Here's what I found, from the U.S. Govt. Printing Office.
An impeachment may proceed only when Congress is in session. 3
Hinds Sec. Sec. 2006, 2462. However, an impeachment proceeding does
not expire with adjournment. An impeachment proceeding begun in the
House in one Congress may be resumed in the next Congress. 3 Hinds
Sec. 2321; 111-1, Jan. 13, 2009, p __. An official impeached by the
House in one Congress may be tried by the Senate in the next Congress.
Manual Sec. 620; 3 Hinds Sec. Sec. 2319, 2320.
Although impeachment proceedings may continue from one Congress to
the next, the authority of the managers appointed by the House expires
at the end of a Congress; and managers must be reappointed when a new
Congress convenes. Manual Sec. 620. Managers on the part of the House
are reappointed by resolution. Manual Sec. 604; Deschler Ch 14
Sec. 4.2.