J. Phys. Chem. 1987, 91, 1307-1316
1307
FEATURE ARTICLE Ionization of Clusters in Collision with High-Rydberg Rare Gas Atoms Tamotsu Kondow Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan (Received: October 29, 1986)
An overview is given on our systematic study of electron attachment to a variety of van der Waals clusters in collision with high-Rydberg rare gas atoms, which are utilized as a source of subthermal electrons with kinetic energy less than 10 meV and a narrow energy distribution. This collisional electron transfer from high-Rydberg atoms provides a unique method for efficient and gentle production of negative cluster ions. The design principle and the operation for this ionization method are described. Various negative cluster ions which can scarcely be generated by conventional techniques are observed, and the mechanism of cluster-ion formation is discussed with the aid of a theoretical consideration. Other techniques used for formation of negative cluster ions from gaseous clusters are reviewed.
1. Introduction
A remarkable advancement of molecular beam techniques in recent years has enabled us to produce atomic and molecular clusters of essentially any size and composition under vacuum1-5 and triggered an explosion of research activities in the studies of clusters.6-8 The properties of clusters vary with the number of component particles (cluster size) more or less continuously, from those of isolated molecules to condensed matters. Namely, the clusters can be treated as “pseudomolecules”when the size is small. On the other hand, clusters with sizes of more than lo3 behave like condensed materials. In between there is a regime where the cluster systems cannot be treated by either the conventional molecular concept or the solid-state concept; that is, the clusters form systems of intermediate nature, bridging the gap between the gas phase and the liquid or solid phase. In this connection, the fundamental properties and applications of such clusters have been investigated by use of a wide variety of spectroscopic techn i q u e ~incorporated ~ with the theories developed in the field of molecular and solid-state chemistry and physics.’&I2 Optical spectroscopy, for example, has revealed that even in a simple system of clusters there exist several metastable structures having similar stabilities,I3-l7 and detailed dynamical information on
(1) Hagena, 0 . F. In Molecular Beams and Low Densify Gas Dynamics; Wegener, P. P., Ed.; Dekker: New York, 1974; p 93. (2) Bowla, R. S.; Kolstad, J. J.; Calo, J. M.; Andres, R. P. SurJ Sci. 1981, 106. 117. (3) Kappes, M. M.; Schir, M.; Radi, P.; Schumacher, E. J . Chem. Phys. 1986,84, 1863. (4) Dietz, T. G.; Duncan, M. A.; Powers, D. E.; Smalley, R. E. J . Chem. Phys. 1981, 74, 6511. (51 Geusic, M. E.; Morse, M. D.; OBrien, S. C.; Smalley, R. E. Reu. Sci. Insbum. 1985, 56, 2123. (6) Proceedings of the Second International Meeting on Small Particles and Inorganic Clusters (Lausanne, 1980). Invited and contributed papers are published in Surf. Sci. 1981, 106. (7) Proceedings of the Third International Meeting on Small Particles and Inorganic Clusters (Berlin, 1984). Invited and contributed papers are published in Surf. Sci. 1985, 156, Parts I and 11. ( 8 ) Ber. Bunsenges. Phys. Chem. 1984, 88. A special volume for cluster studies. (9) Delacretaz, G.; Grant, E. R.; Whetten, R. L.; Wiiste, L.; Zwanziger, J. W. Phys. Rev. Left. 1986, 56, 2598. (10) Frohlich, H. Physica (Amsterdam) 1937, 4, 406. (11) Kubo, R. J . Phys. SOC.Jpn. 1962, 17, 975. (12) Perenboom, J. A. A. J.; Wyder, P.; Meier, F. Phys. Rep. 1981, 78, 173.
0022-3654/87/2091-1307$01.50/0
intracluster processes has been derived by laser and mass spectroscopie~.’~J~ In particular, electron attachment and detachment involving clusters have attracted much attention for the following reasons: (1) In many cases, clusters are characterized and detected by way of ionization, so that spectroscopic methods such as mass spectroscopy are applicable, and (2) the processes of relaxation induced by the electron attachment and detachment are by themselves challenging problems of intracluster dynamics. One of the crucial and yet unsolved problems in the application of mass spectroscopy to studies of van der Waals clusters is to avoid dissociation of clusters when they are ionized for detection. Since the clusters are bound by relatively weak intermolecular forces, van der Waals bonds are likely to be broken by even a small perturbation associated with ionization, and a part of the basic information on the properties of parent neutral clusters is inevitably lost. Therefore, it is necessary to devise a method for ionization which can best prevent such dissociation. Besides threshold photoionization,20 a practicable technique that is closest to this goal seems to be to detect negative cluster ions formed by attachment of “zero-energy” electrons to “cold” neutral clusters. The cluster ions thus formed are, in general, expected to be “vibrationally hot”, and the process of relaxation follows. In analogy with acid-base reactions, introduction of an electron to a cluster gives rise to an increment in the cluster basicity and, as a result, certain intracluster reactions may be favored. These intracluster dynamics in negative cluster ions are contrasted to the dynamics of solvated electrons in the condensed phase. Our study of the collisional electron transfer from high-Rydberg rare gas atoms to van der Waals clusters has been initiated with this prospect. Rydberg atoms having very high principal quantum numbers have been used as a source of electrons with near-zero (13) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H.J . Chem. Phys. 1983, 79, 1581. (14) Carrasquillo, E.; Zwier, T. S.; Levy, D. H. J . Chem. Phys. 1985, 83, 4990 (15) Ondrechen, M. J.; Berkovitch-Yellin, Z.; Jortner, J. J . Am. Chem. SOC.1981, 103, 6586. (16) Schauer, M.; Law, K. S.; Bernstein, E. R. J . Chem. Phys. 1985,82, 736. (17) Wanna, J.; Bernstein, E. R. J . Chem. Phys. 1986, 84, 927. (18) Miller, R. E. J . Phys. Chem. 1986, 90, 3301. (19) Alexander, M. L.; Johnson, M. A,; Levinger, N. E.; Lineberger, W. C. Phys. Reu. Lert. 1986, 57, 976. (20) Rohlfing, E. A,; Cox, D. M.; Kaldor, A,; Johnson, K. H. J . Chem. Phys. 1984, 81, 3846.
0 1987 American Chemical Society
Kondow
1308 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
-
kinetic energy, 10 meV. The cross section for electron transfer from such a Rydberg atom to a target molecule having a positive electron affinity, such as SF,, is known to be as large as cm2. Cross sections of similar orders of magnitude have indeed been observed when clusters having positive vertical electron affinities are used as targets, and many cluster ions have thus been produced efficiently and gently. For example, (H20),- (n 2 13) and (CH,CN),- (n I13), which can scarcely be generated by conventional techniques, have been observed. In the present article, the author has restricted himself to topics of collisional electron transfer to van der Waals clusters from high-Rydberg rare gas atoms and related topics on the formation of negative cluster ions on the basis of negative-ion mass spectrometry. In order to aid the readers in surveying various important areas of cluster chemistry, several review articles are cited in the list of r e f e r e n c e ~ . ~ l - ~ ~
2. High-Rydberg Atoms Rare gas atoms can be excited by electron impact to highRydberg states with principal quantum numbers, np, of 20-40 in a laboratory system.30 In such a high-Rydberg atom, Rg**, its outermost electron (Rydberg electron) is very weakly bound to its core ion, Rg+, by Coulomb interaction because the electron moves slowly around Rg’ in a remote Rydberg orbit, whose radius ranges 102-103nm. Accordingly, the ionization potential and the kinetic energy of the Rydberg electron can be made less than 10 meV. In the collisional processes involving Rg** with a target molecule, M, the Rydberg electron is readily liberated as a result of rotational or vibrational deexcitation of M.,O In particular, when M has a positive electron affinity, the Rydberg electron of Rg** is attached gently to M with high efficiency and a transient negative ion state, M-*, is formed. In such a collision, an effective distance for interaction between M and the Rydberg electron is much smaller than the radius of the Rydberg orbit, and consequently, the collision occurs almost exclusively with the Rydberg electron and the core ion, Rg’, simply behaves like a spectator. Therefore, the Rydberg electron can be treated as a free electron which has momentum and energy distributions identical with the Rydberg electron which is bound to Rg+ (essentially free electron m ~ d e l ) . ~ ~ ~ ~ ~ The essentially free electron model is indeed verified by several typical systems, such as Rg** + SF, or CC14.31-33 In the case of SF, target Rg** SF6 Rg+ + SF6(1)
-
+
the cross section, u(u), for electron attachment at very low energy, 5 10 meV, or np higher than -30, is inversely proportional to the electron velocity, u, in units of cm/s: u(u) = (4.2 f 1.0) X lO-’/u cm2. No dissociation occurs after the electron attachment, because the excess energy is dissipated into its internal degrees of freedom. In the Rg** CCl, system, the negative-ion state, CC14-(2Al), produced by the electron transfer dissociates into C1- and CCl,: Rg** CC14 CC14-(*A1)+ Rg+ (2)
+
+
--
CC14-(2Al)
C1-
+ CCl,
lE
Figure 1. Potential energy curves, vk and v‘, associated with an electron in the kth free electron state and in the ith affinity state, as a function of the interaction coordinate, Qi. The k i transition can only proceed at the crossover points between vk and V .
-
In this case, the excess energy generated by the electron attachment is dissipated through the dissociation potential leading to C1- CC13. In summary, a Rydberg electron with kinetic energy less than 10 meV is captured by M if (1) M has a positive electron affinity and ( 2 ) relaxation of the nuclear motion occurs very readily.
+
3. Attachment of Rydberg Electron to a van der Waals Cluster 3.1. Theoretical Consideration. In the capture of a Rydberg electron by a molecular cluster, (M)m,the kinetic energy of the electron must be transmitted to the nuclear motions of the cluster, since the kinetic energy (510 meV) is too small to excite the component molecules, M, into an electronic excited state. In addition, the cluster cannot be deformed so rapidly as to capture the electron in the cluster. Then, its vertical electron affinity should be positive for the efficient capture of the electron; otherwise, the electron is reemitted before the cluster is deformed to stabilize the affinity states. As the cluster size increases, the energies of the extended affinity states, Le., the states with an electron not localized on any of the single molecules, are expected to be lowered significantly. Namely, the vertical electron affinity of the cluster increases with its size. In the first step of the electron transfer or the capture of the Rydberg electron by the cluster, the Rydberg electron is accommodated in an extended state of the cluster. Such a cluster state, represented by (M),-*, is an excited state, where (M),-* retains the geometry of (M),. The cluster system tends to relax toward its ground state by localization of the captured electron and associated nuclear rearrangement. The stabilization energy associated with this relaxation is transmitted to the internal degrees of freedom of the cluster, Le., the cluster is “heated”. In some cases, evaporation of the component molecules and chemical reactions may occur, by which the excess energy may be released out of the cluster. Tsukada and K ~ n d o w have , ~ described the above-mentioned processes in terms of the Hamiltonian H = xc(k)akfak k
I
(3)
(21) Bartell, L. S. Chem. Reu. 1986, 86, 491. (22) Celii, F. G.; Janda, K. C. Chem. Rev. 1986, 86, 507. (23) Beuhler, R.; Friedman, L. Chem. Rev. 1986, 86, 521. (24) Kouteckf, J.; Fantucci, P. Chem. Rev. 1986, 86, 539. (25) Castleman, Jr., A. W.; Keesee, R. G. Chem. Rev. 1986, 86, 589. (26) Phillips, J. C. Chem. Reu. 1986, 86, 619. (27) Legon, A. C.; Miller, D. J. Chem. Reu. 1986, 86, 635. (28) Mlrk, T. D.; Castleman, Jr., A. W. Adu. A t . Mol. Phys. 1985, 20, 65. (29) Levy, D. H. Annu. Reu. Phys. Chem. 1980, 31, 197. (30) Matsuzawa, M. In Rydberg States of Atoms and Molecules; Stebbings, R. F., Dunning, F. B., Eds.; Cambridge University Press: Cambridge, :r, C. W.; Gray, L. G.; Smith,
K. A.; Dunning, F. B.; Stebbings, R. F. Phys. Rev. A 1985, 32, 3330. (32) Foltz, G. W.; Latimer, C. J.; Hildebrandt, G. F.; Kellert, F. G.; Smith,
K. A,; West, W. P.; Dunning, F. B.; Stebbings, R. F. J. Chem. Phys. 1977, 67, 1352. (33) Dimicoli, I.; Botter, R. J . Chem. Phys. 1981, 74, 2346.
+ x{ti+ x:lli(bx++ bx)}ni+ai+ x(v(k)aj+ak + hc) + ChUxb~+bx(4) ik
Here, ak and ai represent the electron annihilation operators for the kth free electron state and the ith affinity state with the energies e(k) and ei, respectively. The third term represents the coupling between these two states. Intra- and intermolecular vibrations are assumed to be harmonic with their boson operators, bh.and bA+,and the energy, h w A . Since the affinity energies are greatly affected by these vibrational modes, this effect is taken into account as the linear coupling term in { }. For the sake of simplicity, the intramolecular modes are not distinguished from the intermolecular modes, nor the extended affinity states from the localized ones. In the present single-electron problem, the Hamiltonian is rewritten as k
i
ik
(34) Tsukada, M.; Kondow, T., unpublished results.
The Journal of Physical Chemistry, Vol. 91, No. 6,1987 1309
Feature Article where
m
II
I
m
Figure 1 shows the potential energies, vk and V , of the boson Hamiltonians, H I kand H i , respectively, as a function of an interaction coordinate, Q, defined by
An electron in the kth free electron state is attached to the cluster ( k i transition) only at the crossover point, Q,’(k) (see Figure l ) , and no longer detach from the cluster when the system on V‘ passes through the critical point, Q,‘(O). The system dissipates its energy to other vibrational modes so rapidly that the system is not likely to return to the electron reemission region, IQiI < lQJ(O)l, after this p i n t , Q,‘(O), is passed. On the basis of Sumi’s treatment?’ the attachment cross section, u, is roughly calculated to be -+
lT%
(9) on the assumption that the stabilization energy, AEl C,&L12/hwx,is larger than the width of the affinity levels, Here, r Lis defined as
C(7L)’ coth ( h w x / k T ) x
E
rl.
- ri
the electron-trapping site. The cross section increases sharply with decreasing kinetic energy. For the Rydberg electron the cross section has a maximum value for ti E E E 0, since t for the Rydberg electron is practically zero. Furthermore, the cross section is larger if the electron-trapping site has a high vibrational frequency and a large stabilization energy. In section 5.2, the prediction given by this theoretical cross section is described in comparison with the results of the C 0 2 clusters. 3.2. Ionization Processes. As mentioned in sections 4 and 5, negative cluster ions are indeed produced from (M), by Rg** impact. The argument in section 3.1 and the electron-transfer collisions between electron-attracting molecules and Rg* * indicate that these negative cluster ions are produced via the following processes:
(M),
(10)
In addition, P k represents the density of states for the free electron having a momentum hk, and 7: is the time required to leave from the electron reemission region. The cross section given by eq 9 is further simplified as
r
Figure 2. Schematic diagram of the apparatus. It is composed of four chambers, I-IV, pumped separately: G, inlet of source gas; N, sonic nozzle; S, skimmer; C, collimator; I, ion source; Q,quadrupole mass spectrometer; D, ion detector equipped with a charge conversion dynode; P, oil diffusion pumps.
(14)
where N, and N are the number of molecules on the cluster surface and the cluster size, respectively, 2, and 2 represent the coordination numbers for the surface and the interior molecules, respectively, and W shows the width of the affinity band. The following approximations are used for deriving eq 11
where wo is the order of the vibrational frequency and A€ is the difference in energy between Q;(O) and the bottom of V (see Figure 1). As shown in eq 9 and 11, theoretical equations of the attachment crass section can predict qualitatively the dependences of the cross section of (1) the kinetic enprgy of the incoming electron, (2) the energies of the affinity levels related to the vertical electron affinity, and (3) the stabilization energy and the vibrational frequency of
+ Rg**
(M),-*
-
-
-
(M);
(M),-*
+ Rg+
+ ( m - n)M
intracluster reactions
(18) (19) (20)
As discussed in section 3.1, the vertical electron affinity of (M), should be positive, so that process 18 proceeds with sufficiently high efficiency. When (M),-* is stabilized, the excess energy generated is transmitted to the internal degrees of freedom of the cluster, and the effective temperature of the cluster increases. If the temperature does not exceed the sublimation temperature of the cluster, no substantial evaporation occurs (nonevaporative). Otherwise, a number of the component molecules are evaporated (evaporative) or the excess energy is released by the rupture of the chemical bonds of the component molecules (dissociative).
4. Experimental Aspects 4.1. General Design. In the studies of electron attachment processes to neutral clusters, negative-ion mass spectrometry is used universally. One of experimental issues is how efficiently electrons can be attached to the neutral clusters with a less extent of dissociation. Very slow electrons having a narrow energy spread are necessary to achieve this goal, since the cross sections for the attachment increase greatly with the decrease in the incident electron energy and the excess energy due to the electron attachment can be minimized by reduction of the kinetic energy of the incident electrons. The basic principle of the present ionization method is to use Rydberg electrons which are transferred to the weakly bound clusters. An ion source for formation of negative cluster ions has been designed, a cluster beam produced by supersonic free-jet expansion is allowed to collide with high-Rydberg rare gas atoms, Rg**. The van der Waals clusters, ( M ) m ,produced in the free jet are ionized by Rg** in the central collision region of the ion source36 via processes 18-20. A mass spectrometer is employed to detect the negative cluster ions formed in these processes. A schematic diagram of the apparatus is shown in Figure 2. The apparatus consists of (1) a cluster beam source, (2) a triple-grid ion source, (3) a quadrupole mass spectrometer, and (4) (36) Mitsuke, K.; Kondow, T.; Kuchiutsu, K. J . Phys. Chem. 1986, 90,
(35) Sumi, H. J . Phys. SOC.Jpn. 1980, 49, 1701.
1552.
1310 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Kondow
Figure 3. Schematic diagram of the ion source. Three concentric grids, G,, Gg, and Gc, are installed for preventing charged species from penetrating into the central collision region. F and H denote filaments and a housing, respectively.
( cc14 )"Ci-
n n n , ( CC14)n-
I
I
! 1
2
3
500 MASS
i l lI
I
I
I
l
i
4
5
6
7
8
1000
NUMBER
0.1 0.2 0.3 Kr -Pressure I Torr
Figure 5. Dependence of the intensities of (CCI4),CI- (n = I , 4 and 8) on the pressure of the Kr gas used as a source of Kr** atoms. Typical error bars are shown. The intensities for different ions are normalized at the Kr pressure of 0.2 Torr.
impact
Kr"
0
J
9
1500
( m i z )
Figure 4. Mass spectrum of negative cluster ions produced by impact of Kr** atoms on (CC14)m.
a CAMAC system based on an LSI-l1/23 microcomputer. 4.2. Formation of Neutral Clusters. The beam source which produces van der Waals clusters consists of a sonic nozzle having an aperture of 50-pm diameter and a channel length of 0.2 mm. A sample gas is seeded in He, Ar, or HI gas at room temperature with a stagnation pressure of 100-3000 Torr, and the mixed gas is expanded through the nozzle into a beam expansion chamber (chamber I). This chamber is maintained at a pressure of lo-, Torr by use of a 6-in. oil diffusion pump. The central portion of the expansion, which contains larger clusters, is sampled by a conical skimmer (Beam Dynamics) with an entrance hole of 0.31-mm diameter and is injected into a collimation chamber (chamber 11). The pressure of this chamber, evacuated by a 4-in. Torr. The cluster beam oil diffusion pump, does not exceed passing through the skimmer is further collimated by an aperture of 5-mm diameter into an ionization chamber (chamber 111), where a concentric triple-grid ion source is installed. 4.3. Ionization of Clusters. The clusters are ionized in collision with Rg** in the triple-grid ion source (chamber 111). The reaction chamber is evacuated by a 6-in. oil diffusion pump with a liquid nitrogen trap. The pressure of the chamber, which is originally Torr when the rare about lo6 Torr, is increased to about gas atoms are admitted to the ion source. The ion source has a housing of 20-mm length and 60-mm diameter, on which three concentric cylindrical grids and filaments are mounted (see Figure 3). The grids, GA, GB, and Gc, are made of stainless steel mesh with a transparency of 80%. The collision region surrounded by the inner grid, GA,has a length of 20 mm and a diameter of 10 mm. The four pieces of helical filaments made of thoriated tungsten wire of 0.15-mm diameter form a rectangular square. The cluster beam passing through the collision region is ionized by impact of Rg** atoms or electrons. Argon or krypton gas with more than 99.95% purity is introduced to the ion source through a stainless steel pipe of 4-mm diameter. The rare gas atoms are excited in the exterior of G B by 50-eV electrons. Ionic species and electrons are retarded by application of appropriate potentials to the three grids, and only neutral species including Rg** are allowed to enter the central
10
20
30
40
50
60
70
Impact energy lev
Figure 6. Dependence of the intensities of (CCl,),CI- (n = 1, 4, and 8) on the energy of the electrons used for excitation of Kr atoms to highRydberg states. The intensities for different ions are normalized at the electron impact energy of 25 eV.
collision region. Typical potentials applied are -20, -1 50, -50, and -100 V for GA, GB, Gc, and the filaments, respectively. The pushing pressure of the rare gas introduced to the ion source is typically 0.05-0.2 Torr. The following test experiments have shown that single-collision conditions are found to be fulfilled in this pressure region and that the observed negative ions originate from transfer of the Rydberg electrons to the neutral clusters in collision with Rg**: (1) The (CCl,),CI- ions produced from CCI, clusters were measured, as shown in Figure 4; their intensities were proportional to the pushing pressure of the Kr gas (see Figure 5). (2) The impact energy of electrons, given by the potential difference between the center pole of the filaments and grid Gc, was varied at a fixed pressure of the Kr gas in this pressure range (see Figure 6). The ion intensities started in the vicinity of the ionization potential of Kr, increased with increasing impact energy, and decreased gradually at energies higher than 70 eV. (3) The cluster ions with different sizes, n, showed nearly identical trends. (4) The signals decreased when the potential applied to GBwas increased; this observation was interpreted as the field ionization of Rg**. The principal quantum numbers, np, of the Rg**atoms are estimated to be in the range of 25-35, since Rg** atoms having np 2 35 are ionized by the field of 330 V/cm between G, and GB, while those having np I25 decay radiatively before reaching the collision region.37 In order to confirm this estimate, the relative cross sections for the production of (CC14),C1- by Kr** impact were estimated by field ionization (see above). Figure 7 shows the relative cross sections for the cluster ions with different ( 3 7 ) Gallagher, T.F. In Rydberg States of Atoms and Molecules; Stebbings, R. F., Dunning, F. B., Eds.; Cambridge University Press: Cambridge, U.K., 1983; p 165.
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Feature Article
1311
TABLE I: Negative Cluster Ions Observed by Mass Spectroscopy' M
type E
co2 ocs cs2
ion(RA1) (C02)n(0CS)n(CS2)n-
E
H20
(H2O)n-
D20
no data
nL
3 2 1 -13
CD3CN CH3CN + H 2 0 CSHJN CSHSN + H2O
N
E
(CD3CN)L (CH3CN),H,O(C5HSN)n(Cd%Wn(H2O)d-
SF6
N
(SF6)n-
N2O
E R
11 -7 4 n+n'=4 1
(N2O)n(N2O)no-
6 1
E N
N 2 0 + H2O CCI4
R R
(HzO)(N20),0(CC14)n(cc14)nc1-
3 1 0
ion(E1) (C02)nno data (CS2)n(CS2)S (H2O)n(H2O)nOH(D2O)n(D20)"ODundetectable undetectable (CSH" (CSHsN)n(H2O)d-
(SF,),SFj(N2O)n(N2O)nO(N,O),NO(H2O)(N2O)"O(cc14),cI-
nl 2, I* 1
0 1le 0' 12c 0'
-3 n+n'=4 1 Id
0,
3, 1, 0
Od Od
Od
'RAI and E1 stand for Rydberg atom and electron impact, respectively. N = nonevaporative electron attachment, E = evaporative electron attachment, and R = reactive electron attachment. bStamatovic,A.; Leiter, K.; Ritter, W.; Stephen, K.; Mark, T. D. J . Chem. Phys. 1985,83, 2942. 'Knapp, M.; Echt, 0.; Kreisle, D.; Recknagel, E. J . Chem. Phys. 1986, 85, 636. dKnapp, M.; Echt, 0.;Kreisle, D.; Mark, T.D.; Recknagel, E. Chem. Phys. Lett. 1986, 126, 225. I
1
(CCIL,)~
-? I -
Kr** ( n p )
+
(CC14)nCI-
* Kr'
o n.1
n.4 n.8
A
1
,
,
,
,
,
,
/
,
I
31 35 Principal quanum number 27
Figure 7. Dependence of the relative cross sections for formation of (CC14),CI- (n = 1 , 4 , and 8) from the neutral clusters of CC14 by impact of Kr** on the principal quantum number of Kr** atoms estimated by field ionization. The relative cross sections for different n are normalized at np = 31.
sizes, n, plotted against the principal quantum number, np,of Kr**. Since the observed trends are almost identical, the high-Rydberg atoms with different np are expected to result in essentially the same size distribution. These npdependences of the cross sections for the cluster ions agree with that for the monomer given by state-selected Rg* * .32-33 Electron impact ionization of the clusters can also be studied in the same ion source, where the potentials applied to the grids and the filaments are adjusted so as to allow free electrons to enter the collision region. The average electron energy, e, is obtained from the potential difference between grid GAand the center pole of the filaments. The energy spread (fwhm) of the electrons, which mainly arises from the potential drop across each filament, is estimated to be about 2 eV at F = 4 eV by comparison of the measured cross section curve for the 0- production from C 0 2 with that reported.38 4.4. Detection of Negative Cluster Ions. The negative ions produced in the ion source are mass-analyzed by a quadrupole mass spectrometer (Extranuclear, 162-8) mounted coaxially with the cluster beam in the detection chamber (chamber IV). The maximum mass-to-charge ratio of the mass-selected ions is m / z (38) Chantry, P. J. J . Chem. Phys. 1972, 57, 3180.
1650, and a typical mass resolution is about 300 at m / z 1460. The ions after the mass spectrometer are focused by a lens and converted to positive ions by allowing the incoming ions to collide with an ion conversion dynode at a voltage of about 5 kV. The positive ions ejected from the dynode are detected by a Ceratron (Murata, EMS-108 1B). The use of this ion conversion dynode, which is made of a stainless steel disk of 22-mm diameter, has improved the signal-to-noise ratio by more than 3 orders of magnitude, probably because the essential part of the stray electrons is eliminated. The transmission and detection efficiencies of the mass spectrometer and the detector are calibrated by use of the known fragmentation patterns of the positive and negative ions produced by electron impact on perfluorokerosene (PFK).39 The massto-charge ratios are calibrated by the fragment ions of PFK and the cluster ions of C02.39*40The detection chamber is evacuated by a 4-in. oil diffusion pump with a liquid nitrogen trap; the pressure is about 5 X lo-' Torr under the operating conditions. 4.5. Data Acquisition. Measurement and data acquisition systems are based on a CAMAC-crate-mounted LSI-l1/23 microcomputer. The signals from the detector are amplified by a preamplifier (ORTEC 9301) and registered in a multichannel analyzer (Canberra 3 100). The mass spectrometer is operated by a mass programmer, which selects ions having a desired mass-to-charge ratio. The measurement system containing the multichannel analyzer, the quadrupole power supply, and the mass programmer is controlled by a computer via an automatic control interface for the CAMAC online system (ACICOS). The fluctuation in the signal intensity is less than 10% for a period of several hours.
5. Mass Spectra of Negative Cluster Ions Produced by Rydberg Atom Impact The negative-ion mass spectrometry by use of Rg** impact has been examined for various van der Waals clusters whose monomer molecules have positive and negative electron affinities. The results are summarized in Table I. In all the clusters studied, except for those of C C 4 and N20,the product ions are (M)n-, as shown in Table I. The mass spectra have the following common features: (1) The spectra obtained by use of different rare gases are almost identical, (2) the spectra have a threshold size, nL, and a broad distribution having a maximum at about nL + 10, (3) the intensities of (C02),- ions with 11 In I13 are much weaker than (39) Gohlke, R. S.; Thompson, L. H. Anal. Chem. 1968, 40, 1004. (40) Kondow,T.; Mitsuke, K. J . Chem. Phys. 1985, 83, 2612.
Kondow
1312 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
1
(CD3CN),
15
11
20
25
,
30
!
'
500
1
500
1000'
M A S S NUMBER
Figure 8. Mass spectrum of (CD,CN); produced by impact of Kr** atoms on the neutral clusters of CD3CN.
those of the neighboring ions, and (4) several outstanding peaks are discernible at nM (magic number). On the other hand, the clusters of N 2 0 and CC14 are dissociatively ionized and several complex ions, such as (N,O),O-, (N,O),NO-, and (CCl4),C1-, are detected. These experimental features can be explained in terms of the ionization processes: nonevaporative, evaporative, and dissociative/intracluster reaction. The ionization processes depend crucially on (1) the vertical electron affinity, EA,, and (2) the stadefined in section 3.1. In sections 5.1-5.4, bilization energy, Mi, several typical mass spectroscopic studies made in our laboratory are described. 5.1. CD3CN Clusters. Nonevaporative Electron Attachment. Ionization of CD3CN clusters,41(CD,CN),, in collision with fi** is described in the present section. All the negative ions observed are (CD,CN),- ( n L 1l ) , as shown in Figure 8. The threshold size, nL = 11, is independent of the stagnation pressure, Po, in the pressure range between 1000 and 2600 Torr. The size distribution for (CD,CN)[ varies with the stagnation pressure, probably because the size distribution of the neutral clusters, (CD,CN),, changes with the stagnation pressure. When 2.5-eV electrons are used instead of Kr** for the ionization of (CD,CN),, no ion signal is detected. This finding indicates that the cross section for formation of a negative cluster ion depends critically on the kinetic energy of the electron to be captured by the neutral cluster; it is clear that the smallness of the kinetic energy of the Rydberg electron of Kr** enhances the cross section. As described in the foregoing, the cluster ions, (CD3CN);, are formed via the following p r o c e s s e ~ ~ ~ * ~ ~ ~ (CD,CN), (CD,CN),-*
+ Kr**
-
-
(CD,CN),-*
(CD,CN)[
1000 (mlz)
MASS NUMBER
(miz)
+ Kr'
( m = n 2 11)
(21) (22)
where no significant evaporation from (CD3CN),-* is expected for the following reasons. The attached electron is trapped in the cluster probably as a dimeric anion, which is reported by an ESR study to be a stable entity in a negatively charged crystal of CH3CN.43 In this anion the two molecules are likely to be oriented antiparallel with each other. The CD3CN molecules surrounding the trapped electron are polarized and reoriented. The excess energy, which is nearly equal to the stabilization energy, AE,,is transmitted to at least 6m - 6 intermolecular vibrational modes, and consequently the effective temperature of the cluster is increased. Under the assumption that this excess energy is distributed statistically among the 6m - 6 vibrational modes, the increment in the effective vibrational temperature is estimated to be4I about 240 K for m 1 1 1. Since the effective vibrational temperature of the original neutral cluster is very low, the effective temperature of the cluster ion is much lower than the boiling temperature of liquid CD3CN (355 K); the effective boiling temperature of the cluster ion a t which substantial evaporation from (CD3CN),-* is expected is estimated to be even higher. (41) Mituske, K.; Kondow, T.: Kuchitsu, K. J . Phys. Chem. 1985, 90, 1505.
(42) Kondow, T. In Electronic and Atomic Collisions-Invited Papers; Lorents, D. C., Meyerhof, W. E., Peterson, J. R., Eds.: Elsevier: Amsterdam, 1986; p 517. (43) Williams, F.; Sprague, E. D. Acc. Chem. Res. 1982, 15. 408.
Figure 9. Mass spectrum of (CO2)[ produced from (CO,), in collision with Kr**. The stagnation pressure of the gaseous mixture of C 0 2 + He used was 2000 Torr.
1 I
tvac
I
-5 Figure 10. Location of the affinity levels of (C02),* (n = 4 and 13). The energy position of the vacuum level (Evac)changes slightly by use of different basis functions.
Therefore, the cluster size is likely to remain essentially unchanged in the process of relaxation. The mass spectrum of protonated cluster ions of CD,CN41 indicates that the neutral clusters, (CD3CN),, are generated in the range of small m where no negative cluster ions are observed. Therefore, the sharp rise in the mass spectrum of the negative cluster ions can be ascribed to a sudden increase in the cross section for electron attachment. The presence of this clear threshold can be explained by the assumption that the vertical electron affinity, EA,, of (CD3CN),, which is known to be negative for the monomer ( m = l ) , turns out to be positive at m = l l . In this estimation the kinetic energy of the Rydberg electron can be disregarded. This explanation is confirmed from the theoretical cross section for the electron attachment to van der Waals clusters (see eq 9 and 11) with t E t i and e = 0 eV, where t is the kinetic energy of the incoming electron and ti corresponds to EA, of (CD3CN)11. 5.2. CO, Clusters. Evaporative Electron Attachment. Figure 9 shows the spectrum of (CO,); produced from (CO,), in collision with Kr**. The signal-to-noise ratio is greatly improved so that weak peaks are better resolved than that reported p r e v i o ~ s l y . ~ ~ The features of the spectrum are as follows: (1) the threshold size is 3, (2) there is a region where the peak intensities are weak (11 In I13), and (3) n = 14 and 16 are magic numbers. In the electron impact ionization, the peaks with n = 14 and 16 are enhanced with increasing impact energy.* This behavior indicates that n = 14 and 16 are the magic numbers for (CO,),- ( n = 14 and 16). It seems that a dimeric cluster ion is solvated by 12 CO, molecules, which make an icosahedron structure. The COz monomer cannot capture the Rydberg electron because the vertical electron affinity of C 0 2 is -3.8 eV. However, (M),- ( m 2 mL) can capture the Rydberg electron with a large cross section because (1) the vertical electron affinity of (CO,), increases with m and becomes positive beyond mL, (2) the state density of (CO,),-* which takes part in the electron attachment increases with m, and (3) the lifetime for autodetachment from (CO,),-* also increases with m. The attached electron is trapped in one of the C 0 2 molecules (possibly with a bent s t r ~ c t u r e ~ ~ . ~ ~ ) (44) Knapp, M.; Kreisle, D.; Echt, 0.: Sattler, K.; Recknagel, E. Surf: Sci. 1985, 56, 313. (45) Compton, R. N.; Reinhardt, P. W.; Cooper, C. D. J . Chem. Phys. 1975, 63, 3821.
Feature Article
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
(A) 4
1313
1
Figure 11. Structures of the C02 clusters estimated from the crystal structure of C02: (A) (CO,),, (B) (C02)13.a-b distance is 7.47 au and c-d distance is 10.6 au.
in the cluster. Since the resulting excess energy is estimated to be 3.5 eV at maximum, evaporation of C 0 2 molecules should take place; the effective vibrational temperature of (CO,),-* is estimated to be about 900 K, at which temperature appreciable evaporation is expected. From observed feature 2, the number of molecules to be evaporated, m - n, is estimated to be at least 4, and hence, mL is estimated to be at least 7 . Since the peak intensities for 3 In I 6 are very weak, these may be ascribed to cluster ions produced by evaporation of more than four C 0 2 molecules. Figure 10 shows the affinity levels of (CO,),-* ( m = 4 and 13) calculated by the DV-Xa cluster method,,’ where the symmetric cluster structures are postulated (see Figure 11). The level positions relative to the vacuum level cannot be determined conclusively at the present stage, since they are found to depend somewhat on the choice of the basis set. These affinity levels indicate that the vertical electron affinity of (CO,), turns to be positive at a cluster size m between m = 4 and 13, since (C02),-* is unstable while (C02)13-*is stable with respect to be electron detachment from the clusters. By taking advantage of the energy level diagrams of (CO,), and (CO,),-*, the theoretical cross sections for the electron attachment are estimated as a function of electron impact energy, as shown in Figure 12.33 These cross section curves suggest a sudden increase in the cross section for electron attachment in the size range between 4 and 13. This theoretical prediction is consistent with the finding that mL is 7 . More detailed calculations on the cluster structures, the affinity levels, and the cross sections for the CO, clusters are being carried out by Tsukada. 5.3. CCl, Clusters. Dissociative Electron Attachment. Figure 4 shows a mass spectrum of the negative ions produced from a CC14 cluster by Kr** impact4* Two kinds of ions, (CCl,),Cland (CCl4)[, are detected. The spectra of (CCl,),Cl- are similar to those produced by electron impact. Other ions, (CC14),,C12and (CCl4),CC1;, are detected additionally in the electron impact ionization. The intensities of (CC14),Cl- depend on the stagnation pressure, Po, in the nozzle. As shown in Figure 13, the Po dependences for Kr** impact agree in shape with those for impact of 2.5-eV electrons: The threshold pressures at which the ion signals appear also agree with each other. In the Kr** impact ionization, the shape of the Po dependence for CC14- agrees fairly well with those for (CCl4)C1-. This indicates that (CC1,)Cl- and (46) Pacansky, J.; Wehlgren, V.;Bayus, P. S.J . Chem. Phys. 1975,62,
2740. (47) Adachi, H.; Tsukada, M.; Satoko, C. J . Phys. Soc. Jpn. 1978,45,875. (48) Kondow, T. In Highly Excited States of Atoms and Molecules; Kano,
S . S., Matsuzawa, M., Eds.; University of Electro-Communications: Tokyo, 1986; p 131.
Figure 12. Cross sections (in atomic units) for the electron attachment to (CO,), (m = 4 and 13) as a function of electron impact energy, with several plausible sets of the parameter values (Cl, C,,, (Q;(O)/Qt)I/,: a = (C02)13(25, 45, 0.2), b = (C02),3(SO, 90, 0.2), c = (C02)13(25, 45, l.O), d = (CO& (44, 81, l.O), e = (CO,), (88, 162, 1.0).
VIS-
.-c
(CCI
n5 -o L
m
-
v
a.- 0
-
VI
C
500
1000
2000
Po I T o r r
Figure 13. Intensities of (CC14),C1- produced by Kr** impact (data points) and impact of 2.5-eV electrons (solid lines) plotted as a function of the stagnation pressure, Po,in the nozzle. Each digit represents the n value. The broken line represents the Po dependence for CCl,-.
C C 4 - originate from the same precursor. These findings can be explained in terms of the following ionization processes by taking into consideration the ionization of the monomer given by processes 2 and 3: Rg**
+ (CCI,),
(CC14),-*
-
-+
(CC14),-*
(CC14),-1CI-
+
+ Rg+
+ CC13
(23) (24)
(CCl4),-1CC14 (25) Further evaporation of CCl, from (CC14)mlC1-is unlikely because (1) the excess energy due to the dissociative attachment (-0.6 eV) is partitioned to the translational and internal modes of the CC13fragment and (2) no sufficient energy is left for the excitation of the intermolecular vibrations of (CC14),C1- which gives rise to evaporation. The absence of any further evaporation by impact of 2.5-eV electrons is also confirmed by the finding that the n and Po dependences for (CC14),Cl- produced by Kr** impact accord in shape with those by electron impact. As mentioned above, CCl, has the same precursor as (CCl,)Cl-, which originates from (CC14)
