Mars Attacks!

In Kentucky, a family watches in confusion as a herd of cattle starts running past their home, on fire. A spaceship explodes before it returns to Mars, and hundreds of other Martian ships leave Mars and head towards Earth. President James Dale along with his aides addresses the United States concerning the historic event. The President's science aides set up a first contact meeting with the Martians in Pahrump, Nevada as President Dale watches the development on TV with his wife Marsha and his daughter Taffy.
Using a translator, the Ambassador of Mars announces that they "come in peace". When a hippie releases a dove as a symbol of peace, the Ambassador shoots it, then he and the other Martians slaughter a large number of people at the event including General Casey, news reporter Jason Stone, and Billy-Glenn Norris before capturing chat show host Nathalie Lake and her pet Chihuahua Poppy whose heads they later transpose back on one of their flying saucers.
Thinking that the Martians assumed that the dove was a symbol of war, President Dale tells Professor Donald Kessler to renegotiate with the Martians, whose ambassador is invited to address the United States Congress. At this meeting, the Martians massacre most of Congress. Donald begs the Martian Ambassador to stop, but is rendered unconscious and taken aboard their ship where he is later shown with his body parts dismembered and his disembodied head remaining animated. General Decker tries to convince President Dale to retaliate with nuclear warfare, but the President refuses.
After a failed attempt to assassinate President Dale fails, the Martians invade Earth, starting with Washington, D.C. As they attack the White House, the Secret Service evacuates the President, but the First Lady Marsha is crushed to death by the Nancy Reagan chandelier. Taffy vanishes during the chaos. After the U.S. government unsuccessfully attempts a nuclear attack on the mother ship, the Martians expand their invasion to the rest of the Earth, defacing world landmarks for amusement as they do. That night, the Martians breach the bunker where Dale has been taken and the leader reduces Decker to the size of an insect before killing him. The Martians kill everyone else in the bunker except for Dale, who makes an impassioned speech in an attempt to plea for peace and his life. The Martians seem convinced, but then use a false hand to kill the President.
As the Martians ravage Las Vegas with Art Land being among of their victims, Byron Williams, a casino employee and former world champion boxer, leads a small group of survivors consisting of Barbara Land, Tom Jones, a beautiful waitress named Cindy, and a gambling lawyer to an airfield in the hopes of flying a small jet to safety. They barely make it, losing the lawyer in the process, but discover a large group of Martians stationed there as they are preparing to take off. Byron creates a diversion by challenging them to a fistfight. While he succeeds in killing the Ambassador, he is outnumbered and overwhelmed, but Tom, Barbara, and Cindy escape.
Billy-Glenn's brother Richie discovers that the Martians' heads explode when they hear Slim Whitman's "Indian Love Call." He and his grandmother Florence drive around town, using the song to kill Martians. The military thereafter broadcast the song around the globe, killing most of the Martians and their leader, and causing the few remaining survivors to either flee from Earth. Nathalie and Donald disembodied heads kiss while when the Martian spaceship they are on explodes when it crashes into the ocean.
In the aftermath, Richie and Florence are awarded the Medal of Honor by Taffy, she being the only surviving member of the government. Byron, who survived his Martian brawl, arrives in D.C. to reunite with his former wife and their two sons (who saved Dale's life back in the White House using two guns they stole from a dead Martian) as the devastation is being cleaned up. Barbara, Cindy, and Tom Jones emerge from a cave with some animals to see dozens of crashed Martian ships in the adjacent Lake Tahoe.

Question: Who did President Dale ask to renegotiate with the Martians?
Answer: Professor Donald Kessler