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Photoelectron spectroscopy of iron carbonyl cluster anions (Fen(CO)−m, n = 1−4)
29 April 1994
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 22 1 ( 1994) 436-440
ELSEVIER
Photoelectron spectroscopy of iron carbonyl cluster anions (Fe,(CO);, n= l-4) Atsushi Nakajima
‘, Tetsuya Taguwa, Koji Kaya *,l
RIKEN, The Institute of Physical and Chemical Research, Wake, 351-01, Saitama, Japan Received 20 December 1993; in fmal form 23 February 1994
Abstract cluster anions (Fe,( CO); ) were measured in the gas phase at the photon energy of The iron-carbonyl cluster anions were produced by slow electron attachment of less than 1 eV to a supersonic expansion of Fe(CO)5 seeded in a He carrier gas. The photoelectron spectra of the Fe,(CO),_ 1anions show unresolved broad features, and the threshold energy (Er) and the vertical detachment energy (VDE) were determined: The ET of Fe,(CO)&t gradually increases by 0.1-0.3 eV with the cluster size. The study suggests that the iron carbonyl cluster anions produced are van der Waals clusters, in which Fe(C0); works as an ion core. Photoelectron
spectra of iron-carbonyl
4.66 eV, using a magnetic bottle electron spectrometer.
1. Introduction Electron attachment processes to molecules in the gas phase have been studied over two or three decades [ 11. In particular, low-energy electrons are effectively captured by molecules, inducing unimolecular decomposition. Recently, the electron attachment experiments in clusters have been performed, because the produced cluster anions are considered to provide models for excess electrons in the liquid phase [ 2 1. The electron attachment processes to transition metal carbonyls have been also studied extensively [ 31, and dissociative attachment rates [ 41 and photoelectron spectra have been reported [ 5,6 1. The carbonyls studied include Fe (CO) 5, Ni (CO) 4, V(CO),, Cr(CO),, and W(CO)a. Since their elec’ Permanent address: Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohokuku, Yokohama’223, Japan. * Corresponding author.
tron attachment processes are dissociative, parent negative ions are not observed except in V(CO)6. Photodetachment of the negative ions of the iron carbonyls has been observed previously in ICR experiments [ 7,8]. Later, Engelking and Lineberger [ 51 measured the photoelectron spectra of Fe, (CO); (n=O-4) with an Ar ion laser. However, carbonyl cluster anions have been scarcely investigated, and then we studied properties of iron carbonyl cluster anions by using photoelectron spectroscopy (PES) of anions, which is a powerful technique to determine the electron affinities (EAs) of the corresponding neutrals [ 9, lo]. In this Letter, we report mass distribution and PES of iron carbonyl cluster anions, using a magnetic bottle time-of-flight (TOF) spectrometer [ 1l- 141.
2. Experiments Details of the experimental setup have been provided elsewhere [ 15 1. Briefly, the apparatus consists
0009-2614/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)00301-6
437
A. Nakajima et al. /Chemical Physics Letters 221(1994) 436-440
of a pulsed cluster anion source, a TOF mass spectrometer, and a magnetic bottle TOF spectrometer. Iron carbonyl cluster anions were formed by injecting electrons into a supersonic jet of a mixture of Fe( CO) 5 ( z 18 Torr at room temperature) and He carrier, where the total stagnation pressure is z 10 atm. The electrons were produced by gently focusing the second harmonics of a Nd3+ : YAG laser (QuantaRay, GCR-12) onto a Y203 disk surface located at x4 mm downstream from a pulsed valve (400 pm diameter). Molecules in the beam were translationally cooled down to at least z 30 K, which could be estimated by multi-photon ionization for other aromatic molecules [ 161. The cluster anions were coaxially extracted by applying a pulsed electric field ( z 900 eV). After a 1.5 m flight path, the cluster anions were mass selected by their TOF and mass resolution (m/Am) was around 150. The target cluster anion was selected by pulsed deflection plates, and was decelerated to the ion energy of 20-50 eV by a potential elevator located in front of the magnetic bottle TOF spectrometer. The decelerated cluster anion was allowed to be entered in a photodetachment region and the kinetic energy of photoelectrons detached by the fourth harmonics (266 nm, 4.66 eV) of the other Nd3+ : YAG laser (Quanta-Ray, GCR-3) was analyzed by their TOF. The photoelectrons were created in a strong inhomogeneous field ( = 1000 G ) , which was generated by a small cylindrical permanent magnet (Sumitomo Special Metals; NEOMAX), and they were guided by weak magnetic fields ( % 10 G) through a 160 cm drift tube. The photoelectrons were detected by a dual microchannel plate (Hamamatsu) and the signal was accumulated into a multi-channel scaler/averager (Stanford Research Systems, SR430) after amplification. Energy calibration was performed by measuring photoelectron spectra of monomer anions of noble metals, e.g. Cu, Ag, and Au, at two different wavelengths of 355 and 532 nm, where the strong line attributed to the 1S0+2S1,2 transition could be observed [ 17,lS 1. The obtained energy resolution was = 60 meV (fwhm) at 1 eV electron energy. Each spectrum represents an accumulation of lOOOO20000 shots. When a TOF mass spectrum was measured with higher resolution, we mass-analyzed anions produced from the same nozzle source with the other
TOF mass spectrometer equipped with reflectron. Anions in the beam were accelerated with a pulsed electric field were mass-analyzed by a commercial (TOF) spectrometer (Jordan) and the ions were detected by a dual micro-channel plate (Galileo). The ion signal was accumulated in a transient oscilloscope (LeCroy 9400A) coupled with a microcomputer. Mass resolution (m/Am) was typically z 600. 3. Results and discus‘sion 3.1. Mass distribution of iron carbonyl cluster anions
Fig. 1 shows the TOF mass spectrum of Fe,, (CO); (n = l-9 ), where the mass resolution is x 150. An interval between intense mass peaks is 196 u, so that the shifts of mass peaks seem to correspond to an increase of a number of Fe( CO), ( 196 u). However, ambiguity exists in the composition assignment for each mass peak, because both 56Fe and two CO are 56 u. A number of Fe atoms in the peaks can be identified from isotopic abundance of the Fe element: The isotopic abundance of 54Fe/56Fe/57Fe/ 5*Fe is 0.058/0.9172/0.022/0.0028. Therefore, the most abundant ion containing Fe atoms (n < 15 ) is one in which all of the Fe atoms are “jFe, and second in abundance is one in which one of the Fe atoms is n-m=+4 (j-k=l-0) Mass spectrum
Fe,(CO)
of
FeJCO), (j-k)
: [Fe(oO);l,[Fe(CO)&
2-9 (l-1) 3-14 (l-2) I
‘I
,
150
.
4-19
I
1
I
600
1050
1500
I
1950
Mass Number (m/z) Fig. 1. TOF mass spectrum of Fe,(CO); (n= l-9). The observed Fe.(CO); cluster anions are van der Waals clusters of (Fe(CO);)j.(Fe(CO)5)k (_j=l-2 and k=O-8), in which Fe(CO)s works as an ion core (see text). Each peak is labelled according to two notations; one is n-m, denoting the number of iron atoms (n) and that of CO molecules (m), and the other is j-k in parentheses, denoting the number of Fe(CO)r (_j) and that ofFe(CO)5 (k).
A. Nakajima et al. /Chemical Physics Letters 221(1994) 436-440
438
54Fe. The ratios of the former to the latter are 15.8, 7.9, 5.3, and 4.0 in ions containing 1, 2, 3, and 4 Fe atoms, respectively. We used the other TOF mass spectrometer having a mass resolution of = 600, in order to determine the number of iron atoms. Figs. 2a-2d show envelopes of mass peaks around 168,364,560, and 756, respectively. Each intense peak corresponds to the ion in which all of the Fe atoms are 56Fe, and each weak peak, which is 2 u lower than the intense one, corresponds to that in which one of the Fe atoms is 54Fe. The intensity ratios between two peaks are 17 (2 ), 8.6 (9 ), 5.0 ( 7 ), and 3.8 ( 7 ), respectively. Therefore, the mass peaks could be assigned as n-m = l-4,2-9, 3- 14, and 4- 19, respectively, and for larger clusters, similar mass assignments were reasonably done, as labelled in Fig. 1. As reported by many investigators [ 4,5,7,8], an electron attachment process to Fe( CO)s is dissociative and thermoneutral. The dissociative product of Fe (CO) 5 has been examined in gas-phase collision experiments; onset for formation of Fe( CO); occurs at 0 eV, and Fe( CO) 4 continues to increase with
(d)
b
A I
164 166
I
I
I
360 364
I
I
556 560
I
I
I
I
752 756
Mass Number (m/z) Fig. 2. High-resolution TOF mass spectrum of Fe.(CO);; (a) at B 168 u, (b) at ~364 u, (c) at ~560 u, and (d) at ~756 u, respectively. Each intense peak corresponds to an anion in which all of the Fe atoms are “Fe, and each weak peak, which is 2 u lower than the intense one, corresponds to one in which one of the Fe atoms is 54Fe. From an intensity ratio, a number of Fe atoms in the ion can be determined (see text).
electron energy until it reaches a maximum at z 0.5 eV [ 191. At more than mO.5 eV, a channel for producing Fe( CO)< becomes accessible. As shown in Fig. 1, only a small amount of Fe( CO), was observed, so that electron energy in this experiment was presumed to be O-O.5 eV. In addition to the dissociative product of Fe(C0); , a series of iron carbonyl cluster anions, Fe, ( CO) sn_ , , was observed, which can be expressed as [Fe(C0)4*(Fe(CO),)k](k=O-8). Carbonyl compounds in bulk are Fe, ( CO)s, Fe2(CO)9, Fe3 (CO) i2, and so on, where an iron-iron bond constitutes of the frame work in the Fez(C0)9 and Fe3 (CO) i2 compounds. All of these compounds satisfy the 18 electron rule in which all valence electrons of the metal atom and all the electrons donated by the ligands are counted [ 201. On the other hand, the observed iron carbonyl anions in the present study are FeZ( CO), and Fes( CO) 14 for two and three irons, respectively. If the observed cluster anions have the iron-iron bond in the cluster, the Fe,( CO), should not be observed because of the unsatisfaction of the 18 electron rule. Therefore, it is suggested that Fe2(CO), has no iron-iron bond. Similarly, the Fe3( CO) 14 cluster anion should not appear, if the iron-iron bond exists in the cluster. Since the observed anions have more CO molecules in the cluster than those expected by the 18 electron rule, it is presumed that they were van der Waals clusters, in which Fe(C0); works as an ion core. The observed Fe, (CO ) sn_ 1 anions, therefore, should be expressed as (Fe(CO), ).(Fe(CO),)k (k=O-8). Under thermal conditions, Fe(C0); is observed to form Fe2( CO); uniquely in a gas phase reaction with Fe (CO) 5 [ 2 I,22 1. In this experiment, a collision between Fe (CO ) 5 and Fe ( CO ): should contribute to the cluster production, but the Fez (CO), cluster was produced in only a small amount (Fig. 2 ) . The observed cluster anions were produced in an electron attachment process under relatively cold conditions at 4 mm downstream from the nozzle. Therefore, a production of the cluster anions of (Fe(CO), ).(Fe(CO),)k was mainly contributed from the electron attachment process to a (Fe( CO) 5)k+1cluster produced already, and the collision process between Fe( CO), and Fe( CO); is likely to be minor. Otherwise Fe, (CO); should become more abundant. In fact, when electrons were
439
A. Nakajima et al. /Chemical Physics Letters 221 (I 994) 436-440
attached to the beam at sz 10 mm downstream from the nozzle where a clustering process is less efficient, the mass peak of Fez (CO) g became less abundant. 3.2. Photoelectron spectra of iron carbonyl cluster anions Fig. 3 shows photoelectron spectra of Fe,(CO)F,,_i (n=l-4) detached by 266 nm (4.66 eV) photons. In the spectra, the horizontal axis corresponds to the electron binding energy, E,,, defined as E,, = fi v - E,, where Ek is the kinetic energy of the photoelectron. All the spectra exhibit unresolved
Fe3(CO)i4
broad envelopes containing no reproducible fine structure. From these spectra, we derived the threshold binding energies, ET, and vertical detachment energies, VDEs, from the onset and the maximum of each spectrum, respectively. These values are listed in Table 1. The ET corresponds to an estimate of upper limits of adiabatic EAs [ 23,241. When the anion and neutral geometries are similar, the ET almost corresponds to the adiabatic EA. When they are quite different from each other, it is impossible to observe the detached electrons in the vicinity of the adiabatic transition, where the ET only corresponds to the upper limits of the adiabatic EA. The shape of the Fe(CO)r spectrum results from the large geometry change going from DZdin the anion to CZvin the neutral [ 25 1, which causes an excitation to high quanta of vibrational modes. A similar photoelectron spectrum for the Fe (CO)s anion has been measured with an Ar ion laser (363.8 nm; 3.41 eV) [ 51, and the ET value in this work (2.34 +O.l eV) is almost the same as the reported value (2.4 f 0.3 eV) within experimental uncertainties. As shown in Fig. 3, the shapes of the spectra are similar and both ET and VDE increase monotonically with cluster size. The similarity is consistent with the result implicated from the mass distribution of Fe,(CO);; the Fe,(CO)s,_i cluster anions are van derWaalsclustersof (Fe(CO);).(Fe(CO)5)k. Since the observed photoelectron is ejected from an orbital localized mainly at the Fe (CO), core, all of the derived photoelectron spectra are similar. Namely, the ion core of Fe (CO); is solvated by Fe( CO) 5. Differences in solvation energies between anions and the corresponding neutrals are estimated from differences in the ET values: the first, second and the third solvations of Fe(CO)5 to the Fe(CO), core make Table 1 Cluster size dependence of the threshold electron energy (ET) and vertical detachment energy (VDE) of Fe.(CO)&, cluster anions (units: eV)
1.0
2.0
3.0
4.0
Electron Binding Energy I eV Fig. 3. Photoelectron spectra of Fe.( CO)&, cluster anions (n= l-4). The positions of Er and VDE (see text) are indicated by downward and upward arrows, respectively. Peaks indicated by an asterisk express photoelectron from an unassigned multiphoton process (2 or 3 photons).
Cluster size, n
ET
VDE
1 2 3 4
2.34( 10) 2.73( 10) 3.01(12) 3.08(15)
3.02( 13) 3.32( 10) 3.54(13) 3.60( 15)
Numbers in parentheses indicate uncertainties tities; 2.34( 10) represents 2.34kO.10.
in derived quan-
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A. Nakajima et al. /Chemical Physics Letters 221(1994)
the anions 0.39, 0.28 and 0.07 eV more stable than the neutrals, respectively. The increase of both ET and VDE results from an increase in the polarizability of the cluster.
Acknowledgement
We are grateful to Mr. K. Nakao for technical assistance to measure TOF mass spectra.
References [ 1 ] E. Illenberger, Chem. Rev. 92 ( 1992) 1589. [2] 0. Cheshnovsky, C.L. Pettiette and R.E. Smalley, Ion and cluster ion spectroscopy and structure, ed. J.P. Maier (Elsevier, Amsterdam, 1989) p. 373; ST. Arnold, J.G. Eaton, D. Patel-Misra, H.W. Sarkas and K.H. Bowen, Ion and cluster ion spectroscopy and structure, ed. J.P. Maier (Elsevier, Amsterdam, 1989) p. 417. [3] R.R. Squires, Chem. Rev. 87 (1987) 623. [4] P.M. George and J.L. Beauchamp, J. Chem. Phys. 76 ( 1982) 2959. [ 51P.C. Engelking and W.C. Lineberger, J. Am. Chem. Sot. 101 (1979) 5569. [ 6 ] A.E. Stevens, C.S. Feigerle and W.C. Lineberger, J. Am. Chem. Sot. 104 (1982) 5026. [ 71 J.H. Richardson, L.M. Stephenson and J.I. Brauman, J. Am. Chem. Sot. 96 ( 1974) 3073. [8] R.C. Dunbar and B.B. Hutchinson, J. Am. Chem. Sot. 96 (1974) 3816.
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[9] R.R. Corderman and W.C. Lineberger, Ann. Rev. Phys. Chem. 30 (1979) 347. [lo] P.S. Drzaic, J. Marks and J.I. Braunman, in: Gas phase ion chemistry, Vol. 3, ed. M.T. Bowers (Academic Press, New York, 1984) p. 167. [ll]P.KruitandF.H.Read,J.Phys.E16(1983)313. [ 1210. Cheshnovsky, S. Yang, C.L. Pettiette, M.J. Craycraft and R.E. Smalley, Rev. Sci. Instrum. 58 (1987) 2131. [ 131 G. Gantefdr, M. Gausa, K-H. Meiwes-Broer and H.O. Lutz, Faraday Discussions Chem. Sot. 86 (1988) 197. P. Xia, A.J. Cox, Y.A. Yang and L.A. Bloomlield, in: Physics and chemistry of finite systems: from clusters to crystals, Vol. 2, eds. P. Jena, S.N. Khanna, and B.K. Rao (Kluwer, Dordrecht, 1992) p. 1019. A. Nakajima, T. Taguwa, K. Hoshino, T. Sugioka, T. Naganuma, F. Ono, K. Watanabe, K. Nakao, Y. Konishi, R. Kishi and K. Kaya, Chem. Phys. Letters 214 ( 1993) 22. 1’6 A. Nakajima, Y. Kihara, A. Ogawa, T. Taguwa and K. Kaya, unpublished data. [ 171 C-.Y. Cha, G. Gantefdr and W. Eberhardt, Rev. Sci. Instrum. 63 (1992) 5661. [ 181 H. Hotop and W.C. Lineberger, J. Phys. Chem. Ref. Data 4 (1975) 539. [ 191 R.N. Compton and J.A.D. Stockdale, Intern. J. Mass Spectrom. Ion Phys. 22 (1976) 47. [20] D.F. Shriver, P.W. Atkins and C.H. Langford, Inorganic chemistry (Oxford Univ. Press, Oxford, 1990). [21] J. Wronka and D.P. Ridge, Intern. J. Mass Spectrom. Ion Phys. 43 (1982) 23. [22] J. Wronka and D.P. Ridge, J. Am. Chem. Sot. 106 (1984) 67. [ 231 J. Ho, K.M. Ervin and W.C. Lineberger, J. Chem. Phys. 93 (1990) 6987. [ 241 G. Gantefdr, M. Gausa, K-H. Meiwes-Broer and H.O. Lutz, 2. Physik D 9 (1988) 253. [25] J.K. Burdett, Faraday Discussions Chem. Sot. 70 ( 1974) 1599; Inorg. Chem. 14 (1975) 375.
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 22 1 ( 1994) 436-440
ELSEVIER
Photoelectron spectroscopy of iron carbonyl cluster anions (Fe,(CO);, n= l-4) Atsushi Nakajima
‘, Tetsuya Taguwa, Koji Kaya *,l
RIKEN, The Institute of Physical and Chemical Research, Wake, 351-01, Saitama, Japan Received 20 December 1993; in fmal form 23 February 1994
Abstract cluster anions (Fe,( CO); ) were measured in the gas phase at the photon energy of The iron-carbonyl cluster anions were produced by slow electron attachment of less than 1 eV to a supersonic expansion of Fe(CO)5 seeded in a He carrier gas. The photoelectron spectra of the Fe,(CO),_ 1anions show unresolved broad features, and the threshold energy (Er) and the vertical detachment energy (VDE) were determined: The ET of Fe,(CO)&t gradually increases by 0.1-0.3 eV with the cluster size. The study suggests that the iron carbonyl cluster anions produced are van der Waals clusters, in which Fe(C0); works as an ion core. Photoelectron
spectra of iron-carbonyl
4.66 eV, using a magnetic bottle electron spectrometer.
1. Introduction Electron attachment processes to molecules in the gas phase have been studied over two or three decades [ 11. In particular, low-energy electrons are effectively captured by molecules, inducing unimolecular decomposition. Recently, the electron attachment experiments in clusters have been performed, because the produced cluster anions are considered to provide models for excess electrons in the liquid phase [ 2 1. The electron attachment processes to transition metal carbonyls have been also studied extensively [ 31, and dissociative attachment rates [ 41 and photoelectron spectra have been reported [ 5,6 1. The carbonyls studied include Fe (CO) 5, Ni (CO) 4, V(CO),, Cr(CO),, and W(CO)a. Since their elec’ Permanent address: Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohokuku, Yokohama’223, Japan. * Corresponding author.
tron attachment processes are dissociative, parent negative ions are not observed except in V(CO)6. Photodetachment of the negative ions of the iron carbonyls has been observed previously in ICR experiments [ 7,8]. Later, Engelking and Lineberger [ 51 measured the photoelectron spectra of Fe, (CO); (n=O-4) with an Ar ion laser. However, carbonyl cluster anions have been scarcely investigated, and then we studied properties of iron carbonyl cluster anions by using photoelectron spectroscopy (PES) of anions, which is a powerful technique to determine the electron affinities (EAs) of the corresponding neutrals [ 9, lo]. In this Letter, we report mass distribution and PES of iron carbonyl cluster anions, using a magnetic bottle time-of-flight (TOF) spectrometer [ 1l- 141.
2. Experiments Details of the experimental setup have been provided elsewhere [ 15 1. Briefly, the apparatus consists
0009-2614/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)00301-6
437
A. Nakajima et al. /Chemical Physics Letters 221(1994) 436-440
of a pulsed cluster anion source, a TOF mass spectrometer, and a magnetic bottle TOF spectrometer. Iron carbonyl cluster anions were formed by injecting electrons into a supersonic jet of a mixture of Fe( CO) 5 ( z 18 Torr at room temperature) and He carrier, where the total stagnation pressure is z 10 atm. The electrons were produced by gently focusing the second harmonics of a Nd3+ : YAG laser (QuantaRay, GCR-12) onto a Y203 disk surface located at x4 mm downstream from a pulsed valve (400 pm diameter). Molecules in the beam were translationally cooled down to at least z 30 K, which could be estimated by multi-photon ionization for other aromatic molecules [ 161. The cluster anions were coaxially extracted by applying a pulsed electric field ( z 900 eV). After a 1.5 m flight path, the cluster anions were mass selected by their TOF and mass resolution (m/Am) was around 150. The target cluster anion was selected by pulsed deflection plates, and was decelerated to the ion energy of 20-50 eV by a potential elevator located in front of the magnetic bottle TOF spectrometer. The decelerated cluster anion was allowed to be entered in a photodetachment region and the kinetic energy of photoelectrons detached by the fourth harmonics (266 nm, 4.66 eV) of the other Nd3+ : YAG laser (Quanta-Ray, GCR-3) was analyzed by their TOF. The photoelectrons were created in a strong inhomogeneous field ( = 1000 G ) , which was generated by a small cylindrical permanent magnet (Sumitomo Special Metals; NEOMAX), and they were guided by weak magnetic fields ( % 10 G) through a 160 cm drift tube. The photoelectrons were detected by a dual microchannel plate (Hamamatsu) and the signal was accumulated into a multi-channel scaler/averager (Stanford Research Systems, SR430) after amplification. Energy calibration was performed by measuring photoelectron spectra of monomer anions of noble metals, e.g. Cu, Ag, and Au, at two different wavelengths of 355 and 532 nm, where the strong line attributed to the 1S0+2S1,2 transition could be observed [ 17,lS 1. The obtained energy resolution was = 60 meV (fwhm) at 1 eV electron energy. Each spectrum represents an accumulation of lOOOO20000 shots. When a TOF mass spectrum was measured with higher resolution, we mass-analyzed anions produced from the same nozzle source with the other
TOF mass spectrometer equipped with reflectron. Anions in the beam were accelerated with a pulsed electric field were mass-analyzed by a commercial (TOF) spectrometer (Jordan) and the ions were detected by a dual micro-channel plate (Galileo). The ion signal was accumulated in a transient oscilloscope (LeCroy 9400A) coupled with a microcomputer. Mass resolution (m/Am) was typically z 600. 3. Results and discus‘sion 3.1. Mass distribution of iron carbonyl cluster anions
Fig. 1 shows the TOF mass spectrum of Fe,, (CO); (n = l-9 ), where the mass resolution is x 150. An interval between intense mass peaks is 196 u, so that the shifts of mass peaks seem to correspond to an increase of a number of Fe( CO), ( 196 u). However, ambiguity exists in the composition assignment for each mass peak, because both 56Fe and two CO are 56 u. A number of Fe atoms in the peaks can be identified from isotopic abundance of the Fe element: The isotopic abundance of 54Fe/56Fe/57Fe/ 5*Fe is 0.058/0.9172/0.022/0.0028. Therefore, the most abundant ion containing Fe atoms (n < 15 ) is one in which all of the Fe atoms are “jFe, and second in abundance is one in which one of the Fe atoms is n-m=+4 (j-k=l-0) Mass spectrum
Fe,(CO)
of
FeJCO), (j-k)
: [Fe(oO);l,[Fe(CO)&
2-9 (l-1) 3-14 (l-2) I
‘I
,
150
.
4-19
I
1
I
600
1050
1500
I
1950
Mass Number (m/z) Fig. 1. TOF mass spectrum of Fe,(CO); (n= l-9). The observed Fe.(CO); cluster anions are van der Waals clusters of (Fe(CO);)j.(Fe(CO)5)k (_j=l-2 and k=O-8), in which Fe(CO)s works as an ion core (see text). Each peak is labelled according to two notations; one is n-m, denoting the number of iron atoms (n) and that of CO molecules (m), and the other is j-k in parentheses, denoting the number of Fe(CO)r (_j) and that ofFe(CO)5 (k).
A. Nakajima et al. /Chemical Physics Letters 221(1994) 436-440
438
54Fe. The ratios of the former to the latter are 15.8, 7.9, 5.3, and 4.0 in ions containing 1, 2, 3, and 4 Fe atoms, respectively. We used the other TOF mass spectrometer having a mass resolution of = 600, in order to determine the number of iron atoms. Figs. 2a-2d show envelopes of mass peaks around 168,364,560, and 756, respectively. Each intense peak corresponds to the ion in which all of the Fe atoms are 56Fe, and each weak peak, which is 2 u lower than the intense one, corresponds to that in which one of the Fe atoms is 54Fe. The intensity ratios between two peaks are 17 (2 ), 8.6 (9 ), 5.0 ( 7 ), and 3.8 ( 7 ), respectively. Therefore, the mass peaks could be assigned as n-m = l-4,2-9, 3- 14, and 4- 19, respectively, and for larger clusters, similar mass assignments were reasonably done, as labelled in Fig. 1. As reported by many investigators [ 4,5,7,8], an electron attachment process to Fe( CO)s is dissociative and thermoneutral. The dissociative product of Fe (CO) 5 has been examined in gas-phase collision experiments; onset for formation of Fe( CO); occurs at 0 eV, and Fe( CO) 4 continues to increase with
(d)
b
A I
164 166
I
I
I
360 364
I
I
556 560
I
I
I
I
752 756
Mass Number (m/z) Fig. 2. High-resolution TOF mass spectrum of Fe.(CO);; (a) at B 168 u, (b) at ~364 u, (c) at ~560 u, and (d) at ~756 u, respectively. Each intense peak corresponds to an anion in which all of the Fe atoms are “Fe, and each weak peak, which is 2 u lower than the intense one, corresponds to one in which one of the Fe atoms is 54Fe. From an intensity ratio, a number of Fe atoms in the ion can be determined (see text).
electron energy until it reaches a maximum at z 0.5 eV [ 191. At more than mO.5 eV, a channel for producing Fe( CO)< becomes accessible. As shown in Fig. 1, only a small amount of Fe( CO), was observed, so that electron energy in this experiment was presumed to be O-O.5 eV. In addition to the dissociative product of Fe(C0); , a series of iron carbonyl cluster anions, Fe, ( CO) sn_ , , was observed, which can be expressed as [Fe(C0)4*(Fe(CO),)k](k=O-8). Carbonyl compounds in bulk are Fe, ( CO)s, Fe2(CO)9, Fe3 (CO) i2, and so on, where an iron-iron bond constitutes of the frame work in the Fez(C0)9 and Fe3 (CO) i2 compounds. All of these compounds satisfy the 18 electron rule in which all valence electrons of the metal atom and all the electrons donated by the ligands are counted [ 201. On the other hand, the observed iron carbonyl anions in the present study are FeZ( CO), and Fes( CO) 14 for two and three irons, respectively. If the observed cluster anions have the iron-iron bond in the cluster, the Fe,( CO), should not be observed because of the unsatisfaction of the 18 electron rule. Therefore, it is suggested that Fe2(CO), has no iron-iron bond. Similarly, the Fe3( CO) 14 cluster anion should not appear, if the iron-iron bond exists in the cluster. Since the observed anions have more CO molecules in the cluster than those expected by the 18 electron rule, it is presumed that they were van der Waals clusters, in which Fe(C0); works as an ion core. The observed Fe, (CO ) sn_ 1 anions, therefore, should be expressed as (Fe(CO), ).(Fe(CO),)k (k=O-8). Under thermal conditions, Fe(C0); is observed to form Fe2( CO); uniquely in a gas phase reaction with Fe (CO) 5 [ 2 I,22 1. In this experiment, a collision between Fe (CO ) 5 and Fe ( CO ): should contribute to the cluster production, but the Fez (CO), cluster was produced in only a small amount (Fig. 2 ) . The observed cluster anions were produced in an electron attachment process under relatively cold conditions at 4 mm downstream from the nozzle. Therefore, a production of the cluster anions of (Fe(CO), ).(Fe(CO),)k was mainly contributed from the electron attachment process to a (Fe( CO) 5)k+1cluster produced already, and the collision process between Fe( CO), and Fe( CO); is likely to be minor. Otherwise Fe, (CO); should become more abundant. In fact, when electrons were
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A. Nakajima et al. /Chemical Physics Letters 221 (I 994) 436-440
attached to the beam at sz 10 mm downstream from the nozzle where a clustering process is less efficient, the mass peak of Fez (CO) g became less abundant. 3.2. Photoelectron spectra of iron carbonyl cluster anions Fig. 3 shows photoelectron spectra of Fe,(CO)F,,_i (n=l-4) detached by 266 nm (4.66 eV) photons. In the spectra, the horizontal axis corresponds to the electron binding energy, E,,, defined as E,, = fi v - E,, where Ek is the kinetic energy of the photoelectron. All the spectra exhibit unresolved
Fe3(CO)i4
broad envelopes containing no reproducible fine structure. From these spectra, we derived the threshold binding energies, ET, and vertical detachment energies, VDEs, from the onset and the maximum of each spectrum, respectively. These values are listed in Table 1. The ET corresponds to an estimate of upper limits of adiabatic EAs [ 23,241. When the anion and neutral geometries are similar, the ET almost corresponds to the adiabatic EA. When they are quite different from each other, it is impossible to observe the detached electrons in the vicinity of the adiabatic transition, where the ET only corresponds to the upper limits of the adiabatic EA. The shape of the Fe(CO)r spectrum results from the large geometry change going from DZdin the anion to CZvin the neutral [ 25 1, which causes an excitation to high quanta of vibrational modes. A similar photoelectron spectrum for the Fe (CO)s anion has been measured with an Ar ion laser (363.8 nm; 3.41 eV) [ 51, and the ET value in this work (2.34 +O.l eV) is almost the same as the reported value (2.4 f 0.3 eV) within experimental uncertainties. As shown in Fig. 3, the shapes of the spectra are similar and both ET and VDE increase monotonically with cluster size. The similarity is consistent with the result implicated from the mass distribution of Fe,(CO);; the Fe,(CO)s,_i cluster anions are van derWaalsclustersof (Fe(CO);).(Fe(CO)5)k. Since the observed photoelectron is ejected from an orbital localized mainly at the Fe (CO), core, all of the derived photoelectron spectra are similar. Namely, the ion core of Fe (CO); is solvated by Fe( CO) 5. Differences in solvation energies between anions and the corresponding neutrals are estimated from differences in the ET values: the first, second and the third solvations of Fe(CO)5 to the Fe(CO), core make Table 1 Cluster size dependence of the threshold electron energy (ET) and vertical detachment energy (VDE) of Fe.(CO)&, cluster anions (units: eV)
1.0
2.0
3.0
4.0
Electron Binding Energy I eV Fig. 3. Photoelectron spectra of Fe.( CO)&, cluster anions (n= l-4). The positions of Er and VDE (see text) are indicated by downward and upward arrows, respectively. Peaks indicated by an asterisk express photoelectron from an unassigned multiphoton process (2 or 3 photons).
Cluster size, n
ET
VDE
1 2 3 4
2.34( 10) 2.73( 10) 3.01(12) 3.08(15)
3.02( 13) 3.32( 10) 3.54(13) 3.60( 15)
Numbers in parentheses indicate uncertainties tities; 2.34( 10) represents 2.34kO.10.
in derived quan-
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the anions 0.39, 0.28 and 0.07 eV more stable than the neutrals, respectively. The increase of both ET and VDE results from an increase in the polarizability of the cluster.
Acknowledgement
We are grateful to Mr. K. Nakao for technical assistance to measure TOF mass spectra.
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