ABSTRACT
The development of the millisecond and microsecond flash photolysis experiments by George Porter and co-workers [1, 2] in the 1950s marks the true birth of time-resolved spectroscopy. Porter’s work, which provided for the first time a way to capture the absorption spectrum of a short-lived kinetic intermediate in a photochemical reaction, helped to start a new era in physical chemistry, one that was focused on the mechanism and dynamics of chemical reactions. Owing to the subsequent development of mode-locked laser sources, beginning with the picosecond ruby and neodymium-glass lasers in the 1960s, the sub-picosecond passively mode-locked dye laser in the late 1970s and, most recently, the femtosecond self-mode-locked Ti-sapphire laser in the early part of this decade, the time resolution for spectroscopic measurements has advanced three orders of magnitude, from the 10 ps to the 10 fs regime [3]. It is now possible to conduct a wide variety of spectroscopies with ultrashort laser pulses of photons selectable over the entire spectral range from the x-ray region [4, 5] to the terahertz or far-infrared (IR) region [6-10]. A variety of robust methods have been developed to probe the time evolution of populations and coherences. The shortest time scale that is now routinely accessible is comparable to or shorter than the period of molecular vibrations, the fundamental time scale of chemistry.
