SUNScholar - Browsing by Author "Rohwer, E. G."

Browsing by Author "Rohwer, E. G."

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Accurate laboratory wavelengths of the e 3 Σ–(ν' = 5) – X 1 Σ+(ν'' = 0) band of 12C16O
(IOP Science, 2010) Dickenson, G. D.; Nortje, A. C.; Steenkamp, C. M.; Rohwer, E. G.; Du Plessis, A.
The forbidden singlet-triplet transitions of carbon monoxide (CO) are important in the interpretation of vacuum ultraviolet interstellar absorption spectra and in particular for the measurement of large CO column densities. Twenty rovibronic lines of the e 3Σ–(ν' = 5) – X 1Σ+(ν'' = 0) band of 12 C 16O for which laboratory wavelengths were previously unavailable were identified in laser-induced fluorescence excitation spectra. Wavelengths were assigned to five rovibronic transitions to an average accuracy of 0.0028 Å. A further 15 lines could not be fully resolved and average wavelengths were measured for these groups of closely spaced lines. A wavelength difference of 0.011 ± 0.0028 Å between the measured wavelengths and the calculated wavelengths in the atlas of Eidelsberg & Rostas demonstrates the need for more experimental data on CO.
Experimental conditions for vacuum ultraviolet laser spectroscopy
(Academy of Science for South Africa, 2005) Du Plessis, A.; Steinmann, C. M.; Rohwer, E. G.
An efficient wavelength-tunable vacuum ultraviolet (VUV) laser source has been developed at our institute in recent years. This laser source works on the principle of sum-frequency mixing of dye laser pulses in a magnesium vapour medium, which is prepared in a crossed heat pipe system. The laser source is coupled to a pulsed supersonic expansion and is in use for spectroscopic measurements. In this paper, we present a complete characterization of the experimental setup. The source yielded 25-ns-long pulses of VUV radiation with peak power of approximately 1 mW. The wavelength is currently tunable over the range 142.7-146.7 nm with a spectral bandwidth of 0.25 ± 0.05 cm -1. Suggested refinements of the experimental technique include an increase in the repetition rate of the measurements and shorter gas pulses.
Femtosecond laser diagnostics of thin films, surfaces and interfaces
(Academy of Science for South Africa, 2005) Scheidt, T.; Rohwer, E. G.; Bergmann, H. M. V.; Stafast, H.
An overview is given of optical second harmonic generation (SHG) using femtosecond laser pulses to analyse technologically important crystalline materials. The principle of SHG is briefly explained and a typical experimental setup for SHG is outlined. The second harmonic (SH) measurements performed in prototype compound semiconductors reveal the crystalline structure and orientation of monocrystalline SiC, polycrystalline ZnO, and the ternary compound, PbxCd1-xTe, showing the segregation of a Pb-and a Cd-rich phase. Furthermore, SHG can selectively probe the Si/SiO2 interface and the build-up of its electric field induced by laser-activated charge carrier separation. The examples presented demonstrate that SHG is a versatile technique to probe the structural and electronic properties of crystalline materials and particularly surfaces and interfaces.
Femtosecond pump probe spectroscopy for the study of energy transfer of light-harvesting complexes from extractions of spinach leaves
(Academy of Science of South Africa (ASSAf), 2009-10) Ombinda-Lemboumba, S.; Du Plessis, A.; Sparrow, R. W.; Molukanele, P.; Botha, L. R.; Rohwer, E. G.; Steenkamp, C. M.; Van Rensburg, L.
Measurements of ultrafast transient processes, of temporal durations in the picosecond and femtosecond regime, are made possible by femtosecond pump probe transient absorption spectroscopy. Such an ultrafast pump probe transient absorption setup has been implemented at the CSIR National Laser Centre and has been applied to investigate energy transfer processes in different parts of photosynthetic systems. in this paper we report on our first results obtained with Malachite green as a benchmark. Malachite green was chosen because the lifetime of its excited state is well known. We also present experimental results of the ultrafast energy transfer of light-harvesting complexes in samples prepared from spinach leaves. Various pump wavelengths in the range 600-680 nm were used; the probe was a white light continuum spanning 420-700 nm. The experimental setup is described in detail in this paper. Results obtained with these samples are consistent with those expected and achieved by other researchers in this field.
First comparison of electric field induced second harmonic of NIR femtosecond laser pulses in reflection and transmission generated from Si/SiO2 interfaces of a silicon membrane
(Springer, 2011) Nyamuda, G. P.; Rohwer, E. G.; Steenkamp, C. M.; Stafast, H.
For the first time electric field induced second harmonic (EFISH) generation of femtosecond (fs) laser pulses (λ=800 nm, τ=75±5 fs, rep. rate=80 MHz, Epulse≤10 nJ) is observed in transmission through a thin free-standing silicon (Si) membrane of 10-μm thickness and compared to the well-known EFISH results in reflection by use of the z-scan technique. EFISH in reflection and transmission unequivocally originate from the front and rear Si/SiO2 interfaces, respectively, with SiO2 being the natural oxide on the Si surfaces. Frequency conversion is enhanced by photoinduced electric fields across the Si/SiO2 interfaces caused by charge-carrier injection from Si into the oxide. The z-scan results and time-dependent measurements allow comparison of the EFISH signal amplitudes and time constants detected in transmission and reflection, demonstrating the need for further investigation. © 2011 Springer-Verlag.
Free charge carrier absorption in silicon at 800 nm
(Springer, 2016) Heisel, P.-C.; Ndebeka, W. I.; Neethling, P. H.; Paa, W.; Rohwer, E. G.; Steenkamp, C. M.; Stafast, H.
The transmission of a Ti:sapphire laser beam (c.w. and fs pulsed operation at 800 nm) through a 10-μm-thin oxidized silicon membrane at 45° angle of incidence at first increases with the incident laser power, then shows a maximum, and finally decreases considerably. This nonlinear transmission behavior is the same for c.w. and pulsed laser operation and mainly attributed to free charge carrier absorption (FCA) in Si. A simple FCA model is developed and tested.
High-resolution vacuum ultraviolet laser spectroscopy of molecules in a free supersonic jet : in search of rare CO isotopomers and CO-Ar van der Waals molecules
(Academy of Science for South Africa, 2005) Steinmann, C. M.; Du Plessis, A.; Rohwer, E. G.
Tunable vacuum ultraviolet radiation (in the range 142.7-146.7 nm) from a novel laser source was used to probe the electronic excitation spectrum of cold carbon monoxide molecules in a supersonic noble gas jet. Rotationally resolved spectra of 12C16O,13C16O, as well as the rare but astronomically important 12C18O and 12C17O isotopomers, were recorded. Evidence was obtained for the formation of CO-containing van der Waals complexes in the jet.
An improved experimental setup for high-resolution vacuum ultraviolet laser spectroscopy
(Academy of Science for South Africa, 2005) Steenkamp, C. M.; Du Plessis, A.; Rohwer, E. G.
The experimental setup for vacuum ultraviolet (VUV) laser spectroscopy at Stellenbosch University's Laser Research Institute, has been improved by the addition of a VUV monochromator. The VUV source has been characterized over the extended wavelength range 144.2-157.7 nm, and simultaneous measurement of laser-induced fluorescence and absorption spectra has been achieved. We discuss current investigations of carbon monoxide molecules and complexes, facilitated by the narrow bandwidth of the VUV light and the low temperature collision-free conditions in a supersonic jet.
Investigation of the non-linear properties and optical limiting capabilities of C₆₀ and polyfluorene
(Academy of Science for South Africa, 2005) Neethling, P. H.; Rohwer, E. G.; Walters, P. E.
The non-linear properties of C60 and polyfluorene were investigated using the z-scan technique. C60 is a well-known optical limiter and its non-linear properties have been measured and documented. These measurements gave us an indication of the capabilities as well as the accuracy of our own experimental setup. Preliminary absorption measurements of polyfluorene indicated an unexpected time dependence.
Optical second harmonic generation as a probe for internal electric fields at the Si/SiO₂ interface
(Academy of Science for South Africa, 2005) Scheidt, T.; Rohwer, E. G.; Von Bergmann, H. M.
The second harmonic signal generated in native Si/SiO₂ interfaces using femtosecond laser pulses (80 ± 5 fs, 10.5 nJ, 1.59 eV) is time dependent. The temporal evolution of the electric field-induced second harmonic signal (EFISH) shows a steady incline and subsequent saturation for incident laser peak intensities below ∼45 GW/cm2 due to electron injection and trapping as well as defect generation in the SiO₂ layer. We used second harmonic generation imaging to visualize defect sample areas. In contrast, a drastically different behaviour of the time-dependent SH response of Si/SiO₂ interfaces was observed for peak intensities above ∼45 GW/cm2. The SH signal rose to a maximum and showed a subsequent decline over many minutes. We suggest hole injection into the ultra-thin SiO₂ layer as an interpretation of this observation.
Second harmonic generation as a technique to probe buried interfaces
(Academy of Science for South Africa, 2009) Neethling, P. H.; Scheldt, T.; Rohwer, E. G.; Von Bergmann, H. M.; Stafast, H.
Since the advances of femtosecond laser technology during the last decade, optical second harmonic generation (SHG) has proven itself a powerful tool to investigate the electronic and structural properties of semiconductor materials. Its advantage lies in the fact that it is a contact-less, non-intrusive method that can be used in situ. It is sensitive to systems with broken symmetry, in particular interfaces and surfaces. The Si/SiO 2 system is technologically important since it forms a component of most modern electronic equipment. Furthermore, it has been shown that it is possible to induce an electric field across this Interface by means of laser irradiation as a result of defect formation and defect population. This electric field can be measured since it determines the SHG signal. The anisotropy of the SHG signal from the Sl/SiO 2 interface was measured and showed four-fold symmetry, illustrating that the SHG technique was able to characterise the electrical properties of the interface below the 5 nm thick oxide layer.

Browsing by Author "Rohwer, E. G."

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