Ian appeared as Charlie Baker in season four, episode 20 called “Dominance.” The episode aired on NBC on Friday, April 4, 2003 and will also air on USA Network on Sunday, April 13, 2003.
From TV Guide:
Multiple murders at a Manhattan townhouse are the start of a killing spree. Al Baker: Frank Langella. Det. Deuthorn: Erik Palladino. Charlie Baker: Ian Somerhalder. Billy Baker: Jason Ritter. Cragen: Dann Florek. Benson: Mariska Hargitay. Stabler: Christopher Meloni.
“A DEADLY KILLING SPREE SENDS THE SQUAD IN SEARCH OF A MURDERER WITH AN AXE TO GRIND; TONY WINNER FRANK LANGELLA GUEST STARS — When an engagement celebration dinner ends in sexual assault and a gruesome quadruple homicide, Detectives Benson (Mariska Hargitay) and Stabler (Christopher Meloni) search for a violent killer looking for revenge through the domination of his victims. However, when the killer goes on a murder spree aimed at the upper class, it’s a race against time as the squad tries to stop this deadly rampage. Tony Winner Frank Langella (“Fortune’s Fool”) and Ian Somerhalder (“Rules of Attraction”) also guest star. Richard Belzer, Dann Florek, Stephanie March, B.D. Wong and Ice-T also star. TV-14″
CREDITS: IAN THE SOME