Fully and Partially Coherent Pathways in Multiply Enhanced Odd-Order Wave-Mixing Spectroscopy

Direct Probe of Vibrational Fingerprint and Combination Band Coupling

Vibrational fingerprints and combination bands are a direct measure of couplings that control molecular properties. However, most combination bands possess small transition dipoles. Here we use multiple, ultrafast coherent infrared pulses to resolve vibrational coupling between CH3CN fingerprint modes at 918 and 1039 cm-1 and combination bands in the 2750-6100 cm-1 region via doubly vibrationally enhanced (DOVE) coherent multidimensional spectroscopy (CMDS). This approach provides a direct probe of vibrational coupling between fingerprint modes and near-infrared combination bands of large and small transition dipoles in a molecular system over a large frequency range.

Schrödinger Cat State Spectroscopy-A New Frontier for Analytical Chemistry

The invention of the laser generated great excitement, because its ability to create quantum state coherences could form a new family of coherent spectroscopies that were the optical analogue of multidimensional nuclear magnetic resonance (NMR). The full realization of this promise has not yet been realized, but the pathway forward is clear. The path involves the use of multiple, tunable lasers that create a Schrödinger cat state, where the system is simultaneously in a mixture of vibrational and/or electronic states. The multiplicity of these states confers many advantages for analytical methods: high selectivity from the multiple spectral dimensions, line-narrowing, isolation of spectral features where quantum states are coupled, and spectral decongestion. Now that the feasibility of Schrödinger cat spectroscopy has been demonstrated, the future is open for the development of a new frontier in analytical chemistry that creates a new set of tools for studying the complex systems that form the heart of analytical chemistry.

Multidimensional Spectral Fingerprints of a New Family of Coherent Analytical Spectroscopies

Triply resonant sum frequency (TRSF) and doubly vibrationally enhanced (DOVE) spectroscopies are examples of a recently developed family of coherent multidimensional spectroscopies (CMDS) that are analogous to multidimensional NMR and current analytical spectroscopies. CMDS methods are particularly promising for analytical applications because their inherent selectivity makes them applicable to complex samples. Like NMR, they are based on creating quantum mechanical superposition states that are fully coherent and lack intermediate quantum state populations that cause quenching or other relaxation effects. Instead of the nuclear spin states of NMR, their multidimensional spectral fingerprints result from creating quantum mechanical mixtures of vibrational and electronic states. Vibrational states provide spectral selectivity, and electronic states provide large signal enhancements. This paper presents the first electronically resonant DOVE spectra and demonstrates the capabilities for analytical chemistry applications by comparing electronically resonant TRSF and DOVE spectra with each other and with infrared absorption and resonance Raman spectra using a Styryl 9 M dye as a model system. The methods each use two infrared absorption transitions and a resonant Raman transition to create a coherent output beam, but they differ in how they access the vibrational and electronic states and the frequency of their output signal. Just as FTIR, UV-vis, Raman, and resonance Raman are complementary methods, TRSF and DOVE methods are complementary to coherent Raman methods such as coherent anti-Stokes Raman spectroscopy (CARS).

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.

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.

Complete reconstruction of the wave function of a reacting molecule by four-wave mixing spectroscopy

Probing the real time dynamics of a reacting molecule remains one of the central challenges in chemistry. Here we show how the time-dependent wave function of an excited-state reacting molecule can be completely reconstructed from resonant coherent anti-Stokes Raman spectroscopy. The method assumes knowledge of the ground potential but not of any excited potential. The excited-state potential can in turn be constructed from the wave function. The formulation is general for polyatomics and applies to bound as well as dissociative excited potentials. We demonstrate the method on the Li(2) molecule.

Optical N-wave-mixing spectroscopy with strong and temporally well-separated pulses: the doorway-window representation

We have extended the doorway-window representation of optical pump-probe spectroscopy with weak pulses toward N-wave-mixing spectroscopy with temporally well-separated pulses of arbitrary strength. The expressions for the signals in the strong-pulse doorway-window representation are derived in the framework of the nonperturbative theory of N-wave-mixing spectroscopy. The strong-pulse doorway-window representation is complementary to the equation-of-motion phase-matching approach. The latter fully accounts for pulse-overlap effects in signals induced by weak pulses but is computationally more expensive. The performance of the doorway-window approximation for temporally well-separated strong pulses is illustrated for an electronic two-level system with an underdamped Condon-active vibrational mode.

Molecular theory and simulation of coherence transfer in metal carbonyls and its signature on multidimensional infrared spectra

We present a general and comprehensive theoretical and computational framework for modeling ultrafast multidimensional infrared spectra of a vibrational excitonic system in liquid solution. Within this framework, we describe the dynamics of the system in terms of a quantum master equation that can account for population relaxation, dephasing, coherence-to-coherence transfer, and coherence-to-population transfer. A unique feature of our approach is that, in principle, it does not rely on any adjustable fitting parameters. More specifically, the anharmonic vibrational Hamiltonian is derived from ab initio electronic structure theory, and the system-bath coupling is expressed explicitly in terms of liquid degrees of freedom whose dynamics can be obtained via molecular dynamics simulations. The applicability of the new approach is demonstrated by employing it to model the recently observed signatures of coherence transfer in the two-dimensional spectra of dimanganese decacarbonyl in liquid cyclohexane. The results agree well with experiment and shed new light on the nature of the molecular interactions and dynamics underlying the spectra and the interplay between dark and bright states, their level of degeneracy, and the nature of their interactions with the solvent.

High resolution coherent 2D spectroscopy

The purpose of this article is to describe recent progress on the use of coherent two-dimensional spectroscopy for investigating the electronic spectroscopy of gas phase molecules. Unlike conventional high resolution spectra where peaks are distributed along a single axis, high resolution coherent 2D spectra reveal informative patterns along two orthogonal frequency domains. The technique can successfully produce these patterns in situations where one-dimensional spectra appear patternless due to complexity and congestion. Molecular spectra that are difficult to analyze because of strongly perturbing effects (e.g., conical intersections) may be studied using this new technique. Several innovations, such as the ability to graphically separate rotational and vibrational information by clustering peaks and the ability to use multiple clusters to overcome spectral congestion help provide high resolution coherent 2D spectroscopy with the ability to analyze spectra that have previously resisted analysis.