<Tennessee's only woman on death row is asking a federal judge in Knoxville to prevent her execution.Christa Gail Pike's lawyer filed a petition claiming his client's constitutional rights were violated when she was convicted in 1996 of killing Colleen Slemmer.The petition seeks relief from conviction or sentence. It described Pike as a "mentally ill, cognitively impaired, immature adolescent" at the time of the slaying.Slemmer was tortured and fatally struck on the head with a chunk of asphalt.<