Abundances of PAHs in the ISM: Confronting Observations with
  Experimental Results
  We present recent UV laboratory spectra of various polycyclic aromatic
hydrocarbons (PAHs) and explore the potential of these molecules as carriers of
the DIBs. From a detailed comparison of gas-phase and Ne-matrix absorption
spectra of anthracene, phenanthrene, pyrene, 2,3-benzofluorene,
benzo[ghi]perylene, and hexabenzocoronene with new interstellar spectra, we
infer upper limits in the abundance of these PAHs in the interstellar medium.
Upper limits in the column densities of anthracene of $0.8 - 2.8 \times
10^{12}$\ cm$^{-2}$ and of pyrene and 2,3-benzofluorene ranging from $2 - 8
\times 10^{12}$\ cm$^{-2}$ are inferred. Upper limits in the column densities
of benzo[ghi]perylene are $0.9 - 2.4 \times 10^{13}$ and $10^{14}$ cm$^{-2}$
for phenanthrene. The measurements indicate fractional abundances of
anthracene, pyrene, and 2,3-benzofluorene of a few times $10^{-10}$. Upper
limits in the fractional abundance of benzo[ghi]perylene of a few times
$10^{-9}$ and of phenanthrene of few times $10^{-8}$ are inferred. {Toward CPD
$-32^\circ 1734$, we found near 3584 {\AA} an absorption line of OH$^+$, which
was discovered in the interstellar medium only very recently.
  The fractional abundances of PAHs inferred here are up to two orders of
magnitude lower than estimated total PAH abundances in the interstellar medium.
This indicates that either neutral PAHs are not abundant in translucent
molecular clouds, or that a PAH population with a large variety of molecules is
present.
