Utilizing tunable vacuum ultraviolet light and resonance enhanced multiphoton
ionization for isomer specific detection of polycyclic aromatic hydrocarbons fromastrophysical ice analogues
M. J. Abplanalp,1 R. Frigge,1 and R. I. Kaiser1
1Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI, USA

More than 200 molecules have been detected in the interstellar medium (ISM), but nopolycyclic aromatic hydrocarbons (PAHs)–organic molecules consisting of fused aromaticrings–have been observed spectroscopically in the gas phase of the ISM. PAHs have been
proposed to be the parent for up to 20 % of the interstellar carbon budget though, and havebeen linked to the astrobiological evolution of the ISM as well as possible nucleation sites forcarbonaceous interstellar grains. Although many attempts have been made to study complex
ice mixtures there is still a lack of understanding of the chemical complexity from individual
ice constituents such as acetylene. Acetylene has been detected in the ISM, but not within ice
mantles. However, the gas-phase abundance of acetylene was only able to be accounted for bymodels when sublimation from icy grain mantles was included. Furthermore, the processingof methane ice, a known ISM ice constituent, results in the formation of acetylene as aproduct. Also, acetylene ice has been firmly identified as an ice on the surface of Titan. We
will discuss the results of processing pure acetylene ices at 5 K with energetic electrons,
simulating secondary electrons formed via penetration of galactic cosmic rays through ISM
ices. The ices were monitored in-situ via Fourier transform infrared spectroscopy (FTIR) andduring heating via mass spectrometry utilizing a quadrupole mass spectrometer with anelectron impact ionization source as well as with a reflectron time-of-flight mass spectrometercoupled to a tunable photoionization source (PI-ReTOF-MS) used to do single photon
ionization (SPI) as well as resonance enhanced multiphoton ionization experiments (REMPI)
allowing for a comprehensive analysis of the products formed in the acetylene ice. The FTIR
analysis detected several small hydrocarbon products while the extremely sensitive SPIReTOF-MS analysis identified products with general formulae: CnH2n+2, CnH2n, CnH2n-2,CnH2n-4, CnH2n-6, CnH2n-8, CnH2n-10, CnH2n-12, CnH2n-14, and CnH2n-16. These SPI studies at 10.49
eV revealed ion signals that might belong to aromatic molecules like benzene (C6H6; m/z =78), and PAHs like naphthalene (C10H8; m/z = 128) as well as phenanthrene or anthracene(C14H10; m/z = 178) subliming from the irradiated ice. However, to confidently determine
which isomers these signals belonged to SPI cannot be used as not enough information isknown about the other isomers’ ionization energies, and instead REMPI was exploited. These
REMPI experiments were able to confirm several astrochemically interesting molecules andPAHs. With the proof that tunable photoionization is an extremely useful and important tool
for furthering our understanding of astrochemistry a new instrument coupled to a synchrotron
is currently being assembled (Hefei-Shanghai-Hawaii Center for Astrochemistry).

Figure 1: PI-ReTOF-MS data reporting the temperature dependent mass spectra from irradiated acetylene
ice at a photoionization energy of 10.49 eV.