Coherent Stokes Raman Scattering Research Articles

Analysis of vibronic-mode coupling in azulene by fully resonant coherent four wave mixing

Fully resonant, three laser, coherent Stokes Raman scattering (CSRS) is used to probe the vibrational correlations and couplings associated with the S0−S1 transition of azulene in a low temperature mixed molecular crystal. Unlike incoherent resonant emission, CSRS is not complicated by extra resonances due to relaxed emission. The spectra obtained confirm most of the assignments of the S1 vibronic fundamentals made by incoherent spectroscopies. In addition, however, the vibronic counterpart of the vibrational fundamental at 900 cm−1 is correlated to a previously unobserved vibronic resonance at 842.3 cm−1. Evidence of matrix moderated coupling, including possible site splitting and Fermi resonances involving lattice phonons, is given for the 677, 825, 917, 941, and 900 cm−1 vibrational levels.

ChemInform Abstract: Analysis of Vibronic Mode Coupling in Pentacene by Fully Resonant Coherent Four‐Wave Mixing

Fully resonant, three laser, coherent Stokes Raman scattering (CSRS) is used to probe the vibrational correlations and couplings associated with the S0−S1 transition of azulene in a low temperature mixed molecular crystal. Unlike incoherent resonant emission, CSRS is not complicated by extra resonances due to relaxed emission. The spectra obtained confirm most of the assignments of the S1 vibronic fundamentals made by incoherent spectroscopies. In addition, however, the vibronic counterpart of the vibrational fundamental at 900 cm−1 is correlated to a previously unobserved vibronic resonance at 842.3 cm−1. Evidence of matrix moderated coupling, including possible site splitting and Fermi resonances involving lattice phonons, is given for the 677, 825, 917, 941, and 900 cm−1 vibrational levels.

Investigation of librational motions in gas-phase CO 2 clusters by coherent Raman spectroscopy

Coherent Stokes Raman scattering (CSRS) has been employed to investigate the spectroscopy of large CO 2 clusters formed in the expansion of a supersonic jet. In the low-frequency range, around 72 and 88 cm −1, the cluster spectrum features two peaks which can be assigned to the excitation of librational modes. With progressive expansion, i.e. decreasing internal temperature, these peaks shift towards larger wave numbers. Comparison with the data obtained in solid CO 2 shows that the cluster librations are in thermal near-equilibrium with the monomer rotations.

Line-shape analysis of extra resonance signal in coherent Stokes Raman scattering of acridine in the T1 state

The coherent anti-Stokes Raman scattering (CARS) and coherent Stokes Raman scattering (CSRS) line shapes near the 1350 cm−1 region of acridine in the T1 state were recorded in the various resonance (ω1 frequency) conditions over the Tn←T1 absorption band in the n-heptane solution at room temperature. The ω1 frequency dependencies of CARS and CSRS line shapes of the T1 signal at 1368 cm−1 were simulated well on the basis of the theoretical χ(3) expression. On the other hand, the ω1 frequency dependence of the line shape of the extra resonance (ER) signal observed at 1335 cm−1 in CSRS, showing no obvious change of the line shape by changing the ω1 frequencies, could not be simulated at all on the basis of the similar expression. The discrepancy of the ER line-shape simulation was discussed.

Picosecond vibrational cooling in mixed molecular crystals studied with a new coherent raman scattering technique

We demonstrate the pump-induced coherent Stokes Raman scattering (CSRS) technique by measuring vibrational cooling in low temperature crystals of pentacene in naphthalene following excitation of a vibration 747 cm −1 above the S 1 origin. Using picosecond photon echoes and a two-color pump-probe technique, we find that the initial state decays in 33 ps, and reappears at the origin 25 ps later. We show that pump-induced CSRS simultaneously measures the decay from the initial state and reappearance at the origin. This technique has many of the advantages of conventional coherent Raman (e.g. intense coherent signals), but is a direct measure of the population dynamics in the initial and final states.

Some model calculations of the resonance background in coherent Raman spectra

AbstractModel calculations of coherent Raman spectra under resonance pumping conditions for polyatomic molecules have been carried out. The influence of the resonance background, which originates from the ‘extra’ resonance term of the resonance vibronic transition S0S1, is demonstrated. According to the calculations, the coherent anti‐Stokes Raman scattering (CARS) spectra for ωa = 2ωp−ωs are slightly changed when the resonance background is taken into account. The background contribution to the coherent Stokes Raman scattering (CSRS) for ωs = 2ωp−ωa is higher. It is shown that a line reshaping associated with the concentration changes contains information about the relaxation parameters of the molecules, especially about the dephasing Γ.

Coherent Stokes Raman scattering with incoherent light for vibrational-dephasing-time measurement

A new method to measure picosecond or subpicosecond vibrational-dephasing dynamics with the use of incoherent or broadband light is presented theoretically and experimentally. It is based on a transient coherent Raman process with three beams. In our experimental scheme, two of them are incoherent light from a single source, and a delay time between the two is variable. A third beam has a higher frequency than the incoherent light by a vibrational energy in a molecule of interest, and is coherent in the delay-time range of the measurement. The delay-time dependence of coherent Stokes Raman scattering (CSRS) intensity offers the information about the coherence dynamics of the vibrational transition with a resolution time determined only by the correlation time of the incoherent light. For the theoretical calculation, a three-level model of the molecular system with homogeneous broadening is used, and delay-time dependences of the CSRS signal are calculated for off-resonance and resonance cases. Dephasing dynamics of the 2915-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode in dimethylsulfoxide was observed experimentally by the new method with the use of nanosecond laser pulses, and the result agrees well with that obtained with picosecond pulses.

Investigation of broad molecular absorption bands with the help of line shape analysis in coherent Raman scattering experiments

AbstractThe line shape observed in coherent Raman scattering (CRS) with excitation under conditions of electronic resonance is discussed, For the case of A‐type scattering, the complex Raman amplitude is related directly to the molecular one‐photon absorbance of the resonance transition concerned. The basis for this relation is transform theory, a method known from spontaneous resonance Raman scattering to relate the Raman excitation profile to the observed molecular absorbance. Features of the Raman amplitude, determining the observed line shapes for coherent anti‐Stokes Raman scattering (CARS) and for coherent Stokes Raman scattering (CSRS), are discussed qualitatively. The discussion considers two typical forms in which broad molecular absorption spectra of polyatomic molecules in the condensed phases appear, Inhomogeneous broadening leads to characteristic changes of the Raman amplitude in relation to molecular absorbance in the case of CSRS (formolecules in the electronic ground state), whereas for CARS the relation derived in the homogeneous broadened case remains valid. The special advantages of the CRS technique for determining the complex non‐Raman resonant (background) susceptibility from concentration dependence are demonstrated.

Simultaneous observation of rotational coherent Stokes Raman scattering and coherent anti-Stokes Raman scattering in air and nitrogen

We report the first simultaneous observation to our knowledge of pure rotational coherent Stokes Raman scattering (CSRS) and coherent anti-Stokes Raman scattering (CARS) using broadband generation and detection. Three-dimensional small-angle phase-matching conditions for CSRS and CARS processes are discussed.

Four-wave mixing spectroscopy of molecular dimers. Application to dimers of pentacene in p-terphenyl

Dispersive coherent Stokes-Raman scattering (CSRS) experiments on pentacene dimers in p-terphenyl were performed to locate the corresponding singly excited, delocalized, dimer levels. In addition the CNRS technique was used to locate the doubly excited dimer state. Future experiments exploring the dynamics of this novel state are discussed.