3. Lifetime measurements
Evaluation of our LIF signals is done by fitting an exponential decay curve which is convoluted with the response function of our photomultiplier. By this procedure the whole LIF signal can be evaluated including the rising portion. The procedure is described in Engelke et al. (1993). Additionally, we extended the procedure to handle saturation in the fluorescence signals by solving an appropriate rate equation system. Fig. 2 demonstrates the excellent matching between measuring points and fitted curve even if the lifetime is slightly shorter than the FWHM of the laser pulse. During pumping with the laser pulse saturation occurs and also overpumping into third levels. Both transmute the temporal shape of the LIF signal, see also the results of van Lessen et al. (1998). In Fig. 3 a comparison of a fit including saturation with a fit neglecting saturation is given.
In Table 1 seven Fe II level lifetimes are listed together with literature data. Quoted uncertainties refer to the standard deviation of the mean. On each level we made at least 5 independent measurements on up to three transitions using differing buffer gas conditions and laser pulse energies. There is consistency within the mutual error bars, although the measuring procedures are quite different. On the other hand our lifetimes are shorter than the values we used in Kroll & Kock (1987). Therefore we can rescale the absolute transition probabilities presented in Heise & Kock (1990) where we used a reference line from Kroll & Kock.
Table 1. Fe II radiative lifetimes
© European Southern Observatory (ESO) 1999
Online publication: February 22, 1999