A comparison of self broadening and shift of helium,neon and argon emission lines

Self broadening (van der Waals and resonance) and shift of Ne emission lines and van der Waals broadening and shift of He emission lines have been measured using a high pressure (0.5–3.0 atom), low current discharge. These results are compared with previous measurements in Ar and He to obtain a complete comparison of self broadening and shift of He, Ne and Ar emission lines. Oscillator strengths for the resonance transitions are obtained from the resonance broadening coefficients. The trend of the van der Waals broadening coefficients for the three noble gases is correctly predicted by a theory due to HINDMARSHet al.⁽⁴⁾ in which a Lennard-Jones potential is used in the impact theory formalism. The measured line shifts cannot be accounted for by this theory and reflect the need for more accurate quantum mechanical calculations.     

Etude spectroscopique d'un jet de plasma d'argon hors d'equilibre thermodynamique local

We have studied the properties of an argon plasmajet seeded with a few percent of hydrogen. At atmospheric pressure, significant deviations from local thermodynamic equilibrium (L.T.E.) are shown. The plasma parameters (electrondensity N_e, temperature T) have been determined from the broadening and the absolute intensity of hydrogen lines. The transition probabilities and the broadening parameters (width and shift) of some AI lines between 4000 and 6000 Å have been measured.     

Broadening and shift of lines of the principal series of neutral lithium atom produced by collision with helium and argon

Collision broadening and shift produced by argon and helium have been determined in absorption at low pressures for lines of the main series of neutral lithium. The experimental profiles were corrected for instrumental distortions. The actual line profile is well represented by a Voigt function. The collision-broadening and shift rates were derived from the measurements. The experimental data were analyzed by using the impact theory and the Lennard-Jones potential. Interatomic potential constants were thus determined for an excited state Li(I) atom interacting with a ground state Ar or He atom. We have compared experimentally-determined potentials with theoretical “pseudo-potential” curves.     

Interpretation of rare-gas-induced broadening of Rydberg series lines

Approximate expressions have been derived for the cross-sections of rare-gas-induced broadening of the Rydberg series lines on the basis of the interatomic potential which is expressed as a combination of the van der Waals potential, the Fermi potential and the polarization potential. Only the effect of elastic scattering has been taken into account. Multichannel (two-channel) quantum defect theory has been combined with the broadening theory to include the effect of the mutual interaction of the Rydberg series. The resulting cross-sections are in reasonable agreement with the experimental values obtained for Na by Kachru et al. and for Ca and Ba by Ueda et al.     

On the mechanism of atomic diffusion in condensed matter

The mechanism of an elementary diffusion act in condensed matter is analyzed by a molecular dynamics method using the example of a three-dimensional lattice with the Lennard-Jones interatomic interaction potential. It is found that, in a certain temperature range, the diffusion of atoms changes in character from individual hopping to cooperative movement of many atoms (stringlike diffusion). The collective displacement of atoms is associated with the nonlinear interatomic potential and manifests itself at specific relationships between the potential barriers and the temperature. Analogous phenomena must occur in any condensed system.     

Density shift and broadening of transition lines in antiprotonic helium

The density shift and broadening of the transition lines of antiprotonic helium have been evaluated in the impact approximation using an interatomic potential calculated ab initio with the symmetry-adapted perturbation theory. The results help to remove an uncertainty of up to 10 ppm in the laser spectroscopy data on antiprotonic helium and are of importance in experimental tests of bound state QED and CPT invariance.     

Calculated line broadening coefficients in the ν2 band of CH3D perturbed by helium

Line broadening coefficients have been calculated, at room temperature, for lines in the P and R branches of the ν₂ band of monodeuterated methane. A properly symmetrized semiclassical model with parabolic relative trajectories has been used. Two interaction potential models have been considered. The first is a Lennard-Jones type atom-atom potential, while the second one was derived from ab initio calculations. The calculated line widths were compared to the available experimental data and a satisfactory agreement was found, although the model contains no other adjustable parameters than the four atomic Lennard-Jones ones. Nonetheless, failures of calculations have also been evidenced for the highest rotational quantum numbers.     

Impurity spectroscopy in glasses and disordered crystals: Inhomogeneous broadening and electron phonon coupling

Parameters of optical impurity spectra in disordered solids were calculated using the potential energy distribution of the ground state and the guest-host interaction potentials of the lower and upper states. The model can yield inhomogeneous band shapes, pressure shift coefficients of zero phonon lines, (pseudo)local phonon frequencies, and linear and quadratic coupling constants to phonons. Results are compared to properties of Shpol'skii multiplets and zero phonon holes burned over the broad spectra in glasses. Crystal spectra contain discrete lines, but the overall width of multiplets and the bands in weakly polar solvent glasses is similar, and so are the pressure shift coefficients. A decrease of zero phonon transition probability (Debye-Waller factor) with increasing (negative) solvent shift was predicted and confirmed for crystal spectra. In general, no correlation exists between the strength of the first- and second-order couplings, and the vibrational modes involved can be different. Moreover, no relationship was established between the line shift and broadening in a temperature interval from 5 to 100 K.     

Third-order elastic constants of ZnS and ZnSe

The complete set of non-vanishing third-order elastic constants of the semiconductors ZnS and ZnSe is obtained theoretically. The strain energy density is estimated using finite strain elasticity theory by considering the interactions up to two nearest neighbours of each atom in the unit cell of these compounds. This energy density is compared with the strain dependent lattice energy density from the continuum model approximation. The second-order parameter of the potential function φ is obtained from the measured principal axis C_ij. The third-order potential parameter is estimated by assuming a Lennard-Jones type of interatomic potential. The interlattice displacements as well as the second-order elastic constants are evaluated along with the six third-order elastic constants of ZnS and ZnSe. Using these second- and third-order elastic constants of ZnS, the pressure derivatives of second-order elastic constants are evaluated. The second- and third-order elastic constants of ZnSe are compared with the available experimental values. The third-order elastic constants show anisotropy in different directions.     

State equations and properties of various polymorphous modifications of silicon and germanium

The state equations and the pressure dependences of the lattice properties have been obtained for various polymorphous modifications of silicon and germanium using the Mie–Lennard-Jones pair interatomic potential and the Einstein crystal model. It is shown that the elastic-type interatomic potential gives the best results for the semiconductor phase and the plastic-type interatomic potential for the metalized phases whose potential well depth is significantly smaller. The pressure dependences of the lattice properties are calculated along isotherm 300 K and the jumps of the properties during the phase transition from the diamond structure to the β-Sn phase are evaluated for both silicon and germanium. The calculated results agree well with the experimental data.     

Hydrogen-Broadening Coefficients in the ν5 Band of Acetylene at Low Temperature

H₂-broadening coefficients have been measured for 29 lines of C₂H₂ at 173.2 K in the P and R branches of the ν₅ fundamental band near 13.7 μ m, using a tunable diode-laser spectrometer. These lines were individually fitted with a Voigt and a Rautian profile in order to determine the collisional widths. The resulting broadening coefficients are compared with values calculated from a semiclassical theory performed by considering, in addition to electrostatic interactions, the atom-atom Lennard-Jones model. A satisfactory agreement is obtained but only for low and medium J values. By comparing broadening coefficients at 297 and 173.2 K from a simple power law, the temperature dependence of these broadenings has been determined both experimentally and theoretically.     

Lattice dynamics of b.c.c. alkaline-earth metals: Barium

Summary The attractive short-range component of the interatomic potential screens the conventional Born-Mayer potential in the framework of the resonance pseudopotential model. The elastic constants are evaluated at the long-wavelength limit of the phonon spectrum and the obtained results are compared with previous experimental values. The numerical calculations show that the attractive component of the potential explains the soft modes in this body-centred cubic (b.c.c.) alkaline-earth metal barium.     

Bi-resonance functions in the theory of collisional broadening of the spectral lines of molecules

In the model of parabolic trajectories, expressions are derived for the bi-resonance functions that appear in the theory of collisional broadening of molecular spectral lines in the case of their interference. The functions required for calculating the cross-relaxation parameters of molecular spectral lines in the case of their broadening by rare gas atoms are obtained numerically in the model of exact trajectories for the isotropic Lennard-Jones 6–12 potential. Analytic models are proposed for these functions, and the corresponding parameters are obtained.     

Hydrogen-broadening coefficients in the ν7 band of ethylene at low temperature

H₂-broadening coefficients have been measured for 35 lines of C₂H₄ at 173.2 K in the P, Q, and R branches of the ν₇ fundamental band near 10 μm, using a tunable diode-laser spectrometer. These lines were individually fitted with a Voigt and a Rautian profile to determine their collisional widths. The resulting broadening coefficients, as well as those previously measured at room temperature are compared with values calculated on the basis of a semiclassical model of interacting linear molecules, using an atom-atom Lennard-Jones potential in addition to the weak electrostatic contributions. A satisfactory agreement is obtained for the results at room temperature, but the theoretical results at low temperature are generally smaller than the experimental data. Finally, the temperature dependence of the broadening coefficients has been determined both experimentally and theoretically.     

Molecular dynamic investigation of the laws and mechanisms of condensed island film growth

The laws and mechanisms of the epitaxial growth of submicron island films, i.e., films about 0.01 μm in diameter, have been studied by the molecular dynamics method. The interatomic interaction in the simulated system and in the continuous substrate was described by the Lennard-Jones potential. The condensed growth of island films is found to follow the Volmer-Weber mechanism.     

Mesure de la temperature et des coefficients d'absorption de certaines raies de l'argon neutre dans un arc stabilise par parois et optiquement epais

Temperatures have been determined from relative line intensities in an arc stabilized in argon at atmospheric pressure. Proper corrections were made for self-absorption. Absorption coefficients have been measured for argon lines and were compared with calculated values. Good agreement was observed in regions with LTE.     

One-dimensional model of the quasicrystalline alloy

A one-dimensional chain of atoms of two types is investigated. It is proven exactly for the model of attracting hard spheres that if the ratio of the numbers of atoms of the two types is irrational, then the state of absolutely minimal energy is quasicrystalline. The quasicrystalline state is also investigated in the case of the Lennard-Jones interatomic potential. All the microscopic values (interatomic spacing, electronic density, etc.) are shown to be quasiperiodic functions varying on Cantor sets. Diffraction patterns, electronic properties, and vibrational spectra are also discussed.     

Calculated Stark widths of isolated S(III) and S(IV) lines

Calculations have been performed on the electron-impact broadening of isolated lines from doubly-ionized and triply-ionized sulphur emitted from a plasma of electron density 10¹⁷ cm^-3 and temperature 28,500°K. These have been compared with results of measurements performed by Plati?a, Popovi?, Dimitrijevi?, and Konjevi? on a low-pressure, pulsed arc. Good overall agreement has been obtained for both ionization stages, in confirmation of our earlier conclusions based on a similar comparison for oxygen ion lines. The present calculations have been compared with calculations based upon two simplified models of Griem and the classical-path approximation of Sahal-Bréchot. Our calculated widths are in better agreement with experiment than with values obtained from the first two models; for the two multiplets, where comparison is possible, good agreement is found with the widths obtained by Sahal-Bréchot. We conclude that the present model, whereby the effective Gaunt factor is calculated, is capable of predicting reliable values for the Stark widths of isolated ion lines. Comparison with experiment [for S(III) indicates that, provided a sufficiently complete set of perturbing levels is used in the calculations, the present formula for ? may also be used for below-threshold energies in the determination of the elastic contribution to the broadening. For S(IV), an ambiguity remains in this regard, owing chiefly to a scarcity of available experimental data.     

Diode-laser measurements and calculations of N2-broadening coefficients in the ν7 band of ethylene

N₂-broadening coefficients have been measured for 41 lines of C₂H₄ at 173.2 K in the P, Q, and R branches of the ν₇ fundamental band near 10 μm, using a tunable diode-laser spectrometer. These lines were individually fitted with a Voigt and a Rautian profile in order to determine their collisional widths. The resulting broadening coefficients, as well as those previously measured at room temperature for 35 transitions, are compared with values calculated on the basis of a semiclassical model of interacting linear molecules, using an atom-atom Lennard-Jones potential in addition to the electrostatic contributions. An overall satisfactory agreement is obtained for all the results at room and low temperatures. Finally the temperature dependence of the broadening coefficients has been determined through the temperature exponent n, experimentally for the 23 common transitions as well as theoretically for all the studied transitions.     

Calculations of impact broadening and shift of spectral lines for realistic interatomic potentials

We have studied theoretically the impact broadening and shift of the resonance doublets of Rb and Cs for the interatomic potential data of Baylis and Pascale, which were determined from ab initio calculations. We show that, without introducing any adjustable parameter, we obtain good agreement between theoretical and experimental values for both the P_1/2 and the P_3/2 component. In the latter case, the theoretical results are significantly modified when anisotropy of the interaction potential is taken into account.