X光近緣吸收光譜修正的研究：MgO

Study of Correction of X-ray Absorption Near Edge Structure : MgO

王紹宇

指導教授 ： 張秋男

國立臺灣師範大學/理學院/物理學系/碩士(2006年)

摘要

藉由量測X光螢光總額（Total Fluorescence Yield, TFY）可得到某特定元素吸收緣附近的吸收係數（Absorption Coefficient），但自我吸收效應（Self-absorption Effect）會使X光吸收譜的強度改變，故自我吸收修正在X光吸收譜是很重要的。另外一個使X光吸收譜產生變化的效應為內層電洞效應（Core Hole Effect）。我們想藉從事MgO的X光吸收譜的自我吸收效應修正的機會，再重新檢驗此一效應。
我們證實了利用理論計算得到的元素的吸收係數，無法修正樣品的共振吸收譜線的強度。我們嘗試著利用「自我一致」的方法來從事譜線的修正。這個方法確實對共振吸收部分是有效的。樣品膜厚在約1/2平均自由徑以下，可以修正的很好。較厚的樣品則比較不容易修正的很好。在厚樣品中共振吸收強度的各個譜線要達到確實的修正，修正的迴圈數會不同，也就是在同一迴圈數中，譜線不能回復原貌。不過就修正一圈後的譜線而言，確實能夠讓變形的譜線回復一些。
對Mg的K吸收緣與O的K吸收緣附近的X光吸收譜線而言，確實可以看見MgO存在內層電洞效應。MgO雖然被認為是離子鍵結的固體，O的K吸收緣附近的X光吸收譜會有較大的電子屏蔽作用，但仍有相當大的內層電洞效應。

關鍵字

X光近緣吸收光譜修正 ； 自我吸收效應

並列摘要

By using the method of the total fluorescence yield, one can obtain the X-ray absorption near-edge spectrum. But the target self-absorption effect will distort the intensity of X-ray absorption near-edge spectrum, especially at the resonance absorption peaks. It is important to correct the target self-absorption effect in the X-ray absorption near-edge spectrum for some experiments in which the information of the absorption intensity is important. The other effect that may make the X-ray absorption near-edge spectrum distorted is the core hole effect. We shall examine it along with the correction of the target self-absorption effect by studying the X-ray absorption spectrum of MgO.
We have verified that we can’t correct the intensity of the absorption near-edge spectrum at the resonance peaks by using the absorption coefficient obtained by atoms theoretical calculation. A self-consistent method has been developed to correct target self-absorption effect. This method has been demonstrated that it corrects the target self-absorption effect quite well if the thickness of target is less than half of mean free path. However, for a thicker sample, different iteration loops are needed for different resonance peaks to have a reasonable good target self-absorption correction. In other words, it is hard to correct every resonance peaks by the same number of iteration loops. Nevertheless a certain degree of correction can be made by using only one iteration loop.
Both X-ray absorption near-edge spectra of Mg K edge and O K edge show the core hole effect in MgO. MgO is considered as an ionic bond solid and therefore, around oxygen atom in MgO should have greater electron screening and will have less core hole effect. But our data show that there is still quite large core hole effect in the X-ray absorption near-edge spectrum of O K edge

並列關鍵字

correction of XANES ； self-absorption effect