Emission spectroscopy from optically thick laboratory acetylene samples at high temperature

Recently, a high temperature source has been used to produce high temperature emission spectra of acetylene in the 3 μm spectral range, under Doppler limited resolution, and the complete spectral assignment has been performed using a global rovibrational Hamiltonian [Amyay B, Robert S, Herman M, Fayt A, Raghavendra B, Moudens A et al. Vibration-rotation pattern in acetylene (II): Introduction to Coriolis coupling in the global model and analysis of emission spectra of hot acetylene around 3 μm. J Chem Phys 2009;131:114301]. The present investigation focuses on the relative emission line intensities which are observed to be affected. The strongest lines intensity may be considerably reduced for high column density acetylene samples, hence affecting the 3:1 ortho:para intensity ratio. A radiative model is developed to take into account the effects generated by the strong opacity of the acetylene samples including self-absorption and absorption of the radiation emitted by the hot environment. The model is used to extract the absolute concentration of the high temperature acetylene samples from the observed relative spectral intensities. The relevance of the procedure for infrared remote sensing in high temperature astrophysical environments, such as circumstellar envelopes of cool carbon rich evolved stars, is discussed.     

Spiky structure of coherent emission from optically thick media

The spiky structure of the spontaneous emission from praseodymium ions in the LaF₃ lattice on the ³P₀-³H₆ transition under coherent pulsed excitation of the adjacent ³H₄-³P₀ transition has been studied experimentally and theoretically. The experimental results are in good agreement with the theoretical conclusions based on analysis of the emission dynamics of an excited ensemble of three-level systems in the mean field approximation. The generation of a spiky structure is interpreted as the emission of a cnoidal wave pulse. The recorded random pattern of time intervals between the spikes reflects the sensitivity of the cnoidal wave period to fluctuations in the system. The developed theory has been found to be related to models of deterministic chaotic dynamics.     

Determination of the excited state density for an optically thick resonance line

The transverse profile of the monochromatic radiance of an optically thick resonance line from a cylindrical discharge is inverted exactly to give the radial distribution of radiating atoms. In contrast to the Abel transform, this result is valid for all optical depths.     

New FIR laser lines from optically pumped methanol analogues

Several new laser lines using optically pumped deuterated methanol molecules have been discovered. These not only increase the spectral coverage of pumped methanol isotopes but may assist in the assignment of a molecular structure.     

The transport equation in optically thick media

The photon transport equation is transformed into a new form by considering the deviation of the specific intensity from the local equilibrium field. We call the new form of the equations the difference formulation. It is rigorously equivalent to the original transport equation. The difference formulation is particularly suited for thick media, where the radiation field approaches local equilibrium and the deviations from the Planck distribution are small. The difference formulation for photon transport also clarifies the diffusion limit. Preliminary results confirm our expectations of a substantial advantage for accurate numerical calculations in optically thick media.     

New cw FIR laser lines from optically pumped ammonia analogues

Two new cw submillimetre lines optically pumped by the 9 μR(30) CO₂ laser have been observed for¹⁴NH₃ and assigned to thea(6,0)→s(5,0) transition and to its cascade transitions(5,0)→a(4,0) in thev ₂=1 state. In addition, six new emission lines have been observed for NH₂D, and one tentatively assigned to the (11,1)→(10,1) transition of ND₃. Methylamine has also been investigated and twenty-eight new emission lines have been discovered.     

A review of optically pumped far-infrared laser lines from methanol isotopes

The technique of optical pumping in polar molecules is the most efficient for Far-Infrared (FIR) laser generation, providing also a versatile and powerful tool for molecular spectroscopy in this spectral region. Methanol (CH₃OH) and its isotopic varieties are the best media for optically pumped FIR laser, with over thousand lines observed, and the most widely used for investigations and applications. In this sense, it is important organize and make available catalogues of FIR laser lines as complete as possible. Since the last critical reviews of 1984 [1] on methanol and its isotopic varieties [2,3,4], over hundred papers have been published dealing with hundreds of new FIR laser lines. In 1992 a review of FIR laser lines from CH₃OH was presented [5]. In this communication we extend this work to the other methanol isotopes, namely CH₃OD, CD₃OH, CD₃OD,¹³CH₃OH,¹³CD₃OH,¹³CD₃OD, CH₃ ¹⁸OH, CH₂DOH, CHD₂OH and CH₂DOD.Work supported by FAPESP, CNPq, FAEP-Brasil, and CNR-Italia     

Particle effects on the emissivity and temperature of optically thick, mixed media retrieved by mid-IR emission spectroscopy

Passive diagnostics offer new ways of obtaining real-time data for the control and modeling of industrial furnaces. It has been proposed elsewhere that from the intensity profile between 3.8 and 4.7 μm one may derive the temperature of a gas-particle medium and the particle emissivity (ε_p) at 3.95 μm. This technique applies to large columns of combustion products with enough CO₂. The temperature is retrieved by finding the best fit between Planck's function and the intensity profile between 4.56 and 4.7 μm, which is that of a blackbody due to CO₂ saturation. Here we consider the effect of particles on the intensity profile and, therefore, on the retrieved temperature and particle emissivity. We derive an analytic approximation of the effective emissivity for an optically thick gas-particle mixture that includes emission and absorption due to particles and gases, along with isotropic particle scattering. The derivation follows the method of embedded invariance and has been used already for particle-only clouds. It yields a spectral solution that is applicable in other infrared regions where gas and particle optical thicknesses are large. A key parameter (χ) is the ratio of the gas absorption coefficient to the particle extinction coefficient. For χ=1 and ε_p=0.5, particle effects decrease the gas band profile by 5% from that of a blackbody. For χ<1 and ε_p<0.5, particle effects on the calculated temperature and particle emissivity are noticeable and particle effects should be considered. If χ is known, an iterative procedure may be used to calculate temperature and particle emissivity. We illustrate this procedure with data from a coal-fired boiler. Accounting for particle effects, temperatures were 4% higher (at about 1500 K) and particle emissivities 28% lower (for ε_p within 0.3-05) than without considering these effects.     

Coherence effects on photon absorption in optically thick plasmas

A kinetic photon transport model that accounts for spatial coherence is applied to line radiation in optically thick plasmas. It is shown that the photon emission and absorption processes are delocalized in space, which alters the global plasma opacity to spectral lines. Based on this analysis, we demonstrate that spectral profiles and escape factors can be much larger than expected from usual formulas.     

Population inversion in optically thick,recombining hydrogen plasmas

The plasma condition is investigated theoretically for population inversion between the first two excited states of hydrogen atoms in a recombining plasma. The rate equation, including atom-atom collision terms, is solved consistently with the optical escape factors. The upper bound of the ground level population density (n₁)_max necessary for inversion in the optically thick plasma at specified electron density and temperature is nearly inversely proportional to the mean radius of the plasma r_O. With a decrease in the atom temperature, the upper bounds increase in the optically thin plasma but decrease in the optically thick plasma.     

Determination of internal temperature profiles by multifrequency radiothermography in medical applications

Scientific-Research Institute of Radiophysics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 31, No. 9, pp. 1104–1112, September, 1988.     

Long-lived stimulated photon echo in optically thick van vleck paramagnets

Long-lived stimulated photon echo (LSPE) excitation by the sequence ofN pulse pairs in optically thick LaF₃:Pr³⁺ crystals is investigated numerically. Hyperfine structure of the ground and excited states of medium atoms is taken into account. Stable LSPE amplification along the total sample length is realized in the case when the total initial area of the pulse sequence is larger than the threshold value π. Cumulative effect depends on the threshold conditions and the crystal length. At measuring medium spectroscopic parameters the large evaluation errors can be introduced due to neglect of light propagation effects.     

Photonic crystal nanocavity laser in an optically very thick slab

A photonic crystal (PhC) nanocavity formed in an optically very thick slab can support reasonably high-Q modes for lasing. Experimentally, we demonstrate room-temperature pulsed lasing operation from the PhC dipole mode emitting at 1324 nm, which is fabricated in an InGaAsP slab with thickness (T) of 606 nm. Numerical simulation reveals that when T≥800 nm, over 90% of the laser output power couples to the PhC slab modes, suggesting a new route toward an efficient in-plane laser for photonic integrated circuits.     

Frequencies of some optically pumped submillimetre laser lines

Precise frequencies of far infrared (FIR-) laser lines of HCOOH, CH₃OH, CH₃I, CH₃F have been measured by comparison with harmonics of frequency-locked mm-wave klystrons.     

Solution of a multi-level model of an optically thick sodium plasma

A five level model consisting of the ground state, three excited states (3P, 3D, 4S) and the continuum is solved for an optically thick sodium plasma. Radiation trapping is taken into account and dealt with by means of an eigenfunction technique.     

Spatial differencing of the discrete ordinate equations in optically thick media

We present a modification to the linear characteristic method used to finite difference the discrete ordinate (S-N) equations. This modification is designed to produce the correct diffusion limit in the radiative transfer context in the limit of optically thick mesh cells. Numerical tests suggest that this finite difference method is a practical calculational scheme for radiative transfer problems.     

Propagation of light through an optically thick flat homogeneous cloud layer

Abstract

Monte Carlo simulations of light pulse transfer through a flat homogeneous cloud layer are used to compute zero, first and second moments of the light power temporal distribution at the input of a receiver placed at the cloud bottom or removed from it to a distance H in a pure atmosphere. The upper boundary of a cloud layer is considered to be illuminated by an infinitely extended δ-pulsed light beam. Transmittance of the cloud layer T and radiance angular distribution B(θ) at its bottom (these data define the zero moment or the pulse-response energy under steady illumination W), as well as mean propagation time t¯ and variance (width) (Δt)² of the pulse-response are calculated for cloud optical thicknesses τ=0, …, 50, incidence angle θ₀=0, …, 60°, single scattering albedo ω₀=0.995, …, 1.0 and the receiver view angle 2 β=40° and 90°. Monte Carlo simulation results are compared with well known Rosenberg asymptotic formulae for T and B(θ) as well as with a simple model developed for calculating t¯ and Δt. The comparison reveals high accuracy of all formulae considered for the optical thickness τ>10, …, 15. A simple procedure for computing W, t¯ and Δt parameters for a three-layer medium (cloud/pure atmosphere/sea water) is outlined.     

Three-dimensional emission tomography of optically thick plasma for the known absorption

The three-dimensional emission tomography of a plasma partially absorbing intrinsic emission is studied. The distribution of local absorption coefficients is assumed to be known. To determine the distribution of local emission coefficients under the given conditions, the use of algebraic algorithms inverting the projection matrix constructed taking into account absorption is proposed. NDAT and NDFBP algorithms are developed, which are based on the expansion of the product of the operator of energy transport in an absorbing medium and the operator of the solution of the problem of emission tomography of a transparent plasma in a Neumann series. A numerical simulation is made, which shows that the emission field can be reconstructed with a reasonable accuracy for an optical thickness as high as 6–8.