Before 1900 it was already known that short flashes of light permit recording rapid phenomena and uncover truths that the eye is incapable of perceiving. By recording freeze-frame snapshots of a galloping horse via spark photography, Muybridge (1878) was able to show that at certain instants all four legs of the horse were off the ground simultaneously (movie). Toepler (1864) extended spark photography to studying microscopic dynamics. He generated sound waves with a short light spark (pump) and subsequently photographed them with an electronically-delayed second spark (probe). Pump-probe (time-resolved) spectroscopy was born. Abraham and Lemoine (1899) introduced optical synchronism between the pump and the probe flash, improving thereby temporal resolution to the limit set by the flash duration. With these milestones the conceptual framework for studying transient microscopic phenomena was complete. Subsequent progress relied on the development of sources of ever shorter flashes of light and techniques for their measurement.

Time resolution was limited to nanoseconds for more than half a century. A revolution in this technology was required to end this standstill. It came with the invention of the laser. Successive technological developments for the generation and measurement of ultrashort laser pulses improved the resolving power of pump-probe spectroscopy from several nanoseconds to several femtoseconds, by six orders of magnitude within merely two and a half decades. Four decades after the first observation of intermediates of chemical reactions by Eigen, Norrish and Potter (Fig.1), femtosecond technology permitted Ahmed Zewail and coworkers real-time observation of the breakage and formation of chemical bonds (1999: Nobel Prize in Chemistry). Resolving the thousand times faster electronic motion called for advancing time-resolved metrology into the attosecond domain. This second revolution in laser-technology came about at the last turn of the millennium.