The Soviet Union this weekend ordered the expulsion of a US diplomat, saying he was caught carrying out espionage. The Tass news agency said Louis Thomas, a US Embassy attache, had been declared persona non grata for ''activity incompatible with his official status.'' An embassy spokesman confirmed Mr. Thomas was told to leave and said he and his wife Elizabeth were getting ready for their departure.
Meanwhile, the New York Times reported that President Reagan had decided to name Jack F. Matlock as his senior specialist on Soviet affairs. National Security Adviser William Clark reportedly felt a need for an aide who was highly experienced in Soviet affairs. Mr. Matlock is US ambassador to Czechoslovakia