Step-and-Integrate Interferometry in the Mid-Infrared with Photothermal Beam Deflection and Sample-Gas-Microphone Detection

Abstract

The utility of photothermal (PT) detection in Fourier transform infrared spectrometry (FTIR-PTS) has been amply illustrated during the past several years. However, the rapid scanning mode, which is characteristic of all current commercial FTIR instruments, has several inherent disadvantages for PT detection, which have prevented the full realization of the potential of the method. The source of the problem is the temporal modulation of the spectrometer beam intensity, which also naturally creates the PT modulation in rapid scanning FTIR-PTS. The fact that the Fourier frequencies are different for each wavelength and that, for even the lowest practical mirror velocities, these frequencies are relatively high for mid-infrared wavelengths, has three negative consequences for PT detection: (1) lock-in amplification is not possible, (2) the thermal diffusion depth is different for all wavelengths and (3) the modulation frequencies are generally higher than desirable for optimum PT signal strength. These problems are shared by both common types of photothermal detection, sample-gas-microphone and PT beam deflection (the mirage effect). However, the lack of lock-in amplification is particularly serious for mirage detection because of its generally greater sensitivity to external vibration.[1]