PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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Experimental Techniques 36 mode or by varying the the pass energy E p whilst holding the retardation ratio constant known as the constant retard ratio (CRR) mode [10]. i. Constant retard ratio (CRR) mode In the CRR mode electrons entering the analyser system from the sample are retarded by the lens stack by a constant proportion of their kinetic energy so that the ratio of electron kinetic energy to analyser pass energy is kept constant during a spectrum. The retard ratio k is defined as, K = E k − φ a E p ∼ E k E p (2.24) where φ a is the analyser work function. Throughout the scan range the pass energy of the analyser is continuously varied to maintain a constant retard ratio. Sensitivity and resolution are proportional to the pass energy and therefore the kinetic energy in this mode. In this mode the analysed sample area and the emission angle remain almost constant throughout the whole kinetic energy range. ii. Constant analyser energy (CAE) mode In the CAE mode, the analyser pass energy is held constant, and the retarding voltage is changed thus scanning the kinetic energy of detected electrons. The resolution obtained in CAE is constant throughout the whole kinetic energy range. The sensitivity, however is inversely proportional to the kinetic energy and at low kinetic energies is improved over that of CRR. In the CAE mode, the analysed sample area and the emission angle may vary slightly with the kinetic energy, but this is dependent on the lens design. (b) Analyser resolution The analyser is a band pass energy filter for electrons at a specific energy E p and has a finite energy resolution ∆E which is dependent on the chosen mode of operation and specific operating conditions. The energy resolution of the analyser is given approximately by,
Experimental Techniques 37 ∆E = E p ( d 2R 0 + α 2 ) (2.25) where d is the slit width, R 0 is the mean radius of the hemispheres and α is the half angle of electrons entering the analyser at the entrance slit. (c) The electrostatic input lens The input lens collects the electrons from the source and focuses them onto the entrance aperture of the analyser whilst simultaneously adjusting their kinetic energy to match the pass energy of the analyser. The lens is also designed to define the analysis area and angular acceptance of electrons which pass through the hemispherical analyser. The lens design employs double lens concept where two lenses are stacked one above the other. The first lens selects the analysis area (spot size) and angular acceptance. This is an Einzel lens, i.e it does not change the energy of the electrons and therefore has a constant magnification throughout the entire energy range. This lens can be operated in three discrete magnification modes: high, medium and low. In high magnification mode, the focal plane is nearer to the sample and the lens accept a wide angle of electron beams from a small region. In low magnification mode, the focal plane is farther from the sample and the lens accept only a small angle of beams but from a larger area. The medium magnification is in-between the two. The second lens retards or accelerates the electrons to match the pass energy of the analyser and uses the zoom lens function to ensure that the focal point remains on the analyser entrance aperture. The magnification of this lens varies with retard ratio as a result of the law of Helmholtz-Lagrange. The analysis area is defined by the combination of the selected analyser entrance aperture and the magnification of the entire lens. The magnification of the entire lens is a product of the magnifications of the two discrete lenses. (d) The detector A single channel electron multiplier (Channeltron) is placed across the exit plane of the analyser. The channeltron amplifies the current of a single electron by a factor of about 10 8 . The small current pulse present at the output of the channeltron is
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SPECTROSCOPIC INVESTIGATIONS OF THE
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iii To My Parents
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Maharana, P. Khare, D. K. Basa, K.
Page 7 and 8: List of Publications/Preprints [1]
Page 9 and 10: Synopsis The perovskite type mangan
Page 11 and 12: to the t 2g and e g spin-up states
Page 13 and 14: Bibliography [1] R. von Helmolt, J.
Page 15 and 16: Contents CERTIFICATE Acknowledgemen
Page 17 and 18: List of Figures 1.1 Temperature dep
Page 19 and 20: 2.2 Schematic view of the energetic
Page 21 and 22: 3.4 O K edge x-ray absorption spect
Page 23 and 24: Chapter 1 Introduction 1
Page 25 and 26: Introduction 3 The temperature and
Page 27 and 28: Introduction 5 Figure 1.2: Schemati
Page 29 and 30: Introduction 7 Figure 1.4: A schema
Page 31 and 32: Introduction 9 Figure 1.6: Schemati
Page 33 and 34: Introduction 11 Figure 1.8: Differe
Page 35 and 36: Introduction 13 Figure 1.10: A sche
Page 37 and 38: Introduction 15 Figure 1.12: Schema
Page 39 and 40: Introduction 17 Figure 1.13: Phase
Page 41 and 42: Bibliography [1] R. von Helmolt, J.
Page 43 and 44: Introduction 21 [31] Damien Saurel,
Page 45 and 46: Chapter 2 Experimental Techniques 2
Page 47 and 48: Experimental Techniques 25 Figure 2
Page 49 and 50: Experimental Techniques 27 Let us n
Page 51 and 52: Experimental Techniques 29 and G(k,
Page 55 and 56: Experimental Techniques 33 point wi
Page 57: Experimental Techniques 35 negative
Page 63 and 64: Experimental Techniques 41 ( ) dσ
Page 65 and 66: Experimental Techniques 43 The expe
Page 67 and 68: Experimental Techniques 45 are weld
Page 77 and 78: Bibliography [1] H. Hertz, Ann. Phy
Page 79 and 80: Experimental Techniques 57 [35] K.
Page 81 and 82: Chapter 3 Electronic Structure of P
Page 83 and 84: Electronic Structure of Pr 0.67 Ca
Page 95 and 96: Bibliography [1] I. G. Deac, J. F.
Page 99 and 100: Chapter 4 Electronic Structure of P
Page 101 and 102: Electronic Structure of Pr 1−x Ca
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Electronic Structure of Pr 1−x Ca
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Electronic Structure of Ca 0.86 Pr
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Bibliography [1] C. Zener, Phys. Re
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Summary and Conclusions 109 In this
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