Browsing by Author "Ombinda-Lemboumba, Saturnin"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- ItemFemtosecond pump probe spectroscopy of light harvesting complexes and Phthalocyanines(Stellenbosch : Stellenbosch University, 2011-12) Ombinda-Lemboumba, Saturnin; Du Plessis, Anton; Steenkamp, Christine M.; Rohwer, Erich G.; Stellenbosch University. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: The generation of ultrafast light pulses and the development of time resolved spectroscopic techniques, such as the femtosecond pump probe spectroscopy technique, have facilitated the study of ultrafast energy transfer in the photosynthetic systems of green plants and photodynamic therapy drugs. It has allowed the investigation of biological and chemical processes that take place on the ultrafast timescale and has allowed us to obtain spectral and kinetic information on energy transfer. In addition, it has allowed time resolved experiments in which the transient absorption of species under investigation was observed and has elucidated molecular dynamics. In the present work this was done with a temporal resolution of approximately 200 fs and covering a pump-to-probe delay range of 300 fs to 2 ns. The main aims of this study were to characterise the femtosecond pump probe spectroscopy system, to investigate the energy transfer in the natural light harvesting complex II (LHC II) in view of future expansion to the study of synthesized arti cial functional light harvesting complexes and nally to study ultrafast processes in zinc phthalocyanine (ZnPc) systems. In photosynthetic organisms, LHC II is the most abundant light harvesting complex and it plays an important role in light harvesting and photoprotection. The light energy is absorbed by light harvesting complexes and transferred to a reaction centre (RC) in an ultrafast timescale. Phthalocyanines are a new class of photosensitiser used for photodynamic therapy. These drugs are used to treat small and super cial tumours. The energy transfer from the singlet excited state to the triplet excited state occurs on an ultrafast timescale. However, recent work done on zinc phthalocyanine has proved that the determination of the ultrafast component remains a challenge. Several ultrafast studies carried out on ZnPc in solvents have been not only unsuccessful to give a clear picture of the ultrafast dynamics but have also produced divergent results. In this study, a characterisation of the femtosecond pump probe spectroscopy setup was done. The samples under investigation were probed by a white light continuum. The generation of the white light continuum introduced chirp, which in uenced the temporal evolution of the transient absorption results. The technique used to correct the chirp introduced by white light generation is discussed in detail. Our femtosecond pump probe spectroscopy setup was benchmarked by using a well known dye, namely malachite green. In addition, the investigation of the transient absorption change of LHC II, an active component in photosynthesis, as extracted from spinach leaves and the ultrafast dynamics of a promising photosensitiser ZnPc in dimethyl sulfoxide (DMSO) as well as in dimethyl formamide (DMF) was done. The spectral and dynamic results obtained using these three samples are described and exponential ts to the absorbance decay curves used to estimate the timescales of the energy transfer processes are presented. In this experiment, the dynamics and measured time constants related to the energy transfer between the different types of chlorophyll in LHC II was monitored, whereas with ZnPc, the dynamics and the measured time constants associated with solvation dynamics and vibrational relaxation was examined.
- ItemLaser induced chlorphyll fluorescence of plant material(Stellenbosch : University of Stellenbosch, 2007-03) Ombinda-Lemboumba, Saturnin; Von Bergmann, H. M.; Rohwer, Erich G.; Steenkamp, Christine M.; University of Stellenbosch. Faculty of Science. Dept. of Physics.Imaging and spectroscopy of laser induced chlorophyll fluorescence (LICF) are emerging as useful tools in plant physiology and agriculture since these methods allow an early detection of plant stress and transformation of plant tissue, before visual symptoms appear. Chlorophyll fluorescence is governed by photosynthetic efficiency and it depends on the plant species and physiological state. In addition, the laser induced fluorescence of chlorophyll molecules in the red and far red spectral range is also used to study basic processes and phenomena in photo-excited molecules. In the work reported here experimental setups used for laser induced chlorophyll fluorescence imaging and spectroscopy techniques were developed to investigate chlorophyll fluorescence under constant illumination and also to detect green-fluorescent protein (GFP) by looking at the chlorophyll fluorescence spectrum and image. He-Ne (wavelength 632 nm), tunable argon ion (wavelength 455 nm), and excimer (wavelength 308 nm) lasers were used as excitation sources. An Ocean Optics spectrometer was used to record the spectrum of the chlorophyll fluorescence and the variation of the chlorophyll fluorescence spectrum with time. The chlorophyll fluorescence spectrum of tobacco leaves expressing GFP was compared to that of control leaves. A charge-coupled device (CCD) camera was used to image the fluorescence from GFP expressing and control tobacco leaves to investigate the effect of GFP genes on chlorophyll fluorescence in relation to the state of the plant material. The spectral analysis technique and image processing procedures were elaborated in order to obtain better information on chlorophyll fluorescence. The results of this work show that the experimental setups and analytical procedures that were devised and used are suitable for laser induced chlorophyll fluorescence analysis. Fluorescence bleaching could be obtained from the time variation of the fluorescence spectrum, and plant expressing GFP can be distinguished from control plants by differences in the laser induced chlorophyll fluorescence.