Solvation Stokes-Shift Dynamics Studied by Chirped Femtosecond Laser Pulses

Unveiling Coherent Control of Halomethane Dissociation Induced by a Single Strong Ultraviolet Pulse

We present a theoretical investigation into the coherent control of photodissociation reactions in halomethanes, specifically focusing on CH2BrCl by manipulating the spectral phase of a single femtosecond laser pulse. We examine the photodissociation of CH2BrCl under an ultrashort pulse with a quadratic spectral phase and reveal the sensitivity of both the total dissociation probability and the resulting radical products (Br+CH2Cl and Cl+CH2Br) to chirp rates. To gain insights into the underlying mechanism, we calculate the population distributions of excited vibrational states in the ground electronic state, demonstrating the occurrence of resonance Raman scattering (RRS) in the strong-field limit regime. By utilizing chirped pulses, we show that this RRS phenomenon can be suppressed and even eliminated through quantum destructive interference. This highlights the high sensitivity of photodissociation into Cl+CH2Br to the spectral phase, showcasing a phenomenon that goes beyond the traditional one-photon photodissociation of isolated molecules in the weak-field limit regime. These findings emphasize the importance of coherent control in the exploration and utilization of photodissociation in polyatomic molecules, paving the way for new advancements in chemical physics and femtochemistry.

Linear and Nonlinear Optical Processes Controlling S2 and S1 Dual Fluorescence in Cyanine Dyes

We report on the changes in the dual fluorescence of two cyanine dyes IR144 and IR140 as a function of viscosity and probe their internal conversion dynamics from S₂ to S₁ via their dependence on a femtosecond laser pulse chirp. Steady-state and time-resolved measurements performed in methanol, ethanol, propanol, ethylene glycol, and glycerol solutions are presented. Quantum calculations reveal the presence of three excited states responsible for the experimental observations. Above the first excited state, we find an excited state, which we designate as S₁', that relaxes to the S₁ minimum, and we find that the S₂ state has two stable configurations. Chirp-dependence measurements, aided by numerical simulations, reveal how internal conversion from S₂ to S₁ depends on solvent viscosity and pulse duration. By combining solvent viscosity, transform-limited pulses, and chirped pulses, we obtain an overall change in the S₂/S₁ population ratio of a factor of 86 and 55 for IR144 and IR140, respectively. The increase in the S₂/S₁ ratio is explained by a two-photon transition to a higher excited state. The ability to maximize the population of higher excited states by delaying or bypassing nonradiative relaxation may lead to the increased efficiency of photochemical processes.

Solvation Dynamics in Water. 4. On the Initial Regime of Solvation Relaxation

It is shown, by means of numerical and analytic work, that initial molecular momenta play little significant role in the initial fast solvation relaxation that follows electronic excitation of, and charge creation for, a standard model system of a solute in water. Instead, the nonequilibrium dynamics are predominantly described by noninertial "steering" by the torques directly generated by the newly created charge distribution. It is this process that largely overcomes inertia and drives the relaxation dynamics on a time scale of a few tens of femtoseconds in the key initial regime of the dynamics. These results are discussed in the context of commonly employed descriptions such as inertial, Gaussian, and underdamped dynamical behavior.

Vanishing and Revival of Resonance Raman Scattering

The possibility to manipulate quantum coherence and interference, apart from its fundamental interest in quantum mechanics, is essential for controlling nonlinear optical processes such as high harmonic generation, multiphoton absorption, and stimulated Raman scattering. We show, analytically and numerically, how a nonlinear optical process via resonance Raman scattering (RRS) can be manipulated in a four-level double-Λ system by using pulsed laser fields. We find that two simultaneously excited RRS paths involved in the system can generate an ultimately destructive interference in the broad-bandwidth-limit regime. This, in turn, reduces the four-level system to an equivalent three-level system in a V configuration capable of naturally vanishing RRS effects. We further show that this counterintuitive phenomenon, i.e., the RRS vanishing, can be prevented by transferring a modulated phase of the laser pulse to the system at resonance frequencies. This work demonstrates a clear signature of both quantum destructive and constructive interference by actively controlling resonant multiphoton processes in multilevel quantum systems, and it therefore has potential applications in nonlinear optics, quantum control, and quantum information science.

Quantum transition probabilities during a perturbing pulse: Differences between the nonadiabatic results and Fermi's golden rule forms

For a perturbed quantum system initially in the ground state, the coefficient c_k(t) of excited state k in the time-dependent wave function separates into adiabatic and nonadiabatic terms. The adiabatic term a_k(t) accounts for the adjustment of the original ground state to form the new ground state of the instantaneous Hamiltonian H(t), by incorporating excited states of the unperturbed Hamiltonian H₀ without transitions; a_k(t) follows the adiabatic theorem of Born and Fock. The nonadiabatic term b_k(t) describes excitation into another quantum state k; b_k(t) is obtained as an integral containing the time derivative of the perturbation. The true transition probability is given by b_k(t) ², as first stated by Landau and Lifshitz. In this work, we contrast b_k(t) ² and c_k(t) ². The latter is the norm-square of the entire excited-state coefficient which is used for the transition probability within Fermi's golden rule. Calculations are performed for a perturbing pulse consisting of a cosine or sine wave in a Gaussian envelope. When the transition frequency ω_k0 is on resonance with the frequency ω of the cosine wave, b_k(t) ² and c_k(t) ² rise almost monotonically to the same final value; the two are intertwined, but they are out of phase with each other. Off resonance (when ω_k0 ≠ ω), b_k(t) ² and c_k(t) ² differ significantly during the pulse. They oscillate out of phase and reach different maxima but then fall off to equal final values after the pulse has ended, when a_k(t) ≡ 0. If ω_k0 < ω, b_k(t) ² generally exceeds c_k(t) ², while the opposite is true when ω_k0 > ω. While the transition probability is rising, the midpoints between successive maxima and minima fit Gaussian functions of the form a exp[-b(t - d)²]. To our knowledge, this is the first analysis of nonadiabatic transition probabilities during a perturbing pulse.

Optimal and robust control of quantum state transfer by shaping the spectral phase of ultrafast laser pulses

Achieving fast and efficient quantum state transfer is a fundamental task in physics, chemistry and quantum information science. However, the successful implementation of the perfect quantum state transfer also requires robustness under practically inevitable perturbative defects. Here, we demonstrate how an optimal and robust quantum state transfer can be achieved by shaping the spectral phase of an ultrafast laser pulse in the framework of frequency domain quantum optimal control theory. Our numerical simulations of the single dibenzoterrylene molecule as well as in atomic rubidium show that optimal and robust quantum state transfer via spectral phase modulated laser pulses can be achieved by incorporating a filtering function of the frequency into the optimization algorithm, which in turn has potential applications for ultrafast robust control of photochemical reactions.

Time-resolved signatures across the intramolecular response in substituted cyanine dyes

The optically populated excited state wave packet propagates along multidimensional intramolecular coordinates soon after photoexcitation. This action occurs alongside an intermolecular response from the surrounding solvent. Disentangling the multidimensional convoluted signal enables the possibility to separate and understand the initial intramolecular relaxation pathways over the excited state potential energy surface. Here we track the initial excited state dynamics by measuring the fluorescence yield from the first excited state as a function of time delay between two color femtosecond pulses for several cyanine dyes having different substituents. We find that when the high frequency pulse precedes the low frequency one and for timescales up to 200 fs, the excited state population can be depleted through stimulated emission with efficiency that is dependent on the molecular electronic structure. A similar observation at even shorter times was made by scanning the chirp (frequencies ordering) of a femtosecond pulse. The changes in depletion reflect the rate at which the nuclear coordinates of the excited state leave the Franck-Condon (FC) region and progress towards achieving equilibrium. Through functional group substitution, we explore these dynamic changes as a function of dipolar change following photoexcitation. Density functional theory calculations were performed to provide greater insight into the experimental spectroscopic observations. Complete active space (CAS) self-consistent field and CAS second order perturbation theory calculated potential energy surfaces tracking twisting and pyramidalization confirm that the steeper potential at the FC region leads to the observation of faster wave packet dynamics.

Controlling S2 Population in Cyanine Dyes Using Shaped Femtosecond Pulses

Fast population transfer from higher to lower excited states occurs via internal conversion (IC) and is the basis of Kasha's rule, which states that spontaneous emission takes place from the lowest excited state of the same multiplicity. Photonic control over IC is of interest because it would allow direct influence over intramolecular nonradiative decay processes occurring in condensed phase. Here we tracked the S2 and S1 fluorescence yield for different cyanine dyes in solution as a function of linear chirp. For the cyanine dyes with polar solvation response IR144 and meso-piperidine substituted IR806, increased S2 emission was observed when using transform limited pulses, whereas chirped pulses led to increased S1 emission. The nonpolar solvated cyanine IR806, on the other hand, did not show S2 emission. A theoretical model, based on a nonperturbative solution of the equation of motion for the density matrix, is offered to explain and simulate the anomalous chirp dependence. Our findings, which depend on pulse properties beyond peak intensity, offer a photonic method to control S2 population thereby opening the door for the exploration of photochemical processes initiated from higher excited states.

Multidimensional Electronic Spectroscopy of Photochemical Reactions

Coherent multidimensional electronic spectroscopy can be employed to unravel various channels in molecular chemical reactions. This approach is thus not limited to analysis of energy transfer or charge transfer (i.e. processes from photophysics), but can also be employed in situations where the investigated system undergoes permanent structural changes (i.e. in photochemistry). Photochemical model reactions are discussed by using the example of merocyanine/spiropyran-based molecular switches, which show a rich variety of reaction channels, in particular ring opening and ring closing, cis-trans isomerization, coherent vibrational wave-packet motion, radical ion formation, and population relaxation. Using pump-probe, pump-repump-probe, coherent two-dimensional and three-dimensional, triggered-exchange 2D, and quantum-control spectroscopy, we gain intuitive pictures on which product emerges from which reactant and which reactive molecular modes are associated.

Stochastic Liouville equations for femtosecond stimulated Raman spectroscopy

Electron and vibrational dynamics of molecules are commonly studied by subjecting them to two interactions with a fast actinic pulse that prepares them in a nonstationary state and after a variable delay period T, probing them with a Raman process induced by a combination of a broadband and a narrowband pulse. This technique, known as femtosecond stimulated Raman spectroscopy (FSRS), can effectively probe time resolved vibrational resonances. We show how FSRS signals can be modeled and interpreted using the stochastic Liouville equations (SLE), originally developed for NMR lineshapes. The SLE provide a convenient simulation protocol that can describe complex dynamics caused by coupling to collective bath coordinates at much lower cost than a full dynamical simulation. The origin of the dispersive features that appear when there is no separation of timescales between vibrational variations and the dephasing time is clarified.

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.