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Astron. Astrophys. 328, 752-755 (1997)

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2. Experiment

The spectroscopic source of P II is a low-pressure ICP (inductively-coupled plasma). This type of source has been widely used for observing singly-ionized spectra (Curry et al. 1997, Svendinius et al. 1983, Svendenius 1980) because it can be operated at high input powers and low neutral pressures. Such a combination produces a large and strongly excited ion population. Also important for the present work is the absence of electrodes in contact with the plasma. Electrodes are a potential source of impurity lines and may initiate unwanted chemistry with phosphorus radicals.

The ICP consists of a specially designed fused-silica cell and a cylindrical six-turn RF coil. The coil is driven at 13.56 MHz by a high-power radio-frequency source through a capacitive matching network. The cylindrical fused-silica cell is 2 inches in diameter and 4 inches long, with a long, small diameter tube attached to each end. The small diameter of the tubes act as an effective trap for evaporated phosphorus, condensing the vapor before it diffuses to other parts of the vacuum system. No other traps are necessary.

Closed cells with relatively small neutral fill pressures rapidly become contaminated under high power operation and re-ignition of a discharge becomes impossible. Even without impurities, it is often only possible to initiate a discharge with a higher fill pressure or with a different fill gas than is to be used during the experiment. It is vital to have a flowing-gas cell to limit the build-up of impurities in the discharge and to facilitate ignition of the discharge.

Both a pure He and a He-Ar mixture have been used during discharge operation, at pressures between 50 and 100 mTorr. [FORMULA] is introduced by placing a lump of red phosphorus at a judicious location near the edge of the discharge, so that the plasma provides the heat to produce evaporation. The vapor pressure of the [FORMULA] molecule is [FORMULA] Torr at [FORMULA] C and [FORMULA] Torr at [FORMULA] C (The Characterization of High-Temperature Vapors 1967). By using a fill gas to sustain the discharge, the RF coupling is insensitive to the presence of small amounts of an additive gas, so the [FORMULA] vapor pressure can be controlled with the RF input power.

Helium is chosen as the primary fill gas for this experiment because of its very large excitation energy. The lowest excited state is the [FORMULA] metastable level at 19.8 eV above the ground state. Excited He atoms and He ions dissociate phosphorus molecules and ionize monoatomic phosphorus. The dissociation energy of [FORMULA] into [FORMULA] is 2.4eV(Thermodynamic Properties of the Elements 1956), and the dissociation energy of [FORMULA] is 5.0eV (Huber and Herzberg 1979). The ionization energy of monoatomic phosphorus 11.0 eV. A higher electron temperature is also obtained with He than with other rare gases (Schwabedissen et al. 1997), leading to thorough fragmentation of phosphorus molecules and substantial excitation of electronic energy levels. All of these factors are favorable for maximizing P II emission relative to molecular and neutral atomic emission (Svendenius 1980).

The lines of interest are observed with a 0.5-meter Ebert-Fastie spectrometer equipped with a 316 grooves/mm echelle grating. Both the 221.0nm and 219.6nm lines are observed simultaneously in [FORMULA] order using a photo-diode array. A 0.1-meter Seya-Namioka pre-mono-chromator is proximity-coupled to the Ebert-Fastie spectrometer to eliminate overlapping orders of the echelle grating. A radiometric calibration to correct for the pre-monochromator bandpass function, pixel-to-pixel variations in sensitivity of the photo-diode array, and residual vignetting effects occurring in the Ebert-Fastie spectrometer is determined with a deuterium lamp, in conjunction with a fused-silica optical diffuser.

A small amount of Ar is added to the discharge to facilitate low power operation. When operating the source at high RF input powers, and therefore high [FORMULA] densities, a molecular band from another echelle order appears in the region surrounding the 219.6nm line. The high threshold for H-mode operation of a pure He discharge prevents the source from being operated at lower powers (Schwabedissen et al. 1997, Kortshagen et al. 1996). Changing the buffer gas mix is more convenient than breaking vacuum and trying to move the solid phosphorus just a little farther from the discharge in order to reduce evaporation. A small amount of Ar (1-20 mTorr) is added to lower the H-mode threshold. Operating at lower input powers lowers the wall temperature of the discharge and the [FORMULA] density sufficiently to suppress the interferring molecular band.

There has been no effort to measure the phosphorus vapor pressure directly, but from changes in the RF coupling it is estimated that it can be no higher than 5 mTorr, and is quite likely less than 1 mTorr in all cases. The temperature on the outside of the fused-silica cell at the location of the red phosphorus is measured with a thermocouple, isolated from the ambient by several layers of insulating tape. The measured temperatures do not exceed [FORMULA] C for any of the measurements, although the discharge can easily be run much hotter than this.

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998