Mass-Selective Laser Photoionization R. E. Smalley Rice Quantum institute and Department of Chemistry, Rice University, Houston, TX 77251 Can we detect a single molecule? Suppose we have a tremendously good vacuum such as those just outside the cargo hay doors of the space shuttle. On the average there is only one molecule per cuhic centimeter here, a density less than one part in l O I 9 of that in the air near sea level. Could that one molecule in a cuhic centimeter he detected? The answer is, "Sure, it's simple!" The following incredibly sensitive technique has been known for many years. One simply has to cause an electron to be ejected such that the molecule becomes a singly-charged positive molecular ion. Once that is done, the ion is easy to detect with virtually 100% efficiency. The ion is merely accelerated through a strong electric field and forced to slam into a metal target a t high energy. This causes the metal target to eject a few electrons (called "secondary electrons") which can then be accelerated to hit another target where they eject further secondary electrons. This nrocess can be continued with a series of successive targets (called "dynodes") until an avalanche of over ten million electrons has been oroduced all as a result of heine triggered as the i~n:lr mulecullrr ion slammed intar the first is called an mi,tul target. Th:s arra!. 01 metal turret " dvnodes . electron multiplier-asuperb example of a device which converts a single event on a microscopic, molecular scale into a macroscopic observable. The pulse of 10" secondary electrons is sent down a wire and allowed to drop through an ordinary 50 ohm resistor. If the electron multiplier is well designed, the electron pulse will he quite narrow in time-usuall;only 10-8 sec. A charge of 107 electrons flowing in a time interval of only 10P"'ec constitutes a substantial current of 10'5 electrons per sec or 1.6 X 10-4 ampere. Using Ohm's Law, this current pulse across the 50 ohm resistor can he seen to produce a voltage pulse of 50 ohms X 1.6 X 10-4 amps = 8 X V. Eight millivolts may not sound like much, hut to modern electronics such a voltage pulse is trivially easy to detect. I t is a huge, unmistakable event. The entire detection anoaratus can easilv he made such .. that I I O pulsrs of comparal)li~magnitude i~r; generated by randurn, spurlow events. .411 is quiet until un ion slams into the electron multiplier. Thi, voltage pulse which results can t,c usrd 10 light a hghr, trigger an illarm, ur just quietly tell a computrr. "I have d c r c ~ t e dan ion." The onlv remaining question is how togo ahout ejecting an electron i n m that onr mulecule per cuhic cenrunetrr in order to make the pusitiw i m . 11 is here thnt t h r laser bus made a substantial tmrribution within the past iew ymrs. A i will be d~icus. -"",
