Nice bike! I have an Impulse too, though mine has the "Impulse" decal on the top tube instead of the seat tube. Mine doesn't have a serial number anywhere that I can see, and I've really looked everywhere for one.
I really like mine, as it's the only bike I have that is more or less spot on to my ideal size in seat tube length (65 cm c-c), though if I had a choice, I'd like the top tube a little shorter. Also, whoever brazed/constructed the frame, which has immaculate detailing/filing/mitreing, didn't do the bottle cage braze-ons, as they ain't too straight! Either that or they were lazy at the end of the day. Aside from that, it's a great bike; light, fairly stiff, but supple for longer riding and looks great.
Here's a link to the 1991 catalogue page for the Impulse:
http://bulgier.net/pics/bike/Catalog...dson-91/04.jpg
Yours looks basically the same, so I assume it from the same time give or take a year or so. I think mine is a little later, perhaps 1993-1995 or so.