4th IUC-DAEF CAT
School on Science with Synchrotron Radiation
The school – 4th in a series – was organized in collaboration with Centre for Advanced Technology, Indore. It was devoted primarily to photoelectron spectroscopy (PES) and reflectivity studies in the context of the 450 MeV electron storage ring Indus-1 and the beam lines already commissioned on it. Dr .D. D. Bhawalkar, Director, CAT inaugurated the school. In his inaugural address Dr. Bhawalkar described the DAE programs of development of dedicated accelerator based advanced light sources for the scientific community. He pointed out the very many research applications of synchrotron radiation and highlighted the studies on hollow atoms – i.e. atoms with bound electrons excited to very high energy bound states – the so called Rydberg states. He also described the possible use of pulsed character of synchrotron radiation for studies in the femto second time domains involved in fast chemical reactions. Dr B A Dasannacharya, Director, IUC-DAEF presided over the inauguration function. In his presiding address Dr Dasannacharya described the remarkable growth of synchrotron radiation in the last few decades. He emphasized the role of instrumentation in extending the research frontiers and invited the university community to participate in the development of beam line instrumentation for the upcoming Indus-2 – a 2.5 GeV storage ring giving radiation in the X-ray region. Dr. R. V. Nandedkar, CAT gave the next talk. He described at length the synchrotron source Indus-1 and the various beam lines on it.
The school then started with a lecture by Dr. B. D. Padalia, Emeritus Professor from IIT Mumbai. He gave a panoramic view of the entire field of PES drawing on his extensive work in the field for last three decades, particularly with applications to high temperature superconductors. The school had a mix of pedagogic and research talks. Dr S R Barman, IUC-DAEF gave a series of four lectures covering introduction to the nitty-gritty involved in PES data interpretation with examples from both laboratory and synchrotron work. One of his talks was devoted to his work in collaboration with a group from FHI, Germany on multipole plasma oscillations on alkali metal surfaces. Dr. P. Chaddah, CAT gave two talks on the physics involved in PES showing both limitations and successes of the three step (single electron picture) and single step (many body) models of photoelectron emission process. He also discussed how to extract band structure information at several k-points of the Brillouin zone from angle resolved PES data. In addition he elaborated the work on high temperature superconductors, particularly the anisotropic energy gap opening at the Fermi surface at the transition temperature and the relevance of the Bose condensation models of superconductivity in these materials owing to formation of bosonic pairs. These pairs persist at temperatures well above the transition temperature as seen from the PES data. Dr. K. Maiti, TIFR, Mumbai gave two talks on spin-resolved PES. He described in detail the magnetic interactions in the Hamiltonian and went on to show how to understand the contributions of the different spin states for particular PES spectra. He described in detail the experimental set-up necessary for recording the spin resolved data involving use of spin orbit interaction and spin dependent scattering from a standard gold foil; the so called Mott detector. His examples were mostly taken from Gd. Dr. A. Chainani, IPR; Gandhinagar also gave two talks on high resolution PES. In one talk he considered high resolutions in energy of the ejected photoelectron typically 1.5 to 5 meV with applications to 1-2-3 compounds. He devoted the other talk to applications of PES to solar cell passivation studies by recording impurity states at the interface in a p-n junction. He also discussed studies on nitridation in a similar manner. In all these industrial application he used a laboratory set-up developed at IPR with a spatial resolution in microns. Prof. Mrs. S. K. Kulkarni devoted her two talks on PES to semiconductor studies, particularly, GaAs, structures on Si single crystal, nanoparticles and quantum wells etc.
The main presentations on the facilities available at Indus-1 were by Dr. S. M. Chaudhari, IUC-DAEF and Prof. M. S. Hegde, IISc, Bangalore. Dr. Chaudhari described in detail the IUC beam line based on a toroidal grating monochromator. Using three in situ gratings it is possible to scan the photon energy range 10 – 200 eV. The resolving power available depends upon the exit slit size used at the post mirror end and it is wavelength dependent: 1000 at 304 Å and 650 at 584 Å. He also described briefly the experimental station and presented preliminary spectra demonstrating the capability of the set-up. The obtained spectra were of very good quality – comparable to the best available literature. Even in one pass a count rate of up to 20 K was noted. He declared that the beam line is now available to the user community and invited participants to come up with interesting projects. Prof. Hegde gave details about the instrumentation involved in developing the experimental station for angle integrated PES on IUC beam line. He started with very simple concepts of how to measure the energy of the ejected photoelectron and gradually developed the whole spherical analyser system. With a 3 mm wide slit and a pass energy of 30 eV the station could give energy resolution of about 0.8 eV. He pointed out a variety of problems where even with such limited resolution interesting physics can be obtained. He further remarked that the slit size can be reduced to 1mm and also with reduced pass energy the resolution can be substantially improved by a factor of at least three. In his second talk Prof. Hegde described molecular states and how to study them directly using PES. Dr. D. M. Phase, IUC-DAEF described the passivation studies on semiconductor surfaces in the context of first such studies carried out on IUC beam line for GaAs in collaboration with Pune university. In another seminar talk, Dr. T. Shripathi of IUC-DAEF described resonance effects in PES. Such studies are most attractive because of the tenability of synchrotron radiation.
On the reflectivity side talks were given by Dr. A. V. Pimpale, IUC-DAEF and Dr G S Lodha and Shri M H Modi both from CAT. Dr. Pimpale gave a series of three talks and explained in a pedagogic manner the interactions of light with electrons in matter connecting the reflectivity properties to the microscopic response of the electrons in terms of scattering length. He also described applications involving X-ray standing waves and Kissig fringes. Shri Modi described the experimental station on CAT beam line for reflectivity and metrology studies. His 2-axis goniometer for this purpose had an angle accuracy of 0.012 degree over a wide angular range. The vacuum inside the experimental station was much poorer than in the beam line and hence it was necessary to have several stages of differential pumping. He also showed the first reflectivity data recorded using the set-up. Dr. Lodha gave three talks covering a wide range of applications in the X-ray and XUV region. He concentrated mostly on development and studies on multilayer structures for optical applications.
Dr. Jagannath, BARC, Mumbai gave a seminar describing the BARC beam line for angle resolved PES and described at length the corresponding experimental station.
In addition to the above a number of special talks on different topics of interest were also organized in the school. Dr. A. Banerjee, IUC-DAEF described in his seminar the basic process involved in magnetic circular dichroism – i.e. different photon absorption cross section for different electron spin states involved in transitions caused by incident polarized light. He also described how to set up experimental facilities for such studies. Dr. V. G. Sathe gave a talk on EXAFS spectroscopy. He described the basic absorption process and how fine structure in the absorption coefficient arises due to interference effects involving the ejected photoelectron and its reflection from neighboring atoms. He explained at length the information content in the data and how to analyze with the help of models and numerical techniques. He gave an example of a highly complex organic molecule bringing out the salient features. Dr. P. K. Gupta, CAT gave a very fascinating talk on biomedical applications of lasers. He discussed how photosensitization increases the efficacy of drugs in certain cancer cases by creation of long-lived triplet states. He also described the experiments on early diagnosis of cancer by utilizing different photo absorptions by cancerous and normal tissues. Dr. K. J. S. Sawhney, CAT gave a seminar on X-ray fluorescence and its applications for material characterization. He described the X-ray source developed for such studies at CAT and using the SiLi detector remarked that ppm level impurities could be quantitatively detected. Finally, Dr. S. Krishnagopal, CAT gave a talk on free electron laser – a next generation light source. He described the interaction of the relativistic electron in the accelerator with the spatially periodic magnetic structures and emission of coherent radiation as a consequence. He also described various free electron lasers already available in different laboratories the world over and put the CAT experiments in this field in their context.
The participants were shown the various experimental facilities available in IUC-DAEF and CAT. A feedback session was organized at the end to invite comments and suggestions from the participants. The general response of the participants was very favorable and it appeared that considerable interest has been generated amongst them to work in this field. Several proposals for undertaking PES work are being discussed.
There were in all 23 participants from outside Indore.