Doppler-broadened Spectral Lines Research Articles

Studying Turbulence Using Doppler‐broadened Lines: Velocity Coordinate Spectrum

We discuss a new technique for studying astrophysical turbulence that utilizes the statistics of Doppler-broadened spectral lines. The technique relates the power velocity coordinate spectrum (VCS), i.e., the spectrum of fluctuations measured along the velocity axis in position-position-velocity data cubes available from observations, to the underlying power spectra of the velocity/density fluctuations. Unlike the standard spatial spectra, which are a function of angular wavenumber, the VCS is a function of the velocity wavenumber kv ~ 1/v. We show that absorption affects the VCS to a higher degree for small kv and obtain the criteria for disregarding the absorption effects for turbulence studies at large kv. We consider the retrieval of turbulence spectra from observations for high and low spatial resolution observations and find that the VCS allows one to study turbulence even when the emitting turbulent volume is not spatially resolved. This opens interesting prospects for using the technique for extragalactic research. We show that, while thermal broadening interferes with the turbulence studies using the VCS, it is possible to separate thermal and nonthermal contributions. This allows a new way of determining the temperature of the interstellar gas using emission and absorption spectral lines.

Modeling the spectral line shapes with speed-dependent broadening and Dicke narrowing

Shapes of pressure- and Doppler-broadened spectral lines are obtained by solving exactly a three-dimension transport and (or) relaxation equation. The speed dependence of collisional broadening and shifting caused by dephasing collisions and Dicke narrowing caused by velocity-changing collisions are taken into account within the impact approximation. The Rautian–Sobelman and Keilson–Storer models are used to describe velocity-changing collisions. We show in the high density or hydrodynamic limit that both models lead to a profile that is the weighted sum of Lorentz profiles if the collisional broadening is much greater than the frequency of velocity-changing collisions and to the ordinary Lorentz profile in the opposite case. This shows that the relative size of the optical and kinetic cross section is important in determining the shape of absorption curves. The analytical solutions may be useful in modeling absorption profiles for remote sensing of the atmosphere. PACS No.: 32.70Jz

Shapes of pressure- and Doppler-broadened spectral lines in the core and near wings

The Rautian-Sobelman approach describing the collision-Doppler shape of spectral line that takes into account both soft and hard velocity-changing collisions is extended to the case when correlation between Doppler and collision broadening as well as the dispersion line asymmetry should be considered. Numerical calculations show that speed-dependent effects can produce the additional line narrowing as well as broadening and also the line asymmetry. It is shown that speed-dependent profiles based on the soft and hard collision models differ much more one from another than those which omit the speed-dependent effects.

Linear regime of time-varying double resonance in a medium with Doppler-broadened spectral lines

A time-varying regime of a double resonance in a system with a common upper level, with Doppler-broadened spectral lines, is described theoretically. The light field with the lower frequency (the signal) is assumed to be weak in comparison with that with the higher frequency (the pump). The nature of the interaction between the fields is determined substantially by the parameter n, which is the square root of the ratio of oscillator strengths in the pump and signal channels. At n > 0.5, and at sufficiently large distances, the resultant optical field is described as a superposition of the fields of a 2π pump pulse and an exponentially growing signal pulse of steady-state shape which is coupled with the pump pulse. The signal gain coefficient is described as a function of the pump frequency and the pump pulse energy by means of a probability integral of complex argument. At n < 0.5, the signal pulse undergoes a pronounced dispersive spreading with increasing distance traversed, and the rate of amplification of this pulse is lower than exponential. The shaping of the signal pulse is studied as a function of the detuning from resonance in the signal and pump channels.

Multichordal visible/near-UV spectroscopy on the DIII-D tokamak

A pair of visible/near-UV spectrometers with eight viewing chords apiece have been installed on the DIII-D tokamak. Each system views a neutral heating beam and can acquire up to 250 complete spectra from each chord with 5–20-ms time resolution. Each viewing chord covers 60 Å with 0.27-Å spectral resolution, and the chords span about (2)/(3) of the plasma’s full width. By viewing Doppler-broadened spectral lines from charge exchange recombination (CER) reactions between beam neutrals and plasma ions, ion temperatures up to 4 keV have been measured, and the bulk Doppler shift of these same lines has yielded plasma rotation velocities up to 200 km/s. The constancy of temperature on a magnetic flux surface and the rigid rotor model of a flux surface have been confirmed. These instruments have also been used to measure the neutral beam deposition profile, and preliminary experimental results agree with theoretical calculations of the beam deposition profile.

The determination of the vector correlation between photofragment rotational and translational motions from the analysis of Doppler-broadened spectral line profiles

A theoretical framework is presented to describe the possible correlation between the translational and rotational motion of a molecular photofragment, and to relate this to experimental observables. The correlation is defined by the values of a number of bipolar moments of the translational and rotational angular distributions which reflect the dynamics of the dissociation process. Detailed equations are presented for the polarization dependence of the profiles of recoil Doppler–broadened spectral lines of the photoproduct for two distinct classes of system: (i) molecular photodissociation, followed by spontaneous fluorescence by a molecular product with dispersion of its emission spectrum, and (ii) LIF excitation of a photoproduct, with or without dispersion of the resulting emission. The application of the theory is illustrated by calculating the line profiles for two model systems, each with several excitation–detection geometries. It is pointed out that there may still be detectable correlation between the product motions even for weakly predissociated parent molecule transitions in which the memory of the initial excitation is lost through extensive rotation before dissociation.

Gas temperature determination from doppler-broadened spectral lines with self-absorption

The gas temperature determination from spectral line profiles recorded by means of a Fabry-Perot interferometer requires the deconvolution of the profiles into the influence of the apparatus profile, the natural line broadening and, if the optical thickness of the spectral line is not negligible, self-absorption. In this paper it is considered that self-absorption is important in a lot of practical cases. Folding integrals for the line profiles are calculated with self-absorption taken into account for optical thickness, values k E from 0.1 up to 10. The theoretical results are compared with experiments in a hollow cathode arc discharge.

Measurement of Ion Temperature of a Plasma in Magnetic Field with a Gridded Probe

Effects of the magnetic field on the characteristics of a gridded probe are studied under the plasma condition that electron density, electron temperature, and ion temperature are 108∼109 cm-3, 10∼30 eV, and 3∼10 eV, respectively. No serious errors due to the field arise in determining the ion temperature if the ion gyroradius is not too small as compared with the size of the probe. The experimental values for a hydrogen plasma are consistent with those determined from the Doppler-broadened spectral lines of charge-exchanged neutral atoms.

Formation of excited H by impact of 5-30-keVH1+,H2+, andH3+ions on metal surfaces

When 5- to 30-keV ${\mathrm{H}}_{1}^{+}$, ${\mathrm{H}}_{2}^{+}$, and ${\mathrm{H}}_{3}^{+}$ are incident on Mo, Cu, or stainless-steel targets, a small fraction are backscattered in a neutral excited state. These particles subsequently decay and the emission can be observed as Doppler-broadened spectral lines. Measurements of the total Balmer-$\ensuremath{\beta}$ line intensity are presented as a function of incoming projectile energy. It is found that ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{H}}_{3}^{+}$ projectiles at energy $E$ behave the same as two or three protons at energy $\frac{E}{2}$ or $\frac{E}{3}$, respectively. The energy dependence of the total intensity down to a projectile energy of 1.67 keV is consistent with a model which includes radiationless deexcitation at the surface.

Excited-state formation asH+andHe+ions scatter from metal surfaces

Impact of 10- to 30-keV ${\mathrm{H}}^{+}$ or ${\mathrm{He}}^{+}$ ions on polycrystalline metal surfaces causes some projectiles to be backscattered in a neutral excited state. These projectiles subsequently radiatively decay, emitting Doppler-broadened spectral lines. By analysis of the spectral shape of these lines, we are able to determine the probability of radiationless deexcitation of the excited backscattered atoms. Quantitative measurements of spectral intensity indicate that less than 1% of all projectiles are backscattered in an excited state. The relative variation of total spectral line intensity with angle of projectile incidence and with projectile primary energy has been successfully predicted using a model which assumes that the probability for excited-state formation is independent of the scattered projectile's energy and direction. We also predict the variation in total spectral line intensity with target atomic number. Finally, we examine briefly the sputtering and excitation of Al under ${\mathrm{He}}^{+}$ impact.

The shape of doppler-broadened spectral lines produced by light atom reflection from metals: II. Experimental — protons on copper

During proton bombardment of a copper single crystal the Balmer-β spectral line is observed. This line, at 4861 Å, is considerably broadened, the lineshape has been studied as function of impact energy (10–40 keV) and angle of incidence of the protons with respect to the surface normal. The broadening is attributed to the Doppler effect. Only fast hydrogen atoms can escape from the solid surface in an excited state. The slow moving particles undergo a resonance non-radiative decay process. This is concluded from a comparison between experimental lineshapes and calculated lineshapes according to the theory of part 1 of this two-part paper. Good agreement with the experimental data is obtained when using for the strength of the radiationless de-excitation A' = 1.5 × 10 8 cm/sec. This value appears also to be consistent with the minimum yield in the proton intensity observed under channeling conditions.

The shape of doppler-broadened spectral lines produced by light atom reflection from metals: I. Theory

A model for predicting the shapes of Doppler-broadened spectral lines due to de-excitation by photon emission of excited neutral atoms backscattered from solids is presented. The distribution of backscattered particles is calculated on the basis of single Rutherford collisions within the solid accompanied by energy loss due to electronic stopping during passage of the particle through the solid. The effect of the energy dependence of the stopping power, radiationless de-excitation of the escaping neutral atom and the probability of the particle leaving the surface in the desired excited state are incorporated into the calculation. The broadening of the spectral line is determined by combining the Doppler shift formula with the energy- and angular distribution of the backscattered particles.