Guzmán Tejeda

Experimental and numerical investigation of an axisymmetric supersonic jet

A comprehensive study of a steady axisymmetric supersonic jet of CO 2 , including experiment, theory, and numerical calculation, is presented. The experimental part, based on high-sensitivity Raman spectroscopy mapping, provides absolute density and rotational temperature maps covering the significant regions of the jet: the zone of silence, barrel shock, Mach disk, and subsonic region beyond the Mach disk. The interpretation is based on the quasi-gasdynamic (QGD) system of equations, and its generalization (QGDR) considering the translational–rotational breakdown of thermal equilibrium. QGD and QGDR systems of equations are solved numerically in terms of a finite-difference algorithm with the steady state attained as the limit of a time-evolving process. Numerical results show a good global agreement with experiment, and provide information on those quantities not measured in the experiment, like velocity field, Mach numbers, and pressures. According to the calculation the subsonic part of the jet, downstream of the Mach disk, encloses a low-velocity recirculation vortex ring.

Inelastic collisions in para-H2: Translation-rotation state-to-state rate coefficients and cross sections at low temperature and energy

We report an experimental determination of the k(00-->02) rate coefficient for inelastic H(2):H(2) collisions in the temperature range from 2 to 110 K based on Raman spectroscopy data in supersonic expansions of para-H(2). For this purpose a more accurate method for inverting the master equation of rotational populations is presented. The procedure permits us to reduce the measured k(00-->02) rate coefficient to the corresponding sigma(00-->02) cross section in the range of precollisional energy from 360 to 600 cm(-1). Numerical calculations of sigma(00-->02) carried out in the frame of the coupled channel method are also reported for different intermolecular potentials of H(2). A good agreement is found between the experimental cross section and the numerical one derived from Diep and Johnsons potential [J. Chem. Phys. 112, 4465 (2000)].

Raman spectroscopy of supersonic jets of CO2: Density, condensation, and translational, rotational, and vibrational temperatures

The links between translational, rotational, and vibrational temperatures of supersonic molecular jets, and their density and degree of condensation, are discussed in terms of quantitative Raman scattering experimental data. Such links are established as the result of energy and momentum conservation laws, and of the collisional regime in the jet. Four representative supersonic expansions of CO2, generated at different stagnation pressures are analyzed, showing the potential of quantitative Raman spectroscopy for a complete characterization of the jet.

Overtone Raman spectrum and molecular polarizability surface of CO2

The Q branches of the Raman bands associated to the 2ν1:4ν02:ν1+2ν02 Fermi resonance of 12C 16O2 have been observed in the gas phase at 2543, 2671, and 2797 cm−1 and, in addition, at 2514 cm−1, one hot band from their first excited vibrational state is seen. From the analysis of their cross sections, jointly with those of the main Fermi diad, ν1:2ν02 and its hot bands, an improved description of the molecular polarizability surface has been achieved. The following derivatives of the mean polarizability have been obtained: ∂ᾱ/∂q1=12.43×10−42 CV−1 m2; ∂2ᾱ/∂q22σ=2.81×10−42 CV−1 m2 (σ=a,b); ∂2ᾱ/∂q21=0.45×10−42 CV−1 m2; ∂2ᾱ/∂q23=0.67 or 0.15×10−42 CV−1 m2; ∂3ᾱ/∂q1∂q22σ=−0.06×10−42 CV−1 m2, in terms of the dimensionless normal coordinates, and ∂ᾱ/∂S1=3.15×10−30 CV−1 m; ∂2ᾱ/∂S22σ=0.36×10−20 CV−1; ∂2ᾱ/∂S21=2.9×10−20 CV−1; ∂2ᾱ/∂S23=2.0 or 0.5×10−20 CV−1; ∂3ᾱ/∂S1∂S22σ=−1.7×10−10 CV−1 m−1, in terms of the symmetry coordinates. The thermal dependence of the mean polarizability is discussed in terms of these quantities.

RELAXATION AND CRYSTALLIZATION KINETICS OF AMORPHOUS GERMANIUM FILMS BY NANOSECOND LASER PULSES

Relaxation and crystallization of amorphous germanium films on silicon are induced by nanosecond laser pulses. Real time reflectivity measurements and Raman spectroscopy show that amorphous regrowth occurs upon melting and rapid solidification of the film because the thermal conductivity of the silicon substrate is high enough to extract the laser energy absorbed by the film in a very efficient way. The amorphous regrown film is in a relaxed state when compared to the as‐grown amorphous material. Further pulses induce fast crystallization of the film. An increase of the melting threshold is found upon relaxation and crystallization of the film.Relaxation and crystallization of amorphous germanium films on silicon are induced by nanosecond laser pulses. Real time reflectivity measurements and Raman spectroscopy show that amorphous regrowth occurs upon melting and rapid solidification of the film because the thermal conductivity of the silicon substrate is high enough to extract the laser energy absorbed by the film in a very efficient way. The amorphous regrown film is in a relaxed state when compared to the as‐grown amorphous material. Further pulses induce fast crystallization of the film. An increase of the melting threshold is found upon relaxation and crystallization of the film.

A study of shock waves in expanding flows on the basis of spectroscopic experiments and quasi-gasdynamic equations

A comprehensive numerical and experimental study of normal shock waves in hypersonic axisymmetric jets of N 2 is presented. The numerical interpretation is based on the quasi-gasdynamic (QGD) approach, and its generalization (QGDR) for the breakdown of rotational translational equilibrium. The experimental part, based on diagnostics by high-sensitivity Raman spectroscopy, provides absolute density and rotational temperatures along the expansion axis, including the wake beyond the shock. These quantities are used as a reference for the numerical work. The limits of applicability of the QGD approach in terms of the local Knudsen number, the influence of the computational grid on the numerical solution, the breakdown of rotation-translation equilibrium, and the possible formation of a recirculation vortex immediately downstream from the normal shock wave are the main topics considered

Inelastic collisions in molecular nitrogen at low temperature (2⩽T⩽50K)

Theory and experiment are combined in a novel approach aimed at establishing a set of two-body state-to-state rates for elementary processes ij --> lm in low temperature N(2):N(2) collisions involving the rotational states i,j,l,m. First, a set of 148 collision cross sections is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent potential energy surface for N(2)-N(2). Then, the corresponding rates for the range of 2 < or = T < or = 50 K are derived from the cross sections. The link between theory and experiment, aimed at assessing the calculated rates, is a master equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (approximately 2 x 10(-3) mm(3)) of N(2) traveling along the axis of a supersonic jet. The calculated collisional rates are assessed experimentally in the range of 4 < or = T < or = 35 K by means of the master equation, and then are scaled by averaging over a large set of experimental data. The scaled rates account accurately for the evolution of the rotational populations measured in a wide range of conditions. Accuracy of 10% is estimated for the main scaled rates.

Inelastic collisions in molecular oxygen at low temperature (4 ≤ T ≤ 34 K). Close-coupling calculations versus experiment

Close-coupling calculations and experiment are combined in this work, which is aimed at establishing a set of state-to-state rate coefficients for elementary processes ij → lm in O(2):O(2) collisions at low temperature involving the rotational states i, j, l, m of the vibrational ground state of (16)O(2)((3)Σ(g)(-)). First, a set of cross sections for inelastic collisions is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent ab-initio potential energy surface for O(2)-O(2) [M. Bartolomei et al., J. Chem. Phys. 133, 124311 (2010)]. Then, the corresponding rates for the temperature range 4 ≤ T ≤ 34 K are derived from the cross sections. The link between theory and experiment is a Master Equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. This Master Equation provides a linear function of the rates for each rotational state and temperature. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (≈2 × 10(-4) mm(3)) of O(2) travelling along the axis of a supersonic jet at a velocity of ≈700 m/s. The accuracy of the calculated rates is assessed experimentally for 10 ≤ T ≤ 34 K by means of the Master Equation. The rates, jointly with their confidence interval estimated by Monte Carlo simulation, account to within the experimental uncertainty for the evolution of the populations of the N = 1, 3, 5, 7 rotational triads along the supersonic jet. Confidence intervals range from ≈6% for the dominant rates at 34 K, up to ≈17% at 10 K. These results provide an experimental validation of state-to-state rates for O(2):O(2) inelastic collisions calculated in the close-coupling approach and, indirectly, of the anisotropy of the O(2)-O(2) intermolecular potential employed in the calculation for energies up to 300 cm(-1).

Nonequilibrium Processes in Supersonic Jets of N2, H2, and N2 + H2 Mixtures: (I) Zone of Silence

Number density and rotational temperature in the zone of silence of supersonic jets of N(2), H(2), and their mixtures N(2) + 2H(2) and 2N(2) + H(2), at p(0) = 1 bar and T(0) = 295 K, have been measured by Raman spectroscopy. Translational temperature in the jets has been derived from the experimental data assuming isentropic flow. The density along the jet axis decays at a rate depending on the composition of the expanded gas, which can be explained by the variation of its effective heat capacity ratio. Measurements across the jet axis do not support numerical off-axis density modeling from the literature. A strong nonequilibrium between the rotational degrees of freedom of both species is observed, even between the two spin species ortho-H(2) and para-H(2). From the corresponding rotational temperature data, a relationship between rotational cross sections for the inelastic collisions of the different species is established. In the expansions of the mixtures, an enrichment of N(2) is measured on the axis, which is compared with the predictions from the theory of diffusive separation in jets.

Low-temperature inelastic collisions between hydrogen molecules and helium atoms

Inelastic H(2):He collisions are studied from the experimental and theoretical points of view between 22 and 180 K. State-to-state cross sections and rates are calculated at the converged close-coupling level employing recent potential energy surfaces (PES): The MR-PES [J. Chem. Phys. 100, 4336 (1994)], and the MMR-PES and BMP-PESs [J. Chem. Phys. 119, 3187 (2003)]. The fundamental rates k(2-->0) and k(3-->1) for H(2):He collisions are assessed experimentally on the basis of a master equation describing the time evolution of rotational populations of H(2) in the vibrational ground state. These populations are measured in the paraxial region of supersonic jets of H(2)+He mixtures by means of high-sensitivity and high spatial resolution Raman spectroscopy. Good agreement between theory and experiment is found for the k(2-->0) rate derived from the MR-PES, but not for the BMP-PES. For the k(3-->1) rate, which is about one-third to one-half of k(2-->0), the result is less conclusive. The experimental k(3-->1) rate is compatible within experimental error with the values calculated from both PESs. In spite of this uncertainty, the global consistence of experiment and theory in the framework of Boltzmann equation supports the MR-PES and MMR-PESs, and the set of gas-dynamic equations employed to describe the paraxial region of the jet at a molecular level.