Adobe Systems Inc announced a $1.8 billion deal to buy business software maker Omniture Inc on Tuesday as it looks to counter falling sales from its Photoshop and Acrobat programs.

Adobe, which reported lower fiscal third-quarter sales and profit on Tuesday, has been struggling to grow in the economic downturn as corporations rein in technology spending.

It offered to pay $21.50 per share in cash for Omniture, which sells business software that captures and analyzes information generated on websites. That is a 24 percent premium over Omniture's closing price on Tuesday.

Omniture shares soared 25 percent to $21.74 in after-hours trading, while Adobe shares slid 4.5 percent to $34.06.

Under terms of the deal, Omniture will become a unit of Adobe, headed by its current chief executive Josh James. Adobe said the deal should close in the fourth quarter of fiscal 2009, and would add to Adobe's per-share earnings in fiscal 2010.

At the same time as announcing the deal, Adobe reported that fiscal third-quarter earnings, excluding items, fell to 35 cents per share from 50 cents per share a year ago. But it beat Wall Street's average forecast by a penny, according to Thomson Reuters I/B/E/S.

(Reporting by Bill Rigby and Jim Finkle; Editing Bernard Orr)