Emad Mukhtar

Evaluation of Intracavity Optogalvanic Spectroscopy for Radiocarbon Measurements

Ever since the first publication of intracavity optogalvanic spectroscopy (ICOGS) in 2008, this novel technique for measuring the (14)C/(12)C ratio in carbon dioxide has rendered considerable attention. As a result, there are currently at least five different research groups pursuing research on ICOGS. With a claimed limit of detection of 10(-15) ((14)C/(12)C), i.e., in the same order as accelerator mass spectroscopy, achieved with a relatively inexpensive and uncomplicated table-top system, ICOGS has major scientific and commercial implications. However, during the past 5 years, no research group has been able to reproduce these results or present additional proof for ICOGSs capability of unambiguous (14)C detection, including the authors of the original publication. Starting in 2010, our group has set up a state-of-the-art ICOGS laboratory and has investigated the basic methodology of ICOGS in general and tried to reproduce the reported experiments in particular. We have not been able to reproduce the reported results concerning the optogalvanic signals dependence on (14)C concentration and wavelength and, ultimately, not seen any evidence of the capability of ICOGS to unambiguously detect (14)C at all. Instead, we have found indications that the reported results can be products of measurement uncertainties and mistakes. Furthermore, our results strongly indicate that the reported limit of detection is likely to be overestimated by at least 2 orders of magnitude, based on the results presented in the original publication. Hence, we conclude that the original reports on ICOGS cannot be confirmed and therefore must be in error.

Two-photon excitation and time-resolved fluorescence: I. The proper response function for analysing single-photon counting experiments

An accurate instrumental response function is needed to deconvolute fluorescence data obtained by time-correlated single-photon counting (TCSPC) upon multi-photon excitation. Hitherto the response function was obtained by measuring Rayleigh scattering (RS) from colloidal solutions, as is also used in one-photon excited fluorescence. We show that hyper Rayleigh scattering (HRS) provides a better choice for deconvolution of fluorescence decays, as obtained by TCSPC and two-photon excitation (TPE). The one- and two-photon response functions were monitored as RS and HRS from colloidal gold particles at 800 and 400 nm, respectively.

Electronic energy transfer in anisotropic systems. Part 5.—Rhodamine–lipid derivatives in model membranes

The electronic energy transfer between molecules located at lipid/water interfaces of a model membrane has been investigated. The donor–donor systems were lipid derivatives of rhodamine B and rhodamine 101 solubilized in vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The systems were studied at low and high dye concentrations, as well as at different temperatures by means of steady-state and time-resolved fluorescence spectroscopy. The time-dependent fluorescence anisotropy, r(t), obtained at low dye concentrations is solely determined by rotational motions of the rhodamine moiety and its orientation. The orientation of the rhodamine chromophore with respect to the normal of the membranes was determined by using linear dichroism (LD) spectroscopy. It is found that the electronic absorption and emission transition dipoles of the dyes are preferentially oriented in the plane of the lipid bilayer. At high dye concentration, r(t) depends on rotational motions of the rhodamine dye as well as energy transfer between adjacent chromophores. An analytical model of energy transfer and a semi-empirical approach of r(t) were used to analyse the experimental results. The models and the analysis of experimental data were tested against simulated data. Our results are compatible with energy transfer among chromophores located in the lipid/water interfaces of a lipid bilayer with their transition dipoles oriented parallel to the membranes, and furthermore undergoing rapid reorientation compared with the rate of energy transfer. The model predicts that the rhodamine B group is located in the water region at ca. 3.5 A from the lipid/water boundary, while the rhodamine 101 dyes are located in this boundary. Furthermore, the dye concentrations predicted from the analysis of the experiments agree with those calculated from independent data.

Photochemically Induced Isomerisation of Ruthenium Polypyridyl Complexes

The synthesis and characterisation of a series of ruthenium polypyridyl complexes containing pyridyltriazole ligands in different coordination modes are described. The electrochemical and electronic properties of the compounds with respect to the coordination mode of the pyridyltriazole ligand are reported. Upon photolysis of the complex containing the 1-methyl-3-(pyridin-2-yl)-1,2,4-triazole ligand, irreversible ligand isomerisation is observed.

Excited-state symmetry and reorientation dynamics of perylenes in liquid solutions : Time-resolved fluorescence depolarization studies using one- and two-photon excitation

The excited-state symmetry and molecular reorientation of perylene, 1,7-diazaperylene, and 2,5,8,11-tetra- tert-butylperylene have been studied by different fluorescence depolarization experiments. The first excited electronic singlet state was reached through one-photon excitation (OPE) and two-photon excitation (TPE). A 400 and 800 nm femtosecond laser pulse was used for this purpose, and data were collected by means of the time-correlated single-photon counting technique. It is found that the rotational correlation times for each perylene derivative are very similar in the OPE and TPE depolarization experiments. For the determination of the two-photon absorption tensor, a recently described theoretical model has been applied (Ryderfors et al. J. Phys. Chem. A 2007, 111, 11531). It was found that the two-photon process can be described by a 2 x 2 absorption tensor for which the components are solvent dependent and exhibit mixed vibronic character. In the dipole approximation this is compatible with a parity-forbidden two-photon absorption into the first excited singlet state.

Fluorescence study of the weak interaction between pyrene and quaternary ammonium groups

Abstract Systems containing pyrene and tetraalkylammonium halides (R 4 NX) were investigated using time-resolved and steady state fluorescence techniques. The existence of a previously suggested weak interaction between pyrene and quaternary ammonium groups is confirmed and is further verified by solubility measurements. The quenching of pyrene fluorescence by R 4 NBr is a synergistic effect, requiring Br − and enhanced by the simultaneous presence of the quaternary ammonium ion. The presence of tetraalkylammonium halides also affects the efficiency of pyrene excimer formation.

A rod-like polymer containing {Ru(terpy)2} units prepared by electrochemical coupling of pendant thienyl moieties

A new rod-like coordination polymer consisting of (Ru-(terpy)2) motifs bridged by bithiophene units has been prepared by electrochemical polymerisation in acidic organic medium.

Two-photon excited fluorescence depolarisation and electronic energy migration within donor–donor pairs

A unified theoretical description is presented for one- and two-photon excited fluorescence depolarisation and electronic energy migration within pairs of chromophores. Two weakly coupled donor groups are linked via a rigid macromolecule with the ability to undergo restricted reorienting motions. Describing these reorienting motions as well as their influence on the coupling is rather complex, but can be accounted for by using the extended Förster theory. Here explicit expressions have been derived for chromophores belonging to the point groups D(2h), D(2) and C(2v) when residing in uniaxial potentials (i.e. C(infinity v) symmetry). From the given basic equations, it is possible however, to derive the relevant equations for molecules of arbitrary symmetry in any uniaxial orienting potential. The expected time-resolved fluorescence anisotropy for different two-photon absorption tensors are compared for reorienting fluorophores in liquids, as well as in anisotropic systems. Simulated fluorescence depolarisation data are also displayed that mimic energy migration within pairs of two-photon excited donor molecules, which simultaneously undergo reorienting motions within effectively isotropic and uniaxially anisotropic environments. The obtained results demonstrate that the time-resolved fluorescence anisotropy strongly depends on the properties of the two-photon absorption tensor, as well as on using a linear or a circular polarisation of the excitation field.

Ultrafast multiphoton ionization dynamics and control of NaK molecules

The multiphoton ionization dynamics of NaK molecules is investigated experimentally using one-color pump-probe femtosecond spectroscopy at 795 nm and intermediate laser field strengths (about 10 GW ...

Second-harmonic-generation measurements on ZnSe under high pressure

Second-harmonic-generation (SHG) measurements on ZnSe at high pressure, up to 7 GPa, have been reported. The zinc-blende–rock-salt transition pressure has been determined at room temperature from the SHG in ZnSe using a femtosecond laser. The pressure required to induce transformation from a zinc-blende to a rock-salt structure decreases from 11.5 to 1.07 GPa in a femtosecond laser field. SHG can be used to monitor structural changes under pressure of some materials with nonlinear optical properties.