electron-impact dissociation of ammonia: formation of nh+ ions in excited states

Volume 170, number 4 CHEMICAL PHYSICS LETTERS 13 July 1990

Electron-impact dissociation of ammonia: formation of NH+ ions in excited states

U. Miiller and G. Schulz FB. Experimemale Physik, Clniversitci’t des Saarlandes, D-6600 Saarbriicken, Federal Republic ofGermany

Received 28 October 1989; in tinal form 17 May 1990

The emission spectrum following single-electron impact on NH, has been investigated with the objective to search for emissions from excited NH+ ions produced by single-step dissociative excitation/ionization of NH,. The NH+ (C *E+-X ‘l-I, O-O) transition with an appearance potential of 28 k 1.5 eV was identified and an apparent emission cross-section of (7.0 4 2) X 1O-2o cm* at 100 eV incident electron energy was determined for this transltion.

1. Introduction

The formation of neutral fragments in electroni-

cally excited states following electron-impact dissociation of molecules has been investigated by many groups. Fragments in excited states can be detected unambiguously by optical methods using the emitted radiation. Excitation functions and absolute emis-

sion cross-sections for various dissociation processes have been analyzed in detail [ l-101.

The dissociative electron-impact ionization of molecules has also been investigated in great detail [ 1 l- 161. The total cross-section for the formation of charged species is usually determined by a measure-

ment of the total ion current sometimes combined with mass analysis to separate specific fragments. These techniques cannot, however, distinguish between ions or fragment ions formed in the ground state and ions formed in electronically excited states with subsequent decay. Formation of ions in excited states can be an appreciable contribution as pointed out in the case of CF4 by Aarts [ 171 and in the case

of CCl,F, by Jabbour et al. [ 5 1. Detailed knowledge of the formation mechanisms is required to under- stand the processes in plasmas and in the upper

atmosphere. The objective of the present work is to investigate

the formation of NH+ fragments in the electroni-

cally excited states C 2C+, B ‘A, and A 2C- following

single-electron impact on NH,. The corresponding rovibronic transitions to the ground state X 211 have been investigated by high-resolution spectroscopy of gas discharges. Line positions and transition mo- ments can be calculated from molecular constants. Therefore, an unambiguous assignment of the band systems can be achieved by comparing the observed

bands to a synthetic spectrum.

2. Experimental

A crossed electron-molecular-beam apparatus similar to the one described in ref. [ 71 has been

modified to allow for the measurement of absolute emission cross-sections.

The electron beam is produced by an electron gun consisting of a tungsten filament, a two-stage anode and an Einzel lens which focuses the beam into the

interaction region. The electron-beam current is col- lected in a Faraday cup which was specially designed to minimize the influence of secondary electrons.

The molecular beam emanates from a multicap- illary orifice made from a 2 x 4 mm’ piece of a mul- tichannel plate. The diameter of the channels is about 12.5 pm and the length is 500 km. A Baratron ca- pacitance manometer was used to monitor the driving pressure. Molecular flow conditions are valid up

to 0.2 mbar as demonstrated by a linear variation of

0009-26 14/90/$ 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland 11 401


the optical signal with driving pressure. The clcctron gun and molecular-beam system are

contained in a stainless sreel UHV chamber which is evacuated to less than IO-* mbar by a Balzers

TPU330 turbo molecular pump. The light emitted in the interaction region is focused by two quarz lenses onto the entrance slit of a 0.75 m Spex monochromator. A low-noise Hamamatsu Photomultiplier tube R2560 connected to a standard photon-counting system is used to detect the light.

The wavelength-dependent sensitivity of the detection system was determined by a calibrated tung-

sten/iodine standard lamp #I. The well known cross- sections for the helium lines 6 ‘S-2 ‘P at 4168.9 A and 5 ‘S-2 ‘P at 4437.5 .& [ 181 were used to determine the absolute sensitivity of the apparatus.

.4mmonia and helium of 99.999% purity (Linde

AG) were used without further purification. The gas- handling system is made from stainless steel and can be evacuated separately. A repeated evacuation ( IO-* mbar) and refilling (2000 mbar) of the gas line re- duces residual air and water contaminations to a negligible level. The residual background pressure in the UHV chamber does not influence the present results.

The experiment is operated under complete computer control (MC68000-based laboratory computer, RHOTRHRON VME-bus). All relevant parameters such as electron energy, electron current and gas pressure are constantly monitored. The energy of the electron beam can be varied by computer control, which allows us to chop the electron beam and to subtract the dark count rate of the PMT. The cor- rected count rate is normalized by the driving pressure and by the electron current. Slight systematic variations of the gas pressure and the electron current cannot affect the results. This allows us to run the experiment for periods of several days without interruption, which is a crucial requirement, if one wants to detect and resolve weak band systems with sufficient statistical accuracy.

The monochromator is scanned by a computer-

controlled stepping motor. The wavelength scale is calibrated using the emission spectrum of a helium gas discharge lamp to better than 0.3 8.

‘I The calibration was performed by the Phys. Techn. Bundesan-

stalt (PTB), Braunschweig.

402

The experiment can be operated in different

modes: spectroscopy at fixed electron energy, rela- tive cross-section measurements at fixed wavelengths, integration on well-defined spectral ranges to determine absolute emission cross-sections, sta- bilization of the calibration lamp and determination of the absolute sensitivity of the detection system.

The computer system is also used in the calculation of synthetic spectra.

3. Results

Fig. la shows the emission spectrum in the spectral range between 2750 and 5500 A with a resolution of 5 A fwhm following 100 eV single-electron

impact on ammonia. The most prominent features are due to the NH

fragments in the c ‘II and A 311 electronic states: the

NH( c ‘H-a ‘A, O-O) band at 3240 A, the NH(A ‘II- X 3Z-, O-O, l-l ) bands at 3360 A and the Balmer

series of the hydrogen atom. The analysis of these emissions has been presented in previous publica- tions [ 7-10, 21, 221. Weaker emissions are due to the 1-O and O-l vibronic bands of the NH(c ‘II-a ‘A, A3H-X3x-) systems. Fig. lb shows the spectral range between 3800 and 4900 A. The vertical scale has been expanded for clarity of presentation. Some features, which appear as a weak background in fig. la, are now clearly identified as well-defined band systems. They coincide with the vibronic progres- sions of the NH(c’Il-b’C) transition [20]. The feature around the Hy hydrogen line can be assigned to the NH+ (B ‘A-X ‘II, O-O) transition. The line positions reported by Colin and Douglas [ 191 are marked in fig. lb. Band calculations are currently underway and a detailed account of the synthetic spectra will be given elsewhere.

A weak emission band is found in the spectral range between 2800 and 3000 A. This band is part of the C 2C+-X 211 system of the NH+ ion. Fig. 2c shows

the same emission spectrum with a higher (2 A fwhm) resolution following 100 eV single-electron impact on ammonia. Signal to noise ratios are about 1: IO and 120 h of continuous data accumulation were required to obtain the spectrum with sufficient

statistical accuracy. The rotational structure of the

Volume 170, number 4 CHEMICAL PHYSICS LETTERS I3 July 1990

3000 3500 4000 4joo 5000

Wavelength / 0.1 nm

_I_

O 3800 4000 4200 4400 4600 4800

Wavelength / 0.1 nm

Fig. 1. (a) Emission spectrum between 2750 and 5500 8, following 100 eV single-electron impact on NH,. The monochromator apparatus

profile width is 5 8, fwhm. (b) Enlarged section of (a) in the spectral range 3750 to 4950 A. Line positions of the NH+(B 2A-X %, O-

0) transition [ 191, of the NH (c ‘II-b ‘C -, O-O, 1-O) transitions [20], and of the hydrogen-atom Balmer series are marked.

band is resolved to the extent that a comparison with a synthetic spectrum is possible.

Fig. 2a shows the calculated line positions of the NH+ (C *IL+-X ‘II, O-O) transition. There are 12 ro-

tational branches due to the doublet structure of the levels. The spectral positions of the rotational lines have been determined using the molecular constants

and formulas given by Colin and Douglas [ 191 and Feast [ 231. The ground state *II is perturbed by a 4);- state, especially in the first vibronic level (v= 1).

These perturbations are much smaller than the bandwidth of the monochromator and therefore have not been included in the current calculations.

Only the main branches appear with appreciable

403


Wavelength / 0.1 nm

Fig. 2. (a) Line posItions of the NH+(C 2,Y+-X 211, O-O) tran-

sition [ 191. The branches R, , and R,, are forming band heads, and the lines of the lowest rotational quantum numbers arc drawn

downwards for clarity. The intensity of the P2, and RI2 branches is too small to be presented in the figure. (b) Calculated spectrum assuming a rotational temperature of 2500 K and an apparatus profile of 2 8. fwhm. (c) Emission spectrum between 2873 and 2957 A with 2 A spectral resolution following 100 eV electron Impact on ammoma.

intensity. Intensity formulas containing the Hijhnl- London factors and the statistical degeneracy have been given by Earls 1241. A pseudo-rotational tem-

perature T,, has been assumed to describe the pop-

Table I ThresholdenergiesforNH+NH+(CZ~+)+H~te-

Electron Energy / eV

Fig. 3. Excitation function of the NH+ (C ‘x+-X ‘II, O-O) tran-

sition at 2900 A with 20 A spectral resolution.

ulation distribution of the excited-state rotational

levels. Fig. 2b shows the calculated rotational lines convoluted with a triangular apparatus profile corresponding to 2 A fwhm. The only tit parameters are the rotational temperature and the normalization factor. The best fit to the experimental results in fig.

2c assumes a rotational temperature of 2500 ? 500 K. Fig. 3 shows the excitation function taken in the

maximum of the NH+ (C *Z4-X ‘Il, O-O) band at 2900 A with a spectral resolution of 20 A fwhm. We find an onset energy of 28 f 1.5 eV.

Table 1 summarizes the results of other investi-

gations which can be used to determine the energy threshold for the formation of the NH+ (C *Z+) fragments from the NH3 mother molecule: values of 27.2 [ 111 and 27.5 eV [25,26] are found. The very good agreement with our result shows that NH+(C'C+ ) is mainly produced by a single-step

Process

NHs+NH+(X211)

NH+(X2n)-+NH+(C2E+)

NH,+NH(X3Z-)

NH(X’C-)+NH+(X%)

WeV)

22.9i0.5 4.3

4.0

4 19.2

Ref.

[III

1251

[261

&iirrct (eV)

27.2 +0.5

404

NH+(X211j+NH+(C*Z+)

NH,-rNH+(C%+)

4.3

this work

~27.5

28?. 1.5


process. In fig. 3a very small signal is found below 27 eV. This is probably due to a contribution from

the very weak NH(c ‘II-a ‘A, 2-O) transition. The absolute cross-section of the NH+(C *Z+-

X *II, O-O) transition was determined by integrating the measured intensity in the spectral range between 2880and3010~.Wefindavalueof(7~2)x10-20

cm’. The band extends further into the range of longer wavelengths, but these contributions are com- paratively small. The limit of 3010 A precludes contributions from the strong NH(c ‘II-a ‘A, 1-O) band head. A contribution of the NH (c ‘II-a ‘A, 2-O) band cannot be excluded, but the excitation function measurements indicate that it is very small.

4. Conclusions

It has been demonstrated that NH+ (C 2Z+) fragments are formed by single-electron impact dissociation of ammonia. The rotational temperature of the (C 2Z+-X ‘II, O-O) rovibronic transition was found to be 2500 I! 500 IL

Excitation-function measurements show an onset energy of 28 eV, in excellent agreement with the thermochemical value for the direct formation process. The absolute emission cross-section of the band is (7.0 t 2) x 1 Oe20 cm’ at 100 eV incident electron energy. A very weak band system is found next to the Hy hydrogen line which is most likely due to the NH+ (B 2A-X 211, O-O) band.

Acknowledgement

Thanks go to Professor F.J.M. Aarts for his inter- est in this work. We gratefully acknowledge helpful discussions with Professor K. Becker.

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electron-impact dissociation of ammonia: formation of nh+ ions in excited states

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