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\documentstyle{laa}
\begin{document}

\title{Atomic data for absorption lines from the ground level at wavelengths 
greater than 228~{\AA}\thanks{All tables are also available in electronic form:
see the Editorial in A\&AS 1994, Vol. 103, No. 1}}

\author{D.A.~Verner\inst{1} \and P.D.~Barthel\inst{1} \and
  David Tytler\inst{2}}

\offprints{D.A.~Verner}

\institute{Kapteyn Astronomical Institute, P.O.~Box 800, 9700 AV
  Groningen, The Netherlands
\and
Department of Physics, CASS-0111, University of California,
  San Diego, La Jolla, CA 92093-0111, USA}

\thesaurus{02.01.3, 11.17.1}

\date{Received date; accepted date}

\maketitle
\label{sampout}

\begin{abstract}
We list wavelengths, statistical weigths and oscillator strengths for
2249 spectral lines arising from the ground {\mbox states} of atoms and ions.
The compilation covers all wavelengths longward of the He{\sc ii} Lyman
limit at 227.838~{\AA} and all the ion states of all elements from
hydrogen to bismuth ($Z=83$) for which reliable data are known. We
assign experimental wavelengths to 1086 lines which have oscillator
strengths calculated in the Opacity Project, and add 1163 lines which
have critically evaluated oscillator strengths from previous compilations.
The presented data are to be used for quasar absorption spectra 
interpretation. We use solar abundances to calculate the expected
relative strengths of all absorption lines, and list separately the
strongest lines above 228~{\AA} expected in QSO absorption systems,
many of which are listed here for the first time. The great majority of
strong lines expected in quasar spectra are at $\lambda < 912$~{\AA}.
We also list the strongest absorption lines of various ions without
wavelength restriction.
%
\keywords{atomic data -- quasars: absorption lines}
\end{abstract}
%
\section{Introduction}
%
Accurate atomic data are essential in various
fields of astrophysics, from solar physics to quasar spectroscopy.
Of particular interest are absorption line spectra of high redshift QSOs,
which are the main source
of information on the chemical composition and physical state of gas in
early epoch galaxies and thereby on the cosmic evolution of interstellar and
intergalactic matter. Reliable vacuum 
wavelengths and oscillator strengths are needed 
to identify absorption lines in QSO spectra
and to derive parameters such as column densities,
temperatures and velocity structure.

In this paper we present new data on atomic absorption lines arising from the
ground state.
The particle and radiation densities in the interstellar medium and in
the gas which produces
quasar absorption systems are usually sufficiently low that {\mbox nearly} all 
atoms and ions are in their ground states, and consequently 
the absorption lines arising in such gas are {\mbox nearly} all
transitions from the ground (zero energy) level.
At higher densities or in more intense radiation absorption may arise from
excited fine structure levels (e.g. C{\sc ii}$^*$ 1335.6672, 1335.7077 -- see
Bahcall \& Wolf 1968; Morton 1991, hereafter M91), and from upper levels
(e.g. the Balmer lines in stellar spectra), but we will not consider such lines.

M91 presented a compilation of atomic data for the lines
involving the ground level and excited fine structure levels of the
ground term. He included allowed and intersystem absorption 
lines of all ion stages of all elements from hydrogen to germanium ($Z=32$)
with wavelengths longward of the hydrogen Lyman limit at 911.75~{\AA}.
Since then there have been three developments. We now have 
quasar spectra which extend down below 300~{\AA},
we have much improved 
signal to noise in both the satellite UV and from the ground,
and there are improved atomic data.

Satellites now routinely observe 
wavelengths well below 912~{\AA}. 
The Hubble Space Telescope (HST) is measuring spectra down to
1150 \AA, which corresponds to rest frame wavelengths
$300 \la \lambda_0 \la 900$~{\AA} for high $z$ QSOs, and
both the existing Hopkins Ultraviolet Telescope and
the proposed Far-Ultraviolet Spectroscopic
Explorer mission will observe in the
$912 \la \lambda \la 1200$~{\AA} range,
covering rest frame wavelengths down to approximately
200~{\AA}. Using HST, 
Reimers et al. (1992) {\mbox identify} many absorption
lines with wavelengths $\lambda_0 \ga 350$~{\AA} in the spectrum of the
bright quasar HS1700+6416, and many other spectra are being obtained.

The HIRES spectrograph on the W.M. Keck telescope
is now producing spectra of quasars with ${\rm SNR}=30-80$
per 0.01~{\AA} in the rest frame which are sensitive to absorption
lines with rest frame equivalent widths of $W_{\rm rest} \leq 1$~m{\AA},
two orders of magnitude weaker than the limit in typical samples of absorption
lines, and one order of magnitude better than the previous record.

In the meantime, a large international collaboration known as the Opacity
Project (OP) has produced a complete set of accurate atomic data for 
all stages of ionization of astrophysically abundant elements:
$1 \leq Z \leq 14$ and 
 $Z=16, \, 18, \, 20, \, 26$, where $Z$ is atomic number 
(Seaton et al. 1992). In particular,
the OP data include oscillator strengths for
all optically allowed transitions between states with $n \leq 10$ and
$l \leq 4$, where $n$ and $l$ are the active-electron principal and
orbital angular momentum respectively. 
The OP data exclude intersystem (spin-forbidden) transitions.
The OP calculations are based on
high level methods such as the R-matrix method (Burke et al. 1971) and
the
new asymptotic techniques developed by Seaton (1985). Extensive comparisons
of the OP data with experiment and theory (Seaton et al. 1992; Mendoza 1992)
have confirmed the high accuracy of the OP calculations.  

Here we present the data needed to 
interpret absorption lines with wavelengths
$\lambda \geq 227.838$~{\AA}, the 
He{\sc ii} Lyman limit. We give
vacuum wavelengths, statistical weights and oscillator strengths for 
absorption lines from the ground level. In a separate table we list the
strongest absorption lines of various ions without wavelength restriction. 
The data are virtually complete for all stages of ionization of all
elements from hydrogen to nickel ($Z=28$). In addition, the compilation 
includes parameters for lines with reliably known oscillator strengths
for heavier elements up to bismuth ($Z=83$). 

We do not apply strict criteria to the line selection; 
rather we include all ground
level absorption lines which have experimental wavelengths and critically
evaluated oscillator strengths, and we
use solar abundances to calculate the expected relative strengths of 
these absorption lines.

In section 2 of this paper we discuss the sources of data and
compare OP oscillator strengths with ones from earlier
compilations. In section 3 we discuss the expected relative strengths of 
absorption lines arising from the ground level. The strongest lines above
228~{\AA} expected in QSO spectra are listed in a table, 
and in a separate table we list
the strongest lines above 228~{\AA} without experimental
wavelengths. In section 4 we explain the four main atomic data tables, 
the last two of which are long.

We present six tables.
\begin{enumerate}
\item Strong lines above 228~{\AA} expected in quasar absorption systems;
\item Strongest lines above 228~{\AA} without experimental wavelengths;
\item Atomic weights and solar-system abundances;
\item Ionization thresholds and strongest lines of atoms and ions;
\item Resonance absorption lines above 228~{\AA} ordered by element;
\item Resonance absorption lines above 228~{\AA} ordered by wavelength.
\end{enumerate}

In keeping with modern practice for quasars, all wavelengths in this paper are
vacuum values. Some older papers use air values above 2000~{\AA}.
The standard conversion formula is Eq. (1) of Morton \& Smith (1973), or
Eq. (3) of M91.

\section{Data}

\subsection{Opacity Project data and experimental wavelengths}

The OP data available through the TOPbase 0.6 database (Cunto et al. 1993)
include $gf$-values for all multiplets of
optically allowed transitions between states with $n \leq 10$ and   
$l \leq 4$ for all ionization states of elements with atomic number
$1 \leq Z \leq 14$, $Z=16, \, 18, \, 20$ and for $N \leq 24$ for $Z=26$ 
(iron), where $N$ is the number of electrons. The total number of multiplets
exceeds $10^6$, but only 6018 involve the ground terms. 
In the OP calculations relativistic effects are neglected, and therefore
$LS$-coupling is assumed throughout. The $f$-values of individual lines
in multiplets based on $LS$-coupling can be easily calculated (see, e.g.,
Allen 1973). The 6018 multiplets include 9569 lines originating
from the ground (zero energy) level, and of these 2189 lines have the 
theoretical wavelengths above 200~{\AA}.
The OP theoretical wavelengths are accurate to a few angstroms, 
which is not good enough for spectroscopy, so they must be replaced 
by experimental wavelengths
and wavelengths calculated from experimental energy levels which are usually 
good to $\sim 0.001$~{\AA}.

Main sources of experimental wavelengths
are Kelly (1987, hereafter K87; lines below 2000~{\AA})
and M91 (resonance lines above 911.75~{\AA}).
We have also used compilations of experimental energy levels
(Bashkin \& Stoner 1975, 1978; Sugar \& Corliss 1985) to 
calculate accurate wavelengths. Finally, we have
undertaken an extensive literature
search (complete to December 1993) to find new and improved experimental 
wavelengths in original papers. 
We attempted to assign accurate {\mbox wavelengths} to
all 2189 OP lines which originate from the ground level
with theoretical wavelengths $\geq 200$~{\AA}. We obtained accurate wavelengths
longward of 227.838~{\AA} for 1086 of these lines:
230 from M91, 540 from
K87, 174 with wavelengths calculated from experimental energy levels,
and 142 lines from new original papers. 

Some of the remaining 1103 lines were rejected because they had accurate
wavelengths below 227.838~{\AA}, 
but for most we could not locate accurate wavelengths and we 
have only the approximate theoretical wavelengths. Fortunately, most such 
lines are weak higher order lines of spectroscopic series, but the few
exceptions which are strong will be specified in Table 2 below.

We subsequently used the experimental wavelengths to improve the $f$-values 
calculated by the OP.  The OP calculations
give line strength $S$ and transition energy $E$, where
$S$ is given by an integral over the atomic wave functions and does not 
involve $E$. Then $gf$-values are calculated from $S$ and $E$ since 
$gf = {\rm const} \times S \times E$ 
(or $gf = {\rm const} \times S/\lambda$). 
The OP used their theoretical values of $E$ to calculate $gf$-values. 
To improve the $gf$-values we now use the more accurate 
experimental wavelengths instead of the theoretical $E$ values. 
We recalculated the OP $f$-values as:
$f_{\rm improved} = f_{\rm OP} \times (\lambda_{\rm OP}/\lambda_{\rm exp})$.

\subsection{ Additional data from Morton (1991)}

We obtained $f$-values of lines from M91 
in addition to those from the OP.
M91 includes oscillator strengths for 907
ground level lines at $\lambda \geq 911.75$~{\AA}.  
Of these, 564 lines belong to species not
covered by the OP (including 155 lines of Fe{\sc i} and Fe{\sc ii}), 
239 lines are in common with the OP, and 104 lines are 
non-$LS$-type or higher order ($n > 10$) lines of the OP species.
Above 911.75~{\AA}, we include 67 lines with OP $f$-values and experimental
wavelengths which are not covered by M91. All are of species with  $E<13.6$~eV
(65 neutrals and 2 lines of Ca{\sc ii}).

Figure 1 shows a comparison of the improved OP $f$-values with those
from M91
for the 239 lines in common. The agreement is generally
good, with some exceptions. The largest discrepancies are for twelve lines of
S{\sc i}
(M91 source: unpublished semi-empirical data by Kurucz) and
for three lines of Ca{\sc i} which do not have reliable identifications:
OP, M91 and Bashkin \& Stoner (1978) all disagree.
Note that M91 erroneously gave $f=0.5459$ for
C{\sc i} 1157.1857~{\AA}; it should be $f=0.00055$.

\subsection{Data from Fuhr \& Wiese (1991) and the NBS data}

Additional lines and $f$-values were obtained from
Fuhr \& Wiese (1991, hereafter FW) and from the U.S. National Bureau of 
Standards (NBS) publications
(Wiese et al. 1966, 1969; Martin et al. 1988; Fuhr et al. 1988).

FW include data for about 8300 selected
lines of all elements up to Bi ($Z=83$) for which reliable data
are available. We selected from this compilation the data for the ground level
lines, and converted transition probabilities $A_{ik}$ into 
oscillator strengths $f_{ik}$ in accordance with the standard formula:
\begin{equation}
   g_i f_{ik} = 1.499 \times 10^{-8} \lambda^2 g_k A_{ik},  \label{eq:1}
\end{equation}
where $\lambda$ is in {\AA} and $A_{ik}$ is in $10^8$~s$^{-1}$.
In most cases FW give wavelengths to two or sometimes only
one decimal place, whereas other compilations give 
more. We replaced the FW wavelengths with those from
K87, Bashkin \& Stoner (1975, 1978), Sugar \& Corliss (1985) for
elements with $Z \leq 36$, and from 
Moore (1950, 1952a,b, 1958, 1959, 1962) for the heavier elements ($Z \geq 37$,
8\% of the data). 
The latter will not always be better than the FW values,
because in some cases Moore's compilations
overestimate accuracy giving 3 decimal places, and in other cases FW
underestimate accuracy, giving values to only 0.1 -- 0.01~{\AA}.
Note that FW give the air wavelengths of lines above 2000~{\AA}.

We supplemented the FW data with those from
previous NBS compilations of atomic transition probabilities 
(Wiese et al. 1966, 1969; Martin et al. 1988; Fuhr et al. 1988).
We replaced wavelengths given by Wiese et al. (1966, 1969) with
improved values from
the more recent compilations of experimental wavelengths described above.

In total we give $f$-values from FW and NBS compilations for 746 lines. 
Of these, 400 lines belong to species not covered by the OP,
251 lines are in common with the OP, and 95 lines are 
non-$LS$-type or higher order ($n > 10$) lines of the OP species.
FW and NBS tables include only 
36 lines with $\lambda > 911.75$~{\AA} from the ground level
of elements with $Z \leq 32$ which were omitted by M91.
Seventeen of them are transitions to 
high orders ($n \geq 14$)  of the hydrogen Lyman series, and
the others are weak lines of Be{\sc i}, B{\sc i}, Ne{\sc vi}, Cu{\sc i},
Zn{\sc i}, Ga{\sc i}, and Ge{\sc i}. Only 3 of these 36 lines are
covered by the OP: Be{\sc i} 1661.478 and 1491.765~{\AA}, and
B{\sc i} 2089.559~{\AA}.

Figure 2 shows a comparison of the improved OP $f$-values with those 
from the NBS compilations for 251 lines in common. The agreement with 
the recent compilations (FW,  Martin et al. 1988,
Fuhr et al. 1988) is generally good, but some values from 
earlier compilations (Wiese et al. 1966, 1969) have large discrepancies.

In the whole, we include data on oscillator strengths of 2249 lines
from three main sources: 1086 lines from the OP, 668
additional lines from M91, and 495 additional lines from FW
and NBS publications
(Wiese et al. 1966, 1969; Martin et al. 1988; Fuhr et al. 1988).
We excluded from our compilation the OP data for all lines of Ne{\sc i}, 
Na{\sc ii}, Ar{\sc i}, and also for three lines of Si{\sc i},
since the OP assumption of $LS$-coupling is not satisfactory for them. 
For these lines we list $f$-values from previous compilations only. 
For 490 OP lines which have been included in the previous
compilations, we list older $f$-values along with the new
OP values. The accuracy of OP data can deteriorate for neutral and
singly ionized species. For one line (Be{\sc i} 1491.765~\AA),
the OP $f$-value is obviously too small.
This line is clearly seen experimentally (see K87 and references therein),
and its intensity fits smoothly in a sequence of other lines in
the spectroscopic series of $1s^2\,2s^2-1s^2\,2s\,np$ transitions.
In the meantime, the OP $f$-value for this particular line does not at
all fit the sequence of other oscillator strengths for this spectroscopic
series.
In most cases,
the OP data are more accurate than other theoretical data from earlier 
compilations,
but experimental $f$-values from older sources may be preferred
over the OP calculations.  
We do not include in the present compilation new oscillator strengths
from recent papers. Some of them may improve the listed
$f$-values, and some others belong to new lines which are important for 
astrophysics (see, e.g., Cardelli et al. 1993). In a subsequent paper, we 
will compile all available oscillator strengths of the 
ground level lines, and will evaluate their accuracy.

\section{ Expected strengths of absorption lines}

In total, we have collected oscillator strengths and experimental wavelengths
for 2249 absorption lines. For all
these lines, we calculated the expected width (strength) parameter
\begin{equation}
  P = \log (\lambda f) + \log(N/N_{\rm H}) +12.00,     \label{eq:2}
\end{equation}
where $N/N_{\rm H}$ is the abundance of the element relative to hydrogen,
and $\lambda$ is in {\AA}. We used the
solar-system abundances of Anders \& Grevesse (1989), which we list
in Table~3. Provided the element is entirely
in the ion of the line in question, and there is no line saturation, 
\begin{equation}
  W_{\rm rest} =8.85 \times 10^{P-33} N_{\rm H} \lambda \label{eq:3}
\end{equation}
where $W_{\rm rest}$ is the (rest frame) line equivalent width in {\AA},
$\lambda$ is in {\AA} and $N_{\rm H}$ is in cm$^{-2}$.
A line with $W_{\rm rest} =1$~m{\AA} at $\lambda = 1000$~{\AA}
would be produced by a transition with
$P=4.15$ in the quasar damped Lyman-$\alpha$ system with  the largest known 
$N_{\rm H} = 8 \times 10^{21}$~cm$^{-2}$.
If the line is saturated, or if some of the element is in different ions,
or if the metal 
abundances are below solar, then the absorption line will be weaker than
given by Eq. (3), so we do not expect to see many lines with $P \leq 4$.

Figure 3 gives a graphical presentation of the relative strengths
of the 2198 metal lines arising from the ground level which have
wavelengths from 228 to 8000~{\AA}. We also show C{\sc i} 841~{\AA} which is 
very strong (but without an experimental wavelength), but we omit all
H and He lines. The wavelength scale is logarithmic, because most lines are in
the far UV. All strengths have been normalized to that of 
the strongest metal line which is O{\sc v} 629.730~{\AA} with $P=11.44$, 
so we plot $D=10^{P-11.44}$, where $D$ is proportional to 
$W_{\rm rest}/\lambda$
for unsaturated lines, from gas with solar abundances and assuming
that the element is in one ionization state.
Most lines are too weak to show up on this plot.
We identify all lines with $D \geq 0.05$ ($P \geq 10.14$)
on the plot, but if two lines in
a doublet both have $D \geq 0.05$, both are shown but the wavelength is given
for only the stronger line.

Morton et al. (1988, hereafter MYJ) presented a list of strong lines
above 912~{\AA} suitable for an initial search of QSO absorption
systems (Table 4 of MYJ). They included lines with 
$P \geq 9.49$ longward of the hydrogen Lyman-$\alpha$, and $P \geq 9.98$ 
shortward. They omitted all species with 
ionization potentials $E<13.6$~eV, 
since low ionization species are not common in QSO absorption spectra.

In Table 1 we extend the MYJ list of strong lines expected in quasar 
absorption spectra down to 228~{\AA}. For species with $E \geq 13.6$~eV, we 
include all lines with 
$P \geq 9.49$ above 1215.67~{\AA} and $P \geq 9.98$ below
1215.67~{\AA}. In addition, Table 1
includes strong lines of ions with $P$ less than these thresholds,
which do not have other stronger lines. Above 912~{\AA}, we added
the doublet S{\sc vi} 933.378/944.523~{\AA} with $P=9.89/9.59$. 
We also added the strongest ($P>10.5$) lines above 912~{\AA} for neutrals 
with $E<13.6$~eV:  Mg{\sc i} 2852.9642~{\AA}, Fe{\sc i} 2484.0205~{\AA},
and four lines of C{\sc i}.  Other differences with  
Table 4 of MYJ at $\lambda \geq 912$~{\AA} are:

(i) Multiplets of N{\sc i} at 1200~{\AA} and O{\sc i} at 989~{\AA} were not 
resolved in MYJ;

(ii) MYJ did not mark O{\sc i} at 1025.76~{\AA} (almost the same wavelength 
as H{\sc i} Lyman-$\beta$ at 1025.7233~{\AA}),
although it had $P = 10.19$ from their data.\\

All other lines above 912~{\AA} in Table 1 are the same as in MYJ.
Small discrepancies in $P$-values are mostly due to differences in the  
chemical abundances used by MYJ and by us. To save space, we list only
the five strongest lines of H{\sc i}, He{\sc i}, and
He{\sc ii}.  Table 1 includes 53 lines above 912~{\AA} and 98 lines
below 912~{\AA}. Note that 27 lines 
below 912~{\AA} were not included in the previous compilations
of atomic transition probabilities and oscillator strengths
(FW, Fuhr et al. 1988, Wiese et al. 1966, 1969).
We have divided Table 1 into six wavelength segments, separated 
by gaps at the Lyman-$\alpha$ and Lyman limit wavelengths
of H{\sc i}, He{\sc i} and He{\sc ii}. Many of the lines are doublets.
We note that work on quasar absorption lines has mostly been restricted to
lines in the largest wavelength segment, and that most 
of the other lines have not yet been observed.
Theoretical far-UV absorption spectra show that these
lines should be seen in HST spectra of quasars (e.g., Verner
et al. 1994).

We have also calculated the strength parameters $P$ for all 
lines from the ground level with OP $f$-values which do not have experimental
wavelengths. Table 2 includes lines with $P \geq 9.00$ for 
the strongest line from the ground level of a multiplet. Their
theoretical wavelengths (good to a few angstroms) are all above
228~{\AA}.  For all these species there are
no experimental data at the corresponding wavelength ranges, except for
Ca{\sc v}, where data are available (see K87 and references
therein) but line identifications are ambiguous, and no lines were
classified in the
$3s^23p^4(^3{\rm P})-3p^3(^4{\rm S}^\circ)4s(^3\rm D^\circ)$ 
multiplet.


\section{Tables of atomic data}

The bulk of this paper contains tables of atomic data, which
are explained below.

\subsection{Table 3. Atomic weights and solar-system abundances}

Table 3 gives standard atomic weights (scaled to the abundance of $^{12}\rm C$
equal to 12.00) recommended by the commission
on Atomic Weights and Isotopic Abundances (Holden \& Martin 1983), 
and the solar-system
abundances $\log(N/N_{\rm H})+12.00$ from Anders \& Grevesse (1989).
The abundances are meteoritic
for all elements except H, C, N, O (solar photosphere abundances) and
He, Ne, Ar, Kr, Xe (solar wind abundances). For completeness, we reproduce
all elements with known abundances.
These abundances were used in the calculation of the line strength $P$ values.

\subsection{Table 4. Ionization thresholds and strongest lines of 
atoms and ions}

Table 4 gives the basic atomic data for all ionization states of atoms.
The columns are:\\

\begin{tabular}{lcl}
Species & & \\
$Z$ & -- & atomic number\\
$N$ & -- & number of electrons\\
Configuration & -- & ground state electronic configuration.\\ 
  & & Only the five outer shells are indicated\\
$E$, eV & -- & ionization potential,\\
 & & from K87 at $Z \leq 36$ and\\
 & & from Moore (1970) at $Z \geq 37$.\\
 & & The values which are based on \\
 & & extrapolation rather than observation\\ 
 & & are enclosed in square brackets\\
\end{tabular}
The last 5 columns list parameters of the strongest resonance line
of the species:

\begin{tabular}{ll}
$\lambda$, {\AA} & -- vacuum wavelength\\
$g_i$ & -- lower (ground) level statistical weight\\
$g_k$ & -- upper level statistical weight\\
$f_{ik}$ & -- oscillator strength\\
$P$ & -- expected strength parameter from Eq. (2)\\
\end{tabular}\\

Since the OP data include $f$-values for all allowed transitions
between states with $n < 11$ and $l < 5$, we can 
choose the strongest line for any ion of elements with $Z \leq 14$, $Z = 16$, 
18, 20, 26. Wavelengths for lines below 228~{\AA} have been taken from
K87. Some lines of elements covered by the OP which have
no experimental wavelengths but must be the strongest ones for given
ions, are enclosed in square brackets. The parameters of the strongest lines 
for Fe{\sc i} and  Fe{\sc ii} have been taken from M91.

For elements not covered by the OP, the strongest lines have been
selected from ones
with known oscillator strengths and experimental wavelengths.
The oscillator strengths have been taken from M91, FW,
Martin et al. (1988), Fuhr et al. (1988), Cheng et al. (1979; 
multiconfiguration Dirac-Fock calculations for Li-like through F-like 
isoelectronic sequences),
Drake (1979; calculations for He-like
isoelectronic sequence),
and Wiese et al. (1969), in this order of priority.
A few species of the elements not covered by the OP
are not included in the table, because we lack oscillator strengths
for these ions (Sc{\sc vi}, Ti{\sc vii}, etc.).

Ionization thresholds can be used to determine which {\mbox ions}
are likely to be
present, while the strongest lines and their $P$ values can be used to
determine which ions of which elements might be detectable.
In many damped Lyman-$\alpha$ systems 
Zn{\sc ii}~$\lambda 2026.136$ with $P=7.67$ has been detected 
with $W_{\rm rest} \simeq 0.1$~{\AA} (e.g., Pettini \& Hunstead 1990), 
so with HIRES on the Keck telescope we should see 
low ionization lines at $\lambda \geq 900$~{\AA}
with $P \geq 5.7$ in such absorption systems. These will include
lines from elements such as B, F, P, Cl, Ar, Sc, Ti, V, Co, Ni, Cu, Ga and Ge,  
which all have low ionization ions with $E \geq 13.6$~eV and lines at
$\lambda \geq 900${\AA}.
For systems with exceptionally high $N_{\rm H}$ and high metal abundances, and 
with high signal to noise ratios we may also see lines with 
$ 4 \leq P \leq 5.7$
from elements such as Br, Sr, Y, Cd, Xe, Ba, Yb and Pb.

\subsection{Table 5. Resonance absorption lines 
above 228~{\AA} ordered by element}

This table is sorted by element,
ionization state (starting with the most electrons),
multiplet (starting with the ground level lines with the largest wavelength),
wavelength (decreasing wavelength within the
multiplet).  The lines of some multiplets overlap in wavelength with
lines of other multiplets,
so lines are not all in order of decreasing wavelength.
Only the ground level lines are included. The columns are as follows:\\

\begin{tabular}{lcl}
Species & & \\
Transition & -- & lines with uncertain classification are\\
 & & marked by :\\
Multiplet & & \\
$\lambda$, {\AA}) & -- & vacuum wavelength\\
$g_i$ & -- & lower (ground) level statistical weight\\
$g_k$ & -- & upper level statistical weight\\
$f_{ik}$ & -- & improved OP oscillator strength\\
$f'_{ik}$ & -- & oscillator strength from other\\ 
 & & compilations taken in the following\\
 & & order of priority: M91, FW,\\
 & & Martin et al. (1988),\\
 & & Fuhr et al. (1988)\\
 & & and Wiese et al. (1966, 1969)\\
$P$ & -- & expected strength parameter from Eq. (2).\\
 & & For the calculation of this parameter in\\
 & & the cases where both $f_{ik}$ and $f'_{ik}$ are\\
 & & available, we used $f_{ik}$. The only\\
 & & exception is 1491.765~{\AA} line of Be{\sc i},\\
 & & where $f'_{ik}$ was used\\
Ref & -- & reference to experimental wavelength\\
 & &  (numbers are explained at the end of\\
 & & the table)\\
\end{tabular}

If the experimental wavelengths are known for the stronger ground level lines
of a multiplet but are not known for weaker ground level lines of this
multiplet, estimated values of the latter
wavelengths are given in square brackets.

\subsection{Table 6. Resonance absorption lines 
above 228~{\AA} ordered by wavelength}

This table is sorted by wavelength. Only the ground level
lines with known oscillator strengths and experimental wavelengths are
included. The entries are the following:\\

\begin{tabular}{lcl}
$\lambda$, {\AA} & -- & vacuum wavelength\\
Species & & \\
$g_i$ & -- & lower (ground) level statistical weight\\
$g_k$ & -- & upper level statistical weight\\
$f_{ik}$ & -- & oscillator strength\\
$P$ & -- & expected strength parameter from Eq. (2)\\
\end{tabular}

Oscillator strengths have been taken from the $f_{ik}$ entry of Table 5, 
if possible, (i.e., the improved OP $f$-values); 
otherwise, from the $f'_{ik}$ entry of Table 5
which come from the other compilations described above.
 The only exception is 
the 1491.765~{\AA} line of Be{\sc i}, where $f'_{ik}$ was used.


\acknowledgements
DAV acknowledges financial support from the Netherlands Organization for
Pure Research (NWO) through their visitor programs, and travel support from
the Leids Kerkhoven-Bosscha Fonds.  PDB acknowledges
support from the Royal Netherlands Academy of Arts and Sciences.
DT was supported in part by NASA grant NAGW-2119,
and by GO-3801.01-91A from the Space Telescope Science Institute, which
is operated by AURA, Inc., under NASA {\mbox contract} NAS5-26555. 
We acknowledge the use of the TOPbase 0.6 database through the Centre de
Donne\'{e}s Astronomiques de Strasbourg.
We are grateful to J. Tully and C. Mendoza for useful comments on
the Opacity Project data, and to E. Verner for the help in preparation
of the tables.  

\begin{thebibliography}{}
\bibitem[1973]{all73}
Allen C.W., 1973, Astrophysical Quantities, 3d ed., Univ. of London,
The Athlone Press, p. 61
\bibitem[1989]{and89}
Anders E., Grevesse N., 1989, Geochim. Cosmochim. Acta, 53, 197
\bibitem[1968]{bah68}
Bahcall J.N., Wolf R.A., 1968, ApJ 152, 701
\bibitem[1975]{bas75}
Bashkin S., Stoner Jr. J.O., 1975, Atomic Energy Levels and Grotrian
Diagrams, v. I, Hydrogen I - Phosphorus XV, North-Holland Publishing Company
\bibitem[1978]{bas78}
Bashkin S., Stoner Jr. J.O., 1978, Atomic Energy Levels and Grotrian
Diagrams, v. II, Sulphur I - Titanium XXII, North-Holland Publishing Company
\bibitem[1971]{bur71}
Burke P.G., Hibbert A., Robb W.D., 1971, J. Phys. B 4, 153
\bibitem[1993]{car93}
Cardelli J.A., Federman S.R., Lambert D.L., Theodosiou C.E., 1993, 
ApJ 416, L41
\bibitem[1979]{che79}
Cheng K.T., Kim Y.K., Desclaux J.P., 1979, At. Data. Nucl. Data Tables 24,
111
\bibitem[1962]{cor62}
Corliss C.H., Bozman W.R., 1962, Experimental Transition Probabilities
for Spectral Lines of Seventy Elements, NBS Monograph 53 
\bibitem[1993]{cun93}
Cunto W., Mendoza C., Ochsenbein F., Zeippen C.J., 1993, A\&A 275, L5
\bibitem[1979]{dra79}
Drake G.W.F., 1979, Phys. Rev. A 19, 1387
\bibitem[1993]{ekb93}
Ekberg J.O., 1993, A\&AS 101, 1
\bibitem[1990]{ekb90}
Ekberg J.O., Smitt R., Skogvall B., 1990, Phys. Scr. 41, 217
\bibitem[1983]{eri83}
Eriksson K.B.S., 1983, Phys. Scr. 28, 593
\bibitem[1986]{eri86}
Eriksson K.B.S., 1986, Phys. Scr. 34, 211
\bibitem[1987]{eri87}
Eriksson K.B.S., 1987, J. Opt. Soc. Am. B 4, 1369
\bibitem[1980]{faw80}
Fawcett B.C., Hatter A.T., 1980, A\&A 84, 78
\bibitem[1991]{fuh91}
Fuhr J.R., Wiese W.L., 1991, Atomic Transition Probabilities. In:
Lide D.R. (ed.) CRC Handbook of Chemistry and Physics, 72th ed., 
CRC Press, p. 10-128 (FW)
\bibitem[1988]{fuh88}
Fuhr J.R., Martin G.A., Wiese W.L., 1988, J. Phys. Chem. Ref. Data 17, 
Suppl. 4
\bibitem[1983]{hol83}
Holden N.E., Martin R.L., 1983, Pure Appl. Chem. 55, 1101
\bibitem[1992]{joh92}
Joelsson I., Zetterberg P.O., Magnusson C.E., 1981, Phys. Scr. 23, 1087
\bibitem[1992]{joh92}
Johansson L., Magnusson C.E., Joelsson I., Zetterberg P.O., 1992, 
Phys. Scr. 46, 221
\bibitem[1993]{jup93}
Jup\'{e}n C., Isler R.C., Tr\"{a}bert E., 1993, MNRAS 264, 627
\bibitem[1987]{kel87}
Kelly R.L., 1987, J. Phys. Chem. Ref. Data 16, Suppl. 1 (K87)
\bibitem[1988]{lod88}
Lodwig V., Bukow H.H., Buttlar H. v., 1988, Z. Phys. D 9, 177 
\bibitem[1988]{mar88}
Martin G.A., Fuhr J.R., Wiese W.L., 1988, J. Phys. Chem. Ref. Data 17, 
Suppl. 3
\bibitem[1992]{men92}
Mendoza C., 1992, in: Smith P.L., Wiese W.L. (eds.) Atomic and Molecular Data 
for {\mbox Space} Astronomy: Needs, Analysis and Availability, Lecture
Notes in Physics, Springer-Verlag, Berlin, 407, p. 85
\bibitem[1950]{moo50}
Moore C.E., 1950, An Ultraviolet Multiplet Table, Circ. NBS 488, Sect. 1
\bibitem[1952]{moo52a}
Moore C.E., 1952a, An Ultraviolet Multiplet Table, Circ. NBS 488, Sect. 2
\bibitem[1952]{moo52b}
Moore C.E., 1952b, Atomic Energy Levels, v. II, Circ. NBS 467
\bibitem[1958]{moo58}
Moore C.E., 1958, Atomic Energy Levels, v. III, Circ. NBS 467
\bibitem[1959]{moo59}
Moore C.E., 1959, A Multiplet Table of Astrophysical Interest, 
NBS Techn. Note 36
\bibitem[1962]{moo62}
Moore C.E., 1962, An Ultraviolet Multiplet Table, Circ. NBS 488, Sect. 3
\bibitem[1970]{moo70}
Moore C.E., 1970, Ionization Potentials, NSRDS-NBS 34
\bibitem[1991]{mor91}
Morton D.C., 1991, ApJS 77, 119 (M91)
\bibitem[1988]{mor88}
Morton D.C., York D.G., \& Jenkins E.B. 1988, ApJS 68, 449 (MYJ)
\bibitem[1988]{ms73}
Morton D.C., Smith W.M.H., 1973, ApJS, 233, 333
\bibitem[1994]{nav94}
Nave G., Johansson S., Learner R.C.M., Thorne A.P., Brault J.W., 1994, ApJS, 
in press
\bibitem[1991]{per91}
Persson W., Wahlstr\"{o}m C.G., J\"{o}nsson L., Di Rocco H.O., 1991,
Phys. Rev. A 43, 4791
\bibitem[1983]{pet82}
Petersson J.E., 1982, Phys. Scr. 26, 296
\bibitem[1983]{pet83}
Petersson J.E., 1983, Phys. Scr. 28, 421
\bibitem[1990]{pet90}
Pettini M., Hunstead R.W., 1990, Aust. J. Phys. 43, 227
\bibitem[1992]{raa92}
Raassen A.J.J., Snoek L.C., Volten H., et al., 1992, A\&AS 95, 223
\bibitem[1992]{rei92}
Reimers D., Vogel S., Hagen H.-J., et al., 1992, Nat 360, 561
\bibitem[1985]{sea85}
Seaton M.J., 1985, J. Phys. B 18, 2111
\bibitem[1992]{sea92}
Seaton M.J., Zeippen C.J., Tully J.A., et al., 1992, Rev. Mex. Astron. 
Astrofis. 23, 19
\bibitem[1985]{smi85}
Smitt R., Ekberg J.O., 1985, Phys. Scr. 31, 391
\bibitem[1985]{sug85}
Sugar J., Corliss C., 1985, J. Phys. Chem. Ref. Data 14, Suppl. 2
\bibitem[1992]{sug92}
Sugar J., Kaufman V., Rowan W.L., 1992, J. Opt. Soc. Am. B 9, 344
\bibitem[1993]{sug93}
Sugar J., Kaufman V., Rowan W.L., 1993, J. Opt. Soc. Am. B 10, 13
\bibitem[1994]{ver94}
Verner D.A., Tytler D., Barthel P.D., 1994, ApJ 431 (in press)
\bibitem[1966]{wie66}
Wiese W.L., Smith M.W., Glennon B.M, 1966, Atomic Transition Probabilities,
v. I, Hydrogen through Neon, NSRDS-NBS 4
\bibitem[1969]{wie69}
Wiese W.L., Smith M.W., Miles B.M, 1969, Atomic Transition Probabilities,
v. II, Sodium through Calcium, NSRDS-NBS 22
\end{thebibliography}

\clearpage
\begin{center}
\bf{Figure captions}
\end{center}

\vspace{0.5cm}
{\bf Fig. 1.} The Opacity Project (OP) oscillator strengths, $f$, against
the oscillator strengths from Morton (1991), $f'$. Triangles:
12 lines of S{\sc i} (Morton's source: unpublished semi-empirical
data by Kurucz). Circles: Ca{\sc i} lines with different classifications
in the OP and Morton (1991).

\vspace{0.5cm}
{\bf Fig. 2.} The Opacity Project (OP) oscillator strengths, $f$, against
the oscillator strengths from other compilations, $f'$, for the lines not
covered by Morton (1991). All lines but three have wavelengths below
912~{\AA}. Dots: Fuhr \& Wiese (1991), Fuhr et al. (1988),
Martin et al. (1988). Triangles: Wiese et al. (1966, 1969). Cross:
Be{\sc i} 1491.765~{\AA} line (bad OP $f$-value).

\vspace{0.5cm}
{\bf Fig. 3a-d.} Relative strengths of the ground level metal absorption 
lines from 228 to 8000~{\AA} against log wavelength: {\bf a}  228--550~{\AA}, 
{\bf b} 550--1349~{\AA}, {\bf c} 1349--3311~{\AA},  {\bf d} 3311--8000~{\AA}.
The strength of the
strongest metal line, O{\sc v} 629.730~{\AA}, is set to 1.0. Hydrogen and
helium lines are not shown, ionization is neglected (we assume that all
atoms of an element are in the ion which produces the line that we show), 
and we assume solar abundances.
The depths of all lines are equal to $10^{P-11.44}$, where $P$ is the
expected width (strength) parameter from Eq. (2).
Note that the C{\sc i} line at 841{\AA} does not have an experimental
wavelength.\\
\\

\end{document}





