Laser–material interactions probed with picosecond infrared spectroscopy

Picosecond infrared spectroscopy in the mid-IR has been applied to the study of laser–material interactions on an ultrafast time scale. We excite the sample with one pulse at 1064 or 266 nm and probe some time later in the mid IR (2900–1400 cm−1). We study photochemical reactions by exciting the sample directly in the UV. Alternatively, we initiate thermal reactions by exciting a `heaterʹ dye at 1.064 μm, which quickly converts the photon energy into heat. The potential of this technique to study reactions in the solid state was demonstrated for the photochemically induced (266 nm) Wolff rearrangement in a polymer matrix. This reaction is the basis of most positive photoresists. We were able to assign the key intermediate (ketene) in real time at room temperature for the first time. Thermal initiation also resulted in Wolff rearrangement. The position, amplitude and width of the product bands changed, probably due to additional side reactions and temperature effects.