Speed-dependent Effects Research Articles

A variable-temperature cavity ring-down spectrometer with application to line shape analysis of CO2 spectra in the 1600 nm region

We present a new cavity ring-down spectroscopy system which was developed for variable-temperature absorption measurements (220–290 K) of atmospheric gases. This laser spectrometer was developed in the framework of the NASA Orbiting Carbon Observatory-2 project to improve our understanding of line shape parameters for carbon dioxide and oxygen. The apparatus consists of a monolithic, fixed-mirror ring-down cavity within a temperature-regulated enclosure, which is interrogated by a tunable, single-frequency diode laser. We experimentally characterize and model the dependence of the spectrum detuning axis at each setpoint temperature, and show that absolute frequencies are stable to within 200 kHz over several hours, corresponding to temperature stabilities better than 1 mK. We measure the R16e (30013-0001) 12C16O2 transition and carry out multi-spectrum analyses using two line profiles incorporating speed-dependent (quadratic approximation) and Dicke narrowing (hard collision assumption) effects. The resulting broadening coefficient and temperature exponent are in excellent agreement (0.05% level) with previous high-resolution Fourier-transform spectroscopy measurements, and the speed dependent broadening parameter is within 3% of the theoretical value.

Kinematic response in limb and body posture to sensory feedback from carpal sinus hairs in the rat (Rattus norvegicus)

One of the most challenging adaptations within the therians has been to ensure dynamic stability of the trunk during rapid locomotion in highly structured environments. A reorganization of limb mechanics and development of new sensors has taken place within their stem lineage. Rats, which have a similar lifestyle to the first therians, possess sinus hairs specialized for tactile sensing. It is supposed that carpal sinus hairs have a role in sensing substrate properties and can thus induce adjustments in limb kinematics and body posture according to the different surface diameters and structures detected. This implies a shared sensorimotor control loop of sinus hairs and body posture. To investigate the role of the carpal sinus hairs during locomotion and to explore a possible interaction between limb and spine motion, spatiotemporal and kinematic parameters as well as the contact mechanics of the hairs with regard to the surface were quantified. Locomotion on a treadmill with continuous and discontinuous substrates was compared in the presence/absence of the carpal sinus hairs across a speed range from 0.2m/s to 0.6m/s. Recordings were taken synchronously using x-ray fluoroscopy and normal-light high-speed cameras. Our investigation revealed that the three tactile hairs made a triangle-like contact with the ground approximately 30ms before touchdown of the forelimb. Within that time, it is likely that both the body posture and its oscillation are adjusted according to the different surface textures. The sensory input of the carpal sinus hairs induces a stabilization of the trajectory of the center of mass and, therefore, improves the dynamic stability of the trunk; conversely, the absence of the sensors results in a more crouched frontal body posture, similar to that seen in rats when they are moving in an unknown terrain. The carpal sinus hairs also sense the animal's speed during surface contact. This implicates an adjustment of the limb and spine kinematics, by increasing the speed-dependent effect or by increasing the distance between the trunk and the ground when the rat is walking faster.

Line positions, pressure broadening and shift coefficients for the second overtone transitions of carbon monoxide in argon

Absolute positions and spectral line-shape parameters of carbon monoxide 0→3 band, P branch transitions are reported. The speed-dependent effects and the influence of velocity-changing collisions were taken into account in the fitted line-shape models. For the first time the values of pressure shift coefficients of CO in argon for this band were determined. The measurements were made with the Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer, with the frequency axis linked through an optical frequency comb to the UTC(AOS) frequency reference based on a hydrogen maser. Achieved uncertainties of line positions are between 70kHz and 420kHz.

Determination of foreign broadening coefficients for Methane Lines Targeted by the Tunable Laser Spectrometer (TLS) on the Mars Curiosity Rover

Molecular line parameters of foreign- broadening by air, carbon dioxide, and helium gas have been experimentally determined for infrared ro-vibrational spectral lines of methane isotopologues (12CH4 and 13CH4) at 3057cm−1 targeted by the Tunable Laser Spectrometer (TLS) in the Sample Analysis at Mars (SAM) instrument suite on the Mars Science Laboratory (MSL) Curiosity rover. From multi-spectrum analyses with the speed-dependent Voigt line profile with Rosenkrantz line-mixing, speed-dependence and line-mixing effects were quantified for methane spectra at total pressures up to 200 mbar. The fitted air-broadening coefficients deviated from 8–25% to those reported in the HITRAN-2012 database.

Isolated line shape of methane with various collision partners

In this work, the spectral profile of an isolated transition of methane broadened by Ar, N2, O2 and He was studied using different line-shape models, from the simple Voigt to the recently recommended Hartmann–Tran profiles. For each collision-partner, absorption spectra of the ν4 2A11-3A21 line of methane were measured at room temperature and pressures ranging from 28.26 to 95.50mbar, with a high resolution tunable diode laser spectrometer. The spectroscopic parameters of each line-shape model used were then retrieved from measured spectra using a multispectrum fitting procedure. The obtained results show that the Voigt profile leads to large deviations with respect to the measured one and that non-Voigt effects are more important for CH4 diluted in Ar, N2 and O2 than in He. We also confirm that both the confinement narrowing and the speed dependence effects must be taken into account in order to correctly reproduce the experimental spectra. In addition, the determination of the instrumental distortions of the spectrometer is also presented and discussed as a main difficulty of this analysis.

CO2 pressure broadening and shift coefficients for the 2–0 band of 12C16O

Fourier transform absorption spectra of the 2–0 band of 12C16O mixed with CO2 have been recorded at total pressures from 156 to 1212hPa and at 4 different temperatures between 240K and 283K. CO2 pressure-induced line broadening and line shift coefficients, and the temperature dependence of the former have been measured including line mixing using a multi-spectrum non-linear least squares fitting technique. Different line shape models have been considered to take into account the Dicke narrowing or speed dependence effects. Measured line-shape parameters were compared with theoretical values, calculated for individual temperatures using a semi-empirical method and the Exponential Power Gap (EPG) law.

Speed-dependent effects and Dicke narrowing in nitrogen-broadened oxygen

We present the line-shape analysis of the nitrogen-broadened P9 P9 oxygen B-band transition measured by the optical frequency comb-assisted Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer. Perturbation by both oxygen and nitrogen molecules is taken into account simultaneously in the line-shape analysis. Several line-shape models describing physical effects such as Dicke narrowing, the speed dependence of collisional broadening and shifting, and the correlation between velocity- and phase-changing collisions were used in the analysis. The comparison between the hypergeometric and quadratic approximations of the speed-dependent effects is presented. Observed line narrowing is mostly determined by the speed dependence of the collisional broadening.

Spectral shape parameters of pure CO2 transitions near 1.6 µm by tunable diode laser spectroscopy

Room temperature spectra of four lines of pure CO2 in the 1.6μm region have been measured in a wide range of pressure using a tunable diode laser spectrometer. The spectra of each line have been adjusted using a multi-spectrum fitting procedure with various line-shape models, including the recently recommended Hartmann-Tran profile (HTP). The results show that the collision-induced velocity changes and the speed-dependence effects are clearly observed. It is also shown that taking into account the correlation between velocity- and internal state-changes is necessary to obtain physically-meaningful line-shape parameters. The latter are reported and discussed.

Cavity ring-down spectrometer for high-fidelity molecular absorption measurements

We present a cavity ring-down spectrometer which was developed for near-infrared measurements of laser absorption by atmospheric greenhouse gases. This system has several important attributes that make it possible to conduct broad spectral surveys and to determine line-by-line parameters with wide dynamic range, and high spectral resolution, sensitivity and accuracy. We demonstrate a noise-equivalent absorption coefficient of 4×10−12cm−1Hz−1/2 and a signal-to-noise ratio of 1.5×106:1 in an absorption spectrum of carbon monoxide. We also present high-resolution measurements of trace methane in air spanning more than 1.2THz and having a frequency axing with an uncertainty less than 100kHz. Finally, we discuss how this system enables stringent tests of advanced line shape models. To illustrate, we measured an air-broadened carbon dioxide transition over a wide pressure range and analyzed these data with a multi-spectrum fit of the partially correlated, quadratic speed-dependent Nelkin–Ghatak profile. We obtained a quality-of-fit parameter in the multispectrum fit equal to 36,000, thus quantifying small-but-measurable limitations of the model profile. This analysis showed that the line shape depends upon collisional narrowing, speed dependent effects and partial correlations between velocity- and phase-changing collisions.

Doppler broadening thermometry of acetylene and accurate measurement of the Boltzmann constant.

In this paper, we present accurate measurements of the fundamental Boltzmann constant based on a line-shape analysis of acetylene spectra in the ν1 + ν3 band recorded using a tunable diode laser. Experimental spectra recorded at low pressures (0.25 - 9 Torr), have been analyzed using a Speed Dependent Voigt model that takes into account the molecular speed dependence effects. This line-shape model reproduces the experimental data with good accuracy and allows us to determine precise line-shape parameters for the P(25) transition of the ν1 + ν3 band. From the recorded spectra we obtained the Doppler-width and then determined the Boltzmann constant, k(B).

Line shapes, positions and intensities of water transitions near 1.28 μm

We present measurements of approximately 70 isolated, self-broadened, water vapor lines which are assigned to the (1,0,1)–(0,0,0), (0,0,2)–(0,0,0), (1,2,0)–(0,0,0), and (2,0,0)–(0,0,0) vibrational bands and which occur in the transparency window region from 7710cm−1 to 7920cm−1. We acquired absorption spectra on room-temperature, water samples over the pressure range 150–800Pa with the frequency-stabilized cavity ring-down spectroscopy technique. In order to optimize measurement accuracy, we integrated mK-level temperature control and SI-traceable pressure measurements into our cavity ring-down spectroscopy measurements. This technique yielded relative uncertainties of 0.04% and 0.20% in sample density and measured line intensity, respectively. We also referenced our spectrum frequency axes to a Cs clock, which provided vacuum line positions with a combined standard uncertainty of 3MHz. Comparison of our measured intensities, positions and self-broadening parameters with literature values reveals that the present work substantially reduces uncertainty in these line parameters. For the (1,0,1)–(0,0,0) vibrational band, the measured line intensities agree at the 1%-level with published ab initio calculations. Our spectra exhibited signal-to-noise ratios up to ≈20,000:1 to enable stringent tests of theoretical line profiles through multispectrum least-squares data analysis. We show that the partially correlated, quadratic-speed-dependent Nelkin–Ghatak profile gives a quality of fit that is commensurate with the high spectrum signal-to-noise ratio, and unlike most other profiles considered here, reproduces the measured line shapes without systematic residuals over the entire pressure range. Our results confirm that mechanisms of (1) collisional narrowing, (2) speed-dependent effects and (3) partial correlation between velocity-changing and dephasing collisions, contribute to the self-broadened line shape of isolated water vapor transitions.

Observations of Dicke narrowing and speed dependence in air-broadened CO₂ lineshapes near 2.06 μm.

Frequency-stabilized cavity ring-down spectroscopy was used to study CO2 lineshapes in the (20013) ← (00001) band centered near 2.06 μm. Two rovibrational transitions were chosen for this study to measure non-Voigt collisional effects for air-broadened lines over the pressure range of 7 kPa-28 kPa. Lineshape analysis for both lines revealed evidence of simultaneous Dicke (collisional) narrowing and speed-dependent effects that would introduce biases exceeding 2% in the retrieved air-broadening parameters if not incorporated in the modeling of CO2 lineshapes. Additionally, correlations between velocity- and phase/state changing collisions greatly reduced the observed Dicke narrowing effect. As a result, it was concluded that the most appropriate line profile for modeling CO2 lineshapes near 2.06 μm was the correlated speed-dependent Nelkin-Ghatak profile, which includes all of the physical effects mentioned above and leads to a consistent set of line shape parameters that are linear with gas pressure.

Pressure broadening, -shift, speed dependence and line mixing in the ν3 rovibrational band of N2O

In this paper, we report measured air-broadening, -shift, speed dependence and Rosenkranz line mixing parameters for the ν3 fundamental rovibrational band of N2O. A Bruker IFS 125HR Fourier transform spectrometer was used with a White-type multipass absorption cell with 46.4m absorption path length to measure four ambient temperature air-broadened absorption spectra at total pressures ranging from 100 to 1000mbar. A multispectrum fitting technique was used to retrieve parameters up to |m|=40 (m=−J″ and m=J″+1 for the P and R branch, respectively) utilizing the partially correlated quadratic speed-dependent hard collision model including Rosenkranz line mixing. Speed dependence of the broadening parameter as well as line mixing could be observed in the spectra. The broadening parameters are compared to HITRAN2012, where deviations can be ascribed to the influence of neglecting speed dependence effects in spectra analyses when using the Voigt line profile. The line mixing coefficients show a smooth dependence on m.

Quadratic speed dependence of collisional broadening and shifting for atmospheric applications

Precise analysis of spectroscopic data requires a proper handling of speed-dependent effects on molecular lines. Recently, new fast algorithms implementing a quadratic speed dependence of collisional width and shift were developed. In this report we show, how to evaluate the effective quadratic speed dependence in multi-component gases for atmospheric applications. Moreover, we derived relations between parameters describing quadratic speed dependence and temperature dependence of an averaged collisional width and shift. These relations can be directly applied in molecular spectra analysis and are useful for construction of new generation of spectroscopic databases.

A Novel Equivalent Circuit Model of Linear Induction Motors Considering Static and Dynamic End Effects

A novel steady-state equivalent circuit model for linear induction motors is presented in this paper. The novelty resides in classification and inclusion of end effects that are considered static and dynamic, namely as speed-independent and speed-dependent end effects. The Duncan model is used to address the dynamic end effect and appropriately modified to account for the saturation of back iron. The static end effect, which manifests itself by the alternating field component, is represented by an additional circuit branch similar to the branches of the motor with an alternating magnetic field. Each type of end effect is modeled and included separately. The results regarding determination of basic performance characteristics are compared with those obtained from finite element (FE) simulation. The procedure to identify the parameters of this circuit using a FE model is also presented.

Line-shape models testing on six acetylene transitions in the ν1+ν3 band broadened by N2

We present a line-shape study of six absorption lines P(21), P(16), P(19), R(16), R(19) and R(21) in the ν1+ν3 band of the acetylene perturbed by nitrogen. The spectra were recorded using a 3-channel diode laser spectrometer. For each transition studied, the N2-broadening, N2-narrowing, N2-pressure induced line shift and N2-line mixing coefficients were retrieved using several line-shape models. Moreover, we compare the N2-narrowing parameter to the dynamical friction parameter. The spectra were recorded over a pressure range from 5 to 250Torr, and were analyzed using the Voigt, Rautian, Correlated Rautian, Galatry, Rautian–Galatry, Speed Dependent Voigt, Speed Dependent Rautian and Correlated Speed Dependent Rautian line-shape models. Furthermore, we have studied the influence of the type of interaction potential in our analysis for line-shape models that take into account speed dependent effects.

Low-pressure line-shape study in molecular oxygen with absolute frequency reference

We present a line-shape analysis of the rovibronic R1 Q2 transition of the oxygen B band resolved by the Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectroscopy technique in the low pressure range. The frequency axis of the spectra is linked by the ultra-narrow diode laser to the optical frequency comb in order to measure the absolute frequency at each point of the recorded spectra. Experimental spectra are fitted with various line-shape models: the Voigt profile, the Galatry profile, the Nelkin-Ghatak profile, the speed-dependent Voigt profile, and the speed-dependent Nelkin-Ghatak profile with quadratic and hypergeometric approximations for the speed dependence of collisional broadening and shifting. The influences of Dicke narrowing, speed-dependent effects, and correlation between phase- and velocity-changing collisions on the line shape are investigated. Values of line-shape parameters, including the absolute frequency of the transition 435685.24828(46) GHz, are reported.

Detection and Discrimination of Motion-Defined Form: Implications for the Use of Night Vision Devices

Superimposed luminance noise is typical of imagery from devices used for low-light vision such as image intensifiers (i.e., night vision devices). In four experiments, we measured the ability to detect and discriminate motion-defined forms as a function of stimulus signal-to-noise ratio at a variety of stimulus speeds. For each trial, observers were shown a pair of image sequences - one containing dots in a central motion-defined target region that moves coherently against the surrounding dots, which moved in the opposite or in random directions, while the other sequence had the same random/uniform motion in both the center and surrounding parts. They indicated which interval contained the target stimulus in a two-interval forced-choice procedure. In the first experiment, simulated night vision images were presented with Poisson-distributed spatiotemporal image noise added to both the target and surrounding regions of the display. As the power of spatiotemporal noise was increased, it became harder for observers to detect the target, particularly at the lowest and highest dot speeds. The second experiment confirmed that these effects also occurred with low illumination in real night vision device imagery, a situation that produces similar image noise. The third experiment demonstrated that these effects generalized to Gaussian noise distributions and noise created by spatiotemporal decorrelation. In the fourth experiment, we found similar speed-dependent effects of luminance noise for the discrimination (as opposed to detection) of the shape of a motion-defined form. The results are discussed in terms of physiological motion processing and for the usability of enhanced vision displays under noisy conditions.

Speed dependence of CH335Cl–O2 line-broadening parameters probed on rotational transitions: Measurements and semi-classical calculations

Relaxation parameters for K-components (K≤6) of six J→J+1 rotational transitions (J=6, 10, 17, 22, 31 and 33) of CH335Cl perturbed by O2 are measured at room temperature with Voigt, speed-dependent Voigt and Galatry profiles in order to probe the speed-dependence effects. With respect to the previous study of CH335Cl–N2 system [Guinet et al., J Quant Spectrosc Radiat Transfer 2012;113:1113], higher active-gas pressures are reached, providing better signal-to-noise ratios, and the exact expression of the Beer–Lambert law is introduced in the fitting procedure, leading, among other advantages, to much more realistic low-pressure results. The broadening parameters of the considered lines are also computed by a semi-classical method for various relative velocities of colliders and the powers characterizing the dependence of the collisional cross-sections on relative speeds are deduced as functions of the rotational numbers J and K. Additional calculations performed with the Maxwell–Boltzmann distribution of velocities show no significant difference with the earlier results [Buldyreva et al., Phys Chem Chem Phys 2011;13:20326] obtained within the mean thermal velocity approximation. Weighted sums of the presently measured Voigt-profile O2-broadening parameters and of the previously published N2-broadening ones are calculated to yield experimental air-broadening coefficients for spectroscopic databases.

Lineshape models testing on CH4 spectral line 6105.6257 cm−1 (R9F1, R9F2) of 2ν3 band broadened by N2 and Ne

The experimental spectra of CH4 absorption lines in the spectral region of 6105–6107cm−1 have been measured in binary mixtures with N2 and Ne by a high-resolution photo-acoustic laser spectrometer. Lineshape models, including Dicke narrowing and speed dependent effects were used to CH4 spectral line (ν0=6105.62573cm−1) fitting. The hard-, soft-collision and speed dependence models lead to better fits of experiments than the Voigt profile, but the systematic residuals and biases in the retrieved line parameters are remained.