September 13, 1986

The stock market's plunge this week was as much a product of computer technology and Wall Street inventiveness as it was a reflection of new worries over the economy. Part of the market's decline clearly relates to emerging concerns on Wall Street over the failure of interest rates to come down further. High interest rates impose a drag on all economic activity, from the purchase of new homes and new cars to investment in factories and equipment, thus presaging a bleak period for the economy. But at least as important, experts say, is the new order on Wall Street, where the combination of exotic financial markets and instruments, and advanced computer capabilities, has introduced a degree of uncertainty -and volatility - heretofore unknown. Yesterday, the widely watched Dow Jones industrial average fell 34.17 points on record volume, on the heels of an 86.61-point drop Thursday, bringing the total decline for the week to 141.03 points, or 7.4 percent. Wall Street was shocked by the abruptness of it all.

The espionage arrest of a Soviet employee of the United Nations was carried out for a number of reasons, including a decision to confront the Soviet Union over the use of the United Nations for spying, according to Reagan Administration officials. They said other reasons for the decision to arrest the Russian, Gennadi F. Zhakarov, despite the possibility of Soviet retaliation, included a judgment by the Federal Bureau of Investigation that he was a relatively minor operative who could not be turned into a double agent or profitably fed false data. In addition, there was a strong feeling in the Administration that any spy caught ''red handed'' should be prosecuted, just as more than a dozen Americans have in the last year. Double Agent Involved The precise timing of the arrest, officials said, was based on Mr. Zhakarov's requests in early August for classified information from the double agent he thought he was using, officials said. The arrest was approved at least a week in advance by the State Department and White House, but the move was made without the advice of some of the Administration's most senior officials.

The United States and the Soviet Union today freed from jail a Soviet physicist in New York and an American journalist in Moscow to the custody of their respective embassies pending their trials. The agreement, orginally proposed by the Soviet Union, was announced by Secretary of State George P. Shultz at the White House, who made clear that Washington had accepted it grudgingly for the well-being of the American journalist, Nicholas S. Daniloff, Moscow correspondent for the magazine U.S. News & World Report. The United States has maintained throughout that Mr. Daniloff was innocent of any charges of spying and had been arrested on Aug. 30 to provide the Soviet side with someone to trade for Gennadi F. Zakharov, a physicist attached to the United Nations who was arrested on espionage charges in Queens on Aug. 23. Shultz Handled Negotiations Mr. Shultz, who handled the negotiations in an effort to head off a rapid deterioration of relations, was often testy today in his comments to reporters about the developments, which he acknowledged had failed to achieve the Administration's goal of securing Mr. Daniloff's immediate safe-passage to the United States.

The House overwhelmingly approved legislation today that would impose new economic sanctions on South Africa, including a ban on all new American investment. The vote was 308 to 77. Since the Senate adopted an identical measure last month, the bill now goes directly to President Reagan's desk. Representative William S. Broomfield of Michigan, ranking Republican on the Foreign Affairs Committee, said he received a ''clear signal'' from the White House today that Mr. Reagan ''doesn't like the bill in its current form'' and was likely to veto it.

President Hosni Mubarak and Prime Minister Shimon Peres ended the first Egyptian-Israeli summit meeting in five years today without resolving their basic differences over the region's central issue, the Palestinian question. But in a blandly worded joint statement, the two men declared that 1987 would be ''a year of negotiations for peace.'' The two leaders also said that there should be an international conference to seek a Middle East peace, and that ''in principle'' a committee should be appointed to look into this possibility. Some Gains in Ties Seen Significantly, however, the call for an international conference did not appear in the joint statement, which was read to reporters in a rococo chamber of the Ras el Tin Palace here. This suggested that the two sides had not been able to agree on who would attend. In the past, Israel, backed by the United States, has rejected participation by the Soviet Union or the Palestine Liberation Organization, which Arab governments have sought.