Luminescence in collision-induced dissociation of ND3 by H+, H2+, and H3+ beams at energies below 1000 eV

Chemical Physics 483–484 (2017) 78–83

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Luminescence in collision-induced dissociation of ND3 by H+, H+2, and H+3 beams at energies below 1000 eV R. Drozdowski a, S. Werbowy a,⇑, A. Kowalski a, B. Pranszke b a b

Institute of Experimental Physics, Faculty of Mathematics, Physics, and Informatics, University of Gdansk, ul. Wita Stwosza 57, PL-80-308 Gdansk, Poland Gdynia Maritime University, ul. Morska 81-87, PL-81-255 Gdynia, Poland

a r t i c l e

i n f o

Article history: Received 23 September 2016 In final form 29 October 2016 Available online 1 November 2016 Keywords: Luminescence Cross sections Ion beams

a b s t r a c t The luminescence of ND radical and hydrogen Balmer series has been observed in the collisions of Hþ n (n = 1, 2, 3) ions with ND3. Absolute luminescence cross sections, excitation functions, as well as electronic ND(c–a)/ND(A–X) and Db =Hb branching ratios were determined. The rotational and vibrational temperatures characterizing populations of the ND ðA3 P; c1 PÞ states were estimated from computer simulations of the spectra. Ó 2016 Published by Elsevier B.V.

1. Introduction The spectroscopic studies of dissociation of ammonia in the past often used photodissociation (see e.g. ref [1], a review of earlier work [2], and ref [3]). Recently some elaborate laser experiments were performed in this area, such as e.g. bond-selective photodissociation of ammonia [4,5] and deuterated ammonia NHD2 [6]. Half-collisions were studied by LIF and action spectroscopy techniques for the Hg–NH3 system [7]. There are fewer papers on collision-induced dissociation (CID) of NH3 studied by optical emission. The projectiles used in the past in the luminescence studies were: electrons (see a comprehensive experimental work [8] and references therein), rare gases (Ar, Kr, Xe) in the metastable 3 P0;2 states [9–11], Cþ ions [12], and rare gas ions (Arþ ; Krþ ; Xeþ ; Xeþþ ) [13]. The optical emission study of ammonia dissociated by Hþ n ions has not been reported before. The projectiles used here are either simple particles (protons) or molecular ions. We want to investigate whether their structure has an influence on the mechanism of dissociation of ammonia and compare the results with those obtained before with other projectiles. þ Several luminescent reactions of Hþ 2 and H3 with other targets have been studied in the past (see the review [14]). There is currently a big renewal of interest in H+3 reactions [15], as observations show that this molecule is highly abundant in molecular clouds, in the Galactic centre, and in the pre-stellar cores [16]. The H+3 ion ⇑ Corresponding author. E-mail address: [email protected] (S. Werbowy). http://dx.doi.org/10.1016/j.chemphys.2016.10.013 0301-0104/Ó 2016 Published by Elsevier B.V.

plays a fundamental role in the interstellar chemistry, for example in the models explaining deuterium fractionation, i.e. the observed enhancement of deuterated molecules, including ND3 [16], in some regions of space. All previous CID studies of ammonia show that most of the luminescence resulting from collisions of various projectiles with NH3 is due to the NH(A–X) and NH(c–a) electronic transitions. However, these emission spectra are under strong influence of perturbations due to predissociation of the NH(A3 P) and NHðc1 PÞ states into the repulsive NHð15 R Þ state [17,18]. As a result, only the (0,0) and (1,1) bands of NH(A–X, c–a) transitions are visible. In the present work, deuterated ammonia, ND3, is used as a target gas. The isotopic effect on vibrational and rotational energy levels causes contraction of vibrational and rotational quanta, so the spectra of ND contain more rovibronic transitions before predissociation sets in. This enables a more precise determination of vibrational and rotational temperatures that characterize population distributions of the diatomic ND products. The goal of the present work is to derive these temperatures from the luminescence spectra and to measure the emission cross sections for all electronically excited products. A comparison of these data with the results of studies using other projectiles (electrons, rare gas atoms, rare gas ions) should help to establish, whether the CID mechanism is valid for the present systems.

2. Experimental The apparatus has been described in Ref. [19]. Briefly, the ion beam machine has three vacuum chambers pumped separately.

R. Drozdowski et al. / Chemical Physics 483–484 (2017) 78–83

The ion source chamber and the mass-selector chamber are evacuated by two 1200 l/s oil diffusion pumps with liquid nitrogen traps, the collision chamber is evacuated by two parallel Leybold turbomolecular pumps, with pumping speeds 1400 l/s and 450 l/ s. The ion source is a hot-cathode Colutron, operated at anodeto-cathode voltage of 100 V and discharge current of 0.5 A. H2 gas (purity 99.999%) was supplied in the source under pressure of 15 Pa. Hydrogen ions were transported to the magnetic mass selector and further to the retarder, which provided deceleration to the required laboratory energy. The ion beam currents were measured at the end of collision cell, they increased with beam energy from 20 eV to 1000 eV, for H+ in the range from 0.1 to + 12 nA, for Hþ 2 in the range 2–60 nA, for H3 in the range 0.2– 20 nA, respectively. The ion beam current was measured with a Keithley picoammeter, probed 10 times per second and averaged by a computer. The ion beam path in the collision cell was 24 mm. Only the central region of the cell, between 6 mm and 18 mm from the entrance slit, was observed by the detection system. The target gas ND3 (Aldrich, purity 99%) was admitted through a Granville-Phillips automatic valve. The pressure in the collision cell was measured with an MKS Baratron capacitance manometer (head type 398 HD), digitalized and computer averaged; it was kept below 0.2 Pa. Luminescence was monitored through an MgF2 window mounted at the bottom of the collision cell and reflected by a concave mirror towards the entrance slit of the spectrograph. The luminescence spectra were recorded with a 1024-channel ‘‘Mepsicron” detector attached to a modified McPherson 218 spectrograph, which was equipped with 300, 1200, and 2400 l/mm interchangeable snap-in gratings blazed at various wavelengths. The spectral range of sensitivity of the detector is 200–600 nm. The sensitivity curve of the optical system was determined with a standard Osram Wi17/G tungsten ribbon lamp and a Hamamatsu L656K deuterium lamp [20]. During the present studies the light signal was about 5–500 cts/s, depending on the ion beam energy and optical slit width used, while the detector dark count rate was 2 cts/s. For the presentation of results we use the energy values in the center-of-mass (CM) frame, which are transformed from the laboratory energies by multiplying Elab by the factor 20/21, 20/22, and 20/23, for H+, H+2, and H+3, respectively. Throughout the energy balance equations given below, we + assume that our Hþ 2 and H3 projectile ions are in the ground state. This is only an approximation, as we do not know the fractions of metastable states of the ions. Molecular hydrogen ion Hþ 2 has long-lived vibrational states in the ground electronic state [21,22]. These excited vibrational states of Hþ 2 can be substantially depleted, e.g. by adding a large excess of neon into the ion source (up to 13 Pa, to make the Ne/H2 pressure ratio of 5) [23]. We did not apply this method because it could work to disadvantage, supporting production of molecular ions in metastable electronic states via energy-transfer reactions with long-lived neon atoms. Metastable electronic states of Hþ 2 can arise from asymptotes associated with protonated metastable hydrogen atoms, however, bound states arising from these asymptotes are expected to have very shallow minima [24] and are only sparsely populated (if at all). The H+3 ions also have long-lived excited vibrational levels [25]. Depletion of these states occurs in collisions with H2, and was found to be very efficient above source gas pressure of 3 Pa [26]. For H+3 ion, theory predicts one bound excited electronic state, a 3 Rþ u , located 5.77 eV above the ground state with the depth of the potential well equal to 0.28 eV [27]. This triplet state was never observed in the laboratory [16]. Experimental study [28] finds no evidence for an electronically excited H+3 product in the reaction H+2 + H2 ? H+3 + H, which is responsible for creation of H+3 in the ion source.

79

A simple practical rule for minimizing the fraction of metastables in an ion beam from a discharge source advises application of high pressure of the source gas and a low discharge voltage UA [29]. The first condition was fulfilled in our experiment, however, only at relatively high UA = 100 V the H+3 beam intensity was sufficient and the ion source operation was stable. Several experimental runs made at laboratory collision energy of 1000 eV for molecular projectiles at UA of 50 V and later at 100 V did not show meaningful differences in both shape of the spectra and their normalized intensity. Under these source conditions we certainly have some metastable states in the molecular hydrogen ion beams, however, their presence is not clearly manifested because either their fraction is low, or their influence on the dissociation of ammonia is not much different from that of the ground state ions. Tests performed in another laboratory for a discharge source of molecular hydrogen ions have shown that of various parameters, such as discharge current, voltage, electrode spacing, and gas pressure, the last one is crucial in diminishing the metastable content of the ion beam [30]. 3. Results and discussion The luminescence spectra resulting from the bombardment of ND3 by all three ion beams, H+, H+2, and H+3, have similar appearance, in particular at collision energies above 200 eV. The spectra consist of the ND(A–X, c–a) molecular transitions plus hydrogen Balmer series. There are, however, substantial differences concerning relative intensities of emissions at various collision energies, there is also a dependence of the light intensity on the identity of projectile ion. A part of the spectrum observed in the collisions of Hþ 2 with ND3 at 1000 eV laboratory energy is shown in Fig. 1. The luminescence emitter is ND radical, with the characteristic NDðA3 P  X 3 R Þ and NDðc1 P  a1 D) bands of the Dv = 0 sequence in the 320–350 nm region. Both upper states of these transitions converge to the same Nð2 DÞ þ Dð2 SÞ asymptote and predissociate to the repulsive NDð15 R Þ state. The spectra for H+ and H+3 projectiles are similar, but their intensities are lower. For determination of the rotational, Trot, and vibrational, Tvib, temperatures describing the populations of internal states of the products, computer simulations of the spectra were performed, using standard equations [24] and spectroscopic data for ND or NH from Refs. [31–36]. For ND(c–a) some spectroscopic constants were derived from the data for NH [37], using the formulas for isotope effect [24]. The Hönl-London factors were taken from Refs.

Fig. 1. Luminescence spectra of ND(A–X, c–a) in the region of Dv = 0 sequence, obtained in the collisions of Hþ 2 with ND3 at the ECM ¼ 909 eV; spectral resolution: 0.13 nm. The simulated spectrum is shifted upwards for a better comparison.

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Table 1 Rotational and vibrational temperatures of the products determined from the observed spectra (uncertainty 10%). Collision system

Collision energy (CM)

Product/Transition ND(A–X)

ND(c–a)

[eV]

Trot (K)

Tvib (K)

Trot (K)

H+ + ND3

48 95 476 952

3500 3500 3500 4500

2900 4000 5000 6000

1000 1000 1000 1400

Hþ 2 + ND3

45 91 455 909

4000 4000 4500 4500

5000 5000 5000 5000

1200 1200 1400 1500

H+3 + ND3

43 87 434 867

4500 4500 4500 4500

5500 5500 5000 4500

1200 1200 1200 1400

[38,39]. The Franck–Condon factors for ND(A–X) were from Ref. [40], for ND(c–a) we calculated the factors using Morse potential and spectroscopic constants from [31–36]. The derived temperatures are collected in Table 1. Considering the size of collisional energies applied in the experiment, which are relatively high in comparison with typical bond strengths, it is somewhat surprising that the highest temperatures correspond to only 0.5 eV. These population parameters differ very little for all systems studied, and are independent of collision energy (with the exception of Tvib(ND (A)) for H+ projectiles). The absolute values of the luminescence cross sections were determined by measuring light intensities from two reference reactions: from the H+ + N2 collisions at 1000 eV [41] and, independently, from the He+ + H2 reaction at 700 eV [42]. The normalization measurements on our apparatus gave almost identical results from both scaling systems. The derived luminescence cross sections had to be corrected for escape of ND products from the observation window. This procedure is necessary, as the radiative lifetimes of ND emitters are relatively large and the velocities of the excited products are also considerable. After calculating for each projectile the transfer of kinetic energy from the Hþ n ion to the ND3 target molecule in the laboratory system, we assumed that only central collisions contribute to the molecular emission, therefore the loss of light should occur only along the direction of the projectile velocity. Tests have shown that in fact at collision energies above 100 eV, most of the luminescence was emitted from the observation volume determined by the entrance slit. We have further assumed that the ND product will have the same velocity as that acquired by ND3 from the ion projectile. To calculate the correction factors for escape of emitters we applied the method described in Ref. [29], using the derived product velocities, dimensions of the observation zone and the radiative lifetimes. The lifetime of ND(A) is s = 415 ns [43], for ND(c) it is s = 500 ns for v0 = 0 and s = 230 ns for v0 = 1 [44]. The corrected luminescence cross sections, r , are given in the form of excitation functions in Fig. 2. The luminescence cross sections for ND increase monotonically with collision energy without reaching a maximum, the values of r + + for Hþ 2 are by a factor of 2 larger than for H and H3 projectile ions.  The r values increasing with collision energy are typical for an endothermic process. A similar shape of excitation functions was observed in Ref. [13] for NH(A–X) obtained in the Xe+ + NH3 and Xe++ + NH3 collisions and the same trend was seen in a less detailed study of Ar+, Kr+ + NH3 [13]. The absolute emission excitation cross sections at ECM ¼ 100 eV were found to be

r ðNHðA  XÞÞ ¼ 0:1  1020 m2 for Xeþ , Xeþþ ; Arþ , and Kr+ +

Fig. 2. Excitation function for ND(A–X) and (c–a) obtained for H+ (triangles), Hþ 2 (open circles), and H+3 (full circles), corrected for escape of emitters (see text). The relative uncertainty is 10%, the absolute values have the uncertainty of ±0.03  1020 m2.

NH3 reactions, this value is about a factor of 2 higher than r (ND(A–X & c–a)) at 100 eV for H+2 + ND3 (see Fig. 2). The latter value is very similar to that obtained for the e + NH3 system at 100 eV [8,45], The total charge–exchange cross sections for collisions of H+, H+2, and H+3 at 500 eV are 26  1020 m2 , 28  1020 m2 [46], and 6  1020 m2 [47], respectively. The emission cross sections at this energy, obtained in the present work (Fig. 2), correspond to luminescence yields, defined as U ¼ r =rCT , of 0.23%, 0.5%, and 1.5%, respectively. The relative intensities of both molecular emissions of ND are shown in the form of the ND(c–a)/ND(A–X) branching ratio (BR) in Fig. 3. The values of BR were determined from the spectra by counting the registered photons with corrections resulting from the fact that the ND(c–a) (Dv = 0) spectrum is partially overlapping with the ND(A–X)(Dv = 0) emission (see Fig. 1). To the detectorsensitivity-corrected sum of photons at wavelengths below 330 nm, which can be undoubtedly attributed to the ND(c–a) (Dv = 0) system, some photons emitted above 330 nm have been added. This extra contribution was estimated using computer simulated spectral contours generated for both electronic transitions. Corrections of BR due to escape of emitters were also made, although the modification has a minor influence. The branching ratios show big diversity. The BR for H+ at low collision energies is small (the light signal was too low for a meaningful measurement at energies below 50 eV), above 200 eV it corresponds roughly to the statistical value of 0.33, expected a priori for a + singlet/triplet state population ratio. For the Hþ 2 and H3 projectiles the BR is clearly smaller. While the luminescence produced by H+3 shows no contribution from ND(c–a) below 50 eV, for Hþ 2 this transition is present even at 14 eV collision energy. It is unclear, why the ND(c 1 P) state is so weakly populated at low energies for all three projectiles, when this state is only 1.65 eV above the ND (A3 P) state and both states are closely related by their electron configuration. The relative deficiency in ND (c) production could be caused by the higher endoergicity of this state, which lies 1.65 eV above the ND (A) state, therefore the ratio of phase space volumes for both channels could be smaller at low collision energies. The branching ratios were measured in Ref. [13] for Arþ ; Krþ , and Xeþ collisions with NH3 and were found equal to 0.10 (at 69 eV), 0.07 (at 101 eV), and 0.04 (at 69 eV), respectively. The observed decrease of BR values with increasing ion mass for the three RG ions became the ground for the suggestion [13] that the NH (c) and NH (A) molecules are formed in two different

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reach each state. Additionally, the calculated DE0 served for the construction of the energy diagram in Fig. 6 that should enable a quick comparison of the systems from the point of view of energy balance DE0 . For clarity, the diagram does not contain levels corresponding to the ND (c) states, which are 1.65 eV above the indicated ND (A) states. The calculated values indicate that the lowest product channels leading to the ND ðA3 P) and ND ðc1 PÞ states are associated with collision-induced dissociation of ND3 by the Hþ n projectiles that remain unchanged. Assuming that the collision in its earliest stage causes electron promotion to the lowest excited states of ND3, which in turn lead to ND molecules in the observed states, we find that the channels are endoergic (i.e. they require additional energy, which is supplied by kinetic energy of the collision):

Hþn þ ND3 ðX 1 A1 Þ ! Hþn þ ND3 ! +

Hþ 2

Fig. 3. The branching ratios for ND(c–a)/ND(A–X) obtained for H (triangles), (open circles), and H+3 (full circles), corrected for escape of emitters (see text). The + uncertainty of BR is ±0.03 for H+ and 0:02 for Hþ 2 or H3.

mechanisms, although both involving excitation of ammonia, which subsequently dissociates. Our results of BR seem to fall into the trend concerning the ion mass, which influences the relative velocity of collision partners (increasing with decreasing mass of the projectile ion) or the amount of kinetic energy transferred to the ammonia target (increasing with the mass of the projectile). If the proposed different mechanisms [13] for the production of NH (c) and NH (A) indeed have place, then apparently only for ionic projectiles. For the neutral Ar ð3 PÞ colliding with NH3, the BR(c/A) = 0.33 at thermal energy [10] and BR(c/A) = 1.2 at about 1 eV collision energy [11], despite the fact that the production of NH (c) is forbidden by the spin conservation rule. The detailed dynamics of luminescent collision-induced dissociation of ND3 is unknown. Some ideas about the early stages of the process can be drawn from the theoretical work on photodissociation of ammonia [48], where several lowest SCF potential curves for the abstraction process NH3 ! NH þ H2 were calculated. The NH3 molecule in its ground state X 1 A1 pyramidal conformation 2

2

4

2

of C3v symmetry has the 1ða1 Þ ð2a1 Þ ð1eÞ ð3a1 Þ electronic configuration. In the two lowest excited states of NH3, reached by the 3a1 ? 3s Rydberg-type electron promotion, ammonia is planar e 3 A00 state is only about 0.15 and has D3h symmetry. Triplet A 2

e 1 A0 state, and it lies 6.3 eV above eV–0.3 eV below the singlet A 2 the ground state of NH3(X 1 A01 ). As the first (and energetically the lowest) step of the dissociation process, two hydrogen atoms come closer to form H2 in C2v symmetry and the energy of the system rises by 1–2 eV. Past this threshold, the transition state evolves towards product states correlating with the NH ðA3 P) + H2 and NH ðc1 PÞ + H2 asymptotes, respectively. At higher collision energy (about 10 eV above the ground state) ammonia has a pair of 1;3 E0 excited states of almost identical energy, resulting from the 1e ? 3s electron promotion. The paper [48] shows that, of this pair of states, the 1 E0 state correlates without a threshold with NH (c) + H2 products. Even higher excited states of NH3 lead to NH⁄(A,c) + 2H(2S). The discussion of RG+ + NH3 luminescent collisions [13] points out to the experimental evidence [49,50] that in these reactions the NH (A) emission can not occur via charge transfer leading to  NHþ 3 , which would further dissociate to the excited NH (A) product. The energy balances for reactions studied in the present work are based on the data from [31,32,51] and are given below in the form of equations which contain information about the spin properties of the states involved and the energy input DE0 necessary to

! Hþn þ ND ðA 3 PÞ þ D2 ðX 1 Rþg Þ; !

Hþn



þ g Þ;

þ ND ðc PÞ þ D2 ðX R 1

1

DE0 ¼ 8:03 eV;

ð1Þ

DE0 ¼ 9:68 eV:

ð2Þ

In this case, the only difference in molecular luminescence yields could stem from the value of the electron spin of the projectile, that would fulfill or violate the electron spin-conservation rule. This rule was found to hold quite strictly in photodissociation of NH3 [1], where the sum of reactant spin was zero and the NH (A–X) spectrum was not observed, as it would be emitted from the triplet state of the product. When ammonia was bombarded with electrons, the NH(A–X) and NH(c–a) spectra were observed, with similar intensity, although the production of ND ðA3 P) via analogous route was forbidden by the same rule. In our case, the 2 þ ground state of the diatomic projectile ion is Hþ 2 ðX Rg ), while for the triatomic ion it is H+3(X 1 1 A01 Þ, thus of the three projectiles used in the present experiment, only the Hþ 2 ion meets the requirement of spin conservation in Eq. (1). There is, however, a similar pair of product channels, at higher energy, in which two D atoms are formed in place of D2 molecule:

Hþn þ ND3 ðX 1 A1 Þ ! ! Hþn þ ND ðA 3 PÞ þ Dð2 SÞ þ Dð2 SÞ;

DE0 ¼ 12:59 eV;

ð3Þ

! Hþn þ ND ðc 1 PÞ þ Dð2 SÞ þ Dð2 SÞ;

DE0 ¼ 14:24 eV:

ð4Þ

The spin conservation rule is fulfilled in these reaction paths for all three projectile ions. A third way to obtain the ND (A) and ND (c) products would involve a charge transfer from the target to the projectile. The energy balance becomes then specific for each projectile, as their recombination energies are different. The lowest product channels in this group are:

Hþ þ ND3 ðX 1 A1 Þ ! ! Hð2 SÞ þ ND ðA 3 PÞ þ Dþ2 ð2 Rþg Þ; DE0 ¼ 9:90 eV; 

Þ þ Dþ2 ð2

! Hð SÞ þ ND ðc P 2

1

R

þ g Þ;

DE0 ¼ 11:55 eV;

ð5Þ ð6Þ

Hþ2 ðX 2 Rþg Þ þ ND3 ðX 1 A1 Þ ! ! H2 ðX 1 Rþg Þ þ ND ðA 3 PÞ þ Dþ2 ðX 2 Rþg Þ; DE0 ¼ 8:07 eV; ! H2 ðX R 1

þ  1 g Þ þ ND ðc

Þ þ Dþ2 ðX 2

P

R

þ g Þ;

DE0 ¼ 9:72 eV;

ð7Þ ð8Þ

Hþ3 ðX 1 A01 Þ þ ND3 ðX 1 A1 Þ ! ! H2 ðX 1 Rþg Þ þ H þ ND ðA 3 PÞ þ Dþ2 ðX 2 Rþg Þ; DE0 ¼ 14:28 eV;

ð9Þ

! H2 ðX 1 Rþg Þ þ H þ ND ðc 1 PÞ þ Dþ2 ðX 1 Rþg Þ; DE0 ¼ 15:93 eV: ð10Þ Considering the excitation functions (Fig. 2) with this background information on the energetics of reactions, we find that the experimental results correlate well with the CID mechanism, but not with the CT scenario. The observed higher intensity of the ND* luminescence for the Hþ 2 þ ND3 collisions and the similarity of rðND ) for the other two projectiles, can be explained by

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Fig. 4. Excitation functions for Hb þ Db obtained for H+ (triangles), Hþ 2 (open circles), and H+3 (full circles). Fig. 5. The branching ratios for Db =Hb obtained for H+ (triangles), Hþ 2 (open circles), and H+3 (full circles)

processes described by Eqs. (1)–(4). The experimental results do not agree with the CT model, as the endothermicities, DE0 , do not correlate with the values of rðND ) for different projectiles. As shown above, the observed luminescence had also atomic component in the form of Balmer series. The light could be emitted by hydrogen atoms of the projectile or by deuterium atoms of the target. The contributions can be distinguished, as the H and D Balmer-b lines are positioned at k ¼ 486:135 nm and k ¼ 486:004 nm, respectively, and they can be separated. The measured cross sections for the sum of luminescence associated with the transition from hydrogen n = 4 state to the n = 2 state (total intensity of Hb and Db lines) is presented in Fig. 4. The components of the Balmer-b line have upper states 4l with lifetimes no longer than 36 ns [51], therefore in this case there is no need to apply a correction for escape of emitters. Generally, the excitation functions for all three projectiles are similar at higher collision energies, but below 150 eV the Balmer-b line is relatively strong only for the Hþ 2 ion. All three excitation functions seem to reach a maximum below 1000 eV collision energy. The measured cross sections are of the same order of magnitude as that determined in [45] for e + NH3 collisions at 100 eV, where r ðHb Þ ¼ 0:005  1020 m2 . Compared to the total charge transfer cross sections [45], this cross section is very small, it corresponds to the luminescence yield U just below 0.02%. The resulting branching ratio of Db /Hb Balmer line intensities is shown in Fig. 5. The excitation of the projectile H atoms dominates over that of target D atoms. The lowest channels for excitation of Balmer-b line are described by:

Fig. 6. Energy diagram for the systems studied. The levels corresponding to the electron transfer (CT) and collision induced dissociation with excitation (CID) are marked by dashed and continuous lines, respectively. In the description of each level, the light emitting particle is always positioned to the far right. For clarity, the levels related to the NH (c) state are not shown; they are 1.65 eV above the NH (A) state.

Hþ þ ND3 ðX 1 A1 Þ ! ! H ð4 2 S; 4 2 PÞ þ NDþ3 ðX 2 A002 Þ; DE0 ¼ 9:23 eV;

ð11Þ

Hþ2 ðX 2 Rþg Þ þ ND3 ðX 1 A1 Þ ! ! Hð2 SÞ þ H ð4 2 S;4 2 PÞ þ NDþ3 ðX 2 A002 Þ; DE0 ¼ 11:99 eV; Hþ3 ðX 1 A01 Þ þ ND3 ðX 1 A1 Þ ! ! H2 ðX 1 þg Þ þ H ð4 2 S;4 2 PÞ þ NDþ3 ðX 2 A002 Þ;

R

DE0 ¼ 13:60 eV

ð12Þ ð13Þ

Since Hb lines are several times stronger than Db , the measured overall Balmer-b intensities in Fig. 4 follow the trend of DE0 in the energy balance in Eqs. (11)–(13). A consistent explanation of Figs. 4 and 5 is possible if we assume that the Db emission results from the dissociation of the ND3 target for all three projectiles:

Hþn þ ND3 ðX 1 A1 Þ ! Hþn þ ND2 ðX 2 B1 Þ þ D ð42 S; 42 PÞ;

DE0 ¼ 17:54 eV:

ð14Þ

The formation of D , similarly as the production of ND , would then occur in the collision induced dissociation of ND3. The observed branching ratios for Db =Hb reflect the fact that for about the same intensity of Db line, the emission of Hb line decreases in þ the order: Hþ ; Hþ 2 ; H3 , in agreement with the corresponding endothermicities in Eqs. (11)–(13). There is an alternate, although somewhat more complicated way to produce Db lines. It requires electron transfer followed by the dissociation of NDþ 3 . The lowest channels for the Db emission via CT in the collisions of H+, H+2, and H+3 projectiles with ND3 have DE0 equal to 14.09 eV, 12.26 eV, and 18.47 eV, respectively, and it means that for H+, H+2 they are less endoergic, and for H+3 ions they are more endoergic than in the CID mechanism of Eq. (14). The main reason for our rejection of the CT scenario in this case, is the lack of clear correspondence between the experimental Db line intensities and the

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CT energy balances leading to deuterium Balmer-b production. The discussion in Ref. [13] suggests that in the RGþ þ NH3 collisions, the identity of the projectile influences the intensity of H emission, therefore the authors suggest that the mechanisms for NH (A) and H excitations are different, although both involve excitation of ammonia, which subsequently dissociates. 4. Conclusions From the literature it is known that overwhelming majority of the Hþ n + NH3 collisions should lead to charge transfer [46] with þ + main product NHþ 3 at low collision energies and NH2 , NH products at higher collision energies [50]. We assume that the same holds for the deuterated ammonia target. The present study is capable of detecting only luminescent products and indicates that there is a parallel mechanism of collision-induced dissociation of ammonia caused by the impact of minority fraction of ions which did not undergo the electron transfer. The observed luminescence is dominated by the ND emission of the target fragment and a weaker H Balmer series emission resulting from the charge transfer. There is also a very weak contribution of D Balmer series from the target. Although the energy balance permits formation of other excited states of the collision products (neutral or ionized), they are not observed. In some cases the absence of possible emitters is due to a lack of spectrum (NDþ 3 ) [52] in the spectral window of the detector used, a very long radiative lifetime (ND2) [53], or a very high excitation energy (N) [51]. In other cases (NDþ ) one can only speculate about unknown obstacles in the dissociation of the NDþ 3 primary product of charge transfer. Similar observations were made earlier in analogous optical experiments with NH3 target, where electrons [8] and rare gas ions [13] were used as projectiles. The same domination of imidogen emitters was also observed in the collisions of neutral metastable rare gases with ammonia [9,10]. All these studies bring almost identical rotational and vibrational temperatures determined for the molecular emitter NH (or ND ), which suggest the same mechanism of excitation, independent of the projectile, whether it is charged or neutral. Acknowledgment This work was financed within the statutory fund DS-5305200-D464-16 and the research project of the National Science Centre (NCN, Poland), DEC-2012/05/D/ST9/03912. References [1] K.H. Becker, K.H. Welge, Z. Naturforsch. 19a (1964) 1006. [2] H. Okabe, Photochemistry of Small Molecules, Wiley-Interscience Publications, New York, 1978. [3] D. Edvardsson, P. Baltzer, L. Karlsson, B. Wannberg, D.M.P. Holland, D.A. Shaw, E.E. Rennie, J. Phys. B At. Mol. Opt. Phys. 32 (1999) 2583. [4] A. Bach, J.M. Hutchinson, R.J. Holiday, F.F. Crim, J. Chem. Phys. 118 (2002) 4955. [5] H. Akagi, K. Yokoyama, A. Yokoyama, J. Chem. Phys. 118 (2003) 3600.

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