Session N26: Focus Session: Non-adiabatic Molecular Dynamics and Control at Conical Intersections IV

Coherent 2D Spectroscopy and Control of Molecular Complexes

Coherent two-dimensional femtosecond spectroscopy is used to investigate electronic couplings within molecular complexes. Third-order optical response functions are measured in a non-collinear three-pulse photon echo geometry with heterodyne signal detection. In combination with suitable simulations this allows recovering the delocalization of excited-state wavefunctions, their coupling, and the corresponding energy transport pathways, with nanometer spatial and femtosecond temporal resolution. Examples of multichromophoric systems are the FMO and the LH3 light-harvesting complexes from green sulfur bacteria and purple bacteria, respectively, for which energy transfer processes have been determined. Additional challenges arise if one is interested in the spectroscopy of photochemical rather than photophysical processes in molecular complexes: The product yields attained by a single femtosecond laser pulse are often very small, and hence time-dependent signals are hard to measure with good signal-to-noise ratio. In the context of coherent control, this implies that bond-breaking photochemistry in liquids is still difficult despite the many successes of optimal control in gas-phase photodissociation. In a novel accumulative scheme, macroscopic amounts of stable photoproducts are generated in an optimal fashion and with high product detection sensitivity. In connection with time-resolved spectroscopy, the accumulative scheme furthermore provides kinetic information on the pathways of low-efficiency chemical reaction channels. This was applied to investigate the photoconversion of green fluorescent protein.   

Watching the electronic motions driven by a conical intersection

In chemistry, the fastest electronic rearrangements proceed through ``conical intersections'' between electronic potential energy surfaces. With sufficiently short pulses, the electronic motion can be isolated by polarized excitation of aligned electronic wavepackets at a conical intersection. Polarized femtosecond probing reveals signatures of electronic wavepacket motion (due to the energy gaps) and of electron transfer between orbitals (due to the couplings) driven by the conical intersection. After exciting a D$_{4h}$ symmetry silicon naphthalocyanine molecule onto a Jahn-Teller conical intersection in the first excited state, electronic motions cause a $\sim $100 fs drop in the pump-probe polarization anisotropy. The polarized vibrational modulations of the signal can be used to deduce the symmetry and stabilization energies for each vibration. The initial decay of the polarization anisotropy can be quantitatively predicted from these vibrational parameters. Both coupling and energy gap variations are important on the $\sim $100 fs timescale. A 1 meV stabilization drives electrons from orbital to orbital in 100 fs, and the theory indicates that a chemically reactive conical intersection with 1000x greater stabilization energy could cause electronic equilibration within 2 fs. We have recently carried out experiments on a nominally D$_{2h}$ symmetry free-base naphthalocyanine for which the splitting between $x$ and $y$ polarized transitions is not resolved in the linear spectrum. For this molecule, the anisotropy also decays on a similar timescale and exhibits damped modulations whose origin (vibrational or electronic) has not yet been determined. The role of the central protons and nominal D$_{2h}$ symmetry in the electronic dynamics will be discussed.   

Electronic relaxation dynamics in (water)$_{n}^{-}$ ($n$=25-100) and (CH$_{3}$OH)$_{n}^{-}$ ($n\sim $140-530) clusters via femtosecond photoelectron imaging

Large clusters of (H$_{2}$O)$_{n}^{-}$ ($n$=25-50), (D$_{2}$O)$_{n}^{-}$ ($n$=25-100) and (CH$_{3}$OH)$_{n}^{-}$ ($n\sim $140-530) are studied with femtosecond time-resolved photoelectron imaging. For all three systems, the excess electron is promoted to an excited state with a pump laser pulse at 1.55 eV. Subsequent dynamics are monitored by observing photoelectrons detached after a variable delay with a probe pulse at 3.1 eV. For all three systems the excess electron is seen to decay via internal conversion back to the ground state with lifetimes of 190-130 fs for (H$_{2}$O)$_{n}^{-}$, 360-150 fs for (D$_{2}$O)$_{n}^{-}$ and 260-170 fs for (CH$_{3}$OH)$_{n}^{-}$. For all three systems, lifetime of the excited state decreases with increasing cluster size and is found to vary linearly with 1/$n$. Extrapolation to the bulk yields lifetimes of 54$\pm $30 fs for H$_{2}$O, 72$\pm $22 fs for D$_{2}$O and $\sim $150 fs for CH$_{3}$OH.   

Quantum dynamics and photochemistry of negative ions via photoelectron imaging and photofragment spectroscopy.

Photochemistry of molecular and cluster anions is studied using photoelectron imaging and photofragment spectroscopy. Photoelectron imaging is used to observe interference effects in electron photoemission and monitor the transformations of the electronic structure in chemical reactions. The transformations of electronic energy levels and the corresponding wavefunctions are studied in the solvent and reaction-coordinate domains. Time-resolved results reflect the electron emission dynamics, establishment of the reaction product electronic identity and provide dynamical tests of the anion electronic potentials and exit-channel interactions in chemical reactions. Photofragment spectroscopy in the solvent domain reveals solvent-enabled and solvent-controlled bond dissociation and ion-molecule association reactions in anionic environments, including state-crossings and Renner-Teller interactions.   

Conformationally controlled chemistry: Excited state dynamics dictate ground state dissociation

Ion imaging results show distinct photodissociation dynamics for propanal cations initially prepared in either the \textit{cis-} or \textit{gauche-} conformation, even though these differ only slightly in energy and there is a small barrier between them. The product kinetic energy distributions for the H elimination channels are bimodal, and the two peaks are readily assigned to propanoyl cation + H and hydroxyallyl cation + H. Ab initio multiple spawning dynamical calculations show that distinct ultrafast dynamics in the excited state leads to internal conversion to the ground state in isolated regions of the potential surface for the two conformers, and from these distinct regions, conformer interconversion does not effectively compete with dissociation.   

Conformer-selected photodissociation: Ab Initio Multiple Spawning Dynamics of Excited Propanal Cation

Recent experiments have shown that pure cis and gauche propanal cations can be prepared using REMPI (Resonance Enhanced Multiphoton Ionization Spectroscopy). The H elimination pathway which results when these conformer-selected cations are photoexcited was found to depend on the conformer (cis vs. gauche). This dependence is very surprising since the interconversion barrier of the two conformers is small compared to the excited energy. We use the ab initio multiple spawning (AIMS) method developed in our group to model the conformer-specific photodissociation and to elucidate its origins.   

A Numerical Study of Pulse-Shape Control of Non-Adiabatic Electron Excitation in the Strong-Field Regime

The electron optical response of several molecular monocations to short strong-field laser pulses was studied using time-dependent Hartree-Fock theory. In addition to the carrier frequency and maximum amplitude (up to 3.75 x 10$^{13}$ W/cm$^{2})$, the short pulses were characterized by pulse shape parameters: the amplitude profile (trapezoidal and gaussian) and the carrier phase shift. The electron response was traced by the evolution of the excited states occupation numbers and by the instantaneous dipole moment of the molecule. In the molecular monocations studied, butadiene, naphthalene, and anthracene, we observed significant modifications in the dipole moment response and in the corresponding excited state spectra, controlled by intensity, frequency, phase, and shape of the laser pulse.   

Adaptive Control Goal Selection for Strong-Field Dissociative Ionization of Polyatomic Molecules

In many settings (for instance, in strong-field mass-spectral sensing technologies) improving control efficiency is more important than achieving specific control goals. In this case, control goals may be adaptively formulated in the process of a strong-field experiment. To determine the pairs of fragment ions in a mass spectrum that are most susceptible to control by adaptive optimization of the laser pulse shapes in the strong-field regime, a statistical method is proposed that is based on covariance analysis of the mass spectral fragmentation patterns generated by a set of random shaped pulses. As a test, the method was applied to fragmentation of a large organic molecule dimethylmethylphosphonate, (CH$_{3}$O)-PO-(OCH$_{3})$-(CH$_{3})$. All possible pairs of the ionized fragments in \textit{tof} mass spectrum were ranked by the value of their correlation coefficients ranging from +1 to --1. A genetic-algorithm based adaptive control was then used to optimize the ion peak ratios in these pairs. Convincingly, the pairs of fragment ions that have higher negative covariances possess a correspondingly higher degree of controllability, while the pairs that have higher positive covariances possess correspondingly lower controllability.   

Probing Strong-Field Electron-Nuclear Dynamics of Polyatomic Molecules Using Proton Motion

Protons ejected from a large polyatomic molecule during its Coulomb explosion can carry information about the dynamics of explosion and pre-explosion processes related to specific molecular structure. To extract this information, the proton kinetic energy distributions were derived from the shape or the time-of-flight proton peak for three structure-related molecules, anthracene, octohydroanthracene, and anthraquinone, subjected to intense 800 nm, 60 fs laser pulses. The kinetic energy distributions are found to be markedly molecular-specific, providing insight into similarities and differences in the nonadiabatic electron-nuclear dynamics in these molecules during the laser pulse. In particular, analysis of the proton energy distributions reveals molecular specificity of non-adiabatic charge localization and field-mediated restructuring of polyatomic molecules polarized by strong laser fields.   

Toward Coherent Control of Cis-Stilbene Photodynamics

Stilbene can undergo photoisomerization between its cis and trans isomers. Non-radiative quenching of excited state population to the ground state can occur at a twisted and pyramidalized conical intersection of S$_{0}$ and S$_{1}$ that is remarkably similar to that found for ethylene. In addition, photo-excited cis-stilbene can undergo a cyclization reaction giving 4a,4b-dihydrophenanthrene. Here, population decay occurs through a conical intersection in the cis-stilbene configuration. Both competing reaction pathways give appreciable reactant recovery. The product branching ratios can be directly related to the location of the conical intersections in nuclear coordinate space and also, more specifically, to the wavepacket dynamics in the nonadiabatic region. A significant effort is currently underway at Los Alamos to achieve coherent control of photo-excited cis-stilbene. The goal will be to design a shaped femtosecond laser pulse that will control photo-product yield. More importantly, we hope to gain an understanding of the important features of the optimized electric field and thus insight into the prospects for more complicated materials. We will detail the potential surfaces of photo-excited cis-stilbene, the initial S$_{1}$ dynamics, as well as opportunities for quantum control.