Photodissociation of HeH⁺by both electronic and vibrational transitions

Cosmological production of H2 before the formation of the first galaxies

“…The cross‐section for photodissociation from the v = 0 level of HeH + is always σ c < 1.5 × 10 −12 cm 3 s −1 (Roberge & Dalgarno 1982). The cross‐section from the higher excited states v = 7 and 8 is σ c ∼ 3 × 10 −9 cm 3 s −1 near threshold (Saha, Datta & Barua 1978) and could, in principle, be important, except that the fraction of HeH + in these states is ≪ 1 (they lie 1.56 and 1.62 eV above the ground state, as compared with k B T CMB = 0.07 eV at z = 300). Hence the HeH + levels are in LTE at the CMB temperature T CMB and HeH + can be followed as an ‘effective one‐level’ molecular ion.…”

Crossed Beam Photodissociation Imaging ofHeH+with Vacuum Ultraviolet Free-Electron Laser Pulses

“…Electronically excited states of HeH (Fig. 1) are reached only by high energy input (20 eV) and have been addressed theoretically [6 -10], including their photodissociation [8][9][10], and experimentally in electron-impact [11,12] and charge-exchange [13] collision studies; however, their photonic excitation is experimentally largely unexplored. This similarly holds for most small cations since they are only accessible at very low target densities and their study therefore requires an intense source of energetic photons that was so far not available.…”

Photodissociation of the HeH+molecular ion

“…This difference is due to the fact that the electronic wavefunctions especially for the ground and the first excited states are more accurately 0.9 1 1.1 1.2 1.3 1.4 1.5 1. 6 Photon energy (a.u.) represented in [8] than in the present calculation, as is concluded from a comparison of the corresponding energies.…”

“…The helium-hydride ion HeH + is the simplest heteronuclear two-electron system and the only well bound molecular system of hydrogen and helium, the most abundant elements in the universe. This motivated many calculations and experiments performed on this molecular ion which is of interest for the chemistry of astrophysical objects, for the tritium neutrino mass experiments, or simply by itself as a model system (see, e. g., [1,2,3,4,5,6,7,8,9,10] and references therein). More recently, HeH + is drawing special attention due to an experiment performed at the free-electron laser FLASH [11] where the absolute photodissociation cross section at a photon energy of 38.7 eV was measured.…”