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Stabilities and dissociation reactions of the Irm(CO)n+ ions observed in TOF-SIMS of Ir4(CO)12 thin layer
Chemical Physics Letters 556 (2013) 44–48
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Stabilities and dissociation reactions of the Irm(CO)n+ ions observed in TOF-SIMS of Ir4(CO)12 thin layer Taisuke Nakanaga ⇑, Hidekazu Nagai, Naoaki Saito, Yukio Fujiwara, Hidehiko Nonaka National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e
i n f o
Article history: Received 30 September 2012 In final form 21 November 2012 Available online 29 November 2012
a b s t r a c t The kinetic energies of Irm(CO)n+ ions observed in the high resolution TOF-SIMS of Ir4(CO)12 layer have been estimated using the post-acceleration electrode method. The observed kinetic energy of secondary ion depends on the number of coordinated CO molecules, but not on the cluster size of Irm+ shell. The signal of Ir5(CO)12+ ion is missing in the observed mass spectrum. The analysis of the uni-molecular dissociation reactions of the complex ions and DFT calculations have shown that this is due to the structural change of Ir5+ shell with the increase of the coordinated CO molecules. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recent developments of the time-of-flight secondary ion mass spectrometry (TOF-SIMS) have enabled its applications to the analyses of a variety of samples including functional polymers and biomaterials [1–4]. However, in some cases, the mass spectrum of such large molecules gives the signals at mass numbers different from the true mass numbers even after mass calibrations. Actually Nonaka et al. reported a high resolution TOF-SIMS of Ir4(CO)12 layer [5], and showed that there are small anomalies in the apparent masses of the secondary ions. These anomalies seemed due to the ionization reactions at the surface. There are several research works about the Ir cluster complex ion and its applications to the mass spectrometry [5–8]. The mass spectrum of Ir5(CO)n+ in the TOF-SIMS of Ir4(CO)12 layer showed two interesting points [5]. First, the highest coordination number of Ir5(CO)n+ is 14, which suggests that the structure of the metal shell Ir5+ of the complex ion should be tetrahedral bi-pyramid (TBP) from the 18 electron rule [9]. However, there are theoretical researches about the Ir clusters [10–12], and the structure of the Ir5+ cluster cation is known to be square-based pyramid (SBP) [12]. Second, the signal of the partly coordinated complex ion Ir5(CO)12+ is missing in the mass spectrum. It is interesting to reveal the relationship between the structural change of the metal shell and the absence of the mass signal of Ir5(CO)12+. The authors recently showed that the kinetic energies of the ions can be estimated by measuring the mass spectra using different post-acceleration potential (DPA method [13]). The post-acceleration (PA) method is widely used to obtain high sensitivity in high mass region [14], where the ions are re-accelerated by the ⇑ Corresponding author. E-mail address: [email protected] (T. Nakanaga). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.11.070
post-acceleration electrodes in front of the micro channel plate (MCP) detector. There are several methods to study the reactions of meta-stable ions by TOF apparatus [15]. However in many cases the mass resolution is sacrificed to obtain the information of the reactions. The merit of DPA method is that we can estimate the kinetic energies of the ions without reducing the mass resolution. We can discuss about the conditions of the generation of secondary ions at the surface and also the uni-molecular dissociation reactions of the secondary ions in the drift region keeping the best conditions for the mass resolution. In the present Letter, the apparent mass shifts of the mass peaks of Irm(CO)n+ have been investigated in terms of the differences among their initial kinetic energies measured by DPA method. The relative stability and the structure of the Ir5+ cluster complex ions have been discussed considering the uni-molecular dissociation reactions and the results of DFT calculations. 2. Experimental A commercial Ir4(CO)12 powder sample (Strem Chemicals, molecular weight: 1104.9) was evaporated at approximately 150 °C and deposited on a silicon wafer. The details of the sample preparations and measuring conditions are given in the previous paper [5]. High resolution mass spectra of the sample were measured by Toray Research Center through a commercial analysis service using a TOF-SIMS machine (Ion-TOF TOF-SIMS 5). The primary ion beam was reported to be Bi32+ at 25 kV with a pulse width of 7.0 ns before bunching. Both positive and negative secondary ions were measured in the mass range of 0–3500 u. The acceleration voltage was 2 kV. To achieve the high detection efficiency of the MCP detector, the TOF-SIMS system has post-acceleration potential in front of the
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T. Nakanaga et al. / Chemical Physics Letters 556 (2013) 44–48
detector. Three values of the post-acceleration potential, 2.5, 5.0 and 10.0 kV, were used to determine the relative kinetic energies of the Ir cluster complex ions. The stabilization energy and the structures of the most or second most stable isomers of Irm(CO)n+ and Irm(CO)n ions were calculated using B3LYP method with LanL2DZ basis set (GAUSSIAN 03 program [16]). 3. Results and discussions 3.1. The features of the TOF-SIMS of Ir4(CO)12 thin layer The high resolution TOF-SIMS spectra of Ir4(CO)12 layer were measured and the features of the apparent mass shifts of the fragment ions Irm(CO)n+ were precisely discussed in Ref. [5]. The origin of the apparent mass shift and the relative intensities of the signals of Ir4(CO)n+ and Ir5(CO)n+ will be discussed here. Figure 1a shows the close-up of the observed mass signals of 191Ir2193Ir2(CO)n+ ions where n is the number of coordinated CO molecules in the complex ion. The horizontal axis is the mass of the ions calibrated by the standard method. Each mass spectrum shows the mass range between 766.5 + n MCO and 768.5 + n MCO, where MCO is the mass of CO (27.9949 u), so that the peaks of the metal cluster complex ion with different number of CO should appear at the same position. The dashed line shows the expected peak position. Figure 1b shows the similar plots of 191Ir2193Ir3(CO)n+ ions. The horizontal axis shows the mass range from 959.5 + n MCO to 961.5 + n MCO. There are two kinds of peaks observed in the mass spectra. One is the sharp peak appearing near the dashed line. This peak is assigned to the ordinal secondary ion Irm(CO)n+. The other is the broad peak that appears in the lower mass side of the sharp peak. This broad peak is assigned to the product ion of the uni-molecular dissociation reactions
Irm ðCOÞþn ! Irm ðCOÞþn þ CO
ðR1Þ
in the drift tube of the TOF-SIMS apparatus [5]. As it is seen in Figure 1, the positions of the sharp peaks of both Ir4(CO)n+ and Ir5(CO)n+ clearly deviate from the expected mass (the dashed line) to the lower mass side when the number of coordinated CO in the complex ions are small. The origin of this apparent mass shift was assumed to be the condition of the generation of the ions at the surface [5]. Interesting point is that this mass shift depends on the number of CO but not on the number of Ir atoms of the metal shell.
(a) 191Ir2193Ir2(CO)n+
n=4
(b)191Ir2193Ir3(CO)n+
As for the intensity distributions, the mass peaks around Ir5(CO)12+ show different features from others. First, the sharp peak of Ir5(CO)12+ is missing in the mass spectrum. Second, the product ion produced by the uni-molecular dissociation in which two CO molecules dissociate from the complex ion,
Ir5 ðCOÞþ13 ! Ir5 ðCOÞþ11 þ 2CO
ðR2Þ
is clearly observed (marked by in Figure 1). This kind of signal, Dn = 2, is also observed in the spectra of other ions but much weaker than those of the related ions. Although it is not known whether this reaction is the elementary reaction or the sequential reactions of Ir5(CO)13+ ? Ir5(CO)12+ + CO and Ir5(CO)12+ ? Ir5(CO)11+ + CO, this result suggests that Ir5(CO)12+ is not so stable as other complex ions. On the other hand, the product ions of Ir5(CO)11+ and Ir5(CO)12+ of the reactions (R1) are observed in the mass spectrum. The presence of Ir5(CO)11+ signal suggests that the secondary ion of Ir5(CO)12+ exists at the entrance of the drift tube, and that of Ir5(CO)12+ implies that the lifetime of this product ion is long enough to travel through the drift tube and the reflector. These facts suggest that the complex ion Ir5(CO)12+ is not so stable as other complex ions but still has a rigid minimum in the potential surface. As mentioned in Section 1, the structure of Ir5+ metal shell should change from SBP to TDP with increasing number of coordinated CO molecules. The unique feature of Ir5(CO)12+ seems to come from this structural change. The structural change and relative instability of Ir5(CO)12+ will be further discussed using DFT calculations of the metal cluster complex ions later. 3.2. Kinetic energies of the observed ions The post-acceleration (PA) method is widely used to obtain high sensitivity of the MCP detector in high mass region [14]. The PA electrodes are set in front of the MCP detector, and accelerate the ions. TOF of an ion changes when different potential is applied to the PA electrodes. The kinetic energy of the ion can be estimated by analyzing this change (DPA method) [13]. Figure 2 shows the effect of PA potential on the TOF spectra of Ir4(CO)12 layer. Figure 2a shows a part of the mass spectrum of Ir4(CO)n+ ions. The horizontal axis is TOF in ls. Since there are two isotopes of 191Ir and 193Ir, the signal of Ir4(CO)n+ consists of five peaks, and the manifold of the signals of Ir4(CO)12+ is observed at about TOF = 112 ls. The strong signal observed between Ir4(CO)10+ and Ir4(CO)11+ is assigned to Ir4C(CO)10+ which seemed to be produced by the reaction of the complex ion and the impurity at the surface. Figure 2b and c shows the effect of the different PA
Ir4(CO)10+
(a) Ir4(CO)11+
n=6
Ir4(CO)12+
(a’)
10kV
(b)
n=9
n=7
(b’) 5kV
* n=10
n=12
Ir4(CO)10+
(c)
2.5kV
n=15
n=13 766.5 +27.995n
(c’)
768.5 +27.995n
m/z
959.5 +27.995n
961.5 +27.995n
m/z
Figure 1. The close-up of the mass signals of the TOF-SIMS of Ir4(CO)12 layer. (a) 191 Ir2193Ir2(CO)n+; (b) 191Ir2193Ir3(CO)n+.
109
110
111
112
113
114
Time of flight (μs) Figure 2. The effect of the applied post-acceleration potential on the observed mass spectra of the TOF-SIMS of Ir4(CO)12 layer. The strong signal between Ir4(CO)11+ and Ir4(CO)10+ is assigned to Ir4C(CO)10+. (a) 10 kV; (b) 5 kV; (c) 2.5 kV.
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T. Nakanaga et al. / Chemical Physics Letters 556 (2013) 44–48
potentials. The applied PA voltages were 10, 5, 2.5 kV for Figure 2a– c, respectively. As it is seen in the spectra, TOF of the complex ion increased about 2 ls with the decrease of the applied voltage from 10 kV (Figure 2a) to 2.5 kV (c). If we assume two parallel electrodes separated by LPA and the potential difference is VPA, TOF of an ion whose mass is M, electric charge is q, and the initial velocity is v0 at the entrance of the electrodes is calculated by the equation
T1 T0 ¼
MLPA v 0 þ qV PA
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 2qV PA v 20 þ M
ð1Þ
where T0 and T1 are the arrival time at the entrance and exit of the electrodes, respectively [13]. Since T0 does not depend on the applied PA potential, there are three parameters of T0, LPA, and v0 for each observation of T1. The value of LPA is common for all ions, we first determined LPA using TOF of small secondary ions. The values of v0 and T0 of the complex ions were determined from T1 using least squares method. Figure 2a0 –c0 shows the zoom-up of the TOF spectra of Ir4(CO)10+. The horizontal axes were aligned so that the same secondary ion should appear at the same position. It is apparent that the relative positions of the broad peaks to the sharp peaks change with the applied PA potential. This is due to the difference in velocity between product ions (broad peaks) and precursor ions (sharp peaks). The number of coordinated CO molecules, n, in the product ion Irm(CO)n+ was estimated by using the least squares method. Several provable values of n were assumed, and T0 and v0 were determined by the least squares calculations. Then we adopted n which gave least residuals in the calculations. The results were consistent with the assignments given in Ref. [5] when the peaks were not seriously overlapped by other signals. DPA method can be used generally as a low resolution MS or to distinguish the product ions from the precursor ions. Figure 3 compares the observed kinetic energies of the secondary ions of metal cluster complex ions with those of several small secondary ions. The vertical axis is the kinetic energies of the secondary ions relative to that of the protonated water H3O+. Since the protonated ion is expected to have low kinetic energy, we used it as the benchmark. The reliability of the observed values depending on the mass and the peak width of the signal was better than a few eV for complex ions. It is interesting that the kinetic energy of atomic or small fragment ion is significantly higher than that of the protonated ions or fully coordinated complex ions. This difference should reflect the physical conditions of the surface where the secondary ions are generated. At the center of the primary ion bombardment the effective temperature should rise much higher than that of the surrounding area and atomic or small ions can be generated. On the other hand larger ions can be generated in
ΔEPA(eV)
C+,O+ 7 6 5 4 3 2 1 0
0 NH4+, H3O+,CxHy+
Ir4(CO)x x=0-12
C2+,CO+,CH2+
Ir3(CO)x+ x=0-10
Ir(CO)x+ Ir2(CO)x x=0-4 x=0-5
200
400
600
+
Ir5(CO)x+ x=6-14
+
800
1000 1200 1400
m/e
Figure 3. Observed kinetic energies of Irm(CO)n+ and some small ions. The ordinate axis is the relative values to that of the protonated water H3O+.
the surrounding area where the elevation of the effective temperature should be rather moderate. The kinetic energy of the fully coordinated metal cluster complex ion is about the same as that of the protonated ions, and it does not depend on the number of metal atoms. On the other hand, when the number of coordinated CO atoms decreases, it increases as shown in Figure 3. Similar correlation was also observed in the apparent mass shift of the TOF-SIMS of Ir4(CO)12 (Fig. 5 in Ref. [5]). This resemblance suggests that the origin of the apparent mass shifts of the metal cluster complex ions Irn(CO)m+ should be the difference of the initial velocity of the secondary ions. The reason why the kinetic energy does not depend on the number of Ir atoms can be explained as follows. Since the bonding energy of Ir–Ir is much larger than the coordination energy of Irn–CO, the reactions that affect the number of Ir atoms in the metal shell require high energy. So they are expected to occur and finish directly after the ion bombardment. At this point some of CO molecules should be emitted from the metal cluster complex. Then the interaction between the metal cluster complex ion and CO molecules should follow the initial reactions, and determine a kind of local temperature of them. The estimated kinetic energy EPA is about 890 eV, which is considerably lower than the acceleration potential of the apparatus (2 kV). This seems due to the repelling potential set in front of the PA electrodes to get rid of the noise from the stray ions. It can be estimated from the kinetic energies of the product ions dissociating from their precursor ions in the drift region. The kinetic energy EPA of the secondary ion at the entrance of PA electrodes is expressed as
EPA ¼ EK þ EA EP ;
ð2Þ
where EK is the initial translational energy of the secondary ion, EA is the acceleration potential and EP is the repelling potential. EA and EP are constant for all secondary ions. When a secondary ion dissociates into fragments, the sum of EK + EA is distributed to each product in proportion to its mass. The kinetic energy of the product ion E1 is expressed as
E1 ¼ E0
M1 M1 ¼ ðEK þ EA Þ M0 M0
ð3Þ
where E0 is the kinetic energy of the precursor ion in the drift tube, M1 and M0 are the masses of the product and precursor ions, respectively. EK is negligibly small and will be neglected in the following analysis. Since the kinetic energy of the product ion, EPA1, obtained by DPA method is the difference of E1 and the potential energy EP as in Eq. (2), Eq. (3) becomes
EPA1 ¼
M1 ðEPA0 þ EP Þ EP : M0
ð4Þ
Eq. (4) also holds for the case of the secondary ion (M1 = M0). The plot of EPA1 against the mass ratio M1/M0 should give a linear correlation if the assignments of the product ions are correct. Figure 4 shows the plot of the observed kinetic energies of precursor and product ions of Irm(CO)n+ (m = 3–5) against M1/M0. The plot clearly gives linear correlation confirming the assignments of the dissociation reactions. The scatter in the plot is due to the term of EK neglected in Eq. (5). The least squares fit gives K1 = 1680 M1/M0 790, and the repelling potential is estimated to be 790(30) eV where the figure in the parentheses is three times of the standard deviations. 3.3. DFT calculations of Ir metal cluster complex ions and structural change of Ir5+ shell The structures and energies of the most stable isomers of the metal cluster complex ions of Irm(CO)n+ and Irm(CO)n up to m = 5
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T. Nakanaga et al. / Chemical Physics Letters 556 (2013) 44–48
900
E PA(eV)
880
860
840
820
800 0.96
0.98
1.00
M1/M 0 Figure 4. The plot of the observed kinetic energies EPAof Irm(CO)n+ against the mass ratio of the product and precursor ions. The data at M1/M0 = 1 are those of the precursor ions.
SBP (Square Based Pyramid)
DTBP( Distorted Trigonal BiPyramid)
SBP
DTBP
+0eV
+0.33eV
TBP (Trigonal BiPyramid) TBP +0.89eV
Figure 5. Stable isomers of bare Ir5+ cluster cations and their relative stabilities calculated by DFT method.
were calculated by DFT method (GAUSSIAN 03 [16]). In the present Letter, the results about the Ir5(CO)n+ will be mainly discussed. Pawluk et al. investigated the Irn cluster by DFT method [12]. The structure of the most stable Ir5 isomer is square based pyramid (SBP, Figure 5). In the present Letter, the various probable structures of the bare metal cluster ions were assumed and optimized by B3lyp method with LanL2DZ basis set. The results were essentially the same as the earlier works. As for Ir5+ cluster cation, three stable isomers were obtained. Figure 5 shows the summary of the calculations for the Ir5+ clusters. The structure of the most stable
(a) Ir 5(CO)14+
isomer is SBP, and the stabilization energy of TBP is the lowest among them. The optimization calculations of the structures of metal cluster complex ions were performed by the following ways. The metal cluster complex ions which are fully coordinated by CO molecules such as I4(CO)12+ and Ir5(CO)14+ were calculated first. As the starting structures for the optimization calculations, the frameworks of the stable isomers of the bare metal cluster ions were used and CO were placed at the Ir atoms. The coordination numbers of CO were two or three per metal atom, and several orientations of CO were assumed. The optimization calculations were first done using HF method with LanL2DZ basis set. Then the optimized structures of some of the stable isomers were used as the starting parameters for the optimizations with the DFT method (B3LYP). Figure 6a shows the optimized structure of the most stable isomer of Ir5(CO)14+. The framework of the metal cluster of this isomer has been found to be TBP. Even if the structure of SBP was assumed for the starting structure, it changed into DTBP during the optimization calculations and the stabilization energy was lower than that of TBP. The most stable structures of the partially coordinated metal cluster complex ions Irm(CO)n+ were calculated using the coordinates of the larger complex ion of Irm(CO)n+1+ as the starting parameters. The coordinates of the larger complex ion in which one of CO was removed were used. Table 1 summarizes the calculated stabilization energies of the complex ions and the framework of the metal shell. The stabilization energy ES(n) was calculated using the equation
ESðnÞ ¼ Ecomplex ðnÞ n ECO ESBP
ð5Þ
where n is the number of the coordinated CO, Ecomplex(n) is the calculated energy of Ir5(CO)n+ and ESBP is the calculated energy of the bare metal cluster ion of Ir5+ whose framework is SBP. This energy is not necessarily equal to the coordination energy because the framework of the metal cluster may change depending on the number of CO molecules. Table 1 also shows the stabilization energy DES(n) defined by
Ir5 ðCOÞþn1 þ CO ! Ir5 ðCOÞþn þ DESCO ðnÞ:
ðR3Þ
It is interesting that the framework of the metal cluster of the smaller complex ions (n 6 11) is SBP while that of the larger complex ions (n P 12) is TBP. In the case of Ir5(CO)12+, we tried to use SBP for the optimization calculation and found this attempt did not give the most stable isomer. On the other hand, the optimization of Ir5(CO)11+ using the framework of TBP gave the structure of the metal cluster changing from TBP to SBP. Figure 6b and c shows the structures of the most stable isomers of Ir5(CO)12+ and Ir5(CO)11+ calculated by this way.
(b) Ir5(CO)12+
(c) Ir5(CO)11+
Figure 6. Some of the most stable isomers of Ir5(CO)n+ complex ions calculated by DFT method. (a) n = 14; (b) n = 12; (c) n = 11.
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T. Nakanaga et al. / Chemical Physics Letters 556 (2013) 44–48
Table 1 The coordination energies of Ir5(CO)n+ ions (eV).
a
n
ES(n)
DES(n)a
Framework of Ir5+ shell
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2.78 5.40 7.99 10.49 13.07 15.48 17.76 20.06 21.57 23.15 24.70 25.69 27.26 28.52
2.78 2.62 2.59 2.50 2.58 2.41 2.28 2.30 1.51 1.58 1.55 0.99 1.57 1.26
SBP SBP SBP SBP SBP SBP SBP SBP SBP SBP SBP TBP TBP TBP
4. Conclusion
3.0
The relative kinetic energy of the complex ion Irm(CO)n+ generated by static SIMS has been estimated by the DPA method. It depends on the number of coordinating CO molecules but not on the number of Ir atoms. The calculated stabilization energies of the complex ions show a good correlation with the observed relative intensities of the mass signals, suggesting that we can expect to some extent of the local equilibrium at the surface after the bombardment of the primary ion. DFT calculation has also shown that the transition of the structure of Ir5+ shell is induced by the coordination of CO molecules between n = 11 and 12.
2.0
Acknowledgment
DES(n) = ES(n) ES(n 1).
4.0
ESCO(n)
work of the metal cluster ion Ir5+ changes from SBP to TBP at n = 12. The small stabilization energy does not necessary mean that the coordination energy is small. As shown in Figure 5, the Ir5+ cluster in SBP form is more stable than TBP by 0.89 eV. This transition of the framework from SBP (n = 11) to TBP (n = 12) seems to cause the decrease of the total stabilization energy. The low stabilization energy is the reason for the absence of Ir5(CO)12+ ion in the mass spectrum.
0.0
This work has been supported by the Ministry of Economy, Trade and Industry.
Ir5(CO)n+
1.0
References
0
2
4
6
8
10
12
14
n Figure 7. A plot of the stabilization energy by the coordination of one CO molecule on the Ir5(CO)n1+, DESCO(n), against the number of coordinating CO molecules. The structure of Ir5+ shell changes from SBP to TBP at n = 12.
Figure 7 shows the plot of the stabilization energy by the coordination of one CO molecule on the complex DESCO(n) against n. The frameworks of the metal cluster are also shown schematically in the figure. The stabilization energy gradually decreases with increasing number of CO, while it shows sharp change at n = 9 and 12. The relative signal intensities of the complex ions show a good correlation with the calculated stabilization energy. The signal of Ir5(CO)9+ is weak and Ir5(CO)12+ is missing in the mass spectra (Figure 1b). This fact suggests that the relative energies of the partially coordinated complex ions calculated by DFT method can be reliable, and we can expect a kind of local equilibrium at the surface after the bombardment of the primary ion. It is apparent that the stabilization energy of Ir5(CO)12+ is exceptionally low compared to other complex ions, and also the calculated frame-
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Stabilities and dissociation reactions of the Irm(CO)n+ ions observed in TOF-SIMS of Ir4(CO)12 thin layer Taisuke Nakanaga ⇑, Hidekazu Nagai, Naoaki Saito, Yukio Fujiwara, Hidehiko Nonaka National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e
i n f o
Article history: Received 30 September 2012 In final form 21 November 2012 Available online 29 November 2012
a b s t r a c t The kinetic energies of Irm(CO)n+ ions observed in the high resolution TOF-SIMS of Ir4(CO)12 layer have been estimated using the post-acceleration electrode method. The observed kinetic energy of secondary ion depends on the number of coordinated CO molecules, but not on the cluster size of Irm+ shell. The signal of Ir5(CO)12+ ion is missing in the observed mass spectrum. The analysis of the uni-molecular dissociation reactions of the complex ions and DFT calculations have shown that this is due to the structural change of Ir5+ shell with the increase of the coordinated CO molecules. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recent developments of the time-of-flight secondary ion mass spectrometry (TOF-SIMS) have enabled its applications to the analyses of a variety of samples including functional polymers and biomaterials [1–4]. However, in some cases, the mass spectrum of such large molecules gives the signals at mass numbers different from the true mass numbers even after mass calibrations. Actually Nonaka et al. reported a high resolution TOF-SIMS of Ir4(CO)12 layer [5], and showed that there are small anomalies in the apparent masses of the secondary ions. These anomalies seemed due to the ionization reactions at the surface. There are several research works about the Ir cluster complex ion and its applications to the mass spectrometry [5–8]. The mass spectrum of Ir5(CO)n+ in the TOF-SIMS of Ir4(CO)12 layer showed two interesting points [5]. First, the highest coordination number of Ir5(CO)n+ is 14, which suggests that the structure of the metal shell Ir5+ of the complex ion should be tetrahedral bi-pyramid (TBP) from the 18 electron rule [9]. However, there are theoretical researches about the Ir clusters [10–12], and the structure of the Ir5+ cluster cation is known to be square-based pyramid (SBP) [12]. Second, the signal of the partly coordinated complex ion Ir5(CO)12+ is missing in the mass spectrum. It is interesting to reveal the relationship between the structural change of the metal shell and the absence of the mass signal of Ir5(CO)12+. The authors recently showed that the kinetic energies of the ions can be estimated by measuring the mass spectra using different post-acceleration potential (DPA method [13]). The post-acceleration (PA) method is widely used to obtain high sensitivity in high mass region [14], where the ions are re-accelerated by the ⇑ Corresponding author. E-mail address: [email protected] (T. Nakanaga). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.11.070
post-acceleration electrodes in front of the micro channel plate (MCP) detector. There are several methods to study the reactions of meta-stable ions by TOF apparatus [15]. However in many cases the mass resolution is sacrificed to obtain the information of the reactions. The merit of DPA method is that we can estimate the kinetic energies of the ions without reducing the mass resolution. We can discuss about the conditions of the generation of secondary ions at the surface and also the uni-molecular dissociation reactions of the secondary ions in the drift region keeping the best conditions for the mass resolution. In the present Letter, the apparent mass shifts of the mass peaks of Irm(CO)n+ have been investigated in terms of the differences among their initial kinetic energies measured by DPA method. The relative stability and the structure of the Ir5+ cluster complex ions have been discussed considering the uni-molecular dissociation reactions and the results of DFT calculations. 2. Experimental A commercial Ir4(CO)12 powder sample (Strem Chemicals, molecular weight: 1104.9) was evaporated at approximately 150 °C and deposited on a silicon wafer. The details of the sample preparations and measuring conditions are given in the previous paper [5]. High resolution mass spectra of the sample were measured by Toray Research Center through a commercial analysis service using a TOF-SIMS machine (Ion-TOF TOF-SIMS 5). The primary ion beam was reported to be Bi32+ at 25 kV with a pulse width of 7.0 ns before bunching. Both positive and negative secondary ions were measured in the mass range of 0–3500 u. The acceleration voltage was 2 kV. To achieve the high detection efficiency of the MCP detector, the TOF-SIMS system has post-acceleration potential in front of the
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detector. Three values of the post-acceleration potential, 2.5, 5.0 and 10.0 kV, were used to determine the relative kinetic energies of the Ir cluster complex ions. The stabilization energy and the structures of the most or second most stable isomers of Irm(CO)n+ and Irm(CO)n ions were calculated using B3LYP method with LanL2DZ basis set (GAUSSIAN 03 program [16]). 3. Results and discussions 3.1. The features of the TOF-SIMS of Ir4(CO)12 thin layer The high resolution TOF-SIMS spectra of Ir4(CO)12 layer were measured and the features of the apparent mass shifts of the fragment ions Irm(CO)n+ were precisely discussed in Ref. [5]. The origin of the apparent mass shift and the relative intensities of the signals of Ir4(CO)n+ and Ir5(CO)n+ will be discussed here. Figure 1a shows the close-up of the observed mass signals of 191Ir2193Ir2(CO)n+ ions where n is the number of coordinated CO molecules in the complex ion. The horizontal axis is the mass of the ions calibrated by the standard method. Each mass spectrum shows the mass range between 766.5 + n MCO and 768.5 + n MCO, where MCO is the mass of CO (27.9949 u), so that the peaks of the metal cluster complex ion with different number of CO should appear at the same position. The dashed line shows the expected peak position. Figure 1b shows the similar plots of 191Ir2193Ir3(CO)n+ ions. The horizontal axis shows the mass range from 959.5 + n MCO to 961.5 + n MCO. There are two kinds of peaks observed in the mass spectra. One is the sharp peak appearing near the dashed line. This peak is assigned to the ordinal secondary ion Irm(CO)n+. The other is the broad peak that appears in the lower mass side of the sharp peak. This broad peak is assigned to the product ion of the uni-molecular dissociation reactions
Irm ðCOÞþn ! Irm ðCOÞþn þ CO
ðR1Þ
in the drift tube of the TOF-SIMS apparatus [5]. As it is seen in Figure 1, the positions of the sharp peaks of both Ir4(CO)n+ and Ir5(CO)n+ clearly deviate from the expected mass (the dashed line) to the lower mass side when the number of coordinated CO in the complex ions are small. The origin of this apparent mass shift was assumed to be the condition of the generation of the ions at the surface [5]. Interesting point is that this mass shift depends on the number of CO but not on the number of Ir atoms of the metal shell.
(a) 191Ir2193Ir2(CO)n+
n=4
(b)191Ir2193Ir3(CO)n+
As for the intensity distributions, the mass peaks around Ir5(CO)12+ show different features from others. First, the sharp peak of Ir5(CO)12+ is missing in the mass spectrum. Second, the product ion produced by the uni-molecular dissociation in which two CO molecules dissociate from the complex ion,
Ir5 ðCOÞþ13 ! Ir5 ðCOÞþ11 þ 2CO
ðR2Þ
is clearly observed (marked by in Figure 1). This kind of signal, Dn = 2, is also observed in the spectra of other ions but much weaker than those of the related ions. Although it is not known whether this reaction is the elementary reaction or the sequential reactions of Ir5(CO)13+ ? Ir5(CO)12+ + CO and Ir5(CO)12+ ? Ir5(CO)11+ + CO, this result suggests that Ir5(CO)12+ is not so stable as other complex ions. On the other hand, the product ions of Ir5(CO)11+ and Ir5(CO)12+ of the reactions (R1) are observed in the mass spectrum. The presence of Ir5(CO)11+ signal suggests that the secondary ion of Ir5(CO)12+ exists at the entrance of the drift tube, and that of Ir5(CO)12+ implies that the lifetime of this product ion is long enough to travel through the drift tube and the reflector. These facts suggest that the complex ion Ir5(CO)12+ is not so stable as other complex ions but still has a rigid minimum in the potential surface. As mentioned in Section 1, the structure of Ir5+ metal shell should change from SBP to TDP with increasing number of coordinated CO molecules. The unique feature of Ir5(CO)12+ seems to come from this structural change. The structural change and relative instability of Ir5(CO)12+ will be further discussed using DFT calculations of the metal cluster complex ions later. 3.2. Kinetic energies of the observed ions The post-acceleration (PA) method is widely used to obtain high sensitivity of the MCP detector in high mass region [14]. The PA electrodes are set in front of the MCP detector, and accelerate the ions. TOF of an ion changes when different potential is applied to the PA electrodes. The kinetic energy of the ion can be estimated by analyzing this change (DPA method) [13]. Figure 2 shows the effect of PA potential on the TOF spectra of Ir4(CO)12 layer. Figure 2a shows a part of the mass spectrum of Ir4(CO)n+ ions. The horizontal axis is TOF in ls. Since there are two isotopes of 191Ir and 193Ir, the signal of Ir4(CO)n+ consists of five peaks, and the manifold of the signals of Ir4(CO)12+ is observed at about TOF = 112 ls. The strong signal observed between Ir4(CO)10+ and Ir4(CO)11+ is assigned to Ir4C(CO)10+ which seemed to be produced by the reaction of the complex ion and the impurity at the surface. Figure 2b and c shows the effect of the different PA
Ir4(CO)10+
(a) Ir4(CO)11+
n=6
Ir4(CO)12+
(a’)
10kV
(b)
n=9
n=7
(b’) 5kV
* n=10
n=12
Ir4(CO)10+
(c)
2.5kV
n=15
n=13 766.5 +27.995n
(c’)
768.5 +27.995n
m/z
959.5 +27.995n
961.5 +27.995n
m/z
Figure 1. The close-up of the mass signals of the TOF-SIMS of Ir4(CO)12 layer. (a) 191 Ir2193Ir2(CO)n+; (b) 191Ir2193Ir3(CO)n+.
109
110
111
112
113
114
Time of flight (μs) Figure 2. The effect of the applied post-acceleration potential on the observed mass spectra of the TOF-SIMS of Ir4(CO)12 layer. The strong signal between Ir4(CO)11+ and Ir4(CO)10+ is assigned to Ir4C(CO)10+. (a) 10 kV; (b) 5 kV; (c) 2.5 kV.
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potentials. The applied PA voltages were 10, 5, 2.5 kV for Figure 2a– c, respectively. As it is seen in the spectra, TOF of the complex ion increased about 2 ls with the decrease of the applied voltage from 10 kV (Figure 2a) to 2.5 kV (c). If we assume two parallel electrodes separated by LPA and the potential difference is VPA, TOF of an ion whose mass is M, electric charge is q, and the initial velocity is v0 at the entrance of the electrodes is calculated by the equation
T1 T0 ¼
MLPA v 0 þ qV PA
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 2qV PA v 20 þ M
ð1Þ
where T0 and T1 are the arrival time at the entrance and exit of the electrodes, respectively [13]. Since T0 does not depend on the applied PA potential, there are three parameters of T0, LPA, and v0 for each observation of T1. The value of LPA is common for all ions, we first determined LPA using TOF of small secondary ions. The values of v0 and T0 of the complex ions were determined from T1 using least squares method. Figure 2a0 –c0 shows the zoom-up of the TOF spectra of Ir4(CO)10+. The horizontal axes were aligned so that the same secondary ion should appear at the same position. It is apparent that the relative positions of the broad peaks to the sharp peaks change with the applied PA potential. This is due to the difference in velocity between product ions (broad peaks) and precursor ions (sharp peaks). The number of coordinated CO molecules, n, in the product ion Irm(CO)n+ was estimated by using the least squares method. Several provable values of n were assumed, and T0 and v0 were determined by the least squares calculations. Then we adopted n which gave least residuals in the calculations. The results were consistent with the assignments given in Ref. [5] when the peaks were not seriously overlapped by other signals. DPA method can be used generally as a low resolution MS or to distinguish the product ions from the precursor ions. Figure 3 compares the observed kinetic energies of the secondary ions of metal cluster complex ions with those of several small secondary ions. The vertical axis is the kinetic energies of the secondary ions relative to that of the protonated water H3O+. Since the protonated ion is expected to have low kinetic energy, we used it as the benchmark. The reliability of the observed values depending on the mass and the peak width of the signal was better than a few eV for complex ions. It is interesting that the kinetic energy of atomic or small fragment ion is significantly higher than that of the protonated ions or fully coordinated complex ions. This difference should reflect the physical conditions of the surface where the secondary ions are generated. At the center of the primary ion bombardment the effective temperature should rise much higher than that of the surrounding area and atomic or small ions can be generated. On the other hand larger ions can be generated in
ΔEPA(eV)
C+,O+ 7 6 5 4 3 2 1 0
0 NH4+, H3O+,CxHy+
Ir4(CO)x x=0-12
C2+,CO+,CH2+
Ir3(CO)x+ x=0-10
Ir(CO)x+ Ir2(CO)x x=0-4 x=0-5
200
400
600
+
Ir5(CO)x+ x=6-14
+
800
1000 1200 1400
m/e
Figure 3. Observed kinetic energies of Irm(CO)n+ and some small ions. The ordinate axis is the relative values to that of the protonated water H3O+.
the surrounding area where the elevation of the effective temperature should be rather moderate. The kinetic energy of the fully coordinated metal cluster complex ion is about the same as that of the protonated ions, and it does not depend on the number of metal atoms. On the other hand, when the number of coordinated CO atoms decreases, it increases as shown in Figure 3. Similar correlation was also observed in the apparent mass shift of the TOF-SIMS of Ir4(CO)12 (Fig. 5 in Ref. [5]). This resemblance suggests that the origin of the apparent mass shifts of the metal cluster complex ions Irn(CO)m+ should be the difference of the initial velocity of the secondary ions. The reason why the kinetic energy does not depend on the number of Ir atoms can be explained as follows. Since the bonding energy of Ir–Ir is much larger than the coordination energy of Irn–CO, the reactions that affect the number of Ir atoms in the metal shell require high energy. So they are expected to occur and finish directly after the ion bombardment. At this point some of CO molecules should be emitted from the metal cluster complex. Then the interaction between the metal cluster complex ion and CO molecules should follow the initial reactions, and determine a kind of local temperature of them. The estimated kinetic energy EPA is about 890 eV, which is considerably lower than the acceleration potential of the apparatus (2 kV). This seems due to the repelling potential set in front of the PA electrodes to get rid of the noise from the stray ions. It can be estimated from the kinetic energies of the product ions dissociating from their precursor ions in the drift region. The kinetic energy EPA of the secondary ion at the entrance of PA electrodes is expressed as
EPA ¼ EK þ EA EP ;
ð2Þ
where EK is the initial translational energy of the secondary ion, EA is the acceleration potential and EP is the repelling potential. EA and EP are constant for all secondary ions. When a secondary ion dissociates into fragments, the sum of EK + EA is distributed to each product in proportion to its mass. The kinetic energy of the product ion E1 is expressed as
E1 ¼ E0
M1 M1 ¼ ðEK þ EA Þ M0 M0
ð3Þ
where E0 is the kinetic energy of the precursor ion in the drift tube, M1 and M0 are the masses of the product and precursor ions, respectively. EK is negligibly small and will be neglected in the following analysis. Since the kinetic energy of the product ion, EPA1, obtained by DPA method is the difference of E1 and the potential energy EP as in Eq. (2), Eq. (3) becomes
EPA1 ¼
M1 ðEPA0 þ EP Þ EP : M0
ð4Þ
Eq. (4) also holds for the case of the secondary ion (M1 = M0). The plot of EPA1 against the mass ratio M1/M0 should give a linear correlation if the assignments of the product ions are correct. Figure 4 shows the plot of the observed kinetic energies of precursor and product ions of Irm(CO)n+ (m = 3–5) against M1/M0. The plot clearly gives linear correlation confirming the assignments of the dissociation reactions. The scatter in the plot is due to the term of EK neglected in Eq. (5). The least squares fit gives K1 = 1680 M1/M0 790, and the repelling potential is estimated to be 790(30) eV where the figure in the parentheses is three times of the standard deviations. 3.3. DFT calculations of Ir metal cluster complex ions and structural change of Ir5+ shell The structures and energies of the most stable isomers of the metal cluster complex ions of Irm(CO)n+ and Irm(CO)n up to m = 5
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900
E PA(eV)
880
860
840
820
800 0.96
0.98
1.00
M1/M 0 Figure 4. The plot of the observed kinetic energies EPAof Irm(CO)n+ against the mass ratio of the product and precursor ions. The data at M1/M0 = 1 are those of the precursor ions.
SBP (Square Based Pyramid)
DTBP( Distorted Trigonal BiPyramid)
SBP
DTBP
+0eV
+0.33eV
TBP (Trigonal BiPyramid) TBP +0.89eV
Figure 5. Stable isomers of bare Ir5+ cluster cations and their relative stabilities calculated by DFT method.
were calculated by DFT method (GAUSSIAN 03 [16]). In the present Letter, the results about the Ir5(CO)n+ will be mainly discussed. Pawluk et al. investigated the Irn cluster by DFT method [12]. The structure of the most stable Ir5 isomer is square based pyramid (SBP, Figure 5). In the present Letter, the various probable structures of the bare metal cluster ions were assumed and optimized by B3lyp method with LanL2DZ basis set. The results were essentially the same as the earlier works. As for Ir5+ cluster cation, three stable isomers were obtained. Figure 5 shows the summary of the calculations for the Ir5+ clusters. The structure of the most stable
(a) Ir 5(CO)14+
isomer is SBP, and the stabilization energy of TBP is the lowest among them. The optimization calculations of the structures of metal cluster complex ions were performed by the following ways. The metal cluster complex ions which are fully coordinated by CO molecules such as I4(CO)12+ and Ir5(CO)14+ were calculated first. As the starting structures for the optimization calculations, the frameworks of the stable isomers of the bare metal cluster ions were used and CO were placed at the Ir atoms. The coordination numbers of CO were two or three per metal atom, and several orientations of CO were assumed. The optimization calculations were first done using HF method with LanL2DZ basis set. Then the optimized structures of some of the stable isomers were used as the starting parameters for the optimizations with the DFT method (B3LYP). Figure 6a shows the optimized structure of the most stable isomer of Ir5(CO)14+. The framework of the metal cluster of this isomer has been found to be TBP. Even if the structure of SBP was assumed for the starting structure, it changed into DTBP during the optimization calculations and the stabilization energy was lower than that of TBP. The most stable structures of the partially coordinated metal cluster complex ions Irm(CO)n+ were calculated using the coordinates of the larger complex ion of Irm(CO)n+1+ as the starting parameters. The coordinates of the larger complex ion in which one of CO was removed were used. Table 1 summarizes the calculated stabilization energies of the complex ions and the framework of the metal shell. The stabilization energy ES(n) was calculated using the equation
ESðnÞ ¼ Ecomplex ðnÞ n ECO ESBP
ð5Þ
where n is the number of the coordinated CO, Ecomplex(n) is the calculated energy of Ir5(CO)n+ and ESBP is the calculated energy of the bare metal cluster ion of Ir5+ whose framework is SBP. This energy is not necessarily equal to the coordination energy because the framework of the metal cluster may change depending on the number of CO molecules. Table 1 also shows the stabilization energy DES(n) defined by
Ir5 ðCOÞþn1 þ CO ! Ir5 ðCOÞþn þ DESCO ðnÞ:
ðR3Þ
It is interesting that the framework of the metal cluster of the smaller complex ions (n 6 11) is SBP while that of the larger complex ions (n P 12) is TBP. In the case of Ir5(CO)12+, we tried to use SBP for the optimization calculation and found this attempt did not give the most stable isomer. On the other hand, the optimization of Ir5(CO)11+ using the framework of TBP gave the structure of the metal cluster changing from TBP to SBP. Figure 6b and c shows the structures of the most stable isomers of Ir5(CO)12+ and Ir5(CO)11+ calculated by this way.
(b) Ir5(CO)12+
(c) Ir5(CO)11+
Figure 6. Some of the most stable isomers of Ir5(CO)n+ complex ions calculated by DFT method. (a) n = 14; (b) n = 12; (c) n = 11.
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Table 1 The coordination energies of Ir5(CO)n+ ions (eV).
a
n
ES(n)
DES(n)a
Framework of Ir5+ shell
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2.78 5.40 7.99 10.49 13.07 15.48 17.76 20.06 21.57 23.15 24.70 25.69 27.26 28.52
2.78 2.62 2.59 2.50 2.58 2.41 2.28 2.30 1.51 1.58 1.55 0.99 1.57 1.26
SBP SBP SBP SBP SBP SBP SBP SBP SBP SBP SBP TBP TBP TBP
4. Conclusion
3.0
The relative kinetic energy of the complex ion Irm(CO)n+ generated by static SIMS has been estimated by the DPA method. It depends on the number of coordinating CO molecules but not on the number of Ir atoms. The calculated stabilization energies of the complex ions show a good correlation with the observed relative intensities of the mass signals, suggesting that we can expect to some extent of the local equilibrium at the surface after the bombardment of the primary ion. DFT calculation has also shown that the transition of the structure of Ir5+ shell is induced by the coordination of CO molecules between n = 11 and 12.
2.0
Acknowledgment
DES(n) = ES(n) ES(n 1).
4.0
ESCO(n)
work of the metal cluster ion Ir5+ changes from SBP to TBP at n = 12. The small stabilization energy does not necessary mean that the coordination energy is small. As shown in Figure 5, the Ir5+ cluster in SBP form is more stable than TBP by 0.89 eV. This transition of the framework from SBP (n = 11) to TBP (n = 12) seems to cause the decrease of the total stabilization energy. The low stabilization energy is the reason for the absence of Ir5(CO)12+ ion in the mass spectrum.
0.0
This work has been supported by the Ministry of Economy, Trade and Industry.
Ir5(CO)n+
1.0
References
0
2
4
6
8
10
12
14
n Figure 7. A plot of the stabilization energy by the coordination of one CO molecule on the Ir5(CO)n1+, DESCO(n), against the number of coordinating CO molecules. The structure of Ir5+ shell changes from SBP to TBP at n = 12.
Figure 7 shows the plot of the stabilization energy by the coordination of one CO molecule on the complex DESCO(n) against n. The frameworks of the metal cluster are also shown schematically in the figure. The stabilization energy gradually decreases with increasing number of CO, while it shows sharp change at n = 9 and 12. The relative signal intensities of the complex ions show a good correlation with the calculated stabilization energy. The signal of Ir5(CO)9+ is weak and Ir5(CO)12+ is missing in the mass spectra (Figure 1b). This fact suggests that the relative energies of the partially coordinated complex ions calculated by DFT method can be reliable, and we can expect a kind of local equilibrium at the surface after the bombardment of the primary ion. It is apparent that the stabilization energy of Ir5(CO)12+ is exceptionally low compared to other complex ions, and also the calculated frame-
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