P. van der Meulen

Ultrafast single and double proton transfer in photo-excited (2,2'-bipyridyl)-3,3'-diol

Abstract We report on a subpicosecond fluorescence study of [2,2′-bipyridyl]-3,3′-diol using the fluorescent up-conversion technique with a time resolution of ≈300 fs. A new short-living fluorescence band, peaking at 568 nm, is observed, which is attributed to the emissive keto-enol tautomer. This tautomer is discussed to be formed in its excited singlet state in a stepwise proton transfer mechanism; eventually the keto-enol tautomer decays into the double-protonated di-keto product, which alternatively could also have been formed in a direct double proton transfer reaction.

Femtosecond studies of charge separation in photo-excited DCM in liquid solution

Abstract The solvatochromism of DCM in the fluorescent state has been studied using the femtosecond fluorescent up-conversion technique with a time-resolution of ≈ 150 fs. It is shown that intramolecular charge separation is not complete within 300 fs after the excitation pulse. A significant portion of the charge separation trajectory ( ≈ 30%) is controlled by the solvation process on a picosecond time scale. It is inferred that in photo-excited DCM the locally excited and charge transfer states mix in the strong adiabatic coupling limit in the inverted region.

Picosecond time-dependent Stokes shift studies of fluoroprobe in liquid solution

We report on a picosecond spectroscopic study of the dynamical Stokes shift of fluoroprobe in the lowest excited state in the solvents diethylether and ethylacetate. Time-resolved emission spectra with a time-resolution of approximately 10 ps are presented. The spectra reflect dynamical Stokes shifts of a few thousand wave numbers within 10-100 ps after the pulsed laser excitation. The time-dependent shifts are representative of the solvation dynamics of fluoroprobe in diethylether and ethylacetate.

Subpicosecond solvation relaxation of DASPI in polar liquids.

Abstract The solvation dynamics of 2-(p-dimethylaminosotyryl)pyridylmethyl iodide (DASPI) dissolved in the solvents methanol, ethylene glycol, ethanol and acetonitrile, has been studied using the fluorescent up-conversion technique with a time resolution of about 100 fs. A bimodal solvent response function is obtained for DASPI dissolved in the polar solvents. In the bimodal response function the ultrafast initial decay component, a Gaussian term with a characteristic frequency of ω G ≊ 10 ps −1 , is attributed to inertial free streaming motions of the solvent molecules. In addition, for all solutions slower decay components with time constants of several picoseconds typical of rotational and translational diffusional motions of the solvent molecules have been found.

Absolute HeIα photoionisation cross sections of O atoms and electronically excited O2(1Δg)

Abstract Absolute photoionisation cross sections for short-lived ground-state oxygen atoms ( 3 P) and electronically excited O 2 ( 1 Δ g ) are obtained at 584 A using a recently developed electron modulation spectroscopy method. The angle-integrated cross section for the transition O + ( 4 S) ← O ( 3 P) is measured relative to O 2 + ( 2 Π g ) ← O 2 ( 3 Σ g − ) which is used as a normalisation standard. Additionally an absolute value for the angle-dependent cross section at 90° for the transition O 2 + ( 2 Π g ) ← O 2 ( 1 Δ g ) is determined. From relative intensity measurements in the photoelectron spectrum of O atoms a total photoionisation cross section, summed over states accessible with HeIα radiation, is obtained.

HeIα photoionisation cross-section determination of Br atoms

Abstract An extended electron modulation spectroscopy method is described which allows the accurate determination of photoionisation cross sections of transient species relative to those of precursor compounds. In this paper cross section at 584 A for atomic and molecular bromine transitions from neutral ground to lowest ionic states have been measured relative to that of the HBr + (X 2 Π 1/2,3/2 )←HBr(X 1 Σ + ) ionisation. Using the cross section of this HBr transition as an absolute standard and with relative cross-section data for ionisations leading to the accessible excited ionic states of Br + and Br + 2 , absolute total angle-integrated cross sections for the valence shell ionisation process in Br + and Br + 2 are presented.

Femtosecond studies of intramolecular charge separation

The solvation dynamics of DCM, dissolved in a number of polar solvents, has been studied using the fluorescent upconversion technique with a time resolution of ≊100 fs. Typically a bimodal dynamical Stokes shift behavior is found, with time constants of about 100 fs and a few picoseconds. The initial component in the solvation relaxation is attributed to the effect of the free streaming motion of the solvent molecules, whereas the second is typical of the rotational diffusion motions of the solvent molecules.

Femtosecond Intramolecular Charge Separation in DCM.

Upon photo-excitation of the laser dye molecule DCM (4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4-H-pyran), an intramolecular electron transfer (ET) process is known to occur. The charge separation causes an electronic polarization of the DCM molecules which in turn results in a Stokes shift of the emission band. Following ultrashort pulsed photoexcitation of DCM in liquid solution, the temporal evolution of the solvation energy of the system to the minimum value characteristic of the charge transfer state is determined by the reorientational motions of the polar solvent molecules. The effect is manifested as a time dependent red shift in the DCM fluorescence spectrum. The shift of the maximum, vmax, of the fluorescence band in time is represented by C(t) = (vmax(t)-vmax(∞))/(vmax(0)-vmax(∞)), where vmax(t) corresponds to the frequency of the fluorescence band maximum of dissolved DCM. In linear response theory the time-dependence of the Stokes shift is determined by the time autocorrelation function of the free energy fluctuations. Thus, as is well known for the solvents commonly used, the time scale for the Stokes shift dynamics will be on the order of tens of femtoseconds up to tens of picoseconds.

Absolute photoionisation cross sections of transient species

Abstract An electron modulation spectroscopy method is described which allows the accurate determination of photoionisation cross sections of transient species relative to those of stable precursor compounds. In this paper absolute angle-integrated HeIα photoionisation cross sections for both atomic and molecular chlorine and bromine are presented using the absolute photoionisation cross sections for HCl and HBr, known from dipole (e, 2e) spectroscopy, as an absolute standard.