\begin{table}
%\centering
\par
\caption{Atomic data references for CEL analysis.\label{refcel}}
\begin{tabular}{lll}
\hline\hline
{Ion} & {Transition probabilities} &{Collision strengths}\\
\hline
C~{\sc ii}  & \citet{Nussbaumer1981}  & \citet{Blum1992} \\
C~{\sc iii} & \citet{Keenan1992}      & \citet{Keenan1992} \\
            & \citet{Fleming1996}     &                     \\
C~{\sc iv}  & \citet{wiese}           & \citet{gau} \\
N~{\sc ii}  & \citet{Nussbaumer1979}  & \citet{stafford1994b} \\
N~{\sc iii} & \citet{Fang1993}        & \citet{Blum1992} \\
            &                           & \citet{stafford1994a} \\
N~{\sc iv}  & \citet{Nussbaumer1979}  & \citet{Mendoza1983} \\
N~{\sc v}   & \citet{wiese}           & \citet{osterbrock} \\
O~{\sc ii}  & \citet{Zeippen1982}     & \citet{Pradhan1976} \\
O~{\sc iii} & \citet{Nussbaumer1981b} & \citet{Aggarwal1983} \\
O~{\sc iv}  & \citet{nussbaumer1982b} & \citet{zhang1994} \\
            &                           & \citet{hayes} \\
Ne~{\sc ii} & \citet{Mendoza1983}     & \citet{Bayes1985} \\
Ne~{\sc iii}& \citet{Mendoza1983}     & \citet{Butler1994} \\
Ne~{\sc iv} & \citet{Zeippen1982}     & \citet{Giles1981} \\
Ne~{\sc v}  & \citet{froese}          & \citet{lennon} \\
S~{\sc ii}  & \citet{Mendoza1982a}    & \citet{Keenan1996} \\
            & \citet{Keenan1993}      &                     \\
S~{\sc iii} & \citet{Mendoza1982b}    & \citet{Mendoza1983} \\
S~{\sc iv}  & Storey (unpublished)      & \citet{Butler1989} \\
Cl~{\sc iii}& \citet{Mendoza1983}     & \citet{Mendoza1983}\\
Cl~{\sc iv} & \citet{Mendoza1982a}    & \citet{Butler1989} \\
Cl~{\sc v}  & \citet{Mendoza1983}     & \citet{Mendoza1983}\\
Ar~{\sc ii} & \citet{pelan}           & \citet{vujnovic}\\
Ar~{\sc iii}& \citet{Mendoza1983a}    & \citet{Johnson1990} \\
Ar~{\sc iv} & \citet{Mendoza1982a}    & \citet{Zeippen1987}\\
Ar~{\sc v}  & \citet{Mendoza1982b}    & \citet{Mendoza1983}\\
Ar~{\sc vi} & Storey (unpublished)      & \citet{saraph} \\
Mg~{\sc iv} & \citet{Mendoza1983a}    & \citet{saraph1994}\\
Mg~{\sc v}  & \citet{galav}           & \citet{Butler1994}\\
Fe~{\sc iii}& \citet{Nahar1996}       & \citet{Zhang1996} \\
Fe~{\sc iv} & \citet{Garstang1958}    & \citet{Zhang1997} \\
            & \citet{fischer}         &                     \\
Fe~{\sc vi} & \citet{nuss1978}        & \citet{nuss1978}\\
Fe~{\sc vii}& \citet{nussbaumer1982a} & \citet{Keenan1987}\\
            &                           & \citet{berrington}\\
\hline\end{tabular}
\end{table}