Formation of H 3 O + in the Ionization and Fragmentation of Ethanol Induced by Electron Beam Irradiation

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Introduction
Particle beam and laser-induced ionization and fragmentation of molecules have attracted considerable interest for several decades . Hydrogen or proton migration is a widely existing phenomenon during these processes, and it plays a vital role in various fields of physics, chemistry, and biology [1,2,7,19,20]. In particular, for small organic molecules [3][4][5] or molecular complexes [6][7][8], the isomerization process induced by intra-and intermolecular hydrogen transfer has been extensively studied due to its high relevance, e.g., in biological processes such as in DNA mutations [9,10]. The ultrafast hydrogen migration can unlock new dissociation channels, such as hydrogen exchange [11], scrambling [12], and roaming [13,14]. Recent studies showed that it can also stabilize the repulsive potential energy surface of dications before the direct Coulomb explosion occurs [15].
The ethanol molecule (C 2 H 5 OH) contains both hydroxyl and methyl groups and is widely applied in the chemical industry as an important solvent. C 2 H 5 OH has attracted significant interest for studying hydrogen migration due to the relatively large number of hydrogen atoms bound to the various sites in the C-C-O skeleton. Here, of particular interest is the double hydrogen migration [23][24][25][26], forming a H 3 O + hydronium ion, which is central in acid-catalyzed reactions as the active protonating agent. Nevertheless, the understanding of the double H migration is incomplete since it is a rather complex process involving more than one C-H bond cleavage and at least two O-H bond formations. H 3 O + formation has been studied by Raalte and Harrison in an electron impact ionization experiment with deuterated ethanol. Two reaction pathways were proposed, i.e., a concerted path and a sequential fragmentation path. For the concerted path, the two hydrogen migration directly follows single ionization of ethanol, while the sequential path proceeds via the intermediate C 2 H 4 OH + cation where initially hydrogen is abstracted and hydrogen migration occurs subsequently [23]. Later on, Niwa et al. performed an experiment on ethanol using the photoelectron-photoion coincidence (PEPICO) technique. By analyzing the breakdown diagrams and appearance energies (AE) for the different fragment ions, they concluded that H 3 O + was mainly formed by the sequential fragmentation [24]. Further studies using the time of flight (TOF) spectra and the quantum chemical calculations confirmed that the sequential process is the dominant pathway for the formation of H 3 O + in ethanol [25,26].
In this work, we study the ionization and dissociation of ethanol irradiated by low-energy electrons using an (e, 2e + ion) method [27][28][29][30] in which all three final-state particles are detected in coincidence. We use the projectile energy of 90 eV, which is close to the mean energy of secondary electrons produced by high-energy primary radiation, such as X-rays, α-rays, ?-rays, fast electron, and ion beams [31]. Here, the momentum vectors and, consequently, kinetic energies of the outgoing electrons (scattered and ejected electrons) and fragment ions are determined. The contributions of different ionized orbitals for formations of C 2 H 5 OH + , C 2 H 4 OH + , COH + , and H 3 O + cations are obtained by measuring the energy deposition in the ionization process, i.e., the binding energy (BE) spectra. The BE is defined as the energy difference between the initial projectile electron energy E 0 and the energy sum of the scattered (E 1 ) and ejected (E 2 ) electrons: BE = E 0 − ðE 1 + E 2 Þ. The BE resolution of ΔE BE = 3:7 eV, full width of half maximum (FWHM), has been achieved with a measurement on the ionization of helium. For ethanol, there are two conformers assigned as the trans and the gauche structures where the main difference is the dihedral angle between the hydroxyl and the carbon skeleton. Here, we consider the trans conformer with C s symmetry as the geometric structure of ethanol [32]. The ground-state electronic structure of the valence shell can be expressed as (4a ′ ) 2 (5a′) 2 (6a′) 2 (7a′) 2 (1a″) 2 (8a′) 2 (9a′) 2 (2a″) 2 (10a′) 2 (3a ″ ) 2 .

Experiment
The experiment was performed using the multiparticle coincident momentum spectrometer (reaction microscope) combined with a pulsed electron beam [33]. The details of the experimental setup can be found in the earlier work [27,34]; thus, only a brief introduction is given here. A wellfocused (about 1 mm in diameter) electron beam with an energy of 90 eV is crossed with an ethanol gas jet. The projectile electron beam is emitted from an electron gun in which a tantalum photocathode is irradiated by a pulsed ultraviolet laser beam. The wavelength, repetition rate, and pulse width of the laser beam are 266 nm, 40 kHz, and 0.5 ns, respectively. The energy per pulse of the laser is about 1~2 nJ, and the flux of the projectile electron is about 5 nA/cm 2 . The data accumulation time is about 100 hours for the present work. The gas jet is generated by the supersonic gas expansion of He (1 bar) with seeded ethanol vapor through a nozzle with a diameter of 30 μm and a two-stage differential pumping sys-tem. The ethanol gas was produced through the reservoir including the liquid ethanol at room temperature. The supersonic beam was collimated by two sequential skimmers (250 μm and 400 ?m diameter, respectively) and transmitted into the reaction area of the main chamber. After traversing the jet, the nonscattered electron beam is guided into the central hole of the electron detector as a beam dump. The charged particles in the final state (two electrons and one ion) are extracted and guided by homogeneous electric and magnetic fields towards two-dimensional position-and time-sensitive microchannel plate detectors. Three-dimensional momentum vectors of the detected particles are determined from the time-of-flight and positions of the particles hitting the detectors. The ionization process can be expressed as follows:

Quantum Chemistry Calculations
The theoretical calculations are performed utilizing the Gaussian quantum chemistry package [35]. The groundstate equilibrium geometries of the singly charged ethanol molecule, the transition states (TSs), and intermediates (INTs) are optimized by the M06-2X method with the def2TZVP basis set. The zero-point energy (ZPE) correction is acquired by the M06-2X method with the def2TZVP basis set, and the electronic energy was calculated by employing the coupled-cluster single-double and perturbative triple (CCSD(T)) method with the aug-cc-pVQZ basis set. The validity of reaction pathways is confirmed by the NBO population and the intrinsic reaction coordinate (IRC) analysis which is carried out at the M06-2X/def2TZVP level.

Results and Discussion
The BE spectra for the formation of the C 2 H 5 OH + parent ion and C 2 H 4 OH + with H-loss are presented in Figure 1. The experimental data for C 2 H 5 OH + correspond to a single peak located at about 10.8 eV, which is consistent with the ionization energy of the highest occupied molecular orbital (HOMO) of ethanol [32]. This result indicates that the C 2 H 5 OH + parent ion is formed through the HOMO ionization, i.e., the 3a ″ orbital of the trans conformer [32]. For the H-loss channel, i.e., the C 2 H 4 OH + cation, the measured BE spectrum shows a single peak located at about 12.6 eV which can be attributed to the ionization of the of HOMO-1 (10a ′ ) plus some amount of the internal energy (~0.5 eV), leading to the subsequent H-loss. Figure 2 presents the binding energy spectrum for the ionic fragment with a mass-to-charge (m/z) ratio of 29 u. Due to the m/z degeneracy for COH + and C 2 H 5 + , we are unable to distinguish them in our experiment. Hudson and McAdoo obtained the appearance energies (AE) of about 14.2 eV for COH + and 12.7 eV for C 2 H 5 + [24] by comparing the breakdown curves for C 2 H 5 OH + and C 2 D 5 OD + . The present experimental data show a peak structure centered at about 15.3 eV and a shoulder structure at higher BE, and thus, we assigned the ionic product with 29 u to COH + through the pathway of C-C bond breaking and two 2 Laser and Particle Beams hydrogen loss from the C α site [24]. The BE spectrum indicates that the ionization of several valence orbitals leads to the formation of COH + . The main peak located at 15.3 eV corresponds to ionization of 10a ′ , 2a ″ , 9a ′ , 8a ′ , 1a ″ , and 7a ′ orbitals which cannot be resolved energetically. Addi-tionally, ionization of inner-valence orbitals (6a′, 5a′, and 4a ′ ) can also contribute to the formation of COH + . We determine the branching ratios of about 55% and 45% for outervalance and inner-valence ionization, respectively. It is also to be noted that the measured peaks at higher BE, e.g., 35.8 eV, may result also from the ionization plus excitation or from multiple scattering within ethanol clusters [36].
Concerning the production pathway of COH + , Hudson and McAdoo theoretically demonstrated a sequential process involving the H migration. First, a neutral H is ejected, and in the second step, the hydrogen transfers from hydroxyl to the methyl group, and then, the C α -C ? bond breaks with the ejection of neutral methane [26]. The reaction process is expressed as follows: It is considered that the CHO + production with H migration and the COH + formation without H migration are both involved in our experiment. The fragmentation channels induced by the ionization of inner-valence orbitals (6a ′ , 5a ′ , and 4a ′ ) may be concerned with the CHO + formation due to the higher deposited energy supporting for H migration. We notice that the formation of CHO + through hydrogen transfer from oxygen to carbon is different from the widely studied H migration process in ethanol where the migrated H is originated from carbon and moves to the oxygen side [15,[23][24][25].
The measured BE spectrum for the formation of H 3 O + is presented in Figure 3, which exhibits a single-peak structure centered at about 15.1 eV and a tail structure at higher BE. The peak at BE~15.1 eV can be attributed to the ionization of 10a ′ , 2a ″ , 9a ′ , 8a ′ , and 1a ″ orbitals, while the structures at higher BE are caused by the ionization of inner-valence orbitals (6a ′ , 5a ′ , and 4a ′ ) and also the ionization plus excitation or multiple scattering processes in clusters [36]. Previous studies by Niwa et al. [24] and Shirota et al. indicate that  Figure 1: The binding energy spectra related to the C 2 H 5 OH + and C 2 H 4 OH + cations. The black solid circles and the black solid triangles with error bars represent the experimental data of C 2 H 5 OH + and C 2 H 4 OH + , respectively. The data for both ions are normalized to unity at the maximum. The red and blue solid curves are Gaussian fits. At the top of the plot, red solid vertical lines indicate the ionization energies of valence orbitals [32].   3 Laser and Particle Beams the formation of H 3 O + is dominated by the sequential process via the intermediate CH 3 CHOH + cation. In the first step, one C α -H bond breaks and neutral hydrogen is ejected. After that, the fragmentation of CH 3 CHOH + produces H 3 O + . The reaction process is expressed as follows: A theoretical work [26] demonstrated a reaction pathway where the two H which migrate to form H 3 O + originate from the methyl site and sequentially transfer to the hydroxyl site. On the other hand, recent experiments on deuterated ethanol showed a complete scrambling of the four hydrogens at the C-C site before migration to the hydroxyl site [25]. This is in agreement with an earlier study for deuterated CH 3 CD 2 OH which mainly shows yields of H 3 O + and H 2 DO + in accordance with that expected for a CH 3 CDOH + intermediate [23]. In addition, a small yield of HD 2 O + shows that also the direct concerted fragmentation pathway contributes where both H from the C α site migrate: To trace the complete reaction pathways of double H migration processes, we performed quantum chemical calculations for H 3 O + formation in both concerted and sequential ways. The potential energy diagrams are shown in Figure 4 which exhibits the reaction pathways with transition states (TS1 and TS2) and intermediate states (INT1 and INT2). In Figure 4, the energy values are relative to the ionic ground state of CH 3 CH 2 OH + , which is marked by 0.0 eV in Figure 4(a). The single point energy is calculated at the CCSD(T)/aug-cc-pVQZ level including the zero-point vibrational energy corrections. The reaction pathways are confirmed by the intrinsic reaction coordinate (IRC) calculation following routes from transition state to related local minima in two directions. For the concerted pathway in Figure 4( For the sequential pathway in Figure 4(b), the calculated potential energies are consistent with the previous studies [25,26]. The H-loss channel occurs with the C α -H bond cleavage in the first step that forms the CH 3 CHOH + cation (0.4 eV). After that, the double H migration occurs subsequently from methyl to hydroxyl sites through the transition states of TS1 (3.28 eV) and TS2 (2.65 eV), and then, the system dissociates into a H 3 O + cation and a C 2 H 2 neutral fragment. Our calculation indicates that for the sequential The reaction pathway has been confirmed by the intrinsic reaction coordinate (IRC) calculation. The black, blue, and red balls represent carbon, hydrogen, and oxygen atoms, respectively. 4 Laser and Particle Beams process, higher internal energy is needed to overcome the potential barrier (TS1: 3.28 eV) in comparison with the concerted pathway (TS1: 1.05 eV). From this point of view, the concerted pathway is more accessible. However, fast H-loss channel is expected due to its lower energy barrier (0.40 eV). This can cause the observed dominance of the sequential process [23]. Nevertheless, future studies in both experiment and theory are required to unambiguously identify the concerted pathway.

Conclusions
We have presented results from the first (e, 2e + ion) study of the ionization-induced dissociation of ethanol induced by low-energy electron impact (90 eV). A multiparticle coincidence momentum spectrometer is used in which all three charged final-state particles, i.e., two outgoing electrons and one fragment ion, are detected in triple coincidence. The momenta and, consequently, the kinetic energies of all three final-state particles are obtained through the measurement of their TOFs and hit positions. We determine the binding energy (BE) spectra correlated with different ionic fragments, i.e., the parent ion C 2 H 5 OH + , the H-loss channel C 2 H 4 OH + , and the hydrogen migration channels of COH + and H 3 O + cations.
The present results confirm that the nondissociated C 2 H 5 OH + ion product arises due to the HOMO (3a″) orbital ionization, which is consistent with the results of previous appearance energy studies. The BE spectrum for the H-loss channel reveals a single-peak (12.6 eV) structure which can be assigned to the HOMO-1 (10a′) orbital ionization. While for COH + product, both the outer-valence (10a′, 2a″, 9a′, 8a′, 1a″, and 7a′) and inner-valence (6a′, 5a′, and 4a′) orbital ionizations are observed from the measured BE spectrum. The BE spectrum for H 3 O + product shows the similar feature in which the outer-valence (3a″, 10a′, 2a″, 9a′, 8a′, 1a″, and 7a′) and inner-valence (6a′, 5a′, and 4a′) orbital ionizations are involved. For the formation of H 3 O + through double H migration, two fragmentation pathways (concerted and sequential processes) are identified using quantum chemistry calculations. The higher BE peaks can also result from the ionization plus excitation or from multiple scattering of the projectile in ethanol clusters. The present study can have implications for a better understanding of the ionization-induced isomerization mechanisms of molecules.

Data Availability
The experimental data used to support the findings of this study are available from the corresponding author upon request.