Clusters of water and ammonia with excess electrons

Surface Science 156 (1985) 157-164 North-Holland, Amsterdam

CLUSTERS

OF WATER

157

AND AMMONIA

Hellmut HABERLAND, Christoph and Douglas R. WORSNOP

WITH

LUDEWIGT,

EXCESS

ELECTRONS

Hans-Georg

*

SCHINDLER

Fakultiit ftir Physik, Universitiit Freiburg, D - 7800 Freiburg, Fed. Rep. of Germany

Received

10 July 1984: accepted

for publication

10 August

1984

Beams of (H,O), (n = 2. 3, 2 5) and (NH,); (n > 35) have been produced by injecting low-energy electrons into supersonic expansions of Ha0 and NH,. The distribution of (H,O); is very sensitive to the carrier gas in the expansion; n < 10 are observed only when Ha0 is seeded in rare gases. n = 2.67 and n > 11 are particularly stable and may be associated with different types of electron binding. In contrast, the minimum n = 35 for (NH,); is su~~singIy large and virtually independent of expansion conditions.

1. Introduction Injecting electrons into liquid or solid water and ammonia gives rise to a variety of phenomena which have intrigued scientists for over 100 years [1,2]. Excess electrons are localized in sites that have been described as clusters of H,O or NH, molecules within the bulk [2], although the molecules themselves have no stable negative-ion states (both have electronic shells isoelectronic to Ne). Despite much experimental and theoretical interest, the microscopic structure of these bulk phase states remains unclear (1,2]. Their broad absorption spectrum and other properties suggest a structure similar to an F-center in alkali halide crystals [1,2]. An obvious approach to this problem, pursued by this and other research groups [3-Q is to study the interaction of electrons with isolated molecular clusters. One hopes to attach electrons to clusters and investigate their properties as a function of size. If the monomer anion is not stable and yet electrons are trapped in the bulk, the first question is: how Iarge an isolated cluster is necessary to stabilize an electron? Bulk phase experiments have given indirect answers. For example, light absorption in near- and super-critical H,O and NH, vapour has been analysed in terms of electron trapping in clusters formed via density fluctuations [6].

* Dedicated

to Professor

0. Oberhaus

on the occasion

of his 65th birthday.

0039~6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

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H. Haberland et al. / Chters

of water and ammonia

The fluctuations correspond to electrons being trapped in clusters of 6 H,O and 10 NH, molecules [6]. These sizes are consistent with theoretical calculations of electron stability, especially in the case of (H,O), [668]. The molecular beam experiments described here have produced and mass spectrometrically identified beams of isolated (H,O),; and (NH,); clusters [4,5]. We summarize the results to date, which provide the first direct information about the minimum cluster size necessary to stabilize excess electrons in water and ammonia, at least on the 100 ps timescale of the experiment.

2. Experimental As previously described, the key to the experiment is the injection of low-energy electrons into the condensation zone of a supersonic expansion [4,5]. A schematic of the apparatus is shown in fig. 1. Electrons are generated thermionically by placing a hot thoriated iridium filament 2-3 mm from the nozzle. The electron energy is crudely maintained below 0.5-l eV with the filament bias voltage. Negative ions are extracted by ion optics and mass analyzed with a quadrupole mass filter. Photoelectrons have also been used as the electron source, with similar results, albeit at lower signal levels. A new feature is a magnetic field around the nozzle/skimmer volume to confine the electrons and prevent their extraction into the mass filter. The ion optics also contain an electron-impact ionizer to monitor neutrals in the beam.

H,O/Ar, Xe NH, IAr

Fig. 1. Schematic of apparatus. Low-energy electrons emitted from a hot Th-fr filament are injected between the nozzle and skimmer of a differentially pumped supersonic expansion. An electromagnet surrounding the volume between the nozzle and skimmer confines the electrons with a 400 G field. Negative ions are extracted with ion optics (which begin with the skimmer), mass analyzed with a quadrupole mass filter, and pulse counted with a channeltron.

H. Haberiand et al. / Chsters of water and ammonia

159

The expansion is produced with a heatable stainless-steel nozzle (40-150 pm hole diameter). Expansion conditions range from pure H,O or NH, vapour to seeding of H,O or NH, in strong rare-gas expansions: 0.01 to 2 bar H,O in O-12 bar He, Ne, Ar, Kr, Xe at 60-120°C; 0.1-5 bar NH, in O-5 bar Ar at 60°C. Under these conditions, neutral clusters are detected as (H,O),H+ and (NH,),H+ via el~tron-impact ionization. This growth of clusters is consistent with results reported for other H,O and NH, expansions [9]. The primary effect of seeding in rare gases is to cool the seed gas and thus the clusters [lo], which apparently enhances the formation of small (H,O); (n < 11) [S].

3. Results Fig. 2 shows a low-resolution mass spectrum of (H,O); from 2% H,O expanded in 10 bar Ar. Such a strong expansion produces large water clusters, both neutral (detected as positive ions) and negatively charged. The distribution of large clusters is roughly similar for both ions and neutrals. (The size limit of n = 100 is imposed by the mass filter.) In the case of the (H,O);, a threshold is superimposed at small n (in fig. 2, primarily at n = 11 with some intensity at smaller n), corresponding to the necessity of attaching an electron to a finite water cluster. Fig. 3 shows in detail the nature of the threshold for appearance of small (H,O); (n < II), as measured under weaker expansion conditions. The unit mass resolution, combined with appropriately mass-shifted (D,O); spectra

0

500

Fig. 2. Low-resolution mass spectrum Ar (40 pm nozzle, 80°C).

1500 amu

1000 of (H,O);

after electron

injection

into 0.2 bar H,O

in 9 bar

160

H. Huberland

et al. / Clusters of water and wmmma

n

n=2

6

(H,O),

7

I51 !91 10

5

11 12 13

14

11 12 13 14 15

: IH,Ol

I-T-

O

250

amu

Fig. 3. High-resolution mass spectra of (H,O); from: (A) 0.3 bar H,O (150 Frn nozzle, 80°C): (B) 0.2 bar H,O in 7 bar Ar (60 pm, 9O’C); (C) 0.03 bar H,O in 5 bar Xe (60 pm, 85°C). In (B) and (C), Ar(H20);, Xe(H20);. and SF,- (an impurity in the Xe) are labeled as (a). (b) and (c), respectively. Mass filter transmission factor of 10 at 250 amu).

effects

reduce

the signal

at high masses

(e.g.. by about

a

(not shown), unambiguously identifies (H,O); . For example, as shown in figs. 3B and 3C. (H,O); are clearly resolved from (H,O),_ ,OH-. The distribution of small n changes with the carrier gas in fig. 3. In pure H,O expansions (A), a sharp n = 11 threshold is observed [4]. When H,O is seeded in Ar (B), n = 2 and n = 6-10 appear as well; in Xe (C), n = 3, 5 also appear, with enhanced intensity at n = 8, 9 [5]. No signal has been observed at

H. Haberland et al. / Clusters of water and ammonia

161

n = 4. The signals at n = 2, n = 6, 7 and n 2 11 are relatively intense. Seeding in He, Ne or Kr (not shown) gives intermediate results. Similar distributions are observed in D,O expansions with the exception that pure D,O shows an n = 12 threshold for (D,O); [4]. Fig. 4 shows the dependence of the intensity of n = 2, 6 and 13 of (H,O); on the H,O partial pressure in an Ar expansion. The appearance thresholds for the different clusters are clearly separable and data for n > 6 crudely correlate with the growth of neutral clusters. The disappearance of small (n < 10) clusters at higher H,O pressures diverges from the neutral cluster behaviour; under these conditions the ionized mass spectra of the latter show no depletion of small clusters. The appearance of small (H,O); only in strong rare-gas expansions indicate that extra cooling is required for electron attachment to small clusters. The presence of weakly bound Ar(H,O); and Xe(H,O); in figs. 3B and 3C confirms that the rare-gas expansions are indeed cold. As the H,O partial 1000E

- (H,O); + (H,O), m (H,013

+/’

+ + +

0,2

m

I .

:’ 0,5

1

I+

I 2

3

%

PH,O

Fig. 4. Dependence on the H,O partial pressure in an H,O/Ar expansion (7.5 bar total pressure, 60 ps nozzle, 90°C). Signal for n = 2, n = 6 and n = 13 of (H,O); , as labeled.

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H. Haberland et al. / Clusters of water and ammoniu

SO0 Fig. 5. Low resolution 60°C).

mass spectrum

of (NH,);

from 0.5 bar NH,

amu in 5.5 bar Ar (40 pm nozzle.

pressure is increased, the extra heat capacity and condensation of the added Hz0 warm the expansion, leading to the disappearance of Van der Waals bonded clusters and small (H,O); (as shown for the latter in fig. 4). Fig. 5 shows very different results observed for (NH,);. A surprisingly large n = 35 threshold is seen, with none of the structure of the (H,O); threshold. The (NH,); distribution is virtually independent of the expansion conditions. An n = 36 threshold is seen in pure NH,, with the intensity rising much more steeply above threshold than in fig. 5 [4].

4. Discussion The coincident observation of neutral and negatively charged clusters is consistent with reported electron trapping in clusters in critical vapour and with hypothesized “trap sites” for initial localization of electrons in condensed water [6,11,12]. Details of the electron attachment mechanism are clouded in the non-equilibrium nature of cluster growth in supersonic expansions; however, with the possible exception of n = 2, the pressure thresholds of fig. 4 are in a regime typical for neutral cluster growth, as detected via electron-impact ionization [9]. The sensitivity of the formation of small (H,O); (n < 11) to cooling in rare-gas expansions implies that the mass spectral intensity distributions do not simply reflect the relative stability of cluster ions. Dynamic and temperature effects must also be involved. While not so dramatic, the isotope affect seen in pure H,O expansions (n = 11 and n = 12 thresholds for (H,O); and

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163

(D,O);, respectively) and the different shape of the (NH,); threshold in pure NH, expansions [4] provide more evidence of these effects. A further complication is the possibility that metastable species with lifetimes longer than = 100 ~LSwould be detected in these experiments. However, even with these caveats in mind, the (H,O); and (NH,); distributions of figs. 2-5 are intriguing when compared to calculations of binding of excess electrons to water and ammonia clusters. The 2.74 D dipole moment of (H,O), (compared to 1.85 D for H,O) [11,1,2] is above the minimum required to bind an electron, based on calculations for rotating molecular dipoles [13]. A vertical electron affinity of 0.2 meV has been calculated for (H,O); [14]. However, the electric fields in the ion optics and mass filter should detach such a weakly bound electron. as was recently observed for dipole bound excited state of H,C-CHO[15]. The observation of (H,O); agrees with a quantum-mechanical calculations of an n = 6 threshold for stability of (H,O); , relative to an electron and n free H,O molecules [7,8]. However, these calculated (H,O), are metastable with respect to a free electron and a neutral (H,O), cluster, due to rearrangement of the H,O molecules around the electron in (H,O);. Furthermore, the calculated stability increases with increasing n, i.e. nothing changes at n = 8-9, contrary to experimental results [7,8]. The n = 35 threshold for (NH,); presents another case. A cluster of 35 molecules has a partially filled second “solvation shell”, a very different regime than in the smaller (H,O); clusters. This threshold is three times larger than predicted, though the calculations are not as advanced as those for (H,O); [6]. The results of this mass spectrometric observation of (H,O), and (NH,); show little direct correspondence with published calculations. In the case of (H,O);, the separability of n = 2, n = 6, 7 and n 2 11 (via both their intensity distribution and their H,O partial pressure dependence) suggests they may be associated with different types of electron binding. Experiments in progress will more directly probe the nature of these excess electrons in water and ammonia clusters.

Acknowledgements This work was financially supported schungsgemeinschaft and by the Herbert Quandt

by the Foundation

Deutsche Forof Varta AG.

Note added in proof The field detachment threshold for (H,O); was determined to be 31 f 1 kV/cm [16]. For (H,O);Ar, with n = 1 to 6, this value increases nonlinearly to 45 f 1 kV/cm [17].

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