Impersonisation in asp.net to execute .exe

I have an asp.net application(c# ) that invokes an .exe file. The functionality of the .exe file is to execute a dts package. It extracts data from a table to a text file. I need to use impersonisation to create this text file and write it. But it is DTS that creates this text file. When I provide modify permission to IIS_WPG the file creation happens perfectly, but when I use impersonisation it fails. I am struck with this issue for long. Please help.
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