Pure Rotational Raman Spectra Research Articles

Optical design of a reflective double-grating spectrometer to detect one branch in the pure rotational Raman spectrum of N2.

This paper presents the optical design of a high-resolution double-grating spectrometer for extracting the multiple lines in the Stokes or anti-Stokes branch of the pure rotational Raman spectra of nitrogen. The spectrometer is composed of collimating and focusing mirrors, two reflective gratings, and a linear detector. The structural parameters were calculated using geometric configuration, dispersion, and aberrational theory, and conditions for first-order correction of keystone distortion with divergent grating illumination were derived. Based on this method, we simulated a spectrometer with a 16-channel linear array photomultiplier tube, resulting in uniformly distributed single-branch lines on each detector channel. The resolution reached 0.225nm per channel, and the keystone distortion was less than 0.7%. The spectrometer avoids the interference of elastic signals by not detecting them, enabling the extraction of atmospheric temperature profiles via separated single-branch lines with high precision. Our design provides a promising solution to extract atmospheric temperature profiles for pure rotational Raman lidar.

Visualizing ultrafast weak-field-induced rotational revivals of air molecules at room temperature.

The ability to observe quantum coherence and interference is crucial for understanding quantum effects in nonlinear optical spectroscopy and is of fundamental interest in quantum mechanics. Here, we present an experimental study combined with theoretical analysis and numerical simulations to identify the underlying process behind the rotational revivals induced by a pair of time-delayed ultrafast femtosecond laser pulses for air molecules under ambient conditions. Our time-resolved two-dimensional alignment measurements confirm that one-step non-resonant Raman transitions from initial states of mixed molecules play a dominant role, showing a signature of weak-field-induced rotational revivals. Furthermore, we demonstrate that such rotational revival spectra can simultaneously measure the entire pure rotational Raman spectra and observe the quantum interference between two transition pathways from a given initial state. This work provides a powerful tool to observe, control, and identify the rotational dynamics of mixed molecular samples under weak-field excitations.

LITES: rotational Raman spectra of air molecules measured by high-resolution-spectroscopy lidar.

We present the first, to the best of our knowledge, measurements from a new lidar facility that was designed and built at the University of Hertforshire since 2012. LITES (Lidar Innovations for Technologies and Environmental Sciences) allows testing, developing, and measuring of a multitude of climate-change relevant parameters of atmospheric particulate pollution and photochemically reactive trace gases. The core of LITES consists of a lidar spectroscopy instrument. In this first contribution, for example, we present the design and specifications of this instrument, its performance, and potential applications. First, we show examples of the measurements of range-resolved pure rotational Raman spectra and rotational-vibrational Raman spectra of air molecules with a spectral resolution better than 5cm-1. We also present day-time temperature profiles obtained from pure rotational spectroscopic lidar signals. In future work, we aim to explore the potential of our multi-channel high-resolution spectrometric lidar to obtain vertically resolved chemical characterization of aerosols and trace gases.

A Simple Calibration Method of Anti-Stokes-Stokes Raman Intensity Ratios Using the Water Spectrum for Intracellular Temperature Measurements.

Presented here is a facile and practical method for calibrating anti-Stokes-Stokes intensity ratios in low-frequency Raman spectra that is devised specifically for temperature measurements inside cells. The proposed method uses as an intensity standard the low-frequency Raman spectrum of liquid water, a major molecular component of cells, whose temperature is independently measured with a thermocouple. Rather than calibrating pixel intensities themselves, we obtain a correction factor at each Raman shift in the 20-200 cm-1 region by dividing the anti-Stokes-Stokes intensity ratio calculated theoretically from the Boltzmann factor at the known temperature by that obtained experimentally. The validity of the correction curve so obtained is confirmed by measuring water at other temperatures. The anti-Stokes-Stokes intensity ratios that have been subjected to our calibration are well fitted with the Boltzmann factor within ∼1% errors and yield water temperatures in fairly good agreement with the thermocouple temperature (an average difference ∼1 ℃). The present method requires only 15 min of spectral acquisition time for calibration, which is 50 times shorter than that in a recently reported calibration method using the pure rotational Raman spectrum of N2. We envision that it will be an effective asset in Raman thermometry and its applications to cellular thermogenesis and thermoregulation.

Pure rotational coherent anti‐Stokes Raman scattering spectroscopy of nitric oxide: Determination of Raman tensor invariants

AbstractCoherent anti‐Stokes Raman scattering (CARS) measurements of the pure rotational Raman spectrum of nitric oxide were performed. Measurements were performed in a room‐temperature gas cell using a dual‐broadband pure rotational CARS configuration. In this configuration, a broadband dye laser was used to generate the pump and Stokes beams, and the 355‐nm third‐harmonic beam from a Nd:YAG laser was used as the probe beam. The Raman spectrum of NO is of significant theoretical interest because of the spin splitting in the ground electronic level of NO. A detailed model of the pure rotational Raman spectrum of NO was developed based on an irreducible tensor analysis using Hund's case (a) wave functions as basis states for the analysis. The pure rotational Raman polarizability tensor element in the molecular frame was determined for NO based on fitting pure rotational CARS spectra of mixtures of NO and nitrogen (N2). In addition to the pure rotational CARS spectrum featuring 2Π1/2→2Π1/2 and 2Π3/2→2Π3/2 transitions, there is an electronic Raman transition at 121 cm−1 between the spin split 2Π1/2 and 2Π3/2 ground electronic levels that has been the subject of previous theoretical interest, but the value of the tensor invariant that contributes to the intensity of the electronic Raman transitions has not been quantitatively determined. Analysis of our pure rotational CARS spectra indicates that the magnitude of this term is much smaller than indicated in previous work, but the analysis is complicated by the weakness of the electronic Raman transitions compared with pure rotational transitions in the same spectral region. Future experiments to more definitively determine the magnitude and sign of the polarizability tensor element are proposed.

Testing the ab initio quantum‐scattering calculations for the D2–He benchmark system with stimulated Raman spectroscopy

AbstractThis work presents a comparison between experimental and calculated values for the collisional line broadenings and shifts of the S0(0), S0(1), and S0(2) lines of the rotational Raman spectrum of D2 perturbed by He at 77, 195, and 300 K. The pure rotational Raman spectra were obtained by means of a stimulated Raman spectroscopy experimental setup. Close coupling dynamical calculations were performed on the most recent ab initio H2–He potential energy surface. The resulting scattering matrix elements implemented in the general Hess method allow us to provide pressure broadening, pressure shifting, and Dicke coefficients from 10 to 400 K. Experimental and calculated values agree well with each other.

Time-resolved electron density and electron temperature measurements in nanosecond pulse discharges in helium

Thomson scattering is used to study temporal evolution of electron density and electron temperature in nanosecond pulse discharges in helium sustained in two different configurations, (i) diffuse filament discharge between two spherical electrodes, and (ii) surface discharge over plane quartz surface. In the diffuse filament discharge, the experimental results are compared with the predictions of a 2D plasma fluid model. Electron densities are put on an absolute scale using pure rotational Raman spectra in nitrogen, taken without the plasma, for calibration. In the diffuse filament discharge, electron density and electron temperature increase rapidly after breakdown, peaking at ne ≈ 3.5 · 1015 cm−3 and Te ≈ 4.0 eV. After the primary discharge pulse, both electron density and electron temperature decrease (to ne ~ 1014 cm−3 over ~1 µs and to Te ~ 0.5 eV over ~200 ns), with a brief transient rise produced by the secondary discharge pulse. At the present conditions, the dominant recombination mechanism is dissociative recombination of electrons with molecular ions, . In the afterglow, the electron temperature does not relax to gas temperature, due to superelastic collisions. Electron energy distribution functions (EEDFs) inferred from the Thomson scattering spectra are nearly Maxwellian, which is expected at high ionization fractions, when the shape of EEDF is controlled primarily by electron–electron collisions. The kinetic model predictions agree well with the temporal trends detected in the experiment, although peak electron temperature and electron density are overpredicted. Heavy species temperature predicted during the discharge and the early afterglow remains low and does not exceed T = 400 K, due to relatively slow quenching of metastable He* atoms in two-body and three-body processes. In the surface discharge, peak electron density and electron temperature are ne ≈ 3 · 1014 cm3 and Te ≈ 4.25 eV, attained after the surface ionization wave reaches the grounded electrode. The sensitivity of the present diagnostics is too low to measure electron density in the plasma during surface ionization wave propagation (estimated to be below ne ≈ 1013 cm−3). After peaking during the primary current pulse, the electron density decays due to dissociative recombination. Electron temperature decreases rapidly over ~150 ns after the primary current pulse rise, to Te ≈ 0.5 eV, followed by a much more gradual electron cooling between the primary and the secondary discharge pulses, due to superelastic collisions providing moderate electron heating in the afterglow.

A New Method for Measuring Atmospheric Temperature and Aerosol Backscattering Coefficient Using a Pure Rotational Raman Lidar

AbstractIt has been proved to be a high precision method to derive atmospheric temperature with a pure rotational Raman signal of atmospheric molecules and many pure rotational Raman lidars have been established around the world. The sum of all pure rotation Raman spectral lines is independent of temperature. By using this feature, we can determine the aerosol extinction without any assumption. This paper presents a new method for measuring atmospheric temperature and aerosol backscattering coefficient based on the two single lines of pure rotational Raman spectrum (J = 4 and 14) detected by a pure rotation Raman lidar with an additional Mie channel. The concept and mathematical derivation are given in the paper.

Comparison of signal processing methods in remote temperature measurements by pure rotational Raman spectra

Using estimation theory, we study an optimal algorithm for estimating temperature by the rotational Raman spectra of light scattering by molecules of atmospheric nitrogen and oxygen. The simulation technique is used for comparing the optimal estimation algorithm with the parameter fitting procedure and an heuristic algorithm commonly used for calculating temperature by pure rotational spectra.

Daytime operation of a pure rotational Raman lidar by use of a Fabry–Perot interferometer

We propose to use a Fabry-Perot interferometer (FPI) in a pure rotational Raman lidar to isolate return signals that are due to pure rotational Raman scattering from atmospheric nitrogen against the sky background. The main idea of this instrumental approach is that a FPI is applied as a frequency comb filter with the transmission peaks accurately matched to a comb of practically equidistant lines of a pure rotational Raman spectrum (PRRS) of nitrogen molecules. Thus a matched FPI transmission comb cuts out the spectrally continuous sky background light from the spectral gaps between the PRRS lines of nitrogen molecules while it is transparent to light within narrow spectral intervals about these lines. As the width of the spectral gaps between the lines of the PRRS of nitrogen molecules is -114 times the width of an individual spectral line, cutting out of the sky background from these gaps drastically improves the signal-to-background ratio of the pure rotational Raman lidar returns. This application of the FPI enables one to achieve daytime temperature profiling in the atmosphere with a pure rotational Raman lidar in the visible and near-UV spectral regions. We present an analysis of application of the FPI to filtering out the pure rotational Raman lidar returns against the solar background. To demonstrate the feasibility of the approach proposed, we present temperature profiles acquired during a whole-day measurement session in which a Raman lidar equipped with a FPI was used. For comparison, temperature profiles acquired with Vaisala radiosondes launched from the measurement site are also presented.

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.

Analytical expression for the relative intensities of pure rotational raman spectra

The relative intensities of the Stokes and anti-Stokes lines of the pure rotational Raman transitions ( vJ ↔ vJ′) of a diatomic molecule are considered; these are proportional to [ z v ( J)]; where z v(J) = 〈vJ¦y¦vJ + 2〉 〈vJ¦y¦vJ − 2〉 and γ is the polarizability anisotropy. By using the Rayleigh-Schrödinger perturbation theory, it is shown that z v ( J) is given by a simple analytical expression for any potential, whether empirical or of the RKR-type and any operator γ; this expression implies one coefficient δ v , depending on the pure vibrational wavefunction Ψ v and its rotational corrections. From this expression another simple formula, totally independent from Ψ v , is obtained allowing the calculation of z v ( J ± j)( j = 1, 2, 3,…) when one value z v ( J) is provided. This expression is valid for any value of J and j. The numerical application to the ground states of H 2 and CO molecules shows the validity and the accuracy of the present formulation.

Rotational Raman scattering in the instructional laboratory

We describe the use of a single spectrometer and a low power argon laser for the study of the pure rotational Raman spectra of H2, O2, N2, and CO2. Analyses of the spectra allow the student to obtain internuclear separations in the ground state, the rotational temperature of the gas, and the influence of nuclear spin on the allowed rotational states. The four molecules chosen for study have nuclei with spins of I=0 (O2), 1/2 (H2), and 1 (N2) and thus the discussion includes the effects of both Bose and Fermi exchange properties on the symmetry of the overall wave function. To fully illustrate the range of information available from the data, we provide spectra obtained with a high quality double spectrometer which allow even the lowest rotational states to be observed and show the stretching of the H2 molecule due to centripetal forces.

High resolution rotational CARS spectrum of oxygen

A low frequency, high resolution CARS spectrometer is described which has been used to resolve the closely spaced Δ N = 2, Δ J = 2 triplets of oxygen in the pure rotational Raman spectrum. The transition frequencies and intensities from N = 1–19 are extracted from the experimental data with the aid of spectral simulation routines that account for the ∥χ∥ 2 interference effects and the contribution of various line broadening mechanisms to the CARS spectrum. The frequencies match well those predicted using molecular constants obtained from magnetic dipole microwave studies and a Hamiltonian appropriate to a coupling scheme intermediate between Hund's cases (a) and (b). A small, systematic discrepancy is observed between experimental and theoretical intensities which may result from a J dependent linewidth variation within each triplet.

Line widths in the pure rotational Raman spectra of hydrogen and deuterium self-broadened and broadened by foreign gases

The pressure-broadening coefficients for the self-broadened S 0 (0), S 0 (1) and S 0 (2) pure rotational aman lines of hydrogen and the S 0 (0) to S 0 (4) lines of deuterium were determined experimentally over the pressure range 1-100 bar and the temperature range 90-375 K. Broadening coefficients were also determined for the same lines in hydrogen and deuterium perturbed by helium, argon, oxygen, nitrogen, carbon monoxide and hydrogen chloride over the pressure range 2-100 bar, with a partial pressure of the host gas of 1 bar and a temperature of 293 K. In addition, the S 0 (0) pure rotational line of deuterium broadened by carbon monoxide and nitrogen was studied over the same pressure range and a temperature range of 100-293 K. The results are discussed and compared with those of previous studies in the literature

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.

Reinvestigation of the Raman spectra of dihydrogen trapped in rare gas solids. I. H2, HD, and D2 monomeric species

The vibration–rotation and pure rotational Raman spectra of H2, HD, and D2 trapped in Ar, Kr, and Xe matrices have been recorded at 9 K. The frequencies which have been measured within an accuracy of 0.3 cm−1 are compared to the results of recent calculations. Except for the S0(0) line of HD the agreement between observed and calculated matrix shifts is satisfactory. The anomalous blue shift observed for the S0(0) line of HD is well interpreted within the rotation translation coupling (RTC) framework.

Pressure broadening and line coupling in bending bands of CO2

The pressure broadening and line coupling cross sections in the Fano–Ben Reuven theory of line shapes are calculated for bending bands of CO2 in a bath of He atoms. Molecular collision dynamics are simplified by invoking the infinite order sudden (IOS) approximation for molecular rotational and vibrational angular momentum in a manner similar to but not identical with the method developed by Clary and shown to be accurate for CO2–He. Numerical values are obtained using a pairwise additive interaction potential developed by Clary. Predictions are in good accord with data for various infrared bands and pure rotational Raman spectra. It is found that all the pressure broadening and state-to-state cross sections depend on only a few dynamical factors (generalized IOS cross sections) and are therefore closely interrelated. Results are used to assess models developed previously to analyze line shapes in this and similar systems.

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.

ChemInform Abstract: The Pure Rotational Raman Spectrum of 1,3,5‐Trideuterobenzene, C61H32H3.

Abstract The pure rotational Raman spectrum of C 6 1 H 3 2 H 3 has been recorded photographically at 288 K and analysed to yield values of the rotational constants B 0 and D J . In combination with the B 0 values for C 6 1 H 6 and C 6 2 H 6 the bond lengths r (C—H) and r 0 (C—;C) were calculated; r 0 (C—H) = 108.60 ± 0.04 pm; r 0 (C—C) = 139.660 ± 0.008 pm.