Picosecond CARS Research Articles

Non‐collinear optical parametric amplifier for time‐resolved broadband picosecond CARS

We demonstrate generation of broadband high‐intensity picosecond pulses using a non‐collinear optical parametric amplifier, seeded with a broadband cavity‐less dye laser and pumped using the second harmonic of a picosecond Nd:YAG laser. With appropriate choice of the pump‐seed non‐collinearity angle, we observed energy conversion from the pump to the signal wave of around 15%, with the bandwidth of the amplified radiation around 445 cm−1 (20.7 nm) centered around 680 nm. An energy of 1.2 mJ per pulse of duration around 26 ps has been achieved. A light source with such parameters could be suitable for use in time‐resolved broadband picosecond Coherent anti‐Stokes Raman scattering. Copyright © 2014 John Wiley & Sons, Ltd.

Dynamics of Rapid Localized Heating in Nanosecond Pulse Discharges for High Speed Flow Control

Picosecond CARS spectroscopy and phased-locked schlieren imaging are used to measure time-resolved temperature and visualize compression waves in a diffuse filament, nanosecond pulsed discharge sustained between a pair of spherical electrodes in nitrogen and air at P=100 torr. The discharge generates stable plasma with high specific energy loading (up to ~0.5 eV/molecule), and with spatial dimensions sufficiently large to enable laser diagnostic studies. The results demonstrate that significant temperature rise, up to ∆T~200 K, occurs both in nitrogen and in air, on the time scale shorter than the acoustic time scale. The characteristic time for the rapid temperature rise in air, ~100 ns, is significantly shorter compared to that in nitrogen, ~1 µs. In air, a second significant temperature rise, up to ∆T~350 K, occurs on a time scale of ~100-500 µs. This “slow” temperature rise is almost entirely missing in nitrogen. Phase-locked schlieren images demonstrate a near cylindrical shape compression wave formed around the discharge filament, both in nitrogen and in air. An additional, near spherical shape compression wave is formed near the cathode, due to significant energy release in the cathode layer of the discharge. The compression waves, caused by rapid localized heating quantified by the present measurements, are similar to the ones produced by a surface nanosecond pulse discharge in atmospheric air used for high-speed flow control, where comparable temperature rise was detected previously.

Bound states dynamics in disordered N2O crystals

The nitrous oxide crystal is used as a prototype system to analyze the dynamics of bound states in disordered materials. Because of the intramolecular anharmonicity of the potential, a Fermi resonance interaction (Γ) takes place between the v1 and the 2v2 vibron modes. Both are dispersed throughout the crystal by dipolar interactions (Δ). This generally leads, when Γ ↫ Δ, to the appearance of bound states. In a N2O crystal a frozen-in disorder occurs because of the head-tail arrangements of the molecules. Although the treatment of the Fermi resonance in ordered crystals has been well understood the combination of Fermi resonance and of disorder remained a non-trivial problem. A theoretical analysis of the incoherent Raman response of the bound states is provided and compared to experimental data. Also provided are new results obtained by nanosecond CARS experiments in the frequency domain. A comparison between these two types of spectroscopies will be dicussed by taking into account previous picosecond CARS data performed in the time domain.

Temperature-dependent vibrational relaxation in isotopically mixed molecular crystals by picosecond CARS

Temperature-dependent dephasing of 511, 766 and 1385 cm −1 modes in the mixed crystals of naphthalene and perdeuteronaphthalene at various isotopic levels have been studied by picosecond CARS in the region of 6–130 K. In general, adding the isotope shows a significant effect on the low-temperature decay time. However, a small effect on the temperature dependence is observed in the Arrhenius plots of ln[Δν( T)-Δν(6 K)] versus 1/ T for all three modes. This behavior is described by a large change of the thermally activated phonon density of states at low temperature, but by a small change as temperature increases.

Vibrational relaxation in crystal SO 2

The vibrational relaxation of some internal and external phonons in crystal SO 2 as a function of temperature is reported and discussed in terms of depopulation and/or dephasing decay mechanisms. By picosecond CARS spectroscopy the lifetime of the polar ω 1 (A 1) mode (symmetric stretching) was studied in the temperature range 35–170 K. High-resolution Raman spectroscopy was used to measure the linewidth of the second Davydov component, ω 1 (A 2), and of lattice modes at 76 and 102 cm −1 in the same temperature range. The experimental data are analyzed on the basis of current theories on vibrational decay in molecular crystals. Using a recently proposed intermolecular potential for crystal SO 2 including atom-atom and electrostatic interactions, the contributions to linewidth from third-order depopulation processes were calculated. In the case of lattice phonons, the calculated bandwidths are in good agreement with experimental results at low temperature. At higher temperature fourth-order decay mechanisms become efficient. As to the internal phonons, calculations show that depopulation has negligible importance for decay. Higher-order contributions to linewidth must be considered in this case. We have estimated the four-phonon intraband scattering and we have found that this mechanism is mostly responsible for the observed behaviour with temperature. It is suggested that fourth-order intraband processes may be important also for the decay of other well isolated phonons in molecular crystals.

Vibrational dephasing of liquid carbondisulfide investigated by picosecond CARS

Using time-resolved CARS the dephasing rate 1/ T 2 of the ν 1 mode of CS 2 is investigated in the neat liquid and in the solution of n-pentane. A large pressure effect (factor of ⩽ 3) is reported for the range 0–4 kbar similar to spontaneous Raman results. A non-monotonic temperature dependence is reported for an extended interval of 160–450 K and explained by the contributions of vibration-translation and vibration-rotation coupling to 1/ T 2. Estimates of the intermolecular collision frequency from reorientation relaxational data lead to a phenomenological description of vibrational dephasing that accounts semi-quantitatively for the experimental data.

Vibrational Dephasing of Simple Liquids Investigated by Picosecond and Femtosecond CARS

AbstractUsing picosecond CARS the dephasing rate of the ν1‐mode of CS2 is investigated in several solvents and in the neat liquid. A large pressure effect (factor of ≤3) is reported for the range 0 to 4 kbar similar to spontaneous Raman results. For an extended interval of 160 to 450 K, a non‐monotonic temperature dependence is reported for the first time and compared with theoretical models. — A novel non‐degenerate CARS spectrometer is described with 80 fs time resolution and improved measuring accuracy. A dephasing time of T2 = 608 ± 6 fs is reported for the ν1 mode of neat acetone of 293 K; negligible inhomogeneous broadening (< 1%) is indicated by the scattering data for this vibration.

Vibrational dephasing of biphonon satellites in solid CO 2

The vibrational relaxation times of the carbon-13 isotopic satellites, I − and I +, of the two-phonon bound states Ω − and Ω + in the carbon dioxide crystal are measured below 130 K using a time-resolved frequency-selective picosecond CARS system based on a distributed-feedback tunable dye laser. Interpretation of the results requires consideration of both population-change and frequency-modulation mechanisms as sources of impurity dephasing.

Relaxation dynamics of fermi doublets in CS 2 crystals

The vibrational relaxation of CS 2 crystals in the Fermi resonance region is probed by picosecond CARS spectroscopy. The vibrational decay times of the Fermi doublet components have been measured as a function of temperature between 40 and 160 K. Decay mechanisms are proposed to explain the experimental data.

Polarization effects of picosecond CARS in liquids

Coherent Raman scattering of delayed probing pulses after ultrafast excitation is investigated under generalized polarization conditions. Three factors are shown to contribute to the scattering signal: (i) scattering off the isotropic part of the resonant material excitation via the isotropic component of the Raman polarizability (ii) scattering via the anisotropic part of the Raman polarizability from a second, oriented component of the resonant material excitation (iii) four-wave mixing via the non-resonant part χNR of the third-order susceptibility. We demonstrate theoretically and experimentally that different polarization conditions lead to drastic changes of the signal transients in liquids. For the ring breathing mode of C6H5Br the ratio of non-resonant to resonant contributions is measured to be χNR/χres=0.037±0.015.

ChemInform Abstract: A PICOSECOND CARS STUDY OF VIBRON DYNAMICS IN MOLECULAR CRYSTALS: TEMPERATURE DEPENDENCE OF HOMOGENEOUS AND INHOMOGENEOUS LINEWIDTHS

An extensive picosecond CARS and spontaneous Raman investigation of temperature dependent dephasing of the totally symmetric vibrons in naphthalene (h8N), perdeuteronaphthalene (d8N), anthracene (h10A), and perdeuteroanthracene (d10A) crystals is presented. The low temperature picosecond decays, ranging from 128 to less than 10 ps, are due to vibrational relaxation by phonon emission. Unlike optical phonons in these crystals, vibron lifetimes of h8N and h10A do not decrease monotonically with frequency, a pattern which is observed in other molecular systems. The low temperature decay rate is proportional to the square of a cubic anharmonic matrix element and a density of difference states function which has a contribution from phonons and lower frequency vibrons. This function is evaluated for each substance and together with measured lifetimes yields matrix elements which are roughly equal for each vibron. We conclude that vibrational relaxation rates are controlled primarily by the density of final stat...

A picosecond CARS study of vibron dynamics in molecular crystals: Temperature dependence of homogeneous and inhomogeneous linewidths

An extensive picosecond CARS and spontaneous Raman investigation of temperature dependent dephasing of the totally symmetric vibrons in naphthalene (h8N), perdeuteronaphthalene (d8N), anthracene (h10A), and perdeuteroanthracene (d10A) crystals is presented. The low temperature picosecond decays, ranging from 128 to less than 10 ps, are due to vibrational relaxation by phonon emission. Unlike optical phonons in these crystals, vibron lifetimes of h8N and h10A do not decrease monotonically with frequency, a pattern which is observed in other molecular systems. The low temperature decay rate is proportional to the square of a cubic anharmonic matrix element and a density of difference states function which has a contribution from phonons and lower frequency vibrons. This function is evaluated for each substance and together with measured lifetimes yields matrix elements which are roughly equal for each vibron. We conclude that vibrational relaxation rates are controlled primarily by the density of final states. Relaxation in the deuterated crystals is faster than predicted by this model due to residual isotopic impurities and purely intramolecular vibrational relaxation. Three h8N and one h10A vibrons are studied over an extended temperature range. The homogeneous linewidth increases as Tn with n=2.0–2.4. The observed increase is consistent with the above energy relaxation model up to ∼80 K. From ∼80 K to the melting point, this temperature dependence is consistent with a pure dephasing process involving energy exchange with several thermally populated low frequency vibrons. From ∼250 K to the melting point inhomogeneous broadening is observed. It is suggested that the inhomogeneity is due to slowly (≳10 ps) varying density fluctuations which modulate intermolecular attractive interactions. In this respect, molecular crystals near the melting point resemble some molecular liquids.

Vibrational relaxation in molecular crystals. High-resolution Raman band profiles of some naphthalene and naphthalene-d8 phonons at low temperature

Abstract The accurate determination of the shape of phonon Raman bands in crystals is of great importance for the determination of the phonon lifetimes and for the study of their relaxation mechanisms. Several molecular crystals show at low temperatures the occurrence of long-living phonons with Raman bandwidths which are much narrower than the spectral slitwidth of standard Raman instruments. Information on their relaxation processes is thus normally extracted only from time domain experiments using picosecond CARS techniques. In order to extend these measurements to the frequency domain high resolution is needed. For this purpose we have built a high-resolution Raman instrument coupling together a Raman spectrometer and an interferometer. The tandem instrument has a limiting resolution of 0.02 cm −1 , largely sufficient to measure the bandwidths of long-living phonons in molecular crystals. With this instrument we have measured the profile of the six Raman-active phonons of naphthalene- h 8 at 57.5, 69, 511, 514, 766 and 1385 cm −1 and of three corresponding phonons of naphthalene- d 8 at 54, 64 and 493 cm −1 at 5 K. The observed bandwidths are well fitted by lorentzian curves. Comparison with available recent picosecond CARS data is used to prove the competitivity of high-resolution Raman with time-delayed spectroscopy. Bandwidths measured with the two techniques show differences of a few thousands of cm −1 for most of the bands studied. Only in the case of the two lowest phonons of naphthalene- h 8 larger differences are observed. These are interpreted in terms of optical crystal quality. The observed band profiles are interpreted in terms of population decay mechanisms based on anharmonic calculations of three-phonon decay processes. Pure dephasing processes are shown to give vanishing contributions to the low-temperature widths.

Vibrational energy transfer and localization in disordered solids by picosecond CARS spectroscopy

The dephasing rate of the three longest lived vibrons of naphthalene-h8 (T1∼100 ps) is investigated by picosecond CARS in disordered mixed crystals containing naphthalene-d8. Vibrational energy transfer can either occur preferentially to one acceptor state or fairly equally to a number of acceptors. For two of the vibrons studied, the decay occurs preferentially to the lower energy d8N vibron of the same normal coordinate. The other vibron studied appears to couple fairly equally to a large number of acceptors since its corresponding d8N vibration is higher in energy and unavailable for decay. The coupling matrix elements are calculated for the specific decay routes and found to be ∼10 greater than the coupling to other acceptor states. For one of the vibrations, nonexponential decays are observed at high d8N concentrations indicating a distribution of environments and hence localization. A fit to a Voight line shape gives an estimate of the inhomogeneous distribution of frequencies experienced by this localized vibration.

Long-lived vibrational modes in amino acid crystals probed by picosecond CARS spectroscopy

Vibrational relaxation is measured in amino acid crystals at various temperatures by picosecond CARS spectroscopy. Low-frequency (40–125 cm −1) torsional modes, librons, have lifetimes which can exceed 4 ns. The decrease in lifetime with libron frequency and with temperature is determined primarily by the density of states, which shifts to lower frequency in peptide crystals.

Picosecond cars study of the v 2-band of liquid benzene

A picosecond CARS-study, using mode-locked cw-lasers, has been performed to record the spectrum of the CH-stretching mode ( v 2-band) of liquid benzene. From the shape of the spectrum centered at 3061.9 cm -1 a dephasing time τ ph = 0.7 ps has been derived together with a nonresonant background contribution to x (3) of 14% at resonance.

Temperature-dependent vibrational dephasing in molecular crystals: a picosecond cars study of naphthalene

The temperature dependence of picosecond CARS of vibrational excitons in naphthalene is presented. Below ≈40 K relaxation is dominated by spontaneous emission of phonons. Above 40 K high-frequency phonons induce dephasing. The results are consistent with a phonon-promoted energy transfer process.

A study of vibrational and librational dynamics in molecular crystals by picosecond cars

A study of vibrational and librational dynamics in molecular crystals by picosecond cars

Picosecond cars as a probe of the multiphoton photofragmentation of benzene

Picosecond laser pulses have been used to photofragment benzene and to detect the C 2 fragment by a CARS probe technique. The C 2 fragment, generated by a UV multiphoton excitation process, is found to appear extremely rapidly, i.e., within the laser excitation pulse width, <25 ps.

Vibrational relaxation in neat crystals of naphthalene by picosecond cars

Picosecond delayed CARS experiments on totally symmetric modes in naphthalene at 1.5 K are reported. The Raman lineshape of the vibrational excitons is lorentzian and vibrational relaxation can be surprisingly slow. The Raman lineshape of the A g exciton level of the 766 cm −1 vibrational mode reveals that the low-temperature lorentzian lineshape occurs by motional narrowing At higher temperature the exciton is trapped by interaction with lattice phonons.