Mid-infrared laser magnetic resonance using the Faraday and Voigt effects for sensitive detection

Dual-comb optical activity spectroscopy for the analysis of vibrational optical activity induced by external magnetic field

Optical activity (OA) spectroscopy is a powerful tool to characterize molecular chirality, explore the stereo-specific structure and study the solution-state conformation of biomolecules, which is widely utilized in the fields of molecular chirality, pharmaceutics and analytical chemistry. Due to the considerably weak effect, OA spectral analysis has high demands on measurement speed and sensitivity, especially for organic biomolecules. Moreover, gas-phase OA measurements require higher resolution to resolve Doppler-limited profiles. Here, we show the unmatched potential of dual-comb spectroscopy (DCS) in magnetic optical activity spectroscopy (MOAS) of gas-phase molecules with the resolution of hundred-MHz level and the high-speed measurement of sub-millisecond level. As a demonstration, we achieved the rapid, high-precision and high-resolution MOAS measurement of the nitrogen dioxide [Formula: see text]+[Formula: see text] band and the nitric oxide overtone band, which can be used to analyze fine structure of molecules. Besides, the preliminary demonstration of liquid-phase chiroptical activity (as weak as 10^-5) has been achieved with several seconds of sampling time, which could become a routine approach enabling ultrafast dynamics analysis of chiral structural conformations.

Intracavity Faraday modulation spectroscopy (INFAMOS): A tool for radical detection

We present the intra-cavity Faraday modulation spectroscopy technique, whereby optical feedback cavity-enhanced spectroscopy is coupled with Faraday modulation spectroscopy to greatly enhance the interaction path length of a laser beam with a paramagnetic sample in a magnetic field. We describe a first prototype based upon a cw quantum cascade laser targeting a selection of fundamental rovibrational R-branch transitions of nitric oxide (1890 cm^-1), consisting of a linear cavity (finesse F=6300) and a water-cooled solenoid. We demonstrate a minimum detectable Verdet constant of V_min=4.7×10^-14 rad cm^-1 G^-1 Hz^-1/2 (at SNR = 1), corresponding to a single-pass rotation angle of 1.6×10^-10 rad Hz^-1/2 and a limit of detection of 0.21 ppbv Hz^-1/2 NO.

Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π - A2Σ+ band: I. Theory

We give a simple two-transition model of Faraday modulation spectrometry (FAMOS) addressing the electronic X(2)Π(ν('') = 0) - A(2)Σ(+)(ν(') = 0) band in nitric oxide. The model is given in terms of the integrated line strength, S, and first Fourier coefficients for the magnetic-field-modulated dispersive line shape function. Although the two states addressed respond differently to the magnetic field (they adhere to the dissimilar Hund coupling cases), it is shown that the technique shares some properties with FAMOS when rotational-vibrational Q-transitions are targeted: the line shapes have a similar form and the signal strength has an analogous magnetic field and pressure dependence. The differences are that the maximum signal appears for larger magnetic field amplitudes and pressures, ∼1500 G and ∼200 Torr, respectively.

Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π - A2Σ+ band: II. Experiment

A first demonstration of Faraday modulation spectrometry (FAMOS) of nitric oxide (NO) addressing its strong electronic X(2)Π(ν″ = 0) - A(2)Σ(+)(ν(') = 0) band is presented. The instrumentation was constructed around a fully diode-laser-based laser system producing mW powers of ultraviolet light targeting the overlapping Q(22)(21/2) and R(12)Q(21/2) transitions at ∼226.6 nm. The work verifies a new two-transition model of FAMOS addressing the electronic transitions in NO given in an accompanying work. Although the experimental instrumentation could address neither the parameter space of the theory nor the optimum conditions, the line shapes and the pressure dependence could be verified under low-field conditions. NO could be detected down to a partial pressure of 13 µTorr, roughly corresponding to 10 ppb·m for an atmospheric pressure sample, which demonstrates the feasibility of FAMOS for sensitive detection of NO addressing its strong electronic band.

Ultrasensitive detection of nitric oxide at 5.33 microm by using external cavity quantum cascade laser-based Faraday rotation spectroscopy

A transportable prototype Faraday rotation spectroscopic system based on a tunable external cavity quantum cascade laser has been developed for ultrasensitive detection of nitric oxide (NO). A broadly tunable laser source allows targeting the optimum Q(3/2)(3/2) molecular transition at 1875.81 cm(-1) of the NO fundamental band. For an active optical path of 44 cm and 1-s lock-in time constant minimum NO detection limits (1sigma) of 4.3 parts per billion by volume (ppbv) and 0.38 ppbv are obtained by using a thermoelectrically cooled mercury-cadmium-telluride photodetector and liquid nitrogen-cooled indium-antimonide photodetector, respectively. Laboratory performance evaluation and results of continuous, unattended monitoring of atmospheric NO concentration levels are reported.

LMR spectroscopy: a new sensitive method for on-line recording of nitric oxide in breath

Laser magnetic resonance spectroscopy (LMRS) is a sensitive and isotope-selective technique for determining low concentrations of gaseous free radicals with high time resolution. We used this technique to analyze the nitric oxide (NO) concentration profile while simultaneously measuring the flow and expired volume during several single breathing cycles. Eight healthy, nonallergic volunteers were investigated. An initial NO peak was found in all breathing cycles before the NO concentration dropped to a relatively stable plateau in the late phase of expiration. The nasal NO peak was significantly higher than the oral NO peak. The nasal NO plateau was always higher than the oral NO plateau. The height of the initial nasal and oral NO peak rose with increasing duration of breath hold, whereas the late expiratory NO plateau changed only little for either the nasal or the oral breathing cycles. Our findings demonstrate, in line with other reports using other techniques, that the nose is the primary source for NO within the airways.

Faraday Laser Magnetic Resonance Spectroscopy of Vibrationally Excited C2D

We studied the gas phase spectrum of the deuterated ethynyl radical C2D in the region between 3196 and 3243 cm-1 using a Faraday LMR spectrometer in combination with a CO overtone laser. The C2D radicals were generated in a dc glow discharge containing helium, deuterium, and acetylene. We observed a hot band between two vibronic 2Pi states with an origin at 3225 cm-1. The lower level is assigned to the first excited bending level of the electronic X ground state. The upper level corresponds to the first excited electronic state A at 3513 cm-1, which was observed previously [J. Mol. Struct. 190, 41-60 (1988)]. This region is subject to strong vibronic interaction, caused by mixing of the electronic X ground state with the A state at 3513 cm-1. From the analysis of the spectra we could determine the orbital g factor of the upper level, which gave important information about the mixing ratios. In addition we were able to derive a precise term value for the first excited bending level of the electronic X ground state. The experimentally derived molecular parameters are compared with theoretically calculated values, obtained by ab initio calculations. Copyright 1998 Academic Press.

Prognosis for a mid-infrared magnetic rotation spectrometer for the in situ detection of atmospheric free radicals

Mid-infrared magnetic rotation spectroscopy (MRS) experiments on nitric oxide (NO) are quantitatively modeled by theoretical calculations. The verified theory is used to specify an instrument that can make in situ measurements on NO and NO(2) in the Earth's atmosphere at a sensitivity level of a few parts in 10(12) by volume per second. The prototype instrument used in the experiments has an extrapolated detection limit for NO of 30 parts in 10(9) for a 1-s integration time over a 12-cm path length. The detection limit is an extrapolation of experimental results to a signal-to-noise ratio of one, where the noise is considered to be one-half the peak-to-peak baseline noise. Also discussed are the various factors that can limit the sensitivity of a MRS spectrometer that uses liquid-nitrogen-cooled lead-salt diode lasers and photovoltaic detectors.