complexes composed of various aromatic donors and the tetracyanoethytene acceptor, paving the way to systematic nonempirical quantitative studies of charge-transfer excitations in real systems.

Paul, A. K. ; Adhikari, S. ; Mukhopadhyay, D. ; Halasz, G. J. ; Vibok, A. ; Baer, R. ; Baer, M. Photodissociation of H2+ upon Exposure to an Intense Pulsed Photonic Fock State. J. Phys. Chem. A 2009, 113, 7331–7337.Abstract

Producing and controlling nonclassical light states are now the subject of intense experimental efforts. In this paper we consider the interaction of such a light state with a small molecule. Specifically, we develop the theory and apply it numerically to calculate in detail how a short pulse of nonclassical light, such as the high intensity Fock state, induces photodissociation in H2+. We compare the kinetic energy distributions and photodissociation yields with the analogous results of quasi-classical light, namely a coherent state. We find that Fock-state light decreases the overall probability of dissociation for low vibrational states of H2+ as well as the location of peaks and line shapes in the kinetic energy distribution of the nuclei.

Baer, R. Prevalence of the adiabatic exchange-correlation potential approximation in time-dependent density functional theory. Theochem-Journal of Molecular Structure 2009, 914, 19–21.Abstract

Time-dependent (TD) density functional theory (TDDFT) promises a numerically tractable account of many-body electron dynamics provided good simple approximations are developed for the exchange-correlation (XC) potential functional (XCPF). The theory is usually applied within the adiabatic XCPF approximation, appropriate for slowly varying TD driving fields. As the frequency and strength of these fields grows, it is widely held that memory effects kick in and the eligibility of the adiabatic XCPF approximation deteriorates irreversibly. We point out, however, that when a finite system of electrons in its ground-state is gradually exposed to a very a high-frequency and eventually ultra-strong homogeneous electric field, the adiabatic XCPF approximation is in fact rigorously applicable. This result shows that adiabatic XCPF has a larger scope of applicability than previously suspected and in this sense is compliant with recent numerical findings by Thiele et al. [M. Thiele, E.K.U. Gross, S. Kümmel, Phys. Rev. Lett. 100 (2008) 153004] of negligible memory effects in strong-field double ionization.

Andzelm, J. ; Rinderspacher, B. C. ; Rawlett, A. ; Dougherty, J. ; Baer, R. ; Govind, N. Performance of DFT Methods in the Calculation of Optical Spectra of TCF-Chromophores. J. Chem. Theory Comput. 2009, 5 2835–2846.Abstract

We present electronic structure calculations of the ultraviolet/visible (UV?vis) spectra of highly active push?pull chromophores containing the tricyanofuran (TCF) acceptor group. In particular, we have applied the recently developed long-range corrected Baer-Neuhauser-Livshits (BNL) exchange-correlation functional. The performance of this functional compares favorably with other density functional theory (DFT) approaches, including the CAM-B3LYP functional. The accuracy of UV-vis results for these molecules is best at low values of attenuation parameters (\gamma) for both BNL and CAM-B3LYP functionals. The optimal value of \gamma is different for the charge-transfer (CT) and valence excitations. The BNL and PBE0 exchange correlation functionals capture the CT states particularly well, while the ???* excitations are less accurate and system dependent. Chromophore conformations, which considerably affect the molecular hyperpolarizability, do not significantly influence the UV?vis spectra on average. As expected, the color of chromophores is a sensitive function of modifications to its conjugated framework and is not significantly affected by increasing aliphatic chain length linking a chromophore to a polymer. For selected push?pull aryl-chromophores, we find a significant dependence of absorption spectra on the strength of diphenylaminophenyl donors.

Rabani, E. ; Baer, R. Distribution of multiexciton generation rates in CdSe and InAs nanocrystals. Nano Lett. 2008, 8 4488–4492.Abstract

The distribution of rates of multiexciton generation following photon absorption is calculated for semiconductor nanocrystals (NCs). The rates of biexciton generation are calculated using Fermi’s golden rule with all relevant Coulomb matrix elements, taking into account proper selection rules within a screened semiempirical pseudopotential approach. In CdSe and InAs NCs, we find a broad distribution of biexciton generation rates depending strongly on the exciton energy and size of the NC. Multiexciton generation becomes inefficient for NCs exceeding 3 nm in diameter in the photon energy range of 2-3 times the band gap.

Lopata, K. ; Neuhauser, D. ; Baer, R. Curve crossing and negative refraction in simulations of near-field coupled metallic nanoparticle arrays. J. Chem. Phys. 2007, 127, 154714–8.Abstract

We extend our previous results [R. Baer et al., J. Chem. Phys. 126, 014705 (2007).] to develop a simple theory of localized surface plasmon-polariton (LSPP) dispersion on regular arrays of metal nanoparticles in the weak-field and weak-damping limits. This theory describes the energy-momentum as well as the polarization-momentum properties of LSPP waves, both of which are crucial to plasmonic device design. We then explicitly compute the dispersion relation for isotropic and anisotropic two-dimensional square lattices, and show curve crossings between all three levels as well as negative refraction where the phase and group velocities (refractive indices), or at least their projection along the main axis, have different signs. The curve crossing implies that scattering between the different polarizations, and therefore different velocities, is easy at the curve crossing momenta, so that a quick change in wave packet direction can be achieved. Time-resolved wave packet dynamics simulations demonstrate negative refraction and the easy scattering over nanometer length scales. This paper also gives some computational schemes for future applications, such as a way to include source terms and how to efficiently treat dissipative effects.

Baer, R. ; Lopata, K. ; Neuhauser, D. Properties of phase-coherent energy shuttling on the nanoscale. J. Chem. Phys. 2007, 126, 014705–13.Abstract

Recently, the possibility of transporting electromagnetic energy as local-plasmon-polariton waves along arrays of silver nanoparticles was demonstrated experimentally [S. A. Maier et al., Nat. Mater. 2, 229 (2003)]. It was shown that dipole coupling facilitates phase-coherent excitation waves, which propagate while competing against decoherence effects occurring within each dot. In this article the authors study the ideal coherent shuttling in such a system, leaving decoherence for future investigation. In the weak field limit, the waves obey a Schrödinger equation, to be solved using either time-dependent wave-packet or energy resolved scattering techniques. The authors study some dynamical characteristics of these waves, emphasizing intuition and insight. Scattering from barriers, longitudinal-transverse coupling and acceleration methods are studied in detail. The authors also discuss briefly two-dimensional arrays and a simple decoherence model

Livshits, E. ; Baer, R. Time-dependent density-functional studies of the D2 Coulomb explosion. J. Phys. Chem. A 2006, 110, 8443–8450. Publisher's VersionAbstract

Real-time first principle simulations are presented of the D2 Coulomb explosion dynamics detonated by exposure to very intense few-cycle laser pulse. Three approximate functionals within the time-dependent density functional theory (TDDFT) functionals are examined for describing the electron dynamics, including time-dependent Hartree-Fock theory. Nuclei are treated classically with quantum corrections. The calculated results are sensitive to the underlying electronic structure theory, showing too narrow kinetic energy distribution peaked at too high kinetic energy when compared with recent experimental results (Phys. Rev. Lett. 2003, 91, 093002). Experiment also shows a low energy peak which is not seen in the present calculation. We conclude that while Ehrenfest-adiabatic-TDDFT can qualitatively account for the dynamics, it requires further development, probably beyond the adiabatic approximation, to be quantitative.

Shemesh, D. ; Baer, R. ; Seideman, T. ; Gerber, B. R. Photoionization dynamics of glycine adsorbed on a silicon cluster:“On-the-fly” simulations. The Journal of chemical physics 2005, 122, 184704.Abstract

Dynamics of glycine chemisorbed on the surface of a silicon cluster is studied for a process that involves single-photon ionization, followed by recombination with the electron after a selected time delay. The process is studied by “on-the-fly” molecular dynamics simulations, using the semiempirical parametric method number 3 (PM3) potential energy surface. The system is taken to be in the ground state prior to photoionization, and time delays from 5 to 50 fs before the recombination are considered. The time evolution is computed over 10 ps. The main findings are (1) the positive charge after ionization is initially mostly distributed on the silicon cluster. (2) After ionization the major structural changes are on the silicon cluster. These include Si–Si bond breaking and formation and hydrogen transfer between different silicon atoms. (3) The transient ionization event gives rise to dynamical behavior that depends sensitively on the ion state lifetime. Subsequent to 45 fs evolution in the charged state, the glycine molecule starts to rotate on the silicon cluster. Implications of the results to various processes that are induced by transient transition to a charged state are discussed. These include inelastic tunneling in molecular devices, photochemistry on conducting surfaces, and electron-molecule scattering.

Baer, R. ; Seideman, T. ; Ilani, S. ; Neuhauser, D. Ab initio study of the alternating current impedance of a molecular junction. J. Chem. Phys. 2004, 120, 3387–3396.Abstract

The small-bias conductance of the C-6 molecule, stretched between two metallic leads, is studied using time-dependent density functional theory within the adiabatic local density approximation. The leads are modeled by jellium slabs, the electronic density and the current density are described on a grid, whereas the core electrons and the highly oscillating valence orbitals are approximated using standard norm-conserving pseudopotentials. The jellium leads are supplemented by a complex absorbing potential that serves to absorb charge reaching the edge of the electrodes and hence mimic irreversible flow into the macroscopic metal. The system is rapidly exposed to a ramp potential directed along the C-6 axis, which gives rise to the onset of charge and current oscillations. As time progresses, a fast redistribution of the molecular charge is observed, which translates into a direct current response. Accompanying the dc signal, alternating current fluctuations of charge and currents within the molecule and the metallic leads are observed. These form the complex impedance of the molecule and are especially strong at the plasmon frequency of the leads and the lowest excitation peak of C-6. We study the molecular conductance in two limits: the strong coupling limit, where the edge atoms of the chain are submerged in the jellium and the weak coupling case, where the carbon atoms and the leads do not overlap spatially. (C) 2004 American Institute of Physics.

Neuhauser, D. ; Baer, R. A two-grid time-dependent formalism for the Maxwell equation. Journal of Theoretical & Computational Chemistry 2003, 2 537–546. neuhauser2003.pdf
Barash, D. ; Orel, A. E. ; Baer, R. A comment on the stabilization of a one-dimensional short-range model atom in intense laser fields. Journal of Physics B-Atomic Molecular and Optical Physics 2000, 33, 1279–1282. barash2000a.pdf
Barash, D. ; Orel, A. E. ; Baer, R. Laser-induced resonance states as dynamic suppressors of ionization in high-frequency short pulses. Phys. Rev. A 2000, 61, 013402.Abstract

An adiabatic-Floquet formalism is used to study the suppression of ionization in short laser pulses. In the high-frequency limit the adiabatic equations involve only the pulse envelope where transitions are purely ramp effects. For a short-ranged potential having a single-bound state we show that ionization suppression is caused by the appearance of a laser-induced resonance state, which is coupled by the pulse ramp to the ground state and acts to trap ionizing flux.