Session B38: Earle K. Plyler Prize Session I: Spectroscopy

Earle K. Plyler Prize for Molecular Spectroscopy Talk: Coherent Ultrafast Multidimensional Spectroscopy of Molecules; From NMR to X-rays

Multidimensional spectroscopic techniques which originated with NMR in the 1970s have been extended over the past 15 years to the optical regime. NMR spectroscopists have developed methods for the design of pulse sequences that resolve otherwise congested spectra, enhance selected spectral features and reveal desired dynamical events. The major experimental and computational advances required for extending these ideas to study electronic and vibrational motions on the femtosecond timescale will be surveyed. The response of complex molecules and semiconductor nanostructures to sequences of optical pulses provides snapshots of their structure and dynamical processes. Two-dimensional correlation plots of the signals show characteristic cross-peak patterns which carry information about hydrogen bonding, secondary structure fluctuations of proteins and amyloid fibrils, and coherent and incoherent energy and charge transfer in photosynthetic complexes. Double quantum coherence signals that are induced by correlations among electrons or excitons allow the visualization of correlated wavefunctions. Future extensions to the attosecond regime using xray pulses will be discussed. Since core excitations are highly localized at selected atoms, such signals can monitor the motions of valence electron wavepackets in real space with atomic spatial resolution. Common principles underlying coherent spectroscopy techniques for spins, valence electrons, and core electronic excitations, spanning frequencies from radiowaves, infrared, ultraviolet all the way to hard X-rays will be discussed. \\[4pt] [1] ``Coherent Multidimensional Optical Probes for Electronic Correlations and Exciton Dynamics; from NMR to X-rays'', S. Mukamel, D. Abramavicius, L. Yang, W.Zhuang, I.V. Schweigert and D. Voronine. Acct.Chem.Res. Acct.Chem.Res. 42, 553-562 (2009). \\[0pt] [2] ``Coherent Multidimensional Optical Spectroscopy Excitons in Molecular Aggregates; Quasiparticle vs. Supermolecule Perspectives'', D. Abramavicius, B. Palmieri, D. Voronine, F. Sanda and S. Mukamel, Chem. Rev. 109, 2350-2408 (2009).   

Photoelectron spectroscopy of solvated electrons in liquid tetrahydrofuran and methanol microjets

Solvated electrons are an important species in radiation chemistry, biology, and other areas. As the simplest quantum solute, solvated electrons are a critical benchmark to test our understanding of solvation in general. Furthermore, when formed in cells, they are highly reactive and may lead to irreversible damage. It is, therefore, important to understand the energetics associated with electron solvation. To this end, we have undertaken a series of studies directly probe electron vertical binding energies (VBEs) in solvents introduced to vacuum through liquid microjets. Solvated electrons are generated following the excitation of the charge-transfer-to-solvent (CTTS) precursor state of iodide from a millimolar concentration salt included in the solution, detached to vacuum, and measured with our field-free time-of-flight spectrometer. Here we present preliminary results of the measurement of the VBE of electrons solvated in bulk tetrahydrofuran and methanol.   

The Solvated Electron in Acetonitrile

The nature of solvated electrons in liquid acetonitrile is of great interest as it appears that excess electrons in this solvent are stabilized in two forms, a dipole-bound (DB) electron (i.e. a typical solvated electron) and a valence-bound electron (VB) electron (e.g. a solvated CH3CN dimer anion). Previous work has suggested that these two species are in equilibrium and can interconvert. We performed 3-pulse transient hole-burning experiments aimed at better understanding the nature of the VB and DB electrons. We found that photoexcitation of VB electrons produces an increased population of DB electrons, but that exciting DB electrons does not produce VB electrons. This suggests a significant asymmetry in the solvent motions that accompany photoexcitation of the electron: it is easier for a DB electron to relax back into the solvent location from which it came than for the local solvation structure to change enough to create a VB electron, whereas excitation of a VB electron disrupts the local solvent structure to the point where the excited electron can relax into the bulk solvent rather than back to the molecules on which it initially resided.   

Dynamics of electron solvation in I\={ }(CH$_{3}$OH)$_{n}$ clusters (4 $\le \quad n \quad \le $ 11)

The dynamics of electron solvation following excitation of the charge-transfer-to-solvent (CTTS) precursor state in iodide-doped methanol clusters, I\={ }(CH$_{3}$OH)$_{n=4-11}$ are studied with time-resolved photoelectron imaging (TRPEI). This excitation produces a I$^{\ldots }$(CH$_{3}$OH)$_{n}$\={ } cluster that is unstable with respect to electron autodetachment, and whose autodetachment lifetime increases monotonically from $\sim $800 fs to 85 ps as $n$ increases from 4-11. The vertical detachment energy (VDE) and width of the excited state feature in the photoelectron spectrum show complex time dependences during the lifetime of this state. The VDE decreases over the first 100-400 fs, then rises exponentially to a maximum with a $\sim $ 1ps time constant, decreasing by as much as 180 meV with timescales from 4-10 ps. The early dynamics are assigned to electron transfer from the iodide to a localized portion of the methanol cluster, while the longer-time changes in VDE are attributed to solvent reordering, possibly in conjunction with ejection of neutral iodine from the cluster. Changes in the observed width of the spectrum largely follow those of the VDEs; the dynamics of both are attributed to the major rearrangement of the solvent cluster during relaxation. The relaxation dynamics are interpreted as a reorientation of at least one methanol molecule and the disruption and formation of the solvent network in order to accommodate the excess charge.   

Vibronic Enhancement of Exciton Sizes and Energy Transport in Photosynthetic Complexes

This talk investigates the impact of vibronic couplings on the electronic structures and relaxation mechanisms of two cyanobacterial light harvesting proteins, allophycocyanin (APC) and c-phycocyanin (CPC). Both APC and CPC possess three pairs of pigments (i.e., dimers), which undergo electronic relaxation on the sub-picosecond time scale. Electronic relaxation is approximately 10 times faster in APC than in CPC despite the nearly identical structures of their pigment dimers. Femtosecond laser spectroscopies conducted in conjunction with a Frenkel exciton model find that photo-induced electronic relaxation in these two proteins is understood on the same footing only when the vibronic couplings in high-frequency modes are properly taken into account. In addition to incorporating high-frequency intramolecular modes in the spectral density, we simulate electronic relaxation dynamics using a model in which the excitons delocalize in a vibronic basis. General implications of the present findings for energy transport in artificial systems (e.g., crystalline organic semiconductors) are discussed.   

Nonlinear Coherent Optical Imaging for Biomedicine: The Quest for Ultimate Sensitivity

Recent advances in nonlinear coherent optical imaging, particularly stimulated Raman scattering microscopy, have allowed highly sensitive label-free imaging of living cells and organisms based on molecular spectroscopy. Using the ultimate sensitivity of nonlinear optical microscopy, the detection of a single-molecule absorption signal at room temperature has been achieved. These unprecedented sensitivities offer exciting possibilities for biomedicine.   

Optical Control of Conjugated Oligomer Planarity

Using a sequential photo-excitation mechanism we observe the ultrafast conformational planarization of a large fluorene oligomer at $\sim $ 60fs timescale. Novel non-adiabatic excited state molecular dynamics (NA-ESMD) framework incorporating quantum transitions has been used to rationalize this phenomenon. Simulation show the ultrafast relaxation of the photoexcited wavepacket toward the lowest electronic excited state along the torsional coordinate. The process effectively `locks' the oligomer into a planar state within 100~fs, with excess energy being dissipated into other vibrational modes. Ultrafast control of molecular conformation, as demonstrated here, could have impacts for molecular conformational switches for memory or molecular electronics.   

Using 2D Fourier-transform spectroscopy to separate homogeneous and inhomogeneous line widths of heavy- and light-hole excitons in weakly disordered semiconductor quantum wells

Optical two-dimensional Fourier-transform spectroscopy is used to study the heavy- and light-hole excitonic resonances in GaAs quantum wells with weak structural disorder. Homogeneous and inhomogeneous broadening contribute differently to the two-dimensional resonance line shapes, allowing separation of homogeneous and inhomogeneous line widths. The heavy-hole exciton exhibits more inhomogeneous than homogeneous broadening, whereas the light-hole exciton shows the opposite. This situation arises from the interplay between the length scale of the disorder and the exciton Bohr radius, which affects the exciton localization and scattering. Utilizing this separation of line widths, excitation-density-dependent measurements reveal that many-body interactions alter the homogeneous dephasing, while disorder-induced dephasing is unchanged.   

Oxygen atom roaming and multiple dissociation pathways of NO$_{3}$

The role of nitrate radical (NO$_{3})$ photolysis in atmospheric has long been known, but mysteries remain regarding the mechanism of the dissociation. In particular, the NO + O$_{2}$ channel has proven to be a challenge both theoretically and experimentally. High resolution velocity map ion imaging studies reveal that there are two distinct mechanisms to form the NO + O$_{2}$ products. Additionally, the dominant of these mechanisms appears to be the non-traditional state ``roaming'' mechanism recently identified in formaldehyde dissociation. The roaming mechanism involves large amplitude motion associated with a frustrated radical dissociation before roaming oxygen atom abstraction to form O$_{2}$. The identification of roaming in the NO$_{3}$ reaction may imply the widespread importance of this type of mechanism in atmospheric chemistry.   

First Principle Simulations of the Infrared Spectrum of Liquid Water using Hybrid Density Functionals

We report on calculations of the infrared spectrum (IR) of liquid water carried out using first principle molecular dynamics and the hybrid functional PBE0. We find results in much better agreement with experiment than those obtained using semi-local, gradient corrected exchange correlation functionals. In particular the description of the IR stretching band is greatly improved and in good accord with recent measurements. When adopting the PBE0 functional, substantial improvement is also found in the description of the structural properties of the liquid, consistent with a smaller average number of hydrogen bonds, and a reduced molecular dipole moment, as revealed by our analysis of maximally localized Wannier functions. Finally the average electronic gap of the liquid is increased by 60\% with respect to PBE, when computed at the PBE0 level of theory, and is in fair agreement with experiment. Work supported by NSF/OCI-0749217.   

Nonequilibrium Mixed Quantum-Classical simulations of Hydrogen-bond Structure and Dynamics in Methanol-d Carbon tetrachloride liquid mixtures and its spectroscopic signature

Liquid mixtures of methanol-d and carbon tetrachloride provide attractive model systems for investigating hydrogen-bond structure and dynamics. The hydrogen-bonded methanol oligomers in these mixtures give rise to a very broad hydroxyl stretch IR band ($\sim$150 cm$^{-1}$). We have employed mixed quantum-classical molecular dynamics simulations to study the nature of hydrogen- bond structure and dynamics in this system and its spectroscopic signature. In our simulations, the hydroxyl stretch mode is treated quantum mechanically. We have found that the absorption spectrum is highly sensitive to the type of force fields used. Obtaining absorption spectra consistent with experiment required the use of corrected polarizabile force fields and a dipole damping scheme. We have established mapping relationships between the electric field along the hydroxyl bond and the hydrogen-stretch frequency and bond length thereby reducing the computational cost dramatically to simulate the complex nonequilibrium dynamics underlying pump-probe spectra.