Unimolecular decay of metastable Ar cluster ions. Evolution of magic numbers in Ar cluster mass spectra

281

International Journal of Mass Spectrometry and Ion Processes, 74 (1986) 281-301 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

UNIMOLECULAR DECAY OF METASTABLE EVOLUTION OF MAGIC NUMBERS IN Ar CLUSTER MASS SPECTRA

Ar CLUSTER

w.RITTER

* * *, K. STEPHAN

T.D. MARK *, P. SCHEIER, and A. STAMATOVIC ++ Institut fir Experimentalphysik, (First received

27 November

K. LEITER

**,

IONS.

+

Leopold Franzens Universittit, A 6020 Innsbruck (Austria) 1985; in final form 20 October

1986)

ABSTRACT Unimolecular dissociation processes Ar: * Arz (with n up to 25) were investigated with a double focussing sector field mass spectrometer. Ar,: (with n up to 97) and Ar,f+ (with n from 91 up to 191) were produced by electron ionization of Ar clusters formed in a supersonic nozzle expansion under various stagnation gas conditions. In addition to collision-induced dissociation, the existence of metastable dissociation processes in the ps time scale was confirmed. In particular, it was demonstrated that metastable dissociation rates are strongly dependent on the stagnation gas temperature for Arc and Ari (with Ar:* + Arl reaching a record metastable decay rate of 26000 s-l at - 10” stagnation temperature), whereas me&table decays of Ari and Arc are roughly independent of stagnation conditions. Furthermore, we describe the observation that metastable decay not only occurs via the loss of one monomer, but also, for Arc, Ar; , and Arc, via the loss of two monomers. Conversely, larger clusters (Ar& to Ar&,) only decay in a metastable dissociation via the loss of one monomer. For cluster sizes 10 I n I 25, the highest total dissociation rate is found for Ar+*a (8850 s-l). Moreover, because anomalies in the dissociation rate correspond to an abundance anomaly (magic number) in the normal mass spectrum, it is possible to conclude that the occurrence of magic numbers is caused by the intrinsic stability of the cluster ions rather than being due to the neutral growth process. These results are shown to be in agreement with recent calculations. In addition, we have studied collision-induced dissociation as a function of stagnation temperature for Ar; , Ar; , AI-:, and Arc. For Ar$, we were able to deduce an estimate of the collision-induced dissociation cross-section as a function of reaction product all the way from Arii d Arc0 to Ar& + Ar; .

* To whom correspondence should be addressed. ** Permanent address: Ionentechnik GmbH, A 6020 Innsbruck, Austria. *** Permanent address: Planseewerke, A 6600 Reutte, Austria. + Permanent address: Universitltsklinik fiir Hiir-, Stimm- und Sprachstorungen, Franzens Universitlt, A 6020 Innsbruck, Austria. ++ Permanent address: Department of Physics and Meteorology, PMF Beograd, 550, 11001 Beograd, Yugoslavia.

0168-1176/86/$03.50

0 1986 Elsevier

Science Publishers

B.V.

Leopold P.O. Box

282 INTRODUCTION

Recently, there has been considerable interest in the properties of both neutral (van der Waals) and ionized clusters, because clusters form a link between the gas-phase monomer and the condensed phase [l]. The most commonly used cluster production technique (i.e. adiabatic expansion), however, produces a cluster beam containing a distribution of sizes. Thus, a study of the properties of individual clusters is difficult. Moreover, in most studies, the detection method used has been mass spectrometry, the cluster ions being formed by either electron or photon ionization methods. It is clear that the observed cluster ion mass spectra may differ from the neutral distributions due to size-dependent ionization and detection efficiencies [l]. For instance, several experiments have reported and demonstrated the occurrence of large (prompt) dissociative ionization probabilities for van der Waals clusters (see, for example, refs. 2-8; for a comprehensive review, see ref. 1). Next, it has been established that cluster ions produced by electron (or photon) ionization of neutral van der Waals clusters may be metastable due to either electronic, rotational, or vibrational predissociation mechanisms [9-161. Moreover, in some of these cases, the amount of metastable ions compared with stable ions [i.e. Arc, Ar: , (CO,)O-] has been found to depend on the expansion conditions (neutral distribution) [17] and the ionization process employed [14,18]. It was noted in 1982 by Stephan and Mark [17] that “one has to account for the unimolecular dissociation when using impact ionization plus mass spectrometry to probe neutral cluster beams”. Furthermore, State and Moore [19], Echt et al. [20], and Aleksandrov et al. [21] have shown recently that ion abundance anomalies in mass spectra of H,O and Ar clusters, initially attributed to the enhanced stability of certain neutral cluster sizes, are probably caused by delayed evaporation of monomers following electron ionization. In the present study [22], we have extended these previous investigations of State and Moore [19] on the stability of Ar ion clusters (in particular the stability of the ions around Ar&) in several ways. First, the unimolecular into decay of the Arz clusters (10 I n I 25) was separated quantitatively true metastable decay and collision-induced dissociations with the background gas. Metastable decay only proceeds via the loss of one monomer or, in the case of small clusters, two monomers, whereas collision-induced dissociation is observed to proceed via almost every conceivable dissociation channel. Metastable dissociation rates were found to vary strongly from cluster to cluster. It was possible to conclude that the well-known ion abundance anomaly at Ar& (the Ar& ion signal is always much lower than the Ar& and Ar.& ion signals) is probably caused by the large metastable decay of the Ar& compared with the metastable decay of Ar& or Ar& in the

283

above-mentioned time window and before. It is interesting to note that similar results have just been obtained for Xe clusters by Kreisle et al. [23]. Moreover, such behaviour has also been observed recently for alkali halide cluster ions ejected from a surface upon heavy-ion bombardment [24]. In addition, a series of measurements was performed to determine the influence of stagnation conditions (temperature) on the metastable yield of small cluster ions (i.e. Arl, Ar:, Ar4+, and Arl) in order to confirm previous observations showing no influence for Arc and a strong influence for Arc [17,18]. EXPERIMENTAL

The supersonic beam-electron impact ion source-mass spectrometer system and the general experimental technique have been described in detail

Pump

(I

kField

/Skimmer

fr ee region Fi

/

Pump to elec& sector field

ITI

\Tempel .ature bath

Gas inlet

Fig. 1. Schematic view of apparatus. P, Pusher; C, collision chamber; CL, capillary-leak gas inlet; A, aperture; N, 20 pm nozzle for molecular beam gas inlet; L,, collision-chamber exit-slit electrodes (L,, P, and C are at a common source potential of typically +3 kV); L,, penetrating-field extraction electrodes; L, and L,, beam-focussing electrodes; L,, earth slit (end of accelerating region); D, defining slit; L6, L,, Ls, and L9, beam-centering and deflection electrodes; Si, mass-spectrometer entrance slit; S2, entrance magnetic sector field. The length of the field-free region is 41 cm. At a stagnation pressure of 5 bar Ar, the pressure in the differential pumping stage is 5 10e3 Torr and in the ion source (1O-5 Torr, respectively.

(4

,

hhhL*

I

8 I

I

15

12 Cluster

; c

size

20

25'

n

(b)

IO3

c

=! IO2 -2

E IO’ aJ f

IO0

”

-

= 0

12 0

IO

5 Cluster

15

20

size

(cl

. “**s*“.. -**.-- . .. . .it

.$“., *.....”

.

.

“*... -... . 0’ %._ *

A(+.

-2.

a-

“5..

‘... _*_-- 5.s

.

10

20 Cluster

30

CO size

50

60

70

80

/charge

Fig. 2. Mass spectra of Ar clusters produced by supersonic expansion under different experimental conditions. (a) Stagnation pressure 5 bar; stagnation temperature - 60” C; electron energy 70 eV. (b) Stagnation pressure 4 bar; electron energy 70 eV. The spectra are normalized to the monomer. (c) Stagnation pressure 500 Torr; stagnation temperature - 180’ C; electron energy 80 eV.

285

previously [1,10,11,17,18]. Modifications include the addition of an ultra-high pressure gas inlet system and the addition of a differential pumping stage between nozzle and ionization region. A schematic overview of the apparatus is provided by Fig. 1. In short, the experimental set-up consists of a modified [25] three-electrode open type electron ionization source and a high-resolution double focussing reversed geometry sector field mass spectrometer. Neutral rare gas clusters are produced by expanding up to - 10 bar Ar under constant temperature (variable between room temperature and liquid nitrogen temperature) through a 20 pm nozzle. The expanding supersonic gas beam containing the neutral born clusters

TABLE

1

Sampling times for the observation voltage of 3 kV

of molecular

dissociation

of Ar:

with an ion accelerating

tt is the time after production of Ar: to reach the first field-free region; time, i.e. the time for ions travelling through the first field-free region. Cluster ion size, n 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

At W 1.8 2.6 3.2 3.7 4.1 4.5 4.8 5.2 5.5 5.8 6.1 6.3 6.6 6.8 7.1 7.3 7.5 7.8 8.0 8.2 8.4 8.6 8.8 9.0 9.1

3.2 4.6 5.6 6.5 7.2 7.9 8.6 9.2 9.7 10.2 10.7 11.2 11.7 12.1 12.6 13.0 13.4 13.8 14.1 14.5 14.9 15.2 15.6 15.9 16.2

At is the sampling

286

passes a skimmer (differential pumping stage) and is crossed at right angles by an electron beam of variable energy and current. The cluster ions produced are extracted at right angles from the ionization region by a weak electric field, analyzed in the mass spectrometer, and detected with a CuBe conversion dynode followed by a channel electron multiplier used as either analog or counting ion detector. Figure 2 shows typical mass spectra obtained using different expansion conditions. The cluster size distribution is strongly dependent on these conditions [Fig. 2(b)]. As reported previously [26-301, deviations from an otherwise monotonic decrease are discernible at certain cluster sizes, most notably the strong anomaly at Ar&. The present source produces neutral argon clusters beyond Ar,,. The mass spectrometer detection limit lies, however, at around 4000 u, allowing the detection of singly charged cluster ions up to Ar& and of doubly charged cluster ions up to Ar&: [the critical size for the occurrence of doubly charged clusters being Arii+ [31], see also Fig. 2(c)]. It is also interesting to note that the doubly charged cluster ions show characteristic abundance anomalies, most notably the strong anomalies between A$ and Ar$ . The fragment ions produced by unimolecular dissociation of Arc cluster ions in the (first) field-free region (see Fig. 1) of the mass spectrometer

I

I

I

I

I

L

8

12

16

20

Gas pressure

in field free region in 10m5 Torr

Fig. 3. Precursor (AT&) and product (Ar&) ion currents as a function of uncorrected gas pressure in the field-free region for the unimolecular reaction Ar& + Ar& +Ar. Stagnation pressure 4 bar; stagnation temperature - - 60” C.

287

system (see Table 1 for the respective sampling window) are analyzed by the standard method of decoupling the acceleration and analyzer fields [32] (see also ref. 11). As a result of the background pressure in the ion source and in the first field-free region (approx. lop5 Torr at a stagnation pressure of 5 bar), possible metastable dissociation processes Ar,+*+Ar,+_,+Ar

(I)

are accompanied by collision-induced

dissociations

Am+ +Ar-+ArT_,+2Ar

(2)

In order to distinguish between processes (1) and (2), the gas density in the first field-free region (which has a linear relationship with the gas density in the ion source and is measured with an ionization gauge) was varied by throttling the pumping speed in the ion source. Figures 3 and 4 show, as an example, characteristic results for two different cluster ions under two different conditions. In accordance with previous studies of this type [10,11,18,32-341, a linear dependence on pressure was obtained for the ratio,

I

I

I

I

I 2 Gas pressure

I

3 in field

free

L region

I

5 in Id4 Torr

I

L

6

7

Fig. 4. Precursor (Ar&) and product (Ar&, Arc,, and AT&) ion current intensities as a function of uncorrected gas pressure in the field-free region for the unimolecular reactions Ar& -+ Ar& + Ar, AI& -+ Ar&, and Ar+21 + Ar&, respectively. Stagnation pressure 10 bar; stagnation temperature - 60” C.

288

,” .0 L

0.02

-6 E 7 ”

0.01

c z

0

0

1 Pressure

2

3

(lOmL Torr)

Fig. 5. Ratio of product ion current to precursor ion current as a function of uncorrected gas pressure in the field-free region for the unimolecular reactions Arf + Arc +Ar and Ar: + Ari + 2 Ar, respectively. The lines shown represent the computer fit (see text).

Y, between dissociation product and precursor ion (see Figs. 5 and 6), thus indicating a thin target condition for processes (2). Solving the reaction equation leads, in this case, to [34] y=

L%-II = exp( [AC1

At, + UK&) - exp{ A( t, - At)}

Pressure

(lcL

Torr

(3)

1

Fig. 6. Ratio of product ion current to precursor ion current as a function of uncorrected gas pressure in the field-free region for the unimolecular reactions Ar& + Ar& + Ar and Ar$ -+ Ar& + 2 Ar, respectively. The error bars shown symbolize the width of the scatter of the experimental data obtained. The lines shown represent the computer fit (see text).

289

where [Ar,+_,] is the measured fragment ion current, [Arz] is the measured precursor ion current, t, is the flight time of Ar,f from L, (the beginning of the field-free region) to the ion detector at the end of the mass spectrometer, At is the flight time of Ar: in the field-free region, X is the metastable decay rate, CJis the collision-induced cross-section, L is the length of the field-free region, and n is the number of gas atoms per unit volume. Extrapolation of the measured ratio, r, to zero pressure gives the metastable fraction decaying in the time window, At, and also the metastable reaction rate, h. From the slope of r = r(n), an estimate of the cross-section for collision-induced dissociation can be derived (see, for example, refs. 11, 18, 34-36), assuming that the length of the field-free region (41 cm) equals the length of the collision cell and assuming that the uncorrected pressure is representative for this interaction region. This simple analysis (it has been used here only as an additional test) is only possible if h is constant [36], the pressure regime is low, the collisioninduced dissociation cross-section is small (range of validity of thin target condition), and if no other competing reactions occur. In the case of

0

2

4 Pressure

b (16L

t)

IU

Torrid

Fig. 7. Ratio of product ion current to precursor ion current as a function of uncorrected gas pressure in the field-free region for the unimolecular reactions Ar\r2: -+ A& + Ar and Ar& + Arc, +2 Ar, respectively. The lines shown represent the computer fit (see text). The expanding conditions (expansion pressure 10 bar) are different from those in Fig. 6 (expansion pressure 5 bar), thus allowing measurements up to higher pressures. The obtained data for X and u are slightly different from that in Fig. 6. This is probably due to the different expansion conditions (see also similar effects for the smaller cluster ions, e.g. Figs. 8 and 9).

290

competing reactions of the general form Arz* + Ai-:_, + Ar +Arz_,+2Ar

(4

and Arz +Ar-+Ar,+_,+2Ar + Ar,+_, + 3 Ar

(5) the analysis is much more difficult (coupled differential reaction rate equations) and has been made with the help of a computer program [37]. This program yields the corresponding Xi and ui of such a decay reaction system from the measured pressure dependencies of the precursor ion and the various fragment ions (for practical purposes, two measurements at two different pressures are sufficient). In order to check the program and the accuracy of the constants obtained, we have also used this program to compute the pressure dependence of the various ion ratios Ar,+_,/Ar,+, Xi and ai values. These Arz- / Ar,+ , and so on using the determined calculated values are shown for several examples as full lines in Figs. 5-7. It can be seen that there is very good agreement between the calculated curves and the experimental data, even for the high-pressure case (Fig. 7) with a non-linear pressure dependence. RESULTS

Stability

AND DISCUSSION

of small Ar cluster ions: Ar2+, ArJ+, Ar4+, and Ar5’

Figures 8 and 9 and Tables 2-5 give the obtained results (metastable decay rates, X, and estimates of the collision-induced dissociation cross-sec-

200 -; II;:& 0”

M n

40-

, I ( 0 -20 -40 Temperature (“C) Fig. 8. Metastable decay rate, X, as a function of stagnation temperature for the dissociation reaction Ar; * + As+ + Ar. I -GO

291

Tempcruturc

(“C

)

Fig. 9. Metastable decay rate, A, as a function of stagnation temperature for the dissociation reactions Arc * + Ar: + Ar and Ar: * --, Ar+ + 2 Ar (or Ar2), respectively.

TABLE

2

Metastable reaction rate, X, and estimates of the collision-induced dissociation cross-section, 6, as a function of stagnation temperature for the unimolecular dissociation reaction Ar; +Ar++Ar Temperature

x (s-l) u (lo-”

TABLE

m2)

(’ C)

-60

-50

-40

-30

-20

-10

0

+10

50

96 0.08

69 0.08

80 0.10

14 0.11

60 0.15

170 0.14

91 0.17

0.07

3

Metastable reaction rate, h, and estimates e, as a function of stagnation temperature Temperature

of the collision-induced dissociation cross-section, for unimolecular dissociation reactions of Ar:

(O C)

-60

-50

-40

-30

-20

-10

0

105 0.14

50 0.15

130 0.15

180 0.17

490 0.28

1540 0.33

4520 1.1

18 0.03

36 0.02

66 0.03

74 0.04

220 0.01

680 0.14

A r3+ -+ Ar2+

x (s-l) 0(10-~~

m’)

A r3+ -+Ar’ x (s-l) u (10d20 m2)

890 0.14

292 TABLE 4 Metastable reaction rate, h, and estimates u, as a function of stagnation temperature Temperature

of the collision-induced dissociation cross-section, for unimolecular dissociation reactions of Ar;

(O C)

-60

-50

-40

-30

Ar4+ + Ar,+ A (s-l) u (10-20 m*)

130 0.41

190 0.39

220 0.38

180 0.37

Ar4+ + Ar,+ x (s-l) ff (10-20 m’)

280 0.12

210 0.10

260 0.17

740 0.38

-20

-10

210 0.45

4820 2.7

25 900 4.9

tion, a) for the precursor ions Ar: , Arc, Arl , and Arc. It can be seen that, within the experimental error (estimated from repeated measurements to be less than a factor of 2 for a A of 100 s-l and approx. 10% for X 2 1000 s-l), the metastable decay rates are approximately independent from the stagnation gas temperature, the only exceptions being the reactions Arc * --, Arc, Ar: * + Ar+, Art * + Ari , where X strongly increases with increasing temperature. Moreover, in most cases (except for Ai-,+), the loss of one monomer is the most probable dissociation channel, although loss of more than one monomer via metastable and collision-induced dissociation can also be observed. Ar2+. The present result [i.e. the existence of metastable Arl, albeit in only small quantities; i(Ar+)/i(Ar,+) 5 0.05 % and X < 200 s-l] confirms our previous observations [38] of the occurrence of metastable dissociation

TABLE 5 Metastable reaction rate, A, and estimates u, as a function of stagnation temperature At temperatures ful observations.

of the collision-induced dissociation cross-section, for unimolecular dissociation reactions of Arl

higher than - 20” C, the ion intensities

Temperature

are too small to allow any meaning-

( ’ C)

-60

-50

-40

-30

-20

A rsi + Ar,+ A (SC’) u (10m20 m2)

200 0.58

170 0.60

230 0.52

230 0.71

520 0.89

A r,+ --) Ar3+ x (s-l) u (10m20 m*)

130 0.14

74 0.14

43 0.14

56 0.15

293

of Arc (produced via electron-impact ionization of an Ar cluster beam). The occurrence of these metastable dissociation processes [ i(Ar+)/i(ArT) 0.06% [38] at room temperature] was interpreted with the help of ab initio potential diagrams in terms of electronic pre-dissociation and tunnelling. Moreover, very recently we have found [18] that the amount of metastable ions of Ar: (produced in an ion source via associative ionization) is approximately 100 times larger than the amount of metastable ions of Arl formed by electron-impact ionization of an Ar cluster beam. Figure 8 shows X as a function of stagnation temperature and Table 2 gives h and u as functions of stagnation temperature for the unimolecular decay process Ar,f 4 Ar+ + Ar. It can be seen that the metastable reaction rate with values between 50 and 170 s-l shows no particular dependence on the stagnation temperature, whereas the collision-induced dissociation cross-section (at 3 keV collision energy) increases from 0.07 X lo-*’ m* up to 0.17 x lo-*’ m* (the values in the present study are lower than the value obtained previously [38] due to the use of a more realistic length of 41 cm for the interaction region compared with 8 cm previously) as the stagnation temperature is raised from - 60°C up to + 10°C. By changing the stagnation temperature, the internal energy of the neutral precursors of Arz (produced by electron ionization of neutral argon clusters, Ar, + e * At-l) is changed and, in addition, the neutral size distribution of the argon cluster beam is changed [see Fig. 2(b)]. One or both of these effects are likely reasons for the observed dependence of u on the stagnation temperature. The dependence of u on the internal energy of water clusters reported previously by Lau et al. [36] is in accord with this finding. Ar,+ . The strong increase of the metastable decay rate of the reaction Arz * + Arl with increasing temperature is in agreement with a previous observation in our laboratory [17]. Moreover, it can be seen from Fig. 9 that a similar dependence on temperature is also observed for the second (less probable) decay channel, i.e. At-:* into Ar+. In order to understand this temperature dependence, it is imperative to measure the cluster size distribution as a function of stagnation gas temperature. At temperatures around - 60°C (and a stagnation pressure of 4-5 bar), we find Ar, cluster distributions up to n = 20 and even more [e.g. see Fig. 2(a), (b)], whereas for higher temperatures, the abundance of larger clusters decreases drastically. It can thus be concluded that, at low temperatures, most of the Arc ions observed are produced via dissociative ionization of Ar, clusters with n much larger than 3, whereas at higher temperatures, most of the Ar: ions observed are produced via direct ionization of Ar, with negligible contributions from the dissociative ionization from Ar, with n > 3. In the latter case, vertical (Franck-Condon) transitions from the neutral Ar, in a triangular configuration [39] to the linear Arc [40,41] are likely to result in highly excited

294

repulsive ion states leading to prompt and/or delayed fragmentation of Arc. A similar argument has been used by Buck and Meyer [7] in order to account for the almost total absence of Ar: in a quadrupole mass spectrometer after electron ionization of Ar,. Obviously, a possible influence of the internal energy of the neutral precursor cannot be excluded in order to account for this observed temperature dependence. Moreover, it is interesting to note that the ratio of Ar+ to A$ observed in electron ionization of Ar, by Buck and Meyer [7] is close to the present ratio (at higher temperatures) of Ar+ to Ar: p reduced via metastable dissociations from Arc. Finally, as can be seen from Table 4, not only does A increase with stagnation temperature, but also the collision cross-section shows an overall increase with stagnation temperature for both decay channels (see above, Ar:). It is of interest to point out that in other cases also both X and u increase with experimental parameters, indicative of the internal energy transferred in the production process to the precursor ion [14,18]. Arq+ . Results for this ion are given in Table 4. It can be seen that Ar: decays via metastable and collision-induced dissociation into Arl and Arl (the decay into Ar+ is too weak to be analyzed properly). The dominant channel corresponds to a decay into Arz . Moreover, the decay rate for the process Ar: * --) Arl is again very much temperature-dependent and reaches a rate of 26 000 s-l at - 10°C. This is the largest rate observed in argon and is of the same order of magnitude as the record dissociation rate for Xe$ of around 30000 s-l [23]. The absence of any detectable amount of Ar+ and the ratio between Ar: and Ar,i produced by metastable dissociations from Ari is again in good agreement with the observations of Buck and Meyer [7] for the occurrence of ions formed by electron ionization of Ar,. Furthermore, the collision-induced dissociation cross-section for the process Arl + Ar -+ Arc + 3 Ar is also strongly dependent on the stagnation temperature reaching the very large value of 4.9 X lo-*’ m* at - 10°C. Ar,+. Results for this ion are given in Table 5. The dominant dissociation process is Arc* + Ari + Ar, showing no strong temperature dependence and for the metastable and the collision-induced channel. There is also clear evidence for a metastable decay reaction Arc * -+ Ar:. The reactions Ar: -Ar: andAr; -+Ar’ are observed but the signals are too weak to allow a meaningful analysis. Stability of Ar,,+ cluster ions with 10 I n I 25

In contrast to the smaller cluster ions, argon cluster ions with 10 I n 5 25 show a clearly detectable metastable dissociation reaction involving only the loss of one monomer [reaction (l)]. This is in accordance with previous studies on the metastable decay of Ar,+ [42], (CO,): [19], (H,O).H+ [20],

295 TABLE 6 Metastable reaction rate, X, and estimates of the collision-induced dissociation cross-section, u, for the dissociation Ar,+ + Ar,+_1+ Af (loss of one monomer) as a function of cluster size, n (10 I n I 25) Stagnation pressure 4 bar; stagnation temperature

- 60” C.

Reaction Arz + Arz_ 1

x (s-l)

~10-20m*)

25 + 24 24 + 23 23 + 22 22 + 21 214 20 20+19 19418 18+17 17+16 16+15 15 +14 14+13 13 412 12 -+ll ll+lO lo-, 9

5770 5390 5440 4830 3800 8850 4100 3810 2920 2200 2680 2500 2570 2870 2290 2680

1.67 1.81 0.43 0.90 0.90 1.43 0.60 0.62 0.62 0.74 0.69 0.54 0.62 0.68 0.69 0.69

and (N,O)l [43]. Table 6 gives the metastable decay rate, X, and the collision-induced dissociation cross-section for the unimolecular dissociation Arz + Ar,_, + Ar as a function of n. The largest X (8850 s- ‘) is observed for Ar&* + Arc, + Ar and the largest u (1.8 x 10m2’ m2) for AI-& + Ar -+ Arj + 2 Ar. The strong variation of the metastable decay rate can be used to explain the observed ion abundance anomalies in the mass spectrum, in particular the very low abundance of Arlo. Figure 10 shows the ratio of decay product ion current to stable precursor ion current for the loss of one monomer as a function of cluster size, n (10 I n I 25). Also shown are the previous results of State and Moore [19] for the loss of one monomer. These results have been normalized to the present results at n = 15 because State and Moore do not give absolute ratios. Their results show a similar trend with n as the present results, at least up to n = 21; above n = 21, however, their results are significantly different. The most salient feature in these distributions is the strong signal for n = 20, indicating that this cluster size is less stable than others in the metastable time window probed in the present experiment (see also the discussion below). It is instructive to compare the results shown in Fig. 10 (metastable mass spectrum) with the Ar cluster ion distribution

296

25

0

I

10

12

I

lb

I

16

I

18

I

20 CLuster size n

I

I

22

24

Fig. 10. Product to precursor ion current ratio (determined by extrapolation to zero pressure) for the metastable decay reactions Ar,: + Ar,T- t + Ar as a function of precursor ion size, n. Stagnation gas temperature -60°C and stagnation pressure 4 bar. Also shown results by State and Moore [19] (0.005 cm orifice; up to 10 atm stagnation gas pressure) for the loss of one monomer normalized to the present results at n = 15 (see text). 0, State and Moore [19]; 0, present work.

(normal mass spectrum) obtained under identical experimental conditions (e.g. see Fig. 11, which also gives mass spectra obtained previously by other authors [26-301 under various experimental conditions). It can be seen that these two distributions are almost mirror-like (see similar observation for Xe in ref. 23), e.g. the characteristic troughs at n = 15, n = 20, and n = 24 in Fig. 11 correspond to strong metastable signals in Fig. 10 and, conversely, the characteristic troughs at n = 16 and n = 21 in Fig. 10 correspond to strong mass peaks in Fig. 11. With these results in mind, it is possible, for instance for Ar& (the most striking example), to explain the observed anomaly in the normal mass spectrum as being due in part to metastable decay of the Ar$ ion during its flight through the first field-free region (i.e. between 8.2 and 22.7 ~LSafter its production by electron impact) and the rest of the spectrometer, and in part to metastable decay (necessarily) occurring before entering the first field-free region, which is not detectable in the present experiment. From such a comparison between Figs. 10 and 11, it follows that abundance anomalies in the Ar cluster size distribution (normal mass

297

spectrum as shown in Fig. 11) are not due to abundance anomalies in the neutral Ar cluster distribution as argued previously, but rather that they only evolve after Ar cluster ion production as a result of the intrinsic stability of the Ar cluster ions produced *. This is in accordance with similar recent experimental observations in our laboratory on neon [48] and in other laboratories on water and xenon [20,23], although in these latter cases it has been argued that the highly excited cluster ions have to cool and solidify into ordered structures before magic numbers (abundance anomalies) in normal mass spectra can be produced in the size distributions by metastable decay. In the present case, it is not necessary to assume such a two-step model ** because (i) it can be seen that, in the experimental time window, ions display appreciable amounts of metastable dissociations and (ii) only extrapolation of the metastable decay rate back to the time of ion production would yield a completely smooth normal mass spectrum (see above). Hence, it is conjectured that the abundance anomalies in the Ar cluster ion spectrum are produced by metastable decay (vibrational pre-dissociation) of

* It is also interesting to compare the present ,results obtained for electron ionization of a neutral argon jet with those recently found by Harris et al. [44] for mass spectra obtained via the growth of clusters on ions entrained within the jet. There they find strong peaks at n = 13, 19, and 23. This is different from the sequence 14, 16, 19, 21, and 23 which we and others [26-301 find in the same region. The most prominent weak peaks, however, at n = 20 and 24, are the same in both spectra and coincide with large metastable peaks at n = 20 and 24 as seen in Fig. 10. This indicates that this region of the spectrum is most probably determined by the ionic stability, whereas the low portion of the spectrum (obtained for neutral argon jet expansion) might be determined by more than just the charged cluster stability. Most of the observed structure in both cases, i.e. strong peaks at n = 19 and 23 and weak peaks at n = 20 and 24, can be understood in terms of a highly simplified model of charged cluster structure involving only near-neighbour bond counting on lattices presented by Harris et al. [44]. As these authors caution, however, the binding energy differences obtained in this model for the second shell will be the same for neutral as for charged clusters. Similar results (e.g. strong peaks at n =13, 19, and 23; weak peaks at 20 and 24) have recently been obtained by Polymeropoulos et al. [45] who used computer simulations of the dissociation dynamics, yielding dissociation temperatures as a function of cluster size and by Saenz et al. [46] using a Monte Carlo simulation to calculate the stability of ionized clusters and their concentrations as a function of time. See also recent experiments (electron diffraction) and calculations on neutral Ar clusters produced by free jet expansions yielding information on structure and stability of the neutrals [47]. ** In Fig. 1 (top) of ref. 20, the mass spectrum taken at - 4 /.LSafter ion production already exhibits a 25% drop in intensity between the n = 21 peak and the n = 22 peak (not correcting for the overall roughly exponential decrease leading to an approximate ratio between neighbouring peaks of 1.11 [20]). Taking into account that the mass spectrum at the bottom of this figure is caused by metastable decay in the 4-40 ps time window, it could be argued that the small abundance anomalies in the top spectrum are already caused by the corresponding me&table decays prior to 4 ps, thus making the two-step model unnecessary.

298

”

10

I

I

I

I

I

I

I

12

14

16

18

20

22

24

I

CLuster size n

Fig. 11. Cluster ion distribution (ion current versus cluster size, n) for Ar ion clusters produced via electron impact ionization of a neutral cluster beam formed by adiabatic supersonic expansion. 0, Present results: T0 = -6OOC; p0 = 4 bar, Q-, = 20 pm; +, Worsnop et al. [29]: r, = -186°C; p0 = 500 Torr; De =120 pm; X, Ding and Hesslich [28]: T, = 20°C; p0 =lO bar; Do = 30 pm; 0, Milne and Greene [26]: T, = 27OC; p0 = 5 bar; Do = 0.004in; l, Orth et al. [27]: secondary ion mass spectrum of neat solid argon at cryogenic temperature; A, Leiter et al. [30]: T, = - 14O’C; p0 = 4 bar; D,,=lO pm.

the Ar cluster ions due to different metastable decay rates of the various ions. Within the framework of QET, these different metastable decay rates are due (i) to the different excitation energies in the ions after the ionization process (activated complex) because of the different geometry changes from the neutral to the ion and/or (ii) to the different number of states of the activated complex and different density of states. For instance, the relatively large metastable dissociation rate of Ar& is probably due to a larger excitation energy and/or larger number of vibrational and rotational states of the activated complex and/or smaller density of states as compared, for example, with the neighbouring ions Arc, and Arti, respectively. Additional experiments showed that these Arz cluster ions not only dissociate by evaporating one monomer, but that collision-induced dissociation also leads to the loss of up to (n - 2) neutral monomers. It is not possible to ascertain from the present studies whether these loss processes are simultaneous or sequential. Table 7 shows, as an example, estimates of the individual collision-induced dissociation cross-sections for Ar&. It can be seen that the cross-section decreases with the number of neutrals lost.

TABLE 7 Estimates of the collision-induced dissociation Ar\T2:_m+(m +l) Ar as a function of m

cross-section,

u, for the reaction

Stagnation pressure 5 bar; stagnation with a stagnation pressure of 10 bar.

temperature

Reaction Ar& 4 Arzt-m

Collision-induced cross-section, u (10P20 m*)

214 20 21419 21-+18 21417 21+16 21+15 21+14 21413 21-+12 21411 21+10 21+ 9 21~ 8 21+ 7 21~ 6 214 5 214 4 214 3 21-r 2

0.31 0.29 0.18 0.14 0.17 0.15 0.14 0.10 0.08 0.09 0.07 0.06 0.04 0.05 0.02 0.01 0.02 0.03 0.05

-60°C.

Similar

results

ArcI + Ar + were obtained

The dissociation into Ar,f and At-:, however, has a larger cross-section than dissociation into Ar,+, Arc, and At-l. This, together with the observation that At-i is the dominant metastable decay ion of Ar: *, could be evidence of the particular role of Ar: in argon cluster ion production as proposed by Haberland [ 491. The observation of collision-induced dissociation with loss of more than one monomer is in accordance with previous studies [50]. Echt et al. [15] and Kamke et al. [43] recently observed the loss of 5 monomer units after photoionization of ammonia clusters and of 3 monomer units after photoionization of N,O clusters, respectively. Moreover, similar results have been obtained for collision-induced dissociation studies of sputtered nitrogen [51] and alkali iodide [52] cluster ions. ACKNOWLEDGEMENTS

This work was partially supported by the ijsterreichischer Fijrderung der Wissenschaftlichen Forschung.

Fonds

zur

300 REFERENCES

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