I= LASER BASED SPECTROSCOPIC STUDIES OF HELIUM AND LITHIUM
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Title of Thesis
LASER BASED SPECTROSCOPIC STUDIES OF HELIUM AND LITHIUM

Author(s)
Shahid Hussain
Institute/University/Department Details
Department of Physics / Quaid-i-Azam University Islamabad
Session
2007
Subject
Physics
Number of Pages
127
Keywords (Extracted from title, table of contents and abstract of thesis)
helium, lithium, glow discharge geometries, gas discharge, photoabsorption, photoionization, dc discharge, rf discharge

Abstract
In the present work, a detailed experimental study has been conducted to measure the electric field in three different glow discharge geometries, a RF discharge cell, a homemade DC discharge cell and a commercial see-through hollow cathode lamp using the laser optogalvanic spectroscopy. The net electric field in the three discharges has been determined from the termination of 1s2s 1Sº - 1snp 1P1 Rydberg series, from the Stark splitting and also from the observed energy shift of the spectral lines. In the homemade DC glow discharge, the net electric field is measured as a function of applied voltage at a constant gas pressure, as a function of gas pressure at a constant applied voltage and as a function of radial distance from the electrode centers. A comparison of the electric field distribution in the three discharges is described based on the Rydberg series termination, Stark shift, line broadening, and relative intensities of the observed spectral lines.

The next studies are devoted to the measurements of photoionization cross sections of the excited states of helium and lithium using the saturation technique. A DC glow discharge has been used to measure the photoionization cross sections of the 3p 1p and 3p 3p excited states of helium, at threshold and in the near threshold region (0 - 2 eV) using a simple experimental setup. A smooth frequency dependence of the cross section has been observed for both the excited states in accordance to the theoretical calculations. A thermionic diode ion detector has been used for the measurements of photoionization cross sections from the 4s 2S, 4p 2p and 4d 2D excited states of lithium above the first ionization threshold. The behavior of the photoionization cross sections from n = 4 and „“ = 0, 1, 2 excited states have been investigated. A smooth wavelength dependence of photo ionization cross section has been observed for the 4p and 4d excited states, which decrease monotonically with the decrease in the ionizing laser wavelength however the falloff for the 4d excited state is purely hydrogenic. The cross section for the 4s excited state first increases, attains a maximum value and then decreases monotonically with a decrease in the ionizing wavelength-strictly nonhydrogenic. The measured values of the cross sections are in good agreement with the earlier reported theoretical and experimental values.

In addition, the oscillator strength distribution in discrete and continuous regions of the spectra of helium and lithium has been determined. A low-pressure RP glow discharge has been used for the measurement of oscillator strength distribution in helium while a thermionic diode ion detector has been used for the determination of oscillator strength distribution in lithium. The saturation technique has been employed to determine the photoionization cross section from the 2s 1Sº excited state of helium and from 3s 2S1/2 excited state of lithium at and above the first ionization threshold. The measured value of the photo ionization cross section at the ionization threshold has been used to extract the ’-values for the 2s 1Sº †’ np 1P1 Rydberg series of helium from n = 10 to n = 52 and for the 3s 2Sl/2 †’np 2P1/2 Rydberg series of lithium from n = 14 to n = 56. In the continuum region the oscillator strength densities have been estimated by measuring the photoionization cross sections from the 2s 1Sº excited state at five ionizing laser wavelengths while from the 3s 2Sl/2 excited state at four ionizing laser wavelengths above the first ionization threshold. The discrete ’-values smoothly merge into the continuous oscillator strength densities and continuity has been found between the discrete and the continuous spectrum across the ionization threshold.

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S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
344.94 KB
2 1 Introduction 1
851.51 KB
  1.1 Overview 1
  1.2 Gas Discharge 1
  1.3 Rydberg Atoms 9
  1.4 Stark Effect 11
  1.5 Photoabsorption 14
  1.6 Photoionization 18
  1.7 Measurement Of Photoionization Cross Section 19
  1.8 Corss Sectional Area Of Gaussian Beam 24
  1.9 Oscillator Strength 26
  1.10 Layout Of The Thesis 31
3 2 Experimental Setup 33
392.9 KB
  2.1 Discharge Cells 33
  2.2 Thermionic Diode Ion Detector 36
  2.3 Laser System 38
  2.4 Data Acquisition System 41
  2.5 Experimental Setup 42
4 3 Comparative Study Of RF And DC Discharge Based Laser Optogalvanic Spectroscopy Of Helium Rydberg States 47
627.69 KB
  3.1 Studies With RF Discharge 48
  3.2 Studies With DC Discharge 53
  3.3 Studies With See-Through Hollow Cathode 63
  3.4 Comparison Of The Observed Spectra 66
5 4 Measurement Of The Photoionization Cross Section Of Excited States Of Helium 71
281.77 KB
6 5 Measurement Of The Oscillator Strength Distribution In The Helium 81
336.84 KB
7 6 Angular Momentum Dependence Of Photoioinization Cross Section From The Excited States Of Lithium 94
327.8 KB
8 7 Measurement Of Oscillator Strength Distribution In Discrete And Continuous Spectrum Of Lithium 10
580.22 KB
  7.1 Conclusion 120
  7.2 References 122