Experimental and Theoretical Study on Electron Ionization and Fragmentation of Propylene Oxide─the First Chiral Molecule Detected in the Interstellar Medium.

Propylene oxide, CH3CHOCH2, is the first chiral molecule detected in space and the third C3 oxide detected toward the Sagittarius B2 (Sgr B2 (N)) molecular cloud, the others being propanal, CH3CH2CHO, and acetone, (CH3)2CO. With homochirality being ubiquitous in the building blocks of living matter, the formation and decay paths of propylene oxide in space are of specific interest. Motivated by the significant role of photo- and secondary electrons in astrochemistry, we have studied electron ionization and fragmentation of propylene oxide. Ion appearance energies are determined and compared to threshold values for the respective processes calculated at the G4MP2 level of theory, and potential reaction pathways are computed at the DFT level of theory. Electron ionization is found to destabilize propylene oxide, leading to barrierless opening of the C1-C2 bond of the epoxy ring, hydrogen transfer, and fragmentation over the methyl vinyl ether or rupture of the C2-O bond of the epoxy ring and fragmentation of the allyl alcohol cation as an intermediate, rather than direct bond ruptures.

Low-energy electron interaction with 2-(trifluoromethyl)acrylic acid, a potential component for EUVL resist material.

Motivated by the current introduction of extreme ultraviolet lithography (EUVL) into chip manufacturing processes, and thus the transition to electron-induced chemistry within the respective resist materials, we have studied low energy electron-induced fragmentation of 2-(trifluoromethyl)acrylic acid (TFMAA). This compound is chosen as a potential resist component, whereby fluorination enhances the EUV adsorption and may at the same time promote electron-induced dissociation. Dissociative ionization and dissociative electron attachment are studied, and to aid the interpretation of the observed fragmentation channels, the respective threshold values are calculated at the DFT and coupled cluster level of theory. Not surprisingly, we find significantly more extensive fragmentation in DI than in DEA and in fact, the only significant DEA fragmentation channel is the cleavage of HF from the parent molecule upon electron attachment. Rearrangement and new bond formation are substantial in DI and are, in fact, similar to DEA, mainly associated with HF formation. The observed fragmentation reactions are discussed in relation to the underlying reactions and potential implications for the suitability of TFMAA as a component of EUVL resist materials.

Dissociative ionization and electron beam induced deposition of tetrakis(dimethylamino)silane, a precursor for silicon nitride deposition.

Motivated by the use of tetrakis(dimethylamino)silane (TKDMAS) to produce silicon nitride-based deposits and its potential as a precursor for Focused Electron Beam Induced Deposition (FEBID), we have studied its reactivity towards low energy electrons in the gas phase and the composition of its deposits created by FEBID. While no negative ion formation was observed through dissociative electron attachment (DEA), significant fragmentation was observed in dissociative ionization (DI). Appearance energies (AEs) of fragments formed in DI were measured and are compared to the respective threshold energies calculated at the DFT and coupled cluster (CC) levels of theory. The average carbon and nitrogen loss per DI incident is calculated and compared to its deposit composition in FEBID. We find that hydrogen transfer reactions and new bond formations play a significant role in the DI of TKDMAS. Surprisingly, a significantly lower nitrogen content is observed in the deposits than is to be expected from the DI experiments. Furthermore, a post treatment protocol using water vapour during electron exposure was developed to remove the unwanted carbon content of FEBIDs created from TKDMAS. For comparison, these were also applied to FEBID deposits formed with tetraethyl orthosilicate (TEOS). In contrast, effective carbon removal was achieved in post treatment of TKDMAS, while his approach only marginally affected the composition of deposits made with TEOS.

HF Formation through Dissociative Electron Attachment-A Combined Experimental and Theoretical Study on Pentafluorothiophenol and 2-Fluorothiophenol.

In chemoradiation therapy, dissociative electron attachment (DEA) may play an important role with respect to the efficiency of the radiosensitizers used. The rational tailoring of such radiosensitizers to be more susceptive to DEA may thus offer a path to increase their efficiency. Potentially, this may be achieved by tailoring rearrangement reactions into the DEA process such that these may proceed at low incident electron energies, where DEA is most effective. Favorably altering the orbital structure of the respective molecules through substitution is another path that may be taken to promote dissociation up on electron capture. Here we present a combined experimental and theoretical study on DEA in relation to pentafluorothiophenol (PFTP) and 2-fluorothiophenol (2-FTP). We investigate the thermochemistry and dynamics of neutral HF formation through DEA as means to lower the threshold for dissociation up on electron capture to these compounds, and we explore the influence of perfluorination on their orbital structure. Fragment ion yield curves are presented, and the thermochemical thresholds for the respective DEA processes are computed as well as the minimum energy paths for HF formation up on electron capture and the underlying orbital structure of the respective molecular anions. We show that perfluorination of the aromatic ring in these compounds plays an important role in enabling HF formation by further lowering the threshold for this process and through favorable influence on the orbital structure, such that DEA is promoted. We argue that this approach may offer a path for tailoring new and efficient radiosensitizers.

Electron-induced fragmentation mechanisms in organic monomers and their implications for photoresist optimization for EUV lithography.

Secondary electrons generated during the Extreme Ultraviolet Lithography (EUVL) process are predominantly responsible for inducing important patterning chemistry in photoresist films. Therefore, it is crucial to understand the electron-induced fragmentation mechanisms involved in EUV-resist systems to improve their patterning performance. To facilitate this understanding, mechanistic studies were carried out on simple organic EUV-resist monomers, methyl isobutyrate (MIB) and methacrylic acid (MAA), both in the condensed and gas phases. Electron-stimulated desorption (ESD) studies on MIB in the condensed phase showed desorption peaks at around 2 and 9 eV electron energies. The gas-phase study on MIB showed that the monomer followed the dissociative ionization (DI) fragmentation pathway, under single collision conditions, which opened up at electron energies above about 11 eV. No signs of dissociative electron attachment (DEA) were detected for MIB in the gas phase under single collision conditions. However, DEA was an active process in MAA in the gas phase under single collision conditions at around 2 eV, showing that slight modifications of the molecular structures of photoresists may serve to sensitize them to certain electron-induced processes.

Electron-Transfer-Induced Side-Chain Cleavage in Tryptophan Facilitated through Potassium-Induced Transition-State Stabilization in the Gas Phase.

Fragmentation of transient negative ions of tryptophan molecules formed through electron transfer in collisions with potassium atoms is presented for the first time in the laboratory collision energy range of 20 up to 100 eV. In the unimolecular decomposition process, the dominating side-chain fragmentation channel is assigned to the dehydrogenated indoline anion, in contrast to dissociative electron attachment of free low-energy electrons to tryptophan. The role of the collision complex formed by the potassium cation and tryptophan negative ion in the electron transfer process is significant for the mechanisms that operate at lower collision energies. At those collision times, on the order of a few tens of fs, the collision complex may not only influence the lifetime of the anion but also stabilize specific transition states and thus alter the fragmentation patterns considerably. DFT calculations, at the BHandHLYP/6-311++G(3df,2pd) level of theory, are used to explore potential reaction pathways and the evolvement of the charge distribution along those.

The Role of Low-Energy Electron Interactions in cis-Pt(CO)2Br2 Fragmentation.

Platinum coordination complexes have found wide applications as chemotherapeutic anticancer drugs in synchronous combination with radiation (chemoradiation) as well as precursors in focused electron beam induced deposition (FEBID) for nano-scale fabrication. In both applications, low-energy electrons (LEE) play an important role with regard to the fragmentation pathways. In the former case, the high-energy radiation applied creates an abundance of reactive photo- and secondary electrons that determine the reaction paths of the respective radiation sensitizers. In the latter case, low-energy secondary electrons determine the deposition chemistry. In this contribution, we present a combined experimental and theoretical study on the role of LEE interactions in the fragmentation of the Pt(II) coordination compound cis-PtBr2(CO)2. We discuss our results in conjunction with the widely used cancer therapeutic Pt(II) coordination compound cis-Pt(NH3)2Cl2 (cisplatin) and the carbonyl analog Pt(CO)2Cl2, and we show that efficient CO loss through dissociative electron attachment dominates the reactivity of these carbonyl complexes with low-energy electrons, while halogen loss through DEA dominates the reactivity of cis-Pt(NH3)2Cl2.

Dissociation of the FEBID precursor cis-Pt(CO)2Cl2 driven by low-energy electrons.

In this study, we present experimental and theoretical results on dissociative electron attachment and dissociative ionisation for the potential FEBID precursor cis-Pt(CO)2Cl2. UHV surface studies have shown that high purity platinum deposits can be obtained from cis-Pt(CO)2Cl2. The efficiency and energetics of ligand removal through these processes are discussed and experimental appearance energies are compared to calculated thermochemical thresholds. The present results demonstrate the potential effectiveness of electron-induced reactions in the deposition of this FEBID precursor, and these are discussed in conjunction with surface science studies on this precursor and the design of new FEBID precursors.

The role of the dihedral angle and excited cation states in ionization and dissociation of mono-halogenated biphenyls; a combined experimental and theoretical coupled cluster study.

We present a combined theoretical and experimental study on the ionization and primary fragmentation channels of the mono-halogenated biphenyls; 2-chlorobiphenyl, 2-bromobiphenyl and 2-iodobiphenyl. The ionization energies (IEs) of the 2-halobiphenyls and the appearance energies (AEs) of the principal fragments are determined through electron impact ionization, while quantum mechanical calculations at the coupled cluster level of theory are used to elucidate the observed processes and the associated dynamics. The primary fragmentation channels are the direct loss of the halogen upon ionization, the loss of the respective hydrogen halides (HX) as well as loss of the hydrogen halide and an additional hydrogen. We find that the dihedral angle strongly influences the relative potential energy of the neutral and the cation on their respective ground state surfaces, an effect caused by the strong influence of the nuclear motion on the conjugation between the phenyl rings. For the principal dissociative ionization channels from the mono-halogenated biphenyls we reason that these can not be described as statistical decay from the ground state cation, but must rather be understood as direct, state-selective processes from specific excited cationic states characterized through local ionization of either the halogenated or the non-substituted phenyl ring.

Electron induced surface reactions of (η5-C5H5)Fe(CO)2Mn(CO)5, a potential heterobimetallic precursor for focused electron beam induced deposition (FEBID).

Electron-induced surface reactions of (η5-C5H5)Fe(CO)2Mn(CO)5 were explored in situ under ultra-high vacuum conditions using X-ray photoelectron spectroscopy and mass spectrometry. The initial step involves electron-stimulated decomposition of adsorbed (η5-C5H5)Fe(CO)2Mn(CO)5 molecules, accompanied by the desorption of an average of five CO ligands. A comparison with recent gas phase studies suggests that this precursor decomposition step occurs by a dissociative ionization (DI) process. Further electron irradiation decomposes the residual CO groups and (η5-C5H5, Cp) ligand, in the absence of any ligand desorption. The decomposition of CO ligands leads to Mn oxidation, while electron stimulated Cp decomposition causes all of the associated carbon atoms to be retained in the deposit. The lack of any Fe oxidation is ascribed to either the presence of a protective carbonaceous matrix around the Fe atoms created by the decomposition of the Cp ligand, or to desorption of both CO ligands bound to Fe in the initial decomposition step. The selective oxidation of Mn in the absence of any Fe oxidation suggests that the fate of metal atoms in mixed-metal precursors for focused electron beam induced deposition (FEBID) will be sensitive to the nature and number of ligands in the immediate coordination sphere. In related studies, the composition of deposits created from (η5-C5H5)Fe(CO)2Mn(CO)5 under steady state deposition conditions, representative of those used to create nanostructures in electron microscopes, were measured and found to be qualitatively consistent with predictions from the UHV surface science studies.

Low energy electron-induced decomposition of (η5-Cp)Fe(CO)2Mn(CO)5, a potential bimetallic precursor for focused electron beam induced deposition of alloy structures.

The production of alloyed nanostructures presents a unique problem in focused electron beam induced deposition (FEBID). Deposition of such structures has historically involved the mixing of two or more precursor gases in situ or via multiple channel gas injection systems, thereby making the production of precise, reproducible alloy compositions difficult. Promising recent efforts to address this problem have involved the use of multi-centred, heterometallic FEBID precursor species. In this vein, we present here a study of low-energy electron interactions with cyclopentadienyl iron dicarbonyl manganese pentacarbonyl ((η5-Cp)Fe(CO)2Mn(CO)5), a bimetallic species with a polyhapto ligand (Cp) and seven terminal carbonyl ligands. Gas phase studies and coupled cluster calculations of observed low-energy electron-induced reactions were conducted in order to predict the performance of this precursor in FEBID. In dissociative electron attachment, we find single CO loss and cleavage of the Fe-Mn bond, leading to the formation of [Mn(CO)5]-, to be the two dominant channels. Contributions through further CO loss from the intact core and the formation of [Mn(CO)4]- are minor channels. In dissociative ionization (DI), the fragmentation is significantly more extensive and the DI spectra are dominated by fragments formed through the loss of 5 and 6 CO ligands, and fragments formed through cleavage of the Fe-Mn bond accompanied by substantial CO loss. The gas phase fragmentation channels observed are discussed in relation to the underlying processes and their energetics, and in context to related surface studies and the likely performance of this precursor in FEBID.

Low energy electron-induced decomposition of (η3-C3H5)Ru(CO)3Br, a potential focused electron beam induced deposition precursor with a heteroleptic ligand set.

Here we describe in detail low energy electron induced fragmentation of a potential focused electron beam induced deposition (FEBID) precursor, π-allyl ruthenium tricarbonyl bromide, i.e. (η3-C3H5)Ru(CO)3Br, specially designed to allow comparison of the effect of different ligands on the efficiency of low energy electron induced fragmentation of FEBID precursors. Specifically, we discuss the efficiency of dissociative electron attachment (DEA) and dissociative ionization (DI) with respect to electron-induced removal of the allyl, bromide and carbonyl ligands. We place this in perspective with a previous surface study on the same precursor and we propose a design strategy for FEBID precursor molecules to increase their susceptibility towards DEA.

Proton Shuttling and Reaction Paths in Dissociative Electron Attachment to o- and p-Tetrafluorohydroquinone, an Experimental and Theoretical Study.

Here we present a combined experimental and theoretical study on the fragmentation of o- and p-tetrafluorohydroquinone upon low energy electron attachment. Despite an identical ring-skeleton and identical functional groups in these constitutional isomers, they show distinctly different fragmentation patterns, a phenomenon that cannot be explained by distinct resonances or different thermochemistry. Using high-level quantum chemical calculations with the computationally affordable domain localized pair natural orbital approach, DLPNO-CCSD(T), we are able to provide a complete and accurate description of the respective reaction dynamics, revealing proton shuttling and transition states for competing channels as the explanation for the different behavior of these isomers. The results represent a "schoolbook example" of how the combination of experiment and modern high-level theory may today provide a thorough understanding of complex reaction dynamics by computationally affordable means.

State selectivity and dynamics in dissociative electron attachment to CF3I revealed through velocity slice imaging.

In light of its substantially more environmentally friendly nature, CF3I is currently being considered as a replacement for the highly potent global-warming gas CF4, which is used extensively in plasma processing. In this context, we have studied the electron-driven dissociation of CF3I to form CF3(-) and I, and we compare this process to the corresponding photolysis channel. By using the velocity slice imaging (VSI) technique we can visualize the complete dynamics of this process and show that electron-driven dissociation proceeds from the same initial parent state as the corresponding photolysis process. However, in contrast to photolysis, which leads nearly exclusively to the (2)P(1/2) excited state of iodine, electron-induced dissociation leads predominantly to the (2)P(3/2) ground state. We believe that the changed spin state of the negative ion allows an adiabatic dissociation through a conical intersection, whereas this path is efficiently repressed by a required spin flip in the photolysis process.

Quantum superposition of target and product states in reactive electron scattering from CF4 revealed through velocity slice imaging.

Exploiting the technique of velocity slice imaging, we have performed a detailed study of reactive electron scattering with CF4. We have measured the electron impact energy dependence of both the angular and kinetic energy distributions of the channels yielding F- and CF3(-) anions. These data provide an unprecedented insight into the quantum superposition of the target state and product channels, respectively, of Td and C3v symmetry, and shed new light on the dissociation dynamics.

Stabilization, fragmentation and rearrangement reactions in low-energy electron interaction with tetrafluoro-para-benzoquinone: a combined theoretical and experimental study.

Quinones are a class of organic compounds whose extraordinary electron transfer properties are fundamental in ubiquitous processes such as the ATP production and the photosynthesis. Here, we report a combined theoretical and experimental study to shed some light on low-energy electron interaction with tetrafluoro-para-benzoquinone (TFQ) and para-benzoquinone (p-BQ). Similar to it's native counterpart; p-BQ, TFQ forms a metastable molecular anion at unusually high incident energies in electron attachment under single collision conditions. Transition state calculations are used for both p-BQ and TFQ to explore possible stabilization of the molecular anion through isomerization reactions, and stabilization through intramolecular vibrational energy redistribution (IVR) is discussed in relation to the electronic structure of these compounds. We also report exceptionally extensive and complex rearrangement reactions in dissociative electron attachment to TFQ and discuss possible reaction paths based on thermochemical threshold calculations. The observed fragmentation reactions can at large be described by two dissociation series, i.e., the formation of [TFQ - CO -nF](-) (n = 0, 1, 2 or 3) and [TFQ - 2CO -nF](-) (n = 0, 1, 2, 3 or 4). In the latter series we observe, at incident electron energy as low as ~0.5 eV, the excision of two CO molecules followed by recombination of the remaining moiety to form an anionic carbon chain with two terminal CF2 groups. To our knowledge such excision of two non-adjacent carbon atoms and recombination of the remaining moiety have not been observed before in low energy electron attachment.

Molecular rearrangement reactions in the gas phase triggered by electron attachment.

Bond formation and rearrangement reactions in gas phase electron attachment were studied through dissociative electron attachment (DEA) to pentafluorotoluene (PFT), pentafluoroaniline (PFA) and pentafluorophenol (PFP) in the energy range 0-14 eV. In the case of PFA and PFP, the dominant processes involve formation of [M - HF](-) through the loss of neutral HF. This fragmentation channel is most efficient at low incident electron energy and for PFP it is accompanied by a substantial conformational change of the anionic fragment. At higher energy, HF loss is also observed as well as a number of other fragmentation processes. Thermochemical threshold energies have been computed for all the observed fragments and classical trajectories of the electron attachment process were calculated to elucidate the fragmentation mechanisms. For the dominant reaction channel leading to the loss of HF from PFP, the minimum energy path was calculated using the nudged elastic band method.

Dissociative electron attachment to hexafluoroacetylacetone and its bidentate metal complexes M(hfac)2; M = Cu, Pd.

Beta-diketones are a versatile class of compounds that can complex almost any metal in the periodic table of elements. Their metal complexes are found to be fairly stable and generally have sufficient vapor pressure for deposition techniques requiring volatile metal sources. Motivated by the potential role of low energy electrons in focused electron beam induced deposition, we have carried out a crossed electron∕molecular beam study on the dissociative electron attachment and non-dissociative electron attachment (NDEA) to hexafluoroacetylacetone (HFAc) and its bidentate metal complexes: bis-hexafluoroacetylacetonate copper(II), Cu(hfac)2 and bis-hexafluoroacetylacetonate palladium(II), Pd(hfac)2. The relative ion yield curves for the native precursor to the ligand as well as its stable, 16 valence electron Pd(II) complex and open shell, 17 valence electron Cu(II) complex, are presented and compared. For HFAc, the loss of HF leads to the dominant anion observed, and while NDEA is only weakly pronounced for Pd(hfac)2 and loss of hfac(-) is the main dissociation channel, [Cu(hfac)2](-) formation from Cu(hfac)2 dominates. A comparison of the ion yield curves and the associated resonances gives insight into the role of the ligand in the attachment process and highlights the influence of the central metal atom.

Absolute cross sections for dissociative electron attachment and dissociative ionization of cobalt tricarbonyl nitrosyl in the energy range from 0 eV to 140 eV.

We report absolute dissociative electron attachment (DEA) and dissociative ionization (DI) cross sections for electron scattering from the focused electron beam induced deposition (FEBID) precursor Co(CO)(3)NO in the incident electron energy range from 0 to 140 eV. We find that DEA leads mainly to single carbonyl loss with a maximum cross section of 4.1 × 10(-16) cm(2), while fragmentation through DI results mainly in the formation of the bare metal cation Co(+) with a maximum cross section close to 4.6 × 10(-16) cm(2) at 70 eV. Though DEA proceeds in a narrow incident electron energy range, this energy range is found to overlap significantly with the expected energy distribution of secondary electrons (SEs) produced in FEBID. The DI process, on the other hand, is operative over a much wider energy range, but the overlap with the expected SE energy distribution, though significant, is found to be mainly in the threshold region of the individual DI processes.

A combined gas-phase dissociative ionization, dissociative electron attachment and deposition study on the potential FEBID precursor [Au(CH3)2Cl]2.

Motivated by the potential of focused-electron-beam-induced deposition (FEBID) in the fabrication of functional gold nanostructures for application in plasmonic and detector technology, we conducted a comprehensive study on [Au(CH3)2Cl]2 as a potential precursor for such depositions. Fundamental electron-induced dissociation processes were studied under single collision conditions, and the composition and morphology of FEBID deposits fabricated in an ultrahigh-vacuum (UHV) chamber were explored on different surfaces and at varied beam currents. In the gas phase, dissociative ionization was found to lead to significant carbon loss from this precursor, and about 50% of the chlorine was on average removed per dissociative ionization incident. On the other hand, in dissociative electron attachment, no chlorine was removed from the parent molecule. Contrary to these observations, FEBID in the UHV setup was found to yield a quantitative loss and desorption of the chlorine from the deposits, an effect that we attribute to electron-induced secondary and tertiary reactions in the deposition process. We find this precursor to be stable at ambient conditions and to have sufficient vapor pressure to be suitable for use in HV instruments. More importantly, in the UHV setup, FEBID with [Au(CH3)2Cl]2 yielded deposits with high gold content, ranging from 45 to 61 atom % depending on the beam current and on the cleanliness of the substrates surface.