Accurate intensity calibration for low wavenumber (−150 to 150 cm−1) Raman spectroscopy using the pure rotational spectrum of N2

We report on an accurate intensity calibration method for low wavenumber Raman spectroscopy. It uses the rotational Raman spectrum of N₂. The intensity distributions in the rotational Raman spectra of diatomic molecules are theoretically well established. They can be used as primary intensity standards for intensity calibration. The intensity ratios of the Stokes and anti‐Stokes transitions originating from the same rotational levels are not affected by thermal population. Taking the effect of rotation–vibration interactions appropriately into account, we are able to calculate these intensity ratios theoretically. The comparison between the observed and calculated ratios of the N₂ pure rotational spectrum provides an accurate relative sensitivity curve (error ~5 × 10⁻⁴) in the wavenumber region of −150 to 150 cm⁻¹. We determine the temperature of water solely from the low wavenumber Raman spectra, using a thus calibrated spectrometer. The Raman temperature shows an excellent agreement with the thermocouple temperature, with only 0.5 K difference. The present calibration technique will be highly useful in many applications of low wavenumber quantitative Raman spectroscopy. Copyright © 2015 John Wiley & Sons, Ltd.     

Diode-laser spectroscopy of supersonic free jets

A supersonic-free-jet infrared spectrometer has been constructed for investigation of molecular vibrational spectra at low rotational and vibrational temperatures. The sensitivity of measurement in a pulsed jet is increased by employing a phase-sensitive detection method synchronized with the pulse frequency. The performance of the spectrometer is examined for the absorption lines of the NH₃ v ₂ band. A rotational temperature as low as 16K is attained when seeded in He. Cold-jet spectra are demonstrated for thev ₃ bands of PF₅,³⁴SF₆, and¹⁸²WF₆.     

Line-Mixing Effects inQBranches of CO2

A theoretical model based on the energy corrected sudden (ECS) approximation is used in order to account for line-mixing effects in Δ ↔ Π infraredQbranches of¹²C¹⁶O₂. Its quality is demonstrated by comparisons with numerous laboratory spectra of CO₂–He and CO₂–N₂mixtures: threeQbranches in the 4 and 17 μm regions are investigated at room temperature in a wide pressure range. The influence of mixing betweenQ(J) lines associated with odd and even values of the rotational quantum numberJis demonstrated and analyzed in detail. It is shown that, in contrast to available fitting law approaches, the ECS model correctly predicts the influence of the parity of the rotational quantum numbersJandJ′ on coupling between theQ(J) andQ(J′) lines. Comparisons between the effects of collisions of CO₂with N₂and He are made and analyzed. They show that these two systems involve different line couplings within theQbranch.     

Temperature dependence of nitrogen and oxygen broadening of the 16O3 ν 3 band

High-resolution Fourier transform absorption spectra of ozone broadened by N₂ and O₂ have been recorded at room temperature and at 225 K. Nitrogen- and oxygen-broadened half-widths and their temperature dependence for respectively 160 ro-vibrational transitions on the ¹⁶O₃ ν ₃ band with a wide range of rotational quantum numbers J (2–48) and K_a (0–8) have been determined.     

The Rotational Analysis of the Three New Virbrational Bands of NO2 in the Range 5680–5720 Å

The three new vibrational bands in the range 5680–5720 Å of the fluorescence excitation spectra of NO₂ were measured at the normal temperature and were assigned with the rotational quantum numbers. In these bands there are strong spin and rotational forbidden transitions which express the complexity of NO₂ spectra.     

Four-photon polarization spectroscopy of the rotational resonances in liquids in the range 0–3 THz

The spectra of the coherent molecular rotation that coincide with the rotational spectra of the corresponding molecules in the gas phase are measured for the first time using four-photon coherent laser spectroscopy in the range 0–100 cm^?1 in several liquids (CCl₄, H₂O₂, D₂O, and H₂O). The measured spectra make it possible to separate the spectral contributions of the slow rotational molecular motions about the equilibrium and the fast rotations. The selectivity of the action of the microwave radiation on biological objects can be increased using the results obtained.     

Low temperature laser absorption spectra of methane in the near-infrared at 1.65 μm for lower state energy determination

Direct absorption spectra of the 2v₃ band of methane (CH₄) from 6038 to 6050 cm^-1 were studied at different low temperatures using a newly developed cryogenic cell in combination with a distributed feedback (DFB) diode laser. The cryogenic cell can operate at any stabilized temperature ranging from room temperature down to 100 K with temperature fluctuation less than ±1 K within 1 hour. In the present work, the CH₄ spectra in the range of 6038-6050 cm^-1 were recorded at 296, 266, 248, 223, 198, and 176 K. The lower state energy E″ and the rotational assignment of the angular momentum J were determined by a “2-low-temperature spectra method” using the spectra recorded at 198 and 176 K. The results were compared with the data from the GOSAT and the recently reported results from Campargue and co-workers using two spectra measured at room temperature and 81 K. We demonstrated that the use of a 2-low-temperature spectra method permits one to complete the E″ and J values missed in the previous studies.     

Rotationally-Resolved Excitation Spectroscopy of the Methoxy Radical in a Supersonic Jet

Abstract

Laser-induced fluorescence excitation spectra of the methoxy radical have been obtained under sufficiently high resolution in a supersonic jet expansion. The rotational structure associated with its origin band has been identified in the midst of strong overlapping rotational transitions due to the hydroxyl radical in the 31490–31700 cm^?1 spectral region. Rotationally-resolved A ²A₁ - X ²E spectra of the 0₀ ⁰ band of methoxy have been explicitly assigned using the nomenclature for prolate symmetric top transitions in doublet states.     

Temperature Distributions Across the Plasma Layer of Planar Low Pressure Microwave Plasmas — A Comparative Investigation by Optical Emission Spectroscopy

Nowadays low temperature non-equilibrium plasmas received considerable attention in very different fields of plasma processing. The subject of the present paper is the comparative measurement of neutral gas temperature and optical excitation temperature to analyze the temperature distributions across the plasma layer of H₂ non-equilibrium plasmas (p = 0.2 – 1.5 kPa) with small admixtures of hydrocarbons in a novel planar microwave plasma source (2.45 GHz) used for plasmachemical deposition purposes by means of optical emission spectroscopy. Typical microwave power flux densities into the plasma lie within a range of 2 W cm^?2 to 20 W cm^?2. Results of neutral gas temperature measurements derived from H_α line Doppler profiles are compared with rotational temperatures of H₂ and N₂ molecules. The neutral gas temperature (800–1700 K) corresponds to the rotational temperature of the H₂ molecules (Fulcher band, R 0–0 branch) but shows a more distinct spatial gradient. The rotational temperature of admixtured N₂ molecules (2000–3000 K) is much more higher although Boltzmann distribution was ensured. The spatially resolved measured excitation temperature (1–3 eV) determined with the help of line intensity ratios of admixtured Ar well agrees with Langmuir probe measurements. The reported measurements as a whole demonstrate the feasibility of comparative investigations of different optically determined temperatures for expressive characterization of low pressure microwave plasmas.     

Rotational CARS for simultaneous measurements of temperature and concentrations of N2, O2, CO, and CO2 demonstrated in a CO/air diffusion flame

Rotational coherent anti-Stokes Raman spectroscopy (CARS) has over the years demonstrated its strong potential to measure temperature and relative concentrations of major species in combustion. A recent work is the development and experimental validation of a CO₂ model for thermometry, in addition to our previous rotational CARS models for other molecules. In the present work, additional calibration measurements for relative CO₂/N₂ concentrations have been made in the temperature range 294-1246 K in standardized CO₂/N₂ mixtures. Following these calibration measurements, rotational CARS measurements were performed in a laminar CO/air diffusion flame stabilized on a Wolfhard-Parker burner. High-quality spectra were recorded from the fuel-rich region to the surrounding hot air in a lateral cross section of the flame. The spectra were evaluated to obtain simultaneous profiles of temperature and concentrations of all major species; N₂, O₂, CO, and CO₂. The potential for rotational CARS as a multi-species detection technique is discussed in relation to corresponding strategies for vibrational CARS.     

Electron spin relaxation studies of La@C82 in the solid state by EPR

The temperature dependence of EPR spectrum of La@C₈₂ in the powder of empty C₆₀ and C₇₀ mixed crystals was studied by EPR spectroscopy employing X- and Q-band microwave frequencies. The rigid limit spectra (at 4.2 K for the X-band and at 132 K for the Q-band) could be analyzed by static spectral simulation which yielded the EPR parameters,g _‖=2.0021,g _⊥=1.9970,^La A _⊥=7.8 MHz,^La A _‖～0 MHz and an isotropic¹³C coupling value of about 3 MHz. For higher temperatures an appreciable motional averaging effect was observed and the spectra were analyzed by using dynamic spectral simulation based on the stochastic Liouville equation, where we assumed an isotropic rotational motion with the Brownian diffusion. The calculated spectra reproduced the dominant feature of the temperature dependence of the spectra almost satisfactorily for both the X-and Q-band frequencies with the appropriate rotational correlation times. The Arrhenius plots of the correlation time gave two activation energies of 0.9 kcal/mol and 2.9–3.8 kcal/mol for the temperatures below and above 200 K, respectively.     

S1 ← S0 laser excitation spectra of glyoxal in a supersonic jet: High-resolution rotational analysis

The high-resolution rotational spectra of several S₁ ← S₀ vibrational transitions of glyoxal are obtained and analyzed. Undispersed fluorescence excitation spectra are obtained with a CW ring dye laser in a supersonic jet, providing a linewidth down to 100 MHz. Rotational constants and band origins are determined with an asymmetric rotator program. Population distribution in the supersonic jet is studied. The overtones of the torsion mode (ν₇) are examined as an approach to the trans-cis potential barrier. The 8₀¹ and 8₀¹ 7₀² bands of A + B type, which are too congested at room temperature experiments, are well resolved in this jet experiment. As a result, the rotational constants obtained by fitting low-J and -K rotational transitions are in good agreement with the constants obtained by fitting large-J and -K transitions of room temperature spectra. Furthermore, on about 1000 analyzed lines no rotational perturbation has been observed.     

The Rotational Analysis of the Five new Vibronic Bands of No2 in the Range 5050-5200 Å

The five new vibrational bands in the range 5050-5200 Å of the laser induced fluorescence excitation spectra of NO₂ were measured and rotationally assigned at room temperature. Though the spectra were rather congested, we can determine the band origins, and rotational and spin-rotation constants for these bands. All rotational structures analyzed are of the parallel type. It was shown that the electronic excited state òB₂ were heavily perturbed by the high lying vibration levels of ground state [Xtilde] ² A ₁ and that the interactions between these two electronic states was the main rationale for the complexity of NO₂ visible spectra.     

Possible mechanisms of dielectric relaxation of liquid water and calculation of the temperature dependence of the complex permittivity of water

The spectra of the complex dielectric permittivity and absorption of water (H₂O) in the frequency range 0–1000 cm^?1 are calculated for a wide temperature interval. Using the method of autocorrelation functions, the dielectric response of dipoles rotating in potential wells of three types is found. The majority of dipoles (about 90%) rotate in a deep and comparatively narrow potential well, whose profile resembles an upside-down hat. Such a potential models a molecular structure with strongly bent and/or broken hydrogen bonds. The hat model describes the complex permittivity in the low-frequency (Debye) range and in the range 300–1000 cm^?1. The remaining dipoles (～10%) execute harmonic vibrations of two types: rotational vibrations about the equilibrium direction of a hydrogen bond and translational vibrations along this direction. These types of motion yield the dielectric response in the frequency range 10–300 cm^?1. This response is described by the Lorentz lines in terms of the harmonic oscillator model and the truncated parabola model. The hat–harmonic oscillator–truncated parabola composite model provides good agreement with experimental spectra. The lifetimes of the three types of motion considered are about 10, 0.2, and 0.05 ps, respectively. They characterize (i) tetrahedral translations of molecules accompanied by their rotations, (ii) librations of dipoles in the hatlike potential well, and (iii) elastic interactions of hydrogen-bonded molecules. Based on data of independent methods of investigation, it is concluded that the temperature 300 K is a singular point with respect to the properties of liquid water.     

Measurements of N2- and O2-broadening and shifting parameters of methane spectral lines in the 5550–6236 cm−1 region

The absorption spectra of mixtures of methane (CH₄) with N₂ and O₂ at different partial pressures of both CH₄ and buffer gases for three temperatures 240, 267, and 296 K have been recorded using the Bruker IFS 125 HR FTIR spectrometer in the 5550–6236 cm^?1 region. The multispectrum fitting procedure has been applied to these spectra to recover the spectral line parameters. The main goal of this procedure was the determination of the N₂- and O₂-broadening and shifting coefficients and the exponents of their temperature dependences. These parameters have been derived for 452 assigned lines with good values of the signal to noise ratio. The rotational dependence of the mean values of these parameters is discussed. The temperature dependence exponents were observed for both N₂ and O₂ buffer gases.     

Coherent laser spectroscopy of RF resonances in liquid

Four-photon polarization spectra of double distilled water subjected to a special treatment in a cavitation chamber and 20% aqueous solution of hydrogen peroxide were recorded in the range ±8 cm⁻¹. All recorded spectra contain narrow (< 0.3 cm⁻¹) resonances corresponding to the frequencies of the rotational spectrum of ortho and para spin isomers of the H₂O molecule. Numerical simulation of the spectra obtained made it possible to quantitatively estimate the contribution of the rotational spectrum to the coherent scattering signal. It was found that the contribution of the para spin isomer of the H₂O molecule to the rotational line spectrum decreases in an aqueous solution of the α-chymotrypsin protein. Apparently, this decrease indicates the selectivity of interaction of biopolymer molecules with different spin isomers.     

Low frequency four-wave mixing spectroscopy of biopolymer hydration layers in aqueous solutions

Four-wave mixing (FWM) spectroscopy has been applied to detection of H₂O₂ molecules rotational resonances in both DNA and denatured DNA aqueous solutions in the range ±100 cm⁻¹. A considerable growth of rotational lines intensity of H₂O and H₂O₂ has been observed in comparison with distilled water. This fact was interpreted as an exhibition of specific property of a hydration layer formation at DNA/water and denatured DNA/water interfaces. The fitting of four-wave mixing spectra shows the increasing of the H₂O₂ rotational line’s amplitude by a factor of ∼3 in DNA solutions due to denaturizing. The shifting of FWM Brillouin resonances in opposite way in protein solution and SWNT (single wall carbon nanotube) suspension to comparison with water was observed and discussed.     

Raman‐induced Kerr effect spectroscopy of single‐wall carbon nanotubes aqueous suspensions in the range 0.1–10 and 100–250 cm−1

Here, we study a low (less than 0.1 µg/ml) concentration aqueous suspension of single‐wall carbon nanotubes (SWNTs) by Raman‐induced Kerr effect spectroscopy (RIKES) in the spectral bands 0.1–10 and 100–250 cm⁻¹. This method is capable of carrying out direct investigation of SWNT hydration layers. A comparison of RIKES spectra of SWNT aqueous suspension and that of milli‐Q water shows a considerable growth in the intensity of low wavenumber Raman modes. These modes in the 0.1–10 cm⁻¹ range are attributed to the rotational transitions of H₂O₂ and H₂O molecules. We explain the observed intensity increase as due to the production of hydrogen peroxide and the formation of a low‐density depletion layer on the water–nanotube interface. A few SWNT radial breathing modes (RBM)are observed (ω_RBM = 118.5, 164.7 and 233.5 cm⁻¹) in aqueous suspension, which allows us to estimate the SWNT diameters (∼2.0, 1.5, and 1 nm, respectively). Copyright © 2011 John Wiley & Sons, Ltd.     

Pure Rotational Raman Spectra of Vapor Phase Methanols

Abstract

The rotational Raman spectra of four vapor phase isotopic methanols, CH₃OH, CH₃OD, CD₃OH and CD₃OD, have been reported for the first time in the wavenumber regions from 5 to 100–120 cm^?1. The major parts of the spectra consist of bands equispaced at 3.19, 3.04, 2.56 and 2.46 cm^?1 intervals, respectively, and have been interpreted as the pure rotational S-branch transitions.     

The assignment of quantum numbers in the theoretical spectra of the H2 16O, H2 17O, and H2 18O molecules calculated by variational methods in the region 0–26000 cm−1

Quantum numbers have been assigned in the theoretical spectra of three isotopologues of the water molecule: H₂ ¹⁶O, H₂ ¹⁷O, and H₂ ¹⁸O. The spectra were calculated by variational methods in the region 0–26000 cm^?1 at a temperature of 296 K. For each molecule, the quantum numbers are assigned to more than 28000 levels. The quantum numbers are assigned to 216766, 210679, and 211073 spectral lines of the H₂ ¹⁶O, H₂ ¹⁷O, H₂ ¹⁸O molecules, respectively. The theoretical spectra with the assigned quantum numbers are available in the Internet.