Joseph T. Hodges

Laser bandwidth effects in quantitative cavity ring-down spectroscopy

We have investigated the effects of laser bandwidth on quantitative cavity ring-down spectroscopy using the (r)R transitions of the b(ν = 0)?X(ν = 0) band of molecular oxygen. It is found that failure to account properly for the laser bandwidth leads to systematic errors in the number densities determined from measured ring-down signals. When the frequency-integrated expression for the ring-down signal is fitted and measured laser line shapes are used, excellent agreement between measured and predicted number densities is found.

Recommended isolated-line profile for representing high-resolution spectroscopic transitions (IUPAC Technical Report)

Abstract The report of an IUPAC Task Group, formed in 2011 on “Intensities and line shapes in high-resolution spectra of water isotopologues from experiment and theory” (Project No. 2011-022-2-100), on line profiles of isolated high-resolution rotational-vibrational transitions perturbed by neutral gas-phase molecules is presented. The well-documented inadequacies of the Voigt profile (VP), used almost universally by databases and radiative-transfer codes, to represent pressure effects and Doppler broadening in isolated vibrational-rotational and pure rotational transitions of the water molecule have resulted in the development of a variety of alternative line-profile models. These models capture more of the physics of the influence of pressure on line shapes but, in general, at the price of greater complexity. The Task Group recommends that the partially Correlated quadratic-Speed-Dependent Hard-Collision profile (pCqSD-HCP) should be adopted as the appropriate model for high-resolution spectroscopy. For simplicity this should be called the Hartmann–Tran profile (HTP). The HTP is sophisticated enough to capture the various collisional contributions to the isolated line shape, can be computed in a straightforward and rapid manner, and reduces to simpler profiles, including the Voigt profile, under certain simplifying assumptions.

Frequency-Stabilized Single-Mode Cavity Ring-Down Apparatus for High-Resolution Absorption Spectroscopy

We present a cavity ring-down spectroscopy apparatus suitable for high-resolution absorption spectroscopy. The central feature of the spectrometer is a ring-down cavity whose comb of eigenfrequencies is actively stabilized with respect to a tuneable, frequency-stabilized reference laser. By using dichroic ring-down cavity mirrors that are designed to have relatively high losses and low losses at the respective wavelengths of the reference laser and probe laser, the cavity stabilization dynamics are decoupled from frequency jitter of the probe laser. We use the cavity eigenfrequencies as markers in spectral scans and achieve a frequency resolution of ≈1 MHz. Five rovibrational transitions in the (2,0,1) vibrational band of water vapor near 0.935 μm are probed with a continuous-wave external-cavity diode laser, and their line strengths are determined and compared to literature values. Collisional narrowing effects and pressure shifting are observed, illustrating the applicability of the method for quantitat...

Response of a ring-down cavity to an arbitrary excitation

An eigenmode analysis of the response of an empty ring‐down cavity to an arbitrary laser excitation is presented. By explicitly taking into account both the mode structure of the ring‐down cavity and the spectral content of the laser pulse, it is found that the complicated ring‐down signals commonly observed in the laboratory can be interpreted in terms of cavity mode beating. Some conclusions drawn from this analysis are verified experimentally by measurements of the time and frequency response of empty cavities. These observations provide clear evidence for the existence of longitudinal and transverse mode structures in ring‐down cavities.

Pulsed, single-mode cavity ringdown spectroscopy

We discuss the use of single-mode cavity ringdown spectroscopy with pulsed lasers for quantitative gas density and line strength measurements. The single-mode approach to cavity ringdown spectroscopy gives single exponential decay signals without mode beating, which allows measurements with uncertainties near the shot-noise limit. The technique is demonstrated with a 10-cm-long ringdown cavity and a pulsed, frequency-stabilized optical parametric oscillator as the light source. A noise-equivalent absorption coefficient of 5 x 10(-10) cm(-1) Hz(-1/2) is demonstrated, and the relative standard deviation in the ringdown time (sigma(tau)/tau) extracted from a fit to an individual ringdown curve is found to be the same as that for an ensemble of hundreds of independent measurements. Repeated measurement of a line strength is shown to have a standard deviation <0.3%. The effects of normally distributed noise on quantities measured using cavity ringdown spectroscopy are discussed, formulas for the relative standard deviation in the ringdown time are given in the shot- and technical-noise limits, and the noise-equivalent absorption coefficient in these limits are compared for pulsed and continuous-wave light sources.

Multiheterodyne spectroscopy with optical frequency combs generated from a continuous-wave laser

Dual-drive Mach-Zehnder modulators were utilized to produce power-leveled optical frequency combs (OFCs) from a continuous-wave laser. The resulting OFCs contained up to 50 unique frequency components and spanned more than 200 GHz. Simple changes to the modulation frequency allowed for agile control of the comb spacing. These OFCs were then utilized for broadband, multiheterodyne measurements of CO2 using both a multipass cell and an optical cavity. This technique allows for robust measurements of trace gas species and alleviates much of the cost and complexity associated with the use of femtosecond OFCs produced with mode-locked pulsed lasers.

The air-broadened, near-infrared CO2 line shape in the spectrally isolated regime: Evidence of simultaneous Dicke narrowing and speed dependence

Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) was employed to measure air-broadened CO(2) line shape parameters for transitions near 1.6 μm over a pressure range of 6.7-33 kPa. The high sensitivity of FS-CRDS allowed for the first measurements in this wavelength range of air-broadened line shape parameters on samples with CO(2) mixing ratios near those of the atmosphere. The measured air-broadening parameters show several percent deviations (0.9%-2.7%) from values found in the HITRAN 2008 database. Spectra were fit with a variety of models including the Voigt, Galatry, Nelkin-Ghatak, and speed-dependent Nelkin-Ghatak line profiles. Clear evidence of line narrowing was observed, which if unaccounted for can lead to several percent biases. Furthermore, it was observed that only the speed-dependent Nelkin-Ghatak line profile was able to model the spectra to within the instrumental noise level because of the concurrent effects of collisional narrowing and speed dependence of collisional broadening and shifting.

Photoacoustic Measurements of Amplification of the Absorption Cross Section for Coated Soot Aerosols

A quantitative understanding of the absorption and scattering properties of mixed soot and aerosol particles is necessary for evaluating the Earths energy balance. Uncertainty in the net radiative forcing of atmospheric aerosols is relatively large and may be limited by oversimplified models that fail to predict these properties for bare and externally mixed soot particles. In this laboratory study of flame-generated soot, we combine photoacoustic spectroscopy, particle counting techniques, and differential mobility analysis to obtain high-precision measurements of the size-dependent absorption cross section of uncoated and coated soot particles. We investigate how the coating of soot by nonabsorbing films of dibutyl phthalate (chosen as a surrogate for sulfuric acid) affects the particles’ morphology and optical properties. Absorption measurements were made with photoacoustic spectroscopy using a laser at λ = 405 nm. We report measurements and model calculations of the absolute cross section, mass absorption coefficient, and amplification of the absorption cross section. The results are interpreted and modeled in terms of a core–shell geometry and Lorenz–Mie theory of scattering and absorption. We discuss evidence of soot particle and collapse as a result of the coating process and we demonstrate the ability to resolve changes in the coating thickness as small as 2 nm.

Spectroscopic Line Parameters of Water Vapor for Rotation-Vibration Transitions near 7180 cm^(-1)

We present low uncertainty measurements of line parameters for 15 rotation-vibration transitions of water vapor in the wave number range of 7170.27-7183.02 cm{sup -1}. These experiments incorporated frequency-stabilized cavity ring-down spectroscopy and a primary standard humidity generator which produced a stable and accurately known amount of water vapor in a nitrogen carrier gas stream. Intensities and line shape factors were derived by fitting high-resolution spectra to spectral models that account for collisional narrowing and speed-dependent broadening and shifting effects. For most transitions reported here, we estimate the relative combined standard uncertainty of the line intensities to be <0.4%, of which approximately one half this value we ascribe to limited knowledge of the line shape. Our measured intensities and broadening parameters are compared to experimental and theoretical literature values. Agreement between our experimental intensity measurements and those derived by recent ab initio calculations of the dipole moment surface of water vapor is within 1.5%.

Automated high-resolution frequency-stabilized cavity ring-down absorption spectrometer

We describe a frequency-stabilized cavity ring-down absorption spectrometer with automated scanning capabilities. The system utilizes the comb of ring-down cavity resonances as frequency markers for spectral scans. The comb is actively stabilized with respect to a frequency-stabilized reference laser, and the continuous wave, single-mode probe laser frequency is in turn actively stabilized with respect to resonances of the ring-down cavity frequency comb. Ring-down spectra are obtained by locking the external-cavity diode laser probe to sequential cavity resonances (separated by the ring-down cavity free spectral range), and the ring-down cavity frequency comb is translated by controlled amounts using acousto-optic frequency shifting methods for fine frequency steps less than the free-spectral range. A computer-controlled method for automating frequency tuning and probe laser locking to the cavity resonances is described, and high-resolution near-infrared (vicinity of 10712cm−1) absorption spectra of wate...