Absolute Raman Cross Sections Research Articles

Excited-State Geometry Method for Calculation of the Absolute Resonance Raman Cross Sections of the Aromatic Amino Acids

The time correlator formalism was used to calculate the absolute resonance Raman cross sections for the aromatic amino acids based on density functional theory calculations of the ground-state potential energy surfaces combined with projection along normal mode eigenvectors in the excited state. The geometric difference between the minima of the ground and excited states along each normal mode was calculated to provide inputs for the time correlator in the linear approximation. The calculated dimensionless nuclear displacements, Delta(i), provide the electron-phonon coupling constants, S(i) = Delta(i)(2)/2, for the corresponding Raman active mode of frequency omega(t). The method is generally applicable to molecules that are Franck-Condon active. As an example we have chosen to calculate the absolute resonance Raman cross sections of models of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. We discuss the role played by substituents on the aromatic ring that decrease vibronic activity to a level that permits application of the time correlator. While the method may have limitations for molecules of high symmetry, the current study of excited-state displacements and electronic structure indicates that the L(a),(b) states are Franck-Condon active in the aromatic molecules studied.

Trinuclear Copper(I) Complex Containing 3,4,9,10,15,16-Hexamethyl-1,6,7,12,13,18-hexaazatrinaphthylene: A Structural, Spectroscopic, and Computational Study

The compound [(Cu(PPh(3))(2))(3)(HATNMe(6))](BF(4))(3) has been synthesized and characterized by X-ray crystallography, resonance Raman spectroscopy, and density functional theory (DFT) calculations. The X-ray structure of solvated [(Cu(PPh(3))(2))(3)(HATNMe(6))](BF(4))(3) [rhombohedral, R3, a = b = 21.6404(4) A, c = 53.188(3) A, alpha = beta = 90 degrees, gamma = 120 degrees] shows that the HATNMe(6) ligand is very slightly twisted. The electronic absorption spectrum of the complex in chloroform shows two bands in the visible region attributed to ligand-centered (LC) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. Time-dependent DFT calculations show good agreement with experiment, with two MLCT and one LC transition predicted in the visible region (641, 540, and 500 nm). Resonance Raman spectra of the complex using discrete excitation energies between 647 and 406 nm showed a variation in enhancement patterns consistent with at least two distinct transitions. The absolute Raman cross sections have been evaluated and, through a wavepacket analysis, the amount of distortion along each vibrational mode across the Franck-Condon surface is established from the calculated dimensionless displacement (Delta) values as well as other electronic parameters. The pattern of Delta values shows good agreement with the observed calculated modes, with the MLCT transition, showing much larger Delta values for outer ring modes such as nu(93) and nu(205) than in the LC transition. This is consistent with the molecular orbitals involved in the two transitions; the donor orbitals for the LC transition have similar outer-ring bonding characteristics compared to the MLCT transition, which has no donor orbital bonding characteristics on the ligand because the donor molecular orbitals are dpi orbitals.

Absolute Raman cross-sections of some explosives: Trend to UV

The Raman cross-section dependence on excitation energy in spectral range 620–248 nm have been investigated for UN, TATP, RDX, TNT, and PETN explosives. For all investigated explosive materials, significant pre-resonance enhancement in the UV spectral range has been revealed. Normalized Raman scattering signals are 100–200 times stronger with UV excitation at 248 nm compared to visible excitation at 532 nm. Thus the gated Raman technique with UV excitation has significant advantages compared to IR–VIS excitation for the remote detection of explosives. Certain of the observed Raman lines exhibit deviations from classical λ −4 dependence of Raman cross-section and may totally disappear with UV excitation. The possible explanation for this may be that numerous electronic transitions contribute to the molecular polarizability. These contributions could be of opposite sign and lead to partial cancellation of certain transitions. Another possible reason could be that this is a result of the stronger UV absorption that reduces the sampling volume and therefore the number of scatterers that produce the Raman signal.

Measuring the absolute Raman cross section of nanographites as a function of laser energy and crystallite size

In this paper, the dependence of the differential Raman cross section $\ensuremath{\beta}$ of the $D$, $G$, ${D}^{\ensuremath{'}}$, and ${G}^{\ensuremath{'}}$ bands of nanographites on the excitation laser energy and also on the crystallite size is reported. We show that ${\ensuremath{\beta}}_{G}$ is proportional to the fourth power of the excitation laser energy $({E}_{l})$, as predicted by the Raman scattering theory. For the bands which arise from the double-resonance mechanism ($D$, ${D}^{\ensuremath{'}}$, and ${G}^{\ensuremath{'}}$), the differential cross section does not depend on ${E}_{l}$, explaining the strong dependence of the ratio ${I}_{D}∕{I}_{G}$ on the excitation laser energy ${E}_{l}$ used in the Raman experiment. The ${L}_{a}$ dependence of $D$ and ${D}^{\ensuremath{'}}$ band differential cross sections is measured, confirming that the proportionality ${I}_{D}∕{I}_{G}\ensuremath{\propto}{L}_{a}^{\ensuremath{-}1}$ originates from the strong dependence of ${\ensuremath{\beta}}_{D}$ on the inverse of the crystallite size. In the ${G}^{\ensuremath{'}}$ band case, the data show that its differential cross section increases with the increasing crystallite size ${L}_{a}$, following an opposite behavior when compared with the disorder induced $D$ and ${D}^{\ensuremath{'}}$ bands. An analysis on the dependence of the full width at half maximum $(\ensuremath{\Gamma})$ of the $D$, $G$, ${D}^{\ensuremath{'}}$, and ${G}^{\ensuremath{'}}$ bands on the crystallite size ${L}_{a}$ of nanographites is performed, showing that the phonon lifetime is proportional to the crystallite size.

Study of resonance Raman cross section of aqueous β -carotene at low concentrations

Applying Teflon-AF waveguide cell, we determined the absolute resonance Raman cross sections (RCS) for C=C and C-C stretching modes of β-carotene in aqueous solution at low concentrations (10-6–10-10 M) by measuring Raman intensity. It was unexpected that the RCS increased rapidly with the decrease of concentration at extremely low concentration range (10-8–10-10 M), and it reached the value of 1.22×10-21 cm2 molecule-1Sr-1 at 2.5×10-10 M larger than general RCS (10-30 cm2 molecule-1Sr-1) for 109 times. Additionally, we give the tentative theory exploration for the experimental results that the large RCSs at low concentrations attributed to both resonance Raman effect and dipole-induced dipole interactions between solute and solvent molecules.

External Raman standard for absolute intensity and concentration measurements

A new method of calibrating Raman intensities is described, which uses a sapphire window as an external intensity standard. This method overcomes problems often encountered using conventional internal standards techniques, resulting either from refractive index differences or chemical interactions of the standard with the sample. The new method is demonstrated by measuring solution concentrations and Raman cross sections in various liquids. Concentration measurements are performed using the second derivative variance minimization (SDVM) algorithm to simultaneously subtract the sapphire window spectrum, normalize the solution spectral intensity and quantify solute concentration. The absolute Raman cross sections of several pure liquids were determined by comparing intensities of the sapphire-normalized Raman bands to that of benzene and other liquids whose Raman cross sections are known.

Multistate Photochemical Reaction Dynamics of ClNO in Solution: An Absolute Resonance Raman Intensity Analysis Study

The excited-state reaction dynamics of nitrosyl chloride (ClNO) are studied using absolute resonance Raman intensity analysis. The absolute resonance Raman cross sections for ClNO dissolved in cyclohexane and acetonitrile are measured at several excitation wavelengths spanning the absorption band commonly referred to as the “A band” (λmax ≈ 200 nm). The resonance Raman and absorption cross sections are modeled using the time-dependent formalism. Resonance Raman depolarization ratios are also measured and are found to be consistent with at least two electronic transitions participating in the scattering process. Therefore, the standard time-dependent formalism approach was modified by incorporating two excited states into the analysis, with state contributions deconvolved through modeling of the depolarization ratios in addition to the absolute resonance Raman and absorption cross sections. The spectroscopic observables are well reproduced using this two-state model. The analysis presented here demonstrates that the photoexcitation of solution-phase ClNO results in a substantial evolution of the N−Cl stretch coordinate consistent with the dissociation of the N−Cl bond. Significant structural evolution is also observed along the bend, with minimal excited-state structural evolution observed along the NO stretch. The structural evolution along the dissociative N−Cl stretch coordinate is found to be solvent-dependent, and the origin of this dependence is related to changes in the ground-state equilibrium geometry as a function of solvent environment. Finally, the homogeneous line width undergoes a significant increase in acetonitrile relative to cyclohexane, and this increase is proposed to reflect the modification of the excited-state interactions and nonadiabatic relaxation dynamics.

Excited-state metal-to-ligand charge transfer dynamics of a ruthenium(II) dye in solution and adsorbed on TiO2 nanoparticles from resonance Raman spectroscopy.

The dynamics of metal-to-ligand charge transfer (MLCT) in a cis-bis(4,4'-dicarboxy-2,2'-bipyridine)-bis(isothiocyanato)ruthenium(II) dye (N3) are compared for the free dye in solution and the dye adsorbed on the surface of the TiO(2) nanoparticles from resonance Raman spectroscopy. The 544-nm MLCT absorption band of N3 adsorbed on TiO(2) is slightly blue-shifted from that of the free N3, indicating a weak electronic coupling between N3 and TiO(2). The resonance Raman spectra of N3 and the N3|TiO(2) complex obtained upon excitation within the lowest-lying MLCT singlet state of the dye are similar except for slight shifts in band positions. Resonance Raman cross sections have been obtained for the vibrational modes of both N3 and N3|TiO(2) with excitation frequencies spanning the 544-nm MLCT band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism over two electronic states yields mode-specific vibrational and solvent reorganization energies. Despite the weak electronic coupling between N3 and TiO(2) in N3|TiO(2), adsorption strongly affects the reorganization energies of N3 in the intramolecular MLCT state. Adsorption of N3 onto TiO(2) increases the absolute Raman cross section of each mode by a factor of ca. 1.6 and decreases the vibrational and solvent reorganization energies by factors of 2 and 6, respectively. The excited-state dynamics of N3 adsorbed on the surface of TiO(2) nanoparticles were observed to be independent of the number of N3 molecules adsorbed per TiO(2) nanoparticle. The effect of TiO(2) on the dynamics of the adsorbed N3 is primarily due to both mode-specific vibrational and electronic pure dephasing, with the dominant contribution from the latter process.

Probing the molecular basis for solvent-induced dephasing using resonance Raman spectroscopy: betaine-30 in CH 3OH, CH 3OD, and CD 3OD

Absolute Raman cross-sections are reported for fourteen normal modes of betaine-30 dye in CH 3OH, CH 3OD, and CD 3OD solution, at five different excitation frequencies within the visible absorption band. Consistent with previous results from our lab, the Raman intensities of betaine-30 in CD 3OD solution are generally stronger than in CH 3OH solution. For most of the betaine-30 normal modes, however, the resonance Raman cross-sections are similar in CH 3OD and CH 3OH solution. The results suggest that methyl group motion, more so than hydroxyl group motion, is responsible for dephasing of the resonant electronic transition of betaine-30 in methanol.

Investigation of Short-Time Isomerization Dynamics inp-Nitroazobenzene from Resonance Raman Intensity Analysis

Resonance Raman (RR) spectra are presented for p-nitroazobenzene dissolved in chloroform using 18 excitation wavelengths, covering the region of 1(n → π*) electronic transition. Raman intensities are observed for various totally symmetric fundamentals, namely, C−C, C−N, NN, and N−O stretching vibrations, indicating that upon photoexcitation the excited-state evolution occurs along all of these vibrational coordinates. For a few fundamentals, interestingly, in p-nitroazobenzene, it is observed that the RR intensities decrease near the maxima of the resonant electronic 1(n → π*) transition. This is attributed to the interference from preresonant scattering due to the strongly allowed 1(π → π*) electronic transition. The electronic absorption spectrum and the absolute Raman cross section for the nine Franck−Condon active fundamentals of p-nitroazobenzene have been successfully modeled using Heller's time-dependent formalism for Raman scattering. This employs harmonic description of the lowest energy 1(n → π*...

Resonance Raman intensity analysis of chlorine dioxide dissolved in chloroform: The role of nonpolar solvation

Absolute resonance Raman cross sections for chlorine dioxide (OClO) dissolved in chloroform are obtained at several excitation wavelengths spanning the photochemically relevant B12–A22 optical transition. The absolute scattering cross sections of OClO are determined by reference to the 666 cm−1 transition of chloroform whose absolute scattering cross sections are reported here. The time-dependent theory for Raman and absorption are used to develop a mode-specific description of the A22 excited state surface. This description demonstrates that photoexcitation of OClO leads to significant structural evolution along the symmetric stretch and bend coordinates, with only limited evolution occurring along the asymmetric stretch. This description is similar to that determined for OClO dissolved in cyclohexane and water demonstrating that the excited-state structural evolution of OClO is similar in these solvents. Analysis of the OClO absolute scattering cross sections establishes that the homogeneous linewidth is 95±15 cm−1 in chloroform, essentially identical to the linewidths in cyclohexane and water. To establish the origin of this linewidth, the fluorescence cross section for OClO dissolved in cyclohexane is measured and found to be consistent with an excited-state lifetime of ∼200 fs. Comparison of this lifetime to the homogeneous linewidth establishes that the homogeneous broadening is dominated by solvent-induced pure dephasing. It is proposed that the apparent solvent independence of the homogeneous linewidth reflects the mechanical response of the solvent to the photoinitiated change in solute geometry. In support of this hypothesis, the homogeneous linewidth is reproduced using the viscoelastic continuum model of nonpolar solvation. Finally, it is argued that the restricted evolution along the asymmetric-stretch coordinate is due to dielectric solvent–solute interactions consistent with the increase in inhomogeneous linewidth with an increase in solvent polarity.

Resonance Raman and density functional study of the A-band absorption of C 5H 5[WCCPh]O 2

Resonance Raman spectra including absolute Raman cross-section measurements and density functional theory (DFT) calculations were performed to study the A-band absorption of C 5H 5[WCCPh]O 2. The three most intense Franck–Condon active modes, the nominal WO stretch, CC stretch and CC stretch, show noticeable intensity in their overtones as well as their combination bands with each other. This is consistent with the changes in the HOMO–LUMO electron densities obtained from the DFT calculations. This suggests that the excited-state experiences significant structural changes simultaneously in the WO, CC and CC groups and the charge transfer is delocalized in the electronic transition.

Far-ultraviolet resonance Raman spectroscopy of nitrate ion in solution

Resonance Raman spectra are presented for the nitrate anion, NO3−, in water, ethylene glycol, methanol, and acetonitrile solution at six excitation wavelengths from 246 to 204 nm, on resonance with the lowest π→π* excitation. Absolute Raman cross sections for the CH stretches of ethylene glycol and methanol at these wavelengths are also reported. The nitrate spectra in all four solvents are dominated by fundamentals, overtones, and combination bands of the totally symmetric NO stretch (ν1) near 1043 cm−1 and the out-of-phase NO stretches (ν3) at 1340–1400 cm−1, consistent with substantial changes in NO bond length upon π-electron excitation. The intensity in ν3 and the ≈60 cm−1 splitting of this nominally degenerate vibration are indicative of pronounced breaking of the isolated molecules D3h symmetry by the local solvent environment. Intensity in the overtone of the out-of-plane mode (ν2) near 830 cm−1 suggests a change in the equilibrium geometry from planar to pyramidal upon electronic excitation. The absorption spectra and absolute Raman cross sections are simulated with a model that considers resonance with two orthogonally polarized electronic states whose degeneracy is broken by the locally asymmetric environment. Both solvent reorganization and geometry changes along the nitrate molecular vibrations make major contributions to the breadth of the absorption band. No differences between resonant and nonresonant linewidths are observed for the ν1 band.

Resonance Raman and Emission Investigation of the MLCT Transition in Pt(PEt3)2(C⋮CH)2

Resonance Raman spectra including absolute Raman cross section measurements were acquired with excitation wavelengths within the MLCT absorption band of [Pt(PEt3)2(C⋮CH)2]. The absorption and reson...

Pre-resonance Raman scattering in nitrobenzene vapour

For the measurement of absolute Raman scattering cross-sections in the vapour phase, a sample cell was designed and the procedure for measurements and the details of its design are described. The absolute Raman cross-sections of the 1347 cm−1 mode of nitrobenzene vapour were measured using both the 992 cm−1 mode of benzene and the 802 cm−1 mode of cyclohexane as internal intensity standards for excitation wavelengths in the range 406–230 nm. The results show a maximum in the excitation profile, which is attributed to the charge-transfer state of the molecule at around 240 nm. The long data averaging time needed for signal retrieval reduces the Raman intensity by progressively reducing the concentration of the sample through photochemical processes at resonant excitations. The Raman excitation profile at pre-resonance wavelengths shows more than two orders of magnitude enhancement over that predicted by the λ0−4 dependence of the cross-section. A non-linear regression analysis of the data points to the Albrecht A-term pre-resonance approximation (AATPA) theory was carried out. For the two wavelength ranges 406–290 and 406–260 nm, values of the resonant state were found to be 235 and 232 nm, respectively. Considering the assumptions made in the derivation of the theory and the large measurement errors (±20 absolute and ±10% relative), the theory is considered to be applicable to the present data. Although the analysis shows that the contribution to the enhancement of Raman scattering by electronic states lying higher than the charge-transfer state is negligible, the validity of the wavelength range for the application of the theory to the present data points remains uncertain. Copyright © 2000 John Wiley & Sons, Ltd.

Investigation of the short-time photodissociation dynamics of trans-1-bromo-2-iodoethane in the A -band absorption

We have obtained resonance Raman spectra and absolute Raman cross section measurements at five excitation wavelengths within the A-band absorption for 1-bromo-2-iodoethane in cyclohexane solution. The resonance Raman spectra have most of their intensity in the fundamentals, overtones, and combination bands of six Franck–Condon active vibrational modes; the nominal CCI bend, C–I stretch, C–Br stretch, C–C stretch, CH2 wag with the Br atom attached to the CH2 group, and CH2 wag with the I atom attached to the CH2 group. The resonance Raman intensities and A-band absorption spectrum were simulated using a simple model and time-dependent wave packet calculations. The simulation results and normal mode descriptions were used to find the short-time photodissociation dynamics in terms of internal coordinate displacements. The A-band short-time photodissociation dynamics for trans-1-bromo-2-iodoethane show that the C–I, C–Br, and C–C bonds as well as the CCI, CCBr, HCC, ICH, and BrCH angles have significant changes during the initial stages of the photodissociation reaction. This indicates the photodissociation reaction has a large degree of multidimensional character and suggests that the bromoethyl photofragment receives substantial internal excitation in so far as the short-time photodissociation dynamics determines the energy partitioning. Comparison of our results for 1-bromo-2-iodoethane with the A-band short-time dynamics of iodoethane, 1-chloro-2-iodoethane, and 1,2-diiodoethane and the trends observed for their A-band absorption spectra suggest that both the C–I and C–Br bonds experience a noticeable amount of photoexcitation.

Vibrational reorganizational energies of the A-band LMCT absorption in trans-dichlorobis (triphenylphosphine) palladium (II) determined from resonance Raman intensity analysis

Resonance Raman spectra including absolute resonance Raman cross section measurements have been recorded for four excitation wavelengths within the A-band ligand to metal charge transfer (LMCT) absorption at ˜ 340 nm of trans-dichlorobis(triphenylphosphine) palladium (II). A relatively simple model together with time-dependent wavepacket calculations were used to simulate simultaneously the absolute absorption and resonance Raman intensities in order to get an estimate of the vibrational reorganizational energies associated with the Aband absorption plus the homogeneous and inhomogeneous broadening to the absorption bandwidth. It seems that there is significant interaction of the triphenylphosphine ligands with the Cl/Pd chromophore of the LMCT absorption band. The results for transdichlorobis(triphenylphosphine) palladium(II) are compared with those of other inorganic compounds.

Excited-State Reaction Dynamics of Chlorine Dioxide in Water from Absolute Resonance Raman Intensities

Resonance Raman spectra of aqueous chlorine dioxide (OClO) are measured for several excitation wavelengths spanning the photochemically relevant 2B1−2A2 optical transition. A mode-specific description of the optically prepared, 2A2 potential energy surface is derived by simultaneous analysis of the absolute resonance Raman and absorption cross sections. The resonance Raman spectra are dominated by transitions involving the symmetric stretch and bend coordinates demonstrating that excited-state structural evolution occurs predominately along these degrees of freedom. Scattering intensity is not observed for transitions involving the asymmetric stretch, demonstrating that excited-state structural evolution along this coordinate is modest. The limited evolution along the asymmetric stretch coordinate results in the preservation of C2v symmetry on the 2A2 surface. It is proposed that this preservation of symmetry is responsible for the increase in Cl photoproduct quantum yield in solution relative to the gas ...

Effect of geometrical conformation on the short-time photodissociation dynamics of 1-iodopropane in the A-band absorption

We have taken resonance Raman spectra and made absolute Raman cross section measurements at six excitation wavelengths for 1-iodopropane. The resonance Raman spectra have most of their Raman intensity in features that may be assigned as fundamentals, overtones, and combination bands of three Franck–Condon active vibrational modes (the nominal C–I stretch, the nominal CCC bend, and the nominal CCI bend) for the trans and gauche conformations of 1-iodopropane. The resonance Raman and absorption cross sections of the trans and gauche conformations of 1-iodopropane were simulated using a simple model and time-dependent wave packet calculations. The results of the simulations were used in conjunction with the vibrational normal-mode coefficients to find the short-time photodissociation dynamics of trans and gauche conformers of 1-iodopropane in terms of internal coordinate changes. The trans and gauche conformers display significantly different Franck–Condon region photodissociation dynamics, which indicates that the C–I bond breaking is conformational dependent. In particular, there are large differences in the trans and gauche short-time photodissociation dynamics for the torsional motion (xGBx) about the GB carbon–carbon bond and the GBC angle (where C=α-carbon atom attached to the iodine atom, B=β-carbon atom attached to the α-carbon atom, G=methyl group carbon atom attached to the β-carbon atom). The major differences in the trans and gauche A-band short-time photodissociation dynamics can be mostly explained by the position of the C–I bond in the trans and gauche conformers relative to the plane of the three carbon atoms of the n-propyl group of 1-iodopropane.

Simple Approach to Determining Absolute Raman Cross Sections Using an Optical Parametric Oscillator

In the present investigation, it is shown that use of commercial optical parametric oscillators (OPOs) has allowed for a simple method of determining the absolute Raman cross sections utilizing the measurement of integrated intensity ratios. Standard curves for the absolute Raman cross sections of the commonly used internal standards (viz., acetonitrile 918 cm−1peak and nitrate 1045 cm−1band) have been developed from the relative Raman intensities measured in the wavelength range 450–650 nm. The absolute intensities are fitted to the A-term frequency dependence and found to be consistent with the available literature data. We have demonstrated that, using an OPO, one can develop a standard curve for any internal standard, in order to accurately determine absolute Raman cross sections for a given system. This approach, in general, avoids the use of precalibrated lamps or external standard methods which are cumbersome, in comparison to measuring the integrated intensity changes with respect to an internal standard.