Formation of Highly Rovibrationally Excited ... - ACS Publications

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Formation of Highly Rovibrationally Excited Ammonia from Dissociative Recombination of NH4þ € Patrik U. Andersson,* Jenny Ojekull, and Jan B. C. Pettersson

Department of Chemistry, Atmospheric Science, University of Gothenburg, SE-412 96 G€ oteborg, Sweden

Nikola Markovic Department of Chemical and Biological Engineering, Physical Chemistry, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden § € Fredrik Hellberg,† Richard D. Thomas, Anneli Ehlerding, Fabian Osterdahl, ‡ Vitali Zhaunerchyk, Wolf D. Geppert, Magnus af Ugglas, and Mats Larsson

Department of Physics, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden

Einar Uggerud Mass Spectrometry Laboratory and Centre of Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, POB 1033 Blindern, N-0315 Oslo, Norway

Håkan Danared and Anders K€ allberg Manne Siegbahn Laboratory, Frescativ€ agen 26, SE-114 18 Stockholm, Sweden

ABSTRACT The internal energy distribution of ammonia formed in the dissociative recombination (DR) of NH4þ with electrons has been studied by an imaging technique at the ion storage ring CRYRING. The DR process resulted in the formation of NH3 þ H (0.90 ( 0.01), with minor contributions from channels producing NH2 þ H2 (0.05 ( 0.01) and NH2 þ 2H (0.04 ( 0.02). The formed NH3 molecules were highly internally excited, with a mean rovibrational energy of 3.3 ( 0.4 eV, which corresponds to 70% of the energy released in the neutralization process. The internal energy distribution was semiquantitatively reproduced by ab initio direct dynamics simulations, and the calculations suggested that the NH3 molecules are highly vibrationally excited while rotational excitation is limited. The high internal excitation and the translational energy of NH3 and H will influence their subsequent reactivity, an aspect that should be taken into account when developing detailed models of the interstellar medium and ammonia-containing plasmas. SECTION Dynamics, Clusters, Excited States

N

H3 is among the most abundant constituents of dense interstellar clouds,1 and the formation of NH3 is believed to proceed through a sequence of ion-neutral reactions. The final step in this sequence is the production of NH3 by dissociative recombination (DR) of NH4þ with a free electron.2 NH3 plasmas are also used in various applications in industry and research, including nitridation for semiconductor applications,3 synthesis of carbon nanotubes,4,5 and surface treatment to improve wettability and biocompatibility of polymer surfaces.6 The production of NH2, NH, and H2 in such plasmas has recently been studied, and a model with about 100 elementary reactions including the important DR reaction for NH4þ has been developed.7,8 DR processes may, depending on the conditions, determine the abundances of molecular ions and electrons in the ionized media and act as a

r 2010 American Chemical Society

source of highly reactive species including excited atoms, radicals, and molecules. The advent of heavy ion storage rings with electron cooling has made it possible to study ion-electron collisions with a collision energy resolution down to 1 meV.9,10 The technique has previously been used to investigate the DR of NH4þ and to determine the branching fractions (BFs) for the major product channels (see Table 1) together with the absolute DR cross sections and thermal rate coefficients.11,12 The thermal rate coefficients from these studies are in general agreement with

Received Date: June 17, 2010 Accepted Date: July 19, 2010 Published on Web Date: August 06, 2010

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DOI: 10.1021/jz100828u |J. Phys. Chem. Lett. 2010, 1, 2519–2523

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Table 1. Kinetic Energy Release (KER) and Branching Fractions (BFs) (95% confidence interval) for the DR of NH4þ with 0 eV Electrons NH4þ þ e- f KERa (eV)

BF (ref 11)

BF (ref 12)

broad distribution of interparticle separations measured for the products from the DR reaction at a collision energy of 0 eV. Note that separations smaller than 5 mm were only partially resolved due to signal overlap in the camera frame resulting in a loss of events below 5 mm. Figure 1 also plots a synthetic distance distribution, which assumes a random orientation in space of the dissociating NH4 molecules and a KER of 4.74 eV, that is, NH3 being produced in its lowest-energy state. The calculated distribution peaks are at 25 mm, and the maximum separation is 28 mm under the current experimental conditions. A background distribution due to ions colliding with residual gas molecules is also plotted in Figure 1. This distribution was measured at a 1 eV center-of-mass collision energy where the DR rate was significantly smaller than the background rate. The distribution was normalized to the spectrum taken at a 0 eV collision energy using the signals at separation distances larger than 28 mm. The comparison between experimental data and the analytical distribution in Figure 1 shows that NH3 rarely was formed in its lowestenergy state. The excess energy of 4.74 eV was not enough to produce electronically excited fragments, and we conclude that highly rovibrationally excited NH3 were produced. This observation is in accord with previous experimental and quantum chemical studies of NH4þ and NH4, which, among other features, show that the captured electron enters a quasi3s orbital of N and that the potential barrier toward dissociation to NH3 þ H is negligible.20 To estimate the degree of internal excitation in the NH3 fragments, a basis set of N distance distributions equally spaced in energy between 0 and 4.74 eV was calculated and fitted to the experimental data. In the analysis, the NH4þ ions were assumed to be in their vibrational ground state, and the small contribution from the two-body channel NH2 þ H2 was not taken into account. The mean kinetic energy released, ÆKERæ, was determined using the following expression N P Pi 3 KERi ð1Þ ÆKERæ ¼ i ¼ 1 N P Pi

BF (this study)

NH3 þ H

4.74

0.69 ( 0.03 0.85 ( 0.04

0.90 ( 0.01

NH2 þ H2 NH2 þ 2H

4.78 0.31

0.10 ( 0.02 0.02 ( 0.02 0.21 ( 0.03 0.13 ( 0.01

0.05 ( 0.01 0.04 ( 0.02

NH þ H þ H2

0.77

0.00

0.00

0.00

N þ 2H2

1.85

0.00

0.00

0.00

a

The excess energy was calculated assuming zero collision energy and that both the ion and the products were in their lowest-energy state.

Figure 1. Two-particle distance distributions in DR of NH4þ at a 0 eV center-of-mass collision energy; experimental results for DR þ background (blue b) and the background (green - - -) and simulated distribution assuming a KER of 4.74 eV (red ;).

earlier work that employed flowing afterglow methods.13-17 The DR of NH4þ mainly results in the production of NH3 þ H, with a probability of 0.85 ( 0.04.12 Minor channels resulting in NH2 þ H2 and NH2 þ 2H are also populated, which suggests a substantial redistribution of internal energy during the reaction. The aim of the present study is to investigate the energy redistribution and reaction dynamics of the DR reaction for NH4þ. We describe results from ion storage ring experiments where the kinetic energy release (KER) and internal energy excitation in the major product channel NH3 þ H were studied with an imaging technique. The experimental results are also compared with results from ab initio direct dynamics calculations. We initially redetermined the BFs for the reaction in the same way as in ref 12, and the results are shown in Table 1. Production of NH3 þ H dominated with a probability of 0.90 ( € 0.01, in agreement with an earlier study by Ojekull et al.12 that used the same water-cooled ion source. The results reported by Vikor et al.11 were obtained using a hot filament ion source and a shorter storage time, and the enhanced fragmentation observed in their study may be attributed to the presence of more internally excited molecular ions in the ion beam and their increased likelihood of fragmentation. The three-body channel NH2 þ 2H had a significantly lower probability in this € study compared to that reported by Ojekull et al.,12 suggesting that the ions in the present study were cooled more efficiently. The square root of the separation between two fragments hitting the imaging detector is proportional to the kinetic energy released in the DR process. The projection of this distance onto the two-dimensional detector was registered by the CCD camera, and the data were treated in the same way as in the analysis of DR of diatomic ions.18,19 Figure 1 shows the


i¼1

where KERi is the kinetic energy released and P is the contribution of the ith distribution. In order to investigate the influence of N on ÆKERæ, N was varied between 25 and 250. All values of N in this range gave good agreement with the experimental data, and the best fit to the data is shown in Figure 2. ÆKERæ was determined to be 1.4 ( 0.3 eV, where the uncertainty results from uncertainties in the length of the electron cooler, the distance between the electron cooler and the detector, the background subtraction, the factor used to convert from camera pixels to absolute distance, and the robustness of the fit with respect to variation of the size of the basis set. This implies that internal excitation is very substantial, with about 70% of the available energy on average stored as internal energy in the molecules, while 30% is liberated in the NH3 þ H center-of-mass. Due to the efficiency of the detector, some of the threebody events (NH þ 2H) were detected as two-body events.

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Figure 2. Two-particle distance distributions in DR of NH4þ; experimental results for DR (blue b), fit to the experimental distribution (red ;), and simulated distribution for the channel NH2 þ H þ H assuming a KER of 0.31 eV (green - - -). A background has been removed from the experimental data (see Figure 1).

Figure 3. Kinetic energy release distribution for the NH3 þ H channel from a fit to the experimental results (blue b) and from ab initio direct dynamics simulations (red ;).

energy for this channel, which is estimated to be 5.67 eV (5.71 eV at the MP2 level),12 and taking ZPE into account, this value is reduced to 4.74 eV (4.76 eV at the MP2 level). The ZPE is not conserved in the calculations. However, only one trajectory ended up with an internal energy less than the ZPE. A continuous translational energy distribution was constructed by centering a Gaussian function of width 0.18 eV about each data point. This distribution is shown in Figure 3 and compares favorably with the translational energy distribution obtained from the fit to the experimental data described above. The ab initio direct dynamics calculations suggest that the high internal excitation observed in the experiments is mainly due to vibrational excitation, while rotational excitation only plays a minor role. The formation of highly vibrationally excited NH3 is consistent with the experimentally observed sensitivity of the NH þ 2H channel to internal excitation of NH4þ (Table 1). The results are consistent with recent DR studies of H5O2þ and D5O2þ performed in CRYRING, where the dominating channel is 2X2O þ X (X = H, D).23 This reaction is exothermic by 5.1 eV, and as much as 4 eV ended up as internal excitation in the produced water molecules. The high internal excitation is also in agreement with the recently observed propensity for fragmentation observed in DR of complex molecular ions and cluster ions.24-29 To conclude, DR of NH4þ is dominated by the production of highly rovibrationally excited NH3 and energetic H atoms. The average energy stored as internal energy in NH3 was determined to be 3.3 ( 0.4 eV, which corresponds to 70% of the total available energy. On the basis of this result and the results of our theoretical calculations, it is tempting to speculate that deposition of the recombination energy into the total symmetric molecular vibration (Td) will lead to an even distribution of vibrational energy between the four equivalent N-H bonds of NH4.20 The 70% fraction is close to the fraction obtained in DR of D5O2þ,22 and the limited information available indicates that highly excited molecular fragments could be a general characteristic of DR of complex molecular ions and cluster ions. The internal energy distribution compares favorably with results from ab initio direct dynamics calculations, and the calculations suggest that the internal energy is mainly in the form of vibrational excitation, while rotational excitation is limited. The high degree of internal excitation in NH3 and the kinetic energy distribution of NH3 and H may influence their further reactivity and should be taken into consideration when developing detailed plasma models.

The efficiency, D, of the detector has been measured to be 0.6. This suggests that D 3 D 3 0.90 = 0.32 of all DR two-body events were detected, and 3 3 D2 3 (1 - D) 3 0.04 = 0.02 of the threebody breakups were detected as two-body events. This means that 5% of all counts in the two-body distribution may have originated from the three-body channel. A Monte Carlo simulation similar to that used to analyze three-body breakup of triatomic ions such as H2Oþ 21 was therefore performed to simulate the contribution from the NH2 þ 2H channel. The kinetic energy released in the process was assumed to be the maximum value 0.31 eV, that is, all fragments were in their lowest-energy levels. The resulting distance distribution after normalization to the experimental data is shown in Figure 2, and the three-body channel is the likely source of the small hump observed in the experimental distribution at a separation distance of 3 mm. Correcting for this contribution leads to an increase in ÆKERæ for the NH3 þ H channel of slightly less than 0.1 eV. Analysis of the experimental data indicates that the NH3 products are substantially internally excited. To provide further insight into the DR dissociation dynamics, the experimental data were compared with reanalyzed results from ab initio molecular dynamics simulations reported by € Ojekull et al.12 The potential energy surfaces of the ion and neutral ground state are well-separated in the FranckCondon region, and therefore, the DR reaction is likely to proceed via tunneling. Nonrotating NH4 species with an internal energy equal to 5.450 eV were generated with at least 90% of this energy in the form of potential energy. The fragmentation of the highly excited NH4 molecules was simulated by ab initio molecular dynamics at the UMP2/6311þG(d,p) level using the Gaussian 98 program package.22 In these calculations, the fragmentation dynamics were assumed to take place in the ground electronic state. The total energy corresponds to the energy difference for the vertical DR including the zero-point energy (ZPE) for NH4. A total set of 110 trajectories was propagated until the distance between the ejected hydrogen atom and the nitrogen atom was equal to 9 Å, and the KER was calculated from the final velocities of the fragments. The calculations gave the averages of translational energy, Etrans = 1.5 ( 0.2 eV, vibrational energy, Evib = 4.0 ( 0.2 eV, and rotational energy, Erot = 0.2 ( 0.1 eV.12 These energies can be compared to the classically available


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camera frame contained only one DR event. In total, approximately 400 000 images from the CCD camera were stored on a computer and analyzed.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: pan@ chem.gu.se. Phone: þ46 31 786 9073. Fax: þ46 31 772 31 07.

Present Addresses: †

Figure 4.

Manne Siegbahn Laboratory, Frescativ€ agen 26, SE-114 18 Stockholm, Sweden, ‡ Institute for Molecules and Materials, Radboud University Nijmegen, PO Box 9010, NL-6500 GL Nijmegen, The Netherlands.

Imaging detector used to detect the DR products of NH4þ.

EXPERIMENTAL METHODS The experiments have been performed in the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm University, and the experimental procedure has been described in detail elsewhere.9,10,30 NH4þ ions were produced from pure ammonia gas in a water-cooled hollow cathode ion source.31 The ion source was operated at a pressure of a few mbar, which should allow for rotational relaxation of the produced ions and limit effects of vibrational excitations. The ions were accelerated to an energy of 40 keV, mass selected with a bending magnet, and injected into the ion storage ring (circumference of 51.6 m). The ions stored in the ring were subsequently accelerated to 5.2 MeV using a radio frequency unit. The ion beam was merged with an electron beam over a distance of 0.85 m in the electron cooler. The electron velocity distribution can be described by a Maxwell-Boltzmann distribution with a longitudinal and transversal temperature of about 0.1 and 2 meV, respectively. During the first 9 s after acceleration, the electrons and ions were kept at the same average velocity to allow heat transfer from the ions to the electrons in order to reduce the translational temperature of the ions and to produce a narrower and better defined ion beam before the experiment started. The total storage time of 10 s used in this study should allow almost complete relaxation of any vibrationally excited mode. However, NH4þ has three infrared-inactive vibrational modes, and effects of remaining vibrational excitations can therefore not be entirely excluded. Neutral products were generated in the electron cooler by DR between electrons and NH4þ ions. A background of neutral fragments was also created by collisions between ions and residual gas molecules present in the electron cooler. The fragments from individual DR events were detected by an imaging detector situated 6.3 m from the center of the electron cooler. This detector has been described in detail elsewhere19,32 and is schematically illustrated in Figure 4. The DR fragments hit a stack of three microchannel plates, which was mounted in combination with a phosphor screen. The light from the phosphor was imaged onto an image intensifier (II) outside of the vacuum, and the output from the II was imaged onto a 64 64 pixels CCD camera. A photomultiplier tube simultaneously viewing the phosphor screen was used as a trigger for the system. It switched off the II to reduce background and triggered the readout of the camera. The detector sampled DR products on an event-by-event basis, and each


Notes §

Deceased.

ACKNOWLEDGMENT This work was supported by the Swedish Research Council. We are grateful to the staff members of the Manne Siegbahn Laboratory for their support during the experiments.

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