Kalyar, Mazhar Ali (2009) Two-step Laser Excitation Studies of Bound and Auto lonizing States in Barium. PhD thesis, Quaid-i-Azam University, Islamabad.
Abstract We report detailed experimental studies on the spectra of barium using the two-step excitation scheme to measure the absolute photoionization cross section from excited states and the optical oscillator strengths of the Rydberg transitions. In addition, the line shapes of the autoionizing resonances have been studied using various channels approached by different intermediate levels and extensive new measurements of the highly excited even parity autoionizing resonances is presented. Furthermore, the proper line shape of the 6p2 1S0 autoionizing resonance has been reported experimentally for the first time. In the first set of experiments, the photoionization cross section from the 6s6p 1,3P1 excited states have been measured in the energy region from the first ionization threshold up to 1.69 eV excess energy. The experiments have been performed using a thermionic diode ion detector in conjunction with a Nd: YAG laser system. The absolute values of the photoionization cross section from the 6s6p 1P1 and 3P1 excited states at the first ionization threshold have been determined as 90 ± 14 Mb and 102 ± 15 Mb respectively.The studies have been extended to determine the measurements of the oscillator strengths of the 6s6p 1P1 → 6snd 1D2 (20 ≤ n ≤ 76) and 6s6p 3P1 → 6snd 3D2 (20 ≤ n ≤ 63) Rydberg transitions. The measured absolute photoionization cross-sections from the 6s6p 1P1 and 6s6p 3P1 states have been used to calibrate the f-values of the Rydberg transitions. The oscillator strength densities in the continuum corresponding to the 6s6p 3P1 excited state have also been determined. In the next set of experiments, we have investigated how the line shape and width of an autoionizing resonance varies by choosing different excitation paths and a systematic line shape analysis of the 6p7p configuration based 1P1, 3D1 and 3P1 autoionizing resonances are presented. These even parity isolated autoionizing resonances have been accessed via four different intermediate states 6snp 1P1 (6 ≤ n ≤ 8) and 5d6p 1P1. A comparison of the Fano parameters of the resonance profiles reveals that the width of an autoionizing resonance remains constant while the line profile parameter changes with the selection of different intermediate states. The next studies are devoted to the highly excited even parity autoionizing states using the two-step laser excitation via the 5d6p 1P1 and 6s7p 1P1 intermediate levels from the first ionization threshold to the 5d 2D5/2 limit. Atomic beam system coupled with a time of flight mass spectrometer is used to explore the appropriate line shape of the 6p2 1S0 resonance and its adjacent autoionizing resonances have been explored for the first time. Four channels (one open and three closed) MQDT analyses has also been performed to extract the line shape parameters for the broad 6p2 1S0 resonance and other J = 0 resonances in this region. The highly excited even parity autoionizing states close to 5d 2D3/2, 5/2 thresholds have been recorded using a thermionic diode ion detector. Extensive new data for the 5d3/2,5/2nd J = 0, 1, 2 series are presented. Besides the data for the 5dng J = 2 autoionizing resonances is extended up to n = 44 and n = 51 terminating at the 5d3/2 and 5d5/2 threshold respectively. The 5dns J = 2 autoionizing resonances have been observed up to n = 38 and n = 35 built on the 5d3/2 and 5d5/2 threshold respectively. The identification of autoionizing states is made exploiting the parallel and perpendicular polarizations of the two dye laser beams and monitoring the consistency of the quantum defects.
|Item Type:||Thesis (PhD)|
|Uncontrolled Keywords:||Laser, Bound, Autoionizing, Barium, Spectra, Thermionic, Spectrometer, Rydberg, Photoionization, Oscillato, Excitation, Strength, Photoionization|
|Subjects:||Physical Sciences (f) > Physics(f1)|
|Deposited By:||Mr. Javed Memon|
|Deposited On:||19 Aug 2010 11:13|
|Last Modified:||30 Jun 2011 09:15|
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