Stimulated Raman Spectra Research Articles

MCNOX: A code for computing and interpreting ultrafast nonlinear X-ray spectra of molecules at the multiconfigurational level

This work describes a program for computing and analyzing the ultrafast (attosecond and femtosecond) nonlinear X-ray spectra of molecules at the multiconfigurational quantum chemistry level, called MCNOX. It is aimed at cutting-edge current and future photochemistry/photophysics applications enabled by X-ray free-electron lasers and high harmonic generation light sources. It can compute steady-state X-ray absorption spectroscopy (XAS) and three types of ultrafast nonlinear X-ray spectra: transient XAS, all-X-ray four-wave mixing, and stimulated Raman spectra. It is especially capable of picking out major electronic transitions, and further computing the natural transition orbitals for these transitions, which help finally yield the physical and chemical insights from complex signals. Following a research paradigm of “electronic structure→molecular dynamics→signal”, in this paper, methods for the former two steps are reviewed, and then the theory, implementations, and technical details for signal simulations are presented along illustrative examples on uracil.

Diffuse Reflectance-Based Femtosecond Stimulated Raman Spectroscopy of Opaque Suspensions.

By augmentation of the collection optics utilized in transmission-based femtosecond stimulated Raman spectroscopy (FSRS), two novel diffuse reflectance-based femtosecond stimulated Raman spectroscopy (drFSRS) techniques were developed. These techniques were then used to collect the Raman spectra of opaque systems, those being cyclohexane-intercalated poly(tetrafluoroethylene) microbeads and ethanol in 1% intralipid solutions. The resulting drFSRS data from the cyclohexane:PTFE system show significant distortion of the depolarization ratio of the 803 cm-1 cyclohexane peak, indicating a loss of incident pump:probe polarization in a scattering environment. The drFSRS data from the ethanol in 1% intralipid solution demonstrate less signal strength but equal spectral resolution when compared to transmission-based FSRS of the same sample. The results presented in this Technical Note demonstrate the current capabilities of collecting stimulated Raman spectra of opaque systems using drFSRS.

Conformational Relaxation Dynamics of Poly(3-hexylthiophene) Photoexcited in Solution as Studied by Femtosecond Time-Resolved Stimulated Raman Spectroscopy in 1190-1550 nm Region.

When a conjugated polymer is photoexcited in solution, its effective conjugation length in the singlet exciton state often increases through the conformational relaxation of the polymer main chain and/or hopping of the excitation. We measured femtosecond time-resolved near-IR stimulated Raman spectra of poly(3-hexylthiophene) (P3HT) photoexcited in four organic solvents for understanding the dynamics of the exciton elongation through the conformational relaxation separately from that through the exciton hopping. In the ring CC stretch frequency region, a band appears at around 1415 cm-1 and decays, while a new band rises at around 1370 cm-1. The average time constant of the change is estimated to be 8.7-19 ps and correlated almost linearly with the viscosity of the solvents. These results suggest that the main chain of P3HT in the singlet exciton state relaxes from a twisted form to a planar form in the 0-100 ps range when it surmounts an activation barrier of 5.8-7.8 kJ mol-1, generated possibly by the steric effect of the hexyl side group. When the rise of the 1370 cm-1 band is analyzed in detail, it is reproduced with two exponential rise functions with time constants of 0-3.3 and 16-22 ps. The two rise components suggest that a portion of P3HT forms a cluster in solution, while the other portion of P3HT is isolated.

Positive and negative signal and line shape in stimulated Raman spectroscopy: Resonance femtosecond Raman spectra of diphenylbutadiene.

Resonance stimulated Raman signal and line shape are evaluated analytically under common electronic/vibrational dephasing and exponential Raman/probe pulse, exp(-|t|/τ). Generally, the signal from a particular state includes contributions from higher and lower electronic states. Thus, with S0 → S1 actinic excitation, the Raman signal consists of 15 Feynman diagrams entering with different signs. The negative signindicates vibrational coherences in S1 or higher Sn, whereas the positive signreveals coherences in S0 or Sn via S1 → Sn → Sm (n < m) coupling. The signal complexity is in contrast to spontaneous Raman with its single diagram only. The results are applied to femtosecond stimulated Raman spectra of trans-trans, cis-trans (ct), and cis-cis (cc) 1,4-diphenyl-1,3-butadiene, the ct and cc being reported for the first time. Upon actinic excitation, the Stokes spectra show negative bands from S1 or Sn. When approaching higher resonances Sn → Sm, some Raman bands switch their signfrom negative to positive, thus, indicating new coherences in Sn. The results are discussed, and the measured Raman spectra are compared to the computed quantum-chemical spectra.

(INVITED) Stimulated Raman lineshapes in the large light–matter interaction limit

Stimulated Raman scattering (SRS) represents a powerful tool for accessing the vibrational properties of molecular compounds or solid state systems. From a spectroscopic perspective, SRS is able to capture Raman spectra free from incoherent background processes and typically ensures a signal enhancement of several orders of magnitude with respect to its spontaneous counterpart. Since its discovery in 1962, SRS has been applied to develop technological applications, such as Raman-based lasers, frequency shifters for pulsed sources and Raman amplifiers. For the full exploitation of their potential, however, it is crucial to have an accurate description of the SRS processes under the large gain regime. Here, by taking as an example the stimulated Raman spectrum of a model solvent, namely liquid cyclohexane, we discuss how the spectral profiles and the lineshapes of Raman excitations critically depend on the pump excitation regime. In particular, we show that in the large light–matter interaction limit the Raman gain undergoes an exponential increase (decrease) in the red (blue) side of the spectrum, with the Raman linewidths that appear sharpened (broadened).

Multi-Band Elastic Optical Networks: Inter-Channel Stimulated Raman Scattering-Aware Routing, Modulation Level and Spectrum Assignment

Recently, multi-band elastic optical networks (MB-EONs) have been introduced to increase the transmission capacity of optical networks by expanding the transmission frequency range beyond the conventional C-band (e.g., O-, E-, S-, and L-band). Thus, by utilizing the idle frequency bands of the existing standard single mode fibers (SMFs), the capacity improvement in MB-EONs is achieved with lower capital expenditure compared to other techniques, such as space division multiplexing (SDM) in multi-core fibers (MCFs). The routing, modulation level and spectrum assignment (RMLSA) problem in MB-EONs is more challenging than that of the conventional single band EONs, and the inter-channel stimulated Raman scattering (ISRS) process should be considered in signal-to-noise ratio (SNR) analysis. In this article, ISRS-aware RMLSA problem in MB-EONs is studied. We obtain the optimal solution of this problem by formulating a path-based integer linear programming (ILP) model. Since the number of slots and demands are too large in a multi-band system, the ILP problem is too complex to be solved in polynomial time. Therefore, we propose heuristic algorithms to solve the ISRS-aware RMLSA problem. Then, performance of the proposed schemes is evaluated through exhaustive simulations. The results reveal that among all the proposed heuristic algorithms, the algorithm named as enhanced in-process estimation (EIPE) has the best performance in saving more frequency slots for future demands, while achieving higher spectral efficiency and SNR estimation accuracy. Moreover, for a special case of small-scale scenario, the performance of EIPE is close to the results of ILP formulation. Finally, our results indicate that by ignoring ISRS process in the SNR estimation, outage occurs for some of lightpaths, where the outage percentage depends on the launch power.

Origin of dispersive line shapes in plasmon-enhanced stimulated Raman scattering microscopy

Abstract Plasmon-enhanced stimulated Raman scattering (PESRS) microscopy has been recently developed to reach single-molecule detection limit. Unlike conventional stimulated Raman spectra, dispersive-like vibrational line shapes were observed in PESRS. Here, we propose a theoretical model together with a phasor diagram to explain the observed dispersive-like line shapes reported in our previous study. We show that the local enhanced electromagnetic field induced by the plasmonic nanostructure interferes with the molecular dipole-induced field, resulting in the dispersive profiles of PESRS. The exact shape of the profile depends on the phase difference between the plasmonic field and the molecular dipole field. We compared plasmon-enhanced stimulated Raman loss (PESRL) and plasmon-enhanced stimulated Raman gain (PESRG) signals under the same pump and Stokes laser wavelength. The PESRL and PESRG signals exhibit similar signal magnitudes, whereas their spectral line shapes show reversed dispersive profiles, which is in an excellent agreement with our theoretical prediction. Meanwhile, we verify that the nonresonant background in PESRS mainly originates from the photothermal effect. These new insights help the proper use of PESRS for nanoscale bio-imaging and ultrasensitive detection.

A new imaging-based method for alignment of multiple laser beams.

A new method for multiple laser beams alignment, useful in a wide range of spectroscopies, is proposed and demonstrated. The method, based on the coupling of spatial map imaging (SMI) with velocity map imaging (VMI), aided beams visualization, through interrogation of the ionization signal of different species in a time-of-flight mass spectrometer. This approach is very effective for alignment and for evaluating the spatial overlap of laser beams with a molecular beam. This was demonstrated by monitoring the resonant two-photon ionization spectrum of 2-phenylethylamine (PEA) monomer and its hydrated (PEA-H2O) cluster and the ionization-loss stimulated Raman spectrum of the cluster, via VMIs accumulation, as a function of the exciting laser wavelength. The former permitted immediate classification of the features in the spectrum, corresponding to the molecular ion or the cluster. The proposed methodology will be useful in other challenging multiple laser beam experiments for spectroscopic studies and is expected to improve extensively their outcome.

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional π-conjugate Systems.

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near infrared (near-IR) transitions, because it can overcome the low sensitivity of spontaneous Raman spectrometers in the near-IR region. Here, we describe technical details of a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer that we have recently developed. A description of signal generation and optimization, measurement, data acquisition, and calibration and correction of recorded data is provided as well. We present an application of our spectrometer to analyze the excited-state dynamics of β-carotene in toluene solution. A C=C stretch band of β-carotene in the second lowest excited singlet (S2) state and the lowest excited singlet (S1) state is clearly observed in the recorded time-resolved stimulated Raman spectra. The femtosecond time-resolved near-IR stimulated Raman spectrometer is applicable to the structural dynamics of π-conjugate systems from simple molecules to complex materials.

Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer.

Time-resolved femtosecond stimulated Raman spectra (FSRS) of a prototypical organometallic photosensitizer/photocatalyst ReCl(CO)3(2,2'-bipyridine) were measured in a broad spectral range ∼40-2000 (4000) cm-1 at time delays from 40 fs to 4 ns after 400 nm excitation of the lowest allowed electronic transition. Theoretical ground- and excited-state Raman spectra were obtained by anharmonic vibrational analysis using second-order vibrational perturbation theory on vibrations calculated by harmonic approximation at density functional theory-optimized structures. A good match with anharmonically calculated vibrational frequencies allowed for assigning experimental Raman features to particular vibrations. Observed frequency shifts upon excitation (ν(ReCl) and ν(CC inter-ring) vibrations upward; ν(CC, CN) and ν(Re-C) downward) are consistent with the bonding/antibonding characters of the highest occupied molecular orbital and the lowest unoccupied molecular orbital involved in excitation and support the delocalized formulation of the lowest triplet state as ReCl(CO)3 → bpy charge transfer. FSRS spectra show a mode-specific temporal evolution, providing insights into the intersystem crossing (ISC) mechanism and subsequent relaxation. Most of the Raman features are present at ∼40 fs and exhibit small shifts and intensity changes with time. The 1450-1600 cm-1 group of bands due to CC, CN, and CC(inter-ring) stretching vibrations undergoes extensive restructuring between 40 and ∼150 fs, followed by frequency upshifts and a biexponential (0.38, 21 ps) area growth, indicating progressing charge separation in the course of the formation and relaxation of the lowest triplet state. Early (40-150 fs) restructuring was also observed in the low-frequency range for ν(Re-Cl) and δ(Re-C-O) vibrations that are presumably activated by ISC. FSRS experimental innovations employed to measure low- and high-energy Raman features simultaneously are described and discussed in detail.

Near-infrared resonance stimulated Raman study of short-lived transients in PTB7 films

Low-bandgap conjugated polymers, which have an alternating structure of electron-donating and electron-withdrawing monomer units in their main chains, are expected to show high photovoltaic efficiency when the monomer units are appropriately designed. For understanding key factors of the photovoltaic efficiency, we have investigated the structure and dynamics of short-lived transients in films of a low-bandgap conjugated polymer with an alternating structure of thieno[3,4-b]thiophene and benzo[1,2-b:4,5-b′]dithiophene, PTB7, by time-resolved near-infrared stimulated Raman spectroscopy as well as time-resolved near-infrared absorption spectroscopy. Positive polarons of PTB7 are clearly distinguished from its singlet excitons in stimulated Raman spectra by the peak position of a CC stretch band of the thiophene rings in benzodithiophene and intensity of a CSC deformation band of them. These differences strongly suggest that the repeated structure of thiophene rings in the main chain predominantly contributes to the charge transfer characteristics of PTB7 in films. Time-resolved absorption and stimulated Raman spectra of PTB7 films blended with a fullerene derivative show that spectral changes associated with the initial charge separation complete within 1 ps while effects of conformational changes and intermediate species are absent. We suggest that alternating copolymers composed of electron-donating and electron-accepting units commonly undergo the initial charge separation process without conformational changes or intermediates.

Modeling the Ultrafast Response of Two‐Magnon Raman Excitations in Antiferromagnets on the Femtosecond Timescale

AbstractIlluminating a magnetic material with femtosecond laser pulses induces complex ultrafast dynamical processes. The resulting optically detectable response usually contains contributions from both the optical properties and the magnetic degrees of freedom. Disentangling all the different components concurring to the generation of the total signal is a major challenge of contemporary experimental solid‐state physics. Here, this problem is tackled, addressing the purely optical, nonmagnetic artifacts on the time resolved two‐magnon stimulated Raman spectrum of an antiferromagnet, rationalizing the recent observation on the exchange energy modification upon photo‐excitation. It is demonstrated how the genuine dynamics of the magnetic eigenmode can be disentangled from the nonlinear optical effects, generated by cross phase modulation, on the femtosecond timescale. The introduced approach can be extended for the investigation of &lt;100 fs dynamic processes by means of coherent Raman scattering.

Volatility measurements of 1, 2, 6-hexanetriol in levitated viscous aerosol particles

The partitioning of semi-volatile organic compounds (SVOCs) between particle and gas phases is a primary focus in the study of the formation and lifetime of secondary organic aerosols (SOAs). To predict the temporal and spatial distribution of SOAs, it is essential to retrieve the fundamental parameters of SVOCs that govern the distribution of organic compounds between the condensed and gas phases. In the present work, the volatility of 1,2,6-hexanetriol in sucrose/1,2,6-hexanetriol/water mixed aerosol droplets was studied by employing optical tweezers coupled with cavity-enhanced Raman spectroscopy. The radii and refractive indexes of the levitated droplets were determined in real time using the wavelength positions of stimulated Raman spectra, and the effective vapor pressures of 1,2,6-hexanetriol at different relative humidity (RH) were obtained according to Maxwell equation. For the droplets with sucrose/1,2,6-hexanetriol molar ratio of 1:1, the effective vapor pressure of 1,2,6-hexanetriol decreased with the decrease of RH. When the RH decreased from 70% to 10%, the effective vapor pressure of 1,2,6-hexanetriol decreased from (2.16±0.229)×10−4Pa to (6.72 ±0.7)×10−6Pa. Compared to the vapor pressure of pure 1,2,6-hexanetriol (1.16 ± 0.025) ×10−4Pa~(3.82±1.16) ×10−4Pa, the evaporation of 1,2,6-hexanetriol in the mixed droplet was suppressed by sucrose, especially at low RH. For the mixed droplets with sucrose/1,2,6-hexanetriol molar ratio of 3:1, 1:3, a similar phenomenon was observed. Besides, for the mixed droplets under constant RH, the effective vapor pressure of 1,2,6-hexanetriol decreased with the sucrose increasing, and the presence of 1,2,6-hexanetriol caused viscosity decline of sucrose in the meantime. To characterize the water mass transfer of mixed droplets at different RH, the characteristic time ratios between the droplet radius and the RH were calculated. Compared to the characteristic time ratio of pure sucrose droplets, the characteristic time ratio of the mixed droplets apparently decreased. And for constant RH, the characteristic time of sucrose/1,2,6-hexanetriol/H2O decreased with the increase of 1,2,6-hexanetriol. In general, we report the studies of both volatility and molecular diffusion process in the aerosol phase for the sucrose/1,2,6-hexanetriol/H2O ternary system, these results may provide a theoretical basis for subsequent research concerning the volatility of SVOCs in super viscous aerosols.

Generalized Huang-Rhys factors for molecular aggregates

Stimulated Raman spectra of molecular aggregates are often remarkably dissimilar to other vibrational spectra e.g. resonance Raman and hole-burning. Particularly in photosynthetic proteins, extremely sparse and predominantly low-frequency spectra are common, unlike the rich structure typical in spectra obtained by frequency-domain techniques. Thus, a mechanism which selectively enhances the intensity of only a small subset of vibronic transitions under stimulated Raman conditions in molecular complexes seems to be required. In this work, we explore how pigment-localized vibrations couple to the excitonic states of molecular aggregates – and the consequences for spectral observables. To aid in the analysis we introduce the concept of a generalized Huang-Rhys factor for molecular aggregates, we derive an analytical expression for this quantity, and establish two conditions required for strong vibronic coupling. We illustrate the effect of such coupling by simulation of absorption and resonance Raman spectra of a two-mode model dimer.

Spontaneous and Stimulated Raman Scattering in Protium and Deuterium Water

The Raman spectra of different specimens of protium and deuterium water have been compared. The spectra have been recorded using a fiber-optic technique and a compact BWS465-785H spectrometer. The spectra of spontaneous Raman scattering have been excited using a cw laser with a wavelength of 785 nm. Significant differences in the low-frequency Raman spectra from different specimens of water have been observed, which were explained by the presence of structural changes and impurities in analyzed specimens. Stimulated Raman spectra in protium and deuterium water have been measured upon excitation by picosecond laser pulses at a wavelength of 532 nm. The presence of low-frequency satellites in the spectrum of stimulated Raman scattering, indicating the structuring of water due to the formation of clusters consisting of a few molecules, has been revealed.

Raman Scattering in LiOH and LiOD Polycrystals

We have studied spontaneous and stimulated Raman spectra of polycrystalline LiOH and LiOD. The observed spontaneous Raman spectra contain low-frequency high-quality-factor lattice modes corresponding to translational and librational oscillations of the ions. The stimulated Raman spectrum has been found to contain several Stokes and anti-Stokes satellites with a large frequency shift.

Vibrational spectroscopy via stimulated Raman induced Kerr lensing

We present a new method for the measurement of the stimulated Raman spectrum based on time-dependent spatial modulation of a laser beam as it passes through a Raman active medium. This effect is similar to the instantaneous Kerr lensing and Kerr deflection yet involves resonant vibrations which result in a time-dependent refractive index change. We use sub-nanojoule pulses together with a sensitive pump-probe measurement apparatus to excite and detect the fine (10−5–10−4) temporal and spatial variations in intensity resulting from the Raman-induced Kerr effect. We demonstrate the effect by changing the spatial overlap between the pump and probe at the sample and measuring the time-dependent deformation of the probe beam’s cross section. This method is particularly useful for detection of low-frequency Raman lines, as we demonstrate by measuring the Raman spectrum of neat liquids in a cuvette.

Difference Bands in Time-Resolved Femtosecond Stimulated Raman Spectra of Photoexcited Intermolecular Electron Transfer from Chloronaphthalene to Tetracyanoethylene.

The time-resolved femtosecond stimulated Raman spectra (FSRS) of a charge transfer (CT) excited noncovalent complex tetracyanoethylene:1-chloronaphthalene (TCNE:ClN) in dichloromethane (DCM) is reported with 40 fs time resolution. In the frequency domain, five FSRS peaks are observed with frequencies of 534, 858, 1069, 1392, and 1926 cm-1. The most intense peaks at 534 and 1392 cm-1 correspond to fundamentals while the features at 858, 1069, and 1926 cm-1 are attributed to a difference frequency, an overtone and a combination frequency of the fundamentals, respectively. The frequency of the 1392 cm-1 fundamental corresponding to the central C═C stretch of TCNE•- is red-shifted from the frequency of the steady state radical due to the close proximity and electron affinity of the countercation. The observation of a FSRS band at a difference frequency is analyzed. This analysis lends evidence for alternative nonlinear pathways of inverse Raman gain scattering (IRGS) or vertical-FSRS (VFSRS) which may contribute to the time-evolving FSRS spectrum on-resonance. Impulsive stimulated Raman measurements of the complex show coherent oscillations of the stimulated emission with frequencies of 153, 278, and 534 cm-1. The 278 cm-1 mode corresponds to Cl bending of the dichloromethane solvent. The center frequency of the 278 cm-1 mode is modulated by a frequency of ∼30 cm-1 which is attributed to the effect of librational motion of the dichloromethane solvent as it reorganizes around the nascent contact ion pair. The 153 ± 15 cm-1 mode corresponds to an out-of-plane bending motion of TCNE. This motion modulates the intermolecular separation of the contact ion pair and thereby the overlap of the frontier orbitals which is crucial for rapid charge recombination in 5.9 ± 0.2 ps. High time-frequency resolution vibrational spectra provide unique molecular details regarding charge localization and recombination.

Two-dimensional electronic-Raman spectroscopy.

We present a new technique, two-dimensional electronic-Raman spectroscopy (2DER), which combines femtosecond stimulated Raman spectroscopy and a pulse-shaper-assisted 2D spectroscopic scheme for the actinic pump. The 2DER spectrum presents the initial actinic excitation wavelength with nanometer spectral resolution in the first axis and the detected stimulated Raman spectra in the second axis. We measured the correlation of the electronic and vibrational states in the photosynthetic accessory pigment β-carotene and reveal its photoexcited state manifold.

Ultrafast Intramolecular Proton Transfer of Alizarin Investigated by Femtosecond Stimulated Raman Spectroscopy.

We report time-resolved stimulated Raman spectra of alizarin in DMSO solution with 403 nm excitation. Upon photoexcitation, the intramolecular proton transfer reaction of alizarin occurs in 70-80 fs, which is confirmed by both the population growth and the frequency and bandwidth changes of skeletal vibrational modes of alizarin. Interestingly, the vibrational frequencies of ν(C═C) and ν(C═O) show opposite shifts during the reaction, which may implicate changes in the resonance structure of anthraquinone and the attached carbonyl group. Vibrational relaxation in the potential surface of the proton transferred tautomer of alizarin and the population decay occurring with two distinct time scales were also observed in addition to the solvation dynamics of DMSO solvent molecules.