Dissociative electron attachment studies on acetone

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Dissociative electron attachment studies on acetone Vaibhav S. Prabhudesai, Vishvesh Tadsare, Sanat Ghosh, Krishnendu Gope, Daly Davis, and E. Krishnakumar Citation: The Journal of Chemical Physics 141, 164320 (2014); doi: 10.1063/1.4898144 View online: http://dx.doi.org/10.1063/1.4898144 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dissociative electron attachments to ethanol and acetaldehyde: A combined experimental and simulation study J. Chem. Phys. 142, 064316 (2015); 10.1063/1.4907940 Dissociative electron attachment to furan, tetrahydrofuran, and fructose J. Chem. Phys. 125, 044304 (2006); 10.1063/1.2222370 Dissociative electron attachment near threshold, thermal attachment rates, and vertical attachment energies of chloroalkanes J. Chem. Phys. 118, 2562 (2003); 10.1063/1.1535891 Dissociative electron attachment to molecules in the gas phase and in rare gas solids J. Chem. Phys. 116, 5471 (2002); 10.1063/1.1458536 Dissociative electron attachment to gas-phase 5-bromouracil J. Chem. Phys. 113, 2517 (2000); 10.1063/1.1306654

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THE JOURNAL OF CHEMICAL PHYSICS 141, 164320 (2014)

Dissociative electron attachment studies on acetone Vaibhav S. Prabhudesai,a) Vishvesh Tadsare, Sanat Ghosh, Krishnendu Gope, Daly Davis, and E. Krishnakumar Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India

(Received 5 May 2014; accepted 25 September 2014; published online 29 October 2014) Dissociative electron attachment (DEA) to acetone is studied in terms of the absolute cross section for various fragment channels in the electron energy range of 0–20 eV. H− is found to be the most dominant fragment followed by O− and OH− with only one resonance peak between 8 and 9 eV. The DEA dynamics is studied by measuring the angular distribution and kinetic energy distribution of fragment anions using Velocity Slice Imaging technique. The kinetic energy and angular distribution of H− and O− fragments suggest a many body break-up for the lone resonance observed. The ab initio calculations show that electron is captured in the multi-centered anti-bonding molecular orbital which would lead to a many body break-up of the resonance. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4898144] I. INTRODUCTION

Studies of dissociative electron attachment to various simple organic molecules have attracted a lot of attention recently. A part of the motivation behind these studies has been to understand the role of low energy electrons in radiation induced damage in biological cells.1 Also, the site selective fragmentation observed in dissociative electron attachment (DEA) studies on simple organic molecules is indicative of the potential for controlling the chemistry driven by low energy electrons.2 An important class of organic compounds that has relevance to the above motivations is ketones. Three out of four DNA bases form hydrogen bonds between base pairs using the ketonic carbonyl groups present in them. Many biologically relevant sugar molecules are ketones. This makes the study of DEA to simple ketones very important from the radiation biology point of view. Acetone is the simplest ketone one can use as a model for DEA studies on ketone functional group. It is also the first 10 atom molecule observed in the interstellar medium.3 It is a key compound to understand the chemistry of carbonyl group containing molecular anions. The VUV absorption spectra of carbonyl group containing compounds show a close similarity indicating resemblance in their neutral excited states. Particularly for acetone ((CH3 )2 CO) and acetaldehyde (CH3 CHO) where the difference is one of the methyl groups in acetone being replaced by H atom in acetaldehyde, the VUV absorption spectra4, 5 as well as the electron impact excitation spectra are very similar.6 The only difference is that the excited states are systematically shifted to the lower energy side for acetone as compared to acetaldehyde by about 0.5 eV. The question is can we observe similar behaviour in the formation of negative ion resonances in these molecules, at least for the valance excited shape and Feshbach resonances. Considering this, it is worth comparing the DEA process in acetone with that in acetaldehyde.

a) Electronic mail: [email protected]

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On the other hand, in the recent past, methylation of hydrogen sites in big molecules like DNA bases has been used to identify the source of hydride ions or the site of hydrogen abstraction in DEA.7–9 Although this is a very effective method to pin point the DEA dynamics, it is also important to explore the effect of methylation of a hydrogen site on the overall properties of the resonance shown by the molecules in electron attachment. Particularly, it is important to understand the effect of methylation on the contribution of a specific resonance to the DEA outcome as it depends strongly on the resonance lifetime. The lifetime may get influenced by the presence of methyl group in place of a hydrogen atom. Acetaldehyde and acetone are very simple candidates to understand such effects and comparison of the DEA process in these molecules can throw light on these less explored features. DEA to acetaldehyde has been studied quite extensively.10–14 There is a limited literature available on the DEA studies of gas phase acetone.12–15 In the recent measurements of the total cross section of electron scattering from acetone and acetaldehyde, the similarity in the cross section curves has been noted.16 Both the molecules show the contribution of various resonances to the cross section in the energy range of 8–10 eV. Dorman reported H− (8.5 eV), CHCO− (9.2 eV), CH2 CO− (9.2 eV), CH3 − (9 eV), OH− (8.8 eV), and O− (9.2 eV) fragments on DEA to acetone.12 Naff et al. reported CH2 CO− , CHCO− , OH− , CH3 − , and O− fragments all peaking at 9 eV electron energy.13 Muftakhor and Fokin reported CH3 − (6.25 eV) and H− (6.8 eV and 8.8 eV).14 On comparison with acetaldehyde they attribute the 6.8 eV peak to a shape resonance where the incoming electron is captured into the vacant σ * orbital. The latest of these studies by Illenberger et al. report the formation of O− , OH− , CHCO− , C3 H4 O− , and C3 H5 O− .15 All these ion signals are found to peak near 9 eV. For C3 H4 O− and C3 H5 O− fragments, they report a very weak peak between 2 and 3 eV. They do not report CH3 − and their experimental set up is not capable of making reliable measurements for the H− .15 Besides, the

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relative cross sections of O− and OH− channels are reversed in comparison with the other reported studies. However, so far no absolute cross sections have been reported for DEA to acetone. Here we report a detailed investigation of the DEA process in acetone that includes the first ever measurement of the absolute cross section for the various fragment channels as a function of electron energy and their angular and kinetic energy distributions across the resonance peak. The angular distributions and kinetic energies of the ions were measured using a velocity slice imaging (VSI) spectrometer, in the electron energy range of 0–20 eV. The absolute cross sections, also in the same electron energy range, were measured using the same spectrometer but in the improved mass resolution mode. We also report the ab initio calculations on the resonance states that could contribute to DEA to acetone and draw comparison with DEA to acetaldehyde. II. EXPERIMENTAL

The details of the experimental set-up have been discussed earlier.17 The setup consisted of a three element electron gun of Pierce geometry, a Faraday cup to measure the electron beam current, a capillary array to produce an effusive molecular beam, and a Velocity Slice Imaging (VSI) set up to measure the momentum distribution of the fragment ions. To confine the electron beam, a pair of coils mounted outside the vacuum chamber was used in the Helmholtz geometry producing a uniform magnetic field (strength of about 50 Gauss) co-axial to the electron gun in the interaction region. The pulsed electron beam (duration ∼200 ns) was made to cross the molecular beam at right angles and the resulting negative ions were extracted to the VSI spectrometer using a pulsed extraction field in the interaction region delayed (∼100 ns) with respect to the electron beam pulse. The VSI set up consisted of an assembly of the pusher and puller electrodes that flank the interaction region, followed by the lens electrode and a small (50 mm in length) flight tube. The puller electrode has a fine molybdenum wire mesh of 70% transmission. The potentials on various electrodes were optimized for the VSI condition. The negative ions extracted from the interaction region were detected using a two-dimensional (2D) position sensitive detector comprised of a set of micro channel plates (MCP) in Z-stack assembly and a wedge and strip anode. The data were acquired using the program LAMPS.18, 19 The data comprised of the timing signal and the pulses from the wedge, strip, and the meander part of the anode that were amplified using a charge sensitive preamplifier followed by a spectroscopic amplifier. The position information of the ion hit on the detector was obtained in terms of their (X, Y) co-ordinates using the anode signals. The 2D distribution of the ion hits on the detector were displayed online and for each recorded ion hit the position and the timing information was stored in the List mode. The appropriate time slice was obtained by the same program in the offline analysis of the stored data. The fine tuning of the VSI spectrometer was carried out by obtaining the momentum image of O− ions from O2 at 6.5 eV electron energy.17

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The same VSI spectrometer was also used as a time of flight mass-spectrometer (ToFMS) for absolute cross section measurements. The best possible mass resolution was obtained by fine tuning the potentials on the lens and the flight tube. The absolute cross section measurements using the relative flow technique needs the complete collection and detection of all (or a well-defined fraction) of the ions irrespective of their initial kinetic energy and angular distribution. This was ensured in either of the modes of operation of the spectrometer by looking at their positions on the detector. However, while measuring the cross section only timing information of the ions arrived at the detector was used. The acetone vapors were introduced into the vacuum chamber using a capillary array connected to the glass bulb via a needle valve. The pure acetone sample (99.5% Sigma Aldrich) was pumped for a sufficient duration using a different pumping line connected to another roughing pump to get rid of the air trapped in the liquid and the bulb. The background pressure in the vacuum chamber was in the range of 10−9 Torr, which was achieved using an oil-free pumping system comprised of a turbo molecular pump backed by a scroll pump. All the measurements were carried out at low 10−6 Torr which ensured the single collision condition. The normalization of the cross sections to the absolute values required the measurement of pressure behind the capillary array. This was carried out using a capacitance manometer. The counts for a specific fragment anion were measured as a function of electron energy (ion yield curve) using a time-to-amplitude convertor (TAC) based data acquisition system coupled with a digital counter and controlled by a home built multi-scalar data acquisition software. The ion yield curves were also collected using a Multi-hit ToF card of 0.5 ns resolution. This enabled us to store ToF data at each of the steps (in this case 0.2 eV) employed for the ion yield curve measurements. We could also obtain better statistics as all ion yield curves could be measured simultaneously, allowing longer collection time. Some of the ions peaks, in the mass spectra, like around mass 16 could not be properly resolved. However, we could obtain the individual contributions from each of the contributing masses using Gaussian fits. This is shown for the case of CH3 − , O− , and OH− in Fig. 1. The ion yield curves in these cases were obtained based on the Gaussian fits of the mass spectra recorded at all the electron energy steps. All the ion yield curves were then normalized to absolute values using the relative flow technique20 with O− from O2 at 6.5 eV as the standard.21 While the imaging of the ion cloud ensured complete collection and transmission of the ions to the detector, the uniform efficiency of detection of the ions by the Z-stack of MCPs independent of the ion mass was ensured by operating the front plate and the bias across the three plates at appropriate voltages at which no discrimination was observed. The uncertainties in the measured absolute cross section values for various fragments were estimated to be due to (i) the statistical uncertainty in the ion counts, (ii) the uncertainty in the pressure measurement behind the capillary, (iii) the uncertainty in the current measurements, (iv) the uncertainty due to the variation of the detector efficiency in the negative ion mode across the ion masses relevant to these measurements, and (v) the uncertainty in the absolute

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FIG. 1. Mass peak corresponding to CH3 − , O− , and OH− ions taken at 8.6 eV electron energy. The solid curves are the Gaussian fits carried out to determine the relative contribution of each species.

cross section of the standard (cross section of O− from O2 at 6.5 eV). Since the cross sections for all DEA channels are very small, the statistical uncertainty in the ion counts tended to get larger. This was overcome by carrying out the measurements for longer durations till sufficient statistics were obtained. The maximum uncertainty due to this was estimated for the mass 57 ions which had very low count rate (∼10%). For the other masses this uncertainty was less than 5%. The uncertainties in the pressure and electron current measurements were not more than 5% based on their variation during a set of measurements. The operating conditions of the MCP detector were found to cause very little variations in the detection efficiency for the negative ions in the relevant mass range accelerated to about 200 eV which is the case for the present measurements. The estimated uncertainty due to the variation of detector efficiency was not more than 5%. The major contribution to the uncertainty in the absolute cross section came from the reported value of the cross section for O− from O2 which was of the order of 10%.20 Taking all these into account, the overall uncertainty in the absolute cross section is estimated to be about 15% for H− , combined CH3 − , O− , and OH− and mass 41 ions. It was found to be around 17% for the mass 57 ion. As the relative contribution of the CH3 − , O− , and OH− ions were determined using Gaussian fits to the mass spectra an additional uncertainty of about 10% was added to the absolute cross sections of these channels. The final uncertainty in the CH3 − , O− , and OH− channels is about 20%.

III. THEORETICAL CALCULATIONS

A real-valued continuum remover potential to decouple and, hence, to isolate the resonance state from the rest of the continuum was used for calculating the resonance states. The details of the calculation can be found in Refs. 22 and 23. A box-shaped one-electron quadratic potential, which is smoothly turned-on only for the non-interaction region of the physical Hamiltonian of the molecule was used as the continuum remover potential. The electronic bound-states and the bound-like part of the resonance states those are local-

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ized inside the interaction region were unperturbed by the asymptotically defined continuum remover potential. The isolated resonance state, that was decoupled from the continuum, corresponding to the electron attachment was then computed using EA-EOMCCSDt (Electron Affinity- Equation of Motion Coupled Cluster method with Singles, Double and noniterative Triples) code available with the Gamess-US quantum chemical package.24 The EA-EOMCCSDt method is a direct method for the vertical energy difference between the ground state of the target molecule and its electron-attached states. The energy differences were computed from a single set of analytical equations and, hence, a consistent treatment of electron correlation effects is guaranteed. An atomcentered 6-311+G* basis set augmented with an additional even-tempered primitive Gaussian set of s and p function was selected as the one-electron atomic basis set. The EAEOMCCSDt calculations were performed for the equilibrium geometry of the target molecule which was computed at the second-order Moller-Plesset perturbation method.

IV. RESULTS AND DISCUSSIONS A. Absolute cross sections

In DEA to acetone the negative ions with masses 1, 15, 16, 17, 41, and 57 were obtained. They were identified as H− , CH3 − , O− , OH− , CHCO− , and/or CH2 CO− and C3 H4 O− and/or C3 H5 O− , respectively. Due to poor mass resolution of the spectrometer, we were unable to distinguish between mass 56 and 58. However, the large electron energy associated with the appearance of this peak rules out the presence of the parent anion (C3 H6 O− ). Similarly we were unable to distinguish the contributions from CHCO− and CH2 CO− . On comparison with the studies reported using better mass resolution we attribute the mass 41 peak to the combined signal due to CHCO− and CH2 CO− ions and that at the mass 57 peak to the combined signal due to C3 H4 O− and C3 H5 O− .15 In all these channels we observed only one resonance peak between 8 and 9 eV electron energy. We do not observe the low energy peak between 2 and 3 eV as reported by Illenberger et al.15 Though the CH3 − , O− , and OH− mass peaks were not clearly resolved, their relative contribution could be obtained by fitting Gaussians to the combined mass peak as shown in Fig. 1. Based on this, we find that at 8.4 eV the O− signal is about 3 times stronger than OH− . This is consistent with previous reports,12, 13 but not in agreement with the recent results by Illenberger et al.15 where they report the OH− signal to be about 4 times stronger than the O− signal. An explanation for this difference could have been attributed to the relatively large kinetic energy in the O− channel leading to the discrimination in its collection and detection in the measurements by Illenberger et al. However, this could be ruled out as both the ions are found to be formed with very little kinetic energy as seen from the momentum images (see below). Thus the reason for the discrepancy remains unknown. We also do not observe the 6.8 eV peak in the H− ion yield which is reported by Muftakhor and Fokin.14 We suspect the origin of this peak from the background water which in our case has been minimum.

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FIG. 2. Absolute cross sections for various fragment negative ion channels from DEA to acetone.

The absolute cross sections as a function of the elecron energy for different fragment ions are shown in Fig. 2. The individual cross sections for CH3 − , O− , and OH− obtained using Gaussian fits are shown in Fig. 3. The measured cross sections along with the corresponding electron energies are presented in Table I. It can be seen that the O− signal peaks at about 8.8 eV whereas the OH− signal peaks at about 7.8 eV. The CH3 − ion signal peaks at about 8.6 eV with the least cross section among the observed anion fragments. In the earlier reported studies as well, this channel was found to be the weakest one among the observed anion fragments.12, 13 As can be seen from Table I, the formation of H− is the most dominant channel of DEA to acetone. The peak cross section for H− channel which is 2.5 × 10−19 cm2 is about the same as that measured for the compounds like acetic acid25 and about a factor of 6 smaller than that for methane.26 Such a low value of cross section for DEA may be due to the low autodetachment lifetime of the resonance. It may also imply that the dynamics of the dissociation process of the parent anion is complex leading to a relatively slower dissociation.

B. Angular distributions and kinetic energy distributions

To understand the underlying dynamics that leads to the dissociation in DEA process we measured the angular distributions and kinetic energies for various ions using VSI technique. For the heavier fragments the momentum images observed were like a blob which can be understood as a result of many body fragmentation of the anion yielding less kinetic energy release and a lack of unique momentum directions.

FIG. 3. Absolute cross sections for the combined signal of CH3 − , O− , and OH− ions along with the individual contributions of the ions. (The cross section for CH3 − is multiplied by 10.) The relative contributions were obtained by fitting Gaussian peaks in the mass spectra obtained at various electron energies.

Here we discuss the VSI of two lighter fragments namely H− and O− . 1. H−

The momentum distributions at different electron energies for H− fragment are shown in Fig. 4. The arrow indicates the direction of the electron beam. The images show small unresolved distribution which are stretched along the axis of the electron beam. The small size of the images indicates the low kinetic energy of the fragment ion, while the elongation shows anisotropy in the momentum distribution along the axis of the electron beam. The kinetic energy is found to peak at 0 eV and extending up to 0.2 eV. The kinetic energy distribution does not change significantly across the range of the electron energy shown in the figure. The angular distribution of H− obtained resembles that obtained for methane.27 It is known that in the organic molecules there exists the site selectivity in the hydride ion formation based on the functional group present in the molecule.2 In acetone, as the H− is expected to come from the breakage of the C–H bond from either of the methyl groups present, it is expected to resemble the angular distribution obtained from methane. Such features are also reported for the methyl group containing molecules where the hydride ions are found to arise from the C–H site of the methyl group.28 The electron energy at which the hydride ion is obtained in acetone is also in the vicinity of the resonance energy obtained in methane. In methane, the H− formation is understood to be associated with the many body fragmentation of parent anion. The similarity of the H− angular distribution from acetone with that from methane also indicates the many

TABLE I. Absolute cross sections for various anion fragments measured at the electron energies corresponding to the peak in the ion yield curves. Ions Resonance position (eV) Cross section (cm2 )

H−

CH3 −

O−

OH−

8.4 2.5 × 10−19

8.6 1.6 × 10−21

8.8 3.5 × 10−20

7.8 1.3 × 10−20

CHCO− + CH2 CO− 8.6 2.8 × 10−20

C3 H5 O− + C3 H5 O− 8.2 3.7 × 10−21

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FIG. 4. Velocity slice images for H− from acetone at (a) 6.8 eV, (b) 8.4 eV, and (c) 10.2 eV electron energies. (Arrow indicates the direction of the electron beam.)

body break-up mechanism. Knowing the heat of formation (Hf )((CH3 )2 CO) = −218.5 kJ/mol, Hf (CH3 COCH2 ) = 8 kJ/mol (using proton affinity of 2-Oxopropyl radical (820 kJ/mol) and I.P. ((CH3 )2 CO) = 9.7 eV, I.P.(H) = 13.6 eV), Hf (H) = 218 kJ/mol, and EA(H) = 0.75 eV, the threshold for H− via two body break up is found to be 4.2 eV.29 As the H− signal is found to peak at 8.8 eV, the system is expected to carry a lot of excess energy. However, the low kinetic energy observed along with the anisotropy in the angular distribution points to the possibility of two body break up with substantial part of the excess energy in the form of internal energy of the neutral moiety. This neutral fragment may further break into smaller fragments due to the available excess internal energy. There are many such dissociation pathways energetically available. 2. O−

In the velocity slice imaging mode we have to stretch the ion ToF in order to obtain a proper time slice. This process significantly reduces the mass resolution of the spectrometer. Hence, we could not obtain the separate images for O− and OH− fragments as they could not be separated in the flight time. However, as can be seen from Fig. 3, the relative contribution of O− and OH− ions varies between 2.5:1 to 4:1 across the resonance, we can compare the momentum image to find out any possible signature of the individual ions in the momentum distribution. Here we neglect the contribution of CH3 − channel as it is more than one order of magnitude weaker than O− at all the energies. These images at different electron energies are shown in Fig. 5. The images appear as isotropic blobs with very little kinetic energy at all the electron energies around the peak. As there is no substantial change in the image properties across the peak, it can easily be concluded that both the ions are

FIG. 5. Velocity slice images for O− from acetone at (a) 7 eV, (b) 8.4 eV, and (c) 9.6 eV electron energies. (Arrow indicates the direction of the electron beam.)

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formed with very little kinetic energy. The formation of OH− from acetone has to happen through the rearrangement of the atoms, indicating a slow dissociation process and multiple bond breaks. This is likely to lead to the relatively low kinetic energy release although the threshold for this channel is about 2.7 eV.15 The O− formation can go through a two body break up by the direct cleavage of the carbonyl double bond. The threshold for this channel is found to be 6.2 eV.15 On the other hand the threshold for formation of CH3 CHCH2 and O− is about 3.4 eV (Hf (CH3 CHCH2 ) = 20.4 kJ/mol, Hf (O) = 249.2 kJ/mol and EA(O) = 1.46 eV).29 But this requires rearrangement of one of the hydrogen atoms before dissociation. In either of the cases there is substantial excess energy available in the system. As we see very little kinetic energy in the O− channel, the excess energy is likely to be in the form of internal energy of the neutral moiety. This excess energy can cause further break-up of the neutral moiety. We also observed that the size of the blob slightly increases as the electron energy is increased. For the electron energy range shown in Fig. 5, this increase in size is much less than the expected increase in the O− kinetic energy for the two-body dissociation. More than one bond break up could also explain the low kinetic energy of the O− ions. There are many such channels possible with direct multiple bond breakage with the threshold energy in the range of 4–6 eV. We have calculated 40 resonance states in acetone in the energy range 0–10 eV using the method discussed earlier. There are only four resonance states close to 8 eV. Their energies are 7.63 eV, 8.16 eV, 8.8 eV, and 8.90 eV. We have plotted the singly occupied molecular orbitals (SOMOs) of the electron attached states. We looked for the anti-bonding SOMOs because anti-bonding nature of the SOMOs leads to the dissociation after electron attachment. The plots of the SOMOs of these four resonance states show that only one state, that with energy 8.16 eV (Fig. 6(a)), has anti-bonding SOMO on –CO group that leads to the formation of O− . Based on our calculations, this state is a single particle shape resonance. The orbital is plotted using the normalized one-particle contribution computed using the EA-EOMCCSDt method. From the figure it can be seen that the orbital occupied by the incoming

FIG. 6. The molecular orbitals occupied by the incoming electron for (a) acetone at 8.2 eV and (b) acetaldehyde at about 9.5 eV. The resulting resonance leads to the formation of the O− ion in each case. Here acetone undergoes a many body break-up whereas acetaldehyde shows two body break-up (see the text for details).

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electron is a multicenter anti-bonding orbital formed by the mixing of a π * orbital with an orbital from the dipole bound state (the diffused electron density away from the nuclear centre in Fig. 6(a)). This mixing of the orbitals stabilizes the resonance state with a larger life time. Nodes of the antibonding orbital are on C=O and on C–H. Resonant capture of an incoming electron into this anti-bonding orbital decreases the bond order between the carbon and oxygen and also between the carbon and hydrogen atoms and hence initiates the multiple fragmentation of acetone. Electron density in the orbital (Fig. 6(a)) suggests the fragmentation into O− , H2 , and allene (CH2 =C=CH2 ) which has a threshold energy of 5.6 eV (Hf (CH2 =C=CH2 ) = 198.4 kJ/mol, Hf (O) = 249.2 kJ/mol, Hf (H2 ) = 0).29 In any simple ketone, the source of the O− anion would be from the carbonyl group present in the molecule. The same group is also present in the simple aldehydes. Hence, it is natural to compare the dynamics that leads to the formation of O− in aldehydes and ketones. On comparison with the acetaldehyde (CH3 CHO), acetone shows distinctly different angular distribution as well as the kinetic energy distribution for the O− fragment. The velocity slice image of O− and OH− ions from acetaldehyde shows two distinct distributions.11 As in the case of acetone, the relative intensity observed for OH− from acetaldehyde is considerably small as compared to that for the O− ions. Hence we may assume that the momentum image is dominated by O− and the two structures seen in the image is due to the O− fragment being ejected via two different channels.11 The higher energy part shows the six fold symmetry in the angular distribution whereas the low energy part shows almost isotropic distribution. The angular distribution of the O− fragment in acetone has no higher energy component as seen in acetaldehyde. This is indicative of the different nature of the resonances that lead to DEA in these molecules. Also, our calculations show that the relevant resonance state (around 9.5 eV) in acetaldehyde is a Feshbach resonance (2 particle – 1 hole) while that in acetone (around 8.2 eV) is a single particle shape resonance. The orbital plot of the doubly occupied electron attached state (incoming electron orbital) of acetaldehyde (Fig. 6(b)) shows a single node between carbon and oxygen. This is also a mixed dipole bound – π * orbital with a very high contribution from the dipole bound state (diffused electron density away from the nuclear centre in Fig. 6(b)). This orbital from the dipole bound state stabilizes the incoming electron in this orbital and leads to a bond break between the carbon and oxygen atoms leading to a two body fragmentation. On comparison with acetaldehyde, O− in acetone appears at lower electron energy which is consistent with the trend shown in the excited states of the respective neutral molecules. However, the dynamics shown by the DEA process leading to this fragment are different in both the molecules. Also, our ab initio calculations show that the resonances involved in the DEA process are of different nature. Our results show that a substitution of a methyl group by the H atom causes substantial changes to the DEA dynamics. One such possibility for this change is the increase or decrease in the lifetime of the resonances leading to corresponding changes in their contribution to the O− channel.

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This shows that care must be taken in interpreting the DEA data on methylation of hydrogen sites in molecules. V. SUMMARY AND CONCLUSIONS

To summarize, we have measured the absolute partial cross section for DEA to acetone. H− is found to be the most dominant fragmentation channel followed by O− . The kinetic energy and angular distribution of these fragments indicate many body break-up. The H− angular distribution closely resembles that from methane which is a characteristic dissociation pattern for the C–H bond breakage in any organic molecule. It also indicates the possible two step dissociation of the parent anion where the first step leads to the formation of the hydride ion. The O− angular distribution is found to be distinctly different from that in acetaldehyde. Also there is no correlation found in the resonances leading to DEA in this channel between the two molecules. This indicates that the methylation of acetaldehyde substantially changes the resonance characteristics and care must be taken in comparing the data for DEA from compounds where a hydrogen atom is substituted by a methyl group. ACKNOWLEDGMENTS

We acknowledge the technical support of Mr. Satej Tare and Mr. Yogesh Upalekar. We also thank Dr. T. S. Ananthakrishnan (BARC) and Professor S. V. K. Kumar for the data acquisition software. We thank Dr. Sajeev Yashodharan (BARC) for fruitful discussions. 1 B.

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