A 0.27-m imaging monochromator (Spex 270M) was used for
wavelength dispersion of the ICP emission. The spectrometer
was set to monitor a wavelength range of 202.1-231.0 nm, which
covers the Zn II 202.548 nm and Cu I 222.778 nm emission lines.
The ICP plasma was imaged with a biconvex lens (focal length )
50 mm) onto the entrance slit of the monochromator with a
demagnification ratio of 1:0.2. The CCD detector (EG&G Princeton
Applied Research OMA-Vision 4) is approximately 1 cm in
height; therefore, the entire ICP (along its central channel) can
be monitored simultaneously. The emission measurement region
was from 2.5 mm below the top of the load coil to 39.7 mm above
the load coil. The CCD detector was thermoelectrically cooled
to -60 °C to minimize dark current. The exposure time was set
to 10 s, which integrates the emission from 100 laser pulses.