Penning Ionization of Acene Molecules by Helium Nanodroplets

Abstract

Acene molecules (anthracene, tetracene, pentacene) and fullerene (C) are embedded in He nanodroplets (He) and probed by EUV synchrotron radiation. When resonantly exciting the He nanodroplets, the embedded molecules M are efficiently ionized by the Penning reaction He + M → He + M + e. However, the Penning electron spectra are all broad and structureless, largely differing from those measured by binary Penning collisions, as well as from those measured for dopants bound to the He droplet surface. Simulations based on elastic binary electron-He collisions qualitatively reproduce the measured spectra only when assuming unexpectedly large He droplets, indicating that electron spectra of molecules embedded in helium nanodroplets are severely affected by collective electron-helium interactions.

Photoelectron spectroscopy of coronene molecules embedded in helium nanodroplets

“…To test whether elastic scattering of the photoelectrons upon He atoms on their way out of the He droplets could account for the redistribution of peak intensity from peaks A-C to E-G we have also carried out a classical 3-D scattering simulation based on the differential electron-He scattering cross sections. A detailed description of the simulation can be found in our previous work [25]. We find that the result of the simulation (blue curve) resembles the PES of solid Cor and to some extent our measured PES in He droplets.…”

“…The energy resolution of the electron spectra obtained in this way is ∆E/E = 6%. The formation of Cor n ions at hν = 26 eV is due to charge transfer ionization through the ionized He nanodroplet [32], whereas at hν = 21.6 eV, Cor is ionized by Penning ionization as previously reported for other atomic and molecular dopants [23][24][25][26][27]. At hν = 21.6 eV, He nanodroplets are resonantly excited to their 1s2p 1 P state, and Penning ionization occurs after relaxation to the 1s2s 1,3 S state [23,26,33].…”

“…Cor is particularly well-suited for this purpose owing to its high photostability and its large absorption cross section of about 500 Mbarn at hν = 18.5 eV [22]. The resulting electron spectra are compared to Penning ionization electron spectra, which are more intense owing to the large resonantabsorption cross section of He droplets and a high Penning ionization efficiency [23][24][25][26][27]. We find that the Hedroplet PES resemble those previously measured for solid Cor, indicating that mostly Cor clusters are probed in the He droplets.…”

Direct inner-shell photoionization of Xe atoms embedded in helium nanodroplets

“…Using photoelectron-photoion coincidence (PEPICO) detection, we have previously measured high Penning ionization yields for alkali metals, whereas the efficiency of Penning ionization for heavier rare gas atoms was lower than that for charge transfer ionization [21]. The Penning ionization electron spectra were found to feature either sharp lines reflecting the He energy levels and the dopants' E i [15,21], a broad distribution peaked at low energies [22], or a combination of both [20,26].…”

Electron transfer mediated decay of alkali dimers attached to He nanodroplets

“…Helium (He) has a simple electronic structure and the highest ionization energy amongst all elements. Therefore, pure or doped He nanodroplets offer a unique medium where interatomic/molecular decays can be studied [14,15,[20][21][22][23][24][25][26][27][28][29]. Owing to its chemical inertness and low temperature, attached dopant molecules are only weakly perturbed and tend to aggregate into weakly bound complexes inside the droplet or at the droplet surface.…”