Kinetic modelling of electrooptically Q-switched CO2 laser

In this paper, a six-temperature model for CO₂ lasers is used to describe the process of the dynamic emission in the electrooptically Q-switched laser. The dissociation of CO₂ molecules, the electron excitation and the energy-transfer of vibrational levels and the rotational relaxation of rotational levels are involved. The calculated pulse waveforms are in good agreement with the experiment.     

Raman and Infrared Spectra of Matrix Isolated CO2 and CS2

The matrix isolation infrared and Raman spectra of CO₂ and CS₂ in solid krypton at 20°K are reported. The fundamental frequencies are only very slightly red-shifted from the free gas values. Isotopic effects are found in both infrared and Raman spectra, whereas site splittings are evident in the infrared only.     

Rotation-vibration energy cluster formation in XH2D and XHD2 molecules (X = Bi, P, and Sb)

We investigate theoretically the energy cluster formation in highly excited rotational states of several pyramidal XH₂D and XHD₂ molecules (X = Bi, P, and Sb) by calculating, in a variational approach, the rotational energy levels in the vibrational ground states of these species for J?70. We show that at high J the calculated energy levels of the di-deuterated species XHD₂ exhibit distinct fourfold cluster patterns highly similar to those observed for H₂X molecules. We conclude from eigenfunction analysis that in the energy cluster states, the XHD₂ molecule rotates about a so-called localization axis which is approximately parallel to one of the X-D bonds. For the mono-deuterated XH₂D isotopologues, the rotational spectra are found to have a simple rigid-rotor structure with twofold clusters.     

Review of theoretical gain-optimization studies in 10.6µm CO2-N2 gasdynamic lasers

A comprehensive theoretical analysis of optimization of gain in CO₂-N₂ gasdynamic laser employing wedge or conical or hyperbolic nozzles with either H₂O or He as the catalyst is presented. After a review of previous work, the usual governing equations for the steady inviscid quasi-one-dimensional flow in a supersonic nozzle of a gasdynamic laser are used to obtain similar solutions for the various flow quantities, which variables are subsequently used to optimize the small-signal gain on theP(20) line of the (001) → (100) transition of CO₂ at wavelength 10.6μm. The corresponding optimum values like reservoir pressure and temperature and nozzle area ratio also have been predicted and presented in the form of graphs. The analysis predicts that employing of 2D-wedge nozzle results in higher gain values and the CO₂-N₂-H₂O gasdynamic laser employing 2D-wedge nozzle is operationally the best laser system for which the optimum value as high as 3.1 m⁻¹ gain can be obtained.     

A simple empirical model for the collisional spectral shift of air-broadened CO2 lines

A simple empirical model is proposed for the pressure-induced shifts of air-broadened CO₂ absorption lines. It is based on the introduction of components symmetric and antisymmetric with respect to the rotational quantum number m, and on a separation of vibrational and rotational contributions. These components are modelled though empirical analytical laws with parameters that are obtained from fits of the measured shifts in only two different bands. It is then shown that the model leads to satisfactory results for a number of other vibrational transitions in which the shifts take significantly different values. This provides a simple way to generate values for unmeasured transitions and this tool can thus be used to complement spectroscopic databases.     

Effect of collision-induced phase-shifts on the line widths and line shifts of CO2-Ar system

The theoretical calculation of line widths and line shifts for CO₂-Ar system is computed by the Mehrotra-Boggs theory. It is shown for this system that the phase shift effect is very important at large values of |m| wherem is the value of rotational quantum numberJ in the lower vibrational state. It is also pointed out that the Salesky-Korff theory is the same as the Mehrotra-Boggs theory.     

The rotational Raman spectra and cross sections of H2O, D2O, and HDO

We report the experimental rotational Raman spectra of H₂O, and of a mixture of D₂O and HDO in the vapor phase at room temperature, and their interpretation in terms of rotational–vibrational energies, wavefunctions, and transition moments of the molecular polarizability. These transition moments are based on high-level ab initio calculations of the wavelength dependent polarizability surface, and on wavefunctions where the rotational–vibrational coupling is considered in detail. As a byproduct of this analysis several tables have been compiled including scattering strengths and assignments for individual rotational transitions of the three species. From these tables the rotational Raman spectra can be simulated over the range of temperatures up to 2000 K for H₂O, and up to 300 K for D₂O and HDO.     

A Sensitive Isotope Selective Nondispersive Infrared Spectrometer for 13CO2 and 12CO2 Concentration Measurements in Breath Samples

We present a nondispersive infrared spectrometer (NDIRS) for the measurement of the ¹³CO₂/¹²CO₂-ratio in breath samples. A commercial NDIR spectrometer for CO₂ concentration measurements in industrial process control was modified using two separate optical channels for the ¹³CO₂ and ¹²CO₂ detection. Cross interference due to overlapping absorption lines of both isotopic gases was successfully eliminated. The sensitivity of this device is ± 0.4‰ of the ¹³CO₂/¹²CO₂-ratio in a range of 2.5 to 5% of total CO₂. This is sufficient for biomedical applications. Our spectrometer is small in size, cheap and simple to operate and thus a true alternative to isotope ratio mass spectrometers (IRMS). Several biomedical applications with breath samples were demonstrated and were compared in very good agreement with IRMS.     

ESR of Gamma Irradiated C2H204.2H2O Single Crystal

The gamma-irradiated samples of vacuumed powder and single crystal of C₂H₂O₄.2H₂O have been investigated by ESR method. the spectra of these single crystal and unvacuumed powder are attributed to R C HOH radical, but the peaks belonging to this radical in the gamma irradiated powder sample are not observed in the spectrum after having been vacuumed. After a and g tensor values of this radical have been calculated, it was observed to be anisotropic. At low temperature, a considerable change in the spectrum has not been observed.     

Diffusion of CO2/CH4 confined in narrow carbon nanotube bundles

ABSTRACT

The diffusion of a CO₂/CH₄ mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO₂ and CH₄ are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO₂ and CH₄ molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO₂ in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO₂ and CH₄ in the pores. For CO₂, the interaction energies (CO₂–CO₂ and CO₂–CNT) are larger than that of CH₄ in all cases. But only a very small difference in the diffusion coefficient was observed between CO₂ and CH₄. Temperature has a negligible effect on the difference between diffusion coefficients of CO₂ and CH₄ in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO₂/CH₄ in CNT bundle membranes.     

Additivity and non-additivity of dissociation energies in intermolecular interactions. Theoretical studies on (H2)n,n = 2-8, (CO2)n,n = 2-6 and (HF)n,n = 2-8

CCSD(T) and MP2 results using the aug-cc-pV5Z basis set are reported for chain, cyclic and other structures of the clusters (H₂)_n, n?=?2-8, (CO₂)_n, n?=?2-6 and (HF)_n, n?=?2-8. In chain-like structures of (H₂)_n and (CO₂)_n, with the bonding type of the dimer maintained, the dissociation energy D_e of the dimer doubles for the trimer, triples for the tetramer, and so on. Due to these systems being dominated by short-range forces, interactions are essentially restricted to neighbouring monomers. For other types of (H₂)_n and (CO₂)_n structures, the multipliers relative to the dimerisation energy can be much higher. Dissociation energies for the hexamers in S₆ symmetry of both H₂ (379?cm^?1) and CO₂ (4925?cm^?1) are over ten times the respective dimerisation energies. For the chain-like trimer of HF, however, D_e is in excess of double the dimer value. Mainly due to longer-range dipolar forces, the interactions reach beyond the neighbouring monomers. The interaction energy between HF monomers in chains follows an approximate R^?2 (R being the F–F distance) relationship, The calculated dissociation energies of the HF octamer are 15,985?cm^?1 (factor of 10.4) for the chain, and 21,003?cm^?1 (factor of 13.7) for the C_6h cyclic structure.     

Self-, N2- and O2-broadening coefficients for the CO2 transitions near-IR measured by a diode laser photoacoustic spectrometer

Using a high-resolution tunable diode laser photoacoustic spectrometer, self-, N₂ and O₂ pressure broadening coefficients for the first 11 transitions of ¹²C¹⁶O₂ in the R branch of the (30012) ← (00001) overtone band at the 6348 cm⁻¹ have been revisited at room temperature (∼298 K). Air-broadening parameters have also been calculated from the N₂ and O₂ measurements. The dependence of the broadening on rotational quantum number m is discussed. The recorded lineshapes are fitted with standard Voigt line profiles in order to determine the collisional broadening coefficients of carbon dioxide transitions. The results are compared to our previous measurements and to the values reported in the HITRAN04 database and by other research group with a different spectroscopic technique.     

Submillimeterwave spectrum of CH2PH and equilibrium structures of CH2PH and CH2NH

The rotational spectrum of phosphaethene (CH₂PH) was reinvestigated. One hundred and nineteen new lines were measured in the submillimeter range from 500 to 650 GHz. The determination of the centrifugal distortion constants is significantly improved. As the molecule is close to symmetric prolate top, both reduction A and S were compared. The equilibrium structure has been derived from experimental ground state rotational constants and ab initio rovibrational interaction parameters. This semi-experimental structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-zeta quality and a core correlation correction. The structure of CH₂PH was compared to that of CH₂NH which was also determined for this goal. It is found that the semi-experimental structure of CH₂NH is less accurate than the ab initio structure. It is also found that the methylene group is much more asymmetric in CH₂NH than in CH₂PH.     

Dynamics of a Loss-Modulated, Two-Wave CO2 Laser Lasing on Lines of Different Vibrational Bands

The nonlinear dynamics of a loss-modulated, two-wave CO₂ laser lasing on vibrational–rotational lines of different vibrational bands has been investigated theoretically and experimentally. It is shown that the time and energy parameters of laser radiation can be controlled within fairly wide limits by changing the depth and frequency of modulation and the ratio between the constant components of the losses.     

Highly uniform UV preionized CO2 laser

Highly uniform laser pulses have been obtained in CO₂−N₂−He mixtures, between 1 and 5 atm with the help of true volume pre-ionization by single step photoionization of new additives having a low ionization potential.     

High-resolution FTIR measurement of the ν4 band of methylene fluoride-d2

A high-resolution (0.002 cm⁻¹) infrared absorption spectrum of methylene fluoride-d₂ (CD₂F₂) of the lowest fundamental mode ν₄ in the region from 460 to 610 cm⁻¹ has been measured on a Bruker IFS 120-HR Fourier transform infrared spectrometer. More than 3500 transitions have been assigned in this B-type band centered at 521.9 cm⁻¹. The data have been combined with upper state pure rotational measurements in a weighted least-squares fit to obtain molecular constants for the upper state resulting in an overall standard deviation of 0.00018 cm⁻¹. Accurate value for the band origin (521.9578036 cm⁻¹) has been obtained and inclusion of transitions with very high J (?60) and K_a (?34) values has resulted in improved precision for sextic centrifugal distortion constants, in particular D_K, H_KJ, and H_K.     

Experimental and theoretical studies of room-temperature sub-millimetre CH3 35Cl line shapes broadened by H2

Hydrogen-broadening coefficients of methyl chloride rotational lines J?=?6?→?7, 10?→?11, 17?→?18, 22?→?23 and 31?→?32 at 296?K are measured as functions of the quantum number K using a sensitive frequency-modulation technique. As expected for this light perturber, the observed line shapes are well described by Voigt profile model. A clear dependence of the collisional broadening on K is observed for most transitions. From a detailed study of the K-components of the transition J?=?6?→?7 situated at 186?GHz no variation of the broadening of the hyperfine components related to ³⁵Cl quadrupole is stated. Given the absence of refined ab initio computed potential energy surfaces and the impracticality of quantum-mechanical calculations for the considered molecular system, theoretical values of these broadening coefficients are estimated by a semi-classical approach with exact trajectories and a model interaction potential including both long-range and short-range (atom-atom) interactions of the active molecule rigorously treated as a symmetric top. It is shown that the short-range forces yield important contributions to the collisional line width for all values of the rotational quantum numbers J and K. Various models are also tested for the isotropic part of the interaction potential which governs the relative translational motion. It is demonstrated that for the very light perturbing molecule H₂ the calculated line widths, practically independent from the rotational quantum number J (for K?≤?J), are particularly sensitive to the position and slope of the repulsive wall. Modifications required in the semi-classical formalism for a correct application of the cumulant expansion are also tested and it is stated that no difference is observed for the CH₃Cl–H₂ system characterised by quite weak interactions.     

The AB2-XA1 electronic transition of NO2: A rovibronic survey covering 14 300-18 000 cm

More than 250 rotationally resolved vibrational bands of the A²B₂-X²A₁ electronic transition of ¹⁵NO₂ have been observed in the 14 300-18 000 cm⁻¹ range. The bands have been recorded in a recently constructed setup designed for high resolution spectroscopy of jet cooled molecules by combining time gated fluorescence spectroscopy and molecular beam techniques. The majority of the observed bands has been rotationally assigned and can be identified as transitions starting from the vibrational ground state or from vibrationally excited (hot band) states. An exceptionally strong band is located at 14 851 cm⁻¹ and studied in more detail as a typical benchmark transition to monitor ¹⁵NO₂ in atmospheric remote sensing experiments. Standard rotational fit routines provide band origins, rotational and spin rotation constants. A subset of 177 vibronic levels of ²B₂ vibronic symmetry has been analyzed in the energy range between 14 300 and 17 250 cm⁻¹, in terms of integrated density and using Next Neighbor Distribution. It is found that the overall statistical properties and polyad structure of ¹⁵NO₂ are comparable to those of ¹⁴NO₂ but that the internal structures of the polyads are completely different. This is a direct consequence of the X²A₁-A²B₂ vibronic mixing.     

Spectroscopic Study of C2 in Carbon Arc Discharge

The influence of self-absorption on intensity distribution in the rotational structure of the emission d ³II_gII → a ³II_u (0–0) band of C₂ radical was studied. The self-absorption effect was used for the determination of C₂ concentration in a DC carbon arc. The proposed method is general and may be used to other molecular bands of a similar rotational structure.     

Vortex phase diagram studies in the weakly pinned single crystals of YNi2B2C and LuNi2B2C

We present a study of magnetization measurements performed on the single crystals of YNi₂B₂C and LuNi₂B₂C. For both the compounds, we find flux jumps in magnetisation values in the respective field regions, where the structural transitions in the flux line lattice symmetry have been reported in these systems via the small angle neutron scattering experiments. The magnetisation hysteresis loops and the AC susceptibility measurements show pronounced peak effect as well as second magnetisation peak anomaly for both YNi₂B₂C and LuNi₂B₂C. Based on these results, a vortex phase diagram has been constructed for YNi₂B₂C forH∥c depicting different glassy phases of the vortex matter.