Dynamics of intermolecular interactions in CCl4via the isotope effect by femtosecond time-resolved spectroscopy.

We report our study on the ultrafast dynamics of intermolecular interactions in liquid CCl4. A transient transmission time domain signal, obtained in the 40 ps delay range, exhibits beating at the difference frequency of the totally symmetric stretching vibrations of the tetrachloride isotopologues. We show that the spectra obtained as the windowed Fourier transform of different parts of the time domain signal in the range of this totally symmetric vibration, split due to the isotope effect, carry the information about the dynamics of the coherently excited, coupled molecules. We use a simple theoretical model in order to prove that the intermolecular interaction influences the relative amplitudes of the isotopologue peaks in the spectrum. Moreover, we demonstrate that the pump induced coherence in the system leads to additional strengthening of the interaction, which can be observed in the spectra obtained from the experimental time domain signal.

Automodulations in an extended cavity, passively modelocked Ti:Sapphire oscillator--period doubling and chaos.

An extended cavity Ti:Sapphire oscillator exhibits stable operation for positively chirped pulses, while in the negative chirp regime multiple pulses are present in the cavity. At the border of these regimes automodulations, being an effect of the interplay between population inversion, laser medium polarization and the laser pulse field, appear. Two particular instabilities: period doubling and chaotic behavior of the pulse train envelope are observed. Complex temporal evolution of the pulse spectrum within the modulation period is investigated.

Temperature-Dependent Ultrafast Solvation Response and Solute Diffusion in Acetamide-Urea Deep Eutectic Solvent.

In the present paper, we have studied the temperature dependence of translational diffusion and solvation dynamics of a dissolved dipolar dye in the nonionic acetamide-urea deep eutectic solvent (DES), to characterize the viscosity coupling of the measured relaxation times and verify the dynamical heterogeneity aspect of this medium. Three different time-resolved experimental techniques have been employed for this purpose: fluorescence correlation spectroscopy, transient absorption (TA) spectroscopy, and optical Kerr effect (OKE) spectroscopy. The first method provides the proof that the translational diffusion time of a solute in acetamide-urea DES [fCH3CONH2 + (1 - f)CO(NH2)2, f = 0.6] exhibits a fractional viscosity dependence, with exponent 0.758, which, when compared with the viscosity-diffusion relationship for the same solute in common molecular solvents, suggests moderate deviation from the Stokes-Einstein relation. Stokes shift dynamics of a solvatochromic dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran in this DES, followed via femtosecond TA measurements, have been found to be triexponential in nature and dominated by a ∼100 fs component. The other two components, which contribute to a total dynamic Stokes shift magnitude of ∼2500 cm-1, are characterized by time constants in the ∼5 and ∼50 ps regimes. Subsequent comparison with the femtosecond OKE measurements suggests that the relatively slower picosecond solvation components originate from the rapid reorientation of the solvent molecules, while the subpicosecond solvation response arises from the participation of the collective low-frequency solvent modes (such as intermolecular vibrations and librations). We find that the rotational diffusion lifetimes also exhibit fractional power dependence on medium viscosity and thus deviate from the Stokes-Einstein-Debye pprediction. All of these results therefore suggest that the nonionic acetamide-urea DES is a moderately heterogeneous medium.

Identification of the Excited-State C═C and C═O Modes of trans-ß-Apo-8'-carotenal with Transient 2D-IR-EXSY and Femtosecond Stimulated Raman Spectroscopy.

Assigning the vibrational modes of molecules in the electronic excited state is often a difficult task. Here we show that combining two nonlinear spectroscopic techniques, transient 2D exchange infrared spectroscopy (T2D-IR-EXSY) and femtosecond stimulated Raman spectroscopy (FSRS), the contribution of the C═C and C═O modes in the excited-state vibrational spectra of trans-ß-apo-8'-carotenal can be unambiguously identified. The experimental results reported in this work confirm a previously proposed assignment based on quantum-chemical calculations and further strengthen the role of an excited state with charge-transfer character in the relaxation pathway of carbonyl carotenoids. On a more general ground, our results highlight the potentiality of nonlinear spectroscopic methods based on the combined use of visible and infrared pulses to correlate structural and electronic changes in photoexcited molecules.

Water structure in nanopores of agarose gel by Raman spectroscopy.

The evolution of water structure during the gelation process is examined in aqueous solution of agarose using Raman spectroscopy of the O-H stretching band. The measurements have been performed at room temperature for different concentrations of agarose, which yields different dimensions of nanopores in the network of the created gel. Our results show that water confined in the gel pores exhibits evident changes in the local order of molecules in comparison with bulk water and water in the sol state. During the sol-gel transition the number of molecules that participate in the regular tetrahedral H-bond structure increases, and the effect is stronger for higher concentration of the biopolymer.