From Nanosecond to Femtosecond Science

Abstract

The measurement of short time intervals during the first half of this century was limited to intervals longer than one nanosecond. Before 1900 it was already known that electrical sparks and Kerr-cell shutters could have response times as short as 10^‐8 s. Abraham and Lemoine (1899) used a discharge from condenser plates which simultaneously activated a spark gap and a Kerr-cell shutter. The condenser plates were placed in a cell containing carbon disulfide between crossed polarizer and analyzer. The light path between the spark source and the Kerr cell was variable. If the delay path was longer than four meters, no light was transmitted. This proved that the Kerr-cell response was faster than 10^‐8 s. We now know that the response of the Kerr effect in CS₂ is about two picoseconds. Thus Abraham and Lemoine really measured the duration of light emitted by the spark. This experiment used the essential feature of a variation in light path length for the measurement of short time intervals. In most picosecond and femtosecond time-resolved experiments, the delay between a pump and a probe pulse is varied by changes in optical path length. Norrish and Porter (1950) used flash lamps to excite photochemical reactions and probed them spectroscopically with varying delays in the millisecond to microsecond range. This work in “microchemistry” was recognized by the Nobel prize for chemistry in 1967.