J . Phys. Chem. 1987, 91, 2598-2601
2598
Table I summarizes the results of the present measurements and, for comparison, includes the results of photoionization mass spectrometry experiment^^^ and He I-PES experimentsS as well as the results of recent theoretical calculations by Michels et a1.I6 The REMPI-PES allow the first observation of the weakly Re C2113/2uand the D281,2g+states, which were obscured in the He I-PES by the Xe+ 2P03i2and 2P0 atomic states, respectively. In addition, the remaining dimer ion states are observed without interference from the atomic photoelectron peaks for the first time. In general, the dissociation energies of these states are in quite good agreement with the theoretical calculations of Michels et a1.I6 Both theory and experiment show that the B2111i2+state is the only ionic state that is repulsive at all internuclear distances. At Re of the resonant intermediate state (which is approximately equal to R , of the ground electronic state), the potential of the B211,i2gstate of the ion is 0.04 eV above the asymptote, which is in good agreement with the calculations of Michels et a1.I6 that predict a potential 0.034 eV above the asymptote at 4.34 A. Finally, the data in Table I show that the ionization onset for the A2Z+1/2,state observed by using REMPI-PES is much closer to the adiabatic ionization potential than the onset observed by using He I-PES. In general, the large difference in the values of Re for the van der Waals ground state and the bound dimer ion ground state makes the observation of the adiabatic ionization potential difficult for rare gas van der Waals dimers using H e I-PES. However, REMPI-PES makes it possible to select a (32) Ng, C. Y.; Trevor, D. J.; Mahan, B. H.; Lee, Y. T. J. Chem. Phys. 1976, 65. 4327.
resonant intermediate state with a small value of Re and thus improve the Franck-Condon overlap with the u+ = 0 level of the A2Zljzu+state in the ionizing transition.
Summary and Conclusions The (2 + 1) ionization spectrum of Xe2 determined in the wavelength region 2425-2600 A shows a number of molecular band systems that display discrete vibronic structure. Photoelectron spectra taken via a number of these band systems provide the first observation of the weakly bound CZII3/2,,and the D2Z1izg+ states, which are obscured in the H e I-PES by the Xe+ *P03/2 and 2P0 atomic photoelectron peaks, respectively. In addition, by judicious choice of the resonant intermediate state, it is possible to probe the ionic states in regions that are not accessible from the ground state due to poor Franck-Condon overlap. The present data provide lower limits for Dothat are in good agreement with theory and with previous data. Very high resolution rotationally resolved REMPI spectra will be necessary to completely define the potential of the resonant intermediate state before it is practical to perform Franck-Condon factor calculations on the ionizing transition to determine the complete potential energy curves of the ionic states. Acknowledgment. This work was supported by the U S . Department of Energy, Office of Health and Environmental Research, under Contract W-31-109 Eng-38, and by the Office of Naval Research. Registry No. Xe2, 12185-19-2;Xezt, 12185-20-5;Xe, 7440-63-3;Xe', 24203-25-6.
Ionizing Laser Intensity Dependence of the Silicon Cluster Photoionization Mass Spectrum D. J. Trevor, D. M. Cox, K. C. Reichmann, R. 0. Brickman, and A. Kaldor* Corporate Research Laboratory, Exxon Research & Engineering Company, Annandale, New Jersey 08801 (Received: June 17, 1986)
Silicon clusters have been produced by laser vaporization in a pulsed high-pressurehelium beam. Taking into account well-known effects of pulsed laser photoionizationwe have found the silicon cluster distribution to be relatively flat, and ionization thresholds of to be between 4.99 and 6.42 eV, of Si8,9,11-21 to be between 6.42 and 7.87 eV, and of Si2-7,10 greater than 7.87 eV. In addition Si6-11+ and Sil5+are observed to be preferred fragment ions at high ionizing laser intensities.
Introduction Clusters of more than a dozen different elements, alloys, and compounds have been examined by photoionization mass spectroscopy since the development of pulsed laser vaporization cluster source^.^-^ We report the photoionization mass spectrum of silicon clusters produced by laser vaporization using excimer laser photon energies of 7.87, 6.42, and 4.99 eV for ionization. ~~~
~
(1) Rohlfing, E. A,; Cox, D. M.; Kaldor, A. Chem. Phys. Lett. 1983, 99, 161. J . Phys. Chem. 1984, 88, 4497. J. Chem. Phys. 1984, 81, 3322. Rohlfing, E. A.; Cox, D. M.; Petkovic-Luton, R.; Kaldor, A. J. Phys. Chem. 1984,88,6227. Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. Surf. Sci. 1985, 156, 8. (2) Powers, D. E.; Hansen, S. G.;Geusic, M. E.; Michalopoulos, D. L.; Smalley, R. E. J. Chem. Phys. 1983, 78, 2866. Morse, M. D.; Smalley, R. E. Ber. Bunsen-Ges. Phys. Chem. 1984,88,228. Smalley, R. E. Lmer Chem. 1983, 2, 167, and references therein. (3) Bondybey, V.E.; English, J. E. Chem. Phys. Lett. 1984, 111, 195, and references therein. Richtsmeier, S. C.; Parks, E. K.; Lin, K.; Pobo, L. G.; Riley, S. J. J . Chem. Phys. 1985, 82, 3659, and references therein.
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The first photoionization mass spectrum (PMS) of silicon clusters produced by laser vaporization was reported by Bloomfield, Freeman, and Brown4 in a study of the relative photodissociation cross sections of Si,+ for n C 12. They found Si4+ and Si6+ to be especially stable with respect to further photodissociation. Their reported photoionization mass spectrum contained several significant intensity variations. Phillips5 subsequently attempted to explain why certain cluster signals are more intense than others in the PMS of Bloomfield et ale4by drawing analogies with stabilities found in network solid-state systems. H e argued that the drop in cluster ion signal beyond Si,,,' suggested slowing of the cluster growth rate and the weak Sil4+signal to suggest other kinetic limitations on the production of silicon clusters during condensation. ~~~
(4) Bloomfield, L. A,; Freeman, R. R.; Brown, W. L. Phys. Rev. Lett. 1985, 54, 2246. Bloomfield, L. A.; Geusic, M. E.; Freeman, R. R.; Brown, W. L. Chem. Phys. Lett. 1985, 121, 33. (5) Phillips, J. C. J . Chem. Phys. 1985, 83, 3330.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 10, 1987 2599
PMS of Si Clusters I " ' I " ~ I " ' I " ' I " ' 1 " '
1.5
I ~ " 1 " ' ~ " ' -
hv= 7.9 eV; 10+5 p J / c m Z
-
1 .O-
I
hv= 7.9 eV; 325f25 p J / c m Z
>
2
.-0 0 1 Of5 p J / c r n *
(a)
1-
0
5
10
20
15
cluster size
25
Figure 1. Photoionization mass spectra at 7.87 eV (Fz). The upper frame was taken at a low laser intensity of 10 5 pJ/cm2 while the lower frame was obtained with a higher laser intensity of 325 f 25 pJ/cm2. The vaporization laser for the upper frame was 7 m J at 248 nm (KrF) while 15 m J a t 532 nm (doubled Nd:YAG) was used for the lower frame.
*
Figure 3. Photoionization mass spectra of the small clusters a t 7.87-eV photon energy taken a t laser intensities of 10 f 5, 25 f 5, and 55 f 5 pJ/cm2. The vaporization laser was 7 mJ at 248 nm (KrF). 1.0
,
I
I
,
,
I
,
I
I
'
I
'
I f I ' V '
I
j
r
hv= 6.4 eV; 3 2 5 * 2 5 p J / c m 2 /
1 .0
I I I
0.5
f l
l A
c LZ
v1
h
1
I
I
I " ' l " ' / " ' l " ' l " ' 1 " ' 1 " ' 1 " ' 1 " '
1.5
30
cluster size
0.0
.-
h v = 6.4 eV; 1.5i.O.1mJ/cmZ
0.0
40
0.0
20
0.2
0.4
0.6
0.8
laser energy (mJ)
Figure 4. Plots of the integrated ion signals for the indicated cluster ions
n
10
20
30
40
50
60
70
80
90
100
cluster size
Figure 2. Photoionization mass spectra at 6.42 eV (ArF). The upper frame was taken at a moderate laser intensity of 325 f 25 pJ/cmz while the lower frame was obtained with a higher laser intensity of 1.5 0.1 mJ/cm2. The vaporization laser for the upper frame was 7 mJ at 248 nm (KrF) while 15 mJ a t 532 nm (doubled Nd:YAG) was used for the lower frame.
*
Heath et aL6 have recently reported a PMS for silicon and germanium clusters with 7.87- and 6.42-eV one-color photoionization and several two-color experiments. They found the 7.87-eV spectrum very different from the 6.42-eV spectrum and evidence for dissociation on the several hundred nanosecond time scale. They also measured excited-state lifetimes of approximately 100-200 ns for small neutral silicon clusters. Recent theoretical calculations7 have confirmed that both neutral and positively charged silicon clusters from the dimer to the decamer are covalently bound. These calculations predict that the lowest energy structures are distinct minimums, that ionization is from nonbonding molecular orbitals, and that small geometrical changes occur upon ionization. We will infer from the PMS of silicon clusters obtained at 4.99-, 6.42-, and 7.87-eV photoionization energies that the overall cluster (6) Heath, J. R.; Liu, Y.; OBrien, S.C.; Zhang, Q.-L.; Curl, R. F.; Tittel, F. K.; Smalley, R. E J . Chem. Phys. 1985, 83, 5520. (7) Raghavachari, K.; Logovinsky, V. Phys. Reu. Letr. 1985, 55, 2853.
as a function of ionizing laser energy at 6.42-eV photon energy. The vaporization laser was 15 mJ a t 532 nm. The solid line is a linear least-squares fit to the appropriate data, while the dashed lines are nonlinear least-squares fits to the functional form, I", where I is the ionizing laser energy. The best fit values of n were found to be 3.3, 3.3, and 2.8 for Si6+,Si,', and Si,,', respectively, with an uncertainty of f0.5.
distribution is relatively flat. At low laser intensity the PMS does not show the pronounced ion signal variations4 which had previously been interpreted5 as especially stable structures.
Experiments The apparatus used in these experiments has been previously described.' Briefly, the metal clusters are produced by focusing 15 mJ (1 5 ns) of 532-nm or 7 mJ of 249-nm light to a 1.O-mmdiameter spot at the surface of a silicon rod in the leading edge of the main helium beam pulse. This gas pulse quenches the mostly atomic vapor. Primary cluster growth takes place in a 2-mm-diameter 5-cm-long tube (extender) followed by a l-cmdiameter 5-cm- or 7.6-cm-long reactor tube used in chemical reaction studies.8 Reactant pulses were not added during the course of these experiments. The gas flow expands into a vacuum (8) Trevor, D. J.; Whetten, R. L.; Cox, D. M.; Kaldor, A. J . Am. Chem. SOC.1985, 107, 518. Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. Phys. Reu. Lett. 1985, 54, 1494. Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. J . Phys. Chem. 1985, 89, 566. Kaldor, A,; Cox, D. M.; Trevor, D. J.; Whetten, R. L. The New Surface Science i n Catalysis, Gland, J., Ed.; American Chemical Society: Washington, D.C., 1985.
2600 The Journal of Physical Chemistry, Vol. 91, No. 10, 1987
creating the molecular beam which is crossed by a photoionization laser in the extraction region of a time-of-flight mass spectrometer. The ionizing laser intensity dependence of the PMS was measured at 6.42 and 7.87 eV. The laser beam was attenuated either with neutral density filters for 6.42 eV or with metal screens for 7.87 eV. The 3-"diameter silicon rod used was better than 99.999% pure and obtained from Research Organic/Inorganic Chemical Corp. No attempt was made to remove the oxide layer before it was used. Care was taken to clean the rod several times by laser vaporization before data was collected. In contrast to the many transition metals studied with this technique, the silicon oxide clusters did not dominate the PMS before cleaning.
Results Figure 1 shows the PMS for silicon clusters recorded with 7.87-eV ionizing laser photon energy at two different intensities. Figure 2 shows similar PMS using the 6.42-eV ionizing photons. No silicon cluster ions smaller than Siloo (the largest distinguishable signal observed at 6.42 and 7.87 eV) were observed with 4.99-eV ionizing laser energy, even with intensities of 500 J/cm2. Figure 3 shows a series of mass spectra in the small cluster region for different laser intensities a t 7.87-eV photon energy. These spectra together with the spectra in Figure 1 demonstrate that not just Sill+and larger clusters, but also Sis+ and Si9+have nearly linear intensity dependence. Silo+and all clusters less than and including Si7+depend nonlinearly on laser intensity indicating that they are produced by multiple photon processes. Si6+is quite difficult to eliminate by reducing laser intensity as seen in the lowest intensity spectrum of Figure 3. Close examination of the data together with measurements at lower and higher intensities indicate that it also depends nonlinearly on laser intensity. In addition, a background mass peak due to thermal cracking products of the diffusion pump oil appears at 170 f 2 amu, which is not resolved from the Si6+(168.5 amu) signal. This background mass peak has not been removed from the spectra shown in the figures. At the lowest intensity, it contributes more than 50% to the signal observed at this mass. Figure 4 is a plot of several Si,+ ion signals as a function of laser intensity at 6.42 eV. As shown, Si28+,Si3o+,and Si40' all depend linearly on ionizing laser intensity. Similar plots on Si22+ and larger clusters in this same intensity regime also have a linear intensity dependence. In contrast, Si2-,*+and Si,,+, the only other ion signals observed with significant intensity at 6.42-eV photon energies, display a nonlinear dependence on laser intensity. Discussion The low intensity PMS in Figures 1 and 2 show a rather smooth distribution of ion signals, quite in contrast with the higher intensity counterparts. If we assume that (1) dissociative ionization is not important, (2) a linear intensity dependence at low intensity implies single-photon ionization to parent ions, and (3) the photon energy is above the ionization potential, then a mass spectrum obtained at low intensity may be a good reflection of the neutral cluster distribution. Most of these assumptions can be shown to be valid for silicon clusters above Sillwith the 7.87-eV ionizing laser, which will be discussed in a paper on pulsed laser photoionization mass spectroscopy of cluster beams.' Thus the low-intensity spectrum of Figure 1 indicates the silicon cluster distribution is relatively flat from Si,, out to at least Si6owhere the gradual attenuation of the signal is attributed to the finite growth period in the source. As seen in the PMS of Figures 1 and 2 ion signals for many clusters increase significantly in intensity when the laser intensity is increased, for example, Si6+, Silo+. This greater than linear increase in signal arises from one or more of the many possible multiple photon schemes. At even reasonably low ionizing laser fluences of 100 pJ/cm2 the intensity is large due to the narrow pulse width (10 ns) of the laser. At these intensities ( IO4 W/cm2) a cluster can absorb a photon and ionize or dissociate, but the story is not over. At this intensity it is possible for an ion or ~~~
~
(9) Trevor, D. J., manuscript in preparation
Trevor et al. fragment to absorb another 6-8-eV photon likely dissociating the ion or ionizing the fragment. The fragment ions can then also continue absorbing more photons. Such mechanisms should be sensitive to ion or neutral stabilities and appear as cluster ion signal variations in the PMS. From this scenario the large intensity of Si6-,,+ in the 6.42-eV PMS at high intensity suggests that they are preferred photofragment ions from larger clusters in agreement with earlier observations.6 The lower IP of Si,, (see below) compared to Siloalso supports the conclusion that S i l l +should be a preferred fragment. The intensity dependence at 7.87-eV ionizing photon energy displayed in Figure 3 demonstrates that not just Si,, and larger clusters, but also Sis and Si9have IPS less than or equal to 7.87 eV. Silo+,Si7+, and all smaller clusters have a higher order intensity dependence indicating that they are produced by multiple photon processes and have IPS > 7.87 eV. The difficulty in eliminating Si6+ signal is associated with it being a dominant fragment ion. In addition, the ion signals from the "single photon ionized clusters" are not completely free of contamination from higher order processes. For instance, the ratio of Sill+to Sil2+ decreases as the intensity decreases suggesting that the Sill+signal has more of a contribution from higher order processes than Si,,'. The linear intensity dependence observed for Siz2+and larger clusters shown in Figure 4 using 6.42-eV ionizing photon energy implies single-photon processes. If we assume that other interferences such as dissociative ionization are not important, the linear intensity dependence allows an upper bound of 6.42 eV to be placed on the IPS of Si,, and larger clusters. From the low laser intensity PMS, shown in Figure 2, we see that Siz2is the smallest cluster which is detected by single-photon ionization at 6.42 eV. However, even at the moderate laser intensity of 350 pJ/cm2, Sill+and a few other small cluster ions are produced, indicating that a rather efficient multiphoton process must be active. For a variety of metal systems Kappesiohas applied a spherical metal droplet model which predicts IPS to decrease as N-,I3, where N is the cluster size. The ionization potentials for silicon slowly decrease with size, Si,, (7.87 eV), Si,, (6.42 eV), and no cluster even out to Silw is observed with an IP 5 4.99 eV. This is consistent with this model. Molecular orbital calculations find that the adiabatic ionization potentials of silicon clusters up to Si6lie within 7.5 and 7.9 eV.7 Our measurements, however, indicate that the IPSof these clusters are greater than 7.87 eV. For these small clusters we do not have a measured upper bound to the adiabatic I P because their appearance potentials must be above the highest photon energy studied. Instead our measurements represent a lower bound to the vertical IPS. Therefore, strictly speaking there is no inconsistency between the measured and calculated values. It would be informative to have theoretical values of the vertical IPS to compare with the experiment because the effects that can explain this difference in vertical and adiabatic IPS are expected to be small, due to the nonbonding nature of the HOMO. One interpretation of the apparent higher IPS of the small clusters, observed experimentally, is that the HOMO is more bonding than the calculations seem to indicate. The electron bombardment appearance potentials for the dimer and trimer at approximately 2000 K were reported to be 7.4 f 0.3 and 8.2 f 0.3 eV." This dimer value is 0.5 eV less than our lower bound observed by photoionization. If we assume that all of the thermal excitation is available for ionization this value is raised by 0.17 eV, bringing it close to agreement with our lower bound of 7.87 eV. The appearance potential for the trimer is in agreement with our results.
Conclusions By studying the low-intensity PMS of silicon clusters, the IPS of Si2-7and Siioare found to be greater than 7.87 eV while (IO) Kappes, M. M.; Schar, M.; Radi, P.; Schumacher, E. J. Chem. Phys. 1986,84, 1863. ( I I ) Chatillon, C.; Allibert, M.; Pattoret, A. C.R. Hebd. Seances Acad. Sci. Ser. C 1975, 280, 1505. Drowart, J.; DeMaria, G.;Inghram, M. G. J . Chem. Phys. 1958, 29, 1015.
J. Phys. Chem. 1987, 91, 2601-2607 Si,-Silm all have IPSbetween 4.99 and 6.42 eV. Si+ll+are found to be dominant photofragments from the larger silicon clusters. The overall silicon cluster distribution produced from this source is nearly independent of cluster size, showing only minor intensity alterations and the expected decrease to larger sizes. From this
2601
we conclude that the previous interpretation5 of the high-intensity PMS at 6.42 eV is likely incorrect because ion photochemistry was not taken into consideration. Registry No. Si, 7440-21-3.
Electron Attachment to Water Clusters under Collision-Free Conditions Martin Knapp, Olof Echt,* Dietmar Kreisle, and Ekkehard Recknagel Fakultat fur Physik, Universitat Konstanz, 0-7750 Konstanz, West Germany (Received: July 15, 1986)
Long-lived water cluster anions (H20),-, n 2 11, can be formed by electron attachment to preformed, cold water clusters. Attachment occurs resonantly at energies very close to 0 eV. Obviously, the adiabatic electron affinity of these clusters is nonnegative, and electron trapping is not impeded by a potential barrier. Under comparable expansion conditions, attachment to cold, preexisting clusters yields the same minimum cluster anion size as injection of low-energy electrons into the condensation zone of neat water vapor, as reported previously by Haberland and co-workers. The smallest cluster anions being observed show a high probability for electron detachment in the field free drift tube of the time-of-flight mass spectrometer.
Introduction The electron affinity of the free water molecule is exceedingly small, if a bound state exists a t all.l-3 Excess electrons in condensed water, however, are known to be strongly bound (with respect to a free electron in vacuo). In its ground state, the electron will be self-trapped; i.e. the geometrical arrangement of the solvent molecules around the localized electron will be completely different from that of neutral water."1° The molecules have to reorient with respect to the electron; and the local density will be reduced because of dipole-dipole repulsion and weakening of hydrogen bonds. The interaction of the excess charge with molecules beyond the first coordination shell will be crucial to obtain the net stability of the solvated electron. A sufficiently large water cluster will certainly feature a sizeable adiabatic electron affinity, but a large rearrangement of the molecules will be required to solvate the electron. Thus, it has been argued that excess electrons will not bind to preexisting neutral, stable, equilibrium water This picture has been apparently confirmed by the failure to produce (H,O); by electron attachment" or charge transferI2 to free neutral water clusters. A different approach has been recently pursued by Haberland and c o - w o r k e r ~ . ~These ~ ~ ' ~ authors (1) Chipman, D. M. J . Phys. Chem. 1978,82, 1080. ( 2 ) Jordan, K. D.; Wendoloski, J. J. Chem. Phys. 1977, 21, 145. (3) Crawford, 0. H.; Garrett, W. R. J . Chem. Phys. 1977, 66, 4968. (4) Jortner, J. Eer. Bunsen-Ges. Phys. Chem. 1984, 88, 188. (5) Kestner, N. R.; Jortner, J. J. Phys. Chem. 1984, 88, 3818. (6) Rao, B. K.; Kestner, N. R. J . Chem. Phys. 1984, 80, 1587. (7) Krebs, P. J. Phys. Chem. 1984, 88, 3702. Eer. Bunsen-Ges. Phys. Chem. 1984,88, 275. (8) Thompson, J. C. In Physics and Chemistry ofElecrrons and Ions in Condensed Mutter, Acrivos, J. V., et al., Eds.; Reidel: Dordrecht, 1984; p 385. (9) Kenney-Wallace, G.A. Adv. Chem. Phys. 1981,47, 535. (10) Copeland, D. A,; Kestner, N. R.; Jortner, J. J. Chem. Phys. 1970, 53, 1189. (11) Klots, C. E. Radiur. Phys. Chem. 1982, 20, 51. Compton, R. N. In Electronic and Aromic Collisions, Oda, N . , Takayanagi, K., Eds.; North Holland: Amsterdam, 1980; p 251. (12) Herschbach, D. R., private communication to J. Jortner (ref 69 in ref 4). Quitevis, E.L. Ph.D. Thesis, Harvard University, 1980. (13) Haberland, H.; Ludewigt, C.; Schindler, H A . ; Worsnop, D. R. Surf. Sci. 1985, 156, 157. (14) Haberland, H.; Schindler, H.-G.; Worsnop, D. R. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 270. Habedand, H.; Langosch, H.; Schindler, H.-G.; Worsnop, D. R. J . Phys. Chem. 1984, 88, 3903.
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injected electrons (kinetic energy below 2 eV) into the condensation zone of a supersonic jet of water vapor. For an expansion without carrier gas, they observed anions (H20),- for cluster sizes n I 11 (n 1 12 in case of deuteriated water). In these experiments transient cluster anions are presumably formed by electron attachment to warm clusters; they are stabilized and grown further by subsequent collisions with cold water molecules. In this paper we show that, contrary to common belief, electron attachment to free, cold, preexisting water clusters does result in long-lived cluster anions; subsequent stabilization by collisions is not required. The cross section for attachment is surprisingly large, but the electron energy has to be close to 0 eV. The size distributions of the cluster anions are strikingly similar to those obtained by Haberland et al. under comparable expansion conditions. In the time range of microseconds after their formation, the probability for electron detachment from the smallest cluster anions being observed is large, it decreases rapidly with increasing cluster size. This behavior probably reflects the rapid increase of the adiabatic electron affinity of (H,O), for n 1 1 1. A partial account of these results has been published previo~sly.'~ Experimental Section The cluster source and the time-of-flight mass spectrometer Neutral water have already been used in previous e~periments.'~.'~ clusters are formed by adiabatic expansion of neat water vapor through a nozzle (diameter 80 pm) into vacuum. The stagnation pressure is estimated from the temperature of the reservoir containing the liquid water. The temperature of the nozzle is kept slightly higher than that of the reservoir. For most experiments, deuteriated water (obtained from Riedel-de Haen, isotopic purity 99.7%) is used without further purification. The use of deuteriated water allows us to distinguish more easily between "pure" water cluster ions (stoichiometry (H,O); or (D,O);) and oxygenated or hydroxygenated cluster ions. Some experiments were also run with undeuteriated water; this was bidistilled and purified by ion exchange. (15) Knapp, M.; Echt, 0.;Kreisle, D.; Recknagel, E. J . Chem. Phys. 1986, 85, 636. (16) Echt, 0.;Knapp, M.; Sattler, K.; Recknagel, E. Z. Phys. 1983, B53, 71. (17) Echt, 0.; Reyes-Flotte, A,; Knapp, M.; Sattler, K.; Recknagel, E. Eer. Bunsen-Ges. Phys. Chem. 1982, 86, 860.
0 1987 American Chemical Society
