The Geneva summit has temporarily created a new and better "mood" in U.S.-Soviet relations, but this will not last long without progress on basic issues dividing the two superpowers, according to specialists on Soviet affairs and national security.

"This was a purely mood summit, not a substantive summit," former defense secretary James R. Schlesinger said. "It reintroduced civility, which is important, but there is nothing serious to be reported on substance . . . . What do you do for an encore?"

Several experts said they believe that President Reagan outmaneuvered Soviet leader Mikhail Gorbachev in shaping the summit to his liking, that it went "more according to the American than the Soviet script," as Arnold Horelick, director of the Rand-UCLA Center for the Study of Soviet Behavior, put it.

But many also tended to agree with Horelick's observation that "this atmosphere is not likely to endure very long unless at least some interim arms agreement is reached."

"The tone of the meeting was apparently good," Marshall D. Shulman, director of Columbia University's Harriman Institute for Soviet studies, said. "But it is worth keeping in mind the main source of tension is in the uncontrolled arms competition."

Both nations are working on new weapons of mass destruction, Shulman noted. "As they come out of the pipeline and into deployment in the coming months, this will undoubtedly be a tension-begetting process."

Such tension, many of the experts said, may also be produced by events in several Third World "hot spots," such as Angola, Nicaragua and Afghanistan, where the superpowers face important decisions that will decide levels of conflict.

Whether the superpowers are really entering what can be called "a new era" in their relationship, and what its character may be, are widely disputed by experts on Soviet affairs and national security.

Schlesinger predicted a "relaxation of tensions" and said, "Detente is back, and the 'evil empire' is passe," referring to Reagan's earlier description of the Soviet Union.

Richard V. Allen, Reagan's former national security affairs adviser and now a Heritage Foundation distinguished fellow, disagreed with this assessment.

Reagan, he said, has not changed his basic view of the Soviet Union or of "detente as a bankrupt policy" of the 1970s. The president is simply adopting the "practice of detente as a tactic," Allen said.

There is no "new era" in U.S.-Soviet relations, just "a new trend," he added.

Helmut Sonnenfeldt, a top aide to former secretary of state Henry A. Kissinger, was more cautious. The summit, he said, "unclogged" channels of communication between the two superpowers, allowing new opportunities for better communication and possible arms agreements.

But, he quickly added, it is too early to say whether any "new era" in U.S.-Soviet relations is dawning.

Many analysts generally agree that Reagan and Gorbachev returned home with increased prestige in their nations but more vulnerable in one way or another to potential critics.

Reagan made no concessions, as conservative Republicans had said they feared he might. "Nothing was yielded of consequence," said Allen, who presented before the summit the conservative Heritage Foundation's "Briefing Book" of issues and then declared himself "pleased as punch" at the outcome.

The new U.S.-Soviet "dialogue" that Reagan has started with the Soviets also holds open the prospect for a deal on arms control.

This, in the view of many analysts, risks stirring the wrath of his conservative constituency and is likely to stiffen congressional reluctance to spend more on defense, including the controversial Strategic Defense Initiative (SDI), also known as "Star Wars."

The same prospect is also likely to increase pressure on Reagan from West European allies if the Soviets maintain their demand that the SDI be abandoned as part of any agreement on substantial cuts in strategic or European-based intermediate-range nuclear weapons.

"The battle for the mind of the president will go on," said Stanley Hoffmann, professor of international relations at Harvard University, with "very strong pressure from the public and Congress." He referred to the tug-of-war between those in the administration who support or oppose an arms control agreement.

While the summit made Gorbachev "an instant world statesman," as Horelick put it, lack of concrete results on the key arms control issue -- his primary goal in going to Geneva -- has left him "vulnerable," according to Horelick.

Noting that the Soviet leader is still trying to consolidate his domestic position, John D. Steinbruner, head of the Brookings Institution's national security program, said Gorbachev "may have reassured his public, but he's got to deal with his military, too.

"They the Soviet military are going to ask, 'What did you do for us?' The answer is nothing. That's going to be trouble for him."

"Gorbachev can wait one more summit," Hoffmann said. "After that, he is going to be under very strong pressure from his military" to give up on Reagan.