Session B34: Focus Session: Impact of Ultrafast Lasers II: Multidimensional Methods

Control over coherent light fields enables multidimensional coherent spectroscopy and multispectral coherent control

Using a combination of spatial and temporal shaping of optical laser fields, fully coherent spectroscopy and coherent control can be carried out to high order from optical to THz spectral ranges. A single beam with a single femtosecond pulse can be transformed into multiple beams and multiple pulses, reconfigurably under computer control with no human alignment needed, retaining full phase coherence among all the noncollinear fields. This enables multiple-quantum 2D and 3D Fourier transform optical spectroscopy of excitons and exciton-polaritons in inorganic quantum wells and microcavities, in organic J-aggregate films, and in inorganic/organic hybrid structures, the results of which will be discussed. Spatiotemporal shaping also enables coherent control over THz phonon-polariton waves in ferroelectric crystals. The THz waves can be coherently superposed to reach extremely large field amplitudes both in the host crystals and in free space, and the fields can be further enhanced in dipolar antenna and metamaterial structures, enabling highly nonlinear coherent spectroscopy and coherent control in the THz regime. Results from solid, liquid, and gas phases, including multiple-quantum rotational coherences in molecular gases and THz-induced phase transitions in crystalline solids, will be presented. Prospects for further generalization of the approach all the way to the hard x-ray regime will be discussed.   

Solvent Influenced Fluxionality Studied by Ultrafast Chemical Exchange Spectroscopy

Two-dimensional infrared spectroscopy (2DIR) allows unprecedentedly detailed understanding of the dynamics of chemical systems in the condensed phase. Carbonyl vibrations of small transition metal complexes report intramolecular dynamics and solvent-solute interactions due to their strong oscillator strengths and moderate environmental sensitivity. We studed the fluxional dynamics of iron pentacarbonyl (Fe(CO)$_{5}$), which is unique in that it contains nearly perfectly uncoupled vibrational modes. We seek to probe the ``molecularity'' of condensed phase activated barrier crossings beyond the continuum Kramers theory picture. Using 2DIR chemical exchange spectroscopy, we show how the dynamics of Berry pseudorotation, the only significant mechanism for vibrational mode mixing on our experimental timescale, is sensitive to interactions with the environment. In a wide range of solvents, we have investigated the effects of hydrogen bonding with alcohols and friction from high viscosity alkanes. In addition, we have monitored vibrational energy redistribution as a solvation shell probe. Moreover, recently implemented mid-infrared pulse shaper based methods allow increased flexibility in experimental design, enabling experimental techniques that are not possible using passive optics.   

Constant-Speed Vibrational Signaling along Polyethyleneglycol Chain up to 60{\AA} Distance

A series of azido-PEG-succinimide ester oligomers with a number of repeating PEG units of 0, 4, 8, and 12 (azPEG0, 4, 8, and 12) was investigated using a relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy method. The RA 2DIR method relies on the energy transport in molecules and is capable of correlating the frequencies of vibrational modes separated by large through-bond distances. Excitation of the azido group in the compounds at ca. 2100 cm-1 generates an excess energy which propagates in the molecule as well as dissipates into the solvent. We discovered that a part of the excess energy propagates ballistically via the covalent backbone of the molecules with a constant speed of ca. 550 m/s. The transport is described as a propagation of a vibrational wavepacket having a mean-free-path length of ca. 11{\AA}. The discovery has the potential for developing new efficient signal transduction strategies for molecular electronics and biochemistry. It also permits extending the distances accessible in RA 2DIR structural measurements up to ca. 60{\AA}.   

Effects of Symmetry on Intense-field Ionization of Heterocyclic Organic Molecules

We report on the ultrafast photoionization of pyridine, pyridazine, pyrimidine and pyrazine. These four molecules represent a systematic series of perturbations into the structure of a benzene ring which explores the substitution of a C-H entity with a nitrogen atom, creating a heterocyclic structure which remains isoelectronic with benzene. Other than pyridine, each molecule has the same molecular formula, with the only difference being the placement of the perturbing nitrogen atoms (ortho-, meta- or para-substitutions). Differences in the intense-field behavior of these molecules are caused by the symmetry of the perturbation to the benzene system, primarily influenced through the interactions of lone-pair states surrounding the nitrogen atoms. Data is recorded under intense-field, single-molecule conditions. 50 fs, 800 nm pulses are focused into the molecular vapor, and ion mass spectra are recorded for intensities of $\sim $10$^{13}$ W/cm$^{2}$ to $\sim $10$^{15}$ W/cm$^{2}$. We measure ion yields in the absence of the focal volume effect without the need for deconvolution of the data. For all targets, stable singly- and doubly-charged parent ions (C$_{6-n}$H$_{6-n}$N$_{n}^{+(+)})$ are observed with features suggesting resonance enhancement (REMPI).   

Experimental determination of Hamiltonian via 3D Fourier-transform spectroscopy

Prediction and control of quantum mechanical processes requires knowledge about the system Hamiltonian. For coherent control, information about interfering quantum pathways or the underlying Hamiltonian is essential for achieving deterministic control. Even in cases of closed-loop control, a priori knowledge about the system Hamiltonian provides guidance for designing an efficient learning algorithm and a good initial guess for faster convergence. The complete Hamiltonian of a complicated system, especially the effects of inter-particle interactions and coupling to the environments, can only be determined experimentally. Here we demonstrate an experimental determination of the Hamiltonian of an atomic vapor, achieved by using 3-dimensional Fourier transform (3DFT) spectroscopy. The 3DFT spectra provide complete information about the third-order coherent response of the vapor. The contributions from different quantum pathways are unambiguously isolated such that the components of the Hamiltonian, including energy levels, dipole moments and relaxation rates, can be determined. The 3DFT spectroscopy opens an avenue towards identifying the Hamiltonian of complex molecular systems, which can be useful for designing coherent control strategies and for studying the molecular dynamics.   

Nonradiative-decay mechanisms in CdSe nanoparticles: MUPPETS (multiple population-period transient spectroscopy) in excitonic systems

Nonradiative decay in semiconductor nanoparticles on the picosecond to nanosecond time scale is complex and poorly understood. Here, two-dimensional (2D) incoherent spectroscopy (MUPPETS) is applied to these processes in CdSe nanoparticles. For the first time, MUPPETS is extended to multilevel, excitonic systems to yield an analog of 2D coherent correlation spectroscopies. In core-only CdSe particles, the transfer of an excited electron from the core to the surface follows a highly dispersed, power-law decay in 1D measurements. 2D-MUPPETS measurements show that the rate dispersion is not due to relaxation nor due to multi-step kinetics, but results solely from particle-to-particle heterogeneity in the barrier to the surface. A model in which surface defects are distributed within the dipolar electric field of the particle accounts for the power-law decay. A second study of CdSe:ZnS core-shell particles uses correlation MUPPETS to distinguish biexcitons from photoproducts with a fast relaxing single exciton. Even when both species have similar lifetimes, they are distinguishable by having opposite signs and different symmetries in the two time intervals of a 2D experiment. Potential correlations between biexciton and exciton rates are sought, but are not found.