Launching Excel From App?

I have an app that outputs a gridview to excel spreadsheet and adds a couple of rows for accounting. I'm trying to launch the spreadsheet after its saved off, using

Code:

excelApp.ActiveWorkbook.SaveAs(filename);
excelApp.ActiveWorkbook.Saved = true;
excelApp.Quit();
var si= new ProcessStartInfo {FileName = filename, UseShellExecute = true}; //hits here and crashes
Process.Start(si);

it hits the marked line and crashes that it cant find the file, its automatically saving it to the documents folder on my local machine, and i cant seem to get it to save elsewhere (i have a data directory in the project i use for the other file types) that i would like to save to if possible, but it seems that if you have office installed it don't like it. i could always copy the file from documents to the datastore, but that's difficult.

how do i launch excel application using the file i just saved off to show to the bean crunchers?