Dissociation dynamics in the dissociative electron attachment to ammonia molecule
DDissociation dynamics in the dissociative electronattachment to ammonia molecule
Dipayan Chakraborty † , Aranya Giri ∗ and Dhananjay Nandi †† Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India ∗ National Institute of Science Education and Research Bhubaneswar,HBNI, Jatni, 752050, India email: [email protected], [email protected], [email protected] Abstract
Complete dissociation dynamics of low energy electron attachment to ammonia molecule has beenstudied using velocity slice imaging (VSI) spectrometer. One low energy resonant peak around 5.5 eV anda broad resonance around 10.5 eV incident electron energy has been observed. The resonant states mainlydissociate via H − and NH − fragments, though for the upper resonant state, signature of NH − fragmentsare also predicted due to three body dissociation process. Kinetic energy and angular distributions ofthe NH − fragment anions are measured simultaneously using VSI technique. Based on our experimentalobservations, we find the signature of A symmetry in the 10.5 eV resonance energy whereas, the 5.5 eVresonance is associated with the well known A symmetry. Inelastic electron-molecule collisions lead to produc-tion of ions and neutral fragments. Dissociative elec-tron attachment (DEA) is a process in which low en-ergy electron is resonantly captured by the moleculeand a temporary negative ion state (TNI) is formed.Subsequently, the resonance state decays into anionand neutral fragment(s). DEA is a topic of interestthese days and has been studied by different groupsfor several molecules [1, 2, 3, 4]. For example, inter-action of high energy radiation with DNA produceslow energy secondary electrons, which causes dam-age to living cells (like single-double strands breaks,DNA-protein cross-links, Mutation, Apoptosis, etc)via DEA to DNA and to its surrounding molecules[5, 6, 7, 8]. This has provided impetus to rigorousstudies on DEA to DNA and respective biomolecules[9, 10, 11, 12]. Ammonia (NH ) is certainly an es-sential component for many biological and chemicalprocesses. It is the source of nitrogen for plants (apart of nitrogen cycle), therefore approximately 83 %of industrial ammonia is used for the production of fertilizers, and it also serves as a raw material formaking explosives and cleaning fluids. At the cellu-lar level, its ions are present in nucleic acids. Thetoxic effect of ammonia can be seen in all animalswhere it causes neurological dysfunctions [13]. In In-terstellar medium, it is found in the dense molecularclouds of a galaxy [14] and in grain surfaces [15, 16].It is one of the simplest molecules considered whilesimulating for the production of amino acids in inter-stellar ice which also gives answers to the generationof life on earth [17].The study of DEA to ammonia dates back to 1969when Sharp and Dowell [18] and Compton et al. [19]confirmed the two resonances for DEA to ammonia,both producing H − and NH − ions. At the higherresonance, the presence of the NH − ion with com-paratively lower cross-section was also observed. Thecross-sections measured by these two groups differ bya factor of 2 for a particular resonant state anions.Later Rawat et al. [20] reconfirmed that the reso-nant states occurred at 5.5 eV and 10.5 eV incidentelectron energy. The authors also measured the abso-lute cross-sections for both the resonance state anions1 a r X i v : . [ phy s i c s . a t m - c l u s ] M a y igure 1: Excitation functions of NH − ions obtained from DEA to ammonia. The former one is obtainedby using the VSI spectrometer whereas, the second one is from the cross-section spectrometer.using relative flow technique and the reported valuesfor NH − ion for both the resonances are 1.6 × − cm and 0.09 × − cm respectively.In 1986, Burrow et al. [21] analyzed the measure-ments with group theory and suggested that the pla-nar and non-planar dissociation of the lower resonantstate results in H − and NH − decay channels respec-tively. They also predicted the umbrella mode of os-cillation during dissociation. Later, Ram et al. [22]used VSI technique to study the angular distribution(AD) and kinetic energy (KE) distribution of frag-ment anions. The variation in AD of H − ions showedthat electron attachment preferred particular orien-tation of ammonia molecule with C axis along theelectron beam direction (from N side to H side). Thepresence of an umbrella mode of oscillation in lowerresonant state was confirmed by the observed varia-tion in AD of the fragment ions with kinetic energy.Using thermochemical and photo-ionization values,the authors predicted several dissociation channelswith their respective threshold energies. Using theAD measurements the authors obtained the symme- try of the TNI state involved in the process as A and E for the lower and higher resonance respectively.Recently, Rescigno et al. [23] theoretically predictedthat at lower resonance, NH − ion is produced bythe same H − ion dissociation channel but throughan intermediate virtual state where charge is trans-ferred adiabatically at a large internuclear distance.But, in upper resonance there is no mechanism foundwhich is responsible for the formation of NH − ion. Inpresent case, the VSI images have been taken aroundthose resonances to measure the kinetic energy andAD of the fragment ions. From AD measurements,symmetry of the two resonant states is determined. Details of the experimental setup used for veloc-ity slice images (VSI) are present in different pa-pers [2, 24]. In the present context we will discussit briefly. A magnetically collimated pulsed elec-tron beam of 10 kHz repetition rate is producedvia thermionic emission process from a tungsten fila-2igure 2: Time sliced images taken with a 25 ns time window for NH − formed due to DEA to ammoniamolecules for three different electron energy around lower resonance. Electron beam axis is from left to rightshown by a red arrow. X-Y direction corresponds to respective momentum in that direction.ment with energy resolution around 0.8 eV. This elec-tron beam is made to interact perpendicularly withan effusive molecular beam. When low energy elec-trons collide with the molecules, negative ion New-ton spheres are formed. These Newton spheres areprojected to the micro channel plate (MCP) basedtwo dimensional position sensitive detector (PSD)[25] by applying moderate extraction field (2 V/cm).The spectrometer is designed to maintain the velocitymap imaging (VMI) condition i.e. all the ions formedin the interaction region with a given velocity willmap on to a single point on the detector. AD and KEdistribution of the fragment ions are obtained fromthe projections of the Newton spheres. The extrac-tion pulse duration is 2 µ s and is applied 100 ns afterthe electron beam pulse. This delayed extraction pro-vides sufficient time to expand the Newton sphere sothat better time sliced images are extracted. MCP isused to detect the time-of-flight (TOF) of the frag-ment ions and PSD records the corresponding x andy positions. These x and y positions give momentuminformation along that direction and the TOF givesthe z-momentum. Using CoboldPC software one canrecord the x and y positions with corresponding TOFfor off-line analysis.To obtain the ion-yield curve, the MCP signal isamplified by a fast amplifier (FAMP) then fed toa constant fraction discriminator (CFD). This CFD signal provides the stop signal as input to the time-to-amplitude converter (TAC). The start signal isprovided by the pulse generator and is synchronizedwith the electron gun pulse. Time difference betweenthe start and stop signal determines the TOF of thefragments. Output of the TAC is connected to amultichannel analyzer (MCA). Number of ions hit-ting the detector is measured by using MCA. Out-put of the hexanode signals again passes through theFAMP and CFD before it is collected by a time-to-digital converter (TDC) which is directly connectedto a computer. Details of this data acquisition systemare present in a different paper [26]. The aim is tofind the central slice of the Newton Sphere, as it con-tains the kinetic energy and AD information. Duringoff-line analysis, suitable time window is used to se-lect the central one. In the present case 25 ns timewindow is used for the lower resonance whereas, 50ns is used for the higher one. Calibration for the ki-netic energy distribution measurements has been per-formed using the kinetic energy released by O − / O at 6.5 eV [27]. Further this energy calibration hasbeen checked by measuring the kinetic energy of theO − ion produced by DEA to CO [2, 28] at 8.2 eV.3igure 3: Schematic to represent the Feshbach res-onance occurred in the NH molecule around 5.5eV. The blue shaded circles represent the electronspresent in the parent molecular state, and the blackcircle represents the incoming electron. Here the in-coming electron loses its kinetic energy to excites the3a electron, and both are captured in the 4a state.Hence the symmetry of the resonance state is A . v point group The angular distribution of the DEA process is di-rectly related to the symmetry of the TNI state. Thedependence of the DEA cross-section of a diatomicmolecule as a function of dissociating angle is nicelydescribed by O’Malley and Taylor [29]. Based ontheir work, Azria et al. [30] expanded the expressionfor polyatomic molecules.Ammonia belongs to C v point group symmetry. AC v point group has six symmetry operations, iden-tity (E), rotation of 60 ◦ with respect to C axis (C ),rotation of 120 ◦ with respect to C axis (C ) andthree reflections about three mirror planes formed bythe three NH bonds and the C axis. Based on thesimilarities of the operations, there are three symme-try states associated with the C v point group. Thesymmetries are A , A and E. Here A and A areone dimensional representation whereas, E is two di-mensional representation. The ground state configu- ration of NH molecule is A so, the transition ampli-tude from A to A , A and E final state transitionis calculated by considering various partial waves as,A l = (cid:104) Resonant state | Partial wave | Initial state (cid:105) . (1)Here the partial wave denotes the different partialwaves of the incoming electron involved in the tran-sition. This transition amplitude squared and inte-grated over the azimuthal angle to obtain the varia-tion of DEA cross-section with scattering angleI( θ ) = 12 π (cid:90) π | A l | d φ (2)Dissociation occurs in the molecular frame whereas,the measurements of the angular distribution is car-ried out in the lab frame. So, molecular frame to labframe transformation of the partial waves for boththe incident electron beam and the electronic statesis done by the Euler angles ( φ , θ , 0) and (0, β , 0) re-spectively. Here β is the angle between the NH bondand the C axis of NH molecule. In general, thevalue of β is 68.2 ◦ , which is used in the present calcu-lations. The character table for the C v point groupalong with the symmetry states with correspondingbasis functions (described by spherical harmonics)are shown in Table 1. For example, the ground statesymmetry of ammonia molecule given by A can beexpressed by the basis function Y . One can alsoincorporate more than one partial wave by introduc-ing the phase factor between them. The partial waveapproximation used here assumes that the axial re-coil approximation is valid, i.e., the dissociation takesplace on a time scale before the molecule could un-dergo rotation or structural changes. The expressionfor the A to A final state transitions with threelowest partial waves (s+p+d) and A to E final statetransitions with two lowest partial waves are givenbelow [31]4able 1: Character Table for C v symmetry group and their respective basis functionI 2C σ v Basis FunctionA l, ; l = 0,1,2...A , + Y , − E 2 -1 0 (Y l, − , − Y l, ); l=1,2,3...I A s+p+d ( θ ) = a + a (cid:0) sin β sin θ + 2 cos β cos θ (cid:1) + a (cid:0) sin β sin θ + sin β sin θ (cid:1) + a (cid:0) β − (cid:1) (cid:0) − (cid:1) + 4 a a cos β cos θ cos δ + 2 a a
34 sin β sin 2 β sin θ sin 2 θ + a a cos β (cid:0) β − (cid:1) cos θ (cid:0) θ − (cid:1) cos δ + a a (cid:0) β − (cid:1)(cid:0) − (cid:1) cos( δ + δ )(3)I Ep+d =2 b (cid:0) cos β sin θ + 2 sin β cos θ (cid:1) + 32 b (cid:0)
14 sin β sin θ + cos β sin θ (cid:1) + 34 b sin β (3 cos θ − + 2 b b √ β cos 2 β sin θ sin 2 θ cos δ + 2 b b √ β sin 2 β cos θ (3 cos θ −
1) cos δ (4)All the AD data present in this report are fitted usingthese two equations. When low energy electron collides with the NH molecule, then it resonantly captured by the moleculeand forms one temporary negative ion state (TNI),which dissociates via three possible dissociation chan-nels, forming three different negative ions H − , NH − Figure 4: The unweighted kinetic energy distribu-tion of NH − ions around lower resonance state forthree different electron energies represented by dif-ferent color.and NH − :NH + e − → (NH − ) ∗ → H − + NH NH − + H + HNH − + H (5)Excitation functions of NH − ions obtained fromDEA to NH are shown in Fig. 1. The ion yieldcurves are measured using two different spectrome-ters. First one is obtained from the VSI spectrometerwhereas, the second one is obtained using the cross-section spectrometer [32]. In the first ion yield curve,the two resonant peaks are overlapped. The possi-5le reason behind this observation could be the poormass resolution of the VSI spectrometer. In higherresonance, two different fragment anions (NH − andNH − ), are resulting into two broad resonant peaksaround 10 eV and 10.5 eV respectively [23]. In thesecond ion yield curve, one can observe two distinctresonant peaks which confirm the presence of tempo-rary negative ion (TNI) states around those energies.To know the kinetic energy and AD of the fragmentions, VSI images are recorded at six different incidentelectron energies around the two resonances (Fig. 2and 6). The incident electron beam axis is from left toright in each image as indicated by a red arrow. For4.5 and 5.5 eV images (Fig. 2), one can observe twodifferent lobes perpendicular to the electron beam di-rection whereas, for 6.5 eV energy the two lobes arenot prominent. This indicates the possibility of a dif-ferent dissociation mechanism involved at this energy.In the previous studies by Rescigno et al. [23] andRam and Krishnakumar [22], there is a discrepancyin the 5.5 eV image. Our observation at 5.5 eV en-ergy agreed with the Rescigno’s measurement. Fromthe higher resonance images, one can see that theNH − ions are formed mostly in the forward directionof the incident electron beam. A similar observationwas made by Rescigno et al. where the authors foundthe H − momentum distribution and NH − momen-tum distribution to be exact mirror images. Fromthis observation, the authors concluded that both thefragments are produced from the same resonant statewhere the negative charge is transferred from the H − anion to the NH fragment at a large internuclear dis-tance. The two resonances, their corresponding dis-sociation channels, the kinetic energy of the fragmentions and the possible symmetry of the TNI states arediscussed below. The ground state electronic configuration of ammoniamolecule is 1a , resulting A symmetry.It is already documented that 5.5 eV resonance is aFeshbach resonance where the incoming electron losesits energy to excite the occupied 3a valence electronand simultaneously gets captured along with excited Figure 5: Angular distribution of NH − ions (anglewith reference to direction of electron beam axis) fit-ted with (a) A to A transition, taking s, p, d partialwaves, (b) A to A +E transition, taking s, p, d andp, d partial waves for A and E states respectively.6able 2: Fitting parameters for the angular distribution of NH − ion at lower resonance A −→ A transition.4.5 eV 5.5 eV 6.5Weighting ratio ofdifferent partial wavesa :a :a ) δ s − p , δ s − d (rad) 1.65, 3.0 1.55, 3.07 2.17, 1.68R value 0.96 0.91 0.82Table 3: Fitting parameters for the angular distribution of NH − ion at 6.5 eV for A −→ A + E transition.Weighting ratio Phase Phase R of different difference (A ) difference (E) valuepartial wavesa :a :a δ s − p , δ s − d (rad) δ p − d (rad):b :b or-bital (Fig. 3). The dissociation channels produceboth H − and NH − ions [19, 22, 23] in a ratio of 6:4as mentioned by Rescigno et al. [23]. At present wewill focus on NH − ion dissociation channel only. Kinetic energy distribution
Kinetic energies of the fragment ions obtained fromthe VSI images are proportional to its radius. So, inorder to find the distribution, one should integratethe ion counts over the entire 2 π angle and plot itwith respect to the energy. Fig. 4 shows the ki-netic energy distribution of NH − ions, where a peakaround 0.06 eV is observed. The constant kineticenergy peak, which is broad in nature with increas-ing electron energy, reflects internal excitation of theNH − and H fragments. Though, poor electron gunresolution doesn’t allow us to separate different ki-netic energy bands. The experimentally obtained ki-netic energy values of anions are compared with the thermochemical values derived from the given expres-sion KE NH − = (cid:16) − m M (cid:17) [V e − (D − A + E ∗ )] (6)Here m is the mass of the NH − fragment, M is themass of the NH molecule, V e is the incident electronenergy, D is the NH -H bond dissociation energy, A isthe electron affinity of NH atom and E ∗ is the inter-nal energy of the H atom. From literature, D=4.60eV [33], A= 0.77 eV [34] and if we consider the H neu-tral fragments formed in the ground state, then thethermodynamic threshold of the dissociation channelis 3.83 eV. This dissociation channel was previouslyobserved by Sharp and Dowell [18] :NH (A )+e − −→ NH −∗ (A ) −→ H( S)+NH − ( A ) . From Fig. 4, the NH − ion kinetic energy peak canbe observed at 0.06 eV for 5.5 eV resonance. Thustotal kinetic energy release (KER) during the processis (total KER =17 times the kinetic energy of NH − − ions formed due to DEA to ammoniamolecules for three different electron energy around upper resonance state. Electron beam axis is from leftto right shown by red arrow. X-Y direction corresponds to respective momentum in that direction.ions) 1.02 eV. This indicates that at this resonanceNH − ions are produced through the above mentioneddissociation channel, where the threshold is 3.83 eV. Angular distribution
Fig. 5 shows the AD of NH − ions, extracted fromthe VSI images for 4.5, 5.5 and 6.5 eV electron en-ergy. For all the incident electron energies, ions witha kinetic energy range between 0-0.15 eV are consid-ered for the AD measurements. The angle is definedwith respect to the incident electron beam direction.With close inspection, one can observe that most ofthe ions are concentrated within 50 ◦ to 150 ◦ for allthe energies. The VSI images are anisotropic and arefound to be same as the LBNL and Heidelberg ex-perimental VSI images. It was believed that in thisenergy region, separate dissociation channels result-ing both H − and NH − ions are present until Rescignoconcluded that NH − ions are formed due to an adia-batic charge transfer from H − to NH at large inter-nuclear distance, i.e., through a virtual-state channel[23]. This makes the AD of NH − as the AD of H − reflected through 90 o from the electron beam axis( θ NH − = 180 ◦ − θ H − ). We took the AD of H − fromthe measurements of Ram [22] and Rescigno [23] for 4.5 eV and 5.5 eV as a reference and compared itwith our NH − AD curve. At 4.5 and 5.5 eV energies,the AD of H − ion peaked at 85 o and 70 o respectively.Thus in the present measurements for 4.5 eV and 5.5eV incident electron energy θ NH − is equal to 95 ◦ and120 ◦ , confirming the virtual-state channel. Fig. 5also shows that when electron energy increases, back-ward scattering increases, which becomes more dom-inant at higher energy. The observed broad AD canbe explained due to the umbrella mode vibration ofthe TNI state present in this resonance [21, 22]. TheAD results can be discussed further with respect tothe basic structure of ammonia molecule. It has apyramidal shape with the N atom situated at the topand three H atoms at the base. The basic symmetryof the molecule is C v where the C axis is passingthrough the N atom and the center of the triangleformed by the three H atom. The angle between theNH bond and the C axis is 68.2 ◦ and the 3a or-bital which is excited during this 5.5 eV resonancehas the electron density distributed up and down theN atom. Now during the dissociation of the TNI,preferential direction for the higher energetic H − ionis along the N-H bond axis. From the AD measure-ments it is observed that the NH − ions are peakingat 120 ◦ direction i.e. the H − ion is at 70 ◦ (close to8igure 7: The unweighted kinetic energy distributionof NH − ions around the upper resonance state forthree different electron energies.68.2 ◦ ). This clearly implies that the preferential ori-entation of the ammonia molecule during the electronattachment process is along the C axis, from N to Hdirection.To know the symmetry of the associated TNI state,the AD data is fitted with the theoretical expressionas discussed in section 3. Fig. 5 (a) shows the fittedAD curve for A to A transition. It can be observedthat the fitted AD curve is enough to claim that thesymmetry of the resonant state involved is A . Slightdeviation is observed for the 6.5 eV energy. To inves-tigate the possible involvement of any other symme-tries, data points are fitted with A to A +E transi-tion model, which provides a better-fitted AD curvefor 6.5 eV energy. Thus one can predict involvementof E symmetry state around 6.5 eV energy region.Expression 4 represents A to E transition model forthe lowest two partial waves [31]. The values of dif-ferent parameters used in the fit function are listedin Table 2 and Table 3 with corresponding R values. The dynamics involved in the higher resonance isnot as simple as the lower one. To describe thedynamics involved in this resonance process, Ramand Krishnakumar [22] compare it with the VUVabsorption and photo-electron spectrum [35, 36] ofammonia molecule where 1 e → sa Rydberg tran-sition occurred at 10.6 eV energy. This result leadsthem to think that it is a Feshbach resonance wherethe HOMO-1 valence 1 e electron excites and simul-taneously two electrons are captured in the LUMO4a orbital. As a result, the symmetry of the res-onance state involved in the process is E. But fromthe NH − ion AD behavior, the authors could not findany robust signature of E symmetry in the resonantstate. This contrast between the understanding andthe experimental observation is described by the au-thors. The double degeneracy of the 1e orbital couldbe manifested as Jahn-Teller effects or other non-adiabatic effects which lead to the rapid distortionof the molecular geometry. As a result, the AD datadoes not clearly reflect the E symmetry involved inthe process. Now, if the distortion of the moleculargeometry is the reason, then both H − and NH − ADshouldn’t reflect the E symmetry. But, their H − ADreflects the involvement of E symmetry at the sameresonance energy. Later, in the theoretical study byRescigno et al. [23], it was found that axial recoilapproximation breakdown is less severe in this reso-nance as there is no barrier to direct dissociation. Intheir experimental and theoretical study, the authorsconfirmed that H − + NH dissociation channel occursdue to the involvement of E symmetry state whichis in agreement with the present understanding. Butthey are unable to locate any dissociation channel re-sulting to NH − ions if the symmetry of the TNI stateis E. Hence they termed the presence of the NH − ionsin the upper resonance state as a mystery. So, thedynamics of the upper resonance state in ammonia isstill an open question with its symmetry and possibledissociation channels. To address this problem, wetook the study of resonance enhanced multi-photonionization (REMPI) spectrum obtained by Langford et al. [37]. Here the authors found a 1a (cid:48)(cid:48) → (cid:48)(cid:48) Ry-9berg transition to occur within the energy range 9.5to 10.1 eV. The authors represent the states in C s symmetry and the A state in C s symmetry will beeither A or E symmetry on making a comparativesolution comparing the point group C s to C v . ThisRydberg state can be a parent state for the Feshbachresonance that occurred within this energy region.As a consequence one electron from the valence 3 a is excited to a higher a orbital and captured alongwith the incoming electron. As a result, the symme-try of the TNI state will be A , which can be a parentstate for the NH − dissociation channel. Our experi-mental observation clearly shows the involvement ofA symmetry in this resonance. Kinetic energy distribution
Fig. 7 represents the kinetic energy distribution ofthe fragment ions formed around 10.5 eV resonanceenergy. For 9.5, 10.5 and 11.5 eV, one small peak ataround 0.2 eV is observed. The broad distributioncan be in due to the internal excitation of the NH − ions. Sharp and Dowell [18] speculated that NH − ionproduced in this energy range is in its first electron-ically excited state. Though, dissociation channelsfor ground state NH − ions were also predicted in thisresonance energy [22]. By using the thermochemi-cal values in Equation (6), the maximum kinetic en-ergy for ground state NH − ion is found to be 0.4 eV.But observed maximum kinetic energy, in this case,is about 0.55 eV, which is within the electron gun en-ergy resolution range. Another possible contributionto kinetic energy distribution is due to the presenceof NH − ions. With respect to this, if NH − ions areformed with 0.2 eV kinetic energy, total kinetic en-ergy release by the process will be 1.7 eV. This clearlyindicates the presence of H + H + NH − ( Π) chan-nel, whose thermodynamic threshold is 8.08 eV [22].Thus from kinetic energy measurements, one can pre-dict the presence of a three-body dissociation channelin this resonance however, due to poor mass resolu-tion capability of the VSI spectrometer we are unableto separate NH − and NH − fragments. Figure 8: Angular distribution of NH − ions (anglewith reference to direction of electron beam axis) fit-ted with (a) A to E final state transition model, con-sidering lowest two (p and d) partial waves, (b) A toA final state transition model with lowest three (s,p, d) partial waves, (c) A to A +E final state tran-sition model, taking s, p, d and p, d partial waves forA and E states respectively.10igure 9: Schematic to represent the Feshbach resonance occurred in the NH molecule. The blue shadedcircles represent the electrons present in the parent molecule, and the black circle represents the incomingelectron. At 10.5 eV resonance, two different excitation channels are present. The former one is throughthe excitation of an 1e electron and subsequently capture of two electrons to the 4a orbital, resulting anE symmetry of the resonant state (This channel has been observed previously). Whereas the latter one isthrough the excitation of an 3a electron and capture of two electrons into a higher a orbital. Hence, theresulting symmetry of the resonant state is A . The E symmetry state is responsible for the formation ofH − ions whereas, the A symmetry state is responsible for the formation of NH − ions.Table 4: Fitting parameters for the angular distribution of NH − ion at upper resonance A −→ A transition.9.5 eV 10.5 eV 11.5Weighting ratio ofdifferent partial wavesa :a :a ) 0.34,2.37 0.61,1.60 0.74,1.45 δ s − p , δ s − d (rad)R value 0.95 0.97 0.93 Angular distribution
To know the symmetry of the associated resonancestate, AD of the NH − ions is extracted from the VSIimages. From the AD data it can be observed that for9.5 and 10.5 eV energies, most of the ions are formedin the forward direction. The AD data is fitted withthe procedure as discussed in Section 3. Followingthe same procedure, we fit our AD data for A → A , A → E and A → A +E final state transition.The results for the A → E and A → A finalstate transitions are shown in Fig. (8a) and (8b).Here the lowest two partial waves for E final statetransition and lowest three partial waves for A fi-nal state transition are considered because the con-tributions of the higher partial waves are increasinglysmall. From the fitted AD curve, it is clear that A → − ion at 9.5 eV for A −→ A + E transition.Weighting ratio Phase Phase R of different difference (A ) difference (E) valuepartial wavesa :a :a δ s − p , δ s − d (rad) δ p − d (rad):b :b → A final state transi-tion gives us a better fit with good R value (over0.9) which clearly indicates that A state is presentin this resonance. From the current experimental un-derstanding and from the previous studies we proposethat two closely lying resonant states with symmetryA and E are present within this 10.5 eV resonance.The H − ions are formed due to the E symmetry statewhereas, the NH − ions contribution came mainly dueto the A symmetry state. It will be interesting to seewhether the A symmetry is also responsible for theH − ions. But with the current experimental facilities,it is not possible to detect the H − ions. The fittedAD data for A → A + E final state transition is alsoshown in Fig. (8c), which is almost the same as Fig.(8b). Hence the E state contribution can be ruledout. Only for 9.5 eV energy, slightly better-fittedAD curve is observed for A → A +E transition.This little contribution from E state can be in dueto the presence of NH − ions via three body dissocia-tion process, which is possible in this energy region.The fitting parameters for A → A and A → A +Etransition are given in Table 4 and 5. Theoreticalcalculations by Dr. P. C. Minaxi Vinodkumar (pri-vate communications) confirms the presence of A and/or E state around 10.2 eV energy region which,further supports our conclusion [38]. It is to be men-tioned here that, the dissociation dynamics of ammo-nia molecule in higher resonance is complex. Whenthe Rydberg transition occurs, the NH molecule nolonger holds the C v symmetry. So the resonancestarts with a C v geometry before it goes to some other symmetry. A high-level time-dependent the-oretical calculation is imperative to understand thedynamics properly. Complete DEA dynamics of ammonia molecule isstudied using VSI technique. Two resonances around5.5 eV and 10.5 eV are observed. The VSI imagesof NH − fragment ions are measured around the 5.5eV and 10.5 eV resonance energies. KE distributionand AD of the NH − ions are extracted from the sliceimages. From the KE and AD measurements, we re-confirm the presence of A symmetry in the 5.5 eVresonance energy. KE distribution of the 10.5 eV res-onance indicates the involvement of three body dis-sociation process. Our AD measurements clearly in-dicates the presence of A symmetry state in the 10.5eV resonance. D. N. gratefully acknowledges the partial finan-cial support from “Science and Engineering Re-search Board (SERB)” under the project No.“EMR/2014/000457”. DC is thankful to IISERKolkata for providing research fellowship.
References [1] P. Nag and D. Nandi, Phys. Chem. Chem. Phys. , 7130 (2015).122] P. Nag and D. Nandi, Phys. Rev. A , 052705(2015).[3] F. H. ´Omarsson, E. Szymanska, N. J. Mason,E. Krishnakumar, and O. Ing´olfsson, Phys. Rev.Lett. , 063201 (2013).[4] E. Illenberger and J. Momigny, Gaseous Molec-ular Ions: An Introduction to Elementary Pro-cesses Induced by Ionization
Topics in PhysicalChemistry (Steinkopff, 2013).[5] B. Boudaıffa, P. Cloutier, D. Hunting, M. A.Huels, and L. Sanche, Science , 1658 (2000).[6] A. Keller et al. , Sci. Rep. , 7391 EP (2014).[7] E. Alizadeh, T. M. Orlando, and L. Sanche,Annu. Rev. Phys. Chem. , 379 (2015).[8] X. Luo, Y. Zheng, and L. Sanche, J. Chem.Phys. , 155101 (2014).[9] X. Pan, P. Cloutier, D. Hunting, and L. Sanche,Phys. Rev. Lett. , 208102 (2003).[10] H. Abdoul-Carime, P. Cloutier, and L. Sanche,Radiat. Res. , 625 (2001).[11] D. Antic, L. Parenteau, M. Lepage, andL. Sanche, J. Phys. Chem. B , 6611 (1999).[12] B. Liu, P. Hvelplund, S. Brndsted Nielsen, andS. Tomita, J. Chem. Phys. , 4175 (2004).[13] V. Rangroo Thrane et al. , Nat. Med. , 1643EP (2013).[14] A. Cheung, D. M. Rank, C. H. Townes,D. D. Thornton, and W. J. Welch, Phys. Rev.Lett. , 1701 (1968).[15] J. A. Nuth, III, S. B. Charnley, and N. M. John-son, Meteorites and the Early Solar System II (,2006).[16] R. Briggs et al. , Orig. Life Evol. Biosph. , 287(1992).[17] G. M. Mu˜noz Caro et al. , Nature , 403 EP(2002). [18] T. E. Sharp and J. T. Dowell, J. Chem. Phys. , 3024 (1969).[19] R. Compton, J. A. Stockdale, and P. W. Rein-hardt, Phys. Rev. X , 111 (1969).[20] P. Rawat et al. , J Phys: Conf. Ser. , 012018(2007).[21] K. L. Stricklett and P. D. Burrow, J. Phys. B , 4241.[22] N. B. Ram and E. Krishnakumar, J. Chem.Phys. , 164308 (2012).[23] T. N. Rescigno et al. , Phys. Rev. A , 052704(2016).[24] D. Chakraborty, P. Nag, and D. Nandi, Phys.Chem. Chem. Phys. , 32973 (2016).[25] O. Jagutzki et al. , IEEE Trans. Nucl. Sci. ,2477 (2002).[26] P. Nag and D. Nandi, Meas. Sci. Technol. ,095007 (2015).[27] D. Nandi and E. Krishnakumar, Int. J. MassSpectrom. , 39 (2010).[28] D. S. Slaughter et al. , J. Phys. B: At. Mol. Opt.Phys. , 205203.[29] T. F. O’Malley and H. S. Taylor, Phys. Rev. , 207 (1968).[30] R. Azria, Y. L. Coat, G. Lefevre, and D. Simon,J. Phys. B , 679.[31] N. B. Ram, Dissociation dynamics in polyatomicmolecules due to electron attachment
Ph.D the-sis, Tata Institute of Fundamental Research (,2010).[32] D. Chakraborty, P. Nag, and D. Nandi, Rev.Sci. Instrum. , 025115 (2018).[33] D. H. Mordaunt, M. N. R. Ashfold, and R. N.Dixon, J. Chem. Phys. , 6460 (1996).1334] C. T. WickhamJones, K. M. Ervin, G. B. Elli-son, and W. C. Lineberger, J. Chem. Phys. ,2762 (1989).[35] A. D. Walsh and P. A. Warsop, Trans. FaradaySoc. , 345 (1961).[36] M. B. Robin, Higher excitedd states of poly-atomic molecules (Vol. 1) (Academic, 1974).[37] S. R. Langford et al. , J. Chem. Phys. , 6667(1998).[38] P. C. M. Vinodkumar,