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Title of Thesis

Shaukat Mahmood
Institute/University/Department Details
Department of Physics/ Quaid-i-Azam University Islamabad
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Keywords (Extracted from title, table of contents and abstract of thesis)
optogalvanic spectroscopy, inert gases, plasma parameters, optogalvanic signals

In the present work we have studied the time-resolved optogalvanic signals in the spectra of inert gases. The effects on the optogalvanic signals by scanning a dye laser across the dipole transitions in a DC discharge plasma have been investigated. Time-resolved spectra are obtained at fixed wavelengths of the dye laser, resonantly tuned to optically allowed transitions. The temporal evolutions of the signals are registered on a storage oscilloscope. In the first set of experiments we have selected three transitions from the 3s[l/2]2 metastable state of neon corresponding to the ”J = ”K= 0, ± 1 dipole selection rules to investigate the dominant physical processes responsible for the optogalvanic signals. Based on a theoretical model, the observed signals are fitted to an expression,

S(t)= a [e-bt- e-t/ „ ]+ c [e-dt- e-t/ „ ] 1-b„ 1-d „

A non-linear least squares programme has been used to fit the above equation to obtain the parameters that determine the amplitudes and decay rates of the investigated transitions. The change in the signal amplitude as a function of discharge current has been studied. In addition, the electron collisional ionization rate parameter ratios have been determined for the transitions corresponding to the dipole selection rules ”J = ”K = -1,. ”J = ”K = 1 and ”J = ”K = 0, as 1.63,1.75 and 1.00 respectively. The effective lifetimes of the upper levels involved in the aforementioned transitions are also calculated as 62.50 µs, 31.25 µs and 12.85 µs respectively.

We have further extended the work to study the temporal evolution and variation of the laser optogalvanic signals in the spectra of inert gases using commercial hollow cathode lamps. The behavior of the optogalvanic signals for the transitions following the ”J = ”K = 0, ±1 dipole selection rules in the jcK-coupling scheme namely mp5(m + l)p[5/2]3, mps(m +l)p'[3/2]2, mps(m + l)p[3/2]2, mp5(m + l)p'[l/2]1 and mp5(m + l)p[l/2]1 coupled with mp5(m + l)s[3/2]2 (m = 2,3,4 and 5 for neon, argon, krypton and xenon, respectively) metastable states have been investigated. An empirical relation is formulated:

S (t) = Smin + A exp( -α [I -10]), which satisfactorily describes the shift of the optogalvanic signal and the change in the population of the upper level as a function of the discharge current.

In a separate set of experiments, we have determined the oscillator strengths of the 2p5 (2P1/2)nd J =2, 3 auto-ionizing resonances in neon using a DC discharge plasma. The highly excited odd parity states are approached using two-step laser excitation via three intermediate states which are populated from the 2p53s [l/2]2 metastable state, that gets populated by the discharge in the hollow cathode lamp. The f-values have been determined for the nd'[3/2]2. nd'[5/2]2 and nd'[5/2]3 series following the = ”K =”J= +”„“ selection rules. Employing the saturation technique the photoionization cross section at the 2p5 2P1/2 ionization threshold is determined as 5.5(6) Mb and consequently we have extracted the f-values of the auto-ionizing resonances.

In addition, plasma parameters have been determined in the neon hollow cathode discharge lamp. The measurements have been carried out to determine the electron temperature using the intensity ratio method and spatial profile of the electron density using the Stark broadening method. The electron density is measured as a function of discharge current, while the electron temperature 2500(± 10 %) K is obtained at a fixed discharge current.

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2651.25 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
298.8 KB
2 1 Introduction 10
339.86 KB
  1.1 Spectroscopy 10
  1.2 Absorption Spectroscopy 11
  1.3 Laser Optogalvanic Spectroscopy 11
  1.4 Optogalvanic Effect In A Glow Discharge 15
  1.5 Elementary Ionization Processes 16
  1.6 Rydberg Atoms 17
  1.7 Coupling Schemes 18
  1.8 Spectra Of Race Gases 20
3 2 Theoretical Model 22
217.83 KB
  2.1 Introduction 22
  2.2 Rate Equation Model 23
4 3 Experimental Details 32
317.06 KB
  3.1 Introduction 32
  3.2 ND: YAG Laser 34
  3.3 Dye Laser 36
  3.4 Data Acquisition System 39
  3.5 Hollow Cathode Lamp 40
  3.6 Etalon 41
  3.7 Photodiode 42
  3.8 Wavelength Calibration 42
5 4 The Study Of Dominant Physical Processes In The Time-Resolved Optogalvanic Spectra Of Neo 43
383.34 KB
  4.1 Energy Levels 43
  4.2 Experimental Results 46
6 5 Temporal Shift In The Optogalvanic Signals In The Spectra Of Inter Gases 61
302.37 KB
  5.1 Energy Levels 61
  5.2 Shape Of The Optogalvanic Signal 61
  5.3 Experimental Results 64
7 6 Measurements Of Oscillator Strengths Of The 2p 5/2 (P 1/2 ) Nd J=2,3 Autoionizing Resonances In Neo 73
512 KB
  6.1 Term Energies Of Autoionizing Resonance 73
  6.2 Photoionization Cross-Section 78
  6.3 Oscillator Strength 84
8 7 Measurements Of Plasma Parameters In The Neon Hollow Cathode 94
421.46 KB
  7.1 Introduction 94
  7.2 Emission Spectra 95
  7.3 Conclusion 104
  7.4 References 106