Time - Resolved Spectroscopy Comes of Age

Cutting-edge instrumentation is a critical enabler of breakthroughs in every scientific field, and spectroscopy is one of the oldest and most venerated. Time-resolved spectroscopy (TRS) offers a new twist to standard spectroscopic techniques. Although TRS is not new, the development of ultrafast lasers and pulseshaping techniques, among other innovations, has opened up a wide range of nascent application areas, including test and measurement in the semiconductor industry, materials characterization, biological analysis, and archeological dating. What exactly constitutes a TRS technique remains somewhat nebulous. Andrew Monkman, a physicist and co-coordinator of the Center for Time-Resolved Spectroscopy (CTRS) at the University of Durham in England, broadly defines a TRS technique as “anything that allows you to measure the temporal dynamics and the kinetics of photophysical processes.” As an example, he cites the measurement of how an absorption band or fluorescence emission of a given material decays over time. Essentially, TRS uses something akin to a flash of strobe light to freeze a moment in time and a camera or a pulsed beam of light as a detector. The basic technique differs little from other spectroscopic methods. A sample is excited, most commonly by a pulsed laser, although researchers use other excitation methods as well. The resulting emissions and their decay times are then measured as a function of time, either by an ultrafast detector or a second pulse of laser light—an all-optical method also known as pump-probe spectroscopy. “Most spectroscopic techniques will have some sort of time-resolved aspect, because it is just a question of collecting a sequence of spectroscopic information” says Richard Jackson, senior applications scientist and manager of Fourier transform infrared (FTIR) applications for Bruker Optics (Billerica, MA) (Figure 4). TRS has its roots in the demonstration by cine photography that all four legs of a horse leave the ground when it gallops, a finding made late in the 19th century by Time-Resolved Spectroscopy Comes of Age