Absolute Raman Cross Sections Research Articles

Absolute Stimulated Raman Cross Sections of Molecules

Stimulated Raman scattering (SRS) is a fundamental optical process that was discovered more than 60 years ago. While the early SRS spectroscopy studies have provided valuable insights into materials systems, the advent of SRS microscopy has launched a rapidly growing field in biological imaging. However, a fundamental understanding of the molecular response under SRS is still lacking. Herein we present a new framework to introduce molecule-intrinsic stimulated Raman scattering cross sections, σSRS, in the unit of Göppert-Mayer (GM). The absolute SRS cross sections determined for real molecular systems challenge the conventional wisdom that Raman spectroscopy is always a weak process. The enormous rate acceleration of SRS, captured by an apparent SRS cross section, stems from a synergistic effect between the field and the molecule. Our new framework goes beyond the conventional optics-centric view and presents a molecule-inclusive perspective, thus offering a comprehensive foundation for the future growth of SRS spectroscopy and microscopy.

228‐nm quadrupled quasi‐three‐level Nd:GdVO4 laser for ultraviolet resonance Raman spectroscopy of explosives and biological molecules

AbstractWe describe a new compact diode‐pumped solid‐state frequency quadrupled quasi‐three‐level neodymium‐doped gadolinium vanadate (Nd:GdVO4) laser that generates ~50 mW of 228‐nm quasi‐continuous wave light as ns pulses at a tunable kilohertz repetition rate. We developed two generations of this laser. The first generation has a high duty cycle and a tunable repetition rate. The second generation is optimized for maximum output power. We utilize these new lasers to measure ultraviolet resonance Raman (UVRR) spectra of many important chromophores that absorb in deep ultraviolet (UV). We demonstrate the utility of this excitation by measuring the 228‐nm absolute differential Raman cross sections of explosives, peptides, aromatic amino acids, and DNA/RNA nucleotides. Deep UV excitation at 228 nm occurs within the π → π* electronic transitions of these molecules. The 228‐nm resonance excitation enhances the Raman intensities of vibrations of NOx groups, peptide bonds, aromatic amino acid side chains, and DNA/RNA nucleotides. The measured 228‐nm UVRR cross sections of these molecules are 103–104 fold greater than those excited in the visible spectral region. These new lasers should be of great interest for UVRR spectroscopy and for other applications that benefit from compact, high average power deep UV laser light sources with low peak powers.

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.

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...

Polarizability tensor invariants of H2, HD, and D2.

We report an exhaustive compilation of wavelength-dependent matrix elements over the mean polarizability (α¯) and polarizability anisotropy (γ) operators for the rovibrational states of the H2, HD, and D2 molecules together with an accompanying computer program for their evaluation. The matrix elements can be readily evaluated using the provided codes for rovibrational states with J = 0-15 and v = 0-4 and for any laser wavelengths in the interval 182.25-1320.6 nm corresponding to popular, commercially available lasers. The presented results substantially extend the scope of the data available in the literature, both in respect of the rovibrational transitions analyzed and the range of covered laser frequencies. The presented detailed tabulation of accurate polarizability tensor invariants is essential for successful realization of our main long-term goal: developing a universal standard for determining absolute Raman cross sections and absolute Raman intensities in experimental Rayleigh and Raman scattering studies of molecules.

The structural configurations of Ethylene Trithiocarbonate in the binary mixture (SCS2CH2CH2+CHCl3) investigated by polarized Raman: Experimental and quantum chemical results

The isotropic and anisotropic components of the Raman spectra of CS stretching in Ethylene Trithiocarbonate (SCS2CH2CH2, ET) shows noncoincidence effect (NCE). The calculated absolute Raman cross-section (ARCS) of CS stretching initially increases with the dilution of concentration to a maximum and then decreases. This unusual behavior indicates the existence of self-association. Thus we proposed aggregated model and the dimer structure were optimized with DFT/PCM to explain the NCE and ARCS behaviors. Vibrational spectra and depolarization-ratio were calculated for monomer and dimer individually, the dimer is in good accordance with the experimental data. The strong antiparallel CS vibrational coupling in neighboring molecules accounts for the frequency shifts of the CS stretching.

Concentration-dependent frequency shifts of the CS stretching modes in ethylene trithiocarbonate studied by Raman spectroscopy

Resonant with the CS π → π* electronic transition, the intensity of CS stretching and its overtone have been greatly enhanced in the 488- and 319-nm excited resonance Raman spectra. The isotropic and anisotropic parts of the Raman spectra of CS stretching modes of ethylene trithiocarbonate (ET) at different concentrations have been analyzed in order to study the noncoincidence effect (NCE). In neat ET, the experimentally measured values of noncoincidence Δυnc are ~4.60 cm−1 for the CS stretching modes, which reduce to 1.30 cm−1 at the mole fraction χm (ET) = 0.13. Both the isotropic and anisotropic peak frequencies of CS stretching were found to shift to higher wavenumber when the concentrations are diluted, while the value of Δυnc goes on decreasing upon dilution. The absolute Raman cross section of carbonyl stretching was also measured, and their behavior was unusual (first increasing and then decreasing with the decrease of concentration). The experimental result shows that there may exist self-association in the high concentration, and the main NCE mechanism may be due to the transition dipole–transition dipole coupling between the ET molecules. Copyright © 2015 John Wiley & Sons, Ltd.

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.

Assessment of the Metallicity of Single-Wall Carbon Nanotube Ensembles at High Purities

The purity of single-wall carbon nanotube (SWCNT) ensembles is critical, but assessing the purity in terms of metal vs semiconductor content is challenging at high purities. We describe possible bulk Raman spectroscopy based procedures to assess this metallicity and compare it to absorption procedures. A simple metric for metallicity is the G+ band peak intensity ratio at two appropriate wavelengths. Related metallic G– band area derived metrics extend to higher purities. The G band signal scales linearly over orders of magnitude in concentration. However, the ratio-derived metrics may still be nonlinear because of resonance. Therefore, they break down if the tube diameter distributions differ greatly between samples, and other complications are possible. Additionally, the absolute Raman cross sections for each type of SWCNT can be estimated, and the abundance of semiconductors and metals can be tracked independently. Raman-derived metallicity figures of merit can meaningfully evaluate samples of purities...

Resonance Raman Spectra of Individual Excitonically Coupled Chromophore Aggregates

We report resonance Raman spectra of individual porphyrin nanotubular aggregates of meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) deposited on glass. Using a novel internal/external standard method, we show that absolute Raman cross sections of low-frequency vibrational modes are greatly enhanced by J-band excitation. We report single-aggregate resonance Raman spectra obtained without surface enhancement. Variations in the relative intensities of low- and high-frequency Raman modes of different aggregates and images of the resonance light scattering in epi-illumination reveal variations in aggregate structure and allow the possible correlation between Raman intensity and coherence to be explored. Polarized Raman spectra of individual aggregates confirm that the J-band is a composite of two closely spaced vibronically coupled transitions polarized parallel and perpendicular to the long axis of the aggregate, in accordance with our structural model of a hierarchical helical nanotube. The evolution of the Raman spectrum of a single aggregate during laser heating reveals the role of water in the assembly of structural subunits. Our experimental results provide insight into the concept of aggregation-enhanced Raman scattering.

Resonance Raman Intensity Analysis of ClNO2 Dissolved in Methanol

Halogens such as chlorine are converted from halides, including ClNO(2), to reactive radicals by UV solar radiation. These radicals can affect ozone production and destruction in the stratosphere. Recently, it became clear that halogen radicals can also play a significant role in the chemistry of the troposphere. The photochemistry of ClNO(2) has been the subject of several studies in the gas and solid state that demonstrated a clear phase-dependent reactivity. Here, we report our initial studies of nitryl chloride in solution. Resonance Raman (RR) spectra of ClNO(2) dissolved in methanol after excitation within the 1(1)A(1)-2(1)A(1) absorption band (D band) in the region 200-240 nm are presented. RR intensity along the NO symmetric stretch coordinate (v(1)) at 1291 cm(-1) is observed at all excitation wavelengths, whereas limited intensity corresponding to the transition of the N-Cl symmetric stretch (v(3)) was only observed at 199.8 nm, whereas no intensity corresponding to the O-N-O symmetric bend (v(2)) was observed. Depolarization ratios and absolute resonance Raman cross sections for v(1) were obtained at several excitation wavelengths spanning the D band. Depolarization ratios were found to deviate significantly from 1/3, consistent with more than a single dipole-allowed electronic transition contributing to the scattering. RR intensity analysis (RRIA) reveals that two closely spaced excited electronic states contribute to the scattering, which are dissociative along the Cl-N coordinate. In this study the role the solvent environment plays in ClNO(2) state energetics and excited structural evolution along fundamental coordinates is discussed.

Resonance Raman Spectra of TNT and RDX Using Vibronic Theory, Excited-State Gradient, and Complex Polarizability Approximations

Geometries, UV absorption bands, and resonance Raman (RR) cross sections of TNT and RDX are investigated using density functional theory (DFT) in conjunction with the Coulomb attenuated B3LYP exchange-correlation functional. The absorption and RR spectra are determined with use of vibronic (VB) theory, excited-state gradient, and complex polarizability (CPP) approximations. We examined low-energy isomers (two for TNT and four for RDX) whose energies differ by less than 1 kcal/mol, such that they would appreciably be populated at room temperature. The two TNT isomers differ by an internal rotation of the methyl group, while the four conformers of RDX differ by the arrangements of the nitro group relative to the ring. Our theoretical optical properties of the TNT and RDX isomers are in excellent agreement with experimental and recent CCSD-EOM results, respectively. For the two TNT isomers, the ultraviolet RR (UVRR) spectra are similar and in good agreement with recently measured experimental results. Additionally, the UVRR spectra computed using the excited-state and CPP approaches compare favorably with the VB theory results. On the other hand, the RR spectra of the RDX conformers differ from one another, reflecting the importance of the positioning of the NO2 groups with respect to the ring. In the gas phase or in solution, RDX would give a spectrum associated with a conformationally averaged structure. It is encouraging that the computed spectra of the conformers show similarities to recent measured RDX spectra in acetonitrile solution, and reproduce the 10-fold decrease in the absolute Raman cross sections of RDX compared to TNT for the observed 229 nm excitation. We show that in TNT and RDX vibrational bands that couple to NO2 or the ring are particularly resonance enhanced. Finally, the computed RDX spectra of the conformers present a benchmark for understanding the RR spectra of the solid-phase polymorphs of RDX.

Measurement of the absolute Raman cross section of the optical phonons in type Ia natural diamond

The absolute Raman cross section σ RS of the first-order 1332-cm −1 optical phonons in type Ia natural diamond was measured using 785- and 1064-nm pump lasers for the excitation of Raman scattering. A small temperature-controlled blackbody was used for the signal calibration of the 785- and 1064-nm Raman systems. Measurements were made with a 0.9-mm thick (111) natural diamond sample. Values of 2.7±0.6×10 −29 and 0.95±0.2×10 −29 cm 2 per carbon atom were determined for σ RS for 785- and 1064-nm excitation, respectively. The corresponding values of the Raman scattering efficiency S are 3.8±0.8×10 −7 and 1.3±0.3×10 −7 cm −1 sr −1. The values of the Raman polarizability | d | for 785-, and 1064-nm excitation are 6.8±0.7×10 −16 and 8.1±0.8×10 −16 cm 2, respectively. Our measured values of | d | are larger than the previously measured values of 4.4±0.3×10 −16 and 4.6±0.7×10 −16 cm 2 using 514.5- and 694-nm excitation, respectively. | d | vs. excitation photon energy was computed for the range from 1.0 eV (1240 nm) to 4.0 eV (310 nm) using time-dependent density functional theory. The computed values of 5.8×10 −16 and 5.3×10 −16 cm 2 for 514.4- and 694-nm excitation, respectively, are larger than the corresponding measured values of | d | . The computed values of 5.2×10 −16, and 5.0×10 −16 cm 2 for 785- and 1064-nm excitation are smaller than the corresponding values of | d | measured in this work. The computed value of | d | in the static limit is 4.7×10 −16 cm 2. Modification of | d | due to IaA and IaB defects, computed for a 64-atom supercell, is less than 5%.

UV Resonance Raman Study of TTR(105−115) Structural Evolution as a Function of Temperature

UV resonance Raman spectroscopy was used to probe the temperature dependence of the conformation of TTR(105-115) in solution. Resonance Raman spectra with excitation at 239.5 nm, show an increase in the absolute resonance Raman cross section of Tyr with an increase in temperature. This trend is associated with an increase in the hydrophobicity of the Tyr local environment, suggesting a conformational change at 28 °C. Excitation at ~200 nm is known to enhance scattering due to amide vibrations and provides insights as to the secondary structure of a peptide or protein. UVRR spectra at this excitation suggest that in solution the peptide assumes a disordered conformation with frequent formation of β-turns. Explicit-solvent replica-exchange MD simulations of the isolated peptide in the region 15 to 37 °C suggest that the dominant conformation assumed by the peptide corresponds to a coil with β-turns in the central and C-terminal region. In line with the experiments, an increase in temperature induces structural order in the peptide, reflected by an increase in the probability for the formation of β-turns and hydrophobic side-chain contacts, mainly in the 8-11 moiety, and to a lesser extent in the 4-7 moiety.

Measurement of the absolute Raman cross section of the optical phonon in silicon

The absolute Raman cross section σ R S of the first-order 519 cm −1 optical phonon in silicon was measured using a small temperature-controlled blackbody for the signal calibration of the Raman system. Measurements were made with a 25-mil thick (001) silicon sample located in the focal plane of a 20-mm effective focal length (EFL) lens using 785-, 1064-, and 1535-nm CW pump lasers for the excitation of Raman scattering. The pump beam was polarized along the [100] axis of the silicon sample. Values of 1.0±0.2×10 −27, 3.6±0.7×10 −28, and 1.1±0.2×10 −29 cm 2 were determined for σ RS for 785-, 1064-, and 1535-nm excitation, respectively. The corresponding values of the Raman scattering efficiency S are 4.0±0.8×10 −6, 1.4±0.3×10 −6, and 4.4±0.8×10 −8 cm −1 sr −1.The values of the Raman polarizability | d | for 785-, 1064-, and 1535-nm excitation are 4.4±0.4×10 −15, 5.1±0.5×10 −15, and 1.9±0.2×10 −15 cm 2, respectively. The values of 4.4±0.4×10 −15 and 5.1±0.5×10 −15 cm 2 for | d | for 785- and 1064-nm excitation, respectively, are 1.3 and 2.0 times larger than the values of 3.5×10 −15 and 2.5×10 −15 cm 2 calculated by Wendel. The Raman polarizability | d | computed using the density functional theory in the long-wavelength limit is consistent with the general trend of the measured data and Wendel’s model.

Measurement of the absolute Stokes Raman cross sections of the longitudinal optical (LO) phonons of room-temperature GaP

Absolute first-order Stokes Raman cross sections ( σ R ) of the longitudinal optical (LO) phonons of room-temperature GaP at 811 nm (1.53 eV) and 1112 nm (1.12 eV) have been measured, using pump/excitation wavelengths of 785 (1.58 eV) and 1064 nm (1.17 eV), respectively. A temperature-controlled, small-cavity blackbody source was used to calibrate the signal output of the Raman spectrometer system; this allows a direct measurement of the absolute Raman cross section without the need for the use of a known material as the reference standard. Measurements were made on a GaP single-crystal plate with (001) faces, using backscattering geometry. The measured values of σ R are 3.0± 0.6×10 −28 and 1.1± 0.2×10 −28 cm 2 at 811 and 1112 nm, respectively, compared with the corresponding values of 2.8×10 −28 cm 2 and 0.7×10 −28 cm 2 extrapolated from the values measured at 647 nm (1.92 eV) by Calleja et al. in 1981, using the λ − 4 wavelength dependence and decrease of Raman polarizability with wavelength due to resonance enhancement associated with the direct energy gap ( E 0 ) at 2.78 eV and the split-off energy gap ( E 0 + Δ 0 ) at 2.86 eV.

Estimation of peak Raman gain coefficients for Barium-Bismuth-Tellurite glasses from spontaneous Raman cross-section experiments

In this paper we explore the TeO(2)-Bi(2)O(3)-BaO glass family with varied TeO(2) concentration for Raman gain applications, and we report, for the first time, the peak Raman gain coefficients of glasses within this glass family extrapolated from non-resonant absolute Raman cross-section measurements at 785 nm. Estimated Raman gain coefficients show peak values of up to 40 times higher than silica for the main TeO(2) bands. Other optical properties, including index dispersion from the visible to the long wave Infrared (LWIR) are also summarized in this paper.