Coherent population transfer in molecules coupled with a dissipative environment by an intense ultrashort chirped pulse

Chirped Laser Pulse Control of Vibronic Wavepackets and Energy Transfer in Phycocyanin 645

Photosynthetic organisms use light-harvesting complexes to increase the spectrum of light that they absorb from solar photons. Recent ultrafast spectroscopic studies have revealed that efficient (sub-ps) energy transfer is mediated by vibronic coherence in the phycobiliprotein phycocyanin 645 (PC645). Here, we report studies that employ broadband pump-probe spectroscopy with linearly chirped excitation pulses to further investigate the relationship between vibronic state preparation and energy transfer dynamics in PC645. Negatively chirped pulse excitation is found to enhance wavepackets of a high-frequency mode (1580 cm-1) and increase the rate of downhill energy transfer, while on the other hand, positively chirped pulses suppress these oscillatory features and decrease this rate. Model calculations incorporating the influence of the chirped pump pulse are used to understand its effect on initial state preparation. These results provide mechanistic insight into how the overall nonequilibrium rate of energy transfer is influenced by initial state preparation.

A model study of assisted adiabatic transfer of population in the presence of collisional dephasing

Previous studies have demonstrated that when experimental conditions generate non-adiabatic dynamics that prevents highly efficient population transfer between states of an isolated system by stimulated Raman adiabatic passage (STIRAP), the addition of an auxiliary counter-diabatic field (CDF) can restore most or all of that efficiency. This paper examines whether that strategy is also successful in a non-isolated system in which the energies of the states fluctuate, e.g., when a solute is subject to collisions with solvent. We study population transfer in two model systems: (i) the three-state system used by Demirplak and Rice [J. Chem. Phys. 116, 8028 (2002)] and (ii) a four-state system, derived from the simulation studies of Demirplak and Rice [J. Chem. Phys. 125, 194517 (2006)], that mimics HCl in liquid Ar. Simulation studies of the vibrational manifold of HCl in dense fluid Ar show that the collision induced vibrational energy level fluctuations have asymmetric distributions. Representations of these asymmetric energy level fluctuation distributions are used in both models (i) and (ii). We identify three sources of degradation of the efficiency of STIRAP generated selective population transfer in model (ii): too small pulse areas of the laser fields, unwanted interference arising from use of strong fields, and the vibrational detuning. For both models (i) and (ii), our examination of the efficiency of STIRAP + CDF population transfer under the influence of the asymmetric distribution of the vibrational energy fluctuations shows that there is a range of field strengths and pulse durations under which STIRAP + CDF control of population transfer has greater efficiency than does STIRAP generated population transfer.

Investigating the role of human serum albumin protein pocket on the excited state dynamics of indocyanine green using shaped femtosecond laser pulses

When indocyanine green (ICG) is confined inside the pocket of human serum albumin its triplet state formation is mitigated and coherent vibrational motion becomes more observable.

Solvent Environment Revealed by Positively Chirped Pulses

The spectroscopy of large organic molecules and biomolecules in solution has been investigated using various time-resolved and frequency-resolved techniques. Of particular interest is the early response of the molecule and the solvent, which is difficult to study due to the ambiguity in assigning and differentiating inter- and intramolecular contributions to the electronic and vibrational populations and coherence. Our measurements compare the yield of fluorescence and stimulated emission for two laser dyes IR144 and IR125 as a function of chirp. While negatively chirped pulses are insensitive to solvent viscosity, positively chirped pulses are found to be uniquely sensitive probes of solvent viscosity. The fluorescence maximum for IR125 is observed near transform-limited pulses; however, for IR144, it is observed for positively chirped pulses once the pulses have been stretched to hundreds of femtoseconds. We conclude that chirped pulse spectroscopy is a simple one-beam method that is sensitive to early solvation dynamics.

Solvation Stokes-Shift Dynamics Studied by Chirped Femtosecond Laser Pulses

The early optical dynamic response, resulting population, and electronic coherence are investigated experimentally and modeled theoretically for IR144 in solution. The fluorescence and stimulated emission response are studied systematically as a function of chirp. The magnitude of the chirp effect on fluorescence and stimulated emission is found to depend quadratically on pulse energy, even where excitation probabilities range from 0.02 to 5%, in the so-called "linear excitation regime". Interestingly, the shape of the chirp dependence on fluorescence and stimulated emission is found to be independent of pulse energy. The chirp dependence reveals dynamics related to solvent rearrangement following excitation and also depends on electronic relaxation of the chromophore. The experimental results are successfully simulated using a four-level model in the presence of inhomogeneous broadening of the electronic transitions.

Adiabatic Passage in a Three-State System with Non-Markovian Relaxation: The Role of Excited-State Absorption and Two-Exciton Processes

The influence of excited-state absorption (ESA) and two-exciton processes on a coherent population transfer with intense ultrashort chirped pulses in molecular systems in solution has been studied. A unified treatment of adiabatic rapid passage (ARP) in such systems has been developed using a three-state electronic system with relaxation treated as a diffusion on electronic potential energy surfaces. We have shown that ESA has a profound effect on coherent population transfer in large molecules that necessitates a more accurate interpretation of experimental data. A simple and physically clear model for ARP in molecules with three electronic states in solution has been developed by extending the Landau-Zener calculations putting in a third level to random crossing of levels. A method for quantum control of two-exciton states in molecular complexes has been proposed.

Controlled subnanosecond isomerization of HCN to CNH in solution

We report a study of control of the HCN-->CNH isomerization in a liquid Ar solution. We show, using molecular dynamics simulations, nearly complete conversion from HCN to CNH can be achieved in solution on the subnanosecond time scale without requiring laser pulse shaping or molecular alignment. The mechanism of the isomerization reaction involves multiphoton rovibrational excitation on the ground electronic state potential energy surface coupled with rapid rovibrational relaxation in solution. The results demonstrate the important role of rotation-vibration coupling in multiphoton excitation of small molecules and constitute the first realistic computational demonstration of fast, robust, and high-yield laser field manipulation of solution-phase molecular processes.

Adiabatic population transfer in a liquid: Taking advantage of a decaying target state

The feasibility of efficient population transfer between an initial state and a decaying target state of the same parity without populating an intermediate state, in the presence of large-amplitude stochastic energy level fluctuations that mimic the dephasing in a solute molecule due to the influence of a solvent, is demonstrated theoretically. In particular, it is shown that a decaying target state, whose decay rate constant is large compared with the band width of picosecond laser pulses but small compared with the associated peak Rabi frequencies, can dramatically suppress the dephasing-induced nonadiabaticity associated with the dynamics of population transfer, resulting in, irrespective of the correlation time of stochastic energy level fluctuations, negligible population in the intermediate state and complete population transfer to the decaying target state. These results should further motivate experimental studies of optical control of molecular dynamics in a liquid. An interesting connection between our results and the quantum Zeno and anti-Zeno effects is also discussed.