Coherent anti-Stokes Raman Scattering Spectroscopy Research Articles

Femtosecond time‐resolved CARS and DFWM spectroscopy on gas‐phase I2: a wave‐packet description

Femtosecond time-resolved CARS (coherent anti-Stokes Raman scattering) and DFWM (degenerate four-wave mixing) experiments on gas-phase iodine molecules were analyzed within time dependent wave-packet calculations. The recorded signals were interpreted using the ro-vibrational quantum dynamics in different electronic states of I2. Thereby various contributions to the induced third-order polarization were related to overlap integrals between wave packets created by absorption and emission processes. This allows for an intuitive interpretation of the temporal variations in the signals. Copyright © 2000 John Wiley & Sons, Ltd.

Polarization-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy: a new probe of molecular symmetry through accurate determination of the Raman depolarization ratio

A broad-band multiplex CARS spectroscopic system was constructed for precise polarization-controlled measurements. The directions of polarization of the two incident laser beams and that of the CARS signal beam are controlled precisely by two polarizers and an analyzer set on high-precision prism mounts. Polarized CARS spectra were recorded with high signal-to-noise ratios for a number of different analyzer settings, typically at intervals of 1° of the analyzer axis. The observed changes in band shapes are interpreted by a global simulation analysis in which all the band shapes at different analyzer settings are reproduced using a small number of intrinsic bandshape parameters. The analysis leads to very accurate determination of depolarization ratios, accurate within an error of ±0.002 in the most favorable case. The method may well be called polarization-resolved CARS spectroscopy. Depolarization ratios of two typical organic liquids, cyclohexane and 1,2-dichloroethane, were determined. The two non-totally symmetric bands of cyclohexane gave a value of 0.75 within an experimental uncertainty of 0.002. The two totally symmetric bands of 1,2-dichloroethane, which have been reported to be depolarized despite their symmetry, showed depolarization ratios very close to but meaningfully smaller than 0.75. Copyright © 2000 John Wiley & Sons, Ltd.

Laser spectroscopic techniques for combustion diagnostics

During the last decade various laser spectroscopic techniques have shown great potential for combustion diagnostics. The largest advantages with these techniques are that they permit non-intrusive measurements with high spatial, temporal and spectral resolution. The most important parameters measurable are; species concentration, temperature and velocity. In the present review some developments and applications will be briefly described. This includes Laser-Induced Fluorescence, LIF, Coherent anti-Stokes Raman Scattering, CARS, and Polarization spectroscopy, PS

Evidence of strong vibronic coupling in the first excited singlet state of diphenylhexatriene by picosecond CARS spectroscopy

Applying the polarization-sensitive variant of coherent anti-Stokes raman scattering (CARS) with picosecond time resolution solvent-dependent vibrational shifts and depolarization ratios of vibrational resonances of the first excited singlet state of diphenylhexatriene were investigated. The depolarization ratios of all observed Raman lines ( ρ R∼0.72) near the transient absorption of the excited state show the dominance of anisotropic scattering originating from vibrations of the same symmetry species. Vibrational frequencies were determined under polarization conditions avoiding spectral interference with the electronic background. Frequencies of the two broad CC stretching bands in the spectral range above 1600 cm −1 increase with decreasing polarizability of the solvent indicating strong vibronic coupling in the first excited singlet state.

Recent Developments in Ultrafast Time-Resolved Vibrational Spectroscopy of Electronically Excited States

Developments of three new time-resolved vibrational spectroscopies and their applications to electronically excited states are reviewed. Transform-limited picosecond time-resolved Raman spectroscopy has been used to study the vibrational dynamics of trans-stilbene in the lowest excited singlet state. Picosecond time-frequency two-dimensional multiplex Coherent Antistokes Raman Scattering spectroscopy has been used to probe the structure of diphenylacetylene in the lowest and the second lowest excited singlet states. Nanosecond time-resolved dispersive infrared spectroscopy has detected the singlet and triplet intramolecular charge transfer states of 4-(dimethylamino)benzonitrile. Strong evidence for a charge transfer structure has been obtained.

Nanosecond Time-Resolved Resonance CARS Spectroscopy Using the Scanning Multichannel Technique

We report on an experimental setup which has been developed to perform efficient nanosecond time-resolved coherent anti-Stokes Raman scattering (CARS) experiments using either the scanning CARS or the broadband CARS technique. A special electronic device has been constructed that allows the CARS probing laser system to be triggered with a variable delay after the excitation pulse of a nitrogen laser and furthermore prevents any temporal drifts of the nitrogen laser emission. The acquisition of the broadband CARS spectra is based on the recently developed scanning multi-channel technique, which is applied in the present paper to CARS spectroscopy. Several examples of ground- and excited-state CARS spectra are shown to demonstrate the performance of the setup and to point out the benefits and limitations of the various measurement techniques. Moreover, we present CARS spectroscopic investigations of triplet relaxation processes in order to illustrate the feasibility of the designed spectrometer for nanosecond time-resolved CARS experiments.

CHF2Cl and CH3CF2Cl Detection by Coherent Anti-Stokes Raman Scattering and Photoacoustic Raman Spectroscopy

The coherent anti-Stokes Raman scattering (CARS) and photoacoustic Raman spectroscopy (PARS) spectra of gaseous CHF2Cl and CH3CF2Cl in the 3000 cm-1 region are reported for the first time. Several vibrational bands are observed related to C−H stretch fundamentals of CHF2Cl isotopomers, and to CH3 stretches and the first overtone of CH3 deformation of CH3CF2Cl. The simultaneous detection of CARS/PARS signals aids the assignment of the isotopomer bands. Both methods are appropriate for detection of these compounds and can be applied in chemical dynamics studies.

The Application of Femtosecond Time-Resolved Coherent Anti-Stokes Raman Scattering for the Investigation of Ground and Excited State Molecular Dynamics of Molecules in the Gas Phase

In this study, we have employed femtosecond time-resolved CARS (coherent anti-Stokes Raman scattering) spectroscopy in order to gain information about the molecular dynamics evolving on the electronic excited as well as ground state potential energy surfaces (PES) of iodine and bromine in the gas phase. The coherences of the wave packet in the temporal transients of the molecules excited by the ultrashort laser pulses are analyzed by means of fast Fourier transform calculations as well as by a simple model describing the purely rotational contribution. The latter is able to describe the rotational coherence seen for the excited but not for the ground-state dynamics. Information about vibrational dynamics is extracted from the transients reflecting the wave packet motion on both the ground and excited-state PES.

On the coherent vibrational phase in polarization sensitive resonance CARS spectroscopy of copper tetraphenylporphyrin

Polarization sensitive multiplex spectroscopy of resonance coherent anti-Stokes Raman scattering (CARS) of copper(II)-tetraphenylporphyrin in solution (CH2Cl2) is reported. The measurements were performed in the Q band range of the porphyrin electronic absorption. Four polarized CARS spectra were resolved in 1300–1650 cm−1 Raman range and were simultaneously fitted with a single set of vibrational parameters (band positions, bandwidths, amplitudes, depolarization ratios, and phases). The obtained coherent vibrational phases of A1g, A2g, and B1g skeletal modes of the porphyrin macrocycle appeared to correlate strongly with the mode vibrational symmetry. The origin of such correlation is analyzed within a model of the third-order nonlinear electric susceptibility χ(3). The proposed model is based on multidimensional displaced harmonic oscillator in the Herzberg–Teller expansion of Raman polarizability. The coherent vibrational phases of modes of different symmetry classes are directly affected by the symmetry dependent vibronic couplings between the pairs of Qx, Qy and Bx, By electronic transitions of the porphyrin. The phase contrast between the modes of various symmetries is most pronounced near the Q00 resonance.

Coherent spectroscopy of isotropic noncentrosymmetric media based on measurement of the frequency dispersion of a nonlinear optical activity

A phenomenological analysis (in terms of the nonlinear optical susceptibility) is given of a novel form of nonlinear optical spectroscopy based on measurement of a nonlinear optical activity (NOA) under the conditions of coherent anti-Stokes Raman scattering (CARS) in noncentrosymmetric media (nonracemic solutions and mixtures of mirror-asymmetric molecules). The dependence of the NOA on the state of polarisation and directions of propagation of the interacting waves is determined. The conditions are identified for discrimination of a quadrupole NOA—CARS signal from the background of the much stronger dipole CARS and optical-Kerr-effect signals. An estimate is obtained of the role of cascade processes due to lower-order (quadratic) nonlinearities of the investigated medium. A comparison is made of the NOA—CARS and Raman optical activity spectroscopy methods in the case of spontaneous Raman scattering. The intensities of the expected signals are estimated. The proposed nonlinear spectroscopy method is shown to be promising for the study of bioorganic molecules in aqueous solutions.

Phenol adsorption on ZnO (0001) probed by coherent anti-stokes Raman scattering spectroscopy using guided waves

Adsorption of phenol on a well defined (0001) surface of ZnO has been investigated with coherent anti-Stokes Raman scattering spectroscopy (CARS) using planar optical waveguide geometry. Monolayer sensitivity was achieved by establishing an interference condition within the ZnO layer to minimize the CARS signal from the film. At relatively higher surface coverages both physisorbed and chemisorbed phenol were present on the surface; however, with continuous pumping of the vacuum chamber all the physisorbed species were removed from the surface. The chemisorbed species is retained as the phenoxide on ZnO (0001) surface.

Statistics of multimode YAG laser radiation with implications for quantitative coherent anti-Stokes Raman scattering spectroscopy in combustion diagnostics

The autocorrelation coefficient of frequency-doubled YAG laser radiation for different operating conditions and two different laser systems is determined by measurement of the nonresonant coherent anti-Stokes Raman scattering (CARS) intensity as a function of the time delay between two correlated excitation beams. The YAG laser radiation is modeled as the sum of two components, following coherent and Gaussian statistics, respectively. Based on the autocorrelation coefficient and the YAG second-harmonic conversion efficiency, the intensity ratio of these two components is determined, yielding a larger coherent than Gaussian component for all configurations examined. The results and some implications for quantitative CARS spectroscopy are discussed.

Population dynamics of a CO2 laser studied by coherent anti-Stokes Raman scattering

We have studied, both in theory and experiment, the operational behavior of continuous wave CO2 lasers by monitoring the ro-vibrational state manifold of the pumping gas N2 by coherent antiStokes Raman scattering (CARS). Detailed information has been gained about the interaction between cavity photons and vibrationally excited N2(X) states for two types of sealed-off and diffusion-cooled CO2 lasers: a radiofrequency excited waveguide laser and a conventional, direct current excited version. CARS spectroscopy permits for the first time an impressive demonstration of the cooling effect of the intracavity photon flux on the vibrationally coupled v=1 state of N2 and the upper laser level involving the asymmetric stretching mode of the CO2 molecule. The kinetic model calculations indicate that the electron number density and electron temperature in the CO2 waveguide laser are higher than usually assumed in the literature.

CARS spectroscopy of the NaH2 collision complex: the nature of the Na(32 P)H2 exciplex ? ab initio calculations and experimental results

CARS (Coherent Anti-Stokes Raman Scattering) has been used to analyze the rovibronic state distribution of H2 after collision with Na(32 P). New lines, which do not correspond to H2 lines are observed in the CARS spectrum. The experiments point to the formation of a complex of Na(32 P)H2 inA 2 B 2 symmetry. Ab initio calculations of theA 2 B 2 potential were performed. On this surface the vibrational spectra of the exciplex are evaluated. The observed lines can be attributed to vibrational transitions in the complex, in which combinational modes are involved. The connection of experimental and theoretical results indicates that a collisionally stabilized exciplex molecule is formed during the quenching process.

Three-colour CARS spectroscopy of the OH radical at triple resonance

A coherent anti-Stokes Raman-scattering (CARS) study of the OH radical is conducted using resonance enhancement from the A2σ+-X2Π transition. A review of previous work on resonance-enhanced CARS is first given. The OH radical has specific one-photon spectral properties that lead to original CARS features; main and satellite CARS processes are thus defined at resonance. The use of resonance-enhanced CARS for the detection of OH is discussed. Optimum detectivity is obtained at triple resonance. Triple resonance can be nearly achieved using only two lasers on satellite CARS lines, but the best results are obtained on the main CARS lines using three lasers. This is verified experimentally in a discharge and in a one-atmosphere flat flame. Extra resonances from the excited electronic-state vibration are clearly seen. The results indicate that detection of OH in flames at high pressures should be easily feasible.

Coherent anti-Stokes Raman scattering by excited ions in a laser plasma

The coherent anti-Stokes Raman scattering (CARS) method was used in observation of excited Al II, Al III, In II and N II in an optical breakdown plasma. The feasibility of CARS spectroscopy of multiply charged ions in a laser plasma was established.

CARS study of the OH( 2Π) radical product of the photodissociation of H 2O 2 AT 266 nm

The nascent quantum state distribution of the OH radical product of the photodissociation of H 2O 2 at 266 nm has been measured using coherent anti-Stokes Raman scattering (CARS) spectroscopy. The OH radical is produced in ν=0 with a fairly narrow rotational state distribution and with high translational energy. These results agree with previous experiments using laser-induced fluorescence detection, confirming the ability of CARS spectroscopy to measure the quantum state distributions of radical species like OH. This represents the first comparison of a product state distribution measured by CARS with that measured by a more established spectroscopic technique, and demonstrates that accurate quantum state distributions can be obtained via CARS.

Use of CARS spectroscopy for diagnostics in a high-temperature environment

Different applications of coherent anti-Stokes Raman scattering (CARS) in nonintrusive diagnostics at high temperatures are reviewed. Its application to small laboratory flames and large-scale (10 × 10 m) furnaces are described as well as probing in electric discharges. Finally, the potential of using pure rotational CARS and its applicability to flame measurements are described.

CARS spectroscopy in supersonic jets of ammonia monomers and clusters

Supersonic molecular beams of ammonia seeded with various rare gases are investigated in the expansion region using one-dimensional coherent anti-Stokes Raman scattering (CARS). A Raman spectrum of the ν3 vibration of the NH3 monomer with resolved rotational structure and inversion splitting is reported. Spectroscopy of this vibration is ideally suited to determine the rotational temperature in the beam. In the spectral region between 3100 and 3500 cm−1, broad structures are observed which can be attributed to clusters. These structures arise at strong stagnation conditions and increase when going from pure NH3 expansions to NH3/He and NH3/Ne mixtures. Analysis of the employed beams by mass spectrometry suggests that large clusters containing ∼5–50 NH3 molecules are responsible for the observed features. A narrow-peaked structure, red shifted by 3.5 cm−1 from the ν1 Q branch, is attributed to small clusters or NH3 molecules residing on the outside of the cluster whose H atoms are free and not bound to the complex.

Photofragmentation dynamics of formaldehyde: H2(v, J) distributions

Coherent anti-Stokes Raman Scattering (CARS) is used to monitor the rovibrational population distribution of prompt H2 produced from H2CO+hν→H2+CO. Photolysis is performed on K=1 lines of the 2141 S1←S0 transition. Scanning CARS spectroscopy is employed, with suppression of the nonresonant background of the H2CO. The detection sensitivity is ∼1012 cm−3 per quantum state. Helium buffer is added to cool the product translation to near room temperature. States v=0–4, J=1–9 are detected. The vibrational distribution peaks at v=1. The rotational distributions in the vibrational states are somewhat Boltzmann with ‘‘temperatures’’ ranging from 1700 to 2500 K for v=3–0. The total energy, 29 500 cm−1, is chosen near the threshold for dissociation. The internal energy of H2(v, J) sums with that found earlier for CO(v, J) and for translation to this total. An average impact parameter of 0.9 Å with a spread of a few tenths Å is found. H2 pushes away from a point about 0.3 Å beyond the C nucleus of CO. The distributions are controlled by the dynamics of the fragments as they separate in the steep, repulsive exit valley of the potential energy surface; they are entirely different from the statistical distributions typical for bond fission without a barrier. Rotational relaxation of H2 in collisions with He occurs at 300 K with a rate constant of (9±3)×10−13 cm3 molecule−1 s−1 for J=3→J=1 and (2.6±1)×10−13 cm3 molecule−1 s−1 for J=5→J=3.