Interactions of protons with furan molecules studied by collision-induced emission spectroscopy at the incident energy range of 50–1000 eV

Eur. Phys. J. D (2016) 70: 175 DOI: 10.1140/epjd/e2016-70308-1 THE EUROPEAN PHYSICAL JOURNAL D Regular Article Interactions of protons with furan molecules studied by collision-induced emission spectroscopy at the incident energy range of 50–1000 eV Tomasz J. Wasowicz1,a and Boguslaw Pranszke2,b 1 Department of Physics of Electronic Phenomena, Gdansk University of Technology, ul. G. Narutowicza 11/12, 80-233 Gdansk, Poland 2 Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 59, 80-952 Gdansk, Poland Received 6 May 2016 / Received in final form 5 July 2016 Published online 30 August 2016 c The Author(s) 2016. This article is published with open access at Springerlink.com  Abstract. Investigations of the ion-molecule reactions provide insight into many fields ranging from the stellar wind interaction with interstellar media, up to medicine and industrial applications. Besides the applications, the understanding of these processes is itself a problem of fundamental importance. Thus, interactions of protons with the gas-phase furan molecules have been investigated for the first time in the energy range of 50–1000 eV exploiting collision-induced emission spectroscopy. Recorded spectra reveal emission of the atomic Hβ to Hθ lines of the hydrogen Balmer series and the molecular bands of vibrationally and rotationally excited diatomic CH fragments created in the A2 Δ and B2 Σ − electronic states. The measurements of the emission yields of the excited fragments by recording their intensities at different projectile energies have been performed. The highest yields have been observed for production of hydrogen atoms which intensities rapidly decreased with increasing principal quantum number n. From the H (n = 4–7) intensity ratios depopulation factors of hydrogen excited states have been determined at each impact energy and possible collisional mechanisms leading to enhanced production of the hydrogen atoms have been suggested. We compare and discuss our results with improved data set of proton collisions with tetrahydrofuran (THF) molecules, the hydrogenated derivatives of furan. 1 Introduction It is well-known that apart from photons, stars emit constant stream of charged particles, mainly protons, electrons, H+ 2 and He ions, a few percent of nuclei with Z < 10 and very few heavier nuclei [1,2] that can attack nearby medium, i.e planetary and cometary atmospheres, dust grains, meteorites etc. In this viewpoint, studies on resistance and stability of the prebiotic building blocks developed in catalytic reactions on the surfaces of interstellar dust grains against this destructive bombardment are highly required, because they may give knowledge how these species can be formed and preserved in astrophysically relevant environments before being delivered to the terrestrial planets, thus making reliable their contribution to the origin and evolution of life on Earth [3,4]. Interactions of protons with biomolecular systems are also of  Contribution to the Topical Issue “Low-Energy Interactions related to Atmospheric and Extreme Conditions”, edited by S. Ptasinska, M. Smialek-Telega, A. Milosavljevic, B. Sivaraman. a e-mail: b Present address: Gdynia Maritime University, ul. Morska 81-87, 81-225 Gdynia, Poland great importance in the studies of radiation damage of the living cells, because, on the one hand, the proton beams are effectively used in hadrontherapy [5], which facilitates H+ projectiles to cure cancer, and on the other hand, they are considered for planning future crewed space missions to assess the risk of the exposure of astronauts to the solar wind ejected by the Sun [1]. Furthermore, an increasing number of studies have been devoted to reveal the properties of the ion beam processing and fabrication [6,7]. Furan (C4 H4 O) is one of the most fundamental fivemembered heterocyclic aromatic compounds that provides a structural unit of various organic and biologically active substances. For instance, its five-membered ring system consisting of four carbon atoms and one oxygen atom (see Fig. 1) may be regarded to be a building unit in the vitamin B12, biotin [8], conducting polymers [9] and the simple sugars ribose and deoxyribose (dR), the backbone molecules of the RNA and the DNA helix. Moreover, furan and its derivatives play an important role in combustion chemistry, as second-generation biofuels [10,11] or in food and nutrition engineering as a product of thermal degradation of a heat-treated commercial foods [12,13]. Taking into account the relevance of furan molecules in many different fields of astrochemistry, biology and Page 2 of 9 (a) Eur. Phys. J. D (2016) 70: 175 (b) Fig. 1. (a) Furan (C4 H4 O) and (b) tetrahydrofuran (C4 H8 O) molecules. The colour code: carbon atom is grey, oxygen atom is red, and hydrogen atom is white. industry, they seem to be ideal candidates to characterize the mechanisms of ion-molecule collisions, particularly in the context presented above. Apart from ours conference communication on He+ -induced fragmentation of furan molecules [14], to the best of our knowledge, no experimental or theoretical data were reported previously on cationic interactions with either gas- or condensed-phase furan molecules. The present work concentrates on the analysis of the collisional processes occurring in the gasphase furan molecules under protons impact. These mechanisms have been studied using collision-induced emission spectroscopy that allowed identification of the collision products by detecting their emission. Thus, the atomic Hβ to Hθ lines of the hydrogen Balmer series and molecular bands of vibrationally and rotationally excited diatomic CH fragments created in the A2 Δ and B2 Σ − electronic states have been recognized in the recorded spectra. The energetic dependences of the intensities of the excited fragments i.e. the emission yields have been recorded. For comparison, the emission yields in the H+ + tetrahydrofuran collisions have been also remeasured in the 20–1000 eV energy range, but with smaller energy step size than in our previous studies [15]. For both impact systems the highest yields have been observed for production of the excited H atoms which intensities rapidly decrease with increasing principal quantum number n. Moreover, the intensity ratios of the H (n = 4) emissions and the A2 Δ → X2 Πr bands of the CH show that the H (n = 4) are more abundant fragments in the entire studied energy range. These observations give clear indication that collision processes are dominated by an electron transfer from the target molecules to projectiles leading to enhanced production of the hydrogen atoms arising mostly from neutralization of protons and excited to lower lying states. Depopulation studies of higher excited states of hydrogen also support this picture. 2 Experiment The experiment was carried out at the University of Gdansk exploiting the collision-induced emission spectroscopy developed in Göttinge (...truncated)