A variety of frequency-modulation methods for high-sensitivity absorption detection of gas-phase species has evolved in recent years. The distinctions among these methods are mostly semantic. The mathematical derivations for wavelength-modulation spectroscopy and one- and two-tone frequency-modulation spectroscopies are presented; a common terminology is used to permit a comprehensive comparison of predicted detection sensitivities. Applying this formalism, I compare the optimum detection sensitivities of these different methods for a typical laser system, using the same parameters. As long as residual amplitude modulation is minimized by proper adjustment of the detection phase angle, high-frequency wavelength modulation and one- and two-tone frequency-modulation methods all achieve approximately the same sensitivities. The choice among techniques is most strongly driven by the individual laser tuning characteristics, the absorption linewidth, and the detection bandwidth. It is shown that excess laser noise cannot always be excluded from consideration, even at megahertz detection frequencies. Also, detection at harmonics of the modulation or beat frequency may present certain advantages in minimizing residual amplitude-modulation noise.
Wavelength modulation spectroscopy (WMS) and one-tone and two-tone frequency modulation spectroscopy (FMS) are compared by measuring the minimum detectable absorbances achieved using a mid-IR lead-salt diode laser. The range of modulation and detection frequencies spans over 5 orders of magnitude. The best results, absorbances in the low-to-mid 10(-7) range in a 1-Hz bandwidth, are obtained by using high-frequency WMS (10-MHz detection frequency) and are limited by detector thermal noise. This sensitivity can provide species detection limits well below 1 part per billion for molecules with moderate line strengths if multiple-pass cells are used. High-frequency WMS is also tested by measuring the absorbance due to tropospheric N(2)O at 1243.795 cm(-1). WMS at frequencies < 100 kHz is limited by laser excess (1/f) noise. Both of the FMS methods, which require modulating the laser at frequencies >/= 150 MHz, give relatively poor results due to inefficient coupling of the modulation waveform to the laser current. The re ults obtained agree well with theory. We also discuss the sensitivity limitations due to interference fringes from unintentional étalons and the effectiveness of étalon reduction schemes.
[1] A vertical cavity diode laser hygrometer using two absorption lines near 1854 nm has been developed for the National Science Foundation Gulfstream-V aircraft. This instrument operates from the surface to the lower stratosphere, measuring 6 orders of magnitude in water vapor concentration. The optical system consists of an open-path multiple-pass cell mounted on an aerodynamic pylon. The self-operating hygrometer reports concentration in real time at 25 Hz and uses a novel approach for fitting the data with minimal correction terms for changes in ambient pressure and temperature. The instrument intercompares with existing research grade hygrometers in the 2%-10% range. A minimum detection limit of 3.6 × 10 11 molecules/cm 2 or 0.08 ppmv at 15 km altitude conditions is achieved. The design rationale, operation, and flight performance of the hygrometer are described in this work.
Multiple-pass optical cells with dense spot patterns are useful for many applications, especially when the cell volume must be minimized relative to the optical path length. Present methods to achieve these dense patterns require expensive, highly precise astigmatic mirrors and complex alignment procedures. This work describes a new, simpler, and less demanding mirror system, comprising either a pair of cylindrical mirrors or one cylindrical and one spherical mirror.
The rate constant for the reaction NH2 + NO -* products is measured in a high-temperature flow reactor over the range K and fe(cm3 molecule"1 s"1) = [(4.38 ± 0.70) X 10~6]7'"(2•30±0•02) exp [(-1360 ± 120) cal mol"1/#7], At 298 K, k = 9.0 X 10~12 cm3 molecule"1 s"1. Two major product channels are found, one forming N2 + H20, and the second OH and either N2 + H or N2H. No atomic hydrogen is observed at 300 K by using a Lyman-a resonance lamp. The fraction of room-temperature reactions forming OH has been measured as 0.4 ± 0.1. The importance of identifying product channels is discussed in relation to atmospheric and combustion processes.
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