Fourier transform measurement of two-photon excitation spectra: applications to microscopy and optimal control

We report a novel Fourier transform method for measuring two-photon excitation spectra. We demonstrate this method using simple dye molecules and discuss its applications in two-photon fluorescence microscopy and optimal control. This method facilitates an intuitive interpretation of recent control experiments in terms of tuning the nonlinear spectrum of the exciting laser source.     

Watching solvent friction impede ultrafast barrier crossings: a direct test of Kramers theory

A systematic investigation of the solvent's dynamic influence on activated barrier crossings on an electronic ground state is performed using ultrafast two-dimensional infrared chemical exchange spectroscopy. These measurements facilitate a direct comparison with the widely adopted Kramers theory of condensed phase reaction kinetics, and for the first time avoid the significant complication of electronic excitation to probe directly in the time domain a ground electronic state reaction with a well-defined transition state. The picosecond timescale interconversion between two stable isomers of the metal carbonyl complex Co(2)(CO)(8) in a series of linear alkane solvents shows negligible energetic variation with solvent carbon chain length, providing an exclusive probe of the effects of solvent friction. Relative to the linear alkane series, cyclohexane does alter the potential energy surface by preferentially stabilizing one of the isomers. Despite this pronounced modification of the reaction barrier energetics, combination of experiment and computation enables the removal of the nondynamical barrier contribution to the rate constant, isolating the dynamical influence of solvent friction. The experimental data, supported with quantum and classical computations, show agreement with a simple Markovian Kramers theory for the isomerization rate constant's dependence on solvent viscosity.     

Two-dimensional infrared spectroscopy detected by chirped pulse upconversion

A two-dimensional (2D) infrared spectrum of Mn2(CO)10 is measured by using chirped-pulse upconversion (CPU) of the nonlinear signal field plus a reference local oscillator. By converting the spectrum to the visible, a silicon CCD camera can be used. The method offers an attractive alternative to direct IR detection due to the technological maturity of silicon and its greater intrinsic detectivity over HgCdTe. Using CPU, we acquired a rephasing 2D IR spectrum in a few seconds.     

Local-mode approach to modeling multidimensional infrared spectra of metal carbonyls

We present a general approach for modeling multidimensional infrared spectra based on a combination of phenomenological fitting and ab initio electronic structure calculations. The vibrational Hamiltonian is written in terms of bilinearly coupled Morse oscillators that represent local carbonyl stretches. This should be contrasted with the previous approach, where the anharmonic Hamiltonian was given in terms of normal-mode coordinates ( Baiz et al. J. Phys. Chem. A 2009 , 113 , 9617 ). The bilinearly coupled Morse oscillator Hamiltonian is parametrized such that the frequencies and couplings are consistent with experiment, and the anharmonicities are computed by density functional theory. The advantages of the local-mode versus normal-mode approaches are discussed, as well as the ability of different density functionals to provide accurate estimates of the model parameters. The applicability and usefulness of the new approach are demonstrated in the context of the recently measured multidimensional infrared spectra of dimanganese decacarbonyl. The shifts in local site frequencies, couplings, and anharmonicities due to hydrogen bonding to the individual carbonyls are explored. It is found that, even though the effect of hydrogen bonding is nonlocal, it is additive.     

Ultrafast α-like relaxation of a fragile glass-forming liquid measured using two-dimensional infrared spectroscopy

Ultrafast two-dimensional infrared (2D-IR) spectroscopy is used to study the picosecond dynamics of a vibrational probe molecule dissolved in a fragile glass former. The spectral dynamics are observed as the system is cooled to within a few degrees of the glass transition temperature (T(g)). We observe nonexponential relaxation of the frequency-frequency correlation function, similar to what has been reported for other dynamical correlation functions. In addition, we see evidence for α-like relaxation, typically associated with long-time, cooperative molecular motion, on the ultrafast time scale. The data suggests that the spectral dynamics are sensitive to cooperative motion occurring on time scales that are necessarily longer than the observation time.     

Mid-infrared electric field characterization using a visible charge-coupled-device-based spectrometer

We characterize ultrashort mid-infrared pulses through upconversion by using the stretched pulses obtained from the uncompressed output of a chirped-pulse amplifier. The power spectrum thus translated into the visible region can be readily measured with a standard silicon CCD camera-based spectrometer. The spectral phase is also characterized by a variant of zero-added-phase spectral phase interferometry for direct electric field reconstruction. This is a general method that provides a multiplex advantage over conventional infrared detector array-based methods.     

Diffractive optics implementation of six-wave mixing

Diffractive optics are applied to six-wave mixing processes to provide a single optic approach to attaining the required, relatively complex, phase-matching geometry to discriminate against lower-order nonlinear responses. The diffractive optics were designed specifically for broad-bandwidth operation and passive phase locking of the appropriate pulse pairs for use in femtosecond two-dimensional Raman studies of the dynamic structure of liquids. The fifth-order signal was studied in liquid CS>(2); two different colors were used for the excitation and the probe to reduce background scatter, as were two different phase-matching geometries with different degrees of suppression of cascaded third-order processes.     

Solvent-hindered intramolecular vibrational redistribution

Ultrafast two-dimensional infrared spectroscopy and molecular dynamics simulations of Mn(2)(CO)(10) in a series of linear alcohols reveal that the rate of intramolecular vibrational redistribution among the terminal carbonyl stretches is dictated by the average number of hydrogen bonds formed between the solute and solvent. The presence of hydrogen bonds was found to hinder vibrational redistribution between eigenstates, while leaving the overall T(1) relaxation rate unchanged.     

Multiple structures and dynamics of [CpRu(CO)2]2 and [CpFe(CO)2]2 in solution revealed with two-dimensional infrared spectroscopy

Two-dimensional infrared (2DIR) spectroscopy is applied to both (Cp)(2)Fe(2)(CO)(4) and its ruthenium analog (Cp)(2)Ru(2)(CO)(4) in order to study the vibrational dynamics of these two systems. Combining the results of 2DIR spectroscopy and DFT calculations, the different structural forms of both the iron and the ruthenium complexes were characterized, furthering the previous assignment of the linear IR spectrum by determining the transition frequencies associated with the different isomeric forms. Monitoring the time-dependent amplitudes of the cross peaks enabled the observation of equilibrium energy transfer dynamics between different vibrational modes of the cis-B (Cp)(2)Fe(2)(CO)(4) and the gauche-NB (Cp)(2)Ru(2)(CO)(4) complexes. Treating the energy transfer as an equilibrium process, we extracted the rate constants associated with both the uphill and the downhill transfer of vibrational energy, finding that the difference in the rate constants of the two metal complexes maps to the difference in the energy gap between the two modes involved.     

Biomolecular hydration dynamics probed with 2D-IR spectroscopy: From dilute solution to a macromolecular crowd

Although it is well known that water is essential for biological function, it has been a challenge to determine how water behaves near biomacromolecular interfaces, and what role water plays in influencing the dynamics of the biochemical machinery. By adopting a vibrational labeling strategy coupled with ultrafast two-dimensional infrared (2D-IR) spectroscopy, it has recently become possible to study hydration dynamics, site specifically at the surface of proteins and model membranes. We review our recent progress in measuring hydration dynamics in contexts ranging from small-molecule solutes to biomacromolecules in dilute, viscous, and crowded environments.