Microcarbonation of Naphthalene: An Experimental and Computational Study of Photoionization in Naphthalene-Carbon Dioxide Clusters

The photoionization of naphthalene (N)-carbon
dioxide (CO2) clusters was studied using tunable vacuum
ultraviolet (VUV) radiation from a synchrotron in the photon range
of 8.0 to 13.7 eV, in combination with time-of-flight mass spectrometry.
Clusters of monomer, dimer, and trimer naphthalene with CO2 (N(CO2)0–6, N2(CO2)0–3, N3) were observed. The lowest-energy conformers
were obtained via a conformer search, followed by geometry optimizations
at the ωB97X-V2/aug-cc-pVTZ (monomer) and ωB97X-V2/aug-cc-pVDZ
(dimer) levels of theory. Carbon dioxide was found to preferentially
cluster on top of the naphthalene molecule (in an out-of-plane configuration).
From the mass spectra, photoionization intensity curves (PICs) were
constructed, and appearance energies (AEs) were determined. No substantial
trend in AE was observed with increasing size of the naphthalene-carbon
dioxide clusters; rather, AE oscillations around the value for pure
naphthalene were observed. These AE oscillations are also observed
in a recently studied naphthalene-water cluster system, though in
this system a slight downward trend in AE for pure naphthalene clusters
(N1/2/3/4) was observed. The differences between the two systems are
attributed to differing interaction strengths. Understanding these
differences may aid in determining how photoprocessing can proceed
differently depending on the dominant matrix component in interstellar
ices.