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Mukhtar, Emad
Alternative names
Publications (10 of 11) Show all publications
Jacobsson, J. T., Viarbitskaya, S., Mukhtar, E. & Edvinsson, T. (2014). A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots. Physical Chemistry, Chemical Physics - PCCP, 16(27), 13849-13857
Open this publication in new window or tab >>A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots
2014 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 27, p. 13849-13857Article in journal (Refereed) Published
Abstract [en]

The time resolved UV-fluorescence in ZnO quantum dots has been investigated using femtosecond laser spectroscopy. The measurements were performed as a function of particle size for particles between 3 and 7 nm in diameter, which are in the quantum confined regime. A red shift in the fluorescence maximum is seen while increasing the particle size, which correlates with the shift in band gap due to quantum confinement. The energy difference between the UV-fluorescence and the band gap does, however, increase for the smaller particles. For 3.7 nm particles the fluorescence energy is 100 meV smaller than the band gap energy, whereas it is only 20 meV smaller for the largest particles. This indicates a stabilization of the excitons in the smallest particles. The lifetime of the UV fluorescence is in the picosecond time scale and interestingly, it is discontinuous with respect to particle size. For the smallest particles, the exciton emission life time reaches 30 ps, which is three times longer than that for the largest particles. This demonstrates a transition between two different mechanisms for the UV-fluorescence. We suggest that this is an effect of surface trapping and stabilization of the excitons occurring in the smallest particles but not in the larger ones. We also discuss the time scale limit for slowed hot carrier dynamics in ensembles of quantum confined ZnO particles.

National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-221255 (URN)10.1039/C4CP00254G (DOI)000338116700036 ()
Available from: 2014-03-27 Created: 2014-03-27 Last updated: 2019-02-18Bibliographically approved
Persson, A., Eilers, G., Ryderfors, L., Mukhtar, E., Possnert, G. & Salehpour, M. (2013). Evaluation of Intracavity Optogalvanic Spectroscopy for Radiocarbon Measurements. Analytical Chemistry, 85ASAP(14), 6790-6798
Open this publication in new window or tab >>Evaluation of Intracavity Optogalvanic Spectroscopy for Radiocarbon Measurements
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2013 (English)In: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 85ASAP, no 14, p. 6790-6798Article in journal (Refereed) Published
Abstract [en]

Ever since the first publication of intracavity optogalvanic spectroscopy (ICOGS) in 2008, this novel technique for measuring the 14C/12C 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 (14C/12C), 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 ICOGS’s capability of unambiguous 14C 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 on14C concentration and wavelength and, ultimately, not seen any evidence of the capability of ICOGS to unambiguously detect 14C 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.

Keywords
Intracavity optogalvanic spectroscopy, radiocarbon laser spectroscopy, 14CO2 lasers, 14C detection, CO2 oscillations
National Category
Analytical Chemistry Atom and Molecular Physics and Optics Fusion, Plasma and Space Physics
Research subject
Analytical Chemistry; Ion Physics
Identifiers
urn:nbn:se:uu:diva-202646 (URN)10.1021/ac400905n (DOI)000322059600035 ()
Funder
Swedish Research Council, 2010-4422
Note

Anal. Chem., Article ASAP

Available from: 2013-06-25 Created: 2013-06-25 Last updated: 2017-12-06Bibliographically approved
Eilers, G., Persson, A., Gustavsson, C., Ryderfors, L., Mukhtar, E., Possnert, G. & Salehpour, M. (2013). The Radiocarbon Intracavity Optogalvanic Spectroscopy Setup at Uppsala. Radiocarbon, 55(3-4), 237-250
Open this publication in new window or tab >>The Radiocarbon Intracavity Optogalvanic Spectroscopy Setup at Uppsala
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2013 (English)In: Radiocarbon, ISSN 0033-8222, E-ISSN 1945-5755, Vol. 55, no 3-4, p. 237-250Article in journal (Refereed) Published
Abstract [en]

Accelerator mass spectrometry (AMS) is by far the predominant technology deployed for radiocarbon tracer studies. Applications are widespread from archaeology to biological, environmental, and pharmaceutical sciences. In spite of its excellent performance, AMS is expensive and complicated to operate. Consequently, alternative detection techniques for 14C are of great interest, with the vision of a compact, user-friendly, and inexpensive analytical method. Here, we report on the use of intracavity optogalvanic spectroscopy (ICOGS) for measurements of the 14C/12C ratio. This new detection technique was developed by Murnick et al. (2008). In the infrared (IR) region, CO2 molecules have strong absorption coefficients. The IR-absorption lines are narrow in line width and shifted for different carbon isotopes. These properties can potentially be exploited to detect 14CO2, 13CO2, or 12CO2 molecules unambiguously. In ICOGS, the sample is in the form of CO2 gas, eliminating the graphitization step that h is required in most AMS labs. The status of the ICOGS setup in Uppsala is presented. The system is operational but not yet fully developed. Data are presented for initial results that illustrate the dependence of the optogalvanic signal on various parameters, such as background and plasma-induced changes in the sample gas composition.

Keywords
Intracavity optogalvanic spectroscopy, radiocarbon laser spectroscopy, 14CO2 lasers, oscillatory reactions, CO oxidation
National Category
Atom and Molecular Physics and Optics Fusion, Plasma and Space Physics Analytical Chemistry
Research subject
Ion Physics; Analytical Chemistry
Identifiers
urn:nbn:se:uu:diva-202651 (URN)10.2458/azu_js_rc.55.16226 (DOI)000325752100006 ()
Funder
Swedish Research Council, 2010-4422
Available from: 2013-06-25 Created: 2013-06-25 Last updated: 2019-02-28Bibliographically approved
Opanasyuk, O., Mikaelsson, T., Ryderfors, L., Mukhtar, E. & Johansson, L.-Å. B. (2012). On the analyses of fluorescence depolarisation data in the presence of electronic energy migration. Part II: Applying and evaluating two-photon excited fluorescence. Physical Chemistry, Chemical Physics - PCCP, 14(6), 1917-1922
Open this publication in new window or tab >>On the analyses of fluorescence depolarisation data in the presence of electronic energy migration. Part II: Applying and evaluating two-photon excited fluorescence
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2012 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 6, p. 1917-1922Article in journal (Refereed) Published
Abstract [en]

Electronic energy migration within a bifluorophoric molecule has been studied by time-resolved two-photon excited (TPE) fluorescence depolarisation experiments. Data were analysed by using a recently developed quantitative approach [O. Opanasyuk and L. B.-A. Johansson, On the Analyses of Fluorescence Depolarisation Data in the Presence of Electronic Energy Migration. Part I: Theory and General Description, Phys. Chem. Chem. Phys., submitted]. The energy migration occurs between the 9-anthrylmethyl groups of the bifluorophoric molecule, bis-(9-anthrylmethylphosphonate) bisteroid. These groups undergo local reorientations, while overall tumbling of the bisteroid is strongly hampered in the used viscous solvent, 1,2-propanediol. To solely obtain information about local reorientations of the 9-anthrylmethyl group, also the mono-(9-anthrylmethylphosphonate) bisteroid was studied, which enabled modelling of the ordering potential shape. The analysis of data is partly performed in the Fourier domain and the best-fit parameters are determined by using an approach based on a Genetic Algorithm. The energy migration process was described by an extended Forster theory (EFT). A reasonable value of the distance between the 9-anthrylmethyl groups, as well as for the mutual orientation of the ordering potentials, is found. Furthermore, values of the two-photon tensor components were obtained.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-169617 (URN)10.1039/c2cp23177h (DOI)000299290700010 ()
Available from: 2012-03-07 Created: 2012-03-05 Last updated: 2017-12-07Bibliographically approved
Ryderfors, L., Mukhtar, E. & Johansson, L.-Å. B. Å. (2007). The Symmetry of Two-Photon Excited States as Determined by Time-Resolved Fluorescence Depolarisation Experiments. Journal of Physical Chemistry A, 111(45), 11531-11539
Open this publication in new window or tab >>The Symmetry of Two-Photon Excited States as Determined by Time-Resolved Fluorescence Depolarisation Experiments
2007 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 111, no 45, p. 11531-11539Article in journal (Refereed) Published
Abstract [en]

A new experimental and theoretical approach is presented for the quantitative determination and assignment of the two-photon absorption tensor of fluorophores dissolved in liquid solutions. Two linearly independent time-resolved fluorescence anisotropies and the two-photon polarization ratio were determined from experiments based on using the time-correlated single photon counting technique. The data were analyzed in a global manner under the assumption of prevailing diffusive molecular reorientations and when accounting for the influence of rapid unresolved reorientations. The method has been applied in fluorescence studies of perylene, two-photon excited at 800 nm. The analysis suggests that the two-photon transition is mediated via vibronic coupling including at least two vibrations of different symmetry, and also that the first singlet excited electronic state acts as a dominating intermediate state.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-97540 (URN)10.1021/jp074506g (DOI)000250809400018 ()17944444 (PubMedID)
Available from: 2008-09-18 Created: 2008-09-18 Last updated: 2017-12-14Bibliographically approved
Ryderfors, L., Mukhtar, E. & Johansson, L.-Å. B. Å. (2007). Two-Photon Excited Fluorescence and Molecular Reorientations in Liquid Solutions. Journal of Fluorescence, 17(5), 466-480
Open this publication in new window or tab >>Two-Photon Excited Fluorescence and Molecular Reorientations in Liquid Solutions
2007 (English)In: Journal of Fluorescence, ISSN 1053-0509, E-ISSN 1573-4994, Vol. 17, no 5, p. 466-480Article in journal (Refereed) Published
Abstract [en]

Theoretical expressions are derived that relate the two-photon excited fluorescence depolarisation experiments to the molecular symmetry and the rotational motions of fluorescent molecules. Diffusive rotational motions in liquid solvents are considered, as well as the influence of fast unresolved motions (e.g. librations). The results obtained are compared with one-photon excited fluorescence depolarisation experiments. The derived theoretical expressions can be applied for detailed analyses of the molecular rotation in solvent. Several of the results are useful for determining and assigning the components of two-photon absorption tensors.

Keywords
Two-photon excited fluorescence, Two-photon excited fluorescence depolarisation, Two-photon excited anisotropy, Theory of rotational motion, Rotational diffusion, Librations, Perylene
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-97539 (URN)10.1007/s10895-007-0189-x (DOI)000248912100003 ()17578656 (PubMedID)
Available from: 2008-09-18 Created: 2008-09-18 Last updated: 2017-12-14Bibliographically approved
Burza, M., Welch, K., Strömme, M. & Mukhtar, E. (2006). Molecular dynamics of biomimetic ionomers studied using second harmonic generation. In: : . Paper presented at The 7th Nordic Femtochemistry Conference, Uppsala,.
Open this publication in new window or tab >>Molecular dynamics of biomimetic ionomers studied using second harmonic generation
2006 (English)Conference paper, Published paper (Other academic)
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Nanotechnology and Functional Materials
Identifiers
urn:nbn:se:uu:diva-24925 (URN)
Conference
The 7th Nordic Femtochemistry Conference, Uppsala,
Available from: 2007-02-08 Created: 2009-02-26 Last updated: 2016-11-30Bibliographically approved
Borgström, M., Blart, E., Boschloo, G., Mukhtar, E., Hagfeldt, A., Hammarström, L. & Odobel, F. (2005). Sensitized Hole Injection of Phosphorus Porphyrin into NiO: Toward New Photovoltaic Devices. Journal of Physical Chemistry B, 109(48), 22928-22934
Open this publication in new window or tab >>Sensitized Hole Injection of Phosphorus Porphyrin into NiO: Toward New Photovoltaic Devices
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2005 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 109, no 48, p. 22928-22934Article in journal (Refereed) Published
Abstract [en]

This paper describes the preparation and the characterization of a photovoltaic cell based on the sensitization of a wide band gap p-type semiconductor (NiO) with a phosphorus porphyrin. A photophysical study with femtosecond transient absorption spectroscopy showed that light excitation of the phosphorus porphyrin chemisorbed on NiO particles induces a very rapid interfacial hole injection into the valence band of NiO, occurring mainly on the 2-20 ps time scale. This is followed by a recombination in which ca. 80% of the ground-state reactants are regenerated within 1 ns. A photoelectrochemical device, prepared with a nanocrystalline NiO electrode coated with the phosphorus porphyrin, yields a cathodic photocurrent indicating that electrons indeed flow from the NiO electrode toward the solution. The low incident-to-photocurrent efficiency (IPCE) can be rationalized by the rapid back recombination reaction between the reduced sensitizer and the injected hole which prevents an efficient regeneration of the sensitizer ground state from the iodide/triiodide redox mediator. To the best of our knowledge, this work represents the first example of a photovoltaic cell in which a mechanism of hole photoinjection has been characterized.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-75770 (URN)10.1021/jp054034a (DOI)16853987 (PubMedID)
Available from: 2006-02-24 Created: 2006-02-24 Last updated: 2017-12-14Bibliographically approved
Holm, A.-K., Mohammed, O. F., Rini, M., Mukhtar, E., Nibbering, E. T. & Fidder, H. (2005). Sequential merocyanine product isomerization following femtosecond UV excitation. J Phys Chem A Mol Spectrosc Kinet Environ Gen Theory, 109(40), 8962-8
Open this publication in new window or tab >>Sequential merocyanine product isomerization following femtosecond UV excitation
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2005 (English)In: J Phys Chem A Mol Spectrosc Kinet Environ Gen Theory, ISSN 1089-5639, Vol. 109, no 40, p. 8962-8Article in journal (Refereed) Published
Identifiers
urn:nbn:se:uu:diva-75139 (URN)16331998 (PubMedID)
Available from: 2006-01-16 Created: 2006-01-16 Last updated: 2011-01-11
Ryderfors, L., Mukhtar, E. & Johansson, L. B. A. (2005). Two-photon excited fluorescence depolarisation experiments: II. The proper response function for analysing TCSPC data. Chemical Physics Letters, 411(1-3), 51-60
Open this publication in new window or tab >>Two-photon excited fluorescence depolarisation experiments: II. The proper response function for analysing TCSPC data
2005 (English)In: Chemical Physics Letters, Vol. 411, no 1-3, p. 51-60Article in journal (Refereed) Published
Identifiers
urn:nbn:se:uu:diva-74471 (URN)
Available from: 2005-09-29 Created: 2005-09-29 Last updated: 2011-01-11
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