Abstract:
Sequential formation of a poly-cyclic aromatic hydrocarbon (PAH) dication in the H I regions of the interstellar medium (ISM) is proposed
to be a function of internal energy of the doubly ionized PAHs, which, in turn, is dependent on the single- and double-ionization
potentials of the system. This sets a limit on the single- and double-ionization energies of the system(s) that can further undergo sequential
absorption of two photons, leading to a dication (PAH+2). Here, we report the single-ionization (I+1) and double-ionization (I+2) energies
and the I+2/I+1 ratio for some selected PAHs and conjugated polyenes obtained using the Fock space coupled cluster technique, enabling
simultaneous consideration of several electronic states of different characters. The I+2 to I+1 ratio bears a constant ratio, giving allowance
to determine I+2 from the knowledge of single-ionization (I+1) and vice versa. Our observations are in good agreement with the established
literature findings, confirming the reliability of our estimates. The measured single- and double-ionization energies further demonstrate
that the sequential formation and fragmentation of a PAH dication in the H I regions of the ISM for systems such as benzene and conjugated
polyenes such as ethylene and butadiene are quite unlikely because I+2–I+1 for such system(s) is higher than the available photon
energy in the H I regions of the ISM. Present findings may be useful to understand the formation and underlying decay mechanisms of
multiply charged ions from PAHs and related compounds that may accentuate the exploration of the phenomenon of high-temperature
superconductivity.
