| dc.description.abstract | For those narrow resonances observed without a good resolution, in addition to the familiar problem due to a poor instrumental resolution itself, there is another effect called ``column density effect’’ which will profoundly change the observed spectrum. This effect stemmed from the discrepancy between the formula used in dealing with experimental data over-simplifies the actual interaction; also because mathematical formulas used in dealing with data relies on experimental method incidentally, the various spectra observed by different method will be different accordingly. In order to understand about the column density effect well, we chose two of He resonances --- $(2,1_3)$ and $(2,1_4)$ --- to be our targets. It is well known that the widths of the two resonances are both narrower than Doppler broadening, hence calculation of Doppler effect has to be included into convolution in order to expect a good agreement between theory and experiment. Because takeing into account of Doppler effect, we will found out a particular convoluting procedure which has to be taken in the first place. Judged by the comparison between simulation and experimental results, the proper procedure should be that convolute theoretical resonance with Doppler effect first and then convolute with column density effect afterwards. Based on the comparison between experimental results and simulation results convoluted by the proper convolution procedure, the resonances of He $(2,1_3)$ and $(2,1_4)$ were resolved fully in this work.
The well known effects on spectral features such as --- shift, splitting, broadening and variation of oscillator strength --- due to DC electric field for the resonances, were observed in the Stark photoionization spectra of Mg, Ca, Sr and Ba. For the case of the Stark effects around the first threshold of Mg and Ca, the oscillation features (electric field induced resonances) on the ionization continuum were observed distinctly. The oscillation features were formed with many manifolds and these manifolds were constructed by a plurality of observed narrow peaks. All the similar movements for the manifolds and the narrow peaks within these manifolds were observed as field strength varied. There was a particular manifold --- evolving manifold, the movements of the narrow peaks in this manifold were different from these of another manifolds; once the part of the next evolving manifold is appeared, the movements of those narrow peaks in the manifold will be similar with these of another peaks in another manifolds. Incidently for a particular narrow peak just at the position of the apparent threshold, the cross section increased unexpectedly and soon return to a typical value as its neighboring peaks when field strength increases. This unexpected change in cross section for the particular peak can be observed clearly in our experiment.
Below the first ionization limit tunneling effect was observed in spectra of Mg and Ca. For the case of Stark effects of Ca, Sr and Ba in their autoionization region, the movements for the mostly observed autoionization resonances were red shift, but blue shift was observed just for a few resonances. The oscillation features (electric field induced resonances) above the second threshold was observed in the Stark spectra of both Ca and Sr. The most distinct phenomena for the autoionization states were peak splitting and oscillator strength sharing observed in the Stark spectra of Ca and Sr. Effect of level anti-crossing were observed in Stark spectra for the autoionization states of Sr.
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