Continuum emission

The experimental system for measuring the sonoluminescence spectrum of alkali-metal atom emission from an aqueous solution is similar to that for measuring the MBSL spectrum from water. Degassing the solution is an important procedure because the presence of dissolved air affects the emission intensity. In an air-saturated solution, no observation of alkali-metal atom emission has been reported, whereas continuum emission can be observed. A typical experimental apparatus using ultrasonic standing waves is shown in Fig. 13.3 [8]. The cylindrical sample container is made of stainless steel, and its size is 46 mm in diameter and 150 mm in... 

Alkali-Metal Atom Emission and Continuum Emission... 

Abe and Choi [41] succeeded in the spatiotemporal separation of sodium atom emission from continuum emission in argon-saturated NaCl aqueous solution... 

Fig. 13.13 Photographs of sonoluminescence from NaCl aqueous solution sonicated at 135 kHz in a cylindrical beaker (a). Image (a) was digitally processed to obtain the red component (b), which corresponds to Na atom emission, and the blue component (c), which corresponds to continuum emission [41] (Reprinted from the Japan Society of Applied Physics. With permission)...

For continuum emission, on the other hand, the intensity is extremely low at 28 kHz and increases with increasing frequency. The intensity is highest at 1 MHz. 

The almost featureless spectrum at 1.0 MHz is reminiscent of the result by Matula et al. [35], i.e., the SBSL spectrum from NaCl solution indicated only continuum emission with no atomic lines. Sonoluminescing bubbles at higher frequencies are smaller and interact less with surrounding bubbles. These factors may explain why the MBSL spectrum at 1 MHz is similar to that of SBSL. 

Abe S, Choi P-K (2009) Spatiotemporal separation of Na-atom emission from continuum emission in sonoluminescence. Jpn J Appl Phys 48 07GH02... 

Oblath, S.B., and J. L. Gole. 1980. On the continuum emissions observed upon oxidation of aluminum and its compounds. Gombustion Flame 37 293-312. 

On the other hand, for molecules, the electronic transitions result in bands lO SOnm in width due to the changes in vibrational energy levels which also occur. A third type of radiation emitted by stars in the near-UV visible near-IR region is a continuum emission originating from hot particles e.g. hot AI2O3 particles) but this is considered to be grey body radiation and does not contribute to the colour of the star. 

An obvious area of interest for emission spectrocopy is the area of coatings on metal surfaces 165>. Most metals give a very weak continuum emission of perhaps 5 % of a backbody at the corresponding temperature. Some success has been reported in this area I49,152,160). Emission spectra have been taken from the inside of Aluminium... 

W. M. Fawzy, R. J. Le Roy, B. Simard, H. Niki, and P. A. Hackctt, ]. Chem. Phys., 98, 140 (1993). Determining Repulsive Potentials of InAr from Oscillatory Bound -> Continuum Emission. 

The continuum emission from the source is of a low intensity to enable the detection... 

If a continuum source is needed for absorption spectroscopy, this can be provided by discharge lamps fdled to higher densities, such that pressures can exceed 100 bar at operational temperatures. The result is a broad continuum emission with superimposed line spectra, as shown for several lamps in Fig. 14. In commercial spectrometers the deuterium lamp is commonly used for the UV region below 350 nm while the tungsten-halogen lamp is convenient for the 350 to 900 nm range. The latter is an example of a thermal source whose radiant excitance per unit wavelength closely approximates that predicted by the Planck formula for a blackbody radiator " ... 

LIF from Brj excited by 158 nm radiation (F2 laser) has been observed and interpreted. Oscillatory continuum emission, involving three bound-free transitions, dominates in the region 210—440 nm. Collisional interstate transfer in the presence of SF< Nj, and He was found to be efficient and was discussed in terms of a possible optically pumped Bra laser. 

While dust continuum emission can be used to estimate disk masses, line emission from optically thin molecules (e.g., CO) can be used to map the line-of-sight velocities in disks, using the Doppler shift of the moving gas. Evidence for Keplerian velocity profiles is typically found (e.g., Launhardt and Sargent, 2001), as is to be expected for gas in a stable orbit around a central protostar. These measurements only apply to the gas at considerable distances from the young star, however, typically at several hundred astronomical units or more. The situation inside these disks at planetary distances is not constrained by these observations, and even the outer disk measurements are subject to possible confusion with infalling gas in the envelopes and outflowing gas from stellar winds. 

Depending on the pressure in the lamp, both line and continuum emission may be observed. Low-pressure lamps operated at low current densities and temperatures produce sharp atomic lines with little or no continuous background. Increasing pressure and temperature cause the lines to broaden and increase the intensity of the background continuum. 

A Continuum emission spectra have no line character and are generally produced by heating solids to a high temperature. 

At complete ionization of the hydrogen (e.g. when added to a plasma with another gas as the main constituent) ne = p/(2 x k x Te) has a maximum at a wavelength of X — (7.2 x 107)/Te or at a fixed wavelength, the maximum intensity is found at a temperature Te = (5.76 x 107)/2. Thus, the electron temperature can be determined from the wavelength dependence of the continuum intensity. As Te is the electron temperature, absolute measurements of the background continuum emission in a plasma, e.g. for the case of hydrogen, allow determination of the electron temperature in a plasma, irrespective of whether it is in local thermal equilibrium or not. Similar methods also make use of the recombination continuum and of the ratio of the Bremsstrahlung and the recombination continuum. 

The rare gas excimer lasers are based on bound-continuum transitions from an excited diatomic species to its dissociative ground state. The observed continuum emission is a superposition of the Franck-Condon factors from the vibrational levels of the upper state. Thus these molecular dissociation lasers display relatively broad fluorescence as a consequence of the steeply repulsive ground-state potential, and there is always a population inversion on such transitions. However, the net gain is significantly lower than that for a bound-bound transition because of the distribution of oscillator strength over the broad fluorescence band. Figure 1 illustrates schematic potential energy curves for such transitions in the excimer and exciplex lasers. 

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