Lineshape analysis using THz time domain spectroscopy

Summary form only given. The experimental technique of THz time domain spectroscopy (THz-TDS) has been shown to accurately measure absorption lineshapes of molecular vapors in the far infrared spectral region. In an ideal THz-TDS experiment involving molecular vapors, absorption and phase changes of the electromagnetic field due to molecular rotational transitions modulate the measured THz pulse electric field upon transmission through a gas sample. The amplitude and phase modulation are related through the lineshape, which in turn is determined by collisional dephasing and detailed molecular interactions during the collision through the dipole autocorrelation function. To identify a particular gas species, the complex spectral amplitude of the THz pulse after propagating through the gas sample, E/sub s/(/spl omega/), is obtained from the Fourier transform of the time dependent electric field and compared to known rotational transitions. The pulse is implicitly assumed to have infinite temporal extent. The sample spectrum, E/sub s/(/spl omega/), divided the spectrum of a reference pulse propagated through free space, E/sub r/(/spl omega/), determines a unique transfer function, H(/spl omega/): E/sub r/(/spl omega/)/E/sub r/(/spl omega/)=H(/spl omega/)=e/sup ik(/spl omega/)/L with L the length of the sample. Given this complex transfer function, quantitative measurements of the gas sample environment are extracted from the line strength, width, and shape. Using the more traditional propagation vector, k(/spl omega/), the absorption coefficient is obtained from the magnitude of this ratio, and refractive index from the phase.