Real-Time Studies of Molecular Self-Assembly in Thin Films Using a Planar Array Infrared (PA-IR) Spectrograph

John F. Rabolt
Department of Materials Science and Engineering
Delaware Biotechnology Institute
University of Delaware
Newark, Delaware

The invention of Fourier Transform IR (FT-IR) spectroscopy has proven to be one of the most important advances in modern instrumentation development. Optical spectroscopy utilizing the interference of light waves has made fast, sensitive detection of molecular vibration/rotation possible due to the large throughput and multiplex advantages provided by FT instrumentation. Interestingly enough, the same technological innovations, which have made FT-IR the instrument of choice for over thirty years, have also made FT-IR instruments extremely sensitive to their operating environment. As a result, the need for thermal stability, mechanical vibration isolation and stringent optical alignment has put severe limitations on where and how an FT-IR instrument can be used, confining it, for the most part, to the laboratory environment.

We have recently developed a new kind of IR instrument, which uses a focal plane array (FPA) detector onto which a beam of light dispersed by a prism or grating is focused. In this planar array IR (PA-IR) spectrograph, the broad range of frequencies displayed on the pixel array simultaneously make this a multiplex technique without using the complex scanning mechanism or computational requirements (for Fourier transformation of the data) used in FT-IR instruments. Thus the no-moving parts configuration of the PA-IR instrument provide the ruggedness required to make the instrument the size of a “shoe-box” and hence portable. In addition the increased sensitivity (100-1000X over single element FT-IR detectors) of the FPA allows an IR spectrum to be accumulated in as little as 10 microseconds. This speed in acquisition allows irreversible processes (e.g., film fracture, spontaneous chemisorption, Langmuir film compression, etc.) to be studied “real-time” and several examples of ultrafast IR studies of molecular orientation and organization will be given.