Optical frequency comb photoacoustic spectroscopy

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Sadiek , I , Mikkonen , T , Vainio , M , Toivonen , J & Foltynowicz , A 2018 , ' Optical frequency comb photoacoustic spectroscopy ' , Physical Chemistry Chemical Physics , vol. 20 , no. 44 , pp. 27849-27855 . https://doi.org/10.1039/c8cp05666h

Title: Optical frequency comb photoacoustic spectroscopy
Author: Sadiek, Ibrahim; Mikkonen, Tommi; Vainio, Markku; Toivonen, Juha; Foltynowicz, Aleksandra
Contributor: University of Helsinki, Department of Chemistry
Date: 2018-11-28
Language: eng
Number of pages: 7
Belongs to series: Physical Chemistry Chemical Physics
ISSN: 1463-9076
URI: http://hdl.handle.net/10138/312787
Abstract: We report the first photoacoustic detection scheme using an optical frequency comboptical frequency comb photoacoustic spectroscopy (OFC-PAS). OFC-PAS combines the broad spectral coverage and the high resolution of OFCs with the small sample volume of cantilever-enhanced PA detection. In OFC-PAS, a Fourier transform spectrometer (FTS) is used to modulate the intensity of the exciting comb source at a frequency determined by its scanning speed. One of the FTS outputs is directed to the PA cell and the other is measured simultaneously with a photodiode and used to normalize the PA signal. The cantilever-enhanced PA detector operates in a non-resonant mode, enabling detection of a broadband frequency response. The broadband and the high-resolution capabilities of OFC-PAS are demonstrated by measuring the rovibrational spectra of the fundamental C-H stretch band of CH4, with no instrumental line shape distortions, at total pressures of 1000 mbar, 650 mbar, and 400 mbar. In this first demonstration, a spectral resolution two orders of magnitude better than previously reported with broadband PAS is obtained, limited by the pressure broadening. A limit of detection of 0.8 ppm of methane in N-2 is accomplished in a single interferogram measurement (200 s measurement time, 1000 MHz spectral resolution, 1000 mbar total pressure) for an exciting power spectral density of 42 W/cm(-1). A normalized noise equivalent absorption of 8 x 10(-10) W cm(-1) Hz(-1/2) is obtained, which is only a factor of three higher than the best reported with PAS based on continuous wave lasers. A wide dynamic range of up to four orders of magnitude and a very good linearity (limited by the Beer-Lambert law) over two orders of magnitude are realized. OFC-PAS extends the capability of optical sensors for multispecies trace gas analysis in small sample volumes with high resolution and selectivity.
116 Chemical sciences
114 Physical sciences

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