Coherent Vibrational Spectroscopy Research Articles

Versatile few-cycle high-energy MID-IR OPCPA for nonlinear optics, spectroscopy and imaging

High-power, high-energy, ultrashort, mid-infrared (MID-IR) laser systems operating at high repetition rates are of considerable interest for many science applications, such as coherent vibrational spectroscopy, label-free imaging, time-resolved pump-probe and high-harmonic generation studies. We developed an optical parametric chirped-pulse amplifier (OPCPA) system employing a difference-frequency generation in a lithium gallium sulfide nonlinear crystal in the final amplifier stage, which provides in principle the possibility for passive carrier-envelop-phase (CEP) stability. The OPCPA efficiently down-converts a 1 μm 200 μJ Yb-YAG pump pulse into the MID-IR spectral range generating μJ-level pulses at a repetition rate of 200 kHz. Two modes of operations providing complimentary MID-IR pulse properties are presented. Depending on the envisaged application, one can switch between (a) a wavelength-tunable (4.2–11 μm) source and (b) a broadband source centered at ≈8.5 μm by controlling the group-delay dispersion of the signal pulse. The broadband, high-energy MID-IR pulses have a short pulse duration of 74±2 fs, which corresponds to only ≈3 optical cycles at the central wavelength of 8.5 μm.

Simultaneous Temperature and Pressure Measurements in Compressible Flow Using Nanosecond O2 Coherent Anti-Stokes Raman Spectroscopy.

Simultaneous pure-rotational coherent anti-Stokes Raman spectroscopy (PRCARS) and vibrational O2 CARS spectroscopy (VCARS) were performed at elevated pressure and lowered temperature conditions in non-reacting compressible flow. We applied dual-pump CARS in a three-laser, three-color configuration to simultaneously acquire the PRCARS and VCARS spectra of O2. PRCARS spectra provide excellent sensitivity to temperature at relatively low temperatures. Pressure was extracted using the differential response of collisional effects in the PRCARS and the VCARS spectra. We used an under-expanded jet outside a choked converging nozzle as the compressible flow-field. We numerically analyze the pressure sensitivity of the combined CARS technique. Finally, we compare the collisional narrowing lineshape models of rotational diffusion narrowing and modified-exponential-gap model, for fitting the experimental spectrum.

Femtosecond Time-Resolved Infrared-Resonant Third-Order Sum-Frequency Spectroscopy

We demonstrate a coherent vibrational spectroscopy based on molecular infrared (IR)-active resonance. We apply two femtosecond pulses (one in near-IR and the other in mid-IR) to generate the femtos...

Coherent Vibrational Spectroscopy of Electrochemical Interfaces with Plasmonic Nanogratings.

The fundamental understanding of electrochemistry urges accurate knowledge of all interfacial properties at the molecular level, but the retrieval of such information is a real challenge. Optical spectroscopies facilitated by surface plasmon enhancement can shed light on this field, yet past studies relied on either highly inhomogeneous "hot spots" or planar plasmon modes with limited enhancement. Here we report an in situ sum-frequency vibrational spectroscopy scheme using plasmonic nanogratings, which enable strong, coherent surface plasmon excitation even on planar electrodes. With two classical reactions, the gold oxidation and pyridine adsorption in water, we demonstrate the realization of coherent vibrational spectroscopy in the strong absorption region, revealing the polar orientation and ordering of interfacial species that are crucial toward the mechanistic understanding of electrochemical phenomena.

Two-Dimensional Infrared Spectroscopy and Molecular Dynamics Simulation Studies of Nonaqueous Lithium Ion Battery Electrolytes.

Lithium ion battery (LIB) technology is undoubtedly indispensable to modern life. However, despite enormous and extended effort to improve LIB performance, our understanding of the underlying principles and mechanisms of lithium ion transport in nonaqueous LIB electrolytes remained limited until recently. There is a particular lack of knowledge of the microscopic solvation structures and fluctuation dynamics around charge carriers in real electrolytes. Typical electrolytes found in commercially available LIBs consist of lithium salts and mixed carbonate solvents, with the latter playing an essential role in promoting lithium ion transport and forming an electrically stable solid electrolyte interphase. Although a number of linear spectroscopic studies of LIB electrolytes aiming at understanding the complex nature of lithium ion solvation processes have been reported, the notion that each lithium ion is strongly solvated by carbonate molecules to form a long-lasting solvation sheath structure has remained the subject of intense debate. Here, we present the results of FTIR, fs IR pump-probe, two-dimensional IR spectroscopy, and molecular dynamics simulations reported by us and others and discuss the possible interplay of picosecond solvation dynamics and macroscopic ion transport processes within the framework of the fluctuation-dissipation relationship. Further, by measuring the time-dependent fluctuations and spectral diffusions of carbonate carbonyl stretch modes that act as excellent infrared probes for the local electrostatic environment, we show that lithium cations are not only solvated by carbonate molecules but also interact with counteranions at equilibrium depending on solvent composition. Molecular dynamics simulations support the notion that rapid chemical exchanges between carbonate solvent molecules in the first and outer solvation shells are critical for describing mobile lithium ion transport phenomena. We thus anticipate that time-resolved coherent multidimensional vibrational spectroscopy is capable of providing decisive evidence on the ultrafast solvent dynamics of various electrolytes, which is potentially helpful for designing improved and more efficient LIB electrolytes in the future.

Theory of coherent two-dimensional vibrational spectroscopy.

Two-dimensional (2D) vibrational spectroscopy has emerged as one of the most important experimental techniques useful to study the molecular structure and dynamics in condensed phases. Theory and computation have also played essential and integral roles in its development through the nonlinear optical response theory and computational methods such as molecular dynamics (MD) simulations and electronic structure calculations. In this article, we present the fundamental theory of coherent 2D vibrational spectroscopy and describe computational approaches to simulate the 2D vibrational spectra. The classical approximation to the quantum mechanical nonlinear response function is invoked from the outset. It is shown that the third-order response function can be evaluated in that classical limit by using equilibrium or non-equilibrium MD simulation trajectories. Another simulation method is based on the assumptions that the molecular vibrations can still be described quantum mechanically and that the relevant molecular response functions are evaluated by the numerical integration of the Schrödinger equation. A few application examples are presented to help the researchers in this and related areas to understand the fundamental principles and to use these methods for their studies with 2D vibrational spectroscopic techniques. In summary, this exposition provides an overview of current theoretical efforts to understand the 2D vibrational spectra and an outlook for future developments.

Classical coherent two-dimensional vibrational spectroscopy.

Two-dimensional (2D) ultrafast spectroscopy is a powerful tool for studying the electronic and vibrational structures of complex systems. Unfortunately, the physical interpretation of these experiments is obscured by conceptual problems in classical response theory, i.e., the divergence of classical nonlinear response functions. We demonstrate that these difficulties are avoided by modeling classical 2D experiments nonperturbatively, illustrating that nonlinear spectroscopy and nonlinear response are not synonymous. Numerical simulations allow a direct comparison between classical and quantum 2D spectra for simple, weakly anharmonic systems relevant to vibrational spectroscopy. We find that nonperturbative classical theory-although differing in quantitative details-accurately captures the key qualitative features of the quantum 2D spectrum, including the separation of the signal into wavevector-selected pathways, formation of cross peaks between coupled vibrational modes, and coherent beating in the signal as a function of waiting time (so-called "quantum beats"). These results are discussed in terms of a simple analytical model which captures the key physical features of classical 2D spectroscopy and provides a link between classical and quantum descriptions. One interesting conclusion from this comparison is that the "coherence" observed in ultrafast spectroscopy may (at least in vibrational experiments) be understood as a purely classical phenomenon, without reference to quantum mechanics.

Localized vibrations in superconducting YBa2Cu3O7 revealed by ultrafast optical coherent spectroscopy

The interaction between phonons and high-energy excitations of electronic origin in cuprates and their role in the superconducting mechanisms is still controversial. Here we use coherent vibrational time-domain spectroscopy together with density functional and dynamical mean-field theory calculations to establish a direct link between the c-axis phonon modes and the in-plane electronic charge excitations in optimally doped YBCO. The non-equilibrium Raman tensor is measured by means of the broadband 'coherent-phonon' response in pump-probe experiments and is qualitatively described by our model using DFT in frozen phonon approximation plus single band DMFT to account for the electronic correlations. The major outcome of our experimental and theoretical study is to establish the link between out-of-plane copper ions displacements and the in-plane electronic correlations, and to estimate at few unit cells the correlation length of the associated phonon mode. The approach introduced here could help revealing the complex interplay between fluctuations of different nature and spatial correlation in several strongly-correlated materials.

Surface-Enhanced Impulsive Coherent Vibrational Spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) has attracted a lot of attention in molecular sensing because of the remarkable ability of plasmonic metal nanostructures to enhance the weak Raman scattering process. On the other hand, coherent vibrational spectroscopy triggered by impulsive excitation using ultrafast laser pulses provides complete information about the temporal evolution of molecular vibrations, allowing dynamical processes in molecular systems to be followed in “real time”. Here, we combine these two concepts and demonstrate surface-enhanced impulsive vibrational spectroscopy. The vibrational modes of the ground and excited states of poly[2-methoxy-5-(2-ethylhexyloxy)−1,4-phenylenevinylene] (MEH-PPV), spin-coated on a substrate covered with monodisperse silver nanoparticles, are impulsively excited with a sub-10 fs pump pulse and characterized with a delayed broad-band probe pulse. The maximum enhancement in the spectrally and temporally resolved vibrational signatures averaged over the whole sample is about 4.6, while the real-time information about the instantaneous vibrational amplitude together with the initial vibrational phase is preserved. The phase is essential to determine the vibrational contributions from the ground and excited states.

A quantitative study of coherent vibrational dynamics probed by heterodyned transient grating spectroscopy

Coherent vibrational spectroscopy in the time domain is well-established and complimentary to Raman techniques in the frequency domain. Pump probe spectroscopy as the most widespread time-resolved technique in the femtosecond regime was successfully applied to a wide range of vibrational studies, but proves challenging for highly scattering samples or for low absorption cross sections. Here we discuss heterodyned transient grating spectroscopy as a technique with superior signal to noise compared to pump-probe, while delivering identical signals. We analyze vibrational wavepacket dynamics using theory based on the response function formalism. We thus explain the distribution of vibrational amplitude along detection frequencies, relating local minima to the position of the zero phonon line and the presence of neighboring modes. Our description gives a fully coherent picture of vibrational phases and amplitudes near the zero phonon line as well as around higher lying vibronic transitions.

Picosecond dynamics of shock compressed and flash-heated nanometer thick films of δ-HMX

Progress towards probing molecular dynamics of octahydro-1,3,5,7-tetranitro-1,3,4,7-tetrazocine (HMX) subjected to shock compression of a few GPa and/or temperature excursions exceeding thermal decomposition values (T > 500 K) is described. Due to shock velocities of a few nm/ps, nanometer-thick layers are needed for picosecond time resolution. Therefore, 5-10 nm thick films of S-HMX were deposited on metallic substrates with a template of a 4-nitrobenzoic acid monolayer. A polymer layer a few microns thick was spin-coated on top of S-HMX for shock confinement. The monolayer and HMX layer were probed simultaneously utilizing an ultrafast nonlinear coherent vibrational spectroscopy, termed vibrational sum-frequency generation (SFG). Shock pressures were estimated via comparisons with the monolayer nitro transition frequency blueshift in hydrostatic pressure measurements. Temperature determinations were made based on the reflectance of the metallic substrate.

Real-time observation of vibrational quantum beat in condensed phase by 20 fs time-resolved spectroscopy

Impulsive coherent vibrational spectroscopy is a kind of time-resolved spectroscopy methodology with a pulse duration of the femtosecond laser shorter than the molecular vibrational period. Usually, it is based on pump-probe technique and is utilized to real-time observation of the molecular vibrational dynamics coupled to the electronic ground or excited state. We have built an impulsive coherent vibrational spectroscopy setup based on a sub-20 fs, 550 to 700 nm tunable non-collinear optical parametric amplifier and performed pump-probe experiment with an Oxazine 720 methanol solution as the sample. Two vibrational quantum beats with frequency of 592 and 678 cm-1 were observed and the corresponding vibrational periods are 56.3 and 49.2 fs respectively.

Organic Constituents on the Surfaces of Aerosol Particles from Southern Finland, Amazonia, and California Studied by Vibrational Sum Frequency Generation

This article summarizes and compares the analysis of the surfaces of natural aerosol particles from three different forest environments by vibrational sum frequency generation. The experiments were carried out directly on filter and impactor substrates, without the need for sample preconcentration, manipulation, or destruction. We discuss the important first steps leading to secondary organic aerosol (SOA) particle nucleation and growth from terpene oxidation by showing that, as viewed by coherent vibrational spectroscopy, the chemical composition of the surface region of aerosol particles having sizes of 1 μm and lower appears to be close to size-invariant. We also discuss the concept of molecular chirality as a chemical marker that could be useful for quantifying how chemical constituents in the SOA gas phase and the SOA particle phase are related in time. Finally, we describe how the combination of multiple disciplines, such as aerosol science, advanced vibrational spectroscopy, meteorology, and chemistry can be highly informative when studying particles collected during atmospheric chemistry field campaigns, such as those carried out during HUMPPA-COPEC-2010, AMAZE-08, or BEARPEX-2009, and when they are compared to results from synthetic model systems such as particles from the Harvard Environmental Chamber (HEC). Discussions regarding the future of SOA chemical analysis approaches are given in the context of providing a path toward detailed spectroscopic assignments of SOA particle precursors and constituents and to fast-forward, in terms of mechanistic studies, through the SOA particle formation process.

Contrasting organic aerosol particles from boreal and tropical forests during HUMPPA-COPEC-2010 and AMAZE-08 using coherent vibrational spectroscopy

Abstract. We present the vibrational sum frequency generation spectra of organic particles collected in a boreal forest in Finland and a tropical forest in Brazil. These spectra are compared to those of secondary organic material produced in the Harvard Environmental Chamber. By comparing coherent vibrational spectra of a variety of terpene and olefin reference compounds, along with the secondary organic material synthesized in the environmental chamber, we show that submicron aerosol particles sampled in Southern Finland during HUMPPA-COPEC-2010 are composed to a large degree of material similar in chemical composition to synthetic α-pinene-derived material. For material collected in Brazil as part of AMAZE-08, the organic component is found to be chemically complex in the coarse mode but highly uniform in the fine mode. When combined with histogram analyses of the isoprene and monoterpene abundance recorded during the HUMPPA-COPEC-2010 and AMAZE-08 campaigns, the findings presented here indicate that if air is rich in monoterpenes, submicron-sized secondary aerosol particles that form under normal OH and O3 concentration levels can be described in terms of their hydrocarbon content as being similar to α-pinene-derived model secondary organic aerosol particles. If the isoprene concentration dominates the chemical composition of organic compounds in forest air, then the hydrocarbon component of secondary organic material in the submicron size range is not simply well-represented by that of isoprene-derived model secondary organic aerosol particles but is more complex. Throughout the climate-relevant size range of the fine mode, however, we find that the chemical composition of the secondary organic particle material from such air is invariant with size, suggesting that the particle growth does not change the chemical composition of the hydrocarbon component of the particles in a significant way.

Surface-Amplified Ligand Disorder in CdSe Quantum Dots Determined by Electron and Coherent Vibrational Spectroscopies

This Article reports measurements of the intra- and intermolecular ordering of tight-binding octylphosphonate ligands on the surface of colloidal CdSe quantum dots (QDs) within solid state films, and the dependence of this order on the size of the QDs. The order of the organic ligands, as probed by vibrational sum frequency generation (SFG) spectroscopy, decreases as the radius of the QDs decreases; this decrease is correlated with a decrease in the order of underlying Cd(2+), as detected by X-ray photoelectron spectroscopy (XPS) line width measurements, for radii of the QDs, R > 2.4 nm, and is independent of the disorder of the Cd(2+) for R < 2.4 nm. We believe that, for R < 2.4, the decreasing order of the ligands with decreasing size is due to an increase in the curvature of the QD surfaces. Disorder in the Cd(2+) results from the presence of a shell of Cd(2+)-surfactant complexes that form during synthesis, so this work demonstrates the possibility for chemical control over molecular order within films of colloidal QDs by changing the surfactant mixture.

Vibrational spectroscopy of nitroaromatic self-assembled monolayers under extreme conditions

Vibrational spectra up to 8GPa and 600°C were obtained for self-assembled monolayers (SAMs) on noble metal surfaces of the energetic material simulant 4-nitrobenzenethiol (NBT). The high-pressure measurements used a surface-enhanced Raman sensor. The high-temperature measurements used femtosecond laser flash-heating and nonlinear coherent vibrational spectroscopy. The SAM nitro stretching transition νNO2s is insensitive to flash-heating where there is no time for chemical reactivity or thermal expansion. A large pressure broadening in νNO2s of the SAM but not in solid NBT was associated with surface adlayer rearrangement. The implications for time-resolved measurements of shock-compressed energetic materials are discussed.

Observation of terahertz vibrations in the nitrogenase FeMo cofactor by femtosecond pump-probe spectroscopy.

Pump it up! The FeMo cofactor (see picture) has resisted characterization by resonance Raman spectroscopy. Impulsive coherent vibrational spectroscopy may be an alternative probe of its dynamics. A 15 fs visible laser pulse pumped the sample to an excited electronic state, and a second <10 fs pulse probed the change in transmission as a function of the time delay.

Multiple mode coupling in Cy3 molecules by impulsive coherent vibrational spectroscopy using a few-cycle laser pulse

Multiple vibrational mode coupling that induces the missing mode effect and Duschinsky rotation on the potential energy surface of the S(1) state in the Cy3 molecule was clarified by real-time ultrafast spectroscopy with a few-cycle laser pulse. The contributions of homogeneous and inhomogeneous dephasing to the total dephasing time were found to be 55% and 45%, respectively.

Coherent Multidimensional Vibrational Spectroscopy of Representative N-Alkanes

Coherent Multidimensional Vibrational Spectroscopy of Representative N-Alkanes

Coherent Multidimensional Vibrational Spectroscopy of Representative N-Alkanes

Mixed frequency/time domain, two color triply vibrationally enhanced (TRIVE) four wave mixing (FWM) spectroscopy is used to study the methyl and methylene modes in octane and dotriacontane. The experiments involve scanning different combinations of the two excitation frequencies, the monochromator frequency, and the two time delays between the three excitation pulses while the remaining variables are fixed. Two dimensional spectra of the methyl and methylene stretching region have weak, asymmetrical diagonal- and cross-peaks when the excitation pulses are temporally overlapped. As the time delays change, the spectra change as new peaks appear and their peak intensity and position change. Combined two-dimensional scans of the excitation frequency and time delay show the changes are caused by relaxation of the initially excited populations to other states that are coupled to the methyl and methylene stretching modes. Two dimensional time delay scans show that the coherence dephasing rates are very fast so fully coherent TRIVE FWM pathways involving multiple quantum coherences are not possible without shorter excitation pulses. Similar experiments involving the methyl and methylene bend and stretching modes identify cross-peaks between these modes and population transfer processes that create cross-peaks. The asymmetric methylene stretch/Fermi resonance band is observed to contain unresolved states that couple differently with the symmetric methylene stretching and scissor modes as well as with lower lying quantum states that are fed by population transfer. The TRIVE FWM data show that the multidimensional spectra are dominated by rapid population transfer within the methyl and methylene stretching modes and to lower quantum states that are coupled to the stretching modes.