Absorption Spectrum Of Water Vapor Research Articles

Accurate absorption spectroscopy of water vapor near 1.64 µm in support of the MEthane Remote LIdar missioN (MERLIN)

The MERLIN mission (MEthane Remote LIdar missioN) is a space mission dedicated to the methane atmospheric retrieval based on absorption LIDAR measurements at nadir. The methane columns will be obtained by difference between the LIDAR signal at the on-line wavelength corresponding to the R(6) manifold of the 2ν3 band of methane at 6077 cm−1 and the off-line signal at a wavenumber about 1 cm−1 below. As atmospheric water vapor lines may affect the methane retrievals, the present study is dedicated to a detailed laboratory characterization of the interfering water vapor absorption in the targeted region. To this aim, high sensitivity absorption spectra of pure water vapor and water vapor in air are recorded at room temperature using a cavity ring down spectrometer (CRDS) combined with a self-referenced frequency comb. The CRDS recordings cover the 6071.4–6080.0 cm−1 interval with a noise equivalent absorption, αmin, on the order of 5 × 10−11 cm−1 for a 10−3 cm−1 sampling of the spectra. The measured pure and air-broadened spectra are analyzed using the Nelkin-Ghatak (NG) or the speed-dependent Nelkin-Ghatak (sdNG) profile. A multi-spectrum fitting procedure in which series of spectra recorded at different pressures (between 0.7 and 12 Torr for pure water vapor and between 100 and 700 Torr for H2O in air) are simultaneously adjusted, is used to retrieve the spectroscopic parameters. Accurate line parameters including self- and air-pressure shifts, self- and air-broadenings, Dicke narrowing and the speed-dependent coefficients are determined for most of the lines with intensity larger than 10−28 cm/molecule at 296 K. The list is complemented with literature data (in particular from the HITRAN database) to be complete down to a 10−29 cm/molecule intensity cut-off. The obtained spectroscopic results are discussed in comparison with literature data.

CRDS absorption spectrum of 17O enriched water vapor in the 12,277–12,894 cm−1 range

The absorption spectrum of water vapor enriched in oxygen-17 has been recorded by a continuous-wave cavity ring-down spectrometer (CW-CRDS) at room temperature in the 12,277–12,894 cm−1 region. The typical sensitivity of 2 × 10−10 cm−1 allowed the detection of water vapor transitions with intensities larger than 1 × 10−28 cm−1/(molecule cm−2). In total, more than 3400 lines were observed in the recorded spectra with a typical frequency accuracy of 0.002 cm−1. In which more than 3400 transitions were ro-vibrationally assigned to five water isotopic species (H217O, H216O, H218O, HD16O, and HD17O) using the known empirical energy levels combined with the calculated variational line lists based on the results of Partridge and Schwenke. About 1300 transitions of H217O were assigned, leading to the determination of new energy levels of 17 vibration states. Those assigned H217O transitions are involved with the upper vibration states of (013), (023), (032), (051), (061), (070), (080), (090), (112), (131), (150), (160), (211), (230), (301), (310), and (320). The maximum values of the rotational numbers J and Ka were 16 and 9, respectively. The 5ν1 band of HD17O was assigned for the first time as well. The line positions and energy levels determined in this work were compared with literature, and it shows a significant improvement of the line positions of H217O and HD17O in the 12,277–12,894 cm−1 region.

Broadband terahertz pulse generation driven by an ultrafast thin-disk laser oscillator.

We demonstrate broadband THz generation driven by an ultrafast thin-disk laser (TDL) oscillator. By optical rectification of 50-fs pulses at 61 MHz repetition rate in a collinear geometry in crystalline GaP, THz radiation with a central frequency at around 3.4 THz and a spectrum extending from below 1 THz to nearly 7 THz are generated. We realized a spectroscopic characterization of a GaP crystal and a benchmark measurement of the water-vapor absorption spectrum in the THz range. Sub-50-GHz resolution is achieved within a 5 THz bandwidth. Our experiments show the potential of ultrafast TDL oscillators for driving MHz-repetition-rate broadband THz systems.

Study of the Water Vapor Absorption Spectrum in the Visible Spectral Region from 19480 to 20500 cm−1

The vibrational-rotational absorption spectrum of water vapor was recorded and analyzed in the visible spectral region from 19480 to 20500 cm−1. The measurements were carried out using an IFS-125M Fourier spectrometer with a resolution of 0.05 cm−1 at a pressure of 26.3 mbar, a temperature of (24 ± 1)°C, and optical path 24 m. We used a multipass White cell with a base length of 60 cm. A light-emitting diode was used as a radiation source. The signal-to-noise ratio was about 20000. The list of more than 420 lines has been compiled as a result of the analysis the spectrum, which includes line centers, intensities, and quantum vibrational-rotational numbers. More than 220 vibrational-rotational energy levels of 21 upper vibrational states have been determined from the experimental data.

Instrumentation and signal processing for the detection of heavy water using off axis-integrated cavity output spectroscopy technique.

An experimental setup is developed for the trace level detection of heavy water (HDO) using the off axis-integrated cavity output spectroscopy technique. The absorption spectrum of water samples is recorded in the spectral range of 7190.7 cm-1-7191.5 cm-1 with the diode laser as the light source. From the recorded water vapor absorption spectrum, the heavy water concentration is determined from the HDO and water line. The effect of cavity gain nonlinearity with per pass absorption is studied. The signal processing and data fitting procedure is devised to obtain linear calibration curves by including nonlinear cavity gain effects into the calculation. Initial calibration of mirror reflectivity is performed by measurements on the natural water sample. The signal processing and data fitting method has been validated by the measurement of the HDO concentration in water samples over a wide range from 20 ppm to 2280 ppm showing a linear calibration curve. The average measurement time is about 30 s. The experimental technique presented in this paper could be applied for the development of a portable instrument for the fast measurement of water isotopic composition in heavy water plants and for the detection of heavy water leak in pressurized heavy water reactors.

Study of the water vapor absorption spectrum in the visible region from 19480 to 20500 cm-1

Study of the water vapor absorption spectrum in the visible region from 19480 to 20500 cm-1

Numerical method based on transfer function for eliminating water vapor noise from terahertz spectra.

Water vapor noise in the air affects the accuracy of optical parameters extracted from terahertz (THz) time-domain spectroscopy. In this paper, a numerical method was proposed to eliminate water vapor noise from the THz spectra. According to the Van Vleck-Weisskopf function and the linear absorption spectrum of water molecules in the HITRAN database, we simulated the water vapor absorption spectrum and real refractive index spectrum with a particular line width. The continuum effect of water vapor molecules was also considered. Theoretical transfer function of a different humidity was constructed through the theoretical calculation of the water vapor absorption coefficient and the real refractive index. The THz signal of the Lacidipine sample containing water vapor background noise in the continuous frequency domain of 0.5-1.8THz was denoised by use of the method. The results show that the optical parameters extracted from the denoised signal are closer to the optical parameters in the dry nitrogen environment.

Comb-assisted cavity ring down spectroscopy of 17O enriched water between 7443 and 7921 cm−1

The room temperature absorption spectrum of water vapor highly enriched in 17O has been recorded by Cavity Ring Down Spectroscopy (CRDS) between 7443 and 7921cm−1. Three series of recordings were performed with pressure values around 0.1, 1 and 10Torr. The frequency calibration of the present spectra benefited of the combination of the CRDS spectrometer to a self-referenced frequency comb. The resulting CRD spectrometer combines excellent frequency accuracy over a broad spectral region with a high sensitivity (Noise Equivalent Absorption, αmin∼ 10–11–10−10cm−1).The investigated spectral region corresponds to the high energy range of the first hexade. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Partridge and Schwenke. Overall about 4150 lines were measured and assigned to 4670 transitions of six water isotopologues (H216O, H217O, H218O, HD16O, HD17O and HD18O). Their intensities span six orders of magnitude from 10–28 to 10–22cm/molecule. Most of the new results concern the H217O and HD17O isotopologues for which about 1600 and 400 transitions were assigned leading to the determination of 329 and 207 new energy levels, respectively. For comparison only about 300 and four transitions of H217O and HD17O were previously known in the region, respectively.By comparison to highly accurate H216O line positions available in the literature, the average accuracy on our line centers is checked to be on the order of 3MHz (10−4cm−1) or better for not weak well isolated lines. This small uncertainty represents a significant improvement of the line center determination of many H216O lines included in the experimental list provided as Supplementary Material.

Parameters of broadening of water molecule absorption lines by argon derived using different line profile models

The water vapor absorption spectrum was measured in the spectral region 6700–7650 cm–1 with argon as a buffer gas. The room-temperature spectrum was measured using a Bruker IFS 125-HR Fourier Transform Spectrometer with high signal-to-noise ratio, with a spectral resolution of 0.01 cm–1, at argon pressures from 0 to 0.9 atm. The H2O absorption spectral line parameters are derived by fitting two line shape profiles (Voigt and speed-dependent Voigt) to the experimental spectrum. It is shown that the use of speed-dependent Voigt profile provides the best agreement with experimental data.

Measuring spectral parameters of water vapor at low temperature based on tunable diode laser absorption spectroscopy

Accurate and reliable spectral line parameters of gas are very important for measuring gas concentration and temperature.The mainstream spectrum database (e.g.HITRAN) includes the values from theoretical computation based on different models,which have some inevitable deviations from the corresponding actual values.To address this problem,we develop a low-temperature spectral experimental platform for simulating low temperature and low pressure environment so as to accurately measure gas absorption spectral parameters.The spectral experimental platform uses the static cooling technology combined with the Dewar insulation system to maintain the quartz cell at a constant temperature.Through adjusting the electric heating and liquid helium refrigeration,we can achieve temperature change and stability.Temperature of the low temperature absorption cell can be adjusted in a range of 100-350 K with a precision lower than 0.3 K and the temperature gradient in the cell is lower than 0.01 K/cm.The length of quartz cell is 100 cm,and a reflector can be used to increase optical path for absorption.The window diameter is 76 mm,and the spectral resolution is better than 0.001 cm-1.We use a tunable diode laser spectrometer to measure absorption spectra of pure water vapor with the platform at different temperatures (230-340 K) and different pressures (10-1000 Pa).Voigt profile is the leastsquares fit to the measured spectra by using a multi-spectrum fitting routine.A filter is used to reduce electronic noise of detector signal.As spectral lines in the band of 7240-7246 cm-1 are often used in low temperature wind tunnel flow field measurements,a distributed feedback (DFB) diode laser with a wavelength of 1381 nm is used in the experiment, and five water vapor lines are selected and measured.Firstly,from the linear fitting of line area and the full width at half maximum of collisional broadening (or pressure broadening) we obtain line strengths and self-broadening half-width coefficients at different temperatures.Secondly,from nonlinear fitting of line strengths and self-broadening half-width coefficients at different temperatures we obtain the values of line strengths and self-broadening half-width coefficients at the reference temperature (296 K).In the end,comparison between our experimental results and HITRAN2012 database values shows that the maximum discrepancy between the HITRAN database and the experimental result is 10.96%.A transparent uncertainty analysis is given for the measurement values.Uncertainties of our measured line strengths are in a 1.11%-2.98% range (95% confidence level,k=2),which is smaller than those of HITRAN2012 database values (uncertainties are in a range of 5%-10%).The accurate spectral parameters are obtained experimentally,and of great significance for improving the spectrum measurement accuracy of water vapor in low temperature environment in the future.

The absorption spectrum of water vapor in the 2.2 μm transparency window: High sensitivity measurements and spectroscopic database

The weak absorption spectrum of water vapor in the important 2.2μm transparency window is investigated with very high sensitivity. Overall, about 400 absorption lines were measured by Cavity Ring Down Spectroscopy (CRDS) and Optical-Feedback-Cavity Enhanced Laser Spectroscopy (OF-CEAS) in five spectral intervals: 4248.2–4257.3, 4298.4–4302.6, 4336.8.5-4367.5, 4422.4-4441.2 and 4514.6-4533.7cm−1. The achieved sensitivity of the recordings (noise equivalent absorption, αmin, on the order of 2×10−10cm−1) allowed detecting transitions with intensity values down to 1×10−28 cm/molecule, more than one order of magnitude better than previous studies by Fourier Transform spectroscopy. The rovibrational assignment was performed on the basis of variational calculations and of previously determined empirical energy values. Most of the newly assigned lines correspond to transitions of the ν1, ν3 and 3ν2 bands of H217O in natural isotopic abundance. Fourteen energy levels of H217O, H218O and HD18O are newly determined.An accurate and complete spectroscopic database is constructed for natural water in the 4190–4550cm−1 region (2.39–2.20µm). The list includes about 4500 transitions with intensity greater than 1×10−29cm/molecule, for the six most abundant isotopologues in natural isotopic abundance. Line positions were obtained by difference of empirical energy values determined from literature data and complemented with the present CRDS results. The list is made mostly complete by including weak transitions not yet detected, with positions calculated from empirical levels and variational intensities. The variational intensities computed by a collaboration between the University College London and the Institute of Applied Physics in Nizhny Novgorod are found to improve significantly previous results by Schwenke and Partridge. Examples of comparison of the constructed line list to CRDS spectra and to simulations based on the HITRAN2012 list illustrate the advantages of the new list. The absence of many HDO lines is identified as most important deficiency of the HITRAN2012 list in the region.

CRDS with a VECSEL for broad-band high sensitivity spectroscopy in the 2.3 μm window.

The integration of an industry ready packaged Sb-based Vertical-External-Cavity Surface-Emitting-Laser (VECSEL) into a Cavity Ring Down Spectrometer (CRDS) is presented. The instrument operates in the important 2.3 μm atmospheric transparency window and provides a high sensitivity (minimum detectable absorption of 9 × 10(-11) cm(-1)) over a wide spectra range. The VECSEL performances combine a large continuous tunability over 120 cm(-1) around 4300 cm(-1) together with a powerful (∼5 mW) TEM00 diffraction limited beam and linewidth at MHz level (for 1 ms of integration time). The achieved performances are illustrated by high sensitivity recordings of the very weak absorption spectrum of water vapor in the region. The developed method gives potential access to the 2-2.7 μm range for CRDS.

VALIDATION OF THE REAL-TIME 2D TEMPERATURE MEASUREMENT METHOD USING THE CT TUNABLE DIODE LASER ABSORPTION SPECTROSCOPY

Two dimensional (2D) temperature and concentration distribution plays an important role for the combustion structure and the combustor efficiency in engines, burners, gas turbines and so on. Recently, as a multi-species measurement technique with high sensitivity and high response, tunable diode laser spectroscopy (TDLAS) has been developed and applied to the actual engine combustions. With these engineering developments, transient phenomena such as start-ups and load changes in engines have been gradually elucidated in various conditions. In this study, the theoretical and experimental research has been conducted in order to develop the non-contact and fast response 2D temperature and concentration distribution measurement method. The method is based on a computed tomography (CT) using absorption spectra of water vapor at 1388 nm. The computed tomography tunable diode laser spectroscopy (CT-TDLAS) method was employed in engine exhausts to measure 2D temperature distribution. The measured 2D temperature shows a good agreement with the temperature measured by a thermocouple. The temporal and spatial resolutions of this method have also been discussed to demonstrate its applicability to various types of combustor.

CRDS of 17O enriched water between 5850 and 6671 cm−1: More than 1000 energy levels of H217O and HD17O newly determined

The room temperature absorption spectrum of water vapor highly enriched in 17O has been recorded by Cavity Ring Down Spectroscopy (CRDS) between 5850 and 6671cm−1. Two series of recordings were performed with pressure values of 1.0 and 12.0Torr. The investigated spectral region corresponds to the important 1.55μm transparency window of the atmosphere where water absorption is very weak. The high sensitivity of the recordings (αmin ~ 5×10−11cm−1) allows detecting lines with intensity spanning six orders of magnitude (1.4×10−30–3.6×10−24cm/molecule at room temperature). The experimental list includes more than 10,300 lines. The assignments of water lines were performed using known experimental energy levels as well as calculated line lists based on the results of Partridge and Schwenke. More than 8500 lines were assigned to 9619 transitions of six water isotopologues (H216O, H217O, H218O, HD16O, HD17O and HD18O). All but four transitions of the 16O and 18O isotopologues were assigned using known experimental energy levels. More than half of the assigned H217O and HD17O transitions correspond to new (or corrected) upper energy levels. About 1000 new H217O transitions associated with upper states of the second triad and of the first hexad were identified. Most of the newly assigned HD17O transitions belong to the ν1+ν3 and 2ν2+ν3 bands. The assigned transitions allowed to newly determine or correct 20 highly excited rotational levels of the vibrational ground state of this isotopologue. Overall 791 and 266 energy levels are newly determined for H217O and HD17O, respectively. A few additional levels were corrected compared to literature values. The obtained experimental results are compared to the spectroscopic parameters provided by the HITRAN database and to the empirical energy levels recommended by an IUPAC task group.

2D Temperature Detection Characteristics of Engine Exhaust Gases Using CT Tunable Diode Laser Absorption Spectroscopy

Two dimensional (2D) temperature and concentration distribution is related to the combustion structure, the combustor efficiency in engines, burners, gas turbines and so on. Recently, tunable diode laser absorption spectroscopy (TDLAS) as a multi-species measurement technique with high sensitivity and high response has been developed and applied to industrial process monitoring and control technologies in combustion environments. With these engineering developments, transient phenomena such as start-ups and load changes in engines have been gradually elucidated in various conditions. This paper discusses a study of the fast response 2D temperature distribution measurement method based on the combination of TDLAS and Computed Tomographic (CT) reconstruction using absorption spectra of water vapor at 1388nm. The computed tomography tunable diode laser absorption spectroscopy (CTTDLAS) method was appliedtoengine exhausts for 2D temperature distribution measurements. The measured 2D temperature showed better characteristics compared with the temperature measured by a thermocouple. Theoretical H2O absorption spectra in the 1388 nm near-infrared region calculated by the revised HITRAN database were used for temperature measurement. For accurate measurement of temperature in combustion gases, the spectroscopic databases were modified using experimentally measured spectral parameters that are not found in the databases. Accuracy of temperature measurement using TDLAS have also been discussed to demonstrate its applicability to various types of combustor.

High temperature field application of two dimensional temperature measurement technology using CT tunable diode laser absorption spectroscopy

Two dimensional (2D) temperature and concentration distribution plays an important role for the combustion structure and the combustor efficiency in engines, burners, gas turbines and so on. Recently, as a multi-species measurement technique with high sensitivity and high response, tunable diode laser absorption spectroscopy (TDLAS) has been developed and applied to the actual engine combustions. With these engineering developments, transient phenomena such as start-ups and load changes in engines have been gradually elucidated in various conditions. In this study, the theoretical and experimental research has been conducted in order to develop the non-contact and fast response 2D temperature and concentration distribution measurement method. The method is based on a computed tomography (CT) using absorption spectra of water vapor at 1343nm and 1388nm. High temperature field application of two dimensional temperature measurement technology using CT-TDLAS has also been discussed to demonstrate its applicability for various types of combustor.

Collision-Induced Dipole Moment and Millimeter and Submillimeter Continuum Absorption in Water Vapor

This work is devoted to estimation of the additional absorption of millimeter and submillimeter wavelengths in water vapor arising from collisional interaction of molecules due to the induced dipole moment. Absorption is modeled on the basis of ab initio data on the magnitude of the water molecule dipole moment at high densities, and common knowledge of the water vapor absorption spectrum. Using the model developed, we obtained a simple analytical expression for the absorption coefficient as a function of temperature, pressure, and frequency. Comparison of the results with known experimental data leads to the conclusion that in the range of pressures and temperatures typical of water vapor in the Earth’s atmosphere this type of absorption is negligible compared with the absorption arising due to association or dimerization of the water vapor molecules.

LED-based Fourier transform spectroscopy of H2 16O in the range 15500–16000 cm−1

The vibrational-rotational absorption spectrum of water vapor within the range 15500–16000 cm−1 is measured and analyzed. The spectrum is recoded with an IFS-125M Fourier transform spectrometer with a resolution of 0.03 cm−1, at pressure of 25 mbar, at a temperature of 24°C, and at an optical path length of 34.8 m. The measurements are performed using a multipass White cell with a base length of 60 cm. A light-emitting diode is used as a radiation source. The signal-to-noise ratio is about 104, which makes it possible to measure the parameters of lines with intensities on the order of 10−27 cm/molecule. The centers, intensities, and half-widths of lines are determined by fitting the Voigt profile parameters to measured data set by the least squares method. A list of more than 430 lines is formed based on the analysis of the spectrum. The obtained results are compared with calculated and experimental data of other authors.

High-resolution water vapor spectrum and line shape analysis in the Terahertz region

A coherent broadband high-resolution study of the water vapor absorption spectrum at 1.5THz was performed. The transmittance was recorded for many different water vapor and air pressures at multiple path lengths at a resolution of 5–10MHz. A post-processing routine was developed to filter the acquired data before being subjected to a global multispectral fitting analysis of 145 data sets. The experimental data was fit to multiple different line shapes and a line shape analysis was performed in order to determine the most accurate line shape in the Terahertz range. Five of the strongest water vapor lines in the region were identified and fit to the data. The line center frequencies, absolute intensities, self- and foreign-broadening coefficients, and self- and foreign-continuum coefficients were all experimentally determined along with their statistically determined error bars. The fitted parameters are compared to the values from the literature.

The absorption spectrum of water vapor in the 1.25 μm atmospheric window (7911–8337 cm−1)

The absorption spectrum of water vapor in “natural” isotopic abundance has been recorded at room temperature by high sensitivity Continuous Wave Cavity Ring Down Spectroscopy (CW-CRDS) between 7911 and 8337cm−1. The investigated region covers most of the 1.25µm transparency window of importance for atmospheric applications. The recordings were performed with sensitivity on the order of αmin~2×10–11cm−1, more than two orders of magnitude better than previous investigations by Fourier Transform Spectroscopy (FTS). Measured line intensities cover a range of seven orders of magnitude (3×10–30–2×10–23cm/molecule at room temperature). The experimental line list provided as Supplementary Material includes more than 5000 transitions. As a result of the achieved sensitivity, more than 1150 lines of the experimental list were identified as being due to ammonia present as an impurity at the 5ppm concentration level in the water sample. Although incomplete, the obtained ammonia line list seems to be the first one in the region. More than 3193 water lines were assigned to 3560 transitions of five water isotopologues (H216O, H218O, H217O, HD16O and HD18O). The assignments were performed using known experimental energy levels and calculated spectra based on variational calculations by Schwenke and Partridge. The obtained results are compared to the most relevant previous studies by Fourier Transform Spectroscopy in the region and to the exhaustive review of rovibrational line positions and levels performed recently by an IUPAC sponsored task group. Two-hundred and sixty-six levels are newly determined and 46 are corrected by more than 0.015cm−1 compared to those recommended by the water IUPAC task group. The overall agreement between variational and measured intensities is satisfactory.A complete empirical list of 4473 transitions incorporating all the experimental information at disposal was constructed for water in the studied region. The intensity cut-off was fixed to 1×10–29cm/molecule at 296K. A detailed comparison with the line list as provided by the HITRAN database illustrates the advantages of the new list.