Line-Parameter Measurements and Stringent Tests of Line-Shape Models Based on Cavity-Enhanced Absorption Spectroscopy

<p>Laser methods that are based on cavity-enhanced absorption spectroscopy (CEAS) are well-suited for measuring molecular line parameters under conditions of low optical density, and as such they are complementary to broadband Fourier-transform spectroscopy (FTS) techniques. Attributes of CEAS include relatively low detection limits, accurate and precise detuning axes and high fidelity measurements of line shape. In many cases these performance criteria are superior to those obtained using direct laser absorption spectroscopy and FTS-based systems.</p> <p>In this presentation we will survey several examples of frequency-stabilized cavity ring-down spectroscopy (FS-CRDS)¹ measurements obtained with laser spectrometers developed at the National Institute of Standards and Technology (NIST) in Gaithersburg Maryland. These experiments, which are motivated by atmospheric monitoring and remote-sensing applications that require high-precision and accuracy, involve nearinfrared transitions of carbon dioxide, water, oxygen and methane. We discuss spectra with signal-to-noise ratios exceeding 10⁶, frequency axes with absolute uncertainties in the 10 kHz to 100 kHz range and linked to a Cs clock, line parameters with relative uncertainties at the 0.2 % level and isotopic ratios measured with a precision of 0.03 %. We also present FS-CRDS measurements of CO₂line intensities which are measured at atmospheric concentration levels and linked to gravimetric standards for CO₂in air, and we quantify pressure-dependent deviations between various theoretical line profiles and measured line shapes.</p> <p>Finally we also present recent efforts to increase data throughput and spectral coverage in CEAS experiments. We describe three new high-bandwidth CEAS techniques including frequency-agile, rapid scanning spectroscopy (FARS)², which enables continuous-wave measurements of cavity mode linewidth and acquisition of ringdown decays with no dead time during laser frequency tuning, heterodyne-detected cavity ring-down spectroscopy (HD-CRDS)^3<em>,</em>4, which offers shot-noise-limited statistics by interrogating ring-down decays at high frequencies, and finally multi-heterodyne cavity-enhanced spectroscopy (MH-CEAS)⁵, which provides wavelength-multiplexed measurements of both the amplitude and phase shift of the transmitted field.</p