Collision-induced electronic excitation of NH+ ions studied by optical spectroscopy

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Mass Spectrometry and Ion Processes

ELS EVI E R

International Journal of Mass Spectrometry and Ion Processes 173 (1998) 127 141

Collision-induced electronic excitation of NH + ions studied by optical spectroscopy A. Ehbrecht, A. Kowalski 1, Ch. Ottinger* Max-Planek-lnstitut ~r Str6mungsfi~rschung, G6ttingen, German)' Received 23 September 1997: accepted 13 October 1997

Abstract Electronic excitation o f NH ÷ ions by impact on rare gas atoms as well as H2, N2, 02, NO molecules was investigated by means o f emission spectroscopy in the 10-1000 eVlab collision energy range. The NH + B - X and C - X transitions were observed, while A X emission appears to be absent. The excitation functions for populating the B and the C states o f NH + are quite different. The C-state excitation sets in at - 1 0 0 eVcM and rises monotonically thereafter. The B-state excitation function indicates two distinct mechanisms. One is active from the thermodynamic threshold on, but has low efficiency. At medium energies (50-200 eVcM) it is superseded by a much more efficient mechanism, which has an energy dependence similar to that for C-state excitation, but a - 15-20 times larger cross section than the latter. The vibrational excitation in both states is moderate. It can be explained by Franck-Condon transitions from NH + (X) reactant ions, even at low collision energy, assuming that the latter are vibrationally hot (Tvib = 5000 K). Published by Elsevier Science B.V.

I. I n t r o d u c t i o n

Recently we reported on electronic excitation of CH + ions in collisions with rare gas atoms and some diatomic molecules [1]. In the present paper, an analogous study of NH + is presented. This molecular ion is isoelectronic with the very well known, important radical CH, but has been studied much less in the past. The NH+(A2~ -xzII) and (BZA-xzII) transitions were reported by Colin and Douglas [2]. Perturbations between the ground state X2II and the low-lying metastable a4~ - state were analysed in Refs. [3,4]. The C2~:+-xZII transition was first observed by * Corresponding author. Permanent address: Institute of Experimental Physics, University of Gdafisk, ul. Wita Stwosza 57, PL-80-952 Gdafisk, Poland. 0168-1176/98/$19.00 Published by Elsevier Science B.V. Pll S0168-11 7 6 ( 9 7 ) 0 0 2 8 0 - 2

Lunt et al. [5] and later identified and analysed by Feast [6]. Kusunoki and Ottinger [7] have observed NH+(B-X) emission from chemiluminescent reactions of N + with H2, They found some previously unreported bands, and were able to improve several spectroscopic constants on this basis. Theoretical studies o f N H + are also sparse. Liu and Verhaegen [8] have calculated energies near the potential minima for several low-lying electronic states: Guest and Hirst [9] have obtained electronic energies for a wide range of internuclear distances. Extensive calculations by Rosmus and Meyer [ 10] and results of Reddy et al. [ 11 ] concerned the ground state only. Farnell and Ogilvie [12] have calculated the X and a states and their interaction. Kusunoki et al. [13] performed ab initio calculations on eight low-lying doublet states of NH +. Transition

128

A. Ehbreeht et al./lnternational Journal o[Mass Spectromet O' and Ion Processes 173 (1998) 127-141

dipole moments for NH ÷ were calculated in Refs. [14-16] and derived from experimental data in Ref. [7]. In this paper, we report on the observation of collision-induced excitation (CIE) of the NH + molecular ion: NH + +M--* NH +* + M

(1)

by means of optical emission spectroscopy. M stands for a target atom or a diatomic molecule. Two distinct band systems of NH + were observed. In addition, the spectra showed frequently also emission from excited NH, formed in the charge transfer excitation (CTE) reaction NH + + M ---+ NH* + M +

(2)

However, this process is not investigated in the present work. Surprisingly, there are very few CIE studies of molecular ions (for an overview, see [1 ]). In several cases the electronic excitation was detected by means o f translational spectroscopy. Optical spectroscopy has previously only been applied to N + [17,18], CH + [1], and, without detailed analysis, CO + [19].

1:1 mixture o f NH3 and N2. This was found to give a m a x i m u m NH ÷ beam intensity. The NH + beam current at the target cell was on the order of 1 nA at 10 eVlab and about 20 nA at 1000 eVlab. The beam composition was a subject of extensive studies. The NH + potential energy curves given in Fig. 1 show that there is a metastable state, NH+(a4E-), which lies only 3 5 4 c m < above the NH+(X2II) ground-state. Therefore, a large NH+(a) beam fraction could be anticipated. Various test for the presence of this metastable species were made. They were based on the differential ion beam attenuation technique [21]. From the relative magnitude of the ionization energies of NH and C2H2, I.E.(NH) = 13.47 eV [22] and I.E.(C2H2) = 11.406 eV [23], combined with the term energy of NH(a 1A) of 1.561 eV [24], it is expected that the ground state NH+(X2II) ions will efficiently undergo the spin-allowed near-resonant charge transfer

C 2~+

2. Experimental The experimental set-up is the same as described previously [1,20]. Briefly, NH + ions extracted from a 'Colutron' plasma source were mass selected by a magnet and decelerated to the required laboratory energy in the 10-1000 eV lab range. The ion beam collided with the target gas in a cell 24 m m in length, of which the central 12 mm portion represented the effective observation region. The luminescence leaving the cell through a window was focused by a mirror onto the entrance slit of a spectrograph. The detector was a position-sensitive photon counter tube ('Mepsicron'), whose 2 inch diameter photocathode was arranged to lie in the spectrograph's exit focal plane. The detector dark current was extremely low (2.4 counts/s from the entire photocathode). NH + ions were obtained from a

5

4 ul

3

2 ]

0 1

2 R [•1

3

Fig. 1. RKR-potential energy curves of low-lying electronic states of NH+ (based on molecular constants from Ref. [24]).

A. Ehbrecht et al./lnternational Journal of Mass Spectrometry and hm Processes 173 (l 998) 12 7-141

process NH + (X 2H) + C2 H2 ~ NH(aa A) + C2H~- + 0.50 eV.

(3)

The metastable NH+(a4Z -) ions, on the other hand, should have a smaller CT cross section, since the near-resonant reaction channel NH+(a4G- )+C2H2 "-* NH(al A)+C2Hf + 0.55 eV

(4) is spin forbidden and the nearest spin-allowed channel NH+ (a4G-) + C2H2 ~ NH(X3]~ -) + C2 H+ + 2.11 eV

(5)

is well out of resonance. On this basis a separation of the two beam components was attempted. Following the standard practice used in our earlier work [20], the collision cell was filled with acetylene and the intensity of the ion beam transmitted through the cell was measured as a function of the cell pressure. Measurements were made with an ion source anode voltage UA = 40 V and a source pressure Ps -- 25 Pa (typical conditions to give a low fraction of metastables), as well as with UA = !50 V and Ps = 13 Pa (as expected to favour a higher metastable content). Corresponding to the different charge transfer cross sections of the two NH + states, the overall beam attenuation should exhibit deviations from Beer's law. Opposite to the usual situation [21], however, in the present experiment the metastable beam fraction should be attenuated less than the ground state component. This makes the method less sensitive for the detection of a minority metastable species. Even with the 'high UA/IOw Ps' ion source conditions, no indication of two distinct components was observed in the overall attenuation curve. Tests were then made to see if the NH+(a4E -) metastable content of the beam, if any, affected the observed CIE spectra. The entire magnet chamber was filled with acetylene, until the

129

intensity of the ion beam entering the collision cell was reduced to one-third. The spectrum remained essentially the same as that without beam attenuation. While this method still relies on the efficiency of filtering out certain beam components, the most direct test is a comparison of spectra obtained running the ion source in the two modes (without beam filtering). Again, no significant differences were found and we have, thus, ignored the possible presence ofNH+(a4E -) (see also Section 5). During the accumulation of the CIE spectra, the target gas pressure was typically 2.6 Pa; the dependence of the luminescence signal vs. target gas pressure was confirmed to be linear in the 0 3 Pa range. A complete spectrum in the 200600 nm range was taken for each gas at eight different energies between 10 and 1000 eVlab. Subsequently, the contributions of particular emissions were evaluated by integration of identified spectral contours. The correct normalization of the integral light intensity to the effective ion current at each energy was not without problems. It was found that, for collision energies below 100 eVlab, the ion beam current measured on a plate mounted behind the collision cell did not correctly reflect the NH + number density in the region observed by the optical detector. As a result of space charge repulsion and elastic scattering, the NH + beam spread out considerably within the collision cell. Consequently, only a fraction of the ion beam contributing to the light emission at the centre of the cell was actually recorded on the monitor electrode behind the cell. This - strongly energy dependent - fraction was determined by normalization to another luminescent reaction whose cross section, as a function of energy, is well known. A suitable normalization standard in the 15--1000eV~ab range is the luminescent charge exchange reaction (see Refs. [25,26]) He + + Ar---* Ar +* +He.

(6)

Remeasuring the emission cross sections for both the Ar +* 461 nm and Ar +* 476.5 nm lines, the

130

A. Ehbrecht et al./lnternational Journal t~/ Mass Spectrometry and Ion Processes 173 (1998) 127-141

correction factors necessary for a proper normalization of the NH + CIE signal were thus obtained. Below 15 eVlab, where no calibration data from the literature are available, the correction factors derived from [25] were extrapolated, down to 5 eVlab. In the region < 20 eVlab they are estimated to be accurate to within a factor of two.

NH states for CTE, are shown in Fig. 2. Considering CT, the correct choice o f the ionization energy (I.E.) of NH is somewhat problematic. There are two early experimental values of I.E. (NH), (13.1 _+ 0.2)eV [27] and (13.1 _+ 0.05)eV [28], and a more recent one, I.E.(NH) = (13.47 +_ 0.05) eV [22]. Ab initio results for I.E.(NH) are, in chronological order, 13.67 eV [8], 13.5 eV [10] and 13.66 eV [11]. In the present work the experimental value from Ref. [22] was adopted. The ionization energies of the rare gas atoms are taken from Ref. [ 17], those of the diatomic molecules from Ref. [29]. The electronic term energies shown in Fig. 2 are also from Ref. [29]. It is seen in Fig. 2 that the ordering of CIE versus CTE states varies with the ionization energy of the rare gas atom. For He and Ne

3. Energetics The analysis of the CIE experiments on CH + showed [1] that the relative intensities of the observed spectral features are strongly related to the respective positions of CIE-electronic states versus CTE-states. For this reason the states in question here, NH + states for CIE versus

15

He

10

Ne

eV 5 NH*(C)+M NH*(B)+M NH*(A)+M

NH+(a)+M

-At - - - ~ = - . =

H2

Nz

Kr Xe

0a

NH*(X)+M

NO

-5 Fig. 2. Energy levels of the C1E, CT and CTE reaction channels in collisions of NH* ions with a target particle M. Zero of the energy scale is for the reactant pair of ground state NH + (X2II) + M, bottom left. Two vertical arrows mark the CIE processes studied here, yielding excited NH + in the B2A and CzIC+ state. N H +(AZ]~ ) product was not observed. The rest of the figure shows the energies of the charge transfer reaction channels NH*(X) + M ~ NH + M + for nine target species M as indicated. In each case, the lower bar corresponds to the simple CT process forming the neutral NH product in the ground electronic state, NH(X31~ ). The upper bars correspond to the CTE channel, i.e. charge transfer with simultanous electronic excitation of the NH product into the NH(A31I) state.

A. Ehbrecht et al./lnternational Journal of Mass Spectrometo, and Ion Processes 173 (1998) 127 141

4. Results

Table 1 Conversion factors for transformation of the collision energy from the laboratory to the centre-of-mass frame Target

E l~dE cM

He Ne Ar Kr Xe H2 N2 02 NO

4.75 1.75 1.38 1.18 1.11 8.50 1.54 1.47 1.50

An overview spectrum for NH + + Ar collisions at 1000 eVlab is given in Fig. 3; this spectrum shows most of the features observed also for other target gases. The NH+(B-X) emission is the strongest, with the (0,0) and (1,0) bands (with heads at 434.8 nm and 397.0 nm) easily identifiable. The weak emission between 370 and 397 nm belongs most probably to the (2,0) band. The NH+(C-X) (0,0) band appears near 290 nm. It is here quite weak, partly due to unfavourable detection efficiency of the grating used to take this particular spectrum. Striking is the absence of the NH+(A-X) transition, which would appear in the form of red shaded band heads at 462.89nm [(0,0)-band] 431.27nm [(1,0)-band] and 534.94nm [(0,1)-band] [2]. The peak at 460 nm in Fig. 3 is definitely not due to the NH+(A-X)(0,0) band, but represents a group of unresolved Ar + emission lines resulting from charge transfer excitation of the target. In the high-resolution spectrum of Fig. 4, bottom panel, these lines are clearly separated and can be identified as the strong Ar + transitions 4p' 2F°5/24s'2D3/2 at 458.99 nm and 4p'ZF°7/2-4s'2Ds/2 at 460.96 nm [30]. The intense emission feature seen in Fig. 3

targets the CIE states lie much lower than those associated with neutral NH, and one can expect that the CIE process will dominate the spectra. On the other hand, for targets with low I.E. the CH + example [1] teaches that the CTE spectral features can appear quite strongly. This is also found in the present case of NH +. With Xe, O2 and NO targets the NH(A) emission from CTE is, in fact, the strongest spectral feature. This and other charge transfer channels can then even compete so strongly with CIE that the NH + emission is no longer recognizable. For a convenient transformation of the thresholds for various channels shown in Fig. 2 from the EcM to the Elab scale, Table 1 gives the respective conversion factors.

NH ÷ B-X) 200

NH* + Ar E~ab=1000 eV

(0,0)

~'~

NH(A-X)

131

y . . . . . . ............. .......

t--

"~ 100

..................-'"

(I,0)

-" NH÷(C_X)

~

o '

250

'

'

'

I

300

'

'

1

'

I

350

'

'

'

'

I

400

'

'

'

r

r

450

,

,

'

'

~

500

'

'

,

'

I

'

550

Wavelength [nm] Fig. 3. An overview spectrum for NH + colliding with Ar (recorded using a 150 l/ram grating, spectral resolution 2 nm FWHM, target gas pressure 2.5 Pa). The dotted line shows the relative spectral sensitivity of the detection system.

132

A. Ehbrecht et al./lnternational Journal q/" Mass Spectrometry and Ion Processes 173 (1998) 12 7 141

with a peak at 336 nm is the N H ( A - X ) band system, resulting from the luminescent charge exchange process NH + + Ar ~ NH(A3II) + Ar +. Fortunately, the N H ( A - X ) spectrum is well separated from the NH + band systems and does not interfere with their analysis. In this respect, the conditions for the study of CIE are much more favourable in the present case than they are with CH +, where the CH ÷ ( A - X ) spectrum is heavily overlapped by C H ( A - X ) emission [ 1]. Other luminescent processes also contribute to the spectra. Collision-induced dissociation of NH + gives rise to a series of Balmer lines. Fig. 3 shows an intense H a line, while H v is resolved in '

]

'

'

Fig. 4 (Ha lies outside the detector's sensitivity range). In some cases, N + emission lines also appeared. With Kr and Xe targets, collisioninduced emission of certain prominent lines of these neutral atoms and their ions was observed. All these processes are important and interesting in their own right, but are beyond the scope of the present study. They are summarized and briefly characterized in Table 2. Fig. 4 shows the collisionally excited NH+(B X)(0,0) band at higher resolution and for three collision energies as indicated. From the origin at 437.5 nm, marked by a minimum in the band contour, the P and Q branches run towards longer wavelengths, while the R branch forms a head at '

'

I

'

'

'

'

I

'

'

'

'

NH+(B2A.X2FI)

NH*+Ar E=ab=l5 eV

NH+(B2,~-X~I~)

NH*+Ar Elab=100 eV

NH*(B2A-X2F[)

NH++Ar Elab=1000 eV

o° 100

0

100

600

== O

Ar + w 300

Ar ÷

4,30

440

450

460

470

Wavelength [nm] Fig. 4. High resolution spectrum of the (0,0) band of the NH+(B-X) transition (1200 l/mm grating, 0.3 nm FWHM).

A. Ehbrecht et al./International Journal of Mass Spectrometry and Ion Processes 173 (1998) 127 141 Table 2 Emissions other than from NH + or NH observed in collisions of NH + with various targets Target

Emitter

Intensity at 1000 e V l a b

The lowest appearance energy [eVlab]

He

H N+ H N÷ H Ar + H Kr Kr + H Xe Xe + H H N~ H H

Weak Weak Medium Medium Medium Weak Medium Weak Strong Medium Strong Strong Weak Medium Strong Medium Medium

500 500 250 250 250 250 250 250 250 1000 50 50 250 50 50 100 250

Ne Ar Kr

Xe

H2 N2 02 NO

about 435 nm (for details, see a high-resolution photographic spectrum in Ref. [2]). Fig. 4 shows that the relative intensity in the long-wavelength region of the band, e.g. between 450 and 460 nm, increases markedly towards lower collision energy. In the P-branch, 450 and 460 nm correspond to rotational quantum numbers o f J = 11.5 and 17.5. Thus, lower collision energy favours

600

NH÷(CaF..XaH)

NH++Ar Etab=1000eV

IVVI

o>

0 270

the excitation of the high rotational levels in this region, relative to the lower ones. A similar trend was also observed in the CIE of CH ÷ [1] and N~ [17]. Fig. 5 shows a high-resolution spectrum of the NH+(C-X) CIE emission. It includes the (0,0) and (0,1) bands, with partial resolution of the rotational lines. Unfortunately it is here not possible to study the rotational excitation as a function of energy, since the C - X emission appears only near the high end of the collision range covered. Of particular interest are the excitation functions of the NH+(B-X) and ( C - X ) band systems, i.e. the dependence of the cross section for collisional excitation of NH+(B) and NH+(C) on the impact energy ECM, in the centre-of-mass system. To this end, the relative integral emission intensifies of these two band systems were measured, irrespective of any change of spectral contour due to a possible energy dependence of the rovibrational excitation (the rotational excitation of NH+(B) does, in fact, slightly depend on ECM, see above; the vibrational distribution between NH+(B), v' = 1 and 0 was estimated to be independent of ECM, to within 2 0 30%; for NH+(C) it was not measured at different energies).

(0,0) i/~

~'~

280

133

'1 . . . . . . . . ' l ' ' ' ' ' ' ' ' ' l ' 290 300 310 Wavelength[nm]

Fig. 5. High resolution spectrum of the C X transition in NH ÷ (1200 l/mm grating, 0.3 nm FWHM).

134

A. Ehbrecht et al./lnternational Journal o['Mass Spectrometry and Ion Processes 173 (1998) 12 7-141

For the B-state emission, the integration over the entire B - X band system was performed 'online', during the experiment itself, by operating the Mepsicron detector tube as a broadband, variable-width spectral filter. This is possible by taking advantage of the so-called 'edge-gating' feature of this tube, which allows one to count all photons impinging onto the cathode between two adjustable 'edges', corresponding to wavelengths )kmi n and )kma x. For example, for the NH+(B-X) system the edges would be set at )tram = 360 nm and Xm~x= 540 nm, cf. Fig. 3. In the most relevant part of this range, the detection probability does not depend very much on the wavelength, so the tube can be considered in this mode as a filter with an essentially rectangular transmission profile. This is a condition that a variation of the spectral contour with energy, if any, has no effect on the measured excitation function. The advantage of this 'filter' technique is a significant improvement in sensitivity. The spectrometer can be operated at very low resolution (80 FWHM, with the 150 1/mm grating and a very wide slit width of 500 t~m), which allows short data accumulation times and permits measurements of the integral emission intensifies at many different collision energies. The alternative, i.e. taking conventional spectra at fairly high resolution with subsequent, 'off-line' integration, is much more time-consuming. However, the two techniques complement each other. Fig. 3 and Table 2 show that there are several species other than NH + whose emissions fall within the NH+(B-X) spectral range. Such contributions must be eliminated from the data obtained with the edge gating technique, and this was done by making auxiliary spectral measurements in the conventional manner. With the 150 1/mm grating and a 125 #m slit, spectra were taken at fewer energies and with somewhat lower S/N ratio compared to the edge gating technique. From these, the contributions of the 'foreign' emission relative to the NH+(B-X) intensity were found by standard spectral integration, and were used to correct

the edge-gating results, with some interpolation for those energies where no spectra were taken. Edge-gating across the entire NH+(B-X) band system was only possible in the cases of He, Ne, Ar and H2 targets. With Kr, the contamination with numerous Kr + lines was very severe. With N2, the N~(B-X) emission interfered seriously, and also with 02 there was a strong background of foreign lines. In these cases, it was found advantageous to narrow the edge gating range so that it only included the distinctive (0,0) band peak of the NH+(B) emission. The normalization to the standard filter width as used with He etc. (a factor of 2.67) was again accomplished on the basis of resolved spectra. Note, however, that the narrow edge gating does introduce some uncertainty into the excitation function for Kr, N2 and 02, since energy-dependent features outside the filter range, such as the high-J end of the (0,0) band, are excluded. Towards low energy, where the rotational excitation is higher (see Fig. 4), an increasing fraction of the emission is lost. The relative cross section data in this region are therefore lower limits in these cases (e.g. for Kr, see Fig. 6). In the case of Xe the contamination with Xe ÷ lines was so strong, even in the region of the (0,0) band peak, that no useful data on the pure NH + emission intensity could be obtained. In addition, from Fig. 2 it is to be expected that the CIE process is here largely suppressed by the competition of charge transfer, forming NH(X) and NH(A) efficiently. This is certainly true with NO, where only at Elab = 500 and 1000eV some very weak NH + emission was observed. Thus, no CIE data for the low-ionization potential species Xe and NO are presented below. The excitation functions for the NH+(C X) emission were obtained not from the edge gating technique, but directly through integration of low-resolution °spectra (150 1/mm grating, 125 t~m slit, 20 A FWHM). Since this emission is only observed in the high energy-range, the number of data points is much less, and the time saving of the edge gating technique

A. Ehbrecht et al./lnternational Journal of Mass Spectromet~ and Ion Processes 173 (1998) 127-141

+ --o---al,---?--

~

2

l

He Ne Ar Kr

135

7 / 6 5

E ffl

4 8

3

1

2 1 . . . . . . . .

1

10

100

I

0

1000

ECM[eV] Fig. 6. Excitation function for NH+(B) tbr collisions with rare gas atoms. The Kr points at low energy are to be regarded as lower limits of the relative cross section (see the text).

becomes unimportant. In this case, the emission was unobservable not only with Xe and NO, but also with Kr as a target, in qualitative agreement with Fig. 2. Finally, two corrections to the excitation functions must be mentioned. Due to the finite lifetime of the emitters, part of the emission occurs downstream from the detector's field of view and escapes the observation. This loss was corrected for in a way described in Ref. [1]. Two sets of calculated radiative lifetimes are given in the literature; from Ref. [14] ~'(NH+(B)) = 1.0 t~s and r(NH+(C)) = 0.4 ~s, while Ref. [16] gives 0.6 and 0.3 k~s, respectively. The latter values were used here, because they are more recent. However, the choice of radiative lifetimes affects the shape of the excitation functions only above 100 eVlab: using the other value for NH+(B) would result in about twice as large an excitation cross section at 1000 eVla b (where the difference is the greatest); for NH+(C), the choice between 0.3 ~s and 0.4 t~s makes almost no difference for the cross section. Another correction concerns the NH + beam afterglow. This is an emission of the B - X system from the NH ÷ beam in free flight, with the target cell empty. It is due to long-lived ions which

survive the time of flight from their excitation in the ion source to the passage through the observation zone (see also the case of CH + [1]). Afterglow emission is only observable in the NH+(B-X) system, not in the C - X system, since the radiative lifetime of NH+(C) is too short. The afterglow intensity was measured separately with the edge gating technique at each energy and was subtracted from the collision-induced B - X signals. Its contribution was only significant at low ion beam energies, where the CIE signal is small. The absolute afterglow signal, on the other hand, did not depend much on the ion energy, Elab. Since the beam intensity is much smaller at low energy (see Section 2), the afterglow intensity per ion is much greater at small Elab. This is a result of the larger residence time of the ions within the observation region. Interestingly, the spectrum of the NH+(B-X) afterglow, although very noisy, seemed to indicate a greater v ' = l / v ' = 0 intensity ratio than the C1E spectrum. As with CH + [1], this could be due to a significantly larger lifetime of the v' -- 1 level. Figs. 6 - 9 show the excitation functions obtained as described in the foregoing. In Figs. 6 and 7 the data for NH+(B-X) emission

136

A. Ehbrecht et al./lnternational Journal o/ Mass Spectrometry and Ion Processes 173 (1998) 127-141

12

- - 0 - - N2 10

~,

3

I.-

8

E if)

O F-

E

ffl

,<

<

t-

6

e.-

t-

o (..)

4

E/

O TO

2

I I

,,,,~

,

1

,

,

,

,,r,

I

,

10

,

,

,

0

, , , , , , , , [

,,,

100

1000

ECM [eV] Fig. 7. Same as Fig. 6, for three diatomic molecules as targets.

are given. The points at the highest energies are shown on a reduced scale, in order to present them alongside the important low-energy points. For the C - X emission, Figs 8 and 9, this was not necessary since there is no signal from this process at low energies. The ordinate scale is common to all figures. The 1:2 detection sensitivity ratio of C-state versus B-state emission has been taken into account in the normalization of Figs. 8 and 9 relative to Fig. 6 and Fig. 7. Thus,

the cross sections for excitation of the NH+(C) state is less than one-tenth of that for the NH+(B) state. Furthermore, the units of the ordinate scale give an idea of the absolute number of photon counts recorded in a typical experiment. For example, for NH + + Ar at 1000eVlab,are I is 5.1 counts/nA s mtorr. With an NH + current of 16 nA and an Ar pressure of 20 mtorr (see Section 2), this should give a count rate of 1632 counts/s. However, the light loss due to

0.25

0.20 0 I.-

E ¢,0

0.15

== 0.10 8 e

0.05

0.00

'?

10

100

V

'~

1000

EcM[eV] Fig. 8. Excitation function for NH+(C) for collisions with rare gas atoms.

A. Ehbrecht et al./lnternational Journal o f Mass Spectrometry and Ion Processes 173 (1998) 12 7-141

137

0.4

f

0.3 O I-E

<

0.2

ee-O

o. t~

0.1

0.0 10

100

1000

EoMleVI Fig. 9. Same as Fig. 8, for three diatomic molecules as targets.

the 0.6 #s lifetime of NH+(B) is very large at high energy; at 1000 e V l a b it amounts to a factor of 6.2, so that the actually measured count rate was 268 s -1, corrected for NH + beam afterglow. With an accumulation time of 38 s, the statistical error is then 1%. 5. Discussion

5.1. Excitation functions Compared to the collision-induced excitation (CIE) of CH + studied previously by us [ 1], CIE of NH ÷ is a weaker process. For example, for NH+(X) colliding with He at Elab ---- 1000 eV, the integral photon count from the NH+(B-X) transition i s 3 counts/nA s mtorr (Fig. 6). For CH + + He at the same energy, the corresponding light yield for the (integrated) CH+(A-X) system is about 18 counts/nA s mtorr. However, from an experimental point of view, the smaller cross section in the present case is offset by the approximately six times larger NH + ion current available. In addition, the light loss from the observation region is - 3 0 % smaller with NH +, due to its shorter lifetime. It is interesting to contrast the electronic

rearrangements involved in these two experiments. In the CH + case the excitation from the CH+(XIE +) ground state to the A lII and B lA states represents collision-induced transitions from a ~r2 configuration to oTr and 7r2, respectively. Thus, it is plausible that the latter process, requiring the promotion of two electrons and having a threshold energy more than twice that of the A-state excitation, has a smaller cross section and a higher onset on the collision energy scale, cf. [ 1], Figs. 9 and 10. In the present work, excitation from the NH+(XZII) ground state with the configuration o27r into the A2~ -, B2A and C2~ + states was investigated. Here all three possible product states have the same configuration, aTr2, at least in the region of re [8]. Their energetic ordering is seen in Fig. 1. Since the three states all combine radiatively with the ground state, one would have expected about equally strong collision-induced emission from all of them, with similar excitation functions. Instead, surprisingly, no A-state emission at all was observed. Further, the B-state excitation functions have an anomalous shape; they start at quite low energy ( < 10 eVcM) with a plateau or even a small maximum, which was not observed in the CH + cross section curves (except

138

A. Ehbrecht et al./lnternational Journal o/'Mass Spectromet~" and Ion Processes 173 (1998) 127 141

that for CH + + N 2 collisions). Then, towards higher energy, the B state cross sections rise in a way resembling the CH + case: for the light collision partners a gradual increase sets in at medium energy (20-50 eVcM), while for the heavier targets the rise is more sudden and occurs above - 1 0 0 eVcM. The C-state excitation functions, finally, follow throughout this latter, CH+-like pattern, but the cross sections amount to only a few percent of those for the B-state. Such different behaviour of the A, B and C states, despite their identical electron configurations, is not easy to understand. From a simple pseudopotential approach [31 ] the diabatic interaction potentials and the coupling matrix elements between them are determined only by the electron orbital occupancy and should, therefore, be identical in the three cases. The same should then also be true of the excitation functions. Obviously the non-adiabatic transitions responsible for the CIE are controlled sensitively by finer differences between the potentials and couplings, which are not described by the above crude model. In several ways the three excitation processes are similar: they all must proceed via conical intersections, since the transitions 2II'---+2~-, 2II-"~2A and 2II"+2~+ are all symmetry forbidden for collinear approach of the target particle, but become allowed for bent conformations. For these, the 2II'--'+2]~- excitation can proceed through crossings between surfaces of A" symmetry, 21"1"-+2A through A' as well as A", and 2II ._.+2£+ through A' symmetry surfaces. Also, the overall spin is, of course, preserved in each case. However, the individual orbital spins are different for the A2]~ - state on the one hand and the B2A and C2~ + states on the other. All these states can be viewed as arising from the combination of the 30 electron with a (171-)2 configuration, where the latter two electrons can arrange themselves according to the triad s Z - , l A or 113+ ([32], Table 31, cf. the well-known example of O2). Then the A213 - state (as well as the a 4p. state) derives from the (30)l[sP~ (171"2)] combination, while BzA and C 213+ have the (3o) 1[1,5(17r2)] and

(3o)1[1~3+(17rZ)] parentage, respectively ([32], Table 32). The CIE process consists in a promotion of a 30 to a 17r electron, NH+(X; 30217r)---*NH+(A, B, C; 3olrc 2)

(7) where this 1re-electron enters into a triplet combination with the other 17r-electron in the case of the A-state, but into a singlet combination for the B and C states. Now it is well known that in the former case the two electrons are spatially kept apart by the Pauli principle, and hence interact less strongly (which is the basis of Hund's rule). This may explain why the A-state excitation, although not rigorously forbidden, is actually not observed. The principal characteristic of the B-state excitation functions is that they appear to consist of two components, the low-energy plateau (or maximum), and the usual rise at higher energy. These two components are ascribed to two distinct CIE reaction paths, differing in the location of the avoided intersections between the NH+(X) + M entrance and the NH+(B) + M exit potential energy surface. It is suggestive to associate these two mechanisms with the two different symmetries of the triatomic system, A' and A", which exist in this case. Clearly, the NH+(XZII) + M interaction potential will be very different if the ~r-electron is oriented in the N H - M plane (A') or perpendicular to it (A"). This idea is supported by a comparison with the other three CIE reactions studied by us, i.e. the NH+(C), CH+(A) and CH+(B) excitation. Each of these proceeds via surfaces of only one reflection symmetry (A'), and in each case the cross section curves exhibit only the typical rise at high energy (an exception is CH+(A) formed in N2 collisions [1], but with a molecular target the A', A" classification does not strictly apply). Unfortunately, nothing more definite can be said at this time, not even which symmetry is responsible for the low-energy and which for the high-energy portion of the NH+(B) excitation function.

A. Ehbrecht et al./lnternational Journal c?fMass Spectromet~ and Ion Processes 173 (1998) 127-14l

Considering the C-state excitation, the most striking observation is its low efficiency. This is probably related to the fact that the threshold energy for this process is quite high. There are then many competing reactions, such as collision-induced dissociation or charge transfer with the target (see Fig. 2), which will tend to reduce the yield of NH+(C) product.

5.2. Vibrational product excitation The question is of interest whether, along with the electronic CIE, the NH + ions are also vibrationally excited. A theoretical model [ 18] shows that in the limit of high collision energy the relative cross sections for populating different v' levels should be proportional to the FranckCondon factors between v' and the reactant v" levels ('vertical excitation'). We have tested this prediction for NH + + Ar CIE collisions, both for NH+(B) and NH+(C) formation. The relative cross section for production of NH+(B) in the vibrational levels v' = 0 and 1 were obtained from the spectrum shown in Fig. 3 in the Franck-Condon approximation as 4

4

ao_ Io, o ql,OVl,O/~v'Al,v"_ I0,0 ql,0Vl,07"0 4 o/Xv"Ao, v" Ii,o qo, oVo4,ovl °1 11,0 qo.oUo,

(8)

Here I0,0 and I~.o are the relative intensities of the (0,0) and (1,0) bands read from the spectrum by either of two methods: integration over the band profile, with estimated corrections for band overlap, or simply from the height of the sharp band edges. In the latter case, contributions of atomic lines had to be subtracted, i.e. H7 at the (0,0) band edge (Fig. 4), and a N + line at the (1,0) band edge. These lines appear only at high energy; for collisions with He and Ne, they are much stronger than with Ar. Both methods gave a similar result, l,°-2-°~ 7.5. In Eq. (8), qv' v" and vv,,v,, are the emlssnon Franck-Condon factors and (mean) band frequencies, respectively, and To, ~~ are the radiative lifetimes of the v'=0 and 1 levels. The second factor on the r.h.s, of Eq. (8) is simply •

.

Jl,0

'

139

the relative fraction of the total v' -- 0 and 1 emission which occurs in the observed (0,0) and (1,0) bands. Calculated lifetimes r0 = 604 ns and ~-~ --- 640 ns were taken from Ref. [16]. The result from Eq. (8) was Oo/al = 2.1 for At. Very similar ratios were obtained for He and Ne. According to the 'vertical excitation' mechanism, these experimental results were then modelled by the appropriate Franck-Condon factors. If all NH+(X) reactant ions are assumed to be in the v" -- 0 level, the FC model predicts Oo/Oi = qo.o/q~.o = 0.82:0.16 = 5.1, far greater than the experimental result• However, good agreement can be achieved if, still within the FC model, vibrationally excited reactants are considered. This was already found necessary in our related work on CIE of CH + (see eq. (12) in Ref. [1]). 2 Assuming a Boltzmann distribution over the NH+(v ") levels with Tvib as an adjustable parameter, the FC model then yields o0/a~ = 2.1 for Tvib = 5000 K. This happens to be the same temperature which also gave an optimum fit in the CH ÷ case. From spectra taken with NH + + Ar at 10 eV~ab and 100 eVla b (not shown here), quite similar cross section ratios a0/a~ were obtained, i.e. 1.8 and 2.2, respectively. This is interesting, since it indicates that even at low collision energy the FC model still holds in this case. For CIE of CH + also, the v ' = l / v ' = 0 ratio was found to be essentially independent of the collision energy and the target species [1]. For N~- + He CIE collisions [18], on the other hand, the product vibrational distribution has been reported to change somewhat in the Elab = 100-1000 eV range, tending towards a FC distribution. Finally, we also extracted the vibrational excitation of NH+(C) by CIE from the spectrum shown in Fig. 5, Here an intensity

'

2 In the present work we decided to use the notation 'cross section av,', instead of the equivalent, but somewhat ambiguous term 'vibrational population P(v')' employed in [1]. Note, however, that or, is an effective cross section, which includes all channels populating a given v' level from different v" levels.

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ratio Io,o/I~,o = 5.0 was derived from the step heights at the respective band heads. With the FC factors q0,0 -- 0.7, q 1,0 = 0.23 and the ratio (~l.0/v0,0) 4 = 1.25, Eq. (8) then yields ao/al = 2.1. The 'primitive' FC model (NH+(X) in v"= 0 only) predicts Oo/Ol = 0.7/0.23 = 3.0, which is again too large, while a modified FC model, including v" > 0, can reproduce the experimental result. Interestingly, the best fit was achieved with the same degree of NH+(X) vibrational population as before, i.e. Tvi b : 5000 K. This is a strong support for this model. Unfortunately, the NH+(C) vibrational population could not be followed down to low energies, since the emission is then very weak (Fig. 8 and Fig. 9). In summary, it can be stated that the observed population distribution of NH+(B) and NH+(C) v' levels reflects for the most part merely the vibrational excitation already present in the reactant NH+(X) ions. The collisional process as such does not add significantly to the population of higher v' levels. This is in stark contrast to the production of NH+(B) by the chemical exchange reaction N + + H2 [7]. Here, the chemiluminescent product was found to be highly excited into levels up to v' = 6. 5.3. M e t a s t a b l e N H + ions

In this work we have found no evidence for any contribution ofmetastable NH+(a4E -) ions to the CIE signal. According to Section 2, it is possible that a significant NH ÷ beam fraction was in fact in this state. In the case of collisions with 02 one might then have expected to observe additional NH+(B,C) production, since the extra spin of this target could conserve the overall spin despite the spin change of the NH+: NH + (a4~ - ) + 02 (X3 ]~g ) --~ NH + (B2A, C2E + ) + O2(X3 Eg )

(9)

However, Figs. 7, and 9 show, in fact, rather low cross sections with O2. In related experiments with CH + it was found that 02 (as well as NO) targets

did not enable the spin-changing CIE process

CH+(XI~+)+O2(X3~g),NO(X2II) ---, C H + ( b 3 ~ - ) + O 2 , NO

(10)

to occur, since no CH+(b32 - ---, a3II) emission was seen for this reaction. By analogy, collisions o f N H + with O2 may not promote a spin change of the ion, either. The absence of any enhanced NH+(B, C) emission from Eq. (9) thus does not necessarily indicate that the NH+(a) beam fraction is low, but simply that it does not matter for the excitation of NH+(B or C). Another question is whether the metastable NH + ions can undergo CIE w i t h o u t spin change to some higher NH + quartet state. The analogous excitation of metastable CH +, CH+ (a3II)+M ~ CH+ (b3E-) + M

(11)

was, in fact, observed in [1] by the subsequent CH + (b ---, a) emission, despite the very small metastable beam content in this case. M could here be a spinless particle such as argon. In the present case, however, the first excited NH + quartet state above NH+(a) lies at very high energy. In Ref. [8] this question was specifically addressed. It was found that the first bound quartet state which could radiate into NH+(a) is NH+(4II)( 1 a)2(2a)l(3a)1( 1703. However, its emission is predicted to occur around 1000 and would be unobservable with our detection system. Moreover, at such high energy ( - 1 3 eV) competing processes would almost certainly suppress any emission (cf. the discussion on the C-state above). 6. Conclusions

in this work, the collisional excitation of ground state NH+(X2H) ions into the three radiative states A2~ -, B2A, C212+ was investigated. Since all three of these states derive from the same leading configuration aTr2, it was interesting to see if and to what extent this was reflected by the optical emission properties. Well resolved B - X and C - X

A. Ehbrecht et al./In ternational Journal of Mass Spectrometry and Ion Processes 173 (1998) 127-141

emission spectra were obtained. The emphasis, however, was on measurements of the excitation functions, covering the collision energy range 11000 eVcM. The identity of the collision partner (He, Ne, Ar, Kr, H2, N2, O2) has little effect on the general shape of the cross section curves. However, these functions are very different for the three product states. Surprisingly, no A-state emission was observed at all, at any energy or with any target. So the A-state 'excitation function' is zero throughout, although A2]~- is the lowest-lying excited state of NH*. The B-state excitation function appears to have two separate components: a low, fairly flat pedestal setting in at less than 10 eVcM, followed by a distinct rise at medium (30-100 eVcM) energies. Finally, the C-state emission is concentrated in the regime --> 100 eVcM and increases steeply with energy, but the cross section never reaches more than a few percent of that for B-state emission. Clearly the individual properties of the potential energy surfaces in the three cases have to be considered in detail, going well beyond the simple description of the process as a a2~- ~ aTr2 excitation, which is common to all three CIE reactions. Regarding the A-state, there is no fundamental reason forbidding its excitation. A qualitative argument is put forward as to why the coupling between the entrance and exit surfaces may be weaker than in the other two cases. The two-component nature of the B-state cross section curve is interpreted in terms of two pathways leading from NH+(X2II) to NH+(BZA), via different avoided crossings. One will be along two intersecting diabatic surfaces of A' symmetry, the other, of A" symmetry. The C-state excitation is the simplest case and resembles superficially the previously studied CIE-process C H + ( X l t 2 + ~ All-I), (7 2 ~ O"71" excitation. In both cases there is only one overall symmetry, A', and the excitation functions rise monotonically. Acknowledgements A.K. thanks the gemeinschaft for a

Deutsche Forschungsstipend. Very helpful

141

discussions with Professor E.E. Nikitin are gratefully acknowledged.

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