Two-dimensional infrared spectroscopy detected by chirped pulse upconversion

Chirped pulse upconversion for femtosecond mid-infrared spectroscopy at 100 kHz

We demonstrate that chirped pulse up-conversion (CPU), a method routinely used with systems based on 1-kHz Titanium:Sapphire lasers, can be extended to a repetition rate of 100 kHz with an Ytterbium diode-pumped femtosecond amplifier. Individual mid-infrared spectra can thus be measured directly in the near infrared using a fast CMOS linescan camera. After an appropriate Fourier processing, a spectral resolution of 1.1 cm-1 is reported, currently limited by our spectrometer. Additionally, we demonstrate the application of CPU to a pump-probe measurement of the vibrational relaxation in carboxy-hemoglobin, and we show that the combination of fast scanning and fast acquisition enables a straightforward removal of pump scattering interference.

Anisotropic dynamics of an interfacial enzyme active site observed using tethered substrate analogs and ultrafast 2D IR spectroscopy

Enzymes accelerate the rates of biomolecular reactions by many orders of magnitude compared to bulk solution, and it is widely understood that this catalytic effect arises from a combination of polar pre-organization and electrostatic transition state stabilization. A number of recent reports have also implicated ultrafast (femtosecond-picosecond) timescale motions in enzymatic activity. However, complications arising from spatially-distributed disorder, the occurrence of multiple substrate binding modes, and the influence of hydration dynamics on solvent-exposed active sites still confound many experimental studies. Here we use ultrafast two-dimensional infrared (2D IR) spectroscopy and covalently-tethered substrate analogs to examine dynamical properties of the promiscuous Pyrococcus horikoshii ene-reductase (PhENR) active site in two binding configurations mimicking proposed "inactive" and "reactive" Michaelis complexes. Spectral diffusion measurements of aryl-nitrile substrate analogs reveal an end-to-end tradeoff between fast (sub-ps) and slow (>5 ps) motions. Fermi resonant aryl-azide analogs that sense interactions of coupled oscillators are described. Lineshape and quantum beat analyses of these probes reveal characteristics that correlate with aryl-nitrile frequency fluctuation correlation functions parameters, demonstrating that this anisotropy is an intrinsic property of the water-exposed active site, where countervailing gradients of fast dynamics and disorder in the reactant ground state are maintained near the hydration interface. Our results suggest several plausible factors leading to state-selective rate enhancement and promiscuity in PhENR. This study also highlights a strategy to detect perturbations to vibrational modes outside the transparent window of the mid-IR spectrum, which may be extended to other macromolecular systems.

Shot-to-shot 2D IR spectroscopy at 100 kHz using a Yb laser and custom-designed electronics

The majority of 2D IR spectrometers operate at 1-10 kHz using Ti:Sapphire laser technology. We report a 2D IR spectrometer designed around Yb:KGW laser technology that operates shot-to-shot at 100 kHz. It includes a home-built OPA, a mid-IR pulse shaper, and custom-designed electronics with optional on-chip processing. We report a direct comparison between Yb:KGW and Ti:Sapphire based 2D IR spectrometers. Even though the mid-IR pulse energy is much lower for the Yb:KGW driven system, there is an 8x improvement in signal-to-noise over the 1 kHz Ti:Sapphire driven spectrometer to which it is compared. Experimental data is shown for sub-millimolar concentrations of amides. Advantages and disadvantages of the design are discussed, including thermal background that arises at high repetition rates. This fundamental spectrometer design takes advantage of newly available Yb laser technology in a new way, providing a straightforward means of enhancing sensitivity.

Upconversion-based mid-infrared spectrometer using intra-cavity LiNbO3 crystals with chirped poling structure

Upconversion from mid-infrared (MIR) to near-visible wavelengths using sum-frequency generation enables sensitive and low-noise MIR light detection using near-visible-light detectors. MIR spectroscopy is important for a wide spectrum of compound characterization applications but challenged by the thermal noise inherent in conventional MIR detectors, giving low detectivity. In this Letter, we show an upconversion-based spectroscopy scheme with an unprecedented combination of large bandwidth and short sampling time. Using LiNbO₃ crystals with a chirped poling structure placed inside a laser cavity, we demonstrate simultaneous upconversion from 2.15 μm to 5.3 μm with sampling times as low as 10.8 μs and an average signal-to-noise ratio of >6000 at 1 s. We conduct numerical studies to show the inverse relationship between efficiency and bandwidth of the upconversion process in the chirped crystal structure and find good agreement with our experimental results.

Optimized noise reduction scheme for heterodyne spectroscopy using array detectors

In this work, we optimize and further advance a noise reduction scheme for heterodyne spectroscopy. This scheme linearly combines data from reference detectors to predict the noise statistics in the signal detector through an optimized coefficient matrix. We validate this scheme for visible white-light-continuum and 800-nm light sources using un-matched CMOS arrays and show that the signal-to-noise ratio can approach the noise floor of the signal detector while using only ~5% of the energy for reference detection. We also optimize the strategy for estimating the coefficient matrix in practical applications. When combined with elaborate algorithms to perform pixel data compression and expansion, our scheme is applicable in difficult situations, including when the sample position is rapidly scanned, when detectors exhibit nonlinear response, and/or when laser fluctuations are large. The scheme is generalized to scenarios with complex chopping or phase cycling patterns, and a simple approach is provided for the chopping case. Finally, a robust and computationally efficient method is devised to remove multiplicative noise.

Seeing infrared optical vortex arrays with a nonlinear spiral phase filter

We demonstrate a new method to detect infrared optical vortex arrays efficiently, which is based on simultaneous up-conversion imaging and spiral phase contrast via second-harmonic generation (SHG) in the Fourier domain. In our experiment, we use a spatial light modulator to prepare a variety of 1064 nm structured vortex arrays and employ a vortex phase plate of different topological charges to serve as the nonlinear orbital angular momentum (OAM) filter. The SHG is done by mixing the Fourier spectra of input-structured vortices with a single OAM beam in a type-II potassium titanyl phosphate crystal. Then we can convert the input invisible vortex arrays into the visible SHG light fields, and the vortex cores are mapped and seen by bright Gaussian spots, revealing both their positions and topological charges. Our work has potential in the field of infrared imaging and monitoring.

Polarization-sensitive femtosecond mid-infrared spectrometer with a tunable Fabry-Perot filter and chirped-pulse upconversion

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular structure and vibrational dynamics. By using a polarized narrowband pump pulse in which center frequency is rapidly scanned with an electrically tunable Fabry-Perot filter, we have developed a femtosecond MIR spectrometer that is more sensitive to molecular structure. The pump energy and polarization dependences of transient MIR absorption spectra measured with the spectroscopic system revealed the appearance of a dark (infrared-inactive) mode due to a slight decrease in the molecular symmetry of a metal carbonyl complex, Mn₂(CO)₁₀, in solution. In addition, the rotational relaxation time of Mn₂(CO)₁₀ in solution was determined as approximately 20 ps.

Broadband 2D electronic spectrometer using white light and pulse shaping: noise and signal evaluation at 1 and 100 kHz

We have developed a broad bandwidth two-dimensional electronic spectrometer that operates shot-to-shot at repetition rates up to 100 kHz using an acousto-optic pulse shaper. It is called a two-dimensional white-light (2D-WL) spectrometer because the input is white-light supercontinuum. Methods for 100 kHz data collection are studied to understand how laser noise is incorporated into 2D spectra during measurement. At 100 kHz, shot-to-shot scanning of the delays and phases of the pulses in the pulse sequence produces a 2D spectrum 13-times faster and with the same signal-to-noise as using mechanical stages and a chopper. Comparing 100 to 1 kHz repetition rates, data acquisition time is decreased by a factor of 200, which is beyond the improvement expected by the repetition rates alone due to reduction in 1/f noise. These improvements arise because shot-to-shot readout and modulation of the pulse train at 100 kHz enables the electronic coherences to be measured faster than the decay in correlation between laser intensities. Using white light supercontinuum for the pump and probe pulses produces high signal-to-noise spectra on samples with optical densities <0.1 within a few minutes of averaging and an instrument response time of <46 fs thereby demonstrating that that simple broadband continuum sources, although weak, are sufficient to create high quality 2D spectra with >200 nm bandwidth.

Wide-bandgap nonlinear crystal LiGaS2 for femtosecond mid-infrared spectroscopy with chirped-pulse upconversion

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular vibrational dynamics. A quantitative study on nonlinear media for upconversion is still essential for wide applications, particularly at the frequencies below 2000  cm^-1. We evaluate wide-bandgap nonlinear crystals of Li-containing ternary chalcogenides based on their performance as the upconversion medium for femtosecond MIR spectroscopy. The upconversion efficiency is measured as a function of the MIR pulse frequency and the chirped pulse energy. LiGaS₂ is found to be an efficient crystal for the upconversion of MIR pulses in a wide frequency range of 1100-2700  cm^-1, especially below 2000  cm^-1. By using LiGaS₂ as an efficient upconversion crystal, we develop a MIR pump-probe spectroscopy system with a spectral resolution of 2.5  cm^-1, a time resolution of 0.2 ps, and a probe window of 120  cm^-1. Vibrational relaxation dynamics of CO stretching modes of Mn₂(CO)₁₀ in cyclohexane and bovine serum albumin in D₂O are demonstrated with a high signal-to-noise ratio.

Transient Two-Dimensional Infrared Spectroscopy in a Vibrational Ladder

We report on transient 2D Fourier transform infrared spectroscopy (2DIR) after vibrational ladder climbing induced in the CO-moiety longitudinal stretch of carboxyhemoglobin. The population distribution, spreading up to seven vibrational levels, results in a nonequilibrium 2DIR spectrum evidencing a large number of peaks that can be easily attributed to individual transitions thanks to the anharmonicity of the vibrational potential. We discuss the physical origin of the observed peaks as well as the qualitative behavior of the subsequent dynamics governed by population relaxation in the vibrational ladder.

From Ultrafast Structure Determination to Steering Reactions: Mixed IR/Non-IR Multidimensional Vibrational Spectroscopies

Ultrafast multidimensional infrared spectroscopy is a powerful method for resolving features of molecular structure and dynamics that are difficult or impossible to address with linear spectroscopy. Augmenting the IR pulse sequences by resonant or nonresonant UV, Vis, or NIR pulses considerably extends the range of application and creates techniques with possibilities far beyond a pure multidimensional IR experiment. These include surface-specific 2D-IR spectroscopy with sub-monolayer sensitivity, ultrafast structure determination in non-equilibrium systems, triggered exchange spectroscopy to correlate reactant and product bands, exploring the interplay of electronic and nuclear degrees of freedom, investigation of interactions between Raman- and IR-active modes, imaging with chemical contrast, sub-ensemble-selective photochemistry, and even steering a reaction by selective IR excitation. We give an overview of useful mixed IR/non-IR pulse sequences, discuss their differences, and illustrate their application potential.

Applications of two-dimensional infrared spectroscopy

Two-dimensional infrared (2D IR) spectroscopy has recently emerged as a powerful tool with applications in many areas of scientific research. The inherent high time resolution coupled with bond-specific spatial resolution of IR spectroscopy enable direct characterization of rapidly interconverting species and fast processes, even in complex systems found in chemistry and biology. In this minireview, we briefly outline the fundamental principles and experimental procedures of 2D IR spectroscopy. Using illustrative example studies, we explain the important features of 2D IR spectra and their capability to elucidate molecular structure and dynamics. Primarily, this minireview aims to convey the scope and potential of 2D IR spectroscopy by highlighting select examples of recent applications including the use of innate or introduced vibrational probes for the study of nucleic acids, peptides/proteins, and materials.

Ultrafast Dynamics of Carboxy-Hemoglobin: Two-Dimensional Infrared Spectroscopy Experiments and Simulations

This Letter presents a comparison between experimental and simulated 2D mid-infrared spectra of carboxy-hemoglobin in the spectral region of the carbon monoxide stretching mode. The simulations rely on a fluctuating potential energy surface that includes both the effect of heme and the protein surroundings computed from molecular dynamics simulations. A very good agreement between theory and experiment is obtained with no adjustable parameters. The simulations show that the effect of the distal histidine through the hydrogen bond is strong and is directly responsible for the slow decay of the frequency-frequency correlation function on a 10 ps time scale. This study confirms that fluctuations in carboxy-hemoglobin are more inhomogeneous than those in the more frequently studied carboxy-myoglobin. The comparison between simulations and experiments brings valuable information on the complex relation between protein structure and spectral diffusion.

Time-domain spectroscopy in the mid-infrared

When coupled to characteristic, fingerprint vibrational and rotational motions of molecules, an electromagnetic field with an appropriate frequency and waveform offers a highly sensitive, highly informative probe, enabling chemically specific studies on a broad class of systems in physics, chemistry, biology, geosciences, and medicine. The frequencies of these signature molecular modes, however, lie in a region where accurate spectroscopic measurements are extremely difficult because of the lack of efficient detectors and spectrometers. Here, we show that, with a combination of advanced ultrafast technologies and nonlinear-optical waveform characterization, time-domain techniques can be advantageously extended to the metrology of fundamental molecular motions in the mid-infrared. In our scheme, the spectral modulation of ultrashort mid-infrared pulses, induced by rovibrational motions of molecules, gives rise to interfering coherent dark waveforms in the time domain. These high-visibility interference patterns can be read out by cross-correlation frequency-resolved gating of the field in the visible generated through ultrabroadband four-wave mixing in a gas phase.

Ultrafast infrared spectroscopy in photosynthesis

In recent years visible pump/mid-infrared (IR) probe spectroscopy has established itself as a key technology to unravel structure-function relationships underlying the photo-dynamics of complex molecular systems. In this contribution we review the most important applications of mid-infrared absorption difference spectroscopy with sub-picosecond time-resolution to photosynthetic complexes. Considering several examples, such as energy transfer in photosynthetic antennas and electron transfer in reaction centers and even more intact structures, we show that the acquisition of ultrafast time resolved mid-IR spectra has led to new insights into the photo-dynamics of the considered systems and allows establishing a direct link between dynamics and structure, further strengthened by the possibility of investigating the protein response signal to the energy or electron transfer processes. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

A comparative study on chirped-pulse upconversion and direct multichannel MCT detection

A comparative study is carried out on two spectroscopic techniques employed to detect ultrafast absorption changes in the mid-infrared spectral range, namely direct multichannel detection via HgCdTe (MCT) photodiode arrays and the newly established technique of chirped-pulse up-conversion (CPU). Whereas both methods are meanwhile individually used in a routine manner, we directly juxtapose their applicability in femtosecond pump-probe experiments based on 1 kHz shot-to-shot data acquisition. Additionally, we examine different phase-matching conditions in the CPU scheme for a given mid-infrared spectrum, thereby simultaneously detecting signals which are separated by more than 200 cm(-1).

Site-specific measurements of lipid membrane interfacial water dynamics with multidimensional infrared spectroscopy

One route to accessing site-specific dynamical information available with ultrafast multidimensional infrared spectroscopy is the development of robust and versatile vibrational probes. Here we synthesize and characterize a vibrationally labeled cholesterol derivative, (cholesteryl benzoate) chromium tricarbonyl, to probe model lipid membranes, focusing specifically on the membrane-water interface. Utilizing FTIR and polarized-ATR spectroscopies, we determine the location of the chromium tricarbonyl motif to be situated at the water-membrane interface with an orientation of 46 ± 2° relative to the vector normal to the membrane surface. We test the dynamical sensitivity of the (cholesteryl benzoate) chromium tricarbonyl label with two different nonlinear infrared spectroscopy methods, both of which show that the probe is well-suited to the study of membrane dynamics as well as the dynamics of water at the membrane interface. The metal carbonyl vibrational probe located at the surface of a bicelle exhibits spectral diffusion dynamics induced by membrane hydration water that is roughly three times slower than observed using a nearly identical vibrational probe in bulk water.

Simplified and economical 2D IR spectrometer design using a dual acousto-optic modulator

Over the last decade two-dimensional infrared (2D IR) spectroscopy has proven to be a very useful extension of infrared spectroscopy, yet the technique remains restricted to a small group of specialized researchers because of its experimental complexity and high equipment cost. We report on a spectrometer that is compact, mechanically robust, and is much less expensive than previous designs because it uses a single pixel MCT detector rather than an array detector. Moreover, each axis of the spectrum can be collected in either the time or frequency domain via computer programming. We discuss pulse sequences for scanning the probe axis, which were not previously possible. We present spectra on metal carbonyl compounds at 5 µm and a model peptide at 6 µm. Data collection with a single pixel MCT takes longer than using an array detector, but publishable quality data are still achieved with only a few minutes of averaging.

Two-Dimensional Infrared (2DIR) Spectroscopy of the Peptide Beta3s Folding

Probing underlying free energy landscape, pathways, and mechanism is the key for understanding protein folding in theory and experiment. Recently time-resolved two-dimensional infrared (2DIR) with femtosecond laser pulses, has emerged as a promising tool for investigating the protein folding dynamics on faster timescales than possible by NMR. We have employed molecular dynamics simulations to compute 2DIR spectra of the folding process of a peptide, Beta3s. Simulated non-chiral and chiral 2DIR signals illustrate the variation of the spectra as the peptide conformation evolves along the free energy landscape. Chiral spectra show stronger changes than the non-chiral signals because cross peaks caused by the formation of the β-sheet are clearly resolved. Chirality-induced 2DIR may be used to detect the folding of β-sheet proteins with high spectral and temporal resolution.

2D IR spectroscopy using four-wave mixing, pulse shaping, and IR upconversion: a quantitative comparison

Recent technological advances have led to major changes in the apparatuses used to collect 2D IR spectra. Pulse shaping offers several advantages including rapid data collection, inherent phase stability, and phase-cycling capabilities. Visible array detection via upconversion allows the use of visible detectors that are cheaper, faster, more sensitive, and less noisy than IR detectors. However, despite these advantages, many researchers are reluctant to implement these technologies. Here we present a quantitative study of the S/N of 2D IR spectra collected with a traditional four-wave mixing (FWM) apparatus, with a pulse shaping apparatus, and with visible detection via upconversion to address the question of whether weak chromophores at low concentrations are still accessible with such an apparatus. We find that the enhanced averaging capability of the pulse shaping apparatus enables the detection of small signals that would be challenging to measure even with the traditional FWM apparatus, and we demonstrate this ability on a sample of cyanylated dihydrofolate reductase.

Ultrafast mid-infrared spectroscopy by chirped pulse upconversion in 1800-1000cm(-1) region

Broadband femtosecond mid-infrared pulses can be converted into the visible spectral region by chirped pulse upconversion. We report here the upconversion of pump probe transient signals in the frequency region below 1800cm(-1), using the nonlinear optical crystal AgGaGeS4, realizing an important expansion of the application range of this method. Experiments were demonstrated with a slab of GaAs, in which the upconverted signals cover a window of 120cm(-1), with 1.5cm(-1) resolution. In experiments on the BLUF photoreceptor Slr1694, signals below 1 milliOD were well resolved after baseline correction. Possibilities for further optimization of the method are discussed. We conclude that this method is an attractive alternative for the traditional MCT arrays used in most mid-infrared pump probe experiments.

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.

Solvent-dependent spectral diffusion in a hydrogen bonded "vibrational aggregate"

Two-dimensional infrared spectroscopy (2DIR) is used to measure the viscosity-dependent spectral diffusion of a model vibrational probe, Mn(2)(CO)(10) (dimanganese decacarbonyl, DMDC), in a series of alcohols with time scales ranging from 2.67 ps in methanol to 5.33 ps in 1-hexanol. Alcohol-alkane solvent mixtures were found to produce indistinguishable linear IR spectra, while still demonstrating viscosity-dependent spectral diffusion. Using a vibrational exciton model to characterize the inhomogeneous energy landscape, several analogies emerge with multichromophoric electronic systems, such as J-aggregates and light-harvesting protein complexes. An excitonic, local vibrational mode Hamiltonian parametrized to reproduce the vibrational structure of DMDC serves as a starting point from which site energies (i.e., local carbonyl frequencies) are given Gaussian distributed disorder. The model gives excellent agreement with both the linear IR spectrum and the inhomogeneous widths extracted from 2DIR, indicating the system can be considered to be a "vibrational aggregate." This model naturally leads to exchange narrowing due to disorder-induced exciton localization, producing line widths consistent with our 1D and 2D measurements. Further, the diagonal disorder alone effectively reduces the molecular symmetry, leading to the appearance of Raman bands in the IR spectrum in accord with the measurements. Here, we show that the static inhomogeneity of the excitonic model with disorder successfully captures the essential details of the 1D spectrum while predicting the degree of IR activity of forbidden modes as well as the inhomogeneous widths and relative magnitudes of the transition moments.

Ultrabroadband detection of a mid-IR continuum by chirped-pulse upconversion

Ultrabroadband mid-IR continuum pulses can be detected on a single-shot basis using chirped-pulse upconversion. Converting the mid-IR pulse to the visible reduces the fractional bandwidth and enables use of a silicon CCD camera. Removing the cross-phase modulation of the chirped pulse results in 1 cm(-1) resolution over a 600 cm(-1) detected bandwidth.

Transient vibrational echo versus transient absorption spectroscopy: a direct experimental and theoretical comparison

Transient dispersed vibrational echo (DVE) spectroscopy is a practical alternative to transient-absorption spectroscopy because it affords increased sensitivity as well as greater signal-to-noise ratio without the need to detect a reference spectrum. However, as a third-order nonlinear probe, the extraction of kinetic information from transient-DVE is somewhat cumbersome compared to transient absorption. This article provides a direct experimental and theoretical comparison between transient-absorption and transient-DVE measurements and presents a framework for analyzing kinetic measurements while exploring the implications of making some simplifying assumptions in the data analysis. The equations for computing the signal-to-noise ratios under different experimental conditions are derived and used in the analysis of the experimental data. The results, obtained under the same experimental conditions, show that for a relatively strong terminal carbonyl stretching mode, signal-to-noise ratios in transient-DVE spectroscopy are approximately 2.5 times greater than transient absorption. The experimental results along with the theoretical models indicate that transient-DVE could become an attractive alternative to transient-absorption spectroscopy for measuring the kinetics of light-induced processes.

Orientational dynamics of transient molecules measured by nonequilibrium two-dimensional infrared spectroscopy

Transient two-dimensional infrared (2DIR) spectroscopy is applied to the photodissociation of Mn2(CO)10 to 2 Mn(CO)5 in cyclohexane solution. By varying both the time delay between the 400 nm phototrigger and the 2DIR probe as well as the waiting time in the 2DIR pulse sequence, we directly determine the orientational relaxation of the vibrationally hot photoproduct. The orientational relaxation slows as the photoproduct cools, providing a measure of the transient temperature decay time of 70 +/- 16 ps. We compare the experimental results with molecular dynamics simulations and find near quantitative agreement for equilibrium orientational diffusion time constants but only qualitative agreement for nonequilibrium thermal relaxation. The simulation also shows that the experiment probes an unusual regime of thermal excitation, where the solute is heated while the solvent remains essentially at room temperature.

Removing cross-phase modulation from midinfrared chirped-pulse upconversion spectra

We observe that narrow spectral features in mid-infrared spectra obtained by chirped-pulse up-conversion are strongly distorted by cross-phase modulation between the mid-infrared field and the chirped pulse. We discuss the consequences of this effect on spectral resolution, and introduce a correction method that recovers masked lines. This simple correction can be applied either when the upconverted field is fully characterized, such as in multidimensional spectroscopy, or when causality can be used, such as in absorption spectroscopy, which we demonstrate experimentally.

Two-dimensional infrared spectroscopy of metal carbonyls

Metal carbonyl complexes offer both rich chemistry and complex vibrational spectroscopy due to strong coupling among the carbonyl stretches. Using two-dimensional infrared (2DIR) spectroscopy, it is possible to resolve the underlying transitions between vibrational energy levels, determine the orientations and relative magnitude of the corresponding transition dipole moments, measure the coupling between modes due to the anharmonicity of the potential, and probe energy redistribution among the modes as well as energy relaxation to other degrees of freedom. Measurements on metal carbonyl complexes have played, and continue to play, a crucial role in facilitating the development of 2DIR spectroscopy. These compounds have provided powerful demonstrations of the unique ability of 2DIR spectroscopy to resolve vibrational structure and dynamics in multimode systems. In addition, invaluable new information has been obtained on metal-to-ligand charge transfer processes, solvent-solute interactions and fluxionality. Since transition metal complexes play important roles in catalysis and as dye sensitizers for semiconductor nanoparticle photocatalysis, detailed probes of equilibrium and phototriggered dynamics should aid our understanding of these key catalytic systems. The richness and level of detail provided by the 2DIR spectra of metal carbonyl complexes turn them into extremely useful model systems for testing the accuracy of ab initio quantum chemical calculations. Accurate modeling of the 2DIR spectra of solvated metal carbonyl complexes requires the development of new theoretical and computational tools beyond those employed in the standard analysis of one-dimensional IR spectra, and represents an ongoing challenge to currently available computational methodologies. These challenges are further compounded by the increasing interest in triggered 2DIR experiments that involve nonequilibrium vibrational dynamics on multiple electronic potential surfaces. In this Account, we review the various metal carbonyl complexes studied via 2DIR spectroscopy and outline the theoretical approaches used in order to model the spectra. The capabilities of 2DIR spectroscopy and its interplay with modern ab initio calculations are demonstrated in the context of the metal carbonyl complex Mn(2)(CO)(10) recently studied in our lab. Continued progress in experimental implementation and theoretical analysis will enable transient 2D spectroscopy to provide structurally sensitive details of complex, highly interacting nonequilibrium processes that are central to diverse chemical transformations.

Unobtrusive interferometer tracking by path length oscillation for multidimensional spectroscopy

We track the path difference between interferometer arms with few-nanometer accuracy without adding optics to the beam path. We measure the interference of a helium-neon beam that copropagates through the interferometer with midinfrared pulses used for multidimensional spectroscopy. This can indicate motion, but not direction. By oscillating the path length of one arm with a mirror on a piezoelectric stack and monitoring the oscillations of the recombined helium-neon beam, the direction can be calculated, and the path delay can be continuously tracked.

Applications of 2D IR spectroscopy to peptides, proteins, and hydrogen-bond dynamics

Following a survey of 2D IR principles, this article describes recent experiments on the hydrogen-bond dynamics of small ions, amide-I modes, nitrile probes, peptides, reverse transcriptase inhibitors, and amyloid fibrils.

How to turn your pump-probe instrument into a multidimensional spectrometer: 2D IR and Vis spectroscopies via pulse shaping

We have recently developed a new and simple way of collecting 2D infrared and visible spectra that utilizes a pulse shaper and a partly collinear beam geometry. 2D IR and Vis spectroscopies are powerful tools for studying molecular structures and their dynamics. They can be used to correlate vibrational or electronic eigenstates, measure energy transfer rates, and quantify the dynamics of lineshapes, for instance, all with femtosecond time-resolution. As a result, they are finding use in systems that exhibit fast dynamics, such as sub-millisecond chemical and biological dynamics, and in hard-to-study environments, such as in membranes. While powerful, these techniques have been difficult to implement because they require a series of femtosecond pulses to be spatially and temporally overlapped with precise time-resolution and interferometric phase stability. However, many of the difficulties associated with implementing 2D spectroscopies are eliminated by using a pulse shaper and a simple beam geometry, which substantially lowers the technical barriers required for researchers to enter this exciting field while simultaneously providing many new capabilities. The aim of this paper is to provide an overview of the methods for collecting 2D spectra so that an outsider considering using 2D spectroscopy in their own research can judge which approach would be most suitable for their research aims. This paper focuses primarily on 2D IR spectroscopy, but also includes our recent work on adapting this technology to collecting 2D Vis spectra. We review work that has already been published as well as cover several topics that we have not reported previously, including phase cycling methods to remove background signals, eliminate unwanted scatter, and shift data collection into the rotating frame.