  Embedded atoms or molecules in a photoexcited He nanodroplet are well-known
to be ionized through inter-atomic relaxation in a Penning process. In this
work, we investigate the Penning ionization of acetylene oligomers occurring
from the photoexcitation bands of He nanodroplets. In close analogy to
conventional Penning electron spectroscopy by thermal atomic collisions, the n
= 2 photoexcitation band plays the role of the metastable atomic $1s2s$
$^{3,1}S$ He$^\ast$. This facilitates electron spectroscopy of acetylene
aggregates in the sub-kelvin He environment, providing the following insight
into their structure: The molecules in the dopant cluster are loosely bound van
der Waals complexes rather than forming covalent compounds. In addition, this
work reveals a Penning process stemming from the n = 4 band where
charge-transfer from autoionized He in the droplets is known to be the dominant
relaxation channel. This allows for excited states of the remnant dopant
oligomer Penning-ions to be studied. Hence, we demonstrate Penning ionization
electron spectroscopy of doped droplets as an effective technique for
investigating dopant oligomers which are easily formed by attachment to the
host cluster.
