Frequency‐modulation enhanced magnetic rotation spectroscopy: A sensitive and selective absorption scheme for paramagnetic molecules

High band-width mid-infrared frequency-modulated Faraday rotation spectrometer for time resolved measurement of the OH radical

We present a novel mid-infrared frequency-modulated Faraday rotation spectrometer (FM-FRS) for highly sensitive and high bandwidth detection of OH radicals in a photolysis reactor. High frequency modulation (up to 150 MHz) of the probe laser using an electro-optical modulator (EOM) was used to produce a modulation sideband on the laser output. An axial magnetic field was applied to the multi-pass Herriott cell, causing the linearly polarized light to undergo Faraday rotation. OH radicals were generated in the cell by photolyzing a mixture of ozone (O₃) and water (H₂O) with a UV laser pulse. The detection limit of OH reaches 6.8 × 10⁸ molecule/cm³ (1σ, 0.2 ms) after 3 and falling to 8.0 × 10⁷ molecule/cm³ after 100 event integrations. Relying on HITRAN absorption cross section and line shape data, this corresponds to minimum detectable fractional absorption (A_min) of 1.9 × 10^-5 and 2.2 × 10^-6, respectively. A higher signal-to-noise ratio and better long-term stability was achieved than with conventional FMS because the approach was immune to interference from diamagnetic species and residual amplitude modulation noise. To our knowledge, this work reports the first detection of OH in a photolysis reactor by FM-FRS in the mid-infrared region, a technique that will provide a new and alternative spectroscopic approach for the kinetic study of OH and other intermediate radicals.

Faraday rotation spectroscopy based on permanent magnets for sensitive detection of oxygen at atmospheric conditions

A low-power Faraday rotation spectroscopy system that uses permanent rare-earth magnets has been developed for detection of O₂ at 762 nm. The experimental signals are generated using laser wavelength modulation combined with a balanced detection scheme that permits quantum shot noise limited performance. A noise equivalent polarization rotation angle of 8 × 10⁻⁸ rad/Hz¹/² is estimated from the experimental noise, and this agrees well with a theoretical model based on Jones calculus. A bandwidth normalized minimum detection limit to oxygen of 6 ppmv/Hz¹/² with an ultimate minimum of 1.3 ppmv at integration times of ~1 minute has been demonstrated.

Noninvasive magnetometry based on magnetic rotation spectroscopy of oxygen

An experimental demonstration of a noninvasive optical probe of magnetic fields is presented. The technique used is magnetic rotation spectroscopy of the b(1)?(g)(+) - X(3)?(g)(-) band of oxygen near 760 nm. Ambient concentrations of oxygen at atmospheric pressure are sufficient to observe substantial signals. In addition, a diode laser is used as the light source, making this a simple and compact measurement possibility.

Laser Frequency-Modulated Spectroscopy of a Laser-Guided Plasma in Sodium Vapor: Line Positions for NaH (A1Sigma+-X1Sigma+), Na (9-13d and 11-14s), and Ar (5p-4s)

Laser frequency-modulated (FM) spectroscopy has been used as an axial probe of a laser-guided electric discharge in sodium-argon vapor contained in an optically accessible metal heat pipe oven. Absorption measurements in the region 23 106-23 881 cm-1 provided accurate line positions (&plusmn;<0.006 cm-1) for 141 transitions in the v' = 3-8 <-- v" = 0 and v' = 5-9 <-- v" = 1 bands of NaH (A1Sigma+-X1Sigma+). In addition, 18 transitions of Na (3p to 9-13d and 11-14s) and 10 of argon (5p-4s) were measured. Analysis of the spectrum indicates that perhaps all absorption signals are due to neutrals NaH, Na, and Ar and are observed via "population" modulation. Copyright 1997 Academic Press. Copyright 1997Academic Press

Frequency-modulation spectroscopy with transform-limited nanosecond laser pulses

We demonstrate high-quality FM spectra with nanosecond laser pulses. Transform-limited pulses with FM sidebands are produced by pulsed amplification of a phase-modulated cw laser. The pulses can be shifted to the UV by nonlinear mixing. We report both initial experiments on I(2) and what is to our knowledge the first observation of a far-UV transition by FM spectroscopy, at 214.5 nm. Major advantages of this method include (1) spectral resolution of the order of 0.001 cm(-1), (2) better-defined optical phase, and (3) a much smaller and more easily detected modulation frequency, ~500 MHz. The absorption sensitivity is ~10(-4), and considerable further improvement is expected.