Darin J. Ulness

Competitive events in fifth order time resolved coherent Raman scattering: Direct versus sequential processes

Higher order time resolved nonlinear optical processes can often be obscured by sequential lower order processes that compete with the direct event and give similar time domain behavior though they probe different dynamics. This is true for a certain fifth order coherent anti-Stokes Raman scattering (CARS) spectroscopy designed to probe overtone vibrational dynamics. The homodyned intensity from the two competing processes is calculated and it is shown how only the direct fifth order polarization probes overtone dephasing.

Frequency and time resolved coherent Stokes Raman scattering in CS2 using incoherent light

Abstract A frequency and time resolved coherent Stokes Raman scattering signal from CS2( l ) using noisy light (I(2) CSRS) is presented. A broad range of frequencies are simultaneously sampled at each interferometric delay, allowing acquisition times that are at least two orders of magnitude shorter than previous single frequency detected interferometry experiments. Sub-picosecond time resolution of radiation/matter beats called Rabi detuning oscillations is achieved by controlling the relative delay between twin nanosecond broadband pulses.

Fifth-order nonlinear Raman processes in molecular liquids using quasi-cw noisy light. II. Experiment

Fifth-order analogs of coherent Raman scattering generated in a number of molecular liquids using broadband quasi-cw noisy light are presented. It is seen how the signal for the direct fifth-order process, which probes the dynamics of both a fundamental vibration and its overtone, is often contaminated by a sequential process, that is only capable of probing the vibrational dynamics of the fundamental. Although these two processes are virtually indistinguishable when a single Raman resonance is excited, we find that when a second Raman resonance is available within the experimental window governed by the bandwidth of the noisy light, new frequency components in the signal arise and the two competing fifth-order processes become distinguishable. These new frequency components, as well as their decay, are explained in terms of spectral filtering of the noisy light by the Raman resonances. This spectral filter analogy predicts which of the two competing processes dominates in an equimolar mixture of benzene-...

Fifth-order nonlinear Raman processes in molecular liquids using quasi-cw noisy light. I. Theory

Fifth-order nonlinear Raman processes using broadband, incoherent light are treated for a multiply resonant, multicomponent mixture. In particular, the theoretical development of the direct and the sequential fifth-order analogs of coherent Raman scattering is presented. Of the complete formalism, only the dominant doubly Raman resonant hyperpolarizability contributions to the signal intensity are discussed in this article. Furthermore, application is made to simulate fifth-order signals from a variety of hypothetical molecular liquids. It is seen how the direct and the sequential processes can distinguish themselves in a mixture, in a neat liquid with more than one Raman coherence, and also whenever the Raman active modes are taken to be anharmonic. This theoretical treatment anticipates experimental results presented in the following paper.

Theory of coherent Raman scattering with quasi-cw noisy light for a general line shape function

The theory of electronically nonresonant coherent Raman scattering (CRS) with quasi-cw noisy light (I(2) CRS) is developed for a general material response. The (Raman) resonant–resonant and resonant–nonresonant hyperpolarizability contributions to the I(2) CRS signal are interferometrically separable. It is found that, in general, the interferometric decay of each of these terms exposes the Raman line shape function in a different manner. Only for a Lorentzian line is their decay identical. Thus, in principle, I(2) CRS provides a new way to explore the line shape function that is analytically distinct from frequency domain and time domain methods. By way of illustration, the general theory is applied to three common line shapes: Lorentzian (as in the original I(2) CRS theory), Gaussian, and Voigt. The results are shown to be consistent with the principles of factorized time correlation diagram analysis.

Frequency resolved interferometric coherent Raman spectroscopy with incoherent light: Raman frequency shifts, dephasing rate constants, and nonresonant hyperpolarizabilities of mixtures of benzene in hexane

Abstract A newly developed frequency and time resolved coherent anti-Stokes Raman scattering (CARS) technique is used to study properties of the A1g ring breathing mode of benzene in a dilution series of liquid mixtures with n-hexane. Recent advances in the interferometric use of incoherent light in CARS measurements (I(2)CARS) provide excellent precision in the recovered dephasing rate constants, vibrational frequencies, and the ratios of the resonant to non-resonant contributions to the signal. Using this technique we observe a more than two-fold decrease in the dephasing rate constant of the vibration, accompanied by a 0.54 cm−1 increase in the vibrational frequency in the limit of infinite dilution of benzene in n-hexane. These changes in the vibrational spectrum are discussed in terms of the condensed phase dynamics that are responsible. It is also seen how the non-resonant contributions to the signal manifest themselves quantitatively in these frequency and time resolved I(2)CARS measurements.

Coherent multidimensional optical spectra measured using incoherent light.

Four-wave mixing measurements can reveal spectral and dynamics information that is hidden in linear spectra by the interactions among light-absorbing molecules and with their environment. Coherent multidimensional optical spectroscopy is an important variant of four-wave mixing because it resolves a map of interactions and correlations between absorption bands. Previous coherent multidimensional optical spectroscopy measurements have used femtosecond pulses with great success, and it may seem that femtosecond pulses are necessary for such measurements. Here we present coherent two-dimensional electronic spectra measured using incoherent light. The spectra of model molecular systems using broadband spectrally incoherent light are similar but not identical to those expected from measurements using femtosecond pulses. Specifically, the spectra show particular sensitivity to long-lived intermediates such as photoisomers. The results will motivate the design of similar experiments in spectral ranges where femtosecond pulses are difficult to produce.

Two-dimensional electronic spectroscopy using incoherent light: theoretical analysis.

Electronic energy transfer in photosynthesis occurs over a range of time scales and under a variety of intermolecular coupling conditions. Recent work has shown that electronic coupling between chromophores can lead to coherent oscillations in two-dimensional electronic spectroscopy measurements of pigment-protein complexes measured with femtosecond laser pulses. A persistent issue in the field is to reconcile the results of measurements performed using femtosecond laser pulses with physiological illumination conditions. Noisy-light spectroscopy can begin to address this question. In this work we present the theoretical analysis of incoherent two-dimensional electronic spectroscopy, I((4)) 2D ES. Simulations reveal diagonal peaks, cross peaks, and coherent oscillations similar to those observed in femtosecond two-dimensional electronic spectroscopy experiments. The results also expose fundamental differences between the femtosecond-pulse and noisy-light techniques; the differences lead to new challenges and new opportunities.

Halogen Bonding in Iodo-perfluoroalkane/Pyridine Mixtures

Mole fraction and temperature studies of halogen bonding between 1-iodo-perfluorobutane, 1-iodo-perfluorohexane, or 2-iodo-perfluoropropane and pyridine were performed using noisy light-based coherent anti-Stokes Raman scattering (I((2)) CARS) spectroscopy. The ring breathing mode of pyridine both is highly sensitive to halogen bonding and provides a strong I((2)) CARS signal. As the lone pair electrons from the pyridinyl nitrogen interact with the sigma-hole on the iodine from the iodo-perfluoroalkane, the ring breathing mode of pyridine blue-shifts proportionately with the strength of the interaction. The measured blue shift for halogen bonding of pyridine and all three iodo-perfluoroalkanes is comparable to that for hydrogen bonding between pyridine and water. 2-Iodo-perfluoropropane displays thermodynamic behavior that is different from that of the 1-iodo-perfluoroalkanes, which suggests a fundamental difference at the molecular level. A potential explanation of this difference is offered and discussed.

Ion-Pair Interaction in Pyridinium Carboxylate Solutions

Ion-pair interactions between pyridinium cations and various carboxylate anions are explored using noisy light based coherent anti-Stokes Raman scattering (I(2)CARS). Binary mixtures of pyridine and various carboxylic acids (including halo-acetic acids, straight-chain carboxylic acids, and pivalic acid) are prepared. A Brønsted type acid-base reaction occurs in these mixtures to create pyridinium and carboxylate ions. Both pyridine, itself, and pyridinium have strong I(2)CARS signals originating from their ring breathing modes. The vibrational frequency of the ring breathing mode for pyridine is blue-shifted by hydrogen bonding, and that same mode for pyridinium is red-shifted by ion-pair interaction. Frequency shift data for the ring breathing mode of pyridine and pyridinium are presented. These data are discussed in terms of a simplistic model for the electronic behavior of these compounds.