Heterodyne-detected Raman-induced Kerr Effect Spectroscopy Research Articles

Ultrafast Dynamics of Liquid Poly(ethylene glycol)s and Crown Ethers Studied by Femtosecond Raman-Induced Kerr Effect Spectroscopy

Ultrafast molecular dynamics of liquid poly(ethylene glycol)s, tetra(ethylene glycol), penta(ethylene glycol), and poly(ethylene glycol) with the molecular weight of 600, and crown ethers, 12-crown-4 and 15-crown-5, have been investigated by means of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. Picosecond Kerr transients of poly(ethylene glycol)s and crown ethers are characterized by a biexponential function with the time constants of about 2 and 20 ps. Both the faster and slower time constants do not vary much among the five oligo(ethylene oxide)s. Femtosecond dynamics is discussed based on the Kerr (depolarized Raman) spectra obtained by Fourier transform deconvolution analysis of the high time resolution Kerr transients. The broad low-frequency band (0-200 cm(-1)) in the Kerr spectrum is analyzed by two Brownian oscillators. The spectral shapes of linear poly(ethylene glycol) and cyclic crown ether are very different. Both the low- and high-frequency Brownian oscillators for crown ethers show lower frequency and broader spectral features than those for poly(ethylene glycol)s. The comparison of the low-frequency spectra of poly(ethylene glycol)s and crown ethers shows that the low-frequency spectrum of 15-crown-5 is closer to that of poly(ethylene glycol)s than that of 12-crown-4 is. The difference of the low-frequency spectra between poly(ethylene glycol) and crown ether is discussed with the concepts of molecular conformation and liquid density. The features of the observed intramolecular vibrational bands are also correlated with the molecular conformations.

Ultrafast molecular dynamics of liquid aromatic molecules and the mixtures with CCl4

The ultrafast molecular dynamics of liquid aromatic molecules, benzene, toluene, ethylbenzene, cumene, and 1,3-diphenylpropane, and the mixtures with CCl(4) have been investigated by means of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The picosecond Kerr transients of benzene, toluene, ethylbenzene, and cumene and the mixtures with CCl(4) show a biexponential feature. 1,3-Diphenylpropane and the mixtures with CCl(4) show triexponential picosecond Kerr transients. The slow relaxation time constants of the aromatic molecules and the mixtures with CCl(4) are qualitatively described by the Stoke-Einstein-Debye hydrodynamic model. The ultrafast dynamics have been discussed based on the Kerr spectra in the frequency range of 0-800 cm(-1) obtained by the Fourier transform analysis of the Kerr transients. The line shapes of the low-frequency intermolecular spectra located at 0-180 cm(-1) frequency range have been analyzed by two Brownian oscillators ( approximately 11 cm(-1) and approximately 45 cm(-1) peaks) and an antisymmetric Gaussian function ( approximately 65 cm(-1) peak). The spectrum shape of 1,3-diphenylpropane is quite different from the spectrum shapes of the other aromatic molecules for the low magnitude of the low-frequency mode of 1,3-diphenylpropane and/or an intramolecular vibration. Although the concentration dependences of the low- and intermediate-frequency intermolecular modes (Brownian oscillators) do not show a significant trend, the width of high-frequency intermolecular mode (antisymmetric Gaussian) becomes narrower with the higher CCl(4) concentration for all the aromatics mixtures with CCl(4). The result indicates that the inhomogeneity of the intermolecular vibrational mode in aromatics/CCl(4) mixtures is decreasing with the lower concentration of aromatics. The intramolecular vibrational modes of the aromatic molecules observed in the Kerr spectra are also shown with the calculation results based on the density functional theory.

A comparison of the low-frequency vibrational spectra of liquids obtained through infrared and Raman spectroscopies

Dynamic solvation of charge-distribution rearrangements is often described using a (harmonic) solvent coordinate. It is not a priori clear whether such a solvent coordinate has a real physical meaning. We have studied five polar organic liquids (benzonitrile, benzyl alcohol, N,N-dimethylformamide, ethylene glycol, and glycerol triacetate) with high-resolution high signal-to-noise ultrafast optical heterodyne-detected Raman-induced optical Kerr effect spectroscopy (OHD-RIKES). The data, converted to the frequency domain, were analyzed entirely with a multimode Brownian-oscillator model. The infrared spectra of the same five liquids were obtained with a combination of terahertz spectroscopy and Fourier-transform infrared spectroscopy. The Brownian-oscillator fits to the OHD-RIKES spectra could be converted successfully to IR spectra by using a simple theoretical model and by keeping all Brownian-oscillator parameters the same except for the amplitudes. This suggests that there is a small set of harmonic oscillators describing ultrafast solvent nuclear dynamics that can be used to understand solvation, IR absorption, and Raman scattering spectra.

Low-Frequency Modes of Peptides and Globular Proteins in Solution Observed by Ultrafast OHD-RIKES Spectroscopy

The low-frequency (1–200cm−1) vibrational spectra of peptides and proteins in solution have been investigated with ultrafast optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES). Spectra have been obtained for di-L-alanine (ALA(2)) and the α-helical peptide poly-L-alanine (PLA) in dichloroacetic acid solution. The poly-L-alanine spectrum shows extra amplitude compared to the di-L-alanine spectrum, which can be explained by the secondary structure of the former. The globular proteins lysozyme, α-lactalbumin, pepsin, and β-lactoglobulin in aqueous solution have been studied to determine the possible influence of secondary or tertiary structure on the low-frequency spectra. The spectra of the globular proteins have been analyzed in terms of three nondiffusive Brownian oscillators. The lowest frequency oscillator corresponds to the so-called Boson peak observed in inelastic neutron scattering (INS). The remaining two oscillators are not observed in inelastic neutron scattering, do therefore not involve significant motion of hydrogen atoms, and may be associated with delocalized backbone torsions.

The effects of anion and cation substitution on the ultrafast solvent dynamics of ionic liquids: A time-resolved optical Kerr-effect spectroscopic study

Ultrafast solvent dynamics of room-temperature ionic liquids have been investigated by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) by studying the effects of cation and anion substitution on the low frequency librational modes. The spectra of two series of imidazolium salts are presented. The first series is based on the 1-butyl-3-methylimidazolium salts [bmim]+ containing the anions trifluoromethanesulfate [TfO]−, bis(trifluoromethanesulfonyl)imide [Tf2N]−, and hexafluorophosphate [PF6]−. The second series is based on [Tf2N]− salts containing the three cations 1-butyl-2,3-dimethylimidazolium [bmmim]+, 1-methyl-3-octylimidazolium [omim]+, and [bmim]+. It is found in all five samples that the signal is due to libration of the imidazolium ring at three frequencies around 30, 65, and 100 cm−1 corresponding to three local configurations of the anion with respect to the cation.

Intermolecular spectrum of 4-octyl-4 ′-cyanobiphenyl in n-heptane: OHD-RIKES measurements

The low frequency spectrum of 4-octyl-4 ′-cyanobiphenyl (8CB) in n-heptane was obtained by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The spectrum is characterized by a broad band at ∼61 cm −1 and weaker bands at ∼180 and ∼210 cm −1. The broad band becomes narrower and shifts to lower frequency upon dilution. These spectral changes can be explained by a decrease in the curvature of the effective intermolecular potential upon dilution. The intensities of the weak bands also decrease upon dilution. This behavior is consistent with the bands being due to collective modes of a locally ordered structure.

Low-frequency spectrum of homeotropically aligned liquid crystals: optical heterodyne-detected Raman-induced Kerr effect spectroscopy of 4-octyl-4 ′-cyanobiphenyl

The low-frequency spectrum of homeotropically aligned 4-octyl-4 ′-cyanobiphenyl in the smectic-A (Sm-A) phase was obtained by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The absence of a narrow band at 9 cm −1, corresponding to a pseudo-lattice vibration propagating in a direction perpendicular to the smectic layers, suggests that the spectrum of homeotropically aligned Sm-A is due to orientational modes associated with rotation about the long axis of the molecule and translational modes associated with motion in the smectic layers. The spectrum is described by a bimodal lineshape function in which the lower and higher frequency components are attributed respectively to the translational and orientational modes.

Time-resolved optical Kerr-effect spectroscopy of low-frequency dynamics in Di-L-alanine, poly-L-alanine, and lysozyme in solution.

The low-frequency spectra of peptides and proteins in solution have been investigated with optical heterodyne-detected Raman-induced Kerr-effect spectroscopy. Spectra were obtained for di-l-alanine ALA(2) and poly-l-alanine (PLA) in dichloroacetic acid solution. The conformational dependence of those spectra at low frequency has been analyzed. ALA(2) displays a band centered at 50 cm-1, whereas the alpha-helical PLA shows two shoulders at 60 and 140 cm-1. The similarity of the spectral features observed in PLA to those in water can be explained by analogous acoustic translational modes in the hydrogen network of the PLA alpha-helix. The mostly alpha-helical protein lysozyme in aqueous solution has also been investigated and showed significantly more structure with modes at 10, 35, 73, 106, and 164 cm-1.

Intermolecular Dynamics of Room-Temperature Ionic Liquids: Femtosecond Optical Kerr Effect Measurements on 1-Alkyl-3-methylimidazolium Bis((trifluoromethyl)sulfonyl)imides

Using optically heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 40 fs laser pulses, the transient birefringence in the room-temperature ionic liquids (RTILs), 1-alkyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imides, [Cnmim]NTf2 with n = 2, 4, 5, 6, 8, 10 (C2, C4, C5, C6, C8, C10), has been studied at room temperature and ambient pressure. Near zero delay, the OHD-RIKES response is dominated by the instantaneous electronic response. The nuclear response appears as a shoulder on the electronic response. Between 0 and 1 ps, the nuclear response is dominated by the intermolecular vibrational (nondiffusive) response. For C4, C5, C6, and C8, the 1/e time of the pseudo-exponential tail of the intermolecular response decreases with viscosity, in accord with the hydrodynamic model for vibrational dephasing. Superimposed on the OHD-RIKES response for C4, C5, and C6 is a coherent oscillation with a frequency of ∼140 cm-1. The intermolecular vibrational spectra for these RTILs obtained from the reduced OHD-RIKES data by using a Fourier transform procedure extend from 0 to 200 cm-1 and are bimodal with a low-frequency component at ∼22 cm-1 and a high-frequency component at ∼84 cm-1. The relative contribution of the high-frequency component to the total band increases in going from C2 to C5 and remains constant for C5, C6, and C8. The behavior of the reduced spectral densities is consistent with increasing order in the liquid. It is proposed that the 140 cm-1 oscillation arises from collective motions of locally ordered domains in the liquid.

Aqueous dimethyl sulfoxide solutions: Inter- and intra-molecular dynamics

The inter- and intra-molecular dynamics of aqueous dimethyl sulfoxide (DMSO) solutions have been measured using optical heterodyne-detected Raman-induced Kerr effect spectroscopy. Solutions were studied over the entire range of composition at 294 K. The Kerr transients characterize both the underdamped inter- and intra-molecular vibrational motions, as well as the overdamped, diffusive orientational motions. The longer diffusive relaxation time constant τ2 is assigned to DMSO reorientation, and varies strongly with mole fraction of DMSO. The shorter time constant τ1 is assigned to water reorientation, and the value of 1.0 ps is nearly invariant across the range of solution composition. The solutions deviate substantially from hydrodynamic scaling behavior, since the ratio of DMSO reorientation time constant normalized by shear viscosity τ2/η is not a linear function of mole fraction. The peak frequencies for three of five low frequency intramolecular vibrations decrease with increasing water content. Both anisotropic and isotropic Kerr transients are recorded. The isotropic Kerr transient is dominated by the partially depolarized ν10 symmetric C–S–C stretching vibration at 670 cm−1. Electronic structure calculations for isolated DMSO, perdeuterated DMSO (d6-DMSO), and water are carried out using semi-empirical (PM3), density functional (B3LYP), and MP2 methods. Basis sets up to 6-311++G(d,p) are used and vibrational frequencies are calculated within the harmonic approximation. An unusual hemispherical shape is obtained for the electrostatic potential (ESP) about the DMSO oxygen. This hemispherical ESP was consistently observed for all levels of electronic structure theory and all basis sets that were used. Calculated polarizabilities permit us to estimate the anisotropic Kerr signal for an isolated DMSO molecule to be 79 times greater than for an isolated water molecule.

Ultrafast dynamics in aqueous polyacrylamide solutions.

We have investigated the ultrafast dynamics of aqueous polyacrylamide ([-CH(2)CH(CONH(2))-](n), or PAAm) solutions using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The observed aqueous PAAm dynamics are nearly identical for both M(w) = 1500 and 10 000. Aqueous propionamide (CH(3)CH(2)CONH(2), or PrAm) solutions were also studied, because PrAm is an exact model for the PAAm constitutional repeat unit (CRU). The longest time scale dynamics observed for both aqueous PAAm and PrAm solutions occur in the 4-10 ps range. Over the range of concentrations from 0 to 40 wt %, the picosecond reorientation time constants for the aqueous PAAm and PrAm solutions scale linearly with the solution concentration, despite the fact that the solution shear viscosities vary exponentially from 1 to 264 cP. For a given value of solution concentration in weight percent, constant ratios of measured reorientation time constants for PAAm to PrAm are obtained. This ratio of PAAm to PrAm reorientation time constants is equal to the ratio of the volume for the PAAm constitutional repeat unit (-CH(2)CHCONH(2)-) to the molecular volume of PrAm. For these reasons, we assign the polymer reorientation dynamics to motions of the entire constitutional repeat unit, not only side group motions. Simple molecular dynamics simulations of H[-CH(2)CH(CONH(2))-](7)H in a periodic box with 180 water molecules support this assignment. Amide-amide and amide-water hydrogen-bonding interactions lead to strongly oscillatory femtosecond dynamics in the Kerr transients, peaking at 80, 410, and 750 fs.

Structure and Intermolecular Dynamics of Liquids: Femtosecond Optical Kerr Effect Measurements in Nonpolar Fluorinated Benzenes

A comparative study of the collective polarizability anisotropy dynamics of benzene, 1,3,5-trifluorobenzene, and hexafluorobenzene was carried out by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses. The OHD-RIKES data were analyzed by using a model-dependent approach and a Fourier transform approach, which yields a spectral density for the liquid. From an analysis of the long-time tails, collective reorientation times of 2.45 ± 0.06, 9.05 ± 0.10, and 13.5 ± 0.1 ps were obtained respectively for C6H6, 1,3,5-C6F3H3, and C6F6. The spectral densities are narrower for 1,3,5-C6F3H3 and C6F6 than for C6H6 by roughly a factor of 2. Information about the intermolecular vibrational modes of the liquid is contained in the reduced spectral density which is obtained by subtracting the tail-matched diffusive reorientational response from the OHD-RIKES data. In the case of C6H6 and C6F6, the intermolecular spectral density can be decomposed into at least two b...

Reorientational and intermolecular dynamics in binary liquid mixtures of hexafluorobenzene and benzene: femtosecond optical Kerr effect measurements

The molecular dynamics of binary liquid mixtures of C 6F 6 and C 6H 6 were studied by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses at room temperature and ambient pressure. The data give evidence for the effect of interspecies interactions in the short-time, nondiffusive part of the OHD-RIKES response.

Femtosecond Optical Kerr Effect Studies of Liquid Methyl Iodide

The collective polarizability anisotropy dynamics of liquid methyl iodide at room temperature and ambient pressure was studied by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses. The OHD-RIKES data are analyzed by using both the model-dependent approach, which assumes four distinct temporal responses, and the model-independent Fourier transform approach, which generates a spectral density. Near zero time, the OHD-RIKES transient is dominated by the instantaneous electronic response. The short-time nuclear response is characterized by two components. The first component is interpreted as arising from an inhomogeneously broadened (fwhm ≈ 62 cm-1) underdamped intermolecular vibrational mode with a mean frequency of ∼60 cm-1. The second component is an intermediate quasi-exponential response with a 1/e time constant of ∼200 fs. At longer times, the OHD-RIKES transient decays exponentially with a 1/e time constant of 1.76 ± 0.05 ps, which corresponds to the collective reorientation time of CH3I. The spectral density peaks at ∼24 cm-1 and has a fwhm of ∼80 cm-1. The spectral density can be well fitted by an ohmic distribution function with ωc ≈ 30 cm-1. The spectral density obtained from the OHD-RIKES data is consistent with previously measured depolarized Rayleigh scattering and low-frequency far infrared absorption spectra for liquid CH3I.