Ionization and Fragmentation of DCOOD Induced by Synchrotron

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Ionization and Fragmentation of DCOOD induced by Synchrotron Radiation at the Oxygen 1s Edge: The Role of Dimer Formation Manuela Souza Arruda, Aline Medina, Josenilton Nascimento Sousa, Luiz Antonio Vieira Mendes, Ricardo R. T. Marinho, and Frederico Vasconcellos Prudente J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b01714 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on April 27, 2016

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The Journal of Physical Chemistry

Ionization and Fragmentation of DCOOD induced by Synchrotron Radiation at the Oxygen 1s Edge: The Role of Dimer Formation Manuela S. Arruda,∗,†,‡ Aline Medina,† Josenilton N. Sousa,† Luiz A. V. Mendes,† Ricardo R. T. Marinho,† and Frederico V. Prudente∗,† †Instituto de F´ısica, Universidade Federal da Bahia, 40170-115, Salvador, BA, Brazil ‡Centro de Ciências Exatas e Tecnol´ ogicas, Universidade Federal do Recˆ oncavo da Bahia, 44380-000, Cruz das Almas, BA, Brazil E-mail: [email protected]; [email protected]

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Abstract The ionization and photofragmentation of molecules in the core region has been widely investigated for monomers and dimers of organic molecules in the gas phase. In this study, we used synchrotron radiation to excite electrons of the oxygen K-edge in an effusive molecular beam of doubly deuterated formic acid. We used time-of-flight mass spectrometry and employed the spectroscopic techniques PEPICO and PEPIPICO to obtain spectra of single and double coincidences at different pressures. Our results indicate the presence of ions and ion pairs which have charge-to-mass ratio higher than the monomer DCOOD, as the (DCOOD)·D+ , and pairs (DCO+ , DCO+ ) and (CO+ , DCO+ ). Comparing the spectra obtained for different pressures we can ascertain that these ions are formed by the fragmentation of DCOOD dimers.

Introduction The investigation of ionization and molecular fragmentation processes, induced by radiation absorption or by collisions with charged particles, is important in various research areas, such as plasma chemistry, the chemistry of the interstellar medium, astrophysics, radio astronomy, science of comets, exobiology, and the physics and chemistry of atmospheres. The excitation of molecular core electrons is of special interest because it can produce a site specific fragmentation 1–5 or induce isomerization with a specific pathway. 6 On the other hand, the accurate description of multiple hydrogen bond in the dimers and small clusters of molecules has been a subject of extensive studies (see Refs. 7–9 and references therein), because, among others, it has fundamental importance in the formation of DNA base pairs. 10,11 In particular, several theoretical 12–16 and experimental 17–20 investigations of formic acid dimers and clusters have been carried out because they are the simplest examples of this type of interaction. In particular, Heinbuch et al. 21 presented a study of the single photon ionization of hydrogen bonded clusters of HCOOH employing time-of-flight mass spectroscopy and a soft X-ray laser with 26.5 eV. Their main product was a series 2 ACS Paragon Plus Environment

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of protonated ions, (HCOOH)n H+ , generated from a proton transfer reaction in the parent + ion (HCOOH)+ n+1 directly followed by the ionization of (HCOOH)n+1 . The fragmentation

processes of the (HCOOH)2 and (DCOOD)2 after double ionization induced by femtosecond laser irradiation at 800 nm was studied by Hoshina et al. 22 employing time-of-flight mass spectrometry and quantum chemical calculations. The protonated ion, (HCOOH)·H+ , produced via the dissociative ionization of (HCOOH)2 , was observed in the mass spectra. Tabayashi et al. presented a series of studies 23–26 on the inner-shell excitation and fragmentation of small clusters of organic molecules at the oxygen K-edge region by time-of-flight mass spectroscopy. They obtained total-ion yield spectra, single coincidence mass spectra and partial-ion-yield spectra of formic acid monomers and small clusters in order to investigate the changes in the fragmentation mechanisms upon H-bonded cluster formation. 23 These authors identified the production of (HCOOH)·H+ and H3 O+ cations characteristic of proton transfer reactions within the clusters. In previous studies of the photofragmentation of formic acid 27 and formamide 28 in the valence region, we observed the production of a protonated ion, with a mass-to-charge ratio one unit larger than the parent ion. In a recent work we investigated the protonation process of double deuterated formic acid (DCOOD) as a result of dimers photofragmentation in the valence region. 29 The results of that study indicates that deuteronated ions, (DCOOD)·D+ , are originated from dimer fragmentation, and we could ascertain the experimental appearance energy of the ion (DCOOD)·D+ from the ionization of (DCOOD)2 . In particular, we observed that these ions are detected at energies less than the first ionization potential of the monomer. Moreover, the results show that the production of (DCOOD)·D+ and DCO+ augments with the increase of pressure in the experimental chamber, therefore, with the increase of the dimers concentration. These results motivated the extension of the investigation of the origin of (DCOOD)·D+ and DCO+ ions observed at the DCOOD photofragmentation spectra in the core region in the oxygen K edge. In the present work, we perform the study of photoionization and



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photofragmentation of this molecule at the oxygen K edge, for various pressure values in the experimental chamber, by using the photoelectron-photoion coincidence (PEPICO) and the photoelectron photoion-photoion coincidence (PEPIPICO) techniques. The reason we measure the doubly deuterated formic acid is that, since deuterium is not abundant in nature. Thus, the existence of the deuteronated ion cannot be explained as due to H+ capture during the injection of the sample in the experimental chamber or due to the presence of the isotope

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C (see Ref. 29 for a detailed discussion). An effusive molecular beam of doubly

deuterated formic acid molecules was bombarded by photons with energy in the soft X-ray band, specifically from 530 to 580 eV, so that oxygen 1s orbital electrons are excited to more energetic orbitals or to the continuum, causing ionization or dissociative ionization of the molecule. This paper is organized as follows. The experimental apparatus, as well as corrections applied to PEPICO and PEPIPICO spectra, are shown in the Technical Details section. All spectra are shown in the Results section. Initially, we measured total ion yield spectra (TIY) as a function of photon energy for different pressure values. In this case, all detected ions are collected without discriminating their mass-to-charge ratio. We use these spectra to determine in which photon energies occur the the main resonances on the oxygen K edge. Subsequently, we obtain mass spectra of simple (PEPICO) and double (PEPIPICO) coincidence for these resonance photon energies and for energies below and above the oxygen 1s edge. Furthermore, we present the possible fragmentation routes of the main ion pairs detected in coincidence. We finish the article with our conclusions.

Technical Details The experiment was performed at SGM beamline 30 of the Brazilian Synchrotron Light Laboratory by using a time-of-flight mass spectrometer (TOF-MS). 31 This beamline has a monochromator with spherical diffraction gratings, that selects photons with energies be-


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tween 250 and 1000 eV and energy resolution of E/∆E = 1000, for 100 µm slits. The spectrometer allows to collect the photoelectron photoion coincidence (PEPICO) and photoelectron photoion photoion coincidence (PEPIPICO) techniques. 32,33 The PEPICO and PEPIPICO spectroscopy has been widely used in recent studies 27–29,34–38 to determine the fragmentation pathways of different molecular species that are single and doubly ionized. PEPICO and PEPIPICO spectra obtained in the core region contain spurious events from false coincidences. These false coincidences can have two distinct origins, one associated with the detection efficiency of electrons and ions 39 and the other associated with the resolution of the multi-pulse equipment. 40 We applied on the PEPICO spectra these two corrections, while on the PEPIPICO spectra we performed only the correction associated with the resolution of the multi-pulse equipment. Due to the low number of triple coincidences events was not necessary perform the correction associated with the detection efficiency. A baseline was subtracted from the PEPICO spectra to eliminate the background noise. In PEPIPICO spectra, double coincidence events with only one count were subtracted from the spectra. All data treatments - such as corrections due to false coincidence events, noise subtraction, calculation of peak areas in PEPICO and PEPIPICO spectra, error bars and slopes of the coincidence islands of PEPIPICO spectra - were made in FORTRAN programs especially developed by us for the processing of such data. The DCOOD sample was obtained from Glaser Lab-kemikalier laboratory with isotopic purity of 99.5% and of 90 to 95% in relation to contamination by D2 O, as indicated by the producer. The deuterated formic acid is liquid at ambient temperature and pressure. Measurements were taken for the sample in the gas phase and at ambient temperature. In order to study the influence of the dimers on the DCOOD photofragmentation, the measurements were carried out with different pressures in the experimental chamber. The sample was placed in a glass container for liquids, connected to a gas pumping system. This system reduces the pressure above the liquid surface and the DCOOD gas is driven by a hypodermic needle, through which the effusive beam is inserted in the extraction region of the experi-



mental chamber in order to interact with the synchrotron radiation. The unique purification procedure performed was freeze-pump-thaw cycle.

Results TIY spectra The total ion yield spectra of DCOOD, with the main electronic transitions identified, for two different pressure values in the experimental chamber, 2.4×10−6 and 9.1×10−6 mbar, are shown in Figure 1. These spectra were obtained for the photon energy range from 528.0 to 550.0 eV, step 0.1 eV, and acquisition time of 0.5 s per step. Both spectra were calibrated considering near edge X-ray absorption fine structure spectra (NEXAFS), obtained by Prince et al. 41 The ionization spectra are normalized so that the peaks relating to the fourth resonance, at both spectra, have the same intensity.

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Figure 1: Total ion yield spectrum of deuterated formic acid at the oxygen 1s edge at pressures 2.4×10−6 mbar (−, black) and 9.1×10−6 mbar (· · · , red). In Table 1, the energy values obtained for resonances observed in TIY spectrum for the two mentioned pressures are listed and compared with those obtained for the HCOOH by Tabayashi et al. 23 using effusive and supersonic beams, and from high-resolution spectra NEXAFS 41 and ISEELS. 42 In this Table and in Figure 1 are also presented the assignments, 6 ACS Paragon Plus Environment

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following Ref., 23 of each O1s transition to unoccupied molecular orbitals. At the electronic structure point of view, the transitions of O1s assigned in TIY spectra are not influenced from the substitution of hydrogen by deuterium. According to Table 1, in our TIY spectrum only four resonances can be observed in the oxygen edge. Besides the four peaks determined in our spectrum, between the second and third ones, Prince et al. 41 determined other three weak vibrationally resolved peaks and assigned them as the 4s Rydberg state, suggesting that they have pure Rydberg character, with little or no antibonding component. 41 In turn, two resonances were observed only in the ISEELS spectrum at energies 539.6, and 547 eV, one associated with the O1s(OH)→ 4s (highly excited Rydberg) transition and other associated with the O1s(CO)→ σ ∗ (CO) transition. In particular, there is an uncertainty on the assignment of peak 2. Tabayashi et al. 23 obtained two different resonance energies to O1s(OH)→ π ∗ (CO) and O1s(OH)→ 3s/σ ∗ (OH) transitions, while the others authors and us have identified only one resonance energy. Table 1: Energy and assignments of DCOOD at the O 1s edge. Values in parentheses indicate the energy difference relative to the first resonance. Peak 1 2 2

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this worka 532.2 535.5 535.5 538.7 542.6 -

Energy (eV) This workb TIY 23c TIY 23d NEXAFS 41 532.2 532.01 532.36 532.17 535.3 534.8 534.43 535.37 535.3 535.8 535.37 537.14 537.16 537.34 537.54 538.6 538.13 538.37 542.1 542.48 542.3±0.2 a 2.4×10−6 mbar. b 9.1×10−6 mbar. c Effusive.

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Assignment 23 ISEELS 42 532.1 O1s(CO)→ π ∗ (CO) 535.3 O1s(OH)→ π ∗ (CO) 535.3 O1s(OH)→ 3s/σ ∗ (OH) 537.6 O1s(CO)→4s 4s+ν3 4s+2ν3 538.3 O1s(OH)→ 3p 539.6 O1s(OH)→4s 542.1 O1s(OH)→ σ ∗ (C-OH) 547 O1s(CO)→ σ ∗ (CO) Cluster beam.

From the results shown in Figure 1 and Table 1, we can notice some differences in the TIY spectra obtained for different pressures at the experimental chamber. The first is related to the bandwidth of the peaks 1 and 2. For the pressure of 2.4×10−6 mbar we have found the values 1.5 and 1.9 eV to peaks 1 and 2, respectively, while FWHM values 1.6 and 2.0 eV were determined for the pressure of 9.1×10−6 mbar. A justification for this increase in the peak widths is the existence of additional excited states due to dimers that are formed 7 ACS Paragon Plus Environment


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in the gas at higher pressures. The second noticeable difference in our TIY spectra is the decrease of the energy difference between the first and second peaks when the pressure is increased in the experimental chamber. In our work, the second peak is located at 535.5±0.5 eV in the spectrum obtained with pressure 2.4×10−6 mbar. When the chamber pressure was increased to 9.1×10−6 mbar, we observed a decrease of 0.2 eV. This shift can be interpreted as being due to the contributions from H-bonded dimers existent in the higher pressure case. Another difference between the spectra for both pressures is the increased relative intensity in the continuous region (fourth resonance) for the higher pressure. Qualitatively we can note that the first two peaks are less intense in the spectrum obtained for the highest pressure, which means an increase in the relative intensity of the continuum region. This increase can be explained by the effective formation and fragmentation of multiple-charged clusters at higher energies. 23 We point out that these differences highlighted here, even if they can not be conclusive because they are within the accuracy of the measurement, were also observed by Tabayashi et al. in their TIY spectra using effusive and supersonic beams with energy resolution 0.07 eV. 23 Thus, we stress that our TIY spectrum obtained for higher pressure presents qualitative characteristics that are compatible with TIY spectra of clusters: 23 increasing peak width in the energy region lower than the O1s(CO) and O1s(OH) ionization potentials, smallest relative distance between the first and the second peak and an increase of the relative intensity in the continuum region. Similar characteristics and conclusions were found in the TIY spectra of monomers and dimers of other organic molecules obtained with high resolution, like the acetic acid 24 and acetadhyde 25 with energy resolutions E/∆E = 104 and E/∆E = 2000, respectively.


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PEPICO spectra In this section, we analize the photoelectron photoion coincidence results. We begin with the mass spectra obtained for 1.1×10−5 mbar pressure, Figure 2, where we can see the mass-tocharge ratio of detected ions and compare the peaks profiles for five different photon energies. These energies correspond to the four resonances in the K-edge of oxygen that were observed in this work and at an energy value below the first resonance.

Figure 2: DCOOD mass spectra for different photon energies, at pressures 1.1×10−5 mbar. We observe, for energies above and below the first resonance (532.2±0.5 eV), a peak broadening, that occurs markedly in ions with mass-to-charge ratio 14, 18 and 30. This broadening may be related to the initial kinetic energy distribution and to different orientations of the velocity vector of formed ions. Another cause of peaks broadening may be due to quasi-alignment effect of molecules. 43,44 Moreover, we also observed in mass spectra, ions with odd mass-to-charge ratio. These ions may result from the fragmentation of the non-deuterated formic acid (HCOOH) or be doubly charged ions. Due to the low yield, ions with odd mass-to-charge ratio were not analyzed. Since the intensity of the detected ions are associated with the areas of the peaks in the 9 ACS Paragon Plus Environment


mass spectrum, relative ion yield (RIY) spectra, or branching ratio (BR), were obtained for two different values of pressure, and presented in Figure 3. With these spectra we carried out a better comparison of the variation of intensities of the ions detected as a function of pressure and energy. Expressions for the branching ratio and its associated error are obtained in Ref. 45 Overall, we found that the intensity of production of ions does not change significantly with increasing energy, having a different behavior ion with mass-to-charge ratio 30 to the energy corresponding to the first resonance, in accordance to the observed in the mass spectra.

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Figure 3: Relative ion yield of DCOOD fragments listed in the legend as a function of the photon energy, at pressures 3.6×10−6 mbar [Panel (a)] and 1.1×10−5 mbar [Panel (b)]. We selected the RIY spectra of ions with mass-to-charge ratio 12 (C+ ), 14 (CD+ ), 16 (O+ ), 30 (DCO+ ), 46 (COOD+ ) and 50 ((DCOOD)·D+ ) in Figure 4, in order to obtain a more detailed analysis of their production as a function of photon energy and pressure. We observed that the production of atomic or small fragment ions has a small decrease (m/q = 12, 14 and 16) with increasing pressure. On the other hand the branching ratio of ions with m/q = 30, 46 and 50 increases with pressure. The maximum relative intensity of producing (DCOOD)·D+ increases from 0.32% to 0.95%, while the COOD+ increases from 2.97% to 3.42% and DCO+ increases from 26.84% to 30.04%, when the pressure varies from 3.6×10−6 to 1.1×10−5 mbar. Concerning to the ion with m/q = 30, the increase in


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the relative yield with the pressure is even more pronounced for energies above the second resonance. Specifically, we have shown in Ref. 29 that the appearance of the deuteronated formic acid ((DCOOD)·D+ ) and the increase of DCO+ yield are a direct consequence of the formation of formic acid dimers due to increased pressure. Tabayashi et al. 23 obtained similar results when compared mass spectra obtained with beams of HCOOH and HCOOD monomers and clusters. They observed a decrease in the relative production of small ions such as C+ , O+ , and CO+ and an increase the relative + + + production of cations CO2 H+ and COOD+ in clusters when 3 , CO2 HD2 , CHO , COOH

compared with the monomers. They concluded that the protonated and deuteronated cations originate in formic acid clusters, while the cations CHO+ e COOH+ (COOD+ ) are not always of cluster origin but show strong characteristics of fragments produced from molecules within the clusters through the suppression of further fragmentation due to the transfer of their excess energies to surrounding molecules, including the hydrogen bonding partner. 23 With respect to yield variation with energy, we see in Figure 4 a similar behavior for both pressures, for ions m/q = 30 and 50, with peak yield at the first resonance. Ion m/q = 46, also has a peak at the first resonance, but there is a more intense peak in the fourth resonance. Relative yields for the ions m/q = 12, 14 and 16 have a peak in the third resonance and a reasonable production for photon energies below of the ionization potentials. Finally, in mass spectra obtained in our work for higher pressure, we observed a small production ion D3 O+ (m/q=22), as can be observed in the spectrum associated to the first resonance in Figure 2. However, since the intensity of this peak was very low it was not possible to make an analysis of its production. Similarly the ions H3 O+ and HD2 O+ were observed by Tabayashi et al. 23 The authors indicate that such ions come from the fragmentation of formic acid clusters. According to them, these ion are evidence of the H (D) transfer reaction in the acid formic, with H3 O+ and HD2 O+ being formed via H2 O+ (HDO+ ) precursors of a migration of H (D) in the formic acid molecule. We emphasize that in the previous studies of HCOOH and DCOOD molecules in the valence region, 27,29 we also



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Figure 4: Relative production of ions with mass-to-charge ratios 12 [Panel (a)], 14 [Panel (b)], 16 [Panel (c)], 30 [Panel (d)], 46 [Panel (e)] and 50 [Panel (f)] as a function of the photon energy at pressures 3.6×10−6 mbar (−, black) and 1.1×10−5 mbar (· · · , red). verify the existence of the cations H2 O+ and D2 O+ , whose rise was also due to migration (recombination) of H or D in molecular fragmentation of formic acid. The analysis of the behavior of simple coincidence spectra on the O1s edge, generally confirm that the pressure influences the results obtained. We found that the pattern characteristics of DCOOD fragmentation to the stagnation pressure to 3.6×10−6 and 1.1×10−5 mbar are similar to the characteristics obtained by Tabayashi et al. 23 for the fragmentation of beams of free molecules and clusters, respectively.


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PEPIPICO spectra The PEPIPICO spectrum is a two-dimensional representation, where in the (t2 , t1 ) axes, the time-of-flight of the ions detected in coincidence is reported and a color scale indicates the number of times (intensity) that each coincidence event occurs. The shape of coincidence islands, formed in PEPIPICO spectra, as well as their slopes identify which fragment paths can be followed by the ion pairs. 46–48 In Figure 5, we presented the double coincidence spectrum of DCOOD for the photon energy of 0.7 eV above of the O1s(CO)→ π ∗ (CO) transition, where we can check the coincidence islands. Qualitatively we observed that the most intense and definite islands occur for ions with mass-to-charge ratio 2, 12, 16, 28 and 30 detected in coincidence with deuterium. We also noticed the coincidence between the ion pairs (30,28), (30,30) and, with lower counts, (46,30) and (44,30). The sum of the mass-to-charge ratio of these pairs of coincidence exceeds the mass of the monomer deuterated formic acid (48 u), indicating that these coincidences are necessarily from a dimer fragmentation. As in PEPICO spectra, the intensity of production of ion pairs is associated with areas of the islands formed in PEPIPICO spectra. The partial double coincidence yield (PDCY), or



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Figure 6: Partial double coincidence yield of DCOOD fragments listed in the legend as a function of the photon energy, at pressures 1.1×10−6 mbar [Panel (a)] and 1.1×10−5 mbar [Panel (b)]. branching ratio of double coincidence, is determined in a manner analogous to the percentage yield in simple coincidences by dividing the area of the island related with the ion pair to the sum of the areas of all islands. The PDCY for two values of pressure in the experimental chamber, around 1.1×10−6 mbar and 1.1×10−5 mbar, are displayed in Figure 6 to ions with production percentage higher then 1.5% of total yield. Overall, the intensities vary reasonably between the different photon energies. In particular, it can be noted that the pairs containing at least one more massive ion have a higher intensities to first and fourth resonances, while the pairs of lighter ions have higher intensities outside of these resonances. Quantitatively, the relative intensities of the double coincidences are shown in Tables 2 and 3 for the pressure in the experimental chamber around 1.1×10−6 mbar and 1.1×10−5 mbar, respectively. In these Tables we list the PDCY and the experimental slope of coincidence islands with higher production percentages than 0.5% of total yield. These quantities were obtained for photon energies corresponding to O1s(CO)→ π ∗ (CO) and O1s(OH)→ σ ∗ (C-OH) transitions and the photon energy 3.5 eV below and 50.0 eV above the first resonance. Additional information not included in these tables, because of their volume, were used for building the spectra in this section. Regarding the fractional yield of ions detected in coincidence, we find that, in general, the main detected ion pairs are the same at a given energy. The (D+ , O+ ) is the most detected 14 ACS Paragon Plus Environment

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Table 2: Relative yield of the ion pairs and experimental slopes of coincidence islands of DCOOD for different energies and at pressure 1.1×10−6 mbar. Ions 2 2 2 2 2 2 2 2 2 12 12 12 14 14 14 14 16 16 16 16 18 18 28 28 30 30 30 30

2 12 14 16 18 28 30 44 46 16 18 30 14 16 18 30 16 18 28 30 28 30 28 30 30 32 44 46

528.7 eV PDCY (%) |αexp | 4.25 ± 0.23 0.88 11.83 ± 0.58 1.00 1.36 ± 0.09 2.40 18.71 ± 0.88 1.63 1.57 ± 0.10 2.00 12.39 ± 0.60 0.88 3.82 ± 0.21 1.23 1.58 ± 0.10 0.79 0.81 ± 0.06 1.00 9.96 ± 0.49 1.45 0.92 ± 0.07 1.52 0.06 ± 0.01 0.38 0.61 ± 0.05 1.33 3.98 ± 0.22 0.94 2.11 ± 0.13 0.97 0.03 ± 0.01 0.16 6.94 ± 0.36 0.90 1.22 ± 0.08 1.12 4.79 ± 0.26 1.03 5.44 ± 0.29 0.91 2.80 ± 0.16 0.97 1.66 ± 0.11 1.10 0.20 ± 0.02 0.75 0.30 ± 0.03 1.14 0.96 ± 0.07 1.38 0.06 ± 0.01 0.63 0.03 ± 0.01 0.72 0.03 ± 0.01 0.79

532.2 eV PDCY (%) |αexp | 5.66 ± 0.32 1.25 10.90 ± 0.57 1.00 1.35 ± 0.10 2.46 17.43 ± 0.87 1.42 1.47 ± 0.11 1.29 12.91 ± 0.66 1.00 4.41 ± 0.26 1.15 1.54 ± 0.11 0.69 0.76 ± 0.07 1.27 8.79 ± 0.47 1.37 0.68 ± 0.06 1.30 0.09 ± 0.03 0.83 0.33 ± 0.04 0.83 3.81 ± 0.23 0.83 2.24 ± 0.15 1.00 0.09 ± 0.02 0.19 6.56 ± 0.37 1.15 0.95 ± 0.08 1.09 4.74 ± 0.28 1.00 6.13 ± 0.34 0.93 2.75 ± 0.18 0.91 1.86 ± 0.13 1.00 0.28 ± 0.03 0.88 0.53 ± 0.05 1.70 1.15 ± 0.09 0.90 0.30 ± 0.03 1.90 0.07 ± 0.01 4.14 0.09 ± 0.02 1.61

542.2 eV PDCY (%) |αexp | 4.06 ± 0.23 1.11 8.37 ± 0.43 1.05 1.38 ± 0.10 2.27 13.94 ± 0.68 1.53 1.95 ± 0.13 1.93 11.49 ± 0.57 0.94 5.88 ± 0.32 1.46 2.01 ± 0.13 0.69 3.16 ± 0.19 0.86 6.62 ± 0.35 1.28 0.77 ± 0.06 1.37 0.06 ± 0.01 1.86 0.30 ± 0.03 2.16 3.90 ± 0.22 0.92 2.68 ± 0.16 1.06 0.07 ± 0.01 0.63 5.88 ± 0.32 1.05 1.02 ± 0.08 0.93 3.59 ± 0.21 1.03 5.91 ± 0.32 1.00 6.37 ± 0.34 0.97 6.10 ± 0.33 0.97 0.20 ± 0.02 0.64 0.47 ± 0.04 1.18 0.85 ± 0.07 1.00 0.10 ± 0.01 1.21 0.10 ± 0.01 1.00 0.12 ± 0.02 1.00

587.9 eV PDCY (%) |αexp | 4.93 ± 0.32 1.00 11.19 ± 0.63 1.00 1.10 ± 0.10 2.29 18.56 ± 0.98 1.42 1.63 ± 0.14 1.93 13.82 ± 0.76 0.88 5.25 ± 0.34 1.25 1.94 ± 0.15 0.92 2.65 ± 0.25 1.00 8.26 ± 0.49 1.41 0.35 ± 0.05 1.65 0.01 ± 0.01 1.00 0.09 ± 0.02 0.61 3.33 ± 0.23 0.83 1.72 ± 0.14 1.03 0.01 ± 0.01 1.00 6.16 ± 0.38 1.06 0.41 ± 0.05 1.17 4.09 ± 0.28 0.94 5.43 ± 0.35 1.00 4.32 ± 0.29 0.94 3.46 ± 0.24 1.07 0.05 ± 0.01 1.00 0.10 ± 0.02 1.30 0.64 ± 0.07 0.69 0.02 ± 0.01 1.73 0.02 ± 0.01 1.35 0.01 ± 0.01 0.67

ion pair for photon energies considered. Its partial yield has a maximum value to the photon energy 528.67 eV, corresponding about 18.7% of total production, when fragmentation occurs at lower pressure, and about 16.7% of total production for higher pressure. For the first and fourth resonances, for both pressure values, the PDCY of this ion pair decreases, respectively, approximately 1.0% and 3.5% of the value for the photon energy below of resonances, and increases again its value for energies above the resonances. The second pair of most detected ions is (D+ , CO+ ) at all energies and reported pressures. Its minimum production of 11.49% (lower pressure) and 10.71% (higher pressure) is to O1s(OH)→ σ ∗ (C-OH) excitation, while the maximum yield are 13.82% and 12.89% for photon energy above the resonances. In the PDCY spectrum obtained for the lowest pressure, the third most detected ion pair is (D+ , C+ ), with a minimum of 8.37% at resonance O1s(OH)→ σ ∗ (C-OH), and a maximum of 11.83% below resonance. At higher pressure, this ion pair is also the third most intence double coincidence, except at O1s(CO)→ σ ∗ (C-OH) transition, representing 7.26% of total 15 ACS Paragon Plus Environment


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Table 3: Relative yield of the ion pairs and experimental slopes of coincidence islands of DCOOD for different energies and at pressure 1.1×10−5 mbar. Ions 2 2 2 2 2 2 2 2 2 12 12 12 14 14 14 16 16 16 16 18 18 28 28 30 30 30 30

2 12 14 16 18 28 30 44 46 16 18 30 16 18 30 16 18 28 30 28 30 28 30 30 32 44 46

528.7 eV PDCY (%) |αexp | 4.52 ± 0.26 1.00 10.69 ± 0.54 1.05 1.52 ± 0.10 2.31 16.66 ± 0.81 1.63 2.02 ± 0.13 1.60 10.73 ± 0.54 1.00 4.88 ± 0.27 1.29 1.51 ± 0.10 0.73 0.90 ± 0.07 1.07 8.07 ± 0.42 1.34 0.82 ± 0.06 1.32 0.45 ± 0.04 0.72 3.05 ± 0.18 0.97 1.56 ± 0.11 1.07 0.76 ± 0.06 0.63 5.09 ± 0.28 1.06 0.97 ± 0.07 1.12 3.56 ± 0.21 1.00 4.56 ± 0.26 1.07 2.36 ± 0.15 0.94 1.98 ± 0.13 0.91 0.75 ± 0.06 1.29 2.34 ± 0.15 1.07 5.30 ± 0.29 0.92 0.24 ± 0.03 1.00 0.55 ± 0.05 1.04 0.48 ± 0.04 0.97

532.2 eV PDCY (%) |αexp | 3.50 ± 0.24 1.33 9.42 ± 0.53 1.06 1.44 ± 0.12 2.55 15.50 ± 0.82 1.53 1.67 ± 0.13 1.43 11.61 ± 0.64 1.00 5.95 ± 0.36 1.36 1.28 ± 0.11 0.77 0.69 ± 0.07 0.92 6.56 ± 0.39 1.50 0.58 ± 0.06 1.44 0.61 ± 0.06 0.63 2.91 ± 0.20 0.87 1.72 ± 0.14 0.90 1.17 ± 0.10 0.61 3.42 ± 0.23 0.84 0.76 ± 0.07 0.65 3.38 ± 0.23 1.10 5.00 ± 0.31 1.00 1.93 ± 0.15 0.94 2.44 ± 0.18 0.86 1.11 ± 0.10 1.37 3.29 ± 0.22 0.92 7.21 ± 0.43 1.15 0.75 ± 0.07 1.40 0.69 ± 0.07 4.29 0.78 ± 0.08 1.27

542.3 eV PDCY (%) |αexp | 5.20 ± 0.37 1.33 7.26 ± 0.49 1.11 1.17 ± 0.12 2.50 11.81 ± 0.72 1.47 2.10 ± 0.19 2.90 10.71 ± 0.66 1.00 8.29 ± 0.54 1.58 1.68 ± 0.16 1.00 2.69 ± 0.23 0.75 3.95 ± 0.30 1.06 0.41 ± 0.06 1.64 0.30 ± 0.05 0.77 2.18 ± 0.19 1.00 1.62 ± 0.16 0.88 0.60 ± 0.08 0.78 2.79 ± 0.23 0.96 0.44 ± 0.07 0.77 2.10 ± 0.19 1.00 4.44 ± 0.33 1.00 4.70 ± 0.34 0.97 5.93 ± 0.41 1.00 0.41 ± 0.06 0.82 2.52 ± 0.22 1.04 10.83 ± 0.67 1.18 0.44 ± 0.07 0.57 0.73 ± 0.09 0.94 1.18 ± 0.13 1.15

587.9 eV PDCY (%) |αexp | 5.46 ± 0.39 0.89 9.40 ± 0.60 0.88 1.21 ± 0.13 2.92 16.98 ± 0.98 1.39 2.24 ± 0.20 2.08 12.89 ± 0.77 1.21 7.12 ± 0.48 1.50 1.63 ± 0.16 0.69 2.19 ± 0.19 0.77 5.24 ± 0.37 1.21 0.41 ± 0.06 0.93 0.21 ± 0.04 0.31 2.41 ± 0.21 0.71 1.27 ± 0.13 0.97 0.71 ± 0.09 0.55 2.61 ± 0.22 1.36 0.54 ± 0.07 0.84 2.99 ± 0.24 0.97 4.32 ± 0.32 0.82 2.79 ± 0.23 0.85 3.58 ± 0.28 1.14 0.99 ± 0.11 0.59 2.27 ± 0.20 1.00 6.46 ± 0.44 1.00 0.20 ± 0.04 1.22 0.64 ± 0.08 0.81 0.71 ± 0.09 1.35

production, when (DCO+ , DCO+ ) becomes the second more intense ion pair with 10.83% of the total double coincidence yield. In general, we have found that the most abundant ion pairs have higher yields when obtained at lower pressure in the experimental chamber, 1.1×10−6 mbar. The reduction, when the pressure increases (Table 3), is due mainly to the increased of the PDCY of the last six pairs found in both tables. Together they represent a minimum of 9.87% of total double coincidence yield in the energy below of the resonances and a maximum of 11.16% at the O1s(OH)→ σ ∗ (C-OH) transition. When considering the lower pressure results (Table 2), the maximum production of these six ions is only 3.23% at O1s(CO)→ π ∗ (CO) excitation. The total mass of these ion pairs ranges from 56 u, the lightest, to 76 u, the heavier. Therefore, the masses of these ion pairs are larger than the DCOOD monomer mass (48 u) and smaller than the (DCOOD)2 dimer (96 u). This again indicates that the ionic fragments more massive than the parent ion are formed from dimers fragmentation. 16 ACS Paragon Plus Environment

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For a better visualization of the effect of pressure on the fragmentation pathways, we present in Figure 7 the PDCY spectra as function of photon energy for four ion pairs: (28,30), (30,30), (2,12) and (14,16). The continuous curves in black correspond the double coincidences obtained at the lower pressure and the dotted curves in red correspond the ones obtained at the higher pressure. In the PDCY spectra of ion pairs (30,30) and (28,30), we can observe a considerable growth in their productions with increasing pressure. When the pressure in the experimental chamber is of the order of 1.1×10−6 mbar, the percentage yield of these coincidences is around 1.0 % of total production. And when the pressure increases an order of magnitude (1.1×10−5 mbar), the ion pair (30,30) reaches a maximum yield of 10.83% at the O1s(OH)→ σ ∗ (C-OH) transition, while the ion pair (28,30) has a maximum yield of 3.29% at the first resonance. (a)

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Figure 7: Relative yield spectrum of the ion pairs (30,30) [Panel (a)], (2,12) [Panel (b)], (28,30) [Panel (c)] and (14,16) [Panel (d)] as a function of the photon energy at pressures 1.1×10−6 mbar (−, black) and 1.1×10−5 mbar (· · · , red). The bottom two graphs represent the yield of ions (2,12) and (14,16). We observe, in this case, an inversion of curves, so that both pairs are detected more abundantly for lower pressure in the experimental chamber, 1.1×10−6 mbar. If these ions are derived from monomers and dimers precursors, this behavior is expected, since the increase of the pressure only increases the production of ions that would come from dimers, while the production 17 ACS Paragon Plus Environment


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of ions from monomers should not increase with the pressure and its relative production may decrease. Similar behavior in coincidence spectra was observed in this study for higher pressure and in work of Tabayashi et al. 23 for beams of clusters where atomic fragments or small fragments are removed and the relative production of ion DCO+ grows.

Fragmentation paths We complete the analysis suggesting the fragmentation pathways for most abundant ion pairs detected in coincidence. The determination of these paths is made by comparing the experimental slope of the coincidences islands with the theoretical ones of all possible dissociation mechanisms proposed by Simon et al. 46 The experimental slopes of coincidences islands are obtained by dividing the half-width of spectra of the projection PEPIPICO island, along the axis t2 (∆t2 ), the width at half height of the projection along the t1 (∆t1 ), |αexp | = ∆t2 /∆t1 . The suggested molecular fragmentation mechanism will be the one whose theoretical slope comes closest to the experimental slope. In the Table 4, we present the possible fragmentation pathways for the five most abundant ion pairs detected in coincidence: (2,12), (2,16), (2,28) (12,16) and (30,30), which the PDCY is closed to or greater than 10% for at least one photon energy and pressure values. In general, we find in the Table 3 that there is a greater variation in the experimental slopes as a function of energy values. This variation can occur due to a change of the predominant electronic excitation as a function of photon energy, and partly due to the contributions of the dimer fragmentation products of deuterated formic acid, as (30,30) pair. The pair (2,12) corresponds to D+ and C+ ions detected in coincidence. As can be seen in Table 2 for the pressure 1.1×10−6 mbar, the experimental slopes for the corresponding coincidence island were 1.0, except for the O1s(CO)→ π ∗ (CO) resonance, whose slope was 1.05. In this case, the suggested fragmentation route is the Deferred Charge Separation, whose theoretical slope is 1.0, since the molecule can fragment into three or more bodies. Experimentally, we cannot distinguish in how many fragments the molecule dissociates, 18 ACS Paragon Plus Environment

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Table 4: Possible fragmentation paths of ion pairs ION PAIR (2,12) Deferred Charge Separation |αtheo |=1.0 DCOOD2+ → DCO2+ + OD DCO2+ → DC2+ + O DC2+ → D+ + C+ Deferred Charge Separation |αtheo |=1.0 DCOOD2+ → DCOO2+ + D DCOO2+ → DCO2+ + O DCO2+ → D+ + CO+

ION PAIR (2,16) Secondary Decay after a Deferred Charge Separation |αtheo |=1.75 DCOOD2+ → DCO2+ + OD DCO2+ → D+ + CO+ CO+ → O+ + C ION PAIR (2,28) Secondary Decay after a Deferred Charge Separation |αtheo |=0.93 DCOOD2+ → DCOD2+ + O DCOD2+ → DCO+ + D+ DCO+ → D + CO+

Secondary Decay in Competition |αtheo |=1.05 DCOOD2+ → OD+ + COD+ OD+ → D+ + O COD+ → CO+ + D

Secondary Decay in Competition |αtheo |=1.27 + DCOOD2+ → D+ 2 + COO + + D2 → D + D COO+ → CO+ + O

ION PAIR (12,16) Secondary Decay after a Deferred Charge Separation |αtheo |=1.17 DCOOD2+ → DCO2+ + OD DCO2+ → DC+ + O+ DC+ → D + C+

Secondary Decay in Competition |αtheo |=1.48 DCOOD2+ → DCO+ + OD+ DCO+ → C+ + OD OD+ → O+ + D

Secondary Decay in Competition |αtheo |=1.40 DCOOD2+ → DCO+ + D2 O+ CO+ → C+ + O D2 O+ → O+ + D ION PAIR (30,30) Secondary Competing Decay |αtheo |=1.0 DCOOD2+ → DCOOD+ + DCOOD+ 2 DCOOD+ → DCO+ + OD DCOOD+ → DCO+ + OD

because the neutral fragments are not collected with the PEPIPICO technique. Thus, the suggestion of a possible fragmentation route is shown in Table 4. When the pressure is increased, the experimental slope varies between 0.88 and 1.11, and the theoretical slopes of the other fragmentation processes are too high or below these experimental values, so we conclude that the same process takes place, preferably, when we increase the pressure. The pair (2,16) corresponds to ions D+ e O+ (or CD+ 2 ). In the case of formation of ion CD+ 2 , after fragmentation, recombination should occur since the carbon and one of deuterium atoms are not chemically bonded. Thus, we consider that the ion formed, with mass-tocharge ratio 16, is oxygen. We suggest that the fragmentation process is Secondary Decay after a Deferred Charge Separation, Table 4, for which the obtained theoretical slope is 1.75.



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The largest difference between the theoretical and experimental slope is 0.36, occurring is at energy above the resonances in the spectrum obtained for the higher pressure. The smallest difference, 0.12, occurs for energy below the resonance for both pressures. The pair (2,28) corresponds to D+ and CO+ ions. In this case, the experimental slopes of coincidence islands obtained for the highest pressure was 1.0, except for photon energy above of resonance which was 1.21. For the first three photon energies, the fragmentation process may be due to Deferred Charge Separation, whose theoretical slope is also equal to unity; by Secondary Decay after Deferred Charge Separation, whose theoretical slope is 0.93 or Secondary Decay in Competition, whose theoretical slope is 1.05. For energies above the resonance, and at the highest pressure, whose experimental slope is 1.21, a Secondary Decay process may have occurred in Competition, but with the on order of formation of ions different from the previous process. In this case, the theoretical slope is 1.27. For this coincidence pair generated at lower pressure, the experimental values of the slope in the resonances was 1.00 and 0.94, so that these same processes are likely to have occurred. The experimental slopes, for energy values above and below the resonances and the spectrum obtained at the lowest pressure were both 0.88. The theoretical slope which most closely approximates this result is 0.93, corresponding to Secondary Decay after Deferred Charge Separation process. We list all these possible fragmentation routes in Table 4. The pair (12,16) corresponds to ions C+ and O+ detected in coincidence. As we can see in Table 2 for pressure 1.1×10−6 mbar, the experimental slope ranges from 1.28 to O1s(CO)→ π ∗ (CO) transition and 1.45 for photon energy below resonances. We suggest that, for the first resonance, the fragmentation occurs by Secondary Decay after Deferred Charge Separation, whose theoretical slope is 1.17. For other energy values, the suggestion is the Secondary Decay in Competition process, which can follow two different fragmentation routes, as indicated in Table 4. We now suggest that the fragmentation pathway of the couple (30,30) comes from the dimer DCOOD so as to produce two DCO+ ions and two neutral fragments OD. The slopes


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of the ion pair coincidence island (30,30), when pressure is 1.1×10−5 mbar, were 0.92, 1.15, 1.18 and 1.00 for the energies given in Table 3. These experimental data suggest that the slope is close to 1.0, thereby indicating a possible fragmentation route is the Secondary Competing Decay process occurring as indicated in Table 4. This suggested pathway is consistent with the previous PDCY analysis, with the (30,30) ion pair arises mainly after the O1s(OH)→ σ ∗ (C-OH) transition. Note that the theoretical slope of the Decay Secondary in Competition process can have two values 1.00 or 1.67, depending on which of the two formed ions is heavier and which neutral fragments are not chemically bonded. We did not found in the literature the theoretical value of the slope island formed by this process if the two ions have the same mass. We emphasize that the experimental slopes for this ion pair obtained for the lower pressure (Table 2) are also close to 1.0, except for the slope obtained in the first resonance, 1.70, which also coincides with the theoretical values of the slope.

Conclusions In the present paper we have reported a study of photoragmentation and photoionization of DCOOD on the oxygen K edge. We have analyzed total ion yield spectra and mass spectra of single and double coincidences as a function of photon energies. We obtained these spectra focusing synchrotron radiation on an effusive molecular beam of DCOOD for different pressure values. Our main motivation for this study in the core region is to check the influence of the molecular gas pressure in the total ion yield spectra and mass spectra. We have investigated whether even with the intention of studying the photofragmentation monomers, working with an effusive molecular beam in high vacuum system, the spectra may contain cations from dimer photofragmentation. We found distinct characteristics between the total ion yield spectra and mass spectra obtained for pressures 2.4×10−6 and 9.1×10−6 , in the high vacuum range. The TIY spectrum



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obtained for higher pressure presents qualitative characteristics that are compatible with TIY spectra obtained for clusters: 23 (i) increase of the width of the peaks in the pre-edge, (ii) smallest relative distance between the first two resonances, and (iii) increase of the relative intensity in continuum region. The analysis of coincidence spectra indicates that the relative ion yield of the (DCOOD)·D+ and DCO+ increase with pressure, while the percentages of production of small fragments as C+ , CD+ and O+ decrease or remains approximately constant with increasing pressure. A similar result is observed in the double coincidence mass spectrum: for ions formed in coincidence having the sum of the mass-to-charge ratio larger than that of the monomer is lower than for the dimer, such as (DCO+ , DCO+ ) and (CO+ , DCO+ ), the relative fractional yield increases with increasing pressure. Also in PEPIPICO spectra small fragments formed in coincidence, for which sum of mass-to-charge ratios is smaller than for the monomer, such as (D+ , C+ ) and (CD+ , O+ ), the fractional yield decreases with increasing pressure. Pressure increase favors the formation of hydrogen bonds between two monomers, since it increases the number of molecular collisions in the system. Thus, we concluded that the increase in the relative production of ions or pairs ions having mass-to-charge larger than the monomer, such as (DCOOD)·D+ , (DCO+ , DCO+ ) and (CO+ , DCO+ ) is due to increase of dimer concentration in the molecular beam with pressure growth. There is a high concentration of formic acid dimers in the sample holder at room temperature and normal pressure. Due to the lower pressure in the gas injection system, between the needle valve which controls the pressure and the hypodermic needle that injects the molecular beam, the concentration of dimers in formic acid gas decreases due to break up of hydrogen bonds between the monomers. When the pressure in the no beam is increased, increase the concentration of molecules in the gas injection system, consequently fewer hydrogen bonds are broken and the proportion of dimers injected into the experimental chamber increases. 29 From the analysis of the experimental slopes of coincidences islands we suggested the fragmentation pathways of the main ion pairs detected in coincidence. In particular, we


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emphasize that in the formation of the pair (DCO+ , DCO+ ) for different photon energies occurs via a dimer, followed by a Secondary Decay in Competition process. From this analysis it can be said that the increased production of the DCO+ in PEPICO spectra with increasing pressure is also due to the increase in dimer concentration in the molecular beam. We conclude that in the investigation of the photofragmentation of monomers, the contribution of dimers fragmentations can lead to false coincidences. This result is clear when studying the oxygen K edge of DCOOD, finding that the coincidence of the ions with massto-charge ratio (30,30) is quite intense in PEPIPICO spectra obtained for high pressure, representing 10% of the total production of double coincidences. In PEPICO spectra in the same photon energy region, the peak for the ion with m/q = 30 is more intense, and part of the detected ions results from dimer fragmentation.

Acknowledgement This work has been supported by Coordena¸caõ de Aperfei¸coamento de Pessoal de N´ıvel Superior (CAPES/MEC, Brazil), Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnológico (CNPq/MCT, Brazil), Funda¸cão de Amparo à Pesquisa do Estado da Bahia (FAPESB, Brazil) and Laboratório Nacional de Luz S´ıncrotron (LNLS, Brazil).

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(3) Eland, J. H. D.; Linusson, P.; Mucke, M.; Feifel, R. Homonuclear Site-Specific Photochemistry by an IonElectron Multi-Coincidence Spectroscopy Technique. Chem. Phys. Lett. 2012, 548, 90–94. (4) Bodi, A.; Csontos, J.; Kállay, M.; Borkar, S.; Sztáray, B. On the Protonation of Water. Chem. Sci. 2014, 5, 3057–3063. (5) Bava, Y. B.; Mart´ınez, Y. B.; Betancourt, A. M.; Erben, M. F.; Filho, R. L. C.; Védova, C. O. D.; Romano, R. M. Ionic Fragmentation Mechanisms of 2,2,2Trifluoroethanol Following Excitation with Synchrotron Radiation. ChemPhysChem 2015, 16, 322–330. (6) Nenner, I.; Morin, P.; Lablanquie, P.; Simon, M. Photodissociation of Core Excited Molecules. J. Eletron Spectrosc. Relat. Phenom. 1990, 52, 623–648. (7) Li, Z., Wu, L., Eds. Hydrogen Bonded Supramolecular Structures; Springer: Berlin, 2015. (8) Grabowski, S. J., Ed. Hydrogen Bonded New Insights; Springer: Dordrecht, 2006. (9) Gilli, G.; Gilli, P. The Nature of the Hydrogen Bond ; Oxford University Press: Oxford, 2009. (10) Scheiner, S.; Kern, C. W. Molecular Orbital Investigation of Multiply Hydrogen Bonded Systems. Formic Acid Dimer and DNA Base Pairs. J. Am. Chem. Soc. 1979, 101, 4081– 4085. (11) Nikolova, E. N.; Goh, G. B.; Brooks, III, C. L.; Al-Hashimi, H. M. Characterizing the Protonation State of Cytosine in Transient GC Hoogsteen Base Pairs in Duplex DNA. J. Am. Chem. Soc. 2013, 135, 6766–6769. (12) Novak, J.; Malis, M.; Prlj, A.; Ljubic, I.; Kuhn, O.; Doslic, N. Photoinduced Dynamics


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Ionization and Fragmentation of DCOOD Induced by Synchrotron

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