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Photoionization of clusters of Cs atoms solvated with H2O, NH3 and CH3CN
Volume 188, number 3,4
CHEMICAL PHYSICS LETTERS
10 January 1992
Photoionization of clusters of Cs atoms solvated with H20, NH3 and CH3CN Fuminori Misaizu, Keizo Tsukamoto,
Masaomi Sanekata and Kiyokazu Fuke
Institutefir Molecular Science, Myoda# Okazaki 444, Japan Received 4 August 1991; in final form 25 September 1991
Cesium atoms solvated with polar solvents, Cs(H,O)., Cs(NH,), and Cs(CH,CN),, are studied by one-photon ionization and time-of-IX&t mass spectroscopy. The ionization potentials of Cs(H,O), and Cs(CH,CN), are found to be constant for n& 4 (3. I eV) and n 2 I2 (2.4 eV), respectively, while that for Cs( NH3). decreases monotonically with increasing n to a limit of I .4 eV, which coincides with the bulk value. Enhanced stability at n=20 is also observed for the Cs( H,O). clusters. These features are discussed in connection with the stability of the excess electrons in these solvents.
1. Introduction
Electrons in fluids play important roles in many aspects of chemical phenomena and have been the subject of numerous investigations [ 1,2], Especially, the process of excess electron solvation in the alkali metal-polar solvent systems has been one of the central issues. Experimental and theoretical studies have attempted to understand the nature of solvated electrons and the dynamics of electron solvation in these systems. However, the microscopic aspect of solvated electrons has not yet been fully understood. Advances in molecular beam techniques have opened new approaches to a microscopic investigation of the excess electrons in fluids. Recently, negatively charged water and ammonia clusters, (H,O); and (NHJ);,were prepared via capture of low-energy electrons by solvent clusters [ 3-71. The vertical detachment energies were obtained for (H20); and (NH,); with n up to ~70 [6,7], and the excess electron states were examined using quantum path-integral molecular dynamics simulations [ 8,9]. On the other hand, Hertel and co-workers have prepared prototypes such as solvated sodium atom clusters, i.e. Na(H,O), and Na(NH,), [lo-121, which enable us to link the macroscopic with microscopic properties of alkali metal-solvent systems, In analogy to the bulk behavior of an alkali atom in po0009-26 14/92/$05.00
lar fluids [ 1,2], the valence electron of the alkali metal atom is transferred to a solvent cluster with sufficiently large n, and the ground state may have an ion-pair character. Therefore, the electron-photoejection threshold as a function of n is expected to include size-dependent information on the stability of the solvated atom, and also, on the excess electron state in clusters. In this paper, we present results of photoionization-threshold measurements of Cs( NH3) n (n < 3 1)) Cs(H20),andCs(CH,CN),withn<21.1nthebulk, solvent molecules used here have distinct characteristics in dissolving the alkali atom and stabilizing excess electrons. In bulk ammonia and water, the alkali metal is dissolved and produces stable solvated electrons. In the case of acetonitrile, the dimer-radical anion has been reported to be a stable species instead of the solvated electron. These three clusters examined in this study are found to show individual features in the size dependence of the ionization thresholds. The features are discussed in connection with the stability of the excess electron in molecules, clusters, and the bulk phases of these solvents.
2. Experimental
Details of the experimental apparatus used in the present study will be published separately [ 131, The
0 I992 Elsevier Science Publishers B.V. All rights reserved.
241
Volume 188,number 3,4
system consists of a three-stage differentially evacuated chamber which includes a cluster source and a reflection-type time-of-flight (TOF) mass spectrometer. The pick-up type cluster source [ 111 is composed of two pulse valves arranged at right angles. Solvent molecular clusters were produced by supersonic expansion of 2 atm Ar gas mixed with the sample gas from the first pulse valve (General valve, series 9) at room temperature. A pulsed Cs-atom beam was formed by expansion of neat vapor from the second valve which was heated to about 350°C. This pulse valve is a modified fuel injector suitable for high-temperature experiments up to 500°C [ 141. The Cs-atom beam was injected at 1O-l 5 mm downstream from the first nozzle. Solvated Cs clusters, CsM, (M= H20, NH3 and CH,CN), produced by subsequent multiple collisions, were ionized by crossing an excimer-pumped dye laser (Lambda Physik, EMG103-FL2002) 25 cm downstream from the first nozzle. The laser power was attenuated to less than 1 mJ/pulse in order to avoid multiphoton ionization.
Cluster ions produced
242
3. Results 3.1. Cs(H,O), clusters A typical photoionization mass spectrum of Cs{H,O), clusters is shown in figs. 1a and lb. These spectra were obtained at the laser wavelength of 360 nm. No evaporation of constituent Hz0 molecules n
2 4
II I,I
10
20
30
I
were accelerated
at right angles to the incident neutral beam and introduced to the field-free region of the reflectron TOF mass spectrometer. The ions were reflected by electric fields at the end of the TOF chamber and were detected by dual microchannel plates (Hamamatsu, F1552-23s) after flying back into the field-free region. The output signals were fed into a digital storage oscilloscope (LeCroy 9450) after being amplified by a wide-band amplifier (NF Electronic Instruments, BX-3 1). Mass spectra were obtained by scanning the photon energy with an interval of 0.03 eV in the region 3.65-2.14 eV (340-580 nm). The ionization thresholds were determined within ? 0.06 eV by analyzing the mass spectra. The error caused by field ionization is estimated to be about 0.01 eV in the present experimental conditions. The cesium sample (Rare Metallic Co., 99.95%) was filled into the pulse valve under nitrogen atmosphere without further purification. Ammonia gas (Nippon Sanso, 99.99%) was mixed with Ar gas in a stainless-steel reservoir. Liquid samples were placed in a reservoir where Ar gas was bubbled at room temperature. Water was used after distillation and ion exchange. Acetonitrile (Wako, > 99Oh)was used without further purification.
IOJanuary 1992
CHEMICALPHYSICSLETTERS
n
0 24
20
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Cs+( NH3),., -I
I
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40 TIME-OF-KIGW~s Fig. 1. Typical photoionization mass spectra of cesium atom-sotvated clusters. (a) Cs(H,O), with n up to 30. Ionization laser wavelength (1) is 360 nm. (b) Cs(H,O). with n around 20. I=360 nm. An outstanding peak is observed at n=20. (c) Cs(NHx), with n up to 35. ,I=308 nm.
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10 January I992
CHEMICAL PHYSICS LETTERS
from the cluster is expected after ionization at this photon energy. An anomalously enhanced peak is observed at n=20 (fig. lb). This magic number agrees with the enhanced stabilities observed in the mass spectra of H30f (H,O), [ 15,161 and NH: (HZ0 ) ,, [ 17 1, The ionization potential thresholds, IP (n ), determined for n = l-2 1 are plotted in fig. 2a as a function of (n+ 1)-‘13, which is approximately inversely proportional to the cluster radius (see Appendix). For n=l-4, IP(n) decreases almost linearly with (n + 1) -‘13. However, IP (n ) is almost constant at 3.12 eV for n = 4-2 1. This constant IP value, which agrees with that of Na(H,O), [ 111, is nearly equal to the bulk limit of the vertical detachment energies (VDE) determined by the photoelectron spectroscopy of ( H20); [ 61.
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Fig. lc shows a typical one-photon ionization mass spectrum of Cs ( NH3) ,, clusters obtained by irradiation of a XeCl laser beam of 308 nm. Cluster ions, Cs+ ( NH3),, were observed up to n= 50. A step in the size distribution at n s 10 is observed at any laser wavelength between 308 and 480 nm. This step may correspond to the filling of the first coordination shell around the cesium atom according to the theoretical work by Klein and co-workers [ 181. Fig. 2b shows IP( n ) versus (n + 1) - ‘I’ plots determined by changing the ionization laser wavelength. The observed IP( n) value coincides closely with a theoretically predicted one [ 191 for n= 1, though it is higher for n= 4, 6 and 8. The features of the IP( n ) versus (n + 1 ) -‘I3 plots observed for Cs( NH3), are rather different from those for Cs( H,O),. IP (n) decreases almost linearly with (n + 1) -‘I3 for n 2 3. The intercept of the fitted line at (n+ 1)-1/3=0 (n+co) is 1.4 eV. This value, which agrees also in this case with the results of a photoionization experiment for Na(NH,), [ 121, is higher than the heat of solution of the electron in liquid ammonia, 0.99 eV [20,21 ], which is determined thermodynamically. Instead, it coincides with the intercept value from the fitted line of VDE for (NH3); [7].
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(n+l)-1’3 Fig. 2. Ionization potentials (IP (n) ) of CsM, clusters plotted versus (n t I )-“‘. Error bars, indicated at each point for 1d nd 10 and at 5 point intervals for n b 10, include the uncertainty in determining IP( n) and the deviation caused by field ionization. (a) M=HZO (n621). (b) M=NHS (ng31). The result of the least-squares fitting for II2 2 is also shown. The extrapolated value to (n+ I ) - ‘13=0 (n+oo) is 1.41 eV. The correlation coeffkient of the fitted line, r, is 0.990. (c) M&H&N (n<21).
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Volume 188,number 3,4
3.3. Cs(CH,CN),
CHEMICAL PHYSICS LETTERS
clusters
In contrast to the case of ammonia- and water-solvated clusters, the IP(n) values determined for Cs( CH,CN) n show an anomalous dependence upon (n+l)-‘/‘forn
4. Discussion 4.1. Cs(H,O),, The present results of the n dependence of IP(n) for Cs ( HzO) n (fig. 2a) show that the IP( n ) values decrease linearly with decreasing ( n + 1) - ‘I3 for n d 4 and become constant (IP(c~)=3.12 eV) for n>4. Na(H?O), clusters have been found to show a similar trend in the ionization threshold with the same limiting value (IP(m)=3.17 eV) [ll]. As suggested by Coe et al. [6], this limiting value corresponds to the bulk photoelectric threshold of ice (3.2 eV). These results seem to indicate that, for the alkali atom surrounded by more than four water molecules, the IP does not depend on the metal atom but on the solvent species. In the case of Cs(H,O), (n b 4) clusters, the IP values decrease by only 0.8 eV from that of an isolated atom (3.89 eV), though the solvation energies are reported to be more than 2 eV for Cs+ (H20), (n z 4) [ 221. Thus, the present results of the IP measurement imply that the solvated alkali atom in the ground state is fully screened for n > 4. Kestner and Dhar have recently reported theoretical studies for sodium water clusters in which they predicted the geometrical and electronic structures of clusters in the ground and ionized states [ 23 1. According to their results for the sodium atom surrounded by four water molecules, the electron density of the 3s electron is very diffuse and extends over the region occupied by the solvent molecules. As a result, the sodium atom interacts with HZ0 molecules by an ionic-type bonding. They also predicted that the energy for the fully optimized neutral 244
10 January 1992
Na(H,O), cluster is similar to that for the optimized geometry of the sodium ion-water cluster plus electron. These predictions are consistent with the above observations. An alkali-metal atom has been known to be ionized spontaneously in a polar solvent and converted to a contact ion-pair of a solvated alkali-atom ion plus a solvated electron [ 1,2 1. This state in a bulk fluid may correspond to the lowest ion-pair (or a Rydberg-like) state of the clusters. Then, this bulk behavior can be related to the properties of solvated alkali-atom clusters as follows. Fig. 3 draws schematically the energy levels of the covalent, ion-pair, and ion ground states for a Cs atom-solvent cluster as a function of the cluster size. The energy levels of the ion ground states decrease monotonically with increasing number of solvent molecules, as can be seen from the enthalpies of solvation for the Cs+ ion [ 221. For small clusters, the ground state is of the covalent type and is expected to be stabilized very slowly with increasing number of solvent molecules (fig. 3). On the other hand, the energy level of the ion-pair state is more sensitive to the number of solvent molecules. When the screening of the solvated alkali atom is effective as mentioned previously, this state is stabilized much more rapidly than that of the covalent-type state. Since the ion-pair state corre-
(atom)
(bulk) NUMBER OF SOLVENT MOLECULES (n)
Fig. 3. Schematic diagram of the energies of (a) the lowest COvalent, (b) ion-pair, and (c) the ion-ground states for CsM, (M: solvent molecules) versus n. The ion-pair state is expected to correlate with the ground state of the solvated electron plus the solvated Cs+ ion in bulk fields. The ground-state electronic character of the neutral cluster changes from covalent to ion-pair type at a size n*.
Volume 188, number 3.4
CHEMICAL PHYSICS LETTERS
lates with the ground state of the solvated electron plus the solvated Cs+ ion in a bulk fluid, the electronic character of clusters in the ground state is expected to change from the covalent type to the ionpair type at a certain critical cluster size (n*). Based on the above theoretical results, the curvature of the E versus n curve for the ion-pair state (fig. 3) is expected to be similar to that of the ionic state for clusters with more than four water molecules; the interaction in both states is mostly electrostatic in nature. Therefore, the observed plateau in the IP (n) versus (n+ I ) -II3 plot for the Cs(H,O), clusters (fig. 2a) can be understood by the cluster size dependence of the ion-pair and ionic levels, which suggests that n*=4. The mass spectrum shows the enhanced stability of the Cs(H20)& cluster (fig. lb). This value of n often appears as the magic number in mass spectra of H,O+ (H*O), clusters [ 161. These are several possible explanations for the Cs ( HZ0 )& dominance, such as ( 1) the predominance of the preformed (H,O), clusters and (2) the special stability of Cs(H,O)& or (3) that of Cs(H,0)2,,. The photoionization of the (H,O), clusters has been studied extensively and the distribution of the neutral clusters has been known to have no anomaly in the size range discussed here [ 16 ] ; this may rule out explanation ( 1). Since the radius of the Cs+ ion ( 1.65 A) is close to that of the H,O+ ion ( 1.7 A), one can expect the enhanced stability of Cs+ ( HzO) 20by analogy to the H30+ ( HzO) ,, clusters. However, this possibility can also be excluded because the enhanced stability at n=20 is observed even at the photoionization energy close to the ionization threshold (3.1 eV), where the fragmentation due to the excess energy at ionization seems to be negligible. On the other hand, the theoretical calculations for sodium-water clusters mentioned above predict that the core of the clusters for n> 4 has an ionic character. As with sodium, Cs is expected to interact with water molecules by an ionic-type bonding. Therefore, it is reasonable to ascribe the observed anomaly to the enhanced stability of the Cs( H20)zo cluster in the ground state (explanation (3 ) ); this observation seems to support the scheme shown in fig. 3. It is believed from both experimental [ 171 and theoretical [ 161 studies that the H,O+ (H,O),, cluster forms a clathrate structure (deformed pentagonal dodeca-
10 January 1992
hedron) with the H,O+ ion in the center of the hydrogen-bonding networks of 20 water molecules. Thus, the Cs( H20)20 cluster is expected to be in the ion-pair state in which the ion core has an ion-clathrate structure and the excess electron is distributed outside the core. * 4.2. Cs(NHJ, and Cs(CH,CN), clusters The results of the IP( n ) dependence on n for Cs(NH,), (fig. 2b) show that the IP(n) value decreases linearly with increasing (n + 1)-‘I3 for n < 30. As in the case of Na(NH,), [ 111, the intercept of the fitted line at n=o3 is 1.4 eV, which agrees with the bulk photoelectric threshold estimated from the electron binding energies of (NH,); clusters [ 71. These results imply that the n* value of alkali atomammonia cluster is larger than 30. Since the threshold cluster size of (NH 3); cluster formation is found to be 35 [4], the n+ value of the alkali atom-ammonia cluster is expected to be larger than 35. However, the observed IP ( n ) versus ( n + 1) - ‘I3 plots indicate that the IP values of small clusters can be extrapolated smoothly to that of the bulk value. Therefore, an alternative explanation is that, in the case of ammoniated alkali-atom clusters, the electronic character of the ground state may change from the covalent type to the ion-pair type with the increasing cluster size. The intriguing feature of the IP( n ) versus (n + I )-“3 plots of the Cs(CH,CN),, clusters is the anomalous change of IP( n ) for n < 11 (fig. 2~). The rises near the threshold in the ionization efficiency curves for n = 4-6 are found to be as steep as that for n = 1, while those for n = 2, 3 and for n 3 7 are much less steep. These results suggest that the differences in the geometry of neutral and ionic clusters for n = 46 are much closer than those for n=2 and 3. These anomalous changes in the geometry may be due to the large permanent dipole moment of acetonitrile. As in the case of the Cs( H,O ), clusters, the IP( n) versus (n + 1) -‘I3 plots of Cs( CH,CN), show a plateau for n 2 12 (n*= 12 ), which gives the bulk value of 2.4 eV. The behavior of electrons in the bulk acetonitrile has been known to be rather different from that for water and ammonia, where electrons are delocalized over scores of solvent molecules. Accord245
CHEMICAL PHYSICS LETTERS
Volume 188, number 3,4
ing to the ESR studies for y-irradiated acetonitrile crystal, a dimer radical anion, (CH,CN), , is found
to be stable; electrons are trapped by a deep potential well formed in the dimer and a welldefined anisotropic hypertine structure is observed in the ESR spectrum [ 241. Thus, the behavior of the electrons in the bulk is consistent with the finding that the solvated Cs atom is fully screened by a rather small number of acetonitrile molecules as seen in the IP ( n ) versus (n-t 1 )-‘I3 plots (fig. 2~). In order to understand the n dependence of IP for Cs(CH&N),, it seems necessary to carry out an elaborate theoretical calculation for the neutral and ionic states of these clusters. The critical number n* observed in the present study may have some correlation with the cluster size, nthy at which the negatively charged molecular cluster starts to be observed. Several groups have reported n,,, observed by electrons attachment to molecular clusters: Stable negative ions are produced from n=2 for (H,O), [4], n=lO for (CH,CN), [25] and n=35 for (NH3), [4].
Acknowledgement
The authors thank I.V. Hertel for sending a preprint. They also thank N. Okada for helpful support in constructing the high-temperature pulse valve.
Appendix
According to a classical calculation, the ionization potential IP, of a spherical droplet consisting of m particles is written as IP, = WF+ 3e2/8R ,
(A.11
where WF is the work function of the planar solid, and R is the radius of the droplet [ 26 1. Because R is written using the number density p as R= [ (3j1/4x)m]“~
,
64.2)
IP, is proportional to m - ‘I3 for a cluster with large m sufficient to be approximated as a sphere. Therefore, for CsM, clusters the IP( n) value is expected to be proportional to (n+ 1) - Ii3 (Cs atom plus n solvent molecules). 246
10 January 1992
References [ I] J. Jortner and N.R. Kestner, eds., Electrons in fluids (Springer, Berlin, 1973). (21 R.R. Dogonadze, E. Kalman, A.A. Komyshev and J. Ulstrup, eds., The chemical physics of solvation, Part C (Elsevier, Amsterdam, 1988 ) . [3] H. Haberland, H.-G. Schindler and D.R. Worksnop, Ber. Bunsenges. Physik. Chem. 88 (1984) 270. [4]H. Haberland, C. Ludewigt, H.-G. Schindler and D.R. Worsnop, SurfaceSci. 156 ( 1985) 157. [5] P.J. Campagnola, L.A. Posey and M.A. Johnson, J. Chem. Phys. 92 ( 1990) 3243. [6] J.V. Coe, G.H. Lee, J.G. Eaton, S.T. Arnold, H.W. Sarkas, KH. Bowen, C. Ludewigt, H. Haberland and D.R. Worsnop, J. Chem. Phys. 92 (1990) 3980. [7]G.H. Lee, S.T. Arnold, J.G. Eaton, H.W. Sarkas, K.H. Bowen, C. Ludewigt and H. Haberland, Z. Physik D 20 (1991) 9. [8] R.N. Barnett, U. Landman, C.L. Cleveland and J. Jortner, Chem. Phys. Letters 145 (1988) 382. [9] R.N. Bamett, U. Landman, C.L. Cleveland, N.R. Kestner and J. Jortner, Chem. Phys. Letters 148 ( I988 ) 249. [ lo] C.P. Schulz, R. Haugstitter, H.-U. Tittes and I.V. Hertel, Phys. Rev. Letters 57 (1986) 1703. [ 111C.P. Schulz, R. HaugstBtter, H.-U. Tittesand I.V. Hertel, Z. Physik D 10 ( 1988) 279. [ 121 I.V. Hertel, C. Hiiglin, C. Nitsch and C.P. Schulz, Phys. Rev. Letters 67 (1991) 1767. [ 131 K. Tsukamoto, F. Misaizu, M. Sanekata and K. Fuke, to be published. [ 141 K. Fuke, K. Tsukamoto, F. Misaizu and K. Kaya, I. Chem. Phys., in press. [ 151J.Q. Searcy and J.B. Fenn, J. Phys. Chem. 61 ( 1974) 5282. [ 161 U. Nagashima, H. Shinohara, N. Nishi and H. Tanaka, J. Chem. Phys. 84 (1986) 209. [ 171 H. Shinohara, U. Nagashima, H. Tanaka and N. Nishi, J. Chem. Phys. 83 (1985) 4183. [ 181 M. Marchi, M. Sprik and ML. Klein, J. Phys. C 2 (1990) 5833. [ 191 G.J. Martyne and M.L. Klein, J. Phys. Chem. 95 (1991) 515. [20] G. Lepoutre and J. Jortner, J. Phys. Chem. 76 ( 1972) 683. [21] U. Schindewolf, J. Phys. Chem. 88 ( 1984) 3820. [22] R.G. Keesee and A.W. Castleman Jr., J. Phys. Chem. Ref. Data 15 (1986) 1016. [23] N.R. Kestner and S. Dhar, in: Large finite systems, ed. J. Jortner (Reidel, Dordrccht, 1987) p. 209. [24] F. Williams and E.D. Sprague, Accounts Chem. Res. 15 (1982) 408. [25] T. Kondow, J. Phys. Chem. 91 (1987) 1307. [26] D.M. Wood, Phys. Rev. Letters 46 (1981) 749.
CHEMICAL PHYSICS LETTERS
10 January 1992
Photoionization of clusters of Cs atoms solvated with H20, NH3 and CH3CN Fuminori Misaizu, Keizo Tsukamoto,
Masaomi Sanekata and Kiyokazu Fuke
Institutefir Molecular Science, Myoda# Okazaki 444, Japan Received 4 August 1991; in final form 25 September 1991
Cesium atoms solvated with polar solvents, Cs(H,O)., Cs(NH,), and Cs(CH,CN),, are studied by one-photon ionization and time-of-IX&t mass spectroscopy. The ionization potentials of Cs(H,O), and Cs(CH,CN), are found to be constant for n& 4 (3. I eV) and n 2 I2 (2.4 eV), respectively, while that for Cs( NH3). decreases monotonically with increasing n to a limit of I .4 eV, which coincides with the bulk value. Enhanced stability at n=20 is also observed for the Cs( H,O). clusters. These features are discussed in connection with the stability of the excess electrons in these solvents.
1. Introduction
Electrons in fluids play important roles in many aspects of chemical phenomena and have been the subject of numerous investigations [ 1,2], Especially, the process of excess electron solvation in the alkali metal-polar solvent systems has been one of the central issues. Experimental and theoretical studies have attempted to understand the nature of solvated electrons and the dynamics of electron solvation in these systems. However, the microscopic aspect of solvated electrons has not yet been fully understood. Advances in molecular beam techniques have opened new approaches to a microscopic investigation of the excess electrons in fluids. Recently, negatively charged water and ammonia clusters, (H,O); and (NHJ);,were prepared via capture of low-energy electrons by solvent clusters [ 3-71. The vertical detachment energies were obtained for (H20); and (NH,); with n up to ~70 [6,7], and the excess electron states were examined using quantum path-integral molecular dynamics simulations [ 8,9]. On the other hand, Hertel and co-workers have prepared prototypes such as solvated sodium atom clusters, i.e. Na(H,O), and Na(NH,), [lo-121, which enable us to link the macroscopic with microscopic properties of alkali metal-solvent systems, In analogy to the bulk behavior of an alkali atom in po0009-26 14/92/$05.00
lar fluids [ 1,2], the valence electron of the alkali metal atom is transferred to a solvent cluster with sufficiently large n, and the ground state may have an ion-pair character. Therefore, the electron-photoejection threshold as a function of n is expected to include size-dependent information on the stability of the solvated atom, and also, on the excess electron state in clusters. In this paper, we present results of photoionization-threshold measurements of Cs( NH3) n (n < 3 1)) Cs(H20),andCs(CH,CN),withn<21.1nthebulk, solvent molecules used here have distinct characteristics in dissolving the alkali atom and stabilizing excess electrons. In bulk ammonia and water, the alkali metal is dissolved and produces stable solvated electrons. In the case of acetonitrile, the dimer-radical anion has been reported to be a stable species instead of the solvated electron. These three clusters examined in this study are found to show individual features in the size dependence of the ionization thresholds. The features are discussed in connection with the stability of the excess electron in molecules, clusters, and the bulk phases of these solvents.
2. Experimental
Details of the experimental apparatus used in the present study will be published separately [ 131, The
0 I992 Elsevier Science Publishers B.V. All rights reserved.
241
Volume 188,number 3,4
system consists of a three-stage differentially evacuated chamber which includes a cluster source and a reflection-type time-of-flight (TOF) mass spectrometer. The pick-up type cluster source [ 111 is composed of two pulse valves arranged at right angles. Solvent molecular clusters were produced by supersonic expansion of 2 atm Ar gas mixed with the sample gas from the first pulse valve (General valve, series 9) at room temperature. A pulsed Cs-atom beam was formed by expansion of neat vapor from the second valve which was heated to about 350°C. This pulse valve is a modified fuel injector suitable for high-temperature experiments up to 500°C [ 141. The Cs-atom beam was injected at 1O-l 5 mm downstream from the first nozzle. Solvated Cs clusters, CsM, (M= H20, NH3 and CH,CN), produced by subsequent multiple collisions, were ionized by crossing an excimer-pumped dye laser (Lambda Physik, EMG103-FL2002) 25 cm downstream from the first nozzle. The laser power was attenuated to less than 1 mJ/pulse in order to avoid multiphoton ionization.
Cluster ions produced
242
3. Results 3.1. Cs(H,O), clusters A typical photoionization mass spectrum of Cs{H,O), clusters is shown in figs. 1a and lb. These spectra were obtained at the laser wavelength of 360 nm. No evaporation of constituent Hz0 molecules n
2 4
II I,I
10
20
30
I
were accelerated
at right angles to the incident neutral beam and introduced to the field-free region of the reflectron TOF mass spectrometer. The ions were reflected by electric fields at the end of the TOF chamber and were detected by dual microchannel plates (Hamamatsu, F1552-23s) after flying back into the field-free region. The output signals were fed into a digital storage oscilloscope (LeCroy 9450) after being amplified by a wide-band amplifier (NF Electronic Instruments, BX-3 1). Mass spectra were obtained by scanning the photon energy with an interval of 0.03 eV in the region 3.65-2.14 eV (340-580 nm). The ionization thresholds were determined within ? 0.06 eV by analyzing the mass spectra. The error caused by field ionization is estimated to be about 0.01 eV in the present experimental conditions. The cesium sample (Rare Metallic Co., 99.95%) was filled into the pulse valve under nitrogen atmosphere without further purification. Ammonia gas (Nippon Sanso, 99.99%) was mixed with Ar gas in a stainless-steel reservoir. Liquid samples were placed in a reservoir where Ar gas was bubbled at room temperature. Water was used after distillation and ion exchange. Acetonitrile (Wako, > 99Oh)was used without further purification.
IOJanuary 1992
CHEMICALPHYSICSLETTERS
n
0 24
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Cs+( NH3),., -I
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40 TIME-OF-KIGW~s Fig. 1. Typical photoionization mass spectra of cesium atom-sotvated clusters. (a) Cs(H,O), with n up to 30. Ionization laser wavelength (1) is 360 nm. (b) Cs(H,O). with n around 20. I=360 nm. An outstanding peak is observed at n=20. (c) Cs(NHx), with n up to 35. ,I=308 nm.
Volume 188, number 3,4
10 January I992
CHEMICAL PHYSICS LETTERS
from the cluster is expected after ionization at this photon energy. An anomalously enhanced peak is observed at n=20 (fig. lb). This magic number agrees with the enhanced stabilities observed in the mass spectra of H30f (H,O), [ 15,161 and NH: (HZ0 ) ,, [ 17 1, The ionization potential thresholds, IP (n ), determined for n = l-2 1 are plotted in fig. 2a as a function of (n+ 1)-‘13, which is approximately inversely proportional to the cluster radius (see Appendix). For n=l-4, IP(n) decreases almost linearly with (n + 1) -‘13. However, IP (n ) is almost constant at 3.12 eV for n = 4-2 1. This constant IP value, which agrees with that of Na(H,O), [ 111, is nearly equal to the bulk limit of the vertical detachment energies (VDE) determined by the photoelectron spectroscopy of ( H20); [ 61.
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1
10532
1
III
’ l.O
10532
I
’
I
0
1-l
t b)
3.2. Cs(NHJ, clusters 2
Fig. lc shows a typical one-photon ionization mass spectrum of Cs ( NH3) ,, clusters obtained by irradiation of a XeCl laser beam of 308 nm. Cluster ions, Cs+ ( NH3),, were observed up to n= 50. A step in the size distribution at n s 10 is observed at any laser wavelength between 308 and 480 nm. This step may correspond to the filling of the first coordination shell around the cesium atom according to the theoretical work by Klein and co-workers [ 181. Fig. 2b shows IP( n ) versus (n + 1) - ‘I’ plots determined by changing the ionization laser wavelength. The observed IP( n) value coincides closely with a theoretically predicted one [ 191 for n= 1, though it is higher for n= 4, 6 and 8. The features of the IP( n ) versus (n + 1 ) -‘I3 plots observed for Cs( NH3), are rather different from those for Cs( H,O),. IP (n) decreases almost linearly with (n + 1) -‘I3 for n 2 3. The intercept of the fitted line at (n+ 1)-1/3=0 (n+co) is 1.4 eV. This value, which agrees also in this case with the results of a photoionization experiment for Na(NH,), [ 121, is higher than the heat of solution of the electron in liquid ammonia, 0.99 eV [20,21 ], which is determined thermodynamically. Instead, it coincides with the intercept value from the fitted line of VDE for (NH3); [7].
n lo3 lo2
a -3
b
QQQ
#i*@ 2
i
)...
’
n=21
I
(n+l)-1’3 Fig. 2. Ionization potentials (IP (n) ) of CsM, clusters plotted versus (n t I )-“‘. Error bars, indicated at each point for 1d nd 10 and at 5 point intervals for n b 10, include the uncertainty in determining IP( n) and the deviation caused by field ionization. (a) M=HZO (n621). (b) M=NHS (ng31). The result of the least-squares fitting for II2 2 is also shown. The extrapolated value to (n+ I ) - ‘13=0 (n+oo) is 1.41 eV. The correlation coeffkient of the fitted line, r, is 0.990. (c) M&H&N (n<21).
243
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3.3. Cs(CH,CN),
CHEMICAL PHYSICS LETTERS
clusters
In contrast to the case of ammonia- and water-solvated clusters, the IP(n) values determined for Cs( CH,CN) n show an anomalous dependence upon (n+l)-‘/‘forn
4. Discussion 4.1. Cs(H,O),, The present results of the n dependence of IP(n) for Cs ( HzO) n (fig. 2a) show that the IP( n ) values decrease linearly with decreasing ( n + 1) - ‘I3 for n d 4 and become constant (IP(c~)=3.12 eV) for n>4. Na(H?O), clusters have been found to show a similar trend in the ionization threshold with the same limiting value (IP(m)=3.17 eV) [ll]. As suggested by Coe et al. [6], this limiting value corresponds to the bulk photoelectric threshold of ice (3.2 eV). These results seem to indicate that, for the alkali atom surrounded by more than four water molecules, the IP does not depend on the metal atom but on the solvent species. In the case of Cs(H,O), (n b 4) clusters, the IP values decrease by only 0.8 eV from that of an isolated atom (3.89 eV), though the solvation energies are reported to be more than 2 eV for Cs+ (H20), (n z 4) [ 221. Thus, the present results of the IP measurement imply that the solvated alkali atom in the ground state is fully screened for n > 4. Kestner and Dhar have recently reported theoretical studies for sodium water clusters in which they predicted the geometrical and electronic structures of clusters in the ground and ionized states [ 23 1. According to their results for the sodium atom surrounded by four water molecules, the electron density of the 3s electron is very diffuse and extends over the region occupied by the solvent molecules. As a result, the sodium atom interacts with HZ0 molecules by an ionic-type bonding. They also predicted that the energy for the fully optimized neutral 244
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Na(H,O), cluster is similar to that for the optimized geometry of the sodium ion-water cluster plus electron. These predictions are consistent with the above observations. An alkali-metal atom has been known to be ionized spontaneously in a polar solvent and converted to a contact ion-pair of a solvated alkali-atom ion plus a solvated electron [ 1,2 1. This state in a bulk fluid may correspond to the lowest ion-pair (or a Rydberg-like) state of the clusters. Then, this bulk behavior can be related to the properties of solvated alkali-atom clusters as follows. Fig. 3 draws schematically the energy levels of the covalent, ion-pair, and ion ground states for a Cs atom-solvent cluster as a function of the cluster size. The energy levels of the ion ground states decrease monotonically with increasing number of solvent molecules, as can be seen from the enthalpies of solvation for the Cs+ ion [ 221. For small clusters, the ground state is of the covalent type and is expected to be stabilized very slowly with increasing number of solvent molecules (fig. 3). On the other hand, the energy level of the ion-pair state is more sensitive to the number of solvent molecules. When the screening of the solvated alkali atom is effective as mentioned previously, this state is stabilized much more rapidly than that of the covalent-type state. Since the ion-pair state corre-
(atom)
(bulk) NUMBER OF SOLVENT MOLECULES (n)
Fig. 3. Schematic diagram of the energies of (a) the lowest COvalent, (b) ion-pair, and (c) the ion-ground states for CsM, (M: solvent molecules) versus n. The ion-pair state is expected to correlate with the ground state of the solvated electron plus the solvated Cs+ ion in bulk fields. The ground-state electronic character of the neutral cluster changes from covalent to ion-pair type at a size n*.
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CHEMICAL PHYSICS LETTERS
lates with the ground state of the solvated electron plus the solvated Cs+ ion in a bulk fluid, the electronic character of clusters in the ground state is expected to change from the covalent type to the ionpair type at a certain critical cluster size (n*). Based on the above theoretical results, the curvature of the E versus n curve for the ion-pair state (fig. 3) is expected to be similar to that of the ionic state for clusters with more than four water molecules; the interaction in both states is mostly electrostatic in nature. Therefore, the observed plateau in the IP (n) versus (n+ I ) -II3 plot for the Cs(H,O), clusters (fig. 2a) can be understood by the cluster size dependence of the ion-pair and ionic levels, which suggests that n*=4. The mass spectrum shows the enhanced stability of the Cs(H20)& cluster (fig. lb). This value of n often appears as the magic number in mass spectra of H,O+ (H*O), clusters [ 161. These are several possible explanations for the Cs ( HZ0 )& dominance, such as ( 1) the predominance of the preformed (H,O), clusters and (2) the special stability of Cs(H,O)& or (3) that of Cs(H,0)2,,. The photoionization of the (H,O), clusters has been studied extensively and the distribution of the neutral clusters has been known to have no anomaly in the size range discussed here [ 16 ] ; this may rule out explanation ( 1). Since the radius of the Cs+ ion ( 1.65 A) is close to that of the H,O+ ion ( 1.7 A), one can expect the enhanced stability of Cs+ ( HzO) 20by analogy to the H30+ ( HzO) ,, clusters. However, this possibility can also be excluded because the enhanced stability at n=20 is observed even at the photoionization energy close to the ionization threshold (3.1 eV), where the fragmentation due to the excess energy at ionization seems to be negligible. On the other hand, the theoretical calculations for sodium-water clusters mentioned above predict that the core of the clusters for n> 4 has an ionic character. As with sodium, Cs is expected to interact with water molecules by an ionic-type bonding. Therefore, it is reasonable to ascribe the observed anomaly to the enhanced stability of the Cs( H20)zo cluster in the ground state (explanation (3 ) ); this observation seems to support the scheme shown in fig. 3. It is believed from both experimental [ 171 and theoretical [ 161 studies that the H,O+ (H,O),, cluster forms a clathrate structure (deformed pentagonal dodeca-
10 January 1992
hedron) with the H,O+ ion in the center of the hydrogen-bonding networks of 20 water molecules. Thus, the Cs( H20)20 cluster is expected to be in the ion-pair state in which the ion core has an ion-clathrate structure and the excess electron is distributed outside the core. * 4.2. Cs(NHJ, and Cs(CH,CN), clusters The results of the IP( n ) dependence on n for Cs(NH,), (fig. 2b) show that the IP(n) value decreases linearly with increasing (n + 1)-‘I3 for n < 30. As in the case of Na(NH,), [ 111, the intercept of the fitted line at n=o3 is 1.4 eV, which agrees with the bulk photoelectric threshold estimated from the electron binding energies of (NH,); clusters [ 71. These results imply that the n* value of alkali atomammonia cluster is larger than 30. Since the threshold cluster size of (NH 3); cluster formation is found to be 35 [4], the n+ value of the alkali atom-ammonia cluster is expected to be larger than 35. However, the observed IP ( n ) versus ( n + 1) - ‘I3 plots indicate that the IP values of small clusters can be extrapolated smoothly to that of the bulk value. Therefore, an alternative explanation is that, in the case of ammoniated alkali-atom clusters, the electronic character of the ground state may change from the covalent type to the ion-pair type with the increasing cluster size. The intriguing feature of the IP( n ) versus (n + I )-“3 plots of the Cs(CH,CN),, clusters is the anomalous change of IP( n ) for n < 11 (fig. 2~). The rises near the threshold in the ionization efficiency curves for n = 4-6 are found to be as steep as that for n = 1, while those for n = 2, 3 and for n 3 7 are much less steep. These results suggest that the differences in the geometry of neutral and ionic clusters for n = 46 are much closer than those for n=2 and 3. These anomalous changes in the geometry may be due to the large permanent dipole moment of acetonitrile. As in the case of the Cs( H,O ), clusters, the IP( n) versus (n + 1) -‘I3 plots of Cs( CH,CN), show a plateau for n 2 12 (n*= 12 ), which gives the bulk value of 2.4 eV. The behavior of electrons in the bulk acetonitrile has been known to be rather different from that for water and ammonia, where electrons are delocalized over scores of solvent molecules. Accord245
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ing to the ESR studies for y-irradiated acetonitrile crystal, a dimer radical anion, (CH,CN), , is found
to be stable; electrons are trapped by a deep potential well formed in the dimer and a welldefined anisotropic hypertine structure is observed in the ESR spectrum [ 241. Thus, the behavior of the electrons in the bulk is consistent with the finding that the solvated Cs atom is fully screened by a rather small number of acetonitrile molecules as seen in the IP ( n ) versus (n-t 1 )-‘I3 plots (fig. 2~). In order to understand the n dependence of IP for Cs(CH&N),, it seems necessary to carry out an elaborate theoretical calculation for the neutral and ionic states of these clusters. The critical number n* observed in the present study may have some correlation with the cluster size, nthy at which the negatively charged molecular cluster starts to be observed. Several groups have reported n,,, observed by electrons attachment to molecular clusters: Stable negative ions are produced from n=2 for (H,O), [4], n=lO for (CH,CN), [25] and n=35 for (NH3), [4].
Acknowledgement
The authors thank I.V. Hertel for sending a preprint. They also thank N. Okada for helpful support in constructing the high-temperature pulse valve.
Appendix
According to a classical calculation, the ionization potential IP, of a spherical droplet consisting of m particles is written as IP, = WF+ 3e2/8R ,
(A.11
where WF is the work function of the planar solid, and R is the radius of the droplet [ 26 1. Because R is written using the number density p as R= [ (3j1/4x)m]“~
,
64.2)
IP, is proportional to m - ‘I3 for a cluster with large m sufficient to be approximated as a sphere. Therefore, for CsM, clusters the IP( n) value is expected to be proportional to (n+ 1) - Ii3 (Cs atom plus n solvent molecules). 246
10 January 1992
References [ I] J. Jortner and N.R. Kestner, eds., Electrons in fluids (Springer, Berlin, 1973). (21 R.R. Dogonadze, E. Kalman, A.A. Komyshev and J. Ulstrup, eds., The chemical physics of solvation, Part C (Elsevier, Amsterdam, 1988 ) . [3] H. Haberland, H.-G. Schindler and D.R. Worksnop, Ber. Bunsenges. Physik. Chem. 88 (1984) 270. [4]H. Haberland, C. Ludewigt, H.-G. Schindler and D.R. Worsnop, SurfaceSci. 156 ( 1985) 157. [5] P.J. Campagnola, L.A. Posey and M.A. Johnson, J. Chem. Phys. 92 ( 1990) 3243. [6] J.V. Coe, G.H. Lee, J.G. Eaton, S.T. Arnold, H.W. Sarkas, KH. Bowen, C. Ludewigt, H. Haberland and D.R. Worsnop, J. Chem. Phys. 92 (1990) 3980. [7]G.H. Lee, S.T. Arnold, J.G. Eaton, H.W. Sarkas, K.H. Bowen, C. Ludewigt and H. Haberland, Z. Physik D 20 (1991) 9. [8] R.N. Barnett, U. Landman, C.L. Cleveland and J. Jortner, Chem. Phys. Letters 145 (1988) 382. [9] R.N. Bamett, U. Landman, C.L. Cleveland, N.R. Kestner and J. Jortner, Chem. Phys. Letters 148 ( I988 ) 249. [ lo] C.P. Schulz, R. Haugstitter, H.-U. Tittes and I.V. Hertel, Phys. Rev. Letters 57 (1986) 1703. [ 111C.P. Schulz, R. HaugstBtter, H.-U. Tittesand I.V. Hertel, Z. Physik D 10 ( 1988) 279. [ 121 I.V. Hertel, C. Hiiglin, C. Nitsch and C.P. Schulz, Phys. Rev. Letters 67 (1991) 1767. [ 131 K. Tsukamoto, F. Misaizu, M. Sanekata and K. Fuke, to be published. [ 141 K. Fuke, K. Tsukamoto, F. Misaizu and K. Kaya, I. Chem. Phys., in press. [ 151J.Q. Searcy and J.B. Fenn, J. Phys. Chem. 61 ( 1974) 5282. [ 161 U. Nagashima, H. Shinohara, N. Nishi and H. Tanaka, J. Chem. Phys. 84 (1986) 209. [ 171 H. Shinohara, U. Nagashima, H. Tanaka and N. Nishi, J. Chem. Phys. 83 (1985) 4183. [ 181 M. Marchi, M. Sprik and ML. Klein, J. Phys. C 2 (1990) 5833. [ 191 G.J. Martyne and M.L. Klein, J. Phys. Chem. 95 (1991) 515. [20] G. Lepoutre and J. Jortner, J. Phys. Chem. 76 ( 1972) 683. [21] U. Schindewolf, J. Phys. Chem. 88 ( 1984) 3820. [22] R.G. Keesee and A.W. Castleman Jr., J. Phys. Chem. Ref. Data 15 (1986) 1016. [23] N.R. Kestner and S. Dhar, in: Large finite systems, ed. J. Jortner (Reidel, Dordrccht, 1987) p. 209. [24] F. Williams and E.D. Sprague, Accounts Chem. Res. 15 (1982) 408. [25] T. Kondow, J. Phys. Chem. 91 (1987) 1307. [26] D.M. Wood, Phys. Rev. Letters 46 (1981) 749.