Differential electron emission from polycyclic aromatic hydrocarbon molecules under fast ion impact

www.nature.com/scientificreports OPEN Received: 31 January 2017 Accepted: 24 May 2017 Published: xx xx xxxx Differential electron emission from polycyclic aromatic hydrocarbon molecules under fast ion impact Shubhadeep Biswas1, Christophe Champion2, P. F. Weck3 & Lokesh C. Tribedi1 Interaction between polycyclic aromatic hydrocarbon (PAH) molecule and energetic ion is a subject of interest in different areas of modern physics. Here, we present measurements of energy and angular distributions of absolute double differential electron emission cross section for coronene (C24H12) and fluorene (C13H10) molecules under fast bare oxygen ion impact. For coronene, the angular distributions of the low energy electrons are quite different from that of simpler targets like Ne or CH4, which is not the case for fluorene. The behaviour of the higher electron energy distributions for both the targets are similar to that for simple targets. In case of coronene, a clear signature of plasmon resonance is observed in the analysis of forward-backward angular asymmetry of low energy electron emission. For fluorene, such signature is not identified probably due to lower oscillator strength of plasmon compared to the coronene. The theoretical calculation based on the first-order Born approximation with correct boundary conditions (CB1), in general, reproduced the experimental observations qualitatively, for both the molecules, except in the low energy region for coronene, which again indicates the role of collective excitation. Single differential and total cross sections are also deduced. An overall comparative study is presented. Understanding of few-body collision dynamics has been a subject of intense research in past few decades. The first step towards this was the study of fast charge particle impact ionization of simple atoms and molecules. Nowadays, the extent is being extended to the studies of much more complex molecules of specific interests. For example, biologically relevant complex molecules impacted by energetic ions were investigated extensively from the perspective of hadron therapy1–4. The other problem, that has attracted a great deal of attention in recent times, is the understanding of several unidentified astrophysical features in the realm of microscopic details of the molecules which are present in the interstellar medium (ISM)5. In variety of widely contrasted interstellar (IS) environments a set of ubiquitous, unidentified infrared bands (UIR) are present in the emission spectra. Besides, the origin of features like diffuse interstellar bands (DIBs) in the absorption spectra in optical/near IR wavelength range, or the prominent 217.5 nm bump in the interstellar extinction curve are yet to be understood convincingly5–8. As far as present understanding, the evolution of different polycyclic aromatic hydrocarbon (PAH) molecules is highly plausible candidate to answer all the open questions regarding these unresolved features5–10. PAHs are assembly of benzenoid rings of sp2-hybridized C atoms. The peripheral atoms are attached to requisite number of H atoms. The remaining delocalized electrons together form a π-electron cloud over the C skeleton. This gives an extra stability to these molecules to their survival in the harsh IS environment. Generally, PAHs in ISM are formed in the outflows from C-rich giant stars. These can also be formed from the fragmentation of C dust particles in shocked regions and from photosputtering in diffuse IS clouds. Destruction of larger PAH complex in harsh IS environment is also an important production procedure for smaller PAHs. This continuous processing of PAHs is believed to be one of the important aspects of the evolution of the ISM, which is certainly related to the understanding of galaxy formation9,10. For example, the question of heating and cooling of the IS gases automatically came into picture since the first determination of temperature of the ISM by 21 cm emission/ absorption line measurements. In this respect, C allotropes play an important role because these are the main suppliers of free electrons in the IS clouds, thus contributing to the heating of ISM11. The principal mechanism of the 1 Tata Institute of Fundamental Research, Department of Nuclear and atomic Physics, Homi Bhabha Road, Colaba, Mumbai, 400 005, India. 2Université Bordeaux 1, CNRS/IN2P3 Centre d’Études Nucléaires de Bordeaux Gradignan (CENBG) Chemin du Solarium, BP120, 33175, Gradignan, France. 3Sandia National Laboratories, Albuquerque, New Mexico, 87185, USA. Correspondence and requests for materials should be addressed to L.C.T. (email: lokesh@tifr. res.in) SCIeNTIFIC ReporTs | 7: 5560 | DOI:10.1038/s41598-017-05149-8 1 www.nature.com/scientificreports/ Figure 1. Structure of coronene and fluorene molecules. The dark shaded atoms at the centre part of the molecules are C atoms and the peripheral light shaded atoms are H atoms. heating process is the removal of electron (ionization) from the allotropes by transferring energy from external energetic particles (cosmic rays) to the bound electrons. Then the suprathermal electrons produced in this way heat the other IS gases and eventually thermalize the whole system through elastic collisions. Here the radiative transition plays a minimal role12. Thus, in general, the study of electron emission mechanism of PAH molecules is an important issue which can shed light on our knowledge of different important astrophysical scenarios. In the IS environment, cosmic ray (CR) is also an important component. It consists of protons, α-particles, heavier ions and electrons of energies spanning from few eV to hundreds of GeV. The sources of these energetic particles are different ISM shocks and supernova ruminants. The lower energy part of the spectrum, typically upto few hundreds of MeV, cannot be traced directly even with far ranging spacecraft, because these particles are excluded from the heliosphere or severely slowed down by the solar wind. But it is the most important part of the spectrum, as the intensity distribution sharply falls off with energy, and therefore the high energy (few GeV) cosmic rays have only marginal effect compared to those of few MeV12. Thus, it is extremely important to understand how the IS components are processed by these energetic ionic species, and this can only be possible through laboratory studies5. Particularly, it is instructive to study the interaction of MeV energy ions with PAH molecules as it can provide information about how the PAHs are processed by the low-energy cosmic ray in the ISM, and thus it helps to provide crucial inputs to various astrophysical models13. Other than the astrophysical interest, the researchers are also quite fascinated about these molecules because of their collective electronic behaviour. It was predicted earlier that these molecules are capable of showing low energy (around 17 eV) plasmon like collective excitation following some external per (...truncated)