How to align a text vertically using Xceed.Richtextbox.

Sample for horizontal alignment as Left:

var richTextBox = obj as Xceed.Wpf.Toolkit.RichTextBox;
 var selectedText = richTextBox.Selection;
selectedText.ApplyPropertyValue(Paragraph.TextAlignmentProperty, TextAlignment.Left);

May i know the property to be used for aligning a text Vertically as Top,Middle,Bottom.
I tried Richtextbox properties: VerticalAlignment, VerticalContentAlignment: But these didn’t work