Theory of femtosecond coherent anti-Stokes Raman scattering spectroscopy of gas-phase transitions

A theoretical analysis of coherent anti-Stokes Raman scattering (CARS) spectroscopy of gas-phase resonances using femtosecond lasers is performed. The time-dependent density matrix equations for the femtosecond CARS process are formulated and manipulated into a form suitable for solution by direct numerical integration (DNI). The temporal shapes of the pump, Stokes, and probe laser pulses are specified as an input to the DNI calculations. It is assumed that the laser pulse shapes are 70 fs Gaussians and that the pulses are Fourier-transform limited. A single excited electronic level is defined as an effective intermediate level in the Raman process, and transition strengths are adjusted to match the experimental Raman polarizability. The excitation of the Raman coherence is investigated for different Q-branch rotational transitions in the fundamental 2330 cm(-1) band of diatomic nitrogen, assuming that the pump and Stokes pulses are temporally overlapped. The excitation process is shown to be virtually identical for transitions ranging from Q2 to Q20. The excitation of the Raman coherences is also very efficient; for laser irradiances of 5x10(17) W/m2, corresponding approximately to a 100 microJ, 70 fs pulse focused to 50 microm, approximately 10% of the population of the ground Raman level is pumped to the excited Raman level during the impulsive pump-Stokes excitation, and the magnitude of the induced Raman coherence reaches 40% of its maximum possible value. The theoretical results are compared with the results of experiments where the femtosecond CARS signal is recorded as a function of probe delay with respect to the impulsive pump-Stokes excitation.     

Origin of negative and dispersive features in anti-Stokes and resonance femtosecond stimulated Raman spectroscopy

Femtosecond stimulated Raman spectroscopy is extended to probe ground state anti-Stokes vibrational features. Off resonance, negative anti-Stokes features are seen that are the mirror image of the positive Stokes side spectra. On resonance, the observed dispersive lineshapes are dramatically dependent on the frequencies of the picosecond pump and femtosecond probe pulses used to generate the stimulated Raman spectra. These observations are explained by the contributions of the inverse Raman and hot luminescence four-wave mixing processes discussed by Sun et al. [J. Chem. Phys. 128, 144114 (2008)], which contribute to the overall femtosecond stimulated Raman signal.     

Raman under nitrogen. The high-resolution Raman spectroscopy of crystalline uranocene, thorocene, and ferrocene

The utility of recording Raman spectroscopy under liquid nitrogen, a technique we call Raman under nitrogen (RUN), is demonstrated for ferrocene, uranocene, and thorocene. Using RUN, low-temperature (liquid nitrogen cooled) Raman spectra for these compounds exhibit higher resolution than previous studies, and new vibrational features are reported. The first Raman spectra of crystalline uranocene at 77 K are reported using excitation from argon (5145 A) and krypton (6764 A) ion lasers. The spectra obtained showed bands corresponding to vibrational transitions at 212, 236, 259, 379, 753, 897, 1500, and 3042 cm(-1), assigned to ring-metal-ring stretching, ring-metal tilting, out-of-plane CCC bending, in-plane CCC bending, ring-breathing, C-H bending, CC stretching and CH stretching, respectively. The assigned vibrational bands are compared to those of uranocene in THF, (COT)2-, and thorocene. All vibrational frequencies of the ligands, except the 259 cm(-1) out-of-plane CCC bending mode, were found to increase upon coordination. A broad, polarizable band centered about approximately 460 cm(-1) was also observed. The 460 cm(-1) band is greatly enhanced relative to the vibrational Raman transitions with excitations from the krypton ion laser, which is indicative of an electronic resonance Raman process as has been shown previously. The electronic resonance Raman band is observed to split into three distinct bands at 450, 461, and 474 cm(-1) with 6764 A excitation. Relativistic density functional theory is used to provide theoretical interpretations of the measured spectra.     

Tunable resonance hyper-Raman spectroscopy of second-order nonlinear optical chromophores

Two-photon-resonant hyper-Raman spectra are reported for three "push-pull" conjugated organic chromophores bearing -NO(2) acceptor groups, two dipolar and one octupolar. The excitation source is an unamplified picosecond mode-locked Ti:sapphire laser tunable from 720 to 950 nm. The linear resonance Raman spectra of the same molecules are measured using excitation from the laser second harmonic. Excitation on resonance with the lowest-lying band in the linear absorption spectrum yields nearly identical resonance Raman and resonance hyper-Raman spectra. However, excitation into a region that appears to contain more than one electronic transition gives rise to different intensity patterns in the linear and nonlinear spectra, indicating that different transitions contribute differently to the one-photon and two-photon oscillator strength. The promise of the hyper-Raman technique for examining electronic transitions that are both one- and two-photon allowed is discussed.     

Pre-resonance Raman intensity studies of TCNQ and TCNQ−

Pre-resonance Raman spectra have been obtained for TCNQ and LiTCNQ in acetonitrile solution using an Ar⁺—Kr⁺ laser and a tunable rhodamine 6G dye laser. Using the theory of Albrecht and Hutley, we have calculated frequency factors for the intensity variations for several symmetric vibrational modes of each molecule. The observed spectra for TCNQ and LiTCNQ with violet, blue, and green excitation give evidence for B-type resonance enhancement due to vibronic mixing between at least two violet and ultraviolet transitions. The Raman spectra for LiTCNQ with yellow, orange, and red excitation show A-type enhancement due to a single electronic excitation in the near infrared.     

Depolarization ratio and correlation between the relative intensity data and the abundance ratio of various isotopes of liquid carbon tetrachloride at room temperature

The room temperature Stokes and anti-Stokes Raman spectra of liquid CCl(4) have been recorded. The intensity ratios of anti-Stokes to Stokes Raman bands as a function of Raman shift are obtained in agreement with polarizability theory. The depolarization ratio rho (nu) as a function of Raman shift is obtained also in agreement with automatically scanned depolarization ratio rho (nu). Ratio of the intensity of the isotopic nu(1) bands indicates small deviation from the theoretical relative abundance of CCl(4) isotopes. The intensity ratio of the [nu(3)-nu(4), (nu(1)+nu(4))-nu(4)] and nu(1) bands is obtained. The consequences of the presence of different isotopes of CCl(4) on the depolarization ratio of its vibrational bands are discussed. The effects of impurities in liquid CCl(4) on depolarization ratio of the nu(1) band are estimated.     

Accurate measurement of the pressure broadening of the ν1 Raman line of CH4 in the 1–50 atm region by inverse Raman spectroscopy

The lineshape of the ν₁ Raman band of methane is measured as a function of pressure between 1 and 50 atm by inverse Raman spectroscopy using the 488 nm output of a cw Ar⁺ laser as a probe beam and the output of a pulsed dye laser as a Stokes pump beam. The linewidth is found to increase linearly in this pressure region, and the fwhm Δν (in cm^?1) can be expressed as Δν = 0.32 + 0.012p, p being the methane pressure in atm.     

Observation of Anti-Stokes Resonance Raman Signals of Gaseous Bromine with 5145 Å Excitation

The 5145 Å laser line was used to excite gaseous bromine in order to observe the resonance Raman effect. In the Stokes side, strong resonance fluorescence overwhelm, therefore the resonance Raman scattering could not be detected. However, in the anti-Stokes side, four resonance Raman peaks were observed. The corresponding transitions are Δν= ?1 to ?4. The resonance Raman spectrum excited by the 4880 Å laser line was also presented for comparison.     

The resonance Raman effect of uranyl chloride in dimethyl sulfoxide

A study was carried out of the resonance Raman scattering spectra of uranyl chloride (UO2Cl2) in dimethyl sulfoxide ((CH3)2SO) (DMSO) under laser excitation of the UO2(2+) ion in resonance with the 1sigma(g)+ --> 1phi(g) Laport-forbidden f-f electronic transitions span from 530 to 450 nm by using ten output lines of the argon-ion laser at room temperature. The resonance Raman excitation profile of the totally symmetric stretching vibrational mode of uranyl observed at 832 cm(-1) is presented and analyzed in terms of transform theory within the non-Condon model to give relatively good agreement with experimental results. The disagreement between the experimental data and the calculated resonance Raman excitation profile, at the long-wave part of the the 1sigma(g)+ --> 1phi(g) electronic transitions, may be referred to interference between the weak scattering from the neighboring forbidden electronic states (1delta(g)) and strong preresonance scattering from allowed electronic states at higher levels. An amount of change in the experimental resonance Raman excitation profile of the uranyl-DMSO system depends considerably upon the ligands (L) bound to the uranyl group. Elongation of the U-O equilibrium bond length resulting from the 1sigma(g)+ --> 1phi(g) electronic transitions is related to the magnitude of the change in the excitation profile of UO2L2 (L = NO3, CH3COO, Cl) type uranyl compounds in (DMSO).     

Raman spectra of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 between 295 and 60 K

Raman spectra from polycrystalline samples of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 have been studied in the temperature range 60-295 K. Internal modes of both nitrate and tetrachlorozincate ions show expected band narrowing and intensification at lower temperature but no significant changes in frequency. Two bands in the lattice region of both compounds, assigned to nitrate ion libration and rocking, show linear increases in frequency with lowering temperature. The intensity of the libration mode shows a linear decrease with lowering temperature, but the intensity of the rocking mode is relatively insensitive to temperature change. Ammonium ion bands show greater structure at low temperature, suggesting differentiation between the two crystallographically distinct types of cation. The observed spectral changes are interpreted on the basis of increasing ordering and effectiveness of hydrogen bonds between ammonium ions and nitrate ions at low temperatures. The Raman spectra give no evidence of discontinuous changes in frequency or intensity, which would signal temperature-dependent transitions of the crystal structure. Unlike the related single-anion compounds NH4NO3 and (NH4)2ZnCl4, the room-temperature structure of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 appears to persist at least to 60 K, being stabilized by increasingly ordered hydrogen bonding.     

Two photon excitation of the vacuum ultraviolet fluorescence of carbon monoxide

The vacuum ultraviolet fourth positive band spectrum (A¹Πχ¹σ⁺) of CO was excited by two photon absorption of ultraviolet radiation which was produced by frequency doubling the output of a nitrogen laser pumped tunable dye laser. The (0–0) through (9–0) bands were observed. Position monitoring of the ultraviolet beam provided feedback control for continuous adjustment of the orientation of the angle tuned frequency doubling crystal. In this way the orientation for maximum efficiency of the doubling crystal and the point of focus of the ultraviolet beam in the sample cell was maintained during the wavelength scan of a band. A solar-blind photomultiplier was used to detect the vacuum ultraviolet fluorescence. O, P, Q, R, and S rotational branches were observed, and the new band-head positions agree with those predicted from the previously observed single-photon spectra.     

Resonance Raman studies of beta-substituted porphyrin systems with unusual electronic absorption properties.

Zn(II) and Cu(II) porphyrins with beta-conjugated barbiturate functional groups have low-energy electronic transitions which are unusual in that there are two strong bands in the Soret region. Resonance excitation of the two bands shows that each has features characteristic of both the porphyrin and barbiturate groups, with some perturbation to these features caused by the interaction of the two chromophores. The resonance Raman (RR) spectrum (lambda(exc)=413.1 nm) of the 412 nm band shows two bands at 1722 and 1743 cm(-1) attributable to C==O stretches in the substituent. Changes in frequency of porphyrin core modes due to the differing metal centres are reproduced by density functional theory calculations. The Q band RR spectra show modes with anomalous polarization which may be attributed to A(2g) modes, however no overtone or combination bands are observed.     

Spectral measurement of the caesium D(2) line with a tunable heterodyne interferometer

The optical properties of a caesium atomic beam driven on a resonant hyperfine transition in the D(2) line were studied as a function of the probe laser frequency. Using a third off-resonant laser system, a heterodyne interferometer allowed simultaneous absorption and phase shift measurements of either the probe or the coupling laser. The signal features of the probe and coupling laser transmitted intensities showed strong differences in the vicinity of the hyperfine transitions excited by the probe laser. Regular absorption signals and electromagnetically induced transparency were found in either transmitted intensities. Furthermore, light induced birefringence of the probe laser was measured.     

Theoretical Investigation on SERS of Pyridine Adsorbed on Cn Clusters Induced by Charge Transfer: A Hint that SERS Could be Applied on Many Materials

The SERS spectra of pyridine–C_n (n=1–6) complexes are investigated theoretically. The obtained enhancement factors of about 10²–10³ in the pre‐resonance Raman spectrum calculations are attributed to charge‐transfer transitions from the carbon clusters to pyridine, where a good match of band structures between substrates and probe molecules is essential.     

Relaxation dynamics of the hydrated electron: femtosecond time-resolved resonance Raman and luminescence study

Femtosecond time-resolved resonance Raman measurements were carried out to examine the relaxation process of the hydrated electron in water. The rise of the intra- and intermolecular vibrational Raman bands of the solvating water molecules was successfully time-resolved with a time resolution as high as 250 fs. The temporal intensity change of Raman bands, as well as that of luminescence background, was compared with the time evolution of the transient absorption signal. It was found that (1) the Raman and luminescence signals exhibited the same temporal behavior, (2) the rise time of the Raman bands is faster than the appearance of the equilibrated hydrated electron, indicating that the precursor state also gives rise to resonance Raman signals, and (3) the rise of the transient Raman band is slower than that of the transient absorption at the probe wavelength of 800 nm. Because it has been shown that the Raman intensity enhancement arises from the resonance with the s --> p transition, fact 2 implies that the precursor state is the nonequilibrated s-state electron. The delayed rise of the Raman signal compared to the absorption was explained in terms of the temporal change of the resonance condition. In very early time when the absorption is largely red-shifted, the probe at 800 nm is resonant with the high energy part of the absorption that provides little resonance Raman enhancement. This explanation was consistent with the probe wavelength dependence of the temporal behavior of the Raman signal: the Raman bands measured with the higher energy probe (600 nm) rose even more slowly. The resonance Raman signal in the anti-Stokes side was also examined, but no anti-Stokes band was observable. It suggests that the temperature increase of the solvation structure around the nonequilibrated hydrated electron is less than 100 K.     

Substituent effects on the resonance Raman spectra of bis(dithiobenzil)nickel

The influence of substituents on the resonance Raman spectra of bis(p-substituted dithiobenzil)nickels has been examined. The assigned sulfur—nickel stretching vibrations in the complexes appeared in the range 390–410 cm⁻¹ with a shift to higher frequency being observed for the electron-donating substituent. It was found that Raman intensities at vibrations of the benzene ring for ligands excited with a 457.9 nm laser line are about 1.5–3.0 times larger than with a 514.5 nm laser line. The assignments of electronic transitions in the visible region of the nickel complexes were made on the basis of observed resonance Raman intensity patterns.     

Hybrid four-wave mixing in liquid pyridine

Four-wave mixing spectra for liquid pyridine are obtained with broadband laser beam. A comparison of Stokes and anti-Stokes spectra allows the assignment of several lines (951, 991, 1030 cm^?1) to CARS and CSRS processes. Two additional moderately intense lines on the Stokes side between 951 and 991 cm^?1 as well as 991 and 1030 cm^?1 , which do not appear on the anti-Stokes side, are assigned to a “hybrid four-wave mixing” process, in which two active Raman modes of pyridine are involved. This process previously unrevealed in experiment seems to be important for correct assignment of the spectral lines in four-wave mixing experiments with a broadband laser beam.     

Experimental and DFT generated Raman study of two bent-core monomeric liquid crystalline compounds

The temperature dependence Raman spectra of two liquid crystalline compounds defined by the chemical formula of 3,5-difluoro-4?-(4-pentylcyclohexyl)-(1,1?-biphenyl)-4-carbonitrile and 3,4,5-trifluoro-4?-(4-pentylcyclohexyl)-1,1?-biphenyl is being first reported in this study. These compounds are bent-core monomers and their bent nature has been verified by the Density Functional Theory (DFT). The temperature-dependent Raman spectroscopy has been widely used in understanding the effects of temperature-based phase transitions on the molecular vibrations. The same spectroscopic technique; helps to understand various phase transitions temperature in the liquid crystalline compounds (LC) and also their molecular arrangements during the phase transitions. This study has successfully revealed the nature of intermolecular interactions between the investigated compounds during the phase transitions and the correlation between the observed Raman spectra and the measurement temperature. The contributions of different types of chemical bonds in the investigated LC compounds to their recorded Raman spectra have also been discussed in detail. In predicting the observed Raman spectra, the theoretical Raman spectra obtained from the DFT calculation was used as a reliable tool. In the light of the calculated data, the peak position, line width, and integral intensity data for each band in the observed Raman spectra were reported.