Pure Rotational Raman Lines Research Articles

A simulation comparison of calibration functions for different sets of spectral filter passbands in the traditional pure rotational Raman lidar technique

The accuracy of tropospheric temperature measurements with pure rotational Raman (PRR) lidars is affected by the collisional broadening of N2 and O2 PRR lines. In this paper, we intercompare nine calibration functions (CFs) in the traditional PRR lidar technique via simulation. Taking into account the PRR line broadening, the simulation is performed for five sets of spectral filters (SFs) with different passbands in a PRR lidar receiving system. For simplicity of calculations, the transmission function of each SF is approximated by a rectangular function on the wavenumber interval, within which a SF passes the bulk of the backscattered signal intensity in the corresponding PRR lidar channel. A narrow-linewidth laser operating at wavelengths of 354.67 and 532 nm is considered as a lidar transmitter. The CF best suited for tropospheric temperature retrievals from raw PRR lidar data for each set of SFs and laser wavelength is determined by comparative analysis of calibration errors ΔT produced using these CFs. The absolute error |ΔT| does not exceed the value of 3 × 10–3 K when using the best three-coefficient CF, while |ΔT| < 4 × 10–4 K for the best four-coefficient CF regardless of the laser wavelength and SF set used.

Optical design of a crossed double-grating spectrometer for extracting pure rotational Raman lines of N2

In this paper, we introduce a spectrometer design with crossed double-grating for extracting pure rotational Raman (PRR) lines of N2. The design applies a linear optical fibre array as the entrance slit and a linear array photomultiplier tube (LaPMT) as the detector. The linear dispersion and structural parameters of the proposed configuration were analysed theoretically, and the optical layout of the spectrometer was simulated using the Zemax software. A spectrometer with a 32-channel LaPMT was simulated to obtain the PRR spectrum of N2 when the excitation wavelength was 532 nm. The Stokes and anti-Stokes Raman lines, with rotational quantum states from 0 (2) through 14 (16), were obtained on separate channels of the LaPMT, and the resolution was approximately 0.22 nm per channel.

Miniature double-grating monochromator with Littman configuration for pure rotational Raman LIDAR

Conventional double-grating monochromators used in pure rotational Raman (PRR) temperature LIDAR usually adopt the Littrow configuration to separate backscattered PRR lines. A mirror is added to this configuration, thereby producing the Littman configuration. The dispersion of the modified configuration is greater than thrice that of the Littrow configuration for incident angles larger than 60 deg. A spectroscopy system with a linear dispersion better than 0.9 mm / nm is designed for the 532-nm excitation laser wavelength, and the selected N2 PRR lines are effectively separated. The overall system is simulated using only off-the-shelf optical components, and the results show that the volume is smaller than 240 × 200 × 80 mm3.

Rotational and Rotational-Vibrational Raman Spectroscopy of Air to Characterize Astronomical Spectrographs.

Raman scattering enables unforeseen uses for the laser guide-star system of the Very Large Telescope. Here, we present the observation of one up-link sodium laser beam acquired with the ESPRESSO spectrograph at a resolution λ/Δλ∼140 000. In 900s on source, we detect the pure rotational Raman lines of ^{16}O_{2}, ^{14}N_{2}, and ^{14}N^{15}N (tentatively) up to rotational quantum numbers J of 27, 24, and 9, respectively. We detect the ^{16}O_{2} fine-structure lines induced by the interaction of the electronic spin S and end-over-end rotational angular momentum N in the electronic ground state of this molecule up to N=9. The same spectrum also reveals the ν_{1←0} rotational-vibrational Q-branch for ^{16}O_{2} and ^{14}N_{2}. These observations demonstrate the potential of using laser guide-star systems as accurate calibration sources for characterizing new astronomical spectrographs.

Line-shape parameters for pure rotational Raman lines of D2 in He

We review our recent experimental and theoretical studies on the spectral line shapes of D2 in helium baths. We also provide the most accurate to date experimental line positions of the first pure rotational Stokes lines, S0(j=0-2), of D2.

Accurate wavenumber measurements for the S0(0), S0(1), and S0(2) pure rotational Raman lines of D2

AbstractWe present accurate measurements for the wavenumbers of the S0(0), S0(1), and S0(2) pure rotational Raman lines in D2. Our measurements improve the accuracy of the previously available experimental data by an order of magnitude. They also show complete agreement with the state‐of‐the‐art ab initio calculations of the D2 rotational splitting, which include both relativistic and Quantum electrodynamics (QED) corrections.

A Novel Calibration Method for Pure Rotational Raman Lidar Temperature Profiling

AbstractWe propose a new calibration method for pure rotational Raman (PRR) lidar temperature profiling based on the different temperature sensitivities of Stocks and anti‐Stocks PRR lines. This method reconstructs the expression of the differential backscatter cross section according to the temperature dependencies of each component and forms a temperature factor and a calibration factor in the intensity ratio. With these factors, the temperature is retrievable from the lidar return. The effectiveness and accuracy of the proposed method have been verified through simulations and experiments. The inversion error can be reduced by ~50% compared with the commonly used calibration methods in weak signal‐to‐noise situations.

Comparative analysis of calibration functions in the pure rotational Raman lidar technique

Calibration functions (CFs) are required for the pure rotational Raman (PRR) technique to retrieve atmospheric temperature profiles from raw lidar data. The accuracy of temperature retrievals essentially depends on the selected CF, especially in the troposphere where the molecular collision effect dominates over the Doppler one. In this paper, we theoretically and experimentally compare nine different CFs (five of which are suggested for the first time) to determine the best function for the tropospheric temperature retrievals. All the CFs represent special cases of the general CF that takes into account the collisional broadening of individual atmospheric N2 and O2 PRR lines. The comparative analysis of calibration errors ΔTerr produced using these CFs in a simulation study showed that the best CF among them gives |ΔTerr| < 4 × 10− 5 K. To determine the best CF from a practical standpoint, we apply all the functions to nighttime measurement data obtained in Tomsk (56.48°N, 85.05°E, Western Siberia, Russia) with a PRR lidar designed in the Institute of Monitoring of Climatic and Ecological Systems (IMCES). The CF best-suited for the tropospheric temperature measurements with the IMCES PRR lidar is the same as determined by simulation.

Tropospheric temperature measurements with the pure rotational Raman lidar technique using nonlinear calibration functions

Abstract. Among lidar techniques, the pure rotational Raman (PRR) technique is the best suited for tropospheric and lower stratospheric temperature measurements. Calibration functions are required for the PRR technique to retrieve temperature profiles from lidar remote sensing data. Both temperature retrieval accuracy and number of calibration coefficients depend on the selected function. The commonly used calibration function (linear in reciprocal temperature 1∕T with two calibration coefficients) ignores all types of broadening of individual PRR lines of atmospheric N2 and O2 molecules. However, the collisional (pressure) broadening dominates over other types of broadening of PRR lines in the troposphere and can differently affect the accuracy of tropospheric temperature measurements depending on the PRR lidar system. We recently derived the calibration function in the general analytical form that takes into account the collisional broadening of all N2 and O2 PRR lines (Gerasimov and Zuev, 2016). This general calibration function represents an infinite series and, therefore, cannot be directly used in the temperature retrieval algorithm. For this reason, its four simplest special cases (calibration functions nonlinear in 1∕T with three calibration coefficients), two of which have not been suggested before, were considered and analyzed. All the special cases take the collisional PRR lines broadening into account in varying degrees and the best function among them was determined via simulation. In this paper, we use the special cases to retrieve tropospheric temperature from real PRR lidar data. The calibration function best suited for tropospheric temperature retrievals is determined from the comparative analysis of temperature uncertainties yielded by using these functions. The absolute and relative statistical uncertainties of temperature retrieval are given in an analytical form assuming Poisson statistics of photon counting. The vertical tropospheric temperature profiles, retrieved from nighttime lidar measurements in Tomsk (56.48° N, 85.05° E; Western Siberia, Russia) on 2 October 2014 and 1 April 2015, are presented as an example of the calibration functions application. The measurements were performed using a PRR lidar designed in the Institute of Monitoring of Climatic and Ecological Systems of the Siberian Branch of the Russian Academy of Sciences for tropospheric temperature measurements.

Analytical calibration functions for the pure rotational Raman lidar technique.

We present a calibration function in the general analytical form for the tropospheric temperature retrievals using pure rotational Raman (PRR) lidars. The function is derived within the framework of the semiclassical theory and takes into account the collisional broadening of all PRR lines. We analyze via simulation its four simplest nonlinear (three-coefficient) special cases to determine the function that yields the least error, and therefore, is the best-suited for the temperature retrievals. Two of them are proposed for the first time. The comparative analysis of temperature errors showed that all the special cases yield errors less than 0.1 K in modulus, and therefore, can be applied for the tropospheric temperature retrievals. The best function yields the maximum error less than 0.002 K in modulus and five times smaller compared to the commonly used nonlinear calibration function.

Spectroscopic and thermodynamic properties of molecular hydrogen dissolved in water at pressures up to 200 MPa.

We have measured the Raman Q-branch of hydrogen in a solution with water at a temperature of about 280 K and at pressures from 20 to 200 MPa. From a least-mean-square fitting analysis of the broad Raman Q-branch, we isolated the contributions from the four lowest individual roto-vibrational lines. The vibrational lines were narrower than the pure rotational Raman lines of hydrogen dissolved in water measured previously, but significantly larger than in the gas. The separations between these lines were found to be significantly smaller than in gaseous hydrogen and their widths were slightly increasing with pressure. The lines were narrowing with increasing rotational quantum number. The Raman frequencies of all roto-vibrational lines were approaching the values of gas phase hydrogen with increasing pressure. Additionally, from the comparison of the integrated intensity signal of Q-branch of hydrogen to the integrated Raman signal of the water bending mode, we have obtained the concentration of hydrogen in a solution with water along the 280 K isotherm. Hydrogen solubility increases slowly with pressure, and no deviation from a smooth behaviour was observed, even reaching thermodynamic conditions very close to the transition to the stable hydrogen hydrate. The analysis of the relative hydrogen concentration in solution on the basis of a simple thermodynamic model has allowed us to obtain the molar volume for the hydrogen gas/water solution. Interestingly, the volume relative to one hydrogen molecule in solution does not decrease with pressure and, at high pressure, is larger than the volume pertinent to one molecule of water. This is in favour of the theory of hydrophobic solvation, for which a larger and more stable structure of the water molecules is expected around a solute molecule.

Use of a Fabry–Perot interferometer to isolate pure rotational Raman spectra of diatomic molecules

We propose to use a Fabry-Perot interferometer (FPI) as a comb frequency filter to isolate pure rotational Raman spectra (PRRS) of nitrogen molecules. In making the FPI's free spectral range equal to the spectral spacing between the lines of nitrogen PRRS, which are practically equidistant, one obtains a device with a comb transmission function with the same period. However, to match the FPI transmission comb completely with the comb of nitrogen PRRS lines one should tune the wavelength of the radiation used to excite the PRRS of nitrogen exactly to the position of any minimum in the FPI transmission comb. Thus to achieve this task for the case of nitrogen PRRS one must take the FPI's free spectral range Dnu(f)= 4B(N(2)) and the wavelength of the exciting radiation such that (1/lambda(exc)) = 4B(N(2))(k + 1/2), where B(N(2)) is the rotational constant of the nitrogen molecule and k is an arbitrary integer number. In this case all (odd and even) pure rotational Raman lines of nitrogen will pass through the FPI while the line of exciting radiation is being suppressed. Additionally, a FPI cuts out the spectrally continuous sky background light from the spectral gaps between the PRRS lines.

Pressure Broadening, Shift, and Interference Effect for a Multiplet Line in the Rovibrational Anisotropic Stimulated Raman Spectrum of Molecular Oxygen

High-resolution stimulated inverse Raman spectroscopy has been applied to the study of collisional broadening, shifting, and line mixing for theOO(J,N= 5) triplet line of the fundamental vibrational band of molecular oxygen. Accurate line broadening coefficients for the individualJcomponents within the triplet have been measured for the first time and show a significantJdependence. The line broadening coefficients are larger than those previously obtained for unresolved pure rotational Raman lines. The additional broadening is expected to result from electronic spin relaxation. The pressure-induced line shift has been obtained for this line and compared to the value obtained for the fundamentalQbranch. By applying the Rosenkranz perturbation treatment to the collisionally mixed components of the triplet line, we have been able to obtain an estimate of the coupling parameters.

Collisional broadening and shifting of the pure rotational Raman lines S0(J = 0–4) of H2 at room temperature

The pressure broadening and shifting, of the S0(J = 0–4) pure rotational Raman lines of normal hydrogen, have been determined from 1 to 35 amagat units of density, at 29.8 °C. Over the range of densities explored, the width and shift of the lines are linear functions of the density. Our precise results may be reconciled with earlier measurements at higher densities by adding a term cubic in the density for the widths and quadratic in the density for the shifts. It is argued that the present results are safely within the impact regime and may be compared with calculations when they become available. The accuracy obtained is about 0.2 MHz per amagat [Formula: see text], for the broadening, and ranges from 0.1 to 0.3 MHz per amagat [Formula: see text] for the shifting coefficients. An analysis of the results indicate that it may be possible to combine the shifts of the pure vibrational and the pure rotational Raman lines to disentangle the contributions of the isotropic and anisotropic intermolecular forces to the perturbation of molecular spectra. Departures from linear broadening and shifting, at high density, are discussed.

Linewidths in the pure rotational Raman spectra of hydrogen chloride and carbon monoxide and of nitrogen and carbon monoxide perturbed by hydrogen chloride

AbstractPressure‐broadening coefficients, σp, have been determined for self‐broadening of pure rotational Raman lines of carbon monoxide and for pure rotational Raman lines of hydrogen chloride, and carbon monoxide and nitrogen perturbed by hydrogen chloride at 293 K. The results are discussed and compared with calculated and experimental σp values in the literature.

Line widths in the pure rotational Raman spectra of nitrogen, oxygen and hydrogen broadened by foreign gases

AbstractBroadening coefficients, σp, have been determined for the pure rotational Raman lines of nitrogen and oxygen, perturbed by hydrogen, helium and methane, and of hydrogen broadened by helium, argon, nitrogen and carbon monoxide, over the pressure range 2–100 bar with a host gas partial pressure of 1 bar. The results are summarized below:magnified imageThe broadening coefficients are discussed in relation to the dipole, quadrupole and octopole moments and the polarizabilities of the perturbing species.

Close-coupling calculations of transport and relaxation cross sections for H2 in Ar

We present benchmark close-coupling calculations of relaxation, transport, and Raman line shape cross sections for H2 colliding with Ar, using the BC3 (6,8) potential energy surface of Le Roy and Carley. The experimentally observable cross sections calculated include those for shear viscosity, binary diffusion, depolarized Rayleigh light scattering, and flow birefringence. The agreement with available experimental data is fair, except for the pressure shifting of the pure rotational Raman lines of H2 and the cross section for depolarized Rayleigh light scattering. It is shown that the Raman line shift cross sections are very sensitive to the dependence of the intermolecular potential on the H2 stretching coordinate, and the line shift data should be useful in determining this feature of the potential more accurately. Conversely, line shift data should not be used to determine intermolecular potentials when the vibrational dependence of the surface is neglected. The role of orbiting resonances in transport and relaxation cross sections is also investigated. Their effects are found to be small for transport cross sections, but they can contribute up to 30% to relaxation and linewidth cross sections at very low temperatures (&amp;lt;100 K). The resonance effects decrease quickly as the temperature is increased and are negligible at room temperature and above. Nevertheless, the existence of resonant effects must be remembered when performing thermal averaging of cross sections, since one energy point accidentally on resonance can greatly distort calculated results.

Single-pulse broadband rotational coherent anti-Stokes Raman-scattering thermometry of cold N_2 gas

Coherent anti-Stokes Raman scattering (CARS) from the pure rotational Raman lines of N(2) gas is employed to measure the instantaneous (10-nsec) rotational temperature of the gas at room temperature and below. An entire rotational CARS spectrum is generated by a single laser pulse using a broad-bandwidth dye laser and is recorded on an optical multichannel analyzer. A best-fit temperature is obtained for individual experimental spectra by comparison with calculated spectra. Good agreement between CARS temperatures and thermocouple temperatures is observed.

Experimental and calculated cross sections for pressure broadening of pure rotational Raman lines of HCl

The pure rotational Raman transitions J = 0 → 2 through J = 5 → 7 of HCl have been measured. Results are presented for HCl without admixture at 1 atm and 300 K, and for HCl with argon at pressures of about 10 and 20 atm. The HCl self-broadening is compared with earlier results. The linewidths of HCl broadened by argon are compared with the result of calculations. Cross sections are calculated based on different potentials which have been suggested for the ArHCl system.

High resolution vibrational Raman spectrum of oxygen

AbstractThe fundamental vibrational band of oxygen has been examined with a resolution of 0.05 cm−1 for the Q branch and 0.40 cm−1 for the S and O branches. All lines of the Q branch were clearly resolved except Q(1) and Q(3). Calculated molecular parameters agree with those previously reported from microwave and electronic spectra. Line width measurements made in the Q branch and on three S branch lines, using a resolution of 0.05 cm−1, are in fair agreement with previously measured and calculated line widths for pure rotational Raman lines.