Laser-based isomer identification in the vapor phase

摘要

Infrared spectroscopy and mass spectrometry have long been reported as the most selective analytical techniques. Sensitive laser-based detection schemes for infrared spectroscopy such as photoacoustic, cavity ring-down or multipass transmission can be found in a countless number of gas sensing devices. These methods are however not suitable for recording vapors, particularly when high temperatures are involved. This issue has brought us to design and implement a high-temperature multipass cell (HTMC). This novel type of long path absorption cell is heatable up to 723 K and has a variable optical pathlength of up to 35 m. In addition to gases, a condensed sample can be introduced into the cell and analyzed in the vapor phase. The mirrors are separately heated in order to avoid condensation on their optical surface. Furthermore, a compensation mechanism for thermal expansion has been developed to prevent fatal optical misalignments. Infrared spectra of organic compounds in the vapor phase (using single pass heatable cells) reveal strong absorption bands in the mid-IR between 3 and 4 μm. Fingerprinting these rather large molecules requires wide continuous tuning of the laser wavelength. Promising broadly tunable mid-IR laser sources (operating at room temperature) include Cr{sup}(2+)-doped solid-state lasers, in the mid-IR below 3 μm, and external-cavity quantum-cascade lasers, in the mid-IR range above 4 μm. To reach the 3-4 μm range, we further developed a difference frequency generation-based (DFG) laser source (linewidth = 150 MHz). It employs a Q-switched diode-pumped Nd:YAG laser as pump source, and a fiber coupled external-cavity diode laser as signal. Frequency conversion takes place in a periodically poled Lithium Niobate (PPLN) crystal. In gas sensing applications, the DFG radiation is usually tuned over 1-3 cm{sup}(-1) as a result of a simple piezo scan. To meet our requirements, the continuous tuning range of the DFG laser was improved to over 300 cm{sup}(-1) by simultaneously tuning the crystal temperature and the step motor of the external cavity.