"Deactivate" FileMaker when switch to Finder?

Here's my AppleScript (embedded into the FileMaker file as a calculation field:

tell application "Finder"

activate

if not (exists disk "Engineering") then

beep

display dialog "You must first mount the Engineering volume." with icon 2 buttons ("OK") default button 1

else

if file "050-0000-0108-00D.pdf" of folder "050-0000-01XX-XX" of folder "050-0000-XXXX-XX" of folder "050-XXXX-XXXX-XX" of disk "Engineering" exists then

select file "050-0000-0108-00D.pdf" of folder "050-0000-01XX-XX" of folder "050-0000-XXXX-XX" of folder "050-XXXX-XXXX-XX" of disk "Engineering"

open selection

else

beep

display dialog "There is no PDF for this file." with icon 1 buttons ("OK") default button 1

end if

end if

end tell

My problem is that when the script is activated and the PDF file is opened via the Finder, the FileMaker window still appears as if it is the front-most (active) window. That is, the window's title bar does not gray out like normal.

What is the AppleScript command (if any) that "deactivates" the current application?