What is the duty of the federal government to produce impeachment material or exculpatory material for the purposes of impeaching a witness?

With regard to impeachment material, the government is obligated to disclose information that could be used to impeach its own witnesses. See Giglio v. United States, 405 U.S. 150, 153-55 (1972). With regard to law enforcement officers it intends to call to testify, the government has a duty to examine the personnel files of those law enforcement officers for impeachment or exculpatory material, which may need to be furnished to the court or defendant if the files contain information that is or may be material to the defendant's case. United States v. Henthorn, 931 F.2d 29, 31 (9th Cir. 1991).