Abstract
Using a low power beam of ultrashort THz pulses that propagate in the ambient laboratory environment we have measured the rotational signatures of small molecule vapors at frequencies within the atmospheric transmission windows. We investigate two types of apparatus. In the first type the THz beam propagates along a 6.7 meter round trip path that is external to the spectrometer, and which contains a long sample tube (5.4 meter round trip path) that holds the analyte vapor. The environment of the tube is controlled to simulate dry or humid conditions. In the second apparatus the THz beam propagates over a much longer 170 meter round trip path with analyte vapor contained in a relatively short 1.2 meter round trip path sample chamber. We describe the rotational signatures for each apparatus in the presence of the strong interference from water vapor absorption. For the shorter path long-tube apparatus we find that the peak detection sensitivity is sufficient to resolve a 1% absorption feature. For the more challenging 170 meter path apparatus we find that the peak detection sensitivity is sufficient to resolve a 3-5% absorption feature. The experiments presented here represent a first step towards using ultrashort THz pulses for coherent broad band detection of small molecule gases and vapors under ambient conditions.
Highlights
The remote sensing of molecular gases and vapors in the region between 0.1 – 1.0 THz is a challenging problem, in part because of strong absorption of THz radiation by atmospheric water vapor
In this paper we describe experiments that detect the rotational spectra of small molecule vapors under conditions of ambient pressure and temperature, and where the THz beam propagates over relatively long path lengths that are external to the conventional short THz beam path (~50 cm) within the spectrometer
A single waveform is measured with a peak signal to noise ratio (S/N) of about 5000:1
Summary
The remote sensing of molecular gases and vapors in the region between 0.1 – 1.0 THz is a challenging problem, in part because of strong absorption of THz radiation by atmospheric water vapor. The far-infrared studies of water vapor in the atmosphere have concentrated on determining and understanding the water vapor continuum absorption with respect to its importance to ground-based astronomy, remote sensing, and satellite-based applications [5,6,7,8,9,10] These studies include measurements using single frequency sources [5] and broadband Fourier transform spectrometers (FTS) [6,7,8,9,10]. Even with significant losses due to water vapor absorption, diffraction, and coupling back to the receiver the returned THz pulse could be detected with high signal to noise (~200:1) This demonstration leads to the opportunity to investigate the use of long-path THz-TDS to remotely detect gases and vapors. We discuss the detection sensitivity achievable with the current apparatus and the types of molecular vapors for which specificity is achievable under ambient conditions
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