Resonance Raman Cross Sections Research Articles

Raman Scattering Cross Section of Confined Carbyne

We experimentally quantify the Raman scattering from individual carbyne chains confined in double-walled carbon nanotubes. We find that the resonant differential Raman cross section of confined carbyne is on the order of 10-22 cm2 sr-1 per atom, making it the strongest Raman scatterer ever reported.

Short-time dynamics and decay mechanism of E,E-2,4-hexadienal in the first light-absorbing S2(ππ*) state.

The initial nonadiabatic decay dynamics of E,E-2,4-hexadienal (HAL) in the light absorbing S2(ππ*) state were studied using resonance Raman spectroscopy and complete-active space self-consistent field (CASSCF) calculations. The UV and vibrational spectra were assigned on the basis of the UV absorption, Fourier transform (FT)-Raman and FT-infrared measurements, the density-functional theory computations, and the normal mode analysis. The A-band resonance Raman spectra in cyclohexane and acetonitrile were obtained at 282.4, 273.9, 266.0, 252.7, and 245.9 nm excitation wavelengths, respectively, to probe the corresponding structural dynamics of HAL. The A-band absorption cross section and the corresponding absolute resonance Raman cross sections were simulated using a simple model based on the time-dependent wave-packet theory in a Brownian oscillator model. The geometric structures of the singlet electronic excited states and their curve-crossing points were optimized at the CASSCF level of theory. The obtained short-time structural dynamics in easy-to-visualize internal coordinates were then compared with the CASSCF-predicted structural-parameter changes of S2(ππ*)S1(nπ*)-n (n = 1-4). Our results indicate that the initial population of HAL in the S2 state ramifies in or nearby the Franck-Condon (FC) region, leading to five S2(ππ*) → S1(nπ*) internal conversion pathways due to the flexibility of the molecular chain and the different electronic resonant structures formed nearby FC of the S2 state. Then, the formed S1 transient species, which have different geometric structures and different energy partitions, undergo different photophysical processes, such as S1 → S0 internal conversion, S1 → T1 intersystem crossing, and the S1 → S'1 photoisomerization reaction. The substitution effect on the S2(ππ*) → S1(nπ*) internal conversion dynamics and the trans-cis photoisomerization reaction is proposed in terms of the p-π conjugation interaction or the p-σ superconjugation interaction.

Nonresonant and Resonant Surface-Enhanced Raman Scattering of N-Ethyl-N-(2-hydroxyethyl)-4-(4-nitrophenylazo) Aniline in Poly(methyl methacrylate) on Ag Films with Surface Roughness

Abstract Nonresonant and resonant surface-enhanced Raman scatterings (SERS and SERRS) were studied for N-ethyl-N-(2-hydroxyethyl)-4-(4-nitrophenylazo) aniline (Disperse Red 1, or DR1) in poly (methyl methacrylate) on Ag films with surface roughness. DR1 is a chromophore that consists of azobenzene bridged between electron-donating amine and electron-accepting nitro groups, and it has attracted great attention because of its large molecular hyperpolarizability. DR1 hybridized with metal nanoparticles or nanostructures is promising as a building block for nonlinear plasmonics. Our experimental results demonstrated that the Raman cross sections were highly enhanced both at the molecular nonresonant and resonant excitation wavelengths. The spectroscopic properties of SERRS were taken from resonant Raman (RR), and the enhanced RR cross sections were attributed to electromagnetic enhancements due to surface plasmons (SP). The SERS spectrum was also similar to the RR spectrum, rather than the non-resonant Raman (NR) spectrum, even at the molecular non-resonant excitations. A diagram of energy levels was drawn for the DR1/Ag interfaces by using ultraviolet-visible linear absorption and ultraviolet photoelectron spectroscopic data. The enhanced NR cross sections were explained in terms of the electromagnetic enhancements, as well as the metal-to-molecular charge-transfer, by using the energy diagram.

Resonance Raman Characterization of Tetracene Monomer and Nanocrystals: Excited State Lattice Distortions With Implications For Efficient Singlet Fission.

The characterization of specific phonon modes and exciton states that lead to efficient singlet fission (SF) may be instrumental in the design of the next generation of high-efficiency photovoltaic devices. To this end, we analyze the absolute resonance Raman (RR) cross sections for tetracene (Tc) both as a monomer in solution and as a crystalline solid in an aqueous suspension of nanocrystals. For both systems, a time-dependent wavepacket model is developed that is consistent with the absolute RR cross sections, the magnitude of the absorption cross sections, and the vibronic line shapes of the fluorescence. In the monomer, the intramolecular reorganization energy is between 1500 and 1800 cm-1 and the solvent reorganization energy is 70 cm-1. In nanocrystals, the total reorganization is diminished to less than 600 cm-1. The lowest energy exciton has an estimated intramolecular reorganization energy between 300 and 500 cm-1 while intermolecular librational phonons have a reorganization energy of about 130 cm-1. The diminished reorganization energy of the nanocrystal is interpreted in the context of the delocalization of the band-edge exciton onto about ∼7 molecules. When electron and electron-hole correlations are included within many-body perturbation theory, the polarized absorption spectra of crystalline Tc are calculated and found to be in agreement with experiment. The low-lying exciton states and optically active phonons that contribute to the polarized crystal absorption are identified. The likely role of coherent exciton phonon evolution in the SF process is discussed.

Solvent effect on the initial structural dynamics of benzaldehyde in the S3(ππ*) state—Resonance Raman spectroscopic study

AbstractThe excited‐state structural dynamics of benzaldehyde (BA) after population to the S3(πH‐1πL*) light‐absorbing state were studied using resonance Raman spectroscopy and complete active space self‐consistent field (CASSCF) calculations. The C‐band absorption cross section and the corresponding absolute resonance Raman cross sections in cyclohexane and methanol were simulated using the time‐dependent wave‐packets formalism, respectively. The results indicate that the major structural dynamics of BA and the homogeneous broadening of the absorption spectrum in cyclohexane are similar to those of BA in methanol or acetophenone (AP) in cyclohexane, whereas the inhomogeneous broadening of BA in cyclohexane is significantly different from that in methanol. It is proposed that the initial decay pathway of BA upon the C‐band absorption is mostly towards S3S2 and partly to S3,min. The roles of hydrogen bonding and polar–polar interaction in the spectral broadenings are discussed.

Decay dynamics of 6‐azauracil from the light absorbing S2(ππ*) state

AbstractThe initial decay dynamics of 6AU in the S2 state was investigated by using resonance Raman spectroscopy, the time‐dependent wave‐packet theory in a Brownian oscillator model, and complete‐active space self‐consistent field (CASSCF) protocol. The vibrational spectra and the ultraviolet absorption bands were assigned on the basis of the Fourier transform (FT)‐Raman, FT‐infrared measurements, the density‐functional theory computations, and the normal mode analysis. The absorption cross section and the absolute resonance Raman cross sections were simulated simultaneously by using the time‐dependent wave‐packet theory in a Brownian oscillator model. The obtained normal mode displacements of the Franck–Condon active modes were then converted to the short‐time structural dynamics in easy‐to‐visualize internal coordinates. The roles of two pathways via the S2 → S1 internal conversion and the S2 → T3 intersystem crossing were evaluated through the comparison between the short‐time structural dynamics and the CASSCF/CASTP2 calculated structural changes between FC and S2S1 or between FC and S2T3. The results indicate that the structural dynamics in the Franck–Condon region of the S2 state is mostly toward the S2/S1 conical intersection, which supports the conclusion that the S2 → S1 internal conversion dominants the initial decay pathway of 6AU as revealed by the broadband fs TA spectroscopy, whereas the barrierless S2 → T3 intersystem‐crossing process is negligible owing to weak spin‐orbital coupling and unfavorable subsequent T3 → T2 → T1 decay. The reaction coordinates towards the planar S2(ππ*)/S1(nπ*) and severely distorted S2(ππ*)/S0 conical intersection points are proposed respectively, and the solvent effect on the initial decay mechanisms of 6AU, uracil, and thymine is clarified.

Weak Exciton–Phonon Coupling in CdSe Nanoplatelets from Quantitative Resonance Raman Intensity Analysis

Resonance Raman spectra, cross sections, and depolarization ratios have been measured for 4.5 monolayer thick CdSe nanoplatelets dispersed in chloroform. Five excitation wavelengths between 514.5 and 476.5 nm were employed. The resonance Raman spectra are dominated by the longitudinal optical (LO) phonon near 201 cm–1 and its overtone, as in CdSe quantum dots. The absolute scattering intensity is much higher for excitation on resonance with the sharp, lowest-energy heavy-hole to conduction band transition than with higher energy transitions, decreasing by about a factor of 30 between 514.5 and 496.5 nm excitation. The LO phonon overtone is weak directly on resonance with the heavy-hole transition but much stronger at higher excitation energies, a result that is reproduced by simulations of the spectra using standard resonance Raman intensity theory. The absolute Raman cross sections imply a Huang–Rhys parameter for the LO phonon of about 0.08 on resonance with the lowest heavy-hole transition. This is a f...

Electronic Resonant Stimulated Raman Scattering Micro-Spectroscopy.

Recently we have reported electronic pre-resonance stimulated Raman scattering (epr-SRS) microscopy as a powerful technique for super-multiplex imaging ( Wei, L. ; Nature 2017 , 544 , 465 - 470 ). However, under rigorous electronic resonance, background signal, which mainly originates from pump-probe process, overwhelms the desired vibrational signature of the chromophores. Here we demonstrate electronic resonant stimulated Raman scattering (er-SRS) microspectroscopy and imaging through suppression of electronic background and subsequent retrieval of vibrational peaks. We observed a change of the vibrational band shapes from normal Lorentzian, through dispersive shapes, to inverted Lorentzian as the electronic resonance was approached, in agreement with theoretical prediction. In addition, resonant Raman cross sections have been determined after power-dependence study as well as Raman excitation profile calculation. As large as 10-23 cm2 of resonance Raman cross section is estimated in er-SRS, which is about 100 times higher than previously reported in epr-SRS. These results of er-SRS microspectroscopy pave the way for the single-molecule Raman detection and ultrasensitive biological imaging.

Deep ultraviolet initiated excited state dynamics of riboflavin and flavin mononucleotide

AbstractFlavins are cofactors in several light‐activated enzymes and therefore their excited states are found to involve in many photobiological processes. Excited state dynamics of flavin compounds corresponding to their first singlet state (S1) has been studied using a plethora of techniques, whereas studies related to highly absorbing ultraviolet excited states are lacking. Here, we study the ultrafast excited state dynamics of riboflavin and flavin mononucleotide using resonance Raman intensity analysis upon photoexcitation into their most intense absorption band centered at 266 nm. Resonance Raman cross sections of each flavin band are quantitatively measured across the 266‐nm absorption band (257–280 nm), and Raman excitation profiles are constructed. We have used Lee and Heller's time‐dependent wave packet theory to simulate the experimental Raman cross sections in a self‐consistent manner. The simulation results in instantaneous structural changes along with solvation dynamics, through linewidth broadening within tens of femtoseconds following photoexcitation. Major structural changes were observed through contraction and elongation of several ring stretching coordinates, affecting at a different site when compared with the S1 excitation. The value of the total reorganization energy was determined to be 1,665 cm−1 (and 1,602 cm−1 for flavin mononucleotide) with a contribution of 1,310 cm−1 from the inertial response of water. We find upon excitation, the first solvation shell inertially responds with an ultrafast timescale of <30 fs for both the molecules. Our results can be useful to determine the structure and dynamics of flavoenzymes by using flavin as a probe following excitation within their 266‐nm absorption band.

Initial excited state structural dynamics of lumiflavin upon ultraviolet excitation

Excited state photophysics of flavin compounds has attracted great attention due to their involvement in several photobiological processes as a cofactor in photoreceptor enzymes. In this work, we have determined the initial excited state structural dynamics of flavin related compound, lumiflavin (LF), upon ultraviolet (UV) excitation through resonance Raman intensity analysis. A quantitative measurement of resonance Raman cross-sections across the 265 nm absorption band (257–275 nm) of LF have been performed, and Raman excitation profiles (REPs) are constructed for all the Raman active modes. The REPs and absorption cross sections have been simulated using time-dependent wave packet propagation (TDWP) formalism to extract the ultrafast dynamics of LF within 50 fs after photoexcitation. The total internal reorganization energy is estimated to be 470 cm−1, which is clear evidence for the significant structural distortion in the excited state from that of the ground state. By comparing the excited state structural changes between S1 state and 265 nm excited state, we found that different molecular distortions occur following photoexcitation to those two different states. The total reorganization energy is obtained to be 1542 cm−1, in which the maximum contribution comes from the inertial response of water (1070 cm−1). Additionally, our simulation also yields an instantaneous response of the first solvation shell within an ultrafast timescale, τ ∼ 30 fs, following photoexcitation. The current study provides the basis for future investigations to determine the excited state properties of flavin compounds upon UV excitations.

Resonance Raman excitation profiles of CdS in pure CdS and CdSe/CdS core/shell quantum dots: CdS-localized excitons.

Resonance Raman excitation profiles have been measured for the longitudinal optical phonon in two sizes of CdS quantum dots and in CdSe/CdS core/shell quantum dots. In pure CdS, the resonance Raman cross section for the fundamental transition is sharply peaked around the lowest-energy excitonic transition and becomes weaker with higher-energy excitation even though the absorbance continues to increase to higher energies. This effect, also observed in earlier studies of CdSe quantum dots, is attributed largely to interferences among multiple excitonic transitions contributing to the resonance Raman polarizability. No variation in exciton-phonon coupling strength with exciton energy is required to explain the decrease in the ratio of the Raman cross section to absorption cross section at shorter wavelengths. In core/shell structures, the CdSe phonons are relatively strong when exciting on resonance with the lower excitonic transitions, in which the hole is largely localized to the CdSe core, but become nearly undetectable at excitation wavelengths >0.6 eV above the lowest exciton, where both electrons and holes are largely localized in the CdS shell. The CdS phonon Raman cross section exhibits a maximum 0.6-0.7 eV above the lowest exciton and then decreases at higher energies for the same reasons as in pure CdS.

Short-time dynamics and decay mechanism of 2(1H)-pyridinone upon excitation to the light-absorbing S4(21𝝅𝝅*) state

The excited-state structural dynamics and the decay mechanism of 2(1H)-pyridinone (NHP) after excitation to the S4(21ππ*) light-absorbing state were studied using resonance Raman spectroscopy and complete-active space self-consistent field (CASSCF) calculations. The B-band absorption cross-section and the corresponding absolute resonance Raman cross-sections were simulated using a simple model based on time-dependent wave-packet theory. The geometric structures of the singlet electronic excited states and their curve-crossing points were optimized at the CASSCF level of theory. The obtained short-time structural dynamics in easy-to-visualize internal coordinates were then compared with the CASSCF-predicted structural-parameter changes of S4(21ππ*)/S3(21nπ*)-MIN, S4(21ππ*)/S1(11nπ*)-MIN, and S4(21ππ*)-MIN. Our results indicate that the initial population of NHP in the S4 state bifurcates in or near the Franck-Condon region, leading to two predominant (S4S3-MIN and S4S1-MIN) internal conversion pathways. The lowest-lying S2(11ππ*) excited state is finally formed via subsequent internal conversions S3(21nπ*)/S2(11ππ*)-MIN and S1(11nπ*)/S2(11ππ*)-MIN. The enol-keto tautomeric mechanism does not seem to play a role. The decay mechanism in the singlet realm is proposed.

Probing strong correlations with light scattering: Example of the quantum Ising model

In this paper we calculate the nonlinear susceptibility and the resonant Raman cross section for the paramagnetic phase of the ferromagnetic Quantum Ising model in one dimension. In this region the spectrum of the Ising model has a gap $m$. The Raman cross section has a strong singularity when the energy of the outgoing photon is at the spectral gap $\omega_{f} \approx m$ and a square root threshold when the frequency difference between the incident and outgoing photons $\omega_{i} -\omega_{f} \approx 2m$. The latter feature reflects the fermionic nature of the Ising model excitations.

Size-Dependent Exciton–Phonon Coupling in CdSe Nanocrystals through Resonance Raman Excitation Profile Analysis

Resonance Raman excitation profiles, including absolute Raman cross sections, have been measured for the optical phonons of organic ligand capped spherical CdSe nanocrystals with average diameters ranging from 2.6 to 5.2 nm. The absorption spectra, fundamental excitation profiles, overtone to fundamental intensity ratios, and depolarization ratio dispersion profiles are simulated using a model for the exciton–phonon coupling strengths in the various excitonic transitions that contribute to the resonance enhancement. The resonance Raman cross sections increase strongly with nanocrystal size as expected from the increased transition dipole moments (oscillator strengths) in the larger particles. However, the exciton–phonon coupling strengths, particularly for the lowest energy excitonic transition (1Se–1S3/2), are relatively independent of nanocrystal size over this range, perhaps decreasing slightly with increasing size. This result is consistent with calculations of exciton–phonon coupling using an atomistic model for the phonon modes and an effective mass approximation-envelope function model for the excitonic states if the electron and hole effective masses are adjusted to fit the experimental excitonic transition energies.

Dependence of Raman and Resonance Raman Intensities on Sample Self-Absorption

Resonance Raman cross sections are generally larger than normal or preresonance Raman cross sections. Thus, higher Raman intensities are expected for resonance excitation, especially for backscattering measurements. However, self absorption decreases the observed Raman intensities. In the work here we examine the effect of self absorption on the observed preresonance and resonance Raman intensities. For the simplest case where a single electronic transition dominates the Raman scattering, and where the resonance enhancement scales with the square of the molar absorptivity of the absorption band, theory predicts that for close to resonance excitation the observed Raman intensities monotonically increase as resonance is approached. In the case that an impurity absorbs, the observed Raman intensities may decrease as excitation moves close to resonance for particular conditions of impurity absorption band widths and frequency offsets. Impurity absorption also causes decreases in observed Raman intensities for the more slowly increasing preresonance excitation.

State-by-State Investigation of Destructive Interference in Resonance Raman Spectra of Neutral Tyrosine and the Tyrosinate Anion with the Simplified Sum-over-States Approach

UV resonance Raman scattering is uniquely sensitive to the molecular electronic structure as well as intermolecular interactions. To better understand the relationship between electronic structure and resonance Raman cross section, we carried out combined experimental and theoretical studies of neutral tyrosine and the tyrosinate anion. We studied the Raman cross sections of four vibrational modes as a function of excitation wavelength, and we analyzed them in terms of the contributions of the individual electronic states as well as of the Albrecht A and B terms. Our model, which is based on time-dependent density functional theory (TDDFT), reproduced the experimental resonance Raman spectra and Raman excitation profiles for both studied molecules with good agreement. We found that for the studied modes, the contributions of Albrecht's B terms in the Raman cross sections were important across the frequency range spanning the L(a,b) and B(a,b) electronic excitations in tyrosine and the tyrosinate anion. Furthermore, we demonstrated that interference with high-energy states had a significant impact and could not be neglected even when in resonance with a lower-energy state. The symmetry of the vibrational modes served as an indicator of the dominance of the A or B mechanisms. Excitation profiles calculated with a damping constant estimated from line widths of the electronic absorption bands had the best consistency with experimental results.

Intricate packing in the hydrophobic core of barstar through a CH–π interaction

Identification of specific packing interactions within in the hydrophobic core of proteins is important for understanding the integrity of protein structure. Finding such interactions is challenging because few tools allow monitoring of a specific interaction in the presence of several non‐specific forces that hold proteins together. It is important to understand how and when such interactions develop during protein folding. In this study, we have used the intrinsic tryptophan residue, Trp53, as an ultraviolet resonance Raman probe to elucidate the packing interactions in the hydrophobic core of the protein barstar. Barstar is extensively studied for its folding, unfolding and aggregation properties. The Trp53 residue is known to be completely buried in the hydrophobic core of the protein and is used extensively as an intrinsic probe to monitor the folding and unfolding reactions of barstar. A comparison of the resonance Raman cross sections of some bands of Trp53 with those observed for N‐acetyl‐tryptophanoamide in water suggests that Trp53 in barstar is indeed isolated from water. Intensity ratio of the Fermi doublet suggests that Trp53 is surrounded by several aliphatic amino acid residues in corroboration with the crystal structure of barstar. Importantly, we show that the side chain of Trp53 is involved in a unique CH–π interaction with CH groups of Phe56 as well as a steric interaction with the methyl group of Ile5. Copyright © 2014 John Wiley & Sons, Ltd.

Determination of resonance Raman cross-sections for use in biological SERS sensing with femtosecond stimulated Raman spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) is a promising technique for in vivo bioanalyte detection, but accurate characterization of SERS biosensors can be challenging due to difficulties in differentiating resonance and surface enhancement contributions to the Raman signal. Here, we quantitate the resonance Raman cross-sections for a commonly used near-infrared SERS dye, 3,3'-diethylthiatricarbocyanine (DTTC). It is typically challenging to measure resonance Raman cross-sections for fluorescent dye molecules due to the overwhelming isoenergetic fluorescence signal. To overcome this issue, we used etalon-based femtosecond stimulated Raman spectroscopy, which is intrinsically designed to acquire a stimulated Raman signal without strong fluorescence or interference from signals resulting from other four-wave mixing pathways. Using this technique, we found that the cross-sections for most of the resonantly enhanced modes in DTTC exceed 10(-25) cm(2)/molecule. These cross-sections lead to high signal magnitude SERS signals from even weakly enhancing SERS substrates, as much of what appears to be a SERS signal is actually coming from the intrinsically strong resonance Raman signal. Our work will lead to a more accurate determination of SERS enhancement factors and SERS substrate characterization in the biologically relevant near-infrared region, ultimately leading to a more widespread use of SERS for biosensing and bioimaging applications.

Giant Raman scattering from J-aggregated dyes inside carbon nanotubes for multispectral imaging

Raman spectroscopy uses visible light to acquire vibrational fingerprints of molecules, thus making it a powerful tool for chemical analysis in a wide range of media. However, its potential for optical imaging at high resolution is severely limited by the fact that the Raman effect is weak. Here, we report the discovery of a giant Raman scattering effect from encapsulated and aggregated dye molecules inside single-walled carbon nanotubes. Measurements performed on rod-like dyes such as α-sexithiophene and β-carotene, assembled inside single-walled carbon nanotubes as highly polarizable J-aggregates, indicate a resonant Raman cross-section of (3 ± 2) × 10−21 cm2 sr−1, which is well above the cross-section required for detecting individual aggregates at the highest optical resolution. Free from fluorescence background and photobleaching, this giant Raman effect allows the realization of a library of functionalized nanoprobe labels for Raman imaging with robust detection using multispectral analysis. The use of Raman spectroscopy for high-resolution optical imaging is severely limited by the inherent weakness of the Raman effect. Now, a giant resonant Raman effect is demonstrated from J-aggregated dye molecules encapsulated in single-walled carbon nanotubes, and it is used to realize multispectral Raman imaging.

Doping-Dependent Photon Scattering Resonance in the Model High-Temperature SuperconductorHgBa2CuO4+δRevealed by Raman Scattering and Optical Ellipsometry

We study the model high-temperature superconductor HgBa(2)CuO(4+δ) with electronic Raman scattering and optical ellipsometry over a wide doping range. The dependence of the resonant Raman cross section on the incident photon energy changes drastically as a function of doping, in a manner that corresponds to a rearrangement of the interband optical transitions seen with ellipsometry. This doping-dependent Raman resonance allows us to reconcile the apparent discrepancy between Raman and x-ray detection of magnetic fluctuations in superconducting cuprates. Intriguingly, the strongest variation occurs across the doping level where the antinodal superconducting gap appears to reach its maximum.