Electron Impact Induced Fragmentation of N2H+ and N2D+ - The

Oct 21, 2014 - King Abdulaziz City for Sciences and Technology (KACST), Riyadh 1442, Saudi Arabia. ‡ IPR, Université de Rennes I, UMR No. 6451 du C...
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Electron Impact Induced Fragmentation of N2H+ and N2D+ M. O. A. El Ghazaly,*,†,∥ J. B. A. Mitchell,‡ J. J. Jureta,§,∥ and P. Defrance∥ †

King Abdulaziz City for Sciences and Technology (KACST), Riyadh 1442, Saudi Arabia IPR, Université de Rennes I, UMR No. 6451 du CNRS, 35042 Rennes France § Institute of Physics, University of Belgrade, 11081 Belgrade, Serbia ∥ Institute of Condensed Matter and Nanosciences, Chemin du Cyclotron, Catholic University of Louvain, B-1348 Louvain-la-Neuve, Belgium ‡

ABSTRACT: Electron impact dissociation of protonated and deuterated nitrogen ions has been studied using a crossed beams apparatus. Absolute cross sections for dissociation channels producing N+ and NH+, respectively, are presented. The observations of subthreshold signals in these measurements indicate the presence of ro-vibrationally and possibly electronically excited states in the parent ions. Comparisons with other measurements are given.

remeasurement at CRYRING10 that has found agreement with the afterglow work. Indeed, the problem arose due to contamination of the original experiment by 14N15N+ ions. This can be a significant problem when studying N2H+ as the isotopic abundance of 15N+ is not negligible (0.37%). The astrophysical importance of N2H+ has been discussed in detail in ref 3, and recent theoretical examination of this process has been reported.11 It is an important ion as it can readily participate in proton transfer reactions with other molecules thus being a central player in the chemistry of nitrogen rich environments. It has also significance in terrestrial plasmas having been identified as a component of industrial processing plasmas.12 The ground state surface of the molecule has a minimum potential energy at R(N−N) = 2.977 au and R(N2−H) = 2.080 au with a calculated dissociation energy of 5.466 eV.13 Potential energy surfaces for ground and excited states have been calculated by Vasudevan et al. 14 and Gianturco and collaborators.15,16 Depending upon the energy of the electron, the fragmentation of N2H + can proceed via a number of channels with associated energy thresholds listed in Table 1 using the same values as in ref 17, assuming, as there, that the system starts from the ground ro-vibrational level. Ionic fragment are produced by dissociative excitation (DE) processes denoted DE1−DE6 and by dissociative ionization (DI) denoted DI1−DI4.

1. INTRODUCTION Electron−ion collisions are of fundamental importance for the physics and chemistry of astrophysical and laboratory plasmas. Indeed, electrons initiate and drive most of the key processes in astrophysics, e.g., in planetary ionospheres and interstellar clouds and in many other plasma environments, e.g., thermonuclear fusion plasmas. Electron impact ionization and dissociation processes have been widely studied in the singlepass experiment at the Catholic University of Louvain, Belgium.1 This device has and can be used for the study of reactions which involve light molecular ions playing an important role in astrophysics and fusion research. Indeed, nitrogenated ions have been recently highlighted by the IAEA, as playing an important role in the edge of fusion plasmas2 where nitrogen is an impurity component. Specific data on such molecular ions has been thus recognized as an urgent need for fusion plasma research purposes. The protonated nitrogen ion, N2H+, is widely observed in interstellar clouds and planetary atmospheres and as such has been the subject of a number of theoretical and experimental studies. In particular, a number of measurements have been made concerning the dissociative recombination of this ion, the first being that of Mul and McGowan3 using a single pass merged beam technique. A significant controversy arose when a second merged beam experiment,4 performed at the CRYRING storage ring, seemed to show that the main product channel for recombination was that involving the rupture of the NN bond leading to N + NH products. This was in total disagreement with afterglow experiments where predictions5 and later direct measurements indicated that N2 + H was the dominant exit channel.6−9 This controversy has finally been resolved by a © 2014 American Chemical Society

Received: August 22, 2014 Revised: October 10, 2014 Published: October 21, 2014 10020

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available in the center-of-mass (E, in eV) for the collision is, with the assumption of mi ≫ me, given by m eE = eVe + e (qiVi − eVe) mi (1)

Table 1. Experimental and Thermodynamic (from Ref 17) Energy Thresholds for Dissociation Channels Due to Electron Impact on N2H+ Ions in Its Ground Ro-Vibrational and Electronic States dissociation channel

process

thermodynamic threshold energy (eV)

threshold of this experiment

N2 + H+ N2+ + H N + NH+ N+ + NH N + N + H+ N+ +N + H N2+ + H+ NH+ + N+ N+ + N+ + H N+ + N+ + H +

DE1 DE2 DE3 DE4 DE5 DE6 DI1 DI2 DI3 DI4

5.12 7.12 11.36 12.42 14.50 15.40 20.70 25 0.4 29.0 43.5

not measured not measured 9 eV 9 eV not measured 16 eV not measured 9 eV 30 eV not well resolved

Here, Ve and Vi, me, and mi are the acceleration voltages, charges, and masses of electrons and ions, respectively, qi is the ion charge, and e is the elementary charge. The electron energy is corrected for the contact potential difference (ΔV), which is determined by accurately measuring the threshold potential for the ionization of argon ions. In the present case ΔV was found to be −5.15 ± 0.5 eV. Ions are extracted from the ECR ion source, focused, and accelerated to 8 keV. The primary ions are selected by means of a newly installed, higher magnetic rigidity, 90° double-focusing magnet,18 and additionally focused and deflected by a 45° spherical electrostatic deflector and directed into the collision region, where it crosses a ribbon-shaped, variable energy, electron beam, at right angles. In order to separate ions produced in the region from those generated by the collisions of the beam with the residual gas along the beamline, electrodes surrounding the interaction region are biased at a positive voltage (V0) so that the energy of positive ions formed inside this region is increased. Ionic products of the reaction are massto-charge selected by means of the 90° double-focusing analyzing magnet, and subsequently deflected by the 90° spherical electrostatic deflector and directed into a channeltron detector. A vertical slit placed at the exit of the analyzing magnet serves to define the mass resolution. The additional laboratory kinetic energy resolution is defined by the geometry of the electrostatic deflector. Ions formed outside the collision region do not undergo a kinetic displacement induced by V0 and so are rejected by the analyzing magnet. The primary ion beam is collected in a wide Faraday cup, located inside the magnetic chamber. 2.2. Cross-Section Measurement and Procedure. In the animated beam method,21 the electron beam is spatially swept across the ion beam in a linear seesaw motion, at a constant velocity u. The total number of events K produced during one sweep of the electron beam across the ion beam is related to the cross section σ by the following expression:

Results have recently been reported for the dissociation of N2H+ ion into the NH+ channel.18 In the experiment, ionic fragments are collected, independently of the initial dissociative process, excitation, or ionization, meaning that the results may correspond to processes DE3 or DI2. Inclusive cross sections are thus determined and a specific procedure was developed in order to obtain the individual contribution of each concerned process. These results were found to be in satisfactory agreement with previous work of the Oak Ridge group in the low energy range.17 The present article is concerned with the fragmentation of N2H+ into the N+ channel, that is, by processes DE4, DE6, DI2, and DI3 (Table 1). This study follows previous electron impact dissociation investigations using the same experimental setup as for two nitrogenated molecular ions,19,20 N2+ and ND+. In the next section, the experimental set up and method are first described. Results are then presented and discussed in subsequent sections.

2. EXPERIMENTAL METHOD AND SETUP The measurements were performed using the single-pass experimental setup with the animated crossed electron−ion beam method.21 This well-tested setup together with the experimental method has successively been improved over the last years.19,22 The merit of these improvements goes to the development of the method of measuring the kinetic energy release distributions (KERD) in the dissociation process.22 A comprehensive description introducing this valuable method is given in ref 23. A review on the upgrade of the experimental setup has been reported elsewhere20 and only a brief outline is presented, here. The key element of the equipment is the analyzing section used to separate the collision products from the target ion beam, which is composed of a 90° doublefocusing magnet in combination with a 90° electrostatic spherical deflector. This improves the separation of neighboring fragments and also reduces the noise by rejecting ions scattered along the beam trajectory in the analysis region. The wide angular acceptance (0.1 rad) generally allows full angular product collection, at a given speed, so that the single-pass crossed beam method can be extended in order to allow the determination of kinetic energy release (KER) distributions of dissociation products.22,23 2.1. Experimental Setup. In this crossed beam experiment, a fixed-energy ion beam interacts at right angles with an electron beam whose energy can be tuned from a few electronvolts up to 2.5. keV. In such a configuration, the energy

σ=

uK AIeIiγ

(2)

where γ is the detector efficiency and Ie and Ii are the electron and ion beam current intensities, respectively. A is a kinematic factor, which for beams interacting at a right angle, is given by A=

(vi 2 + ve 2)1/2 veviqie 2

(3)

Here, e and qi,, and ve and vi are the charges and velocities of electrons and ions, respectively. All slits and apertures between the collision region and the ion detector are dimensioned to provide total ion transmission, in nondissociative ionization experiments. In dissociative processes, the internal energy released to fragments may cause a significant widening of both their angular and the velocity distributions in the laboratory frame. In most cases, such distributions are much wider than those of the primary ion beam or simple ionization products. The angular acceptance of the analyzing magnet allows fragments emitted within a cone of 0.1 rad aperture around the center-of-mass velocity, to be transmitted. This is large 10021

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angle θM appears when it is ejected with the maximum velocity wM oriented perpendicularly to the beam axis: w tan θM = M vc (5)

enough to entirely cover angular distributions up to a maximum kinetic energy release (see below). However, the exit slit of the magnet (ΔR = 8 mm wide in the present experiment) does not allow the total transmission of the velocity distribution. Consequently, the collection of the fragments is not complete and only a fraction η of the product signal is collected, at each velocity, i.e., at each analyzing magnetic field value to yield the so-called “apparent cross-section” σapp(Bo) = ησ

In this expression, vc is the center of mass velocity, which also corresponds to the velocity of fragments dissociating with zero KER. Fragment kinetic energies are in the keV range, and kinetic energy releases are in the eV range, so that θm is usually small, and we can write

(4)

⎛ w ⎞2 k θM 2 = ⎜ M ⎟ = 1 E1 ⎝ vc ⎠

Therefore, at a given electron energy, the apparent cross section σapp is measured. The total absolute cross section covering the full product velocity distribution is determined by systematically varying the analyzer magnetic field and integrating the results. 2.3. Angular Acceptance and Molecular Fragmentation. In a dissociative process, the internal potential energy is transferred into the kinetic energy release of the fragments, which flying apart, form a virtual sphere in the momentum space around the center of mass momentum, the so-called dissociation sphere.22 The distribution of the fragments in the dissociation sphere follows both the angular and the kinetic energy release distributions (KERD) that characterize the reaction dynamics. The angular distribution of the fragments, which is inherent to the internal symmetry properties of the molecule, characterizes the collision with respect to the electron beam direction and may reveal some anisotropy with respect to that direction. However, when many electronic excited states are involved, one may consider that the fragmentation becomes globally isotropic due to compensation between their individual possible anisotropies. For a given dissociation product, the kinetic energy release distribution (KERD) in the center of mass contributes to widen both the angular and the velocity distributions (VD) observed in the laboratory. Estimation of the absolute cross section relies on the assumption that, in the experiment, for any observed velocity, the full angular distribution is collected on the detector, so that numerical integration of the VD can be performed to obtain the absolute cross section. Evidently, this procedure must be restricted to those KERD, which induce, in the laboratory frame, angular distributions with opening cone angle θM lower than the angular acceptance of the analyzing system θa. Let us consider (Figure 1) a molecule of mass M fragmenting into two products of mass m1 and m2, respectively; v1 and v2 represent the velocity of fragment 1 and 2, respectively. For fragment 1, which is to be detected, the maximum ejection

(6)

(m1wM2)/(2e)

The maximum KER (eV) is k1 = and E1(eV) = (m1vc2)/(2e). The above-mentioned angular acceptance condition is met if θM < θa, that is, according to 2:

k1 < θa 2E1

(7)

This relationship is modified, considering that, for fragmentation in two fragments, the total KER KT is m1w12 mw2 M + 2 2 = k1 m2 2e 2e m2 k1 = KT M

KT = k1 + k 2 = or

(8)

Next, the initial total kinetic energy of the molecule (E0, in eV) can be written as E0 =

m1vL 2 mv 2 M + 2 L = E1 2e 2e m1

or

E1 = E0

m1 M

(9)

Finally, eq 7 is expressed with respect to the maximum total KER KM and to E0: m KM < θa 2E0 1 m2 (10) A positive bias voltage is applied to the collision region (see above) to reduce the background signal. This postcollision acceleration increases the fragment kinetic energy by V0 (eV) and, additionally, significantly reduces θM such that the final form of the acceptance condition is written as ⎞ ⎛ m KM < θa 2⎜E0 1 + V0⎟ ⎠ ⎝ m2

(11)

In the present experiment, E0 = 8 keV, V0 = 1000 eV, and θa = 0.05 rad. For the present target (N2H+, M = 29), application of eq 11 gives KM values limited to 3.21, 21.17, 23.93, and 562.50 eV, for fragments H+, N+, NH+, and N2+, respectively. KERD are usually observed to cover the low energy range (0−8 eV) for DE processes, but for DI processes, they may extend up to 15−20 eV due to Coulomb explosion. As an example, Figure 2 shows the apparent cross section recorded for production of N+ and NH+ fragments at the interaction of the N2H+ beam with 100 eV electron beam. The absolute cross section for a specific fragment is obtained by integrating the signal of the corresponding peak, taking into account the width of the analyzing slit. This procedure is repeated at some selected electron energies. In the next step, apparent cross sections are measured at the central magnetic field B0 over the full electron energy range, i.e., from reaction

Figure 1. Vector representation of the fragment velocity in the CM (w) and in the laboratory (v1), with respect to the CM velocity (vc). The fragment velocity is first indicated at an arbitrary ejection angle. It is also shown for the 90° ejection angle, resulting in the maximum transverse velocity (wM) at a given w. 10022

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Figure 2. Apparent cross section for electron impact dissociation of N2H+: formation of 14N+ (left peak) and of NH+ (right peak) as a function of the product analyzer magnetic field.

dσ(E ker) 2μv d ⎛ 1 dσ(v) ⎞ = − 2c ⎜ ⎟ dE ker m dv ⎝ v dv ⎠

threshold up to 2.5 keV. In order to put these data onto the absolute scale, one must evaluate the transmission factor given by expression 4. This factor is first computed at those selected electron energies where magnetic field scans are performed. Next, these data are interpolated in order to obtain the transmission at any electron energy. Finally, the measured apparent cross sections are corrected by means of eq 4 to obtain the absolute cross sections as a function of collision energy. The procedure is repeated separately for each fragment. It is essential to check quickly for the full angular product transmission, at a given electron energy. This can easily be performed by looking to the apparent cross section (Figure2). The limits of the dissociation peak correspond to ejection in the forward and in the backward directions, for fragments with the maximum velocity wM (see Figure 1). As the maximum angle corresponding to full angular acceptance is 2θa, the condition will be met if the ratio ΔB/B0, with ΔB representing the full width of the dissociation peak, being also smaller than 2θa, which takes the value here of 0.1 rad. This condition is readily examined on Figure 2, for both peaks. 2.4. Kinetic Energy Release Distributions. In the assumption of a global isotropy of angular distributions, the KERD can be specifically determined via the magnetic scanning spectrum of produced fragment velocities, and a subsequent transposition of the laboratory data into the molecular centerof-mass frame. El Ghazaly and co-workers22,23 demonstrated that in this case the KER distribution dσEKER/dEKER for a specific fragment, is expressed in terms of the velocity distribution of the measured cross section dσ(v)/dv:

(12)

where m is the mass of the molecular fragment and μ its reduced mass. dσ(v)/dv is computed from the result of the scan. EKER is the total kinetic energy released to dissociation fragments. By assuming the fragmentation of the molecular ion to be binary, this total kinetic energy release EKER is given by

E ker =

m2wm 2 2μ

(13)

where wm represents the projection of w onto the ion beam axis (Figure 1). This method was initiated and used for the first time in this kind of single-pass experiment, to determine the KER distributions in a binary fragmentation of small molecular ions.23 The results were benchmarked against the well-known theoretical models of the simplest prototypical hydrogen molecular ions and their available experimental data, providing thus an excellent validation of such useful method.22 It has been used systematically in this single-pass experiment, to determine the KERD for a wide range of small molecular ions of fusion and astrophysical interests.1,24−28 2.5. Experimental Conditions. Protonated nitrogen ions are generated from a mixture of nitrogen and hydrogen prepared in a gas reservoir at the partial pressure of 800 mbar and 200 mbar for N2 and H2, respectively. This mixture is introduced in the ECR ion source having a power of 300 W and a pressure of 3 × 10−3 mbar measured at the entrance to the ion source. Initially, rather than examining the N2H+ ion, we chose to study N2D+ (mass 30), so as to limit the loss of light 10023

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fragments with high transverse velocities. While this avoids the 14 15 + N N (mass 29) isotope effect, the problem of mass contamination is not solved as it is possible to create NO+ (mass 30) ions in the ion source due to reaction of N2+ with residual water molecules. Measurements of the primary beam intensity were made with and without the addition of deuterium to the source, and it was found that the ratio of (NO+) without deuterium and (NO+ + N2D+) with deuterium was 4%. As we shall show later, this contamination does not present a serious problem in this measurement. The total uncertainties (9%) for these measurements are obtained as the quadrature sum of uncertainties on all measured quantities: counting statistics, sweep speed, electron and ion currents, electron and ion energies, and detector efficiency. The electron energy is corrected for contact potentials, and the error on the collision energy is ±0.5 eV.

3. RESULTS: SIMPLE IONIZATION AND DISSOCIATIVE IONIZATION Ions produced by single ionization (SI) processes leading to the doubly ionized molecular ion leave the collision center with a velocity distribution characteristic of the primary ion beam and thus can appear as a central peak (or top hat) in the apparent cross section profile obtained by scanning the analyzer magnet. Dissociative ionization, however, generally gives rise to fragments with high velocities with respect to the center of mass (due to the Coulomb repulsion between the fragments), and thus, contributions from this process can be seen at the extremities of this profile. By examining this region, it is in principle possible in some cases to separate apparent cross sections for dissociative excitation and those from dissociative ionization.23 Such apparent cross sections are characterized by the presence of a new, higher energy threshold. A benchmark application of this procedure was presented through earlier work on the dissociation and ionization of N2+ molecular ion.19 In many further experiments, this procedure allowed the individual DE and DI contributions to be separated and determined, (see ref 20 and references therein). In some cases, DE and DI clearly appear as sufficiently distinct in the apparent cross section or in the KERD (see, for example, refs 20 and 28), so that it is possible to obtain absolute DI cross sections. It is not the case for N2H+, and so, in this article we report only apparent (and not absolute) cross sections for this process, which were measured for a magnetic field close to the higher limit of the cross section profile. In particular for the N2H+ ion, product ions are formed in an incredible number of vibrational states29 that act almost as a continuum, which makes it difficult to separate processes and electronic states. 3.1. Experimental Results. In initial measurements, the deuterated species N2D+ was studied as the collection of fragments is more easily obtained for the heavier ion. While the ro-vibrational energy distribution for N2D+ is different from N2H+; the electronic energy levels are of course the same. Figure 3 presents absolute cross sections (i.e., corrected by the transmission factor) for the fragmentation of N2D+ and N2H+ ions leading to N+ products, plotted as a function of electron energy. We concentrated our measurements initially on this channel and the deuterated ion, as the products are well separated by the analysis system. The N2+ channel is not easily separated from the primary N2D+ ion beam, while D+ products leave the collision center with a velocity such that those emitted

Figure 3. Absolute cross section as a function of incident electron energy for the formation of N+ from the electron impact dissociation of N2D+ ions and N2H+ produced using isotopically pure 14N14N gas in the ion source.

transversely to the primary ion beam can easily be lost from the detection system. It is seen that there is a threshold in the cross section at about 9 eV followed by a rise to a maximum in the vicinity of 90 eV. The thermodynamic energy threshold for the formation of N+ is in fact 12.42 eV. The first question that can be asked is if this difference between the measured threshold and the thermodynamic value is due to contamination from NO+ ions that could be present in the beam. In order to see if there was an effect due to NO+, we performed a second measurement, but this time for N2H+ and using isotopically pure 14N2 source gas for the production of the ions. The results of this measurement are also shown in Figure 3. What is very striking here is that the data for N2H+ and N2D+ essentially overlap each other. There is a very slight shift at threshold with the N2H+ ions being slightly displaced, but it is clear that the effect of NO+ is negligible. From this test, we can conclude that the difference between the measured threshold (9 eV) and the thermodynamic value (12.4 eV) probably indicates that some fraction of the target ions is formed in long-lived excited states. Above 16 eV, the slope increases, and this might be attributed to the opening up of channel DE6 though it is not possible to distinguish this channel directly from channel DE4 nor to place the threshold accurately Figure 4 shows the N2H+ data along with the results for the + N channel of Fogle et al.17 (which we shall refer to as the Oak Ridge data), and it is seen that there is a clear discrepancy between the two measurements. The threshold in the Oak Ridge data seems to be at higher energy than our threshold, but their data point at 10 eV seems puzzling. What is most striking, however, is the difference in the cross section above 20 eV. The Oak Ridge data levels off to a plateau, while our results display a clear maximum at about 80 eV, with more than a factor of 2 difference in absolute value. It is also evident in Figure 4 that there is a distinct decrease of slope in our data above 20 eV. This would suggest that there is competition due to a new reaction channel opening up at this energy, which, from the list shown above, this is likely to be the channel leading to N2+ + H+. We have not studied the channel leading to N2+ since it is difficult in our apparatus to separate 10024

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Figure 4. Comparison of the absolute cross sections for N2H+ dissociation leading to N+ formation, measured in the current experiment, closed (red) circles, and the corresponding data of Fogel et al.,17 inverted solid (black) triangles.

setting in here, but otherwise, away from threshold, the data points practically overlap. Regarding the threshold, it is seen that the large error bars on the Oak Ridge data point preclude further discussion of any comparison. The thermodynamic energy threshold for this data is at 11.36 eV while our data displays a threshold at about 9 eV. Again, given the above discussion, this indicates the presence of internally excited ions in our primary beam. In addition to dissociative excitation (DE), the process of dissociative ionization (DI) will contribute to charged fragment formation above the respective thresholds for these channels. We have a means of identifying this process and of distinguishing it from contributions from dissociative excitation, as described in the experimental section. Under some circumstance if DI is clearly separate from DE, it is possible to obtain absolute cross sections for this process.23 This was not the case in this experiment and so we can only report apparent cross sections. These are shown in Figure 6 for the case of the DI of N2D+ yielding N+. The threshold for this channel appears at about 30 eV, which is close to what one might expect for the channel DI3 from Table 1. This does not rule out the influence of excited states in the primary ions; however, the upper excited state, leading to this ionization channel, can have a descending slope so that the transition energy is higher than the asymptotic energy. This dissociation channel also produced NH+, and in Figure 5, it is seen that there is a distinct change in the cross section arising at this energy. There seems to be another change of slope above about 40 eV, and this might possibly be attributed to the channel DE4. However, the resolution is not sufficient to say this for certain. The kinetic energy release of the N+ fragments has been determined using the procedure outlined above for interaction energy of 100 eV, and the results are shown in Figure 7. What is striking is that this is clearly dominated by low energy release indicating that indeed the upper transition states leading to the

this product from the primary beam. The Oak Ridge group, however, have published data for this channel, which they found to be dominant, at low energy, and to have a value about 20% less than that for N+ production in the energy range where the latter sets in (see Figure 2 in ref 17). This difference in magnitude of the cross sections for the two measurements is made all the more surprising when data is presented for the channel leading to NH+ as shown in Figure 5. (This data has been presented and discussed in a preliminary paper.18) It is seen here that there is really very good agreement between the two measurements regarding the magnitudes for the cross section. The Oak Ridge data displays a peak that begins to form at about 35 eV, while our data displays a plateau

Figure 5. Absolute cross sections for the formation of NH+ from electron impact dissociation of N2H+. Our data,18 solid (green) circles, and that of Fogel et al.,17 solid inverted (black) triangles. 10025

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Heninger et al.30 who have shown that, under formation conditions similar to those in this work, more than 60% of the ions were excited with energies of at least 0.718 eV. Forty-nine percent of the ions have values of v2 states with energies at least up to 1.3 eV and lifetimes of many tens of milliseconds. Thus, while the shift in the measured thresholds seen in the present work can perhaps be explained by the vibrational excitation of the reacting ions, it is not clear why there should be such differences with the work of Fogle et al.17 It is not impossible that quasi-metastable electronic states are also populated in our beam, and these could be partly responsible for the subthreshold cross-section. Such states have been seen by Moran and co-workers31 in beams of N2+ where states with lifetimes on the order of tens of microseconds were able to survive the passage from the ion source to the target region of their apparatus. Thus, it is not impossible to consider that such states might be present in our beam. We shall examine this further in a subsequent publication where other measurements will be analyzed. It is also our intention to supplement this study with synchrotron radiation measurements of soft X-ray photoionization and photodissociation, to explore this hypothesis further.

Figure 6. Apparent cross section for the formation of N+ due to electron impact dissociative ionization of N2D+, measured at the analyzing field equal to 1214 G, that is, 38 G above the central field B0. According to the initial energy of the molecular beam and taking into account the width of the analyzing slit, this shift indicates that only dissociative ionization processes with KER larger than 6.8 eV are taken into account in the measurement.

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Lecointre, J.; Belic, D. S.; Cherkani-Hassani, H.; Jureta, J. J.; Defrance, P. A crossed-beam experiment for electron impact ionization and dissociation of molecular ions: its application to CO+. J. Phys. B: At. Mol. Opt. Phys. 2006, 39, 3275−3279. (2) Chung, H.-K.; Braams, B. J. Light Element Atom, Molecule and Radical Behaviour in the Divertor and Edge Plasma Regions, Report of the Third Research Coordination Meeting, International Atomic Energy Agency, Vienna, 2013. http://www-amdis.iaea.org/ publications/INDC/INDC_NDS-636.pdf. (3) Mul, P. M.; McGowan, J. W. Dissociative recombination of N2H+ and N2D+. Astrophys. J. Lett. 1979, 227, L157−L159 The values for the cross section and rate coefficient presented in this article should be reduced by a factor of 2 due to a later identified calibration error. (4) Geppert, W. D.; Thomas, R.; Semaniak, J.; Ehlerding, A.; Millar, T. J.; Osterdahl, F.; af Ugglas, M.; Djuric, N.; Paal, A.; Larsson, M. Dissociative Recombination of N2H+: Evidence for Fracture of the NN Bond. Astrophys. J. 2004, 609, 459−464. (5) Adams, N. G.; Herd, C. R.; Geoghegan, M.; Smith, D.; Canosa, A.; Gomet, J. C.; Rowe, B. R.; Queffelec, J. L.; Morlais, M. Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H-atom production in the dissociative recombination of some protonated ions. J. Chem. Phys. 1991, 94, 4852−4857. (6) Butler, J. M.; Babcock, L. M.; Adams, N. G. Effects of deuteration on vibrational excitation in the products of the electron recombination of HCO+ and N2H+. Mol. Phys. 1997, 91, 81−90. (7) Rosati, R. E.; Johnsen, R.; Golde, M. F. Yield of electronically excited N2 molecules from the dissociative recombination of N2H+ with e−. J. Chem. Phys. 2004, 120, 8025−8030. (8) Poterya, V.; McLain, J. L.; Adams, N. G.; Babcock, L. M. Mechanisms of electron-ion recombination of N2H+/N2D+ and HCO+/DCO+ ions: temperature dependence and isotopic effect. J. Phys. Chem. A 2005, 109, 7181−7186. (9) Molek, C. D.; McLain, J. L.; Poterya, V.; Adams, N. G. A remeasurement of the products for electron recombination of N2H+ using a new technique: no significant NH + N production. J. Phys. Chem. A 2007, 111, 6760−6765.

Figure 7. Kinetic energy release distribution of the N+ fragments, in the global isotropy assumption, as determined by expression 12

dissociation are rather flat or possess a minimum outside the Franck−Condon region. 3.2. Discussion. The results presented in this article raise questions concerning the reason for the threshold shifts. The answers to these questions point us toward other measurements. First the difference between our measured threshold and the thermodynamic value (which of course is just a minimum energy; the actual transition energy can be greater) indicates that there is considerable excitation in these ions. According to a two-dimensional calculation by Mahapatra et al.,13 N2H+ can support 163 vibrational states within a potential well of depth 5.46 eV. In a subsequent three-dimensional calculation,29 this figure is increased to 4580! Hence it is not surprising that there must be a considerable population of vibrationally excited states present in our measurement. Measurements of radiative lifetimes of N2H+ and N2D+ ions have been performed by 10026

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dx.doi.org/10.1021/jp5084967 | J. Phys. Chem. A 2014, 118, 10020−10027