[Show abstract][Hide abstract] ABSTRACT: The possibility to induce predetermined coherent quantum dynamics by
controlling only the dissipative environmental parameters (such as temperature
and pressure) is studied using the combined optimal control and environment
engineering frameworks. As an example, we consider the problem of transforming
an optically excited donor state into free charge carriers via intermediate
higher-lying bridge state(s), with a view to solar energy conversion. In this
context, vibrational bath engineering allows to promote fast, directional
charge transfer and to suppress recombinative losses.
[Show abstract][Hide abstract] ABSTRACT: We introduce analytical models of torsional alignment by moderately intense laser pulses that are applicable to the limiting cases of the torsional barrier heights. Using these models, we explore in detail the role that the laser intensity and pulse duration play in coherent torsional dynamics, addressing both experimental and theoretical concerns. Our results suggest strategies for minimizing the risk of off-resonant ionization, noting the qualitative differences between the case of torsional alignment subject to a field-free torsional barrier and that of torsional alignment of a barrier-less system (equivalent to a 2D rigid rotor). We also investigate several interesting torsional phenomena, including the onset of impulsive alignment of torsions, field-driven oscillations in quantum number space, and the disappearance of an alignment upper bound observed for a rigid rotor in the impulsive torsional alignment limit.
The Journal of Chemical Physics 08/2015; 143(6):064307. DOI:10.1063/1.4927917 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The problems of optimizing the value of an arbitrary observable of the
two-level system at both a fixed time and the shortest possible time is
theoretically explored. Complete identification and classification along with
comprehensive analysis of globally optimal control policies and traps (i.e.
policies which are locally but not globally optimal) is presented. The central
question addressed is whether the control landscape remains trap-free if
control constraints of the inequality type are imposed. The answer is
astonishingly controversial, namely, although formally it is always negative,
in practice it is positive provided that the control time is fixed and chosen
[Show abstract][Hide abstract] ABSTRACT: Tip-enhanced Raman scattering (TERS) and optically excited tip-enhanced fluorescence (TEF) of a self-assembled porphyrin monolayer on Ag(111) are studied using an ultrahigh vacuum scanning tunneling microscope (UHV-STM). Through selectively exciting different Q-bands of meso-tetrakis- (3,5-di-tertiarybutylphenyl)-porphyrin (H2TBPP), chemical information regarding different vibronic excited states is revealed by a combination of theory and experiment; namely TERS and time-dependent density functional theory (TDDFT) simulations. The observed TEF spectra suggest a weak coupling of H2TBPP to the substrate due to the bulky t-butyl groups and a possible alternative excited state decay path. This work demonstrates the potential of combining TERS and TEF for studying surface-mounted porphyins on substrates, thus providing insight into porphyrin-sensitized solar cells and catalysis.
[Show abstract][Hide abstract] ABSTRACT: A tight binding model is used to investigate photoinduced tunneling current through a molecular bridge coupled to two semiconductor
electrodes. A quantum master equation is developed within a non-Markovian theory based on second-order perturbation theory with respect to the molecule-semiconductor electrode coupling. The spectral functions are generated using a one dimensional alternating bond model, and the coupling between the molecule and the electrodes is expressed through a corresponding correlation function. Since the molecular bridge orbitals are inside the bandgap between the conduction and valence bands, charge carrier tunneling is inhibited in the dark. Subject to the dipole interaction with the laser field, virtual molecular states are generated via the absorption and emission of photons, and new tunneling channels open. Interesting phenomena arising from memory are noted. Such a phenomenon could serve as a switch.
The Journal of Chemical Physics 04/2015; 142(15):154111. DOI:10.1063/1.4917029 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We suggest optical modulation of the dielectric function of a molecular monolayer adsorbed on a metal surface as a potential means of controlling plasmon resonance phenomena. The dielectric function is altered using a laser pulse of moderate intensity and linear polarization to align the constituent molecules. After the pulse, the monolayer returns to its initial state. Time-dependent, optically controlled dielectric function is illustrated by molecular dynamics calculations.Keywords: surface plasmon resonance; self-assembled monolayer; molecular dynamics; molecular alignment; polarizability
[Show abstract][Hide abstract] ABSTRACT: Shaped ultrafast laser pulses were used to study and control the ionization dynamics of electronically excited pyrazine in a pump and probe experiment. For pump pulses created without feedback from the product signal, the ion growth curve (the parent ion signal as a function of pump/probe delay) was described quantitatively by the classical rate equations for internal conversion of the S
2 and S
1 states. Very different, non-classical behavior was observed when a genetic algorithm (GA) employing phase-only modulation was used to minimize the ion signal at some pre-determined target time, T. Two qualitatively different control mechanisms were identified for early (T < 1.5 ps) and late (T > 1.5 ps) target times. In the former case, the ion signal was largely suppressed for t < T, while for t ≫ T, the ion signal produced by the GA-optimized pulse and a transform limited (TL) pulse coalesced. In contrast, for T > 1.5 ps, the ion growth curve followed the classical rate equations for t < T, while for t ≫ T, the quantum yield for the GA-optimized pulse was much smaller than for a TL pulse. We interpret the first type of behavior as an indication that the wave packet produced by the pump laser is localized in a region of the S
potential energy surface where the vertical ionization energy exceeds the probe photon energy, whereas the second type of behavior may be described by a reduced absorption cross section for S
0 → S
2 followed by incoherent decay of the excited molecules. Amplitude modulation observed in the spectrum of the shaped pulse may have contributed to the control mechanism, although this possibility is mitigated by the very small focal volume of the probe laser.
The Journal of Chemical Physics 09/2014; 142(14). DOI:10.1063/1.4916642 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tip-enhanced Raman spectroscopy (TERS) provides chemical information about adsorbates with nanoscale spatial resolution, but developments are still required in order to incorporate ultrafast temporal resolution. In this Letter, we demonstrate that a reliable TER signal of rhodamine 6G (R6G) using picosecond (ps)-pulsed excitation can be obtained in ultrahigh vacuum (UHV). In contrast to our previous observation of irreversible signal loss in ambient TERS (Klingsporn, J. M.; Sonntag, M. D.; Seideman, T.; Van Duyne, R. P. J. Phys. Chem. Lett. 2014, 5, 106-110), we demonstrate that the UHV environment decreases irreversible signal degradation. As a complement to the TERS experiments, we examined the rate of surface-enhanced Raman (SER) signal decay under picosecond irradiation and found that it is also slowed in UHV compared to that in ambient. Signal decay kinetics suggest that the predominant mechanism responsible for signal loss in ps SERS of R6G is surface diffusion. Both diffusive and reactive phenomena can lead pulsed excitation TER signal loss, and a UHV environment is advantageous in either scenario.