S. Pommeret

Primary ultrafast events preceding the photoinduced proton transfer from pyranine to water

Abstract Femtosecond fluorescence and absorption spectroscopies are used to probe the early events of the photoinduced proton transfer (PT) from pyranine to water. The process is found to involve two ultrafast steps (300 fs and 2.5 ps) which precede the relatively slow (87 ps) PT step. From the comparative study of the properties of the excited acid and its conjugate anion in various aqueous and alcoholic media, these ultrafast steps are identified as the solvation dynamics of the locally excited (LE) state of the acid and its subsequent relaxation to an intermediate electronic state, whose nature is discussed.

Cross phase modulation artifact in liquid phase transient absorption spectroscopy

We present experimental results for the cross phase modulation (xpm) induced transient absorption signal in a 1 mm thick fused silica plate using a white light continuum as a probe. The fused silica plate mimics the entrance window of a commercial flow cell commonly used in liquid-phase transient absorption measurements. The experimental results are compared with those obtained theoretically by numerically solving the set of nonlinear coupled wave equations describing the propagation of the pump and the probe. The simulations allow for the different group velocities of the pump and probe pulses, and include the influence of the first and second order dispersion on the continuum probe. From the calculations the physical origin of the complex oscillatory feature observed around the zero time delay of each wavelength of the (chirped) continuum has been accurately identified. The influence of propagation effects arising from the finite thickness of the sample is discussed in great detail, and the necessity to...

A femtosecond fluorescence up-conversion study of the dynamic Stokes shift of the DCM dye molecule in polar and non-polar solvents

Abstract Time-resolved fluorescence spectra of the DCM dye molecule in methanol and chloroform have been studied using the fluorescence up-conversion technique with femtosecond time resolution. There is only one fluorescent excited state for all times and the dynamic Stokes shift observed is only due to solvent relaxation. In methanol, the mean position of the fluorescence spectrum shifts towards the red bi-exponentially (175 fs and 3.2 ps) while in chloroform it remains practically unchanged for all times. A spectral narrowing with a 10 ps time constant in methanol (7 ps in chloroform), due to vibrational energy dissipation from the excited solute to the surrounding solvent, is also observed.

Some evidence of ultrafast H2O+-water molecule reaction in femtosecond photoionization of pure liquid water: Influence on geminate pair recombination dynamics

Abstract The elucidation of detailed mechanisms of primary events in molecular dynamics, charge transfer or reaction dynamics have been made possible by advances in spectroscopic techniques using ultrashort laser pulse generation. In this paper we will center on femtosecond investigations of the very primary processes occurring in pure liquid water following a photoionization of solvent molecules by ultraviolet femtosecond pulses. We have observed in the near ultraviolet region (460, 410 nm), an instantaneous transient absorption with ultrashort lifetime which appears during the initial energy deposition. This induced absorption rises within the pulse, i.e. in less than 100 fs and faster than the precursor of the fully hydrated electron. Its relaxation can be described by a monoexponential law. This ultrashort transient absorption is tentatively assigned to the water cation H2O+. The relaxation would then correspond to the ion-molecule reaction H2O+ + H2O → H3O+ + OH for which the cleavage rate constant is measured to be 1013 s−1 at 294 K. An H/D isotope effect on the dynamics of the ion-water molecule reaction has been observed. These results are analyzed considering the dynamical properties of the protic solvent. The influence of the local dynamical molecular structure of the fluid on the primary reactions involving an ultrafast geminate recombination (e−hyd…X3O+; e−hyd…OX with X = H or D) will be discussed.

Coupling between molecular rotations and OH⋯O motions in liquid water: Theory and experiment

A new theory is proposed to describe spectral effects of the coupling between molecular rotations and OH⋯O motions in liquid water. The correlation function approach is employed together with a special type of development in which the coupling energy of these two motions is the expansion parameter. The isotropy of the liquid medium plays an essential role in this study. Based on this theory, a new infrared pump–probe experiment is described permitting a visualization of molecular rotations at subpicosecond time scales. Full curves relating the mean squared rotational angle and time, and not only the rotational relaxation time, are measured by this experiment. However, very short times where the incident pulses overlap must be avoided in this analysis. The lifetime of OH⋯O bonds in water is rotation–limited.

Ultrafast events in the electron photodetachment from the hexacyanoferrate(II) complex in solution

Abstract Following excitation of the hexacyanoferrate(II) complex in water with a 40 fs laser pulse at 267 nm, the absorption of the hydrated electron rises with a global time constant of 510 fs, whereas the characteristic absorption of the hexacyanoferrate(III) appears almost instantaneously. A transient absorption band around 490 nm is tentatively assigned to the charge-transfer-to-solvent (CTTS) state of the hexacyanoferrate(II). Its ultra-rapid decay (≪60 fs) is due to the electronic repulsion between the electron and its parent core.

Femtosecond laser studies of excited state intramolecular proton transfer in an ultraviolet-filter molecule

Abstract Following excitation by a femtosecond laser pulse at 346 nm, phenol,2-(2H-benzotriazol-2-yl)-4-methyl-6-[2-methyl-3-[1,3,3,3-tetramethyl-1-[(trimethylsilyl)oxy]disiloxanyl]propyl] in n-heptane relaxes towards its ground state in a four-step mechanism. By excited state intramolecular proton transfer (50 fs) the enol S1 state produces the keto S1′ state responsible for a small gain spectrum above 600 nm. The latter relaxes to its keto ground state S0′ (130 fs) and the keto→enol back proton transfer (500 fs) yields the vibrationally hot enol ground state, the cooling of which is a relatively slow process (≫2 ps). We present kinetics and detailed transient absorption and gain spectra between 380 and 720 nm.

Three-photon absorption cross-section of simple molecular liquids

Abstract Three-photon absorption coefficients and the corresponding three-photon absorption cross-sections of several simple molecular liquids at 400 nm are obtained from intensity dependent transmission measurements. The peak power of the femtosecond laser pulses was varied from 10 12 to 10 15 W/m 2 . For all liquids investigated the three-photon absorption coefficient was found to be close to 1×10 −27 m 3 /W 2 .

Femtosecond investigation of single-electron transfer and radical reactions in aqueous media and bioaggregate-mimetic systems

The elucidation of detailed mechanisms of ultrafast complex events that occur in molecular dynamics, charge transfer, or reaction dynamics has been made possible by recent advances in spectroscopy techniques that use ultrashort laser pulse generation. We focus on the implications of femtosecond spectroscopy for the investigation of electron reactivity in homogeneous aqueous solutions and organized assemblies that mimic bioaggregates.

Ultrafast dynamics of the excited methylviologen-iodide charge transfer complexes

Abstract The ultrafast dynamics of the excited charge transfer complexes between methylviologen and iodide ions has been investigated in aqueous solutions of potassium iodide using femtosecond transient absorption spectroscopy. Methylviologen and iodide form 1:1 and 1:2 charge transfer complexes characterized by charge transfer bands in the same spectral region. Transient absorption of the complexes has been studied after ultrafast excitation in their charge transfer band. The excitation of the 1:1 complex results in the formation of the MV + /I radical pair while the excitation of the 1:2 complex is leading to the formation of the MV + /I and MV + /I 2 radical pairs.