Modeling of N2+ first negative spectra excited by electron impact on N2

Previously published intensity formulas for the calculation of N₂⁺ first negative spectra excited by electron impact on N₂ given by Herzberg and Muntz are generalized and adapted to arbitrary N₂X vibrational temperatures. The formulation includes all the quantities necessary for detailed computer modeling. Despite the refinements, the rotational structure and band profiles generated by the different methods agree to within a few percent at both normal (300°K) and elevated (5000°K) rotational and vibrational temperatures. Discrepancies in published A-values are noted and a recalculated array is given.     

Formaldehyde around 3.5 and 5.7-μm: Measurement and calculation of broadening coefficients

Self- and N₂-broadening coefficients of H₂CO have been retrieved in both the 3.5 and 5.7-μm spectral regions. These coefficients have been measured in FT spectra for transitions with various J (from 0 to 25) and K values (from 0 to 10), showing a clear dependence with both rotational quantum numbers J and K. First, an empirical model is presented to reproduce the rotational dependence of the measured self- and N₂-broadening coefficients. Then, calculations of N₂-broadening of H₂CO were made for some for 3296 ν₂ transitions using the semi-classical Robert-Bonamy formalism. These calculations have been done for various temperatures in order to obtain the temperature dependence of the line widths. Finally, self- and N₂-broadening coefficients, as well as temperature dependence of the N₂-widths has been generated to complete the whole HITRAN 2008 version of formaldehyde (available as supplementary materials).     

Time-domain measurements of S-branch N2–N2 Raman linewidths using picosecond pure rotational coherent anti-Stokes Raman spectroscopy

Time-resolved dual-broadband picosecond pure rotational CARS has been applied to measure self-broadened S-branch N₂–N₂ Raman linewidths in the temperature range 294–1466 K. The coherence decays were detected directly in the time domain by following the J-dependent CARS signal decay as a function of probe delay. The rotational Raman N₂–N₂ linewidths were derived from these time-dependent decays and evaluated for thermometric accuracy. Comparisons were made to the energy-corrected sudden (ECS) and modified exponential gap (MEG) dynamical scaling laws, and the results were used to quantify the sensitivity of nanosecond rotational CARS thermometry to the linewidth model employed. The uncertainty based on the linewidth model used in pure N₂ was found to be 2 %. The merits and limitations of this rapid method for the determination of accurate Raman linewidths are discussed.     

Infra-red spectrum,structure and properties of the N2-Ar van der Waals molecule

The first spectroscopic observation of bound N₂-Ar van der Waals molecules has been achieved with a cryogenic long path cell maintained at 87 K. The infra-red spectrum exhibits prominent fine structure near the N₂ stretching frequency which is assigned to hindered internal rotation of N₂ within the weakly bound complex. An analysis of this fine structure yields a T-shaped equilibrium geometry in which the N₂ bond axis is perpendicular to the N₂-Ar van der Waals bond axis. The observed spectrum is shown to be consistent with an internal rotational barrier of 20 cm^-1 (57 cal/mole). Approximately 20 per cent of the bound species are trapped by this rotational barrier and acquire a locked semi-rigid structure. The remaining 80 per cent have ill-defined geometry and undergo hindered internal rotation. The rotational envelope of an infra-red fundamental is analysed to give an estimate of the N₂-Ar bond length as 3·9 Å.     

Zeeman relaxation of N2 + (2Σ+) in collisions with 3He and 4He

We compare the cross sections for the transitions changing the projection of the total angular momentum of N₂ ⁺(²Σ) in collisions with ³He and ⁴He at very low collision energy. The fundamental states of the two nuclear spin isomers of N₂ ⁺ are considered as well as the two fine structure levels of the first excited para level N=2. It is shown that the two fundamental states of the two nuclear spin isomers behave differently. For the fundamental para level N=0 of N₂ ⁺, the projection changing cross section is always negligible compared to the elastic one for both He isotopes. For the fundamental ortho level N=1 of N₂ ⁺, the spin-rotation interaction couples the different spin levels directly so the spin relaxation becomes a first order process. The associated resonances increase the projection changing cross section which remains smaller but becomes comparable with the elastic one. This is in contrast with the excited rotational levels of N₂ ⁺, which for the rotational deactivation and elastic channels are found to be equal around the resonances for the collisions involving ³He. These two channels are always larger than the projection changing one. We also find that, for transitions involving the fundamental rotational state, the domain of validity of the threshold laws discussed by Krems and Dalgarno [Phys. Rev. A 67, 050704 (2003)] for a potential decreasing faster than 1/r2 is shortened, due to the long range charge induced dipole potential. This effect is illustrated for the collisions of ³He with the fundamental para state of N₂ ⁺.     

Spectral study of reactions involving organic vapours and the pink afterglow of nitrogen

Chemiluminescent reactions resulting from addition of C₂H₂ and BrCN to nitrogen showing a pink afterglow have been investigated. The spectra show CN(B→A), CN(B→X), CN(A→X), and N₂(IIP) bands. The CN(B→X) bands are intense and exhibit abnormal rotational distributions with two rotational temperatures (T₁ = 2400°K, T₂ = 400°K). New perturbations and high-J rotational lines are reported. Comparisons of intensities of bands of the CN(B→A) and (B→X) systems yield 1:80 as the ratio of electronic transition probabilities for the two systems.     

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.     

The structure of shock fronts in various gases

The optical reflectivity of shock fronts in A, N₂, O₂, CO, CO₂, N₂O, CH₄, Cl₂ and HCl have been studied. The thickness of shock fronts in A up to the Mach number M = 1·55 is in agreement with the theory of Gilbarg and Paolucci. The rotational relaxation time is about 5·5 collisions in N₂ and equal to or less than that in the other gases. However, for stronger shocks N₂ does not appear to reach rotational equilibrium in the shock front and a qualitative theoretical discussion of this phenomenon is presented. In HCl there appears to be over-excitation of rotation in the shock front. There is no vibrational excitation of any of the gases.     

Raman spectroscopic study of a laminar hydrogen diffusion flame in air

Spontaneous Raman spectroscopy has been employed for time-averaged, spatially-resolved measurements of temperature and species concentration in an axisymmetric, laminar hydrogen diffusion flame in quiescent air. Temperatures were obtained from vibrational Q-branch raman spectra of N₂, O₂, and H₂ and the rotational Raman spectra of N₂ and H₂, and concentrations of H₂, and N₂ were determined. The results are compared to existing numerical nonequilibrium calculations for the conditions of this experiment. Significant differences between experimental and predicted temperature and concentration profiles are observed. In particular, the flame is larger in both diameter and length and the flame zone is thicker than predicted. Some possible sources of the discrepancies are discussed.     

N2- and O2-broadening coefficients and profiles for millimeter lines of N2O

For the purpose of atmospheric applications, we have measured N₂- and O₂-induced broadenings and shapes of rotational lines of N₂O in the 235-350 K temperature range, precisely the J=8←7, J=22←21, and J=23←22 lines, located near 201, 552, and 577 GHz, respectively. The analysis of experimental lineshapes shows up significant deviations from the Voigt profile, which are characteristic of line narrowing processes. In a first step, the Voigt profile was considered for the determination of pressure broadening parameters and of their temperature dependencies. Results are in good agreement with the dependence from rotational quantum number previously observed for other rotational and rovibrational lines. They are well explained by calculations based on a semiclassical formalism that includes the atom-atom Lennard-Jones potential in addition to electrostatic interactions up to hexadecapolar contributions. In a second step, observed lineshapes were analyzed by using the Galatry profile and a speed-dependent Voigt profile. The nonlinear pressure behavior observed for the diffusion rate β involved in the Galatry profile leads to rule out the possible role of velocity/speed changing collisions, and to infer that discrepancies from the Voigt profile result from the dependence of relaxation rates on molecular speeds. This interpretation is supported by the comparison of optical and kinetic radii and confirmed by theoretical calculations of relaxation rates. Finally, it can be claimed that, for the N₂O-N₂ and N₂O-O₂ systems, deviations from the Voigt profile are explained by a speed-dependent Voigt profile.     

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.     

An ab initio potential energy surface for the C2H2–N2 system

An ab initio potential energy surface determined at the CCSD(T) level of theory is presented for the van der Waals complex C₂H₂–N₂. Additional calculations performed with the HF- and DFT- SAPT methods compare well with the CCSD(T) results and allow a better understanding of the main features of this interaction potential surface. An expansion of this surface over spherical harmonics has also been performed. The global energy minimum of the complex is obtained for the linear conformation. The T conformations are the least attractive. Such characteristics mainly arise because of the variation, in sign and in absolute value of the electrostatic energy between all these conformations. The specific role of the quadrupole–quadrupole interaction which involves two moments of opposite signs is therefore examined. The main features derived from the present surface are compared and discussed according to the following relevant systems: N₂–H₂, C₂H₂–H₂, C₂H₂–C₂H₂ and N₂–N₂. Calculated rotational constants for selected conformations of the C₂H₂–N₂ dimer are found to be in good agreement with available values.     

Application of time-resolved spectroscopy to concentration measurements in gas mixtures

Concentration measurements using femtosecond Raman Induced Polarization Spectroscopy (RIPS) are performed in binary gas mixtures CO₂–N₂ and CO₂–N₂O at room temperature. The principle of these measurements is based on the nonlinear rotational time response of each molecular component of the mixture. The general form of this molecular response is a series of periodic transients with a period related to the rotational constant B_e. The relative strength of the individual responses allows an accurate determination of the concentration. Two techniques are presented using either two pulses (one pump and one probe) or three pulses (two pumps and one probe).     

Narrow-band BoxCARS applied to CO2 laser discharges

Summary CARS (Coherent Anti-Stokes Raman Scattering) has developed into a powerful tool for studying molecular systems. One of its possibilities is to derive vibrational and rotational temperatures as well as concentrations of molecules from measurements of the energy level population differences. A very good spatial resolution of CARS technique is one of its important advantages. This feature has been utilized for making spatially resolved measurements of the vibrational and rotational temperatures of N₂ in a d.c.-excited transverse-flow CO₂ laser discharge. Apart from that also spectra of CO₂, CO and O₂ in the discharge have been taken, which allowed us to evaluate the spatial distributions of those components in the discharge. Additionally first investigations of a microwave-excited CO₂ laser module have been performed for comparison. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

N2O同位素的紫外光解：同位素分馏和产物的转动量子态分布

采用时间依赖的量子波包法研究¹⁴N¹⁴N¹⁶O、¹⁴N¹⁵N¹⁶O、¹⁵N¹⁴N¹⁶O、¹⁵N¹⁵N¹⁶O、¹⁴N¹⁴N¹⁷O和¹⁴N^{14相似文献}   

Quantum Mechanical Virial Theorem in Systems with Translational and Rotational Symmetry

Generalized virial theorem for quantum mechanical nonrelativistic and relativistic systems with translational and rotational symmetry is derived in the form of the commutator between the generator of dilations G and the Hamiltonian H. If the conditions of translational and rotational symmetry together with the additional conditions of the theorem are satisfied, the matrix elements of the commutator [G,H] are equal to zero on the subspace of the Hilbert space. Normalized simultaneous eigenvectors of the particular set of commuting operators which contains H, J ², J _z and additional operators form an orthonormal basis in this subspace. It is expected that the theorem is relevant for a large number of quantum mechanical N-particle systems with translational and rotational symmetry.     

The millimeter wave rotational spectrum of N-cyanomethanimine,CH2NCN

The rotational spectrum of the short-lived species N-cyanomethanimine, CH₂NCN, has been measured in the frequency range 100–250 GHz. The observed transitions allow the determination of the rotational and centrifugal distortion constants and the nitrogen quadrupole coupling constants for both nitrogen nuclei. The N-cyanomethanimine spectrum was measured directly in the products of the pyrolysis of trimethylenetetrazole. The rotational constants obtained are A = 63 372.995(11) MHz, B = 5 449.347 90(28) MHz, and C = 5 009.559 86(29) MHz; the quadrupole coupling constants are χ_aa = 2.057(39) MHz and χ_bb ? χ_cc = ?7.205(21) MHz for the imine nitrogen, and χ_aa = ?3.264(33) MHz and χ_bb ? χ_cc = ?1.630(18) MHz for the cyano-group nitrogen. The accurate constants obtained allow the calculation of the line position and hyperfine structure of any rotational transition appropriate for a radioastronomical search.     

The accommodation of the translational and rotational energy of a gas in a Knudsen flow past a thin wire

The paper presents the results obtained in determining the accommodation coefficients for the translational and rotational energy of gas molecules in a Knudsen flow past a thin wire. The method used was based on numerically solving the complete heat balance equation for a wire probe. The accommodation coefficients were determined for H₂, N₂, CH₄, and CO₂ on a gilded tungsten surface. For hydrogen with a quenched rotational energy, a negative accommodation coefficient of rotational energy was obtained due to the conversion of the rotational energy of incident molecules into the translational energy of reflected molecules.     

Calculations of inelastic cross sections for rotational excitation

Cross sections for scattering of N₂ (j=0) molecules on He atoms are calculated for relative energies below the excitation threshold for the N₂ (j=2) rotational state. The close coupling method is used and the coupled differential equations are solved numerically. Very sharp resonances (corresponding to a lifetime of 10^?10 sec) caused by quasistable states of the N₂He system are found in the calculated cross section, when the closed channels corresponding to the N₂ (j=2) states were included in the coupled equations. The position of the resonances is compared with the calculated energy eigenvalues of the corresponding two body potential. Furthermore the equilibrium concentration of the N₂He quasistable and orbiting states is calculated at 80 °K andp _{N 2}=1 Atm. Both concentrations are found to be 1%.     

Rotational diffusion of carbon monoxide in various simple liquids

Infra-red spectra of CO dissolved in liquid N₂, O₂, Ar, Xe and CH₄ have been recorded both in the fundamental and the first overtone. From the band profiles, rotational correlation functions, band moments and intermolecular mean-square torques have been calculated. With the help of Gordon's rotational diffusion model [1] we computed the theoretical correlation functions in the ‘M-diffusion’ (including or not a distribution of angular momentum correlation times) and ‘J-diffusion’ limits. The validity of the Gordon and Debye diffusion models in the various dense gases is discussed.