Title of Thesis

Photoionization Cross Section And Oscillator Strength Measurements Of The Excited States Of Sodium And Magnesium

Author(s)

Muhammad Rafiq

Abstract
In the present work, detailed experimental studies have been conducted to observe the Rydberg series of magnesium and measure the absolute photoionization cross section from the excited states of sodium and magnesium. In addition to these measurements, the optical oscillator strengths of the Rydberg transitions from the 3p and 4s states of sodium have been determined.
In the first set of experiments new experimental data on the highly excited λ = 0, 1, 2 and 3 states of magnesium have been acquired using two-photon and two-step laser excitation technique in conjunction with a thermionic diode ion detector. The new observations include even parity 3sns 1S0 (8 ≤ n ≤ 24) and 3snd 1D2 (7 ≤ n ≤ 62) Rydberg states approached directly from the 3s2 1S0 ground state via two-photon excitation. In comparison, the odd parity 3snp 1P1 (20 ≤ n ≤ 61) and 3snf 1F3 (14 ≤ n ≤ 66) Rydberg states are accessed by two-step excitation via 3s4s 1S0 and 3s3d 1D2 intermediate states. The Rydberg relation fit to the new data of the 3snp 1P1 and 3snf 1F3 series yields the binding energies of the 3s4s 1S0 and 3s3d 1D2 levels as 18167.702 cm-1 and 15267.972 cm-1 respectively. By adding the binding energies to the corresponding energies of the 3s4s 1S0 and 3s3d 1D2 intermediate levels, a precise value of the first ionization potential of magnesium is determined as 61671.04 ± 0.04 cm-1. The quantum defects for the 3sns 1S0, 3snp 1P1, 3snd 1D2 and 3snf 1F3 Rydberg series have been determined as 1.526(2), 1.046(2), 0.602(2) and 0.049(2) respectively.
In the next set of experiments, the absolute photoionization cross section from the 3s3p 1P1 excited states of magnesium isotopes have been measured in the energy region from the first ionization threshold up to 1.4 eV excess energy. For these studies a two-step photoionization and saturated ionization technique has been employed in conjunction with an atomic beam source and a Time of Flight (TOF) mass spectrometer. The Time of Flight mass spectrometer enables us to separate the three stable isotopes of magnesium on the time axis. The absolute value of the photoionization cross sections from the 3s3p 1P1 excited state near the 3s ionization threshold is measured as 90 ± 16 Mb (at 354.5 nm ionizing wavelength) for the dominating isotope (24Mg) whereas the value at the peak of the 3p2 1S0 auto-ionizing resonance is determined as 785 ± 141 Mb. The present experimentally measured photoionization cross sections are compared with the existing experimental and theoretical work showing excellent agreement.
The next studies are devoted to the new measurements of the oscillator strengths for the 4s 2S1/2 → np 2P1/2, 3/2 (19 ≤ n ≤ 57), 3p 2P3/2→ nd 2D5/2, 3/2 (13 ≤ n ≤ 48) and 3p 2P1/2 → nd 2D3/2 (13 ≤ n ≤ 50) Rydberg transitions of sodium. For these measurements the two-photon and two-step laser excitation techniques have been employed using a thermionic diode ion detector in conjunction with Nd: YAG pumped dye lasers. The measured f-values have been calibrated with the photoionization cross-sections determined as 0.65(0.10), 7.9(1.3) and 6.7(1.1) Mb from the 4s 2S1/2, 3p 2P3/2 and 3p 2P1/2 intermediate states at the first ionization threshold respectively. In addition, we have determined the binding energy of the 4s 2S1/2 level as 15709.444(8) cm-1 by employing the Rydberg relation to the observed np 2P1/2, 3/2 transitions. Addition of the binding energy to the known energy of the 4s 2S1/2 level, the value of the first ionization potential of sodium is determined as 41449.44(1) cm-1. A comparison of the experimentally determined results shows good agreement with the existing data.

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1.1 Overview
1.2 Rydberg atoms
1.3 Photoabsorption
1.4 Laser isotope separation
1.5 Photoionization
1.6 Cross sectional area of Gaussian beam
1.7 Oscillator strength

2.1 Thermionic diode ion detector
2.2 Time of flight mass spectrometer
2.3 Laser system
2.4 Data acquisition system
2.5 Experimental setup
2.6 Intensity profile of the laser beam

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