Sensitive and selective detection of OH radicals using Faraday rotation spectroscopy at 28 µm

Abstract

We report on the development of a Faraday rotation spectroscopy (FRS) instrument using a DFB diode laser operating at 2.8 µm for the hydroxyl (OH) free radical detection. The highest absorption line intensity and the largest gJ value make the Q (1.5) double lines of the 2Π3/2 state (υ = 1 ← 0) at 2.8 µm clearly the best choice for sensitive detection in the infrared region by FRS. The prototype instrument shows shot-noise dominated performance and, with an active optical pathlength of only 25 cm and a lock-in time constant of 100 ms, achieves a 1σ detection limit of 8.2 × 10(8) OH radicals/cm3.

Magnetic field interaction with guided light for detection of an active gaseous medium within an optical fiber

“…Faraday Rotation Spectroscopy (FRS) is a highly sensitive and selective, background-free, spectroscopic technique that exploits the magneto-optic Faraday rotation effect to detect paramagnetic trace gases such as nitric oxide (NO) [9,10], molecular oxygen (O 2 ) [11], or free radical species like hydroxyl radicals (OH) [12]. In FRS, background absorption interference from common diamagnetic atmospheric species such as water (H 2 O), carbon dioxide (CO 2 ) and other non-paramagnetic molecules are eliminated.…”

“…In FRS, background absorption interference from common diamagnetic atmospheric species such as water (H 2 O), carbon dioxide (CO 2 ) and other non-paramagnetic molecules are eliminated. As described in [9][10][11][12], FRS probes the change in the state of polarization of a linearly polarized laser beam as it propagates through a gas cell containing paramagnetic molecules exposed to an external magnetic field. When the laser frequency is in resonance with a Zeeman-split absorption line of the paramagnetic molecular species, magnetic circular birefringence (MC-birefringence) and magnetic circular dichroism (MC-dichroism) are observed.…”

Cryogen-free heterodyne-enhanced mid-infrared Faraday rotation spectrometer

“…Since first reported in the 1980s [1], Faraday rotation spectroscopy (FRS) has been used as a sensitive and selective technique for the detection of gas-phase paramagnetic species such as NO [1][2][3][4][5][6][7], NO 2 [8,9], O 2 [10,11], and OH radicals [12,13]. In the presence of magnetic field, the transition states of the paramagnetic molecules split due to the Zeeman Effect causing magnetic circular birefringence (MCB, a difference in refractive indices for lefthanded (LHCP), and right-handed (RHCP) circularly polarized components) and magnetic circular dichroism (MCD, a difference in absorption coefficients for LHCP and RHCP).…”

“…The most common practices reported to-date in the literature to improve FRS system performance were mostly focused on increase in the FRS signal strength, and on optimization of the analyzer offset angle θ. Several strategies focused on the FRS signal increase have been reported including: selection of molecular transitions with higher intensity [4][5][6][7]13], providing optimal magnetic field strength [5,7,10], optimizing the sample pressures [5,10,11], using higher laser power or minimizing the losses in the gas cell [7], increasing the optical path with multi-pass cells [15,16], as well as applying cavity enhanced techniques [17]. Since analyzer offset angle θ (measured from the crossed position) affects both the signal strength as well as the total noise received by the photodetector, optimization of θ must be performed for each FRS system individually.…”

“…This fundamental limit represents the minimum detectable fluctuation of the field at the level of 1/N 1/2 , where N is a mean photoelectron number generated by the photodetector. As recently demonstrated by Zhao et al, when very low optical power (at µW level) is used in combination with a sensitive thermoelectrically cooled (TEC) mercury cadmium telluride (MCT) photodetector, a shot-noise-dominated FRS of OH radicals at ~2.8 μm could be realized [13]. However up to now, no cryogen-free FRS system has been able to approach the fundamental limit in the mid-IR spectral range at wavelengths >3µm.…”