Investigating the collective nature of cavity-modified chemical kinetics under vibrational strong coupling

In this paper, we develop quantum dynamical methods capable of treating the dynamics of chemically reacting systems in an optical cavity in the vibrationally strong-coupling (VSC) limit at finite temperatures and in the presence of a dissipative solvent in both the few and many molecule limits. In the context of two simple models, we demonstrate how reactivity in the collective VSC regime does not exhibit altered rate behavior in equilibrium but may exhibit resonant cavity modification of reactivity when the system is explicitly out of equilibrium. Our results suggest experimental protocols that may be used to modify reactivity in the collective regime and point to features not included in the models studied, which demand further scrutiny.

Shaping the laser control landscape of a hydrogen transfer reaction by vibrational strong coupling. A direct optimal control approach

Controlling molecular reactivity by shaped laser pulses is a long-standing goal in chemistry. Here, we suggest a direct optimal control approach that combines external pulse optimization with other control parameters arising in the upcoming field of vibro-polaritonic chemistry for enhanced controllability. The direct optimal control approach is characterized by a simultaneous simulation and optimization paradigm, meaning that the equations of motion are discretized and converted into a set of holonomic constraints for a nonlinear optimization problem given by the control functional. Compared with indirect optimal control, this procedure offers great flexibility, such as final time or Hamiltonian parameter optimization. A simultaneous direct optimal control theory will be applied to a model system describing H-atom transfer in a lossy Fabry-Pérot cavity under vibrational strong coupling conditions. Specifically, optimization of the cavity coupling strength and, thus, of the control landscape will be demonstrated.

Controlled intramolecular H-transfer in malonaldehyde in the electronic ground state mediated through the conical intersection of 1nπ* and 1ππ* excited electronic states

We report photo-isomerization of malonaldehyde in its electronic ground state (S0), mediated by coupled 1nπ*(S1)-1ππ*(S2) excited electronic states, accomplished with the aid of optimally designed ultraviolet (UV)-laser pulses. In particular, control of H-transfer from a configuration predominantly located in the left well (say, reactant) to that in the right well (say, product) of the electronic ground S0 potential energy surface is achieved by a pump-dump mechanism including the nonadiabatic interactions between the excited S1 and S2 states. An interplay between the nonadiabatic coupling due to the conical intersection of the S1 and S2 states and the laser-molecule interaction is found to be imprinted in the time-dependent electronic population. The latter is also examined by employing optimal fields of varying intensities and frequencies of the UV laser pulses. For the purpose of the present study, we constructed a three-state and two-mode coupled diabatic Hamiltonian with the help of adiabatic electronic energies and transition dipole moments calculated by ab initio quantum chemistry methods. The electronic diabatic model is developed using the calculated adiabatic energies of the two excited electronic states (S1 and S2) in order to carry out the dynamics study. The optimal fields for achieving the controlled isomerization are designed within the framework of optimal control theory employing the optimization technique of a multitarget functional using the genetic algorithm. The laser-driven dynamics of the system is treated by numerically solving the time-dependent Schrödinger equation within the dipole approximation. A time-averaged yield of the target product of ∼40% is achieved in the present treatment of dynamics with optimal laser pulses.

Molecular ground-state dissociation in the condensed phase employing plasmonic field enhancement of chirped mid-infrared pulses

Selective bond cleavage via vibrational excitation is the key to active control over molecular reactions. Despite its great potential, the practical implementation in condensed phases have been hampered to date by poor excitation efficiency due to fast vibrational relaxation. Here we demonstrate vibrationally mediated, condensed-phase molecular dissociation by employing intense plasmonic near-fields of temporally-shaped mid-infrared (mid-IR) pulses. Both down-chirping and substantial field enhancement contribute to efficient ladder climbing of the carbonyl stretch vibration of W(CO)6 in n-hexane solution and to the resulting CO dissociation. We observe an absorption band emerging with laser irradiation at the excitation beam area, which indicates that the dissociation is followed by adsorption onto metal surfaces. This successful demonstration proves that the combination of ultrafast optics and nano-plasmonics in the mid-IR range is useful for mode-selective vibrational ladder climbing, paving the way toward controlled ground-state chemistry.

Toward understanding tautomeric switching in hydroxynaphthaldehydes: Characterization of electronic absorption spectra

Long-wavelength electronic absorption spectra of 4-hydroxy-1-naphthaldehyde, its dimer complexes, and 4-hydroxy-3-(piperidine-1-ylmethyl)-1-naphthaldehyde are investigated using time-dependent density functional theory with the TPSSh functional within a continuum solvation model. The results are correlated to recent experimental findings on solvent-, pH- and concentration-dependent absorption. It is confirmed that with decreasing wavelength the spectrum is dominated by the deprotonated (360 nm–400 nm), the dimer (340 nm–370 nm) and the monomer (< 280 nm) species. The potential use of hydroxynaphthaldehydes for the design of tautomeric switches is discussed.

Toward understanding tautomeric switching in 4-hydroxynaphthaldehyde and its dimers: A DFT and quantum topology study

The electronic structures and stabilities of all benzenoid (enol) and quinonoid (keto) forms of 4-hydroxynaphthaldehyde (ALD-14) have been investigated using density functional theory (DFT) with a range of functionals and basis sets. The anti-enol form represents the global minimum energy structure. Low rotation barriers of both the hydroxyl and the aldehyde groups characterize this form. Fourier analysis of the potential energy function for rotation indicate that the conformational preference of ALD-14 is determined by both the dipole–dipole repulsion and bond moments interactions. Further, three different ALD-14 dimer complexes are investigated, i.e. head-to-tail (HT), head-to-head (HH), and stacked (S) forms. The analysis of natural bond order, quantum topology features of the Laplacian of the electron density, binding energies and structural parameters of these dimers point to comparable stabilities of the HT and S-dimers, with a preference for a stacking contact. The origin of its stability can be traced to π-conjugative, H-bonding and dispersion interactions.

Arresting consecutive steps of a photochromic reaction: studies of β-thioxoketones combining laser photolysis with NMR detection

Laser photolysis coupled with NMR detection was used for the identification of photoproducts and the photoreaction pathway of monothiodibenzoylmethane.

Investigations of an O-H...S hydrogen bond via Car-Parrinello and path integral molecular dynamics

The presence of intramolecular hydrogen bonds influences the binding energy, tautomeric equilibrium, and spectroscopic properties of various classes of organic molecules. This article discusses the O-H...S bridge, one of the less commonly investigated types of intramolecular interactions. 3-mercapto-1,3-diphenylprop-2-en-1-one was considered as the model structure. This compound exhibits photochromic properties. Car-Parrinello molecular dynamics (CPMD) was applied to investigate the spectroscopic and molecular properties of this compound in the gas phase and in the solid state. The second part of the study is devoted to the effects of the quantization of nuclear motions, with special attention to the O-H...S moiety. Path integral molecular dynamics (PIMD) of the molecular crystal of 3-mercapto-1,3-diphenylprop-2-en-1-one was carried out for this purpose. The employment of this fully quantum mechanical technique enables one to study, in a time-averaged sense, the zero-point motion important for flat potential energy surfaces. Finally, the potentials of mean force (Pmfs) were calculated from the CPMD and PIMD data obtained for the solid-state calculations. The effect of including quantum nuclear motion was investigated. In the studied compound, quantum effects shortened the H-bridge and provided a better description of the free energy minimum. The computational results place this uncommon intramolecular H-bonding among the class of strong hydrogen bonds with large red shifts of O-H stretching modes, which correspond well with previously presented experimental data in the literature concerning this structure.

Controlling vibrational excitation with shaped mid-IR pulses

We report selective population of the excited vibrational levels of the T(1u) CO-stretching mode in W(CO)(6) using phase-tailored, femtosecond mid-IR (5.2 microm, 1923 cm(-1)) pulses. An evolutionary algorithm was used to optimize specific vibrational populations. Stimulated emission peaks, indicative of population inversion, could be induced. Systematic truncation of each optimized pulse allowed for increased understanding of the excitation mechanism. The pulses and techniques developed herein will have broad applications in controlling ground state chemistry and enhancing vibrational spectroscopies.

A nonperturbative calculation of nonlinear spectroscopic signals in liquid solution

Nonlinear spectroscopic signals in liquid solution were calculated without treating the field-matter interaction in a perturbative manner. The calculation is based on the assumption that the intermolecular degrees of freedom can be treated classically, while the time evolution of the electronic state is treated quantum mechanically. The calculated overall electronic polarization is then resolved into its directional components via the method of Seidner et al. [J. Chem. Phys. 103, 3998 (1995)]. It is shown that the time dependence of the directional components is independent of laser intensity in the impulsive pulse regime, which allows for flexibility in choosing the procedure for calculating optical response functions. The utility and robustness of the nonperturbative procedure is demonstrated in the case of a two-state chromophore solvated in a monoatomic liquid, by calculating nonlinear time-domain signals in the strong-field, weak-field, impulsive, and nonimpulsive regimes.

Terahertz-Laser Control of Large Amplitude Vibrational Motion in the Sub-One-Cycle Pulse Limit

We investigate the control of state-selective population transfer in the THz spectral range generated by sub-one-cycle pulse excitation. To this end we developed a zero-net-force modification of the optimal control algorithm which allows us to extend the algorithm into the ultrashort pulse domain. By combining the analysis of the control landscapes and that of optimal control theory, we were able to formulate a general mechanism suitable for laser control by ultrashort pulses. The strategy consists of a superposition of two pi-pulses with carrier envelope phases of phi = pi/2. The first pulse is effectively in resonance with the targeted transition, while the second one, fired at around the minimum of the first pulse second lobe, removes leaking to the dipole-coupled background state. To compensate for the pulses ultrashort duration, the carrier frequencies of both pulses are red-shifted from the spectroscopic resonance.

Quantum optimal control of molecular isomerization in the presence of a competing dissociation channel

The quantum optimal control of isomerization in the presence of a competing dissociation channel is simulated on a two-dimensional model. The control of isomerization of a hydrogen atom is achieved through vibrational transitions on the ground-state surface as well as with the aid of an excited-state surface. The effects of different competing dissociation channel configurations on the isomerization control are explored. Suppression of the competing dissociation dynamics during the isomerization control on the ground-state surface becomes easier with an increase in the spatial separation between the isomerization and dissociation regions and with a decrease in the dissociation channel width. Isomerization control first involving transfer of amplitude to an excited-state surface is less influenced by the dissociation channel configuration on the ground-state surface, even in cases where the excited-state surface allows for a moderate spreading of the excited wave packet.