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Seminar, Wednesday, April 21, 2010 at 3:30 pm

Guided wave terahertz characterization of vibrational fingerprint lines in molecular solids
Dr. Joseph Melinger
Naval Research Laboratory

One of the outstanding problems of terahertz (THz) spectroscopy is the measurement of the underlying vibrational spectrum of a molecular solid, where individual vibrational transitions are often merged into broad absorption features by line broadening processes. In this talk I describe how this problem can be addressed using the technique of waveguide terahertz time-domain spectroscopy (THz-TDS). In this technique an analyte molecular film is deposited on one of the inner surfaces of a single-mode metal parallel plate waveguide (PPWG) with a 50 micron gap between the plates. The vibrational spectrum of the film is then measured in a sensitive manner using sub-picosecond THz pulses confined within the PPWG over a relatively long pathlength of a few centimeters. I will show how waveguide THz-TDS (applied at cryogenic temperatures) has proven useful in resolving the underlying vibrational fingerprint spectrum of a variety of explosives solids in the frequency range between 0.2 THz – 4.0 THz, with linewidths of individual transitions as narrow as 7 GHz (0.21 cm 1), and line frequencies determined to a precision of 1 GHz (0.03 cm-1). I’ll also discuss how these highly resolved spectra are currently being modeled using solid state computational methods.