Excitation of L-valine molecules by electrons and photons

Eur. Phys. J. D (2022)76:9 https://doi.org/10.1140/epjd/s10053-021-00331-0 THE EUROPEAN PHYSICAL JOURNAL D Regular Article Atomic and Molecular Collisions Excitation of L-valine molecules by electrons and photons Yu. A. Bandurin1,a , A. N. Zavilopulo1,b , Sh. Molnar2 , and O. O. Shpenik2 1 2 Institute of Electron Physics, National Academy of Sciences of Ukraine, Uzhhorod 88017, Ukraine Ukrainian-Hungarian Educational-Scientific Institute of Uzhhorod National University, Uzhhorod 88000, Ukraine Received 30 August 2021 / Accepted 15 December 2021 © The Author(s), under exclusive licence to EDP Sciences, SIF and Springer-Verlag GmbH Germany, part of Springer Nature 2022 Abstract. Excitation of L-valine molecules was studied by optical spectroscopy. Optical emission spectra of the L-valine molecule and optical excitation functions of molecular bands and the Hβ spectral line were measured in the gas phase using electron impact. In the spectra of optical emission in the wavelength range of 250–500 nm, intense emission bands were found at energies of incident electrons of 30, 50 and 70 eV. Analysis of structural features of the valine molecule suggested a fragmentation scheme with the formation of excited particles in collisions with electrons. A notable feature of the presented optical excitation functions is a different growth dynamics with an increase in the energy of exciting electrons and the presence of a number of features and kinks, which are especially pronounced for λ = 305 nm and λ = 311 nm. The excitation thresholds were determined from the initial sections of the excitation functions of the most intense spectral lines by the least-squares method. The photoluminescence spectra of L-valine were measured for the first time on a Shimadzu RF-6000 spectrofluorophotometer in the spectral range of 400–800 nm for excitation wavelengths of 250, 275, 333, 351, and 380 nm. 1 Introduction Excitation of atoms, ions and molecules by electron impact is a key elementary process that determines the basic properties of matter in the gas and plasma phases. It is the process of particle excitation in collisions with electrons that determine the features of photon emission in the form of molecular bands and spectral lines. Excitation of molecules into repulsive states leads to dissociation, which determines the concentration of various atomic particles. In addition, collisions of electrons with molecules are accompanied by vibrational and rotational excitations of molecular energy levels. Similar processes occur when molecules are excited by photons, and photodissociation is the main accompanying process. The energies of photons of visible and ultraviolet radiation are sufficient both to break a chemical bond and to change the structure of an excited molecule. In this case, the processes of molecular excitation are accompanied by transitions of one or more electrons to higher electronic states, followed by emission in a wide range of wavelengths from infrared to ultraviolet. Investigation of the processes of excitation by electron and photon impact can be carried out by different methods, the most effective one is the optical method. a b Processes occurring in polyatomic organic molecules at their interaction with slow electrons are of considerable interest not only for quantum theory of scattering and theory of electronic structure of molecules but also for solving a number of applied problems [1–3]. One of the most important elementary processes occurring in molecules when they interact with slow electrons is the process of excitation, which leads to the emission of spectral bands by both the whole molecules and their fragments. The study of molecules in the gas phase makes it possible to exclude solvation effects that can affect conformational stability of molecules and redistribution of vibrational excitations in real biological systems. Among the elementary processes occurring at the interaction of low-energy electrons with molecules, the processes of elastic scattering, excitation and ionization are the most effective [4, 5]. Studies of various processes of interaction of lowenergy (slow) electrons with molecules that are part of DNA and RNA are of particular interest. These processes are a subject of intensive research [6–10]. It should be noted that secondary electron emission, which is the result of the primary interaction of electrons with a molecule, plays an important role in the mechanism of radiation damage to DNA. The destructive role of these secondary electrons is the further ionization of the peptide components of DNA, initiating an avalanche effect, leading to damage to the genome. e-mail: bandurin˙ e-mail: (corresponding author) 0123456789().: V,-vol 123 9 Page 2 of 13 Therefore, first of all, attention is paid to the elucidation of mechanisms of processes that damage the structure of these molecules, which lead to the death of individual cells and the organism as a whole. The problem with the basic components of DNA and RNA often requires solutions, since an accurate understanding of the processes occurring at the molecular level is considered an important step towards describing more complex phenomena occurring at the cellular level. According to the prevailing concepts [11], it is slow electrons that are related to the main part of destructive changes at the molecular level of biostructures, with the main target being genetic macromolecules [12]. The latter is important, in particular, for studying mechanisms of mutations in biological objects during a viral pandemic. The relevance of studying these processes by various methods, especially in the case of biomolecules, has acquired particular importance at the present time in relation to the COVID-19 pandemic [13, 14]. The range of low energies of the exciting electrons includes characteristic values of the binding energies of atoms in organic molecules, excitation energies, and ionization potentials of atoms and molecules themselves, which makes it possible to study the elementary processes of excitation, ionization, and dissociation. Consequently, information about processes at atomic and molecular levels can be considered the first stage in the study of the general mechanisms of functioning of a living cell. An applied aspect of this problem is the possibility of a directed change in the parameters of such biostructures, which will lead to the creation of new bioorganic materials. Application of monoenergetic electron beams with scanning of their energy to excite complex molecules in the gas phase enables one to obtain information on the position and structure of energy levels, as well as to estimate the relative probability of their excitation. Investigation of emission of photons and energy dependences of the excitation of electronic-vibrational states provides information on the efficiency of conversion of internal energy of complex molecules into the light. Despite a definite importance of studying the main (...truncated)