Trying President Trump in the Senate after he leaves office would be a bill of attainder. Lovett’s test (328 US 303) says that a bill of attainder has these prongs: 1) specifically identified the people to be punished; 2) imposed punishment; and 3) did so without benefit of judicial trial.
All of the above would apply to a Senate trial. It would be an act of a legislature declaring a person, or a group of persons, guilty of some crime, and punishing them.
Consequently it is explicitly prohibited by the Constitution.
Article II, Section 4, provides:
The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.
Article I, Section II, Section 7, provides for the remedies available in the Judgment after Senate trial:
Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.