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Astron. Astrophys. 342, 610-613 (1999)

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2. Experimental

In our TRLIF experiment we use a modified version of our high-current hollow cathode as a sputtering source for iron atoms and ions and a linear Paul trap for storing the ions (see Fig. 1). The cylindrical cathode insert of 35 mm length and a central bore of 6 mm is stopped down to a 0.3 mm aperture on the low-pressure side. Within the discharge the buffer gas pressure is typically 300 Pa so that the trap is filled continuously through the aperture by pressure gradient. While the neutrals move through the trap as an effusive beam the iron ions are trapped in an oval ion cloud with a diameter of a few millimeters. For obtaining an efficient ion trapping the low-pressure side is filled with 3-40 Pa neon as a cooling gas. A more detailed description is given in Schultz-Johanning et al.(1998).

[FIGURE] Fig. 1a and b. LIF Apparatus. a  Cross-sectional view of the linear Paul trap flanged to the hollow cathode. The fluorescence photons are imaged perpendicular to the laser beam onto a photomultiplier tube. Three laser ports can be seen. b  View from the front of the apparatus.

A Quanta Ray DCR 11-3 Nd:YAG laser-pumped dye laser (Lambda Physik: LPD 3002) produces laser pulses of 4-5 ns duration (FWHM) with a spectral bandwidth of 0.2 cm-1 and a repetition rate of 10 Hz. The spectral range 220-270 nm is obtained by frequency doubling with a BBO crystal leading to a pulse duration of 3-4 ns. The laser beam is crossed with the ion cloud/beam in the Paul trap. Perpendicular to the laser beam the fluorescence photons are imaged by a lens system onto the photocathode of a multiplier using the magic angle arrangement. We need no longer optical filters for excluding unwanted light because scattered laser light is sufficiently suppressed by a stack of diaphragms, and light from the discharge itself is excluded by the 0.3 mm aperture. Our multiplier (Hamamatsu R2496) has a finite risetime of 700 ps and a non-ideal response function with ringing due to the electrical circuitry. The ringing cannot be smoothed without a loss in time-resolution. We therefore measure the response function separately and include it in our evaluation procedure, see also Engelke et al. (1993) and Schnabel et al. (1995). In addition, we measure the temporal shape of our laser pulse by means of a fast photodiode (Hamamatsu R1328U-52) having a risetime of 60 ps. This arrangement is also used to trigger the fluorescence signal. Moreover, the shape of the laser pulse enters our evaluation procedure. The fluorescence curves are recorded time-resolved by means of a fast digitizing oscilloscope (Tektronix TDS 680 B) with an analog bandwidth of 1 GHz and a realtime scanning rate of 2 [FORMULA] 5 Gigasamples. This instrument allows the record of a full decay curve and simultaneously the record of the temporal laser pulse with a 200 ps time increment which is comparable with the risetime of the oscilloscope. In the present experiment we typically added 500 single shots for obtaining a flawless S/N ratio. It is worth mentioning that the measuring time is less than one minute.

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999