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The 4d–4f dipole resonance of the Pr atom in an endohedral metallofullerene, Pr@C82
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Journal of Quantitative Spectroscopy & Radiative Transfer 109 (2008) 1590–1598 www.elsevier.com/locate/jqsrt
The 4d–4f dipole resonance of the Pr atom in an endohedral metallofullerene, Pr@C82 Hideki Katayanagia,b, Bhim P. Kafleb, Junkei Koua, Takanori Moria, Koichiro Mitsukea,b,, Yasuhiro Takabayashic, Eiji Kuwaharac, Yoshihiro Kubozonoc a
Department of Vacuum UV Photo-Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan b Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan c Department of Chemistry, Faculty of Science, Okayama University, Okayama 700-8530, Japan Received 21 September 2007; received in revised form 19 November 2007; accepted 21 November 2007
Abstract The photoion yield spectra of an endohedral metallofullerene Pr@C82 were measured in the photon energy range of 2+ 3+ 100–150 eV by using time-of-flight mass spectrometry. Parent ions Pr@C+ 82, Pr@C82 and Pr@C82 were observed in the 2+ mass spectra. The photoion yield spectra of Pr@C82 showed a broad peak at 120–140 eV that was assigned to the 4d–4f giant dipole resonance of the encapsulated Pr atoms. Absolute photoabsorption cross sections of Pr@C82 were evaluated from the photoion yield spectra to be 37712 Mb at 110 eV (off-resonance) and 52713 Mb at 130 eV (on-resonance). These cross sections of Pr@C82 were compared with the results of Ce@C82, the only metallofullerene whose photoionization properties have ever been studied near the 4d edge of the encapsulated metal atom. The enhancement of photoabsorption due to the giant resonance was found to be similar in Pr@C82 and Ce@C82, but there are marked differences in the peak shapes, which can be explained as due to interference effects between the fullerene cage and the encapsulated metal atoms. r 2007 Elsevier Ltd. All rights reserved. Keywords: Metallofullerenes; Photoionization; Photoabsorption; Mass spectrometry; Synchrotron radiation
1. Introduction Endohedral metallofullerenes have a unique structure in which one or more metal atoms are encapsulated in a ‘‘nano-space’’ inside a fullerene cage [1]. Owing to this novel structure, the metallofullerenes have been expected to be candidates for a new material of curious functionality [2]. The existence of metallofullerenes was reported just after the discovery of C60 [3]. Nevertheless, knowledge about the optical properties of metallofullerenes has been limited because of difficulties in their mass production and purification. This paper
Corresponding author at: Department of Vacuum UV Photo-Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan. Tel.: +81 564 55 7446; fax: +81 564 53 7327. E-mail address: [email protected] (K. Mitsuke).
0022-4073/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2007.11.005
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describes their photoabsorption and photoionization processes that are expected to be of fundamental importance in various contexts of the science and applications of metallofullerenes. A few authors have made theoretical calculations of photoabsorption cross sections of gaseous metallofullerenes. In the absorption spectrum of Ba@C60 Wendin and Wa¨stberg [4] have predicted characteristic peaks between 90 and 150 eV originating from 4d–4f giant dipole resonance transitions. Later the same group calculated a theoretical photoabsorption spectrum of La@C60 and found that the giant resonance peaks can persist regardless of the encapsulation [5]. Analogous giant dipole resonances are wellknown phenomena for the isolated atoms of Xe, Ba and most of lanthanide metals [6]. The above authors [4,5] claimed that the 4d electrons of these metal atoms are scarcely perturbed by the fullerene cages. There had been no report of experimental data on the photoabsorption cross section of gaseous metallofullerenes until the photoionization efficiencies of Ce@C82 were first published in 2005 [7]. A broad peak due to the 4d–4f resonance was observed in the yield curve of Ce@C2+ 82 . This peak was located in almost the same photon energy range as the 4d–4f resonance of the atomic Ce. However, the net increase of photoabsorption cross section due to the resonance was smaller for Ce@C82 (14.3 Mb) than for various lanthanide atoms (40–50 Mb, [6,8–10]). Moreover, an oscillatory structure was overlapped on the peak of Ce@C2+ 82 . From these findings, we concluded that the fullerene cage weakens but does not extinguish the resonance and that interference by the cage modifies the structure of the peak [7]. No comparison has been made with other experimental results, since there is no study except Ref. [7]. Note that we have studied the photoionization processes of Dy@C82 in the photon energy range of 24.5–39.5 eV [11] in order to decide on whether the Dy 5p–5d resonance affects the photoabsorption spectra of Dy@C82; in this study, we did not examine the presence of the Dy 4d–4f giant dipole resonance at 150–160 eV. In the present paper, we describe the photoabsorption spectra of Pr@C82 as another example of the giant resonance in metallofullerenes. Praseodymium is located next to cerium in the periodic table. The metallofullerene Pr@C82 was ionized by synchrotron radiation in the photon energy range of 100–150 eV. 2+ Similarly to Ce@C82, a broad peak was observed in the photoion yield spectra of Pr@C+ 82 and Pr@C82 . The peak was assigned to the 4d–4f giant resonance of encapsulated Pr atoms. The photoabsorption cross section of Pr@C82 was evaluated from the photoion yield spectra within a plausible assumption that the sum of the yields of the singly and doubly charged ions is approximately proportional to the total photoionization cross section and that the quantum yield for photoionization is equal to unity. 2. Experimental details 2.1. Sample preparation A Pr@C82 sample was synthesized by the procedure reported by Kubozono and co-workers [12]. Soot containing Pr@C82 was prepared by an arc discharge of a graphite composite rod with Pr concentration of 0.8 mol% under an ambience of 1.07 104 Pa buffer helium gas. The sample of Pr@C82 was extracted selectively with N,N-dimethylformamide (DMF) from the soot for 30 h by Soxhlet extraction. After removal of DMF at 373 K under reduced pressure, black powder was obtained. The powder was dissolved in toluene with the help of ultrasonication. The solution was filtered with a 0.2 mm membrane filter and injected into an HPLC equipped with a Buckyclutcher I column with toluene as eluent. The flow rate of eluent was 1.3 ml min1. A purified sample of Pr@C82 was obtained by a one-step HPLC operation. The precipitate obtained after evaporating toluene from the fraction of 15–18 min in the HPLC operation was washed with n-hexane and dried by means of dynamical pumping of 104 Pa at 373–573 K for 2 days. Identification of Pr@C82 was carried out using laser desorption mass spectrometry. No impurity peak was observed and hence the purity of our product was found to be very high. In summary, approximately 1 mg of Pr@C82 was produced with purity better than 99% from 10 g of soot after 10 working days. This amount sufficed for five experimental runs taking a photoion yield curve in the photon energy range of 100–150 eV. Conventional spectroscopic and diffraction analyses have provided corroboration for the ‘‘encapsulation’’ of Pr atoms in Pr@C82. The detailed procedure and relevant data were described elsewhere [13]. First, features of the ultraviolet, visible and near-infrared spectra of the present Pr@C82 sample agreed well with those of pure endohedral metallofullerenes reported in the literature [14,15]. Second, the lattice constant, a, of the
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present sample was determined from powder X-ray diffraction to be 15.70 A˚ [13], in good agreement with the a-values of Ce@C82 (isomer I: 15.78 A˚, isomer II: 15.74 A˚) in [16]. Third, we sublimated Pr@C82 molecules of our sample by resistive heating and deposited them on a SiO2/Si substrate. The STM image of Pr@C82 was spherical on the substrate, thus ensuring that the cage structure of Pr@C82 was preserved on heating. All the above evidence goes to show that the Pr atoms in our sample are encapsulated in the fullerene cages. 2.2. Apparatus and measurements The experimental setup for measurement of the photoion yields of Pr@C82 was described elsewhere [17,18], so we will only mention it briefly. The sample powder loaded in a quartz sample holder was heated to 730–790 K to generate a molecular beam. The sample holder was mounted inside a radiation shield made of stainless steel. The molecular beam intersected at right angles with the focused beam of monochromatized synchrotron radiation from a Dragon-type monochromator at the beamline 2B in the UVSOR synchrotron facility in Okazaki. The ionization region defined by the overlap of the two beams is 10 mm in length and 2 1 mm2 in cross section. A gold mesh of 50% transmittance was located at the end of the light path. A picoammeter was equipped to measure the photoelectric current from the mesh and the current was converted into the photon flux of the synchrotron radiation. A crystal-oscillator surface thickness monitor was employed to measure the deposition rate D of Pr@C82 to evaluate the density of the molecular beam in the ionization region. Various ionic species produced in the ionization region were extracted by a pulsed electric field with a width of 6.5 ms and a repetition rate of 10 kHz. The ions were then mass-separated by a double-focusing timeof-flight (TOF) mass spectrometer and detected with a secondary electron multiplier comprising two microchannel plates (MCPs). The distance from the center of the ionization region to a front MCP was nearly equal to 80 mm. The signal from the multiplier was processed by a preamplifier, a discriminator and a PCboard-based multiscaler (Time-to-Digital Converter, FastComtec, P7887E) in series. Ion signal counts were integrated over the respective mass peaks and normalized by the photon flux measured at the same time. A photoion yield curve was obtained by recording the normalized counts by scanning the photon energy from 100 to 150 eV at intervals of 1 eV. The final averaging was fulfilled over five experimental runs. 2.3. Error estimation Through the analysis for obtaining absolute photoabsorption cross sections, we need to take two major sources of error into consideration: the sample number density in the ionization region and the overall detection efficiency, Z, of the TOF mass spectrometer. First, we presumed that the sublimation rate of the sample from the quartz holder was constant throughout an experimental run. Thus we averaged the D values from the surface thickness monitor at the respective photon energies in order to evaluate the number density of the sample in the ionization region. Nevertheless, a slight change or unevenness in the temperature of the sample holderpoccasionally pffiffiffi gave rise to transient fluctuation of the number density. A standard deviation of ffiffiffiffiffi the mean, s= N ¼ s= 5, of D was considered to be propagated to errors in the photoabsorption cross sections. Here, N denotes the number of experimental runs. Second, the detection efficiency Z of our TOF mass spectrometer depends on the mass-to-charge ratio m/z of the ion. The detection efficiency is estimated from the transmittance of the flight tube and the detection efficiency of the MCP ion detector. The transmittance can be determined mainly by the solid angle of the detector. The Pr@C+ 82 ion, which has the longest flight time, entered the detector at 17.4 ms after the leading edge of the pulsed electric field, or the origin of TOF spectra. While traveling through the tube, the Pr@C+ 82 ions moved by 1 mm at most, owing to their thermal motion perpendicular to the normal of the detector. This shift is smaller than the effective radius of the detector (7.25 mm). We therefore can dispense with the correction of the m/z-dependence of the transmittance. The m/z-dependence of the detection efficiency of the MCP detector has been investigated experimentally [19,20] and theoretically [21]. We compared the present experimental conditions with those of the previous studies and concluded that the detection efficiency of our MCP varies with m/z crucially, and hence adopted an empirical formula for the ions with m/z ¼ 1500–35,000 proposed by Twerenbold [19]. In this formula, the
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relative detection efficiencies can be calculated from the velocities of the ions impinging on the front MCP. We have measured the relative detection efficiencies of singly and doubly charged ions from five rare gases (He, Ne, Ar, Kr, Xe) and verified the validity of his functional form [22]. Relative detection efficiencies Z(Pr@Cz+ 82 ) 2+ for Pr@C+ and Pr@C were then calculated from the acceleration voltages of our mass spectrometer to be 82 82 0.032 and 0.10, respectively, when the saturated efficiency is assumed to be unity. We used these values to correct the count rates of the respective mass peaks to evaluate absolute cross sections. 3. Results and discussion 3.1. Mass spectra of the produced ions When the temperature of the sample holder reached around 500 K, an intense and broad background of H2O+ was found in the TOF mass spectra at the flight time of 8.5 ms or earlier. This background can be accounted for by H2O+ formed after the rising edge of the pulsed electric field for ion extraction. At the 2+ temperature of ca. 740 K the peaks of Pr@C+ 82 and Pr@C82 become discernible at 17.36 and 12.32 ms, respectively. Fig. 1 shows a TOF mass spectrum for ions produced from the Pr@C82 beam at the temperature of 781 K. The photon energy is set to 124 eV. We subtracted a raw spectrum at 735 K from that at 781 K to eliminate the background due to H2O+. In Fig. 1 the peaks corresponding to Pr@C+ 82 (m/z ¼ 1125.7) and Pr@C2+ (m/z ¼ 562.9) are clearly observed. Here, no correction was made for the detection efficiency. Since 82 + + Z(Pr@C2+ ) is approximately three times as high as Z(Pr@C ), the abundance of Pr@C is considered to be 82 82 82 rather higher than that of Pr@C2+ . Quantitative discussion will be made in Section 3.3. Another peak at 82 2+ approximately 10 ms could originate either from Pr@C3+ (m/z ¼ 375.2) or from C (m/z ¼ 360.3). These two 82 60 ions cannot be distinguished within the resolution of our mass spectrometer. If the peak at 10 ms is due to C2+ 60 from contaminating C60, an additional peak of C+ 60 must be observed at around 13.9 ms, because the branching 2+ ratio for production of C+ 60 is expected to be approximately 1.4 times as large as that of C60 [18,22–24]. 3+ However, there is no peak at 13.9 ms. The peak at 10 ms can therefore be assigned to Pr@C82 . In addition, this assignment was supported by evidence that all the peaks of Pr@Cz+ 82 (z ¼ 1–3) emerge at nearly the same temperature of the sample holder (ca. 740 K) and that their relative intensities depend similarly on the temperature. z+ z+ Peaks due to other fullerene ions such as Cz+ 70 (z ¼ 1, 2, y) were not detected. The absence of C60 and C70 guarantees the purity of the sample, which is consistent with the result of laser desorption mass analysis
(H2O)
Counts / 100 s
20
10
2+
Pr@C82 3+
+
Pr@C82
Pr@C82
0 0
5
10 Time of flight / µs
15
20
Fig. 1. Time-of-flight mass spectrum of the ions produced by photoionization of Pr@C82. The temperature of the sample holder is 781 K and the photon energy is 124 eV. To eliminate the background due to H2O+ a raw spectrum at 735 K is subtracted from that at 781 K.
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(see Section 2.1). We can also expect that dissociative photoionization of Pr@C82 at around 124 eV leads to z+ z+ z+ scarce Cz+ 60 or C70 . In the case of Ce@C82, the cross section for producing either C60 or C70 was found to be more than 50% of the total photoabsorption cross section of Ce@C82 at the 4d–4f resonance [7]. The branching ratios for photofragmentation from Pr@C82 exhibit marked contrasts to those from Ce@C82. We will explain the mechanism to produce the highly charged Pr@C3+ 82 ions. Hosokawa et al. [13] have concluded that, in neutral Pr@C82, the encapsulated Pr atom exists as a triply charged state on the basis of the results of Raman spectroscopy. After excitation of the 4d electron of this Pr3+ moiety, cascade Auger decay may follow to lead to ions with a charge more than 3+. Dismissing removal of any electron on the fullerene cage, the Pr moiety in the observed Pr@C3+ 82 is expected to have a charge of 6+. Throughout our experiments no signal has been observed associated with the fragment ions Pr@C822nz+ and C822nz+ (nX1: the number of C2 units lost). Under the present resolution of our mass spectrometer the signal of Pr@C822nz+ would give rise to shoulders in the lower mass side of the Pr@Cz+ 82 peaks, rather than apparent peaks. In Fig. 1 the Pr@Cz+ 82 peaks are not accompanied by shoulders, so that the cross sections for formation of Pr@C822nz+ are considered to be small. Still, as a precaution, we set mass ranges on the TOF spectra wide enough to include the ion counts of concomitant fragments when we integrate the signal counts over the Pr@Cz+ 82 peaks. This treatment enables us to accurately estimate the total photoabsorption cross sections of Pr@C82.
3.2. Photoion yield spectra of Pr@C82 + Fig. 2 shows the photoion yield spectra of (a) Pr@C2+ 82 and (b) Pr@C82 before the correction of the m/z-dependence on the detection efficiency of the MCP detector. In panels (a) and (b) the ordinate shows signal counts normalized at the photon flux of 1.0 1011 s1. Here, we plotted values averaged over five experimental runs after normalizing the photon flux for each data point. The error bars correspond to 1s for the five runs. In Fig. 2a, the ion yield of Pr@C2+ 82 at 130 eV is significantly larger than those at 110 and 150 eV. The difference exceeds the experimental uncertainties. There is no bump at around 130 eV in the yield curves z+ of Cz+ 60 and C70 from C60 and C70, respectively [18,23,24]. We thus deduced that the peak in Fig. 2a does not stem from the C82 fullerene cage but from the encapsulated Pr atom. Indeed, the peak lies at almost the same
6
a
Counts / s-1
4 2 0
b
2
Intensity (arb.)
1 0 20
c
0 100
110 120 130 140 Photon energy / eV
150
+ Fig. 2. Photoion yield curves of (a) Pr@C2+ 82 and (b) Pr@C82. The ordinate shows the normalized signal counts at the photon flux of 1.0 1011 s1. The error bars correspond to 1s for five experimental runs. The solid curve in (c) is the absorption spectrum of atomic Pr, taken from Ref. [19].
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energy as a peak of the 4d–4f giant resonance of atomic Pr, whose photoabsorption spectrum is taken from Ref. [25] and drawn in Fig. 2c. The same trend can be seen in the spectrum of Pr@C+ 82 (Fig. 2b), although it is accompanied by much larger errors than that of Pr@C2+ . This is mainly due to the lower detection efficiency 82 of Pr@C+ in spite of its larger abundance (see Section 2.3). 82 Closer inspection reveals that the peak features of the transitions near the Pr 4d edges of Pr@C82 differ from those of atomic Pr. Seemingly, the onset energy of the 4d–4f giant resonance in Fig. 2a is higher by 5 eV for the yield curve of Pr@C2+ 82 than for the photoabsorption cross section of Pr in Fig. 2c. The lower onset of the latter curve is attributed to several weak peaks and shoulders situated between 115 and 120 eV, which have been assigned [25] to Rydberg-type transitions from the 4d orbital of Pr. In the case of Pr@C82 the encapsulation may cause depression of the strength of the Rydberg transitions and lead to reduction of the intensity below 120 eV. This is because Rydberg orbitals suffer from strong perturbation from the fullerene cage owing to their large average radii. Another possible explanation for the different onset energies is an effect of the uneven charge distribution in Pr@C82, as represented by Pr3+@C3 82 (see Section 3.1). Namely, the photoabsorption spectrum of Pr3+ may behave differently from that of neutral Pr. This possibility is, however, not likely to be operative because the positive charge ordinarily tends to enhance the Rydberg transitions and to suppress the giant resonance. Finer discussion of this charge effect must await measurements of the photoabsorption spectrum of gaseous Pr3+, which is available for comparing with our results for Pr@C82. In contrast to the smooth profile of the resonance peak of the bare Pr in Fig. 2c, the yield curve of Pr@C2+ 82 from Pr@C82 in Fig. 2a has an oscillatory structure above 120 eV. This characteristic structure reflects directly the variation of the photoabsorption cross section of Pr@C82. In the Pr@C+ 82 curve the corresponding structure seems to be ambiguous due to the poor statistics of the data points. An analogous structure in the ion yield spectra of Ce@C2+ 82 was reported in Ref. [7] and was understood on the basis of an interference effect of photoelectrons by the fullerene cage that was first proposed in the theoretical study of Ba@C60 [4]. The oscillatory structure of the Pr@C2+ 82 spectrum is attributable to an interference of the same kind. The detailed 2+ shapes of the peaks in Pr@C2+ 82 and Ce@C82 are, however, not identical. This leads to the possibility that the magnitude of the interference depends on the properties of the encapsulated atom, that is, on its electronic structure. Moreover, the difference in the interference effect might imply that the site of the metal atom inside the fullerene cage differs for different species. 3.3. Evaluation of photofragmentation cross sections We will explain the procedure for calculating an absolute partial photofragmentation cross section, s(Pr@Cz+ 82 ). The procedure is essentially the same as that employed in our previous paper for the evaluation of the cross sections of C60 [26]. In the present paper we improved this procedure to obtain more accurate cross sections by introducing a correction for the relative detection efficiencies of the MCPs. For singly and doubly charged states, the cross section can be expressed as zþ zþ sðPr@Czþ 82 Þ ¼ RðPr@C82 Þ=FnLZabs ðPr@C82 ÞF t,
R(Pr@Cz+ 82 )
(1) Pr@Cz+ 82 ,
where is the count rate of the ion signals of F is the photon flux of synchrotron radiation, n is the number density of Pr@C82 in the ionization region, L is the length of the ionization volume along the light path, Zabs( Pr@Cz+ 82 ) is the ‘‘absolute’’ overall efficiency of the apparatus, F is the repetition rate of the pulsed electric field applied to the ionization region, and t is the average residence time of the ions in the ionization volume. The value of R/F can be obtained directly from Figs. 2a and b. Taking account of the experimental geometry, we evaluated n from the averaged sublimation rate and t from the temperature of the sample holder, following the procedure described in Ref. [26]. To avoid difficulties in determining precise values of Zabs(Pr@Cz+ 82 ), we utilized C60 as a standard sample for which absolute photofragmentation cross sections are known. Eq. (1) can be rewritten with relative detection efficiencies Z given in Section 2.3 as sðPr@Czþ 82 Þ ¼
zþ ðRðPr@Czþ 82 Þ=ZðPr@C82 ÞÞ FC60 nC60 tC60 stot ðC60 Þ. P 0 0 zþ zþ F n t z0 ðRðC60 Þ=ZðC60 ÞÞ
(2)
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Table 1 2+ Partial cross sections for the formation of Pr@C+ 82 and Pr@C82 from Pr@C82 and the total photoabsorption cross section of Pr@C82 at photon energies of 110 and 130 eV Partial cross sectiona,b
Photon energy
110 eV (off-resonance) 130 eV (on-resonance) a
s(Pr@C+ 82)
s(Pr@C2+ 82 )
Total photoabsorption cross sectiona,b s tot
22 (11) 28 (11)
15 (4) 24 (7)
37 (12) 52 (13)
All cross sections are in Mb. Numbers in parentheses represent errors estimated in Section 3.3.
b
0
0
zþ zþ Here, RðC60 Þ designates the count rate for C60 ions produced from C60, and stot(C60) is the total photoionization cross section of C60. The physical quantities with a subscript C60 have the same meanings as those for Pr@C82 but are applied to C60. It should be noted that the L values of the two samples cancel out, as 0 2+ do the F values. The relative detection efficiencies ZðCz60þ Þ for C+ 60 and C60 were estimated to be 0.074 and 0.23, respectively, from Twerenbold’s formula [22]. We supposed that stot(C60) is equal to 60 times the absorption cross section of a carbon atom or 60 s(C) and used the value of 24.5 Mb at 108.5 eV reported by Henke et al. [27,28]. 2+ Photofragmentation cross sections for producing Pr@C+ 82 and Pr@C82 were calculated from Eq. (2) and are listed in Table 1. The total photoionization cross section stot of Pr@C82 is assumed to be equal to the sum of these cross sections. Errors associated with the cross sections in Table 1 are derived from (i) the standard deviation of the mean of the number density of the sample (see Section 2.3) and (ii) the error bars in Fig. 2. 3+ 3+ The contribution of s(Pr@C3+ 82 ) was disregarded, since the corrected count rate R(Pr@C82 )/Z(Pr@C82 ) of 3+ + 2+ Pr@C82 was one order of magnitude smaller than those of Pr@C82 and Pr@C82 . The value of stot at the on-resonance position of 130 eV is larger for Pr@C82 (52 Mb) than for Ce@C82 (19:6þ6:5 3:9 Mb) [7]. Nevertheless, introduction of the relative detection efficiency into the data of Ref. [7] makes stot for Ce@C82 closer to that for Pr@C82. The value of stot at the off-resonance position of 110 eV was evaluated to be 37 Mb. An enhancement of the photoabsorption due to the giant resonance was roughly estimated to be 15 Mb. This value agrees well with that of Ce@C82 (14.3Mb) within the experimental uncertainties. Since typical values of cross sections for the lanthanide atoms are 40–50 Mb [6,8–10], it is likely that the giant resonance was weakened by the fullerene cage in Pr@C82 as well as in Ce@C82. If stot at the off-resonance position originates exclusively from the fullerene cage, the values of stot of Pr@C82 and Ce@C82 should be equal to or larger than that of C82. The latter value is estimated to be 33.5 Mb from 82 times the total photoabsorption cross section of a carbon atom or 82 s(C) at 108.5 eV [27]. The value of stot for Pr@C82 obtained experimentally was 37 Mb. This value agrees well with the calculated value of 82 s(C). This confirms the validity of our estimation of the cross sections. Moreover, we have measured the ion yield spectra of Xe and calculated s(Pr@Cz+ 82 ) by applying Eq. (2) replacing C60 by Xe as another standard sample. A reliable total photoionization cross section of Xe has been reported in a paper by Holland et al. [29]. The s(Pr@Cz+ 82 ) values thus obtained agreed with those in Table 1 within the estimated uncertainties. Although the photoionization cross section of Xe has been measured with a higher degree of accuracy than that of C60, we chose C60 rather than Xe as the standard sample in Eq. (2) for the following two reasons. First, the extrapolation of Twerenbold’s formula to the Xez+ ions gives rise to considerable uncertainties in their relative detection efficiencies, since his formula has been empirically determined for ions with m/z larger than 1500. Second, the molecular beam of C60 was produced in a similar manner to that of Pr@C82, and the number densities of both the samples in the ionization region were evaluated similarly from their sublimation rates.
4. Conclusions We have detected a giant dipole resonance of the encapsulated praseodymium atoms in the photoion yield of the endohedral metallofullerenes, Pr@C82. The present results of Pr@C82, together with the previous
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observation on Ce@C82, clearly showed that the resonance due to the 4d–4f transition does not collapse even within the fullerene cages. We evaluated the absolute photoabsorption cross sections of Pr@C82 at photon energies of 110 and 130 eV and found out that the resonance was reduced by encapsulation. These results agreed qualitatively with the theoretical prediction. Moreover, the present study gives the first evidence asserting that the existence and suppression of the giant resonance is not unique to Ce@C82 but almost universal among metallofullerenes. Acknowledgments We are grateful to the members of the UVSOR for their help during the course of the experiments. This work has been supported by the Joint Studies Program (2004–2006) of the Institute for Molecular Science, by Grants-in-Aid for Scientific Research (Grant Nos. 14340188, 18045031, 18350016, and 17750023) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by a grant for scientific research from Research Foundation for Opto-Science and Technology.
References [1] Chai Y, Guo T, Jin C, Haufler RE, Chibante LPF, Fure J, et al. Fullerenes with metals inside. J Phys Chem 1991;95:7564–8. [2] Shinohara H. Endohedral metallofullerenes. Rep Prog Phys 2000;63:843–92. [3] Heath JR, O’Brien SC, Zhang Q, Liu Y, Curl RF, Kroto HW, et al. Lanthanum complexes of spheroidal carbon shells. J Am Chem Soc 1985;107:7779–80. [4] Wendin G, Wa¨stberg B. Many-electron effects in BaC60-collective response and molecular effects in optical conductivity and photoionization. Phys Rev B 1993;48:14764–7. [5] Wa¨stberg B, Wendin G. Theoretical investigations of relaxation and screening in X-ray-absorption and photoionization of C60 and La, Ba-C60. Nucl Instrum Methods Phys Res B 1994;87:162–6. [6] Connerade JP, Esteva JM, Karnatak RC, editors. Giant resonances in atoms, molecules, and solid, NATO ASI series. New York: Plenum; 1987. [7] Mitsuke K, Mori T, Kou J, Haruyama Y, Kubozono Y. 4d-4f dipole resonance of the metal atom encapsulated in a fullerene cage: Ce@C82. J Chem Phys 2005;122:064304-1–5. [8] Dehmer JL, Starace AF. Potential-barrier effects in photoabsorption. 3. Application to 4d-shell photoabsorption in lanthanum. Phys Rev B 1972;5:1792–6. [9] Libermani DA, Zangwill A. A relativistic program for optical-response in atoms using a time-dependent local density approximation. Comput Phys Commun 1984;32:75–82. [10] Tong XM, Kato D, Watanabe T, Ohtani S. Sharp and window resonances in the 4d photoabsorption spectrum of Eu+ ions. Phys Rev A 2001;64:032716-1-6. [11] Mitsuke K, Mori T, Kou J, Haruyama Y, Takabayashi Y, Kubozono Y. Photoion yield curves of Dy@C82 in the vacuum UV region. Int J Mass Spectrom 2005;243:121–5. [12] Shibata K, Rikiishi Y, Hosokawa T, Haruyama Y, Kubozono Y, Kashino S, et al. Structural and electronic properties of Ce@C82. Phys Rev B 2003;68:094104-1–7. [13] Hosokawa T, Fujiki S, Kuwahara E, Kubozono Y, Kitagawa H, Fujiwara A, et al. Electronic properties for the C2v and Cs isomers of Pr@C82 studied by Raman, resistivity and scanning tunneling microscopy/spectroscopy. Chem Phys Lett 2004;395:78–81. [14] Ding J, Yang S. Isolation and characterization of Pr@C82 and Pr2@C80. J Am Chem Soc 1996;118:11254–7. [15] Akasaka T, Okubo S, Kondo M, Maeda Y, Wakahara T, Kato T, et al. Isolation and characterization of two Pr@C82 isomers. Chem Phys Lett 2000;319:153–6. [16] Rikiishi Y, Kubozono Y, Hosokawa T, Shibata K, Haruyama Y, Takabayashi Y, et al. Structural and electronic characterizations of two isomers of Ce@C82. J Phys Chem B 2004;108:7580–5. [17] Mori T, Kou J, Ono M, Haruyama Y, Kubozono Y, Mitsuke K. Development of a photoionization spectrometer for accurate ion yield measurements from gaseous fullerenes. Rev Sci Instrum 2003;74:3769–73. [18] Kou J, Mori T, Kumar SVK, Haruyama Y, Kubozono Y, Mitsuke K. Double photoionization of C60 and C70 in the valence region. J Chem Phys 2004;120:6005–9. [19] Twerenbold D, Gerber D, Gritti D, Gonin Y, Netuschill A, Rossel F, et al. Single molecule detector for mass spectrometry with mass independent detection efficiency. Proteomics 2001;1:66–9. [20] Krems M, Zirbel J, Thomason M, DuBois RD. Channel electron multiplier and channelplate efficiencies for detecting positive ions. Rev Sci Instrum 2005;76:093305-1–7. [21] Fraser GW. The ion detection efficiency of microchannel plates (MCPs). Int J Mass Spectrom 2002;215:13–30. [22] Mitsuke K, Katayanagi H, Kafle BP, Huang C, Yagi H, Prodhan MSI, et al. Relative partial cross sections for single, double, and triple photoionization of C60 and C70. J Phys Chem A 2007;111:8336–43.
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[23] Mitsuke K, Katayanagi H, Kou J, Mori T, Kubozono Y. Fragmentation mechanism of highly excited C70 cations in the extreme ultroviolet. AIP Conf Proc 2006;811:161–6. [24] Reinko¨ster A, Korica S, Pru¨mper G, Viefhaus J, Godehusen K, Schwarzkopf O, et al. The photoionization and fragmentation of C60 in the energy range 26–130 eV. J Phys B 2004;37:2135–44. [25] Richter M, Meyer M, Pahler M, Prescher T, Raven Ev, Sonntag B, et al. Experimental-study of atomic 4d giant-resonances by photoabsorption and photoelectron-spectroscopy—Sm, Eu, and Gd. Phys Rev A 1989;40:7007–19. [26] Mori T, Kou J, Haruyama Y, Kubozono Y, Mitsuke K. Absolute photoabsorption cross section of C60 in the extreme ultraviolet. J Electron Spectrosc Relat Phenom 2005;144–147:243–6. [27] Henke BL, Gullikson EM, Davis JC. X-ray interactions—photoabsorption, scattering, transmission, and reflection at E ¼ 50–30,000 eV, z ¼ 1–92. At Data Nucl Data Tables 1993;54:181–342. [28] Kafle BP, Katayanagi H, Huang C, Yagi H, Prodhan MSI, Mitsuke K. Absolute total photoionization cross section of C60 in the range of 25–120 eV, revisited. J Phys Soc Jpn 2008;77:xxxx-1-5. [29] Holland DMP, Codling K, West JB, Marr GV. Multiple photoionization in the rare-gases from threshold to 280 eV. J Phys B 1979;12:2465–84.
Journal of Quantitative Spectroscopy & Radiative Transfer 109 (2008) 1590–1598 www.elsevier.com/locate/jqsrt
The 4d–4f dipole resonance of the Pr atom in an endohedral metallofullerene, Pr@C82 Hideki Katayanagia,b, Bhim P. Kafleb, Junkei Koua, Takanori Moria, Koichiro Mitsukea,b,, Yasuhiro Takabayashic, Eiji Kuwaharac, Yoshihiro Kubozonoc a
Department of Vacuum UV Photo-Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan b Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan c Department of Chemistry, Faculty of Science, Okayama University, Okayama 700-8530, Japan Received 21 September 2007; received in revised form 19 November 2007; accepted 21 November 2007
Abstract The photoion yield spectra of an endohedral metallofullerene Pr@C82 were measured in the photon energy range of 2+ 3+ 100–150 eV by using time-of-flight mass spectrometry. Parent ions Pr@C+ 82, Pr@C82 and Pr@C82 were observed in the 2+ mass spectra. The photoion yield spectra of Pr@C82 showed a broad peak at 120–140 eV that was assigned to the 4d–4f giant dipole resonance of the encapsulated Pr atoms. Absolute photoabsorption cross sections of Pr@C82 were evaluated from the photoion yield spectra to be 37712 Mb at 110 eV (off-resonance) and 52713 Mb at 130 eV (on-resonance). These cross sections of Pr@C82 were compared with the results of Ce@C82, the only metallofullerene whose photoionization properties have ever been studied near the 4d edge of the encapsulated metal atom. The enhancement of photoabsorption due to the giant resonance was found to be similar in Pr@C82 and Ce@C82, but there are marked differences in the peak shapes, which can be explained as due to interference effects between the fullerene cage and the encapsulated metal atoms. r 2007 Elsevier Ltd. All rights reserved. Keywords: Metallofullerenes; Photoionization; Photoabsorption; Mass spectrometry; Synchrotron radiation
1. Introduction Endohedral metallofullerenes have a unique structure in which one or more metal atoms are encapsulated in a ‘‘nano-space’’ inside a fullerene cage [1]. Owing to this novel structure, the metallofullerenes have been expected to be candidates for a new material of curious functionality [2]. The existence of metallofullerenes was reported just after the discovery of C60 [3]. Nevertheless, knowledge about the optical properties of metallofullerenes has been limited because of difficulties in their mass production and purification. This paper
Corresponding author at: Department of Vacuum UV Photo-Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan. Tel.: +81 564 55 7446; fax: +81 564 53 7327. E-mail address: [email protected] (K. Mitsuke).
0022-4073/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2007.11.005
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describes their photoabsorption and photoionization processes that are expected to be of fundamental importance in various contexts of the science and applications of metallofullerenes. A few authors have made theoretical calculations of photoabsorption cross sections of gaseous metallofullerenes. In the absorption spectrum of Ba@C60 Wendin and Wa¨stberg [4] have predicted characteristic peaks between 90 and 150 eV originating from 4d–4f giant dipole resonance transitions. Later the same group calculated a theoretical photoabsorption spectrum of La@C60 and found that the giant resonance peaks can persist regardless of the encapsulation [5]. Analogous giant dipole resonances are wellknown phenomena for the isolated atoms of Xe, Ba and most of lanthanide metals [6]. The above authors [4,5] claimed that the 4d electrons of these metal atoms are scarcely perturbed by the fullerene cages. There had been no report of experimental data on the photoabsorption cross section of gaseous metallofullerenes until the photoionization efficiencies of Ce@C82 were first published in 2005 [7]. A broad peak due to the 4d–4f resonance was observed in the yield curve of Ce@C2+ 82 . This peak was located in almost the same photon energy range as the 4d–4f resonance of the atomic Ce. However, the net increase of photoabsorption cross section due to the resonance was smaller for Ce@C82 (14.3 Mb) than for various lanthanide atoms (40–50 Mb, [6,8–10]). Moreover, an oscillatory structure was overlapped on the peak of Ce@C2+ 82 . From these findings, we concluded that the fullerene cage weakens but does not extinguish the resonance and that interference by the cage modifies the structure of the peak [7]. No comparison has been made with other experimental results, since there is no study except Ref. [7]. Note that we have studied the photoionization processes of Dy@C82 in the photon energy range of 24.5–39.5 eV [11] in order to decide on whether the Dy 5p–5d resonance affects the photoabsorption spectra of Dy@C82; in this study, we did not examine the presence of the Dy 4d–4f giant dipole resonance at 150–160 eV. In the present paper, we describe the photoabsorption spectra of Pr@C82 as another example of the giant resonance in metallofullerenes. Praseodymium is located next to cerium in the periodic table. The metallofullerene Pr@C82 was ionized by synchrotron radiation in the photon energy range of 100–150 eV. 2+ Similarly to Ce@C82, a broad peak was observed in the photoion yield spectra of Pr@C+ 82 and Pr@C82 . The peak was assigned to the 4d–4f giant resonance of encapsulated Pr atoms. The photoabsorption cross section of Pr@C82 was evaluated from the photoion yield spectra within a plausible assumption that the sum of the yields of the singly and doubly charged ions is approximately proportional to the total photoionization cross section and that the quantum yield for photoionization is equal to unity. 2. Experimental details 2.1. Sample preparation A Pr@C82 sample was synthesized by the procedure reported by Kubozono and co-workers [12]. Soot containing Pr@C82 was prepared by an arc discharge of a graphite composite rod with Pr concentration of 0.8 mol% under an ambience of 1.07 104 Pa buffer helium gas. The sample of Pr@C82 was extracted selectively with N,N-dimethylformamide (DMF) from the soot for 30 h by Soxhlet extraction. After removal of DMF at 373 K under reduced pressure, black powder was obtained. The powder was dissolved in toluene with the help of ultrasonication. The solution was filtered with a 0.2 mm membrane filter and injected into an HPLC equipped with a Buckyclutcher I column with toluene as eluent. The flow rate of eluent was 1.3 ml min1. A purified sample of Pr@C82 was obtained by a one-step HPLC operation. The precipitate obtained after evaporating toluene from the fraction of 15–18 min in the HPLC operation was washed with n-hexane and dried by means of dynamical pumping of 104 Pa at 373–573 K for 2 days. Identification of Pr@C82 was carried out using laser desorption mass spectrometry. No impurity peak was observed and hence the purity of our product was found to be very high. In summary, approximately 1 mg of Pr@C82 was produced with purity better than 99% from 10 g of soot after 10 working days. This amount sufficed for five experimental runs taking a photoion yield curve in the photon energy range of 100–150 eV. Conventional spectroscopic and diffraction analyses have provided corroboration for the ‘‘encapsulation’’ of Pr atoms in Pr@C82. The detailed procedure and relevant data were described elsewhere [13]. First, features of the ultraviolet, visible and near-infrared spectra of the present Pr@C82 sample agreed well with those of pure endohedral metallofullerenes reported in the literature [14,15]. Second, the lattice constant, a, of the
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present sample was determined from powder X-ray diffraction to be 15.70 A˚ [13], in good agreement with the a-values of Ce@C82 (isomer I: 15.78 A˚, isomer II: 15.74 A˚) in [16]. Third, we sublimated Pr@C82 molecules of our sample by resistive heating and deposited them on a SiO2/Si substrate. The STM image of Pr@C82 was spherical on the substrate, thus ensuring that the cage structure of Pr@C82 was preserved on heating. All the above evidence goes to show that the Pr atoms in our sample are encapsulated in the fullerene cages. 2.2. Apparatus and measurements The experimental setup for measurement of the photoion yields of Pr@C82 was described elsewhere [17,18], so we will only mention it briefly. The sample powder loaded in a quartz sample holder was heated to 730–790 K to generate a molecular beam. The sample holder was mounted inside a radiation shield made of stainless steel. The molecular beam intersected at right angles with the focused beam of monochromatized synchrotron radiation from a Dragon-type monochromator at the beamline 2B in the UVSOR synchrotron facility in Okazaki. The ionization region defined by the overlap of the two beams is 10 mm in length and 2 1 mm2 in cross section. A gold mesh of 50% transmittance was located at the end of the light path. A picoammeter was equipped to measure the photoelectric current from the mesh and the current was converted into the photon flux of the synchrotron radiation. A crystal-oscillator surface thickness monitor was employed to measure the deposition rate D of Pr@C82 to evaluate the density of the molecular beam in the ionization region. Various ionic species produced in the ionization region were extracted by a pulsed electric field with a width of 6.5 ms and a repetition rate of 10 kHz. The ions were then mass-separated by a double-focusing timeof-flight (TOF) mass spectrometer and detected with a secondary electron multiplier comprising two microchannel plates (MCPs). The distance from the center of the ionization region to a front MCP was nearly equal to 80 mm. The signal from the multiplier was processed by a preamplifier, a discriminator and a PCboard-based multiscaler (Time-to-Digital Converter, FastComtec, P7887E) in series. Ion signal counts were integrated over the respective mass peaks and normalized by the photon flux measured at the same time. A photoion yield curve was obtained by recording the normalized counts by scanning the photon energy from 100 to 150 eV at intervals of 1 eV. The final averaging was fulfilled over five experimental runs. 2.3. Error estimation Through the analysis for obtaining absolute photoabsorption cross sections, we need to take two major sources of error into consideration: the sample number density in the ionization region and the overall detection efficiency, Z, of the TOF mass spectrometer. First, we presumed that the sublimation rate of the sample from the quartz holder was constant throughout an experimental run. Thus we averaged the D values from the surface thickness monitor at the respective photon energies in order to evaluate the number density of the sample in the ionization region. Nevertheless, a slight change or unevenness in the temperature of the sample holderpoccasionally pffiffiffi gave rise to transient fluctuation of the number density. A standard deviation of ffiffiffiffiffi the mean, s= N ¼ s= 5, of D was considered to be propagated to errors in the photoabsorption cross sections. Here, N denotes the number of experimental runs. Second, the detection efficiency Z of our TOF mass spectrometer depends on the mass-to-charge ratio m/z of the ion. The detection efficiency is estimated from the transmittance of the flight tube and the detection efficiency of the MCP ion detector. The transmittance can be determined mainly by the solid angle of the detector. The Pr@C+ 82 ion, which has the longest flight time, entered the detector at 17.4 ms after the leading edge of the pulsed electric field, or the origin of TOF spectra. While traveling through the tube, the Pr@C+ 82 ions moved by 1 mm at most, owing to their thermal motion perpendicular to the normal of the detector. This shift is smaller than the effective radius of the detector (7.25 mm). We therefore can dispense with the correction of the m/z-dependence of the transmittance. The m/z-dependence of the detection efficiency of the MCP detector has been investigated experimentally [19,20] and theoretically [21]. We compared the present experimental conditions with those of the previous studies and concluded that the detection efficiency of our MCP varies with m/z crucially, and hence adopted an empirical formula for the ions with m/z ¼ 1500–35,000 proposed by Twerenbold [19]. In this formula, the
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relative detection efficiencies can be calculated from the velocities of the ions impinging on the front MCP. We have measured the relative detection efficiencies of singly and doubly charged ions from five rare gases (He, Ne, Ar, Kr, Xe) and verified the validity of his functional form [22]. Relative detection efficiencies Z(Pr@Cz+ 82 ) 2+ for Pr@C+ and Pr@C were then calculated from the acceleration voltages of our mass spectrometer to be 82 82 0.032 and 0.10, respectively, when the saturated efficiency is assumed to be unity. We used these values to correct the count rates of the respective mass peaks to evaluate absolute cross sections. 3. Results and discussion 3.1. Mass spectra of the produced ions When the temperature of the sample holder reached around 500 K, an intense and broad background of H2O+ was found in the TOF mass spectra at the flight time of 8.5 ms or earlier. This background can be accounted for by H2O+ formed after the rising edge of the pulsed electric field for ion extraction. At the 2+ temperature of ca. 740 K the peaks of Pr@C+ 82 and Pr@C82 become discernible at 17.36 and 12.32 ms, respectively. Fig. 1 shows a TOF mass spectrum for ions produced from the Pr@C82 beam at the temperature of 781 K. The photon energy is set to 124 eV. We subtracted a raw spectrum at 735 K from that at 781 K to eliminate the background due to H2O+. In Fig. 1 the peaks corresponding to Pr@C+ 82 (m/z ¼ 1125.7) and Pr@C2+ (m/z ¼ 562.9) are clearly observed. Here, no correction was made for the detection efficiency. Since 82 + + Z(Pr@C2+ ) is approximately three times as high as Z(Pr@C ), the abundance of Pr@C is considered to be 82 82 82 rather higher than that of Pr@C2+ . Quantitative discussion will be made in Section 3.3. Another peak at 82 2+ approximately 10 ms could originate either from Pr@C3+ (m/z ¼ 375.2) or from C (m/z ¼ 360.3). These two 82 60 ions cannot be distinguished within the resolution of our mass spectrometer. If the peak at 10 ms is due to C2+ 60 from contaminating C60, an additional peak of C+ 60 must be observed at around 13.9 ms, because the branching 2+ ratio for production of C+ 60 is expected to be approximately 1.4 times as large as that of C60 [18,22–24]. 3+ However, there is no peak at 13.9 ms. The peak at 10 ms can therefore be assigned to Pr@C82 . In addition, this assignment was supported by evidence that all the peaks of Pr@Cz+ 82 (z ¼ 1–3) emerge at nearly the same temperature of the sample holder (ca. 740 K) and that their relative intensities depend similarly on the temperature. z+ z+ Peaks due to other fullerene ions such as Cz+ 70 (z ¼ 1, 2, y) were not detected. The absence of C60 and C70 guarantees the purity of the sample, which is consistent with the result of laser desorption mass analysis
(H2O)
Counts / 100 s
20
10
2+
Pr@C82 3+
+
Pr@C82
Pr@C82
0 0
5
10 Time of flight / µs
15
20
Fig. 1. Time-of-flight mass spectrum of the ions produced by photoionization of Pr@C82. The temperature of the sample holder is 781 K and the photon energy is 124 eV. To eliminate the background due to H2O+ a raw spectrum at 735 K is subtracted from that at 781 K.
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(see Section 2.1). We can also expect that dissociative photoionization of Pr@C82 at around 124 eV leads to z+ z+ z+ scarce Cz+ 60 or C70 . In the case of Ce@C82, the cross section for producing either C60 or C70 was found to be more than 50% of the total photoabsorption cross section of Ce@C82 at the 4d–4f resonance [7]. The branching ratios for photofragmentation from Pr@C82 exhibit marked contrasts to those from Ce@C82. We will explain the mechanism to produce the highly charged Pr@C3+ 82 ions. Hosokawa et al. [13] have concluded that, in neutral Pr@C82, the encapsulated Pr atom exists as a triply charged state on the basis of the results of Raman spectroscopy. After excitation of the 4d electron of this Pr3+ moiety, cascade Auger decay may follow to lead to ions with a charge more than 3+. Dismissing removal of any electron on the fullerene cage, the Pr moiety in the observed Pr@C3+ 82 is expected to have a charge of 6+. Throughout our experiments no signal has been observed associated with the fragment ions Pr@C822nz+ and C822nz+ (nX1: the number of C2 units lost). Under the present resolution of our mass spectrometer the signal of Pr@C822nz+ would give rise to shoulders in the lower mass side of the Pr@Cz+ 82 peaks, rather than apparent peaks. In Fig. 1 the Pr@Cz+ 82 peaks are not accompanied by shoulders, so that the cross sections for formation of Pr@C822nz+ are considered to be small. Still, as a precaution, we set mass ranges on the TOF spectra wide enough to include the ion counts of concomitant fragments when we integrate the signal counts over the Pr@Cz+ 82 peaks. This treatment enables us to accurately estimate the total photoabsorption cross sections of Pr@C82.
3.2. Photoion yield spectra of Pr@C82 + Fig. 2 shows the photoion yield spectra of (a) Pr@C2+ 82 and (b) Pr@C82 before the correction of the m/z-dependence on the detection efficiency of the MCP detector. In panels (a) and (b) the ordinate shows signal counts normalized at the photon flux of 1.0 1011 s1. Here, we plotted values averaged over five experimental runs after normalizing the photon flux for each data point. The error bars correspond to 1s for the five runs. In Fig. 2a, the ion yield of Pr@C2+ 82 at 130 eV is significantly larger than those at 110 and 150 eV. The difference exceeds the experimental uncertainties. There is no bump at around 130 eV in the yield curves z+ of Cz+ 60 and C70 from C60 and C70, respectively [18,23,24]. We thus deduced that the peak in Fig. 2a does not stem from the C82 fullerene cage but from the encapsulated Pr atom. Indeed, the peak lies at almost the same
6
a
Counts / s-1
4 2 0
b
2
Intensity (arb.)
1 0 20
c
0 100
110 120 130 140 Photon energy / eV
150
+ Fig. 2. Photoion yield curves of (a) Pr@C2+ 82 and (b) Pr@C82. The ordinate shows the normalized signal counts at the photon flux of 1.0 1011 s1. The error bars correspond to 1s for five experimental runs. The solid curve in (c) is the absorption spectrum of atomic Pr, taken from Ref. [19].
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energy as a peak of the 4d–4f giant resonance of atomic Pr, whose photoabsorption spectrum is taken from Ref. [25] and drawn in Fig. 2c. The same trend can be seen in the spectrum of Pr@C+ 82 (Fig. 2b), although it is accompanied by much larger errors than that of Pr@C2+ . This is mainly due to the lower detection efficiency 82 of Pr@C+ in spite of its larger abundance (see Section 2.3). 82 Closer inspection reveals that the peak features of the transitions near the Pr 4d edges of Pr@C82 differ from those of atomic Pr. Seemingly, the onset energy of the 4d–4f giant resonance in Fig. 2a is higher by 5 eV for the yield curve of Pr@C2+ 82 than for the photoabsorption cross section of Pr in Fig. 2c. The lower onset of the latter curve is attributed to several weak peaks and shoulders situated between 115 and 120 eV, which have been assigned [25] to Rydberg-type transitions from the 4d orbital of Pr. In the case of Pr@C82 the encapsulation may cause depression of the strength of the Rydberg transitions and lead to reduction of the intensity below 120 eV. This is because Rydberg orbitals suffer from strong perturbation from the fullerene cage owing to their large average radii. Another possible explanation for the different onset energies is an effect of the uneven charge distribution in Pr@C82, as represented by Pr3+@C3 82 (see Section 3.1). Namely, the photoabsorption spectrum of Pr3+ may behave differently from that of neutral Pr. This possibility is, however, not likely to be operative because the positive charge ordinarily tends to enhance the Rydberg transitions and to suppress the giant resonance. Finer discussion of this charge effect must await measurements of the photoabsorption spectrum of gaseous Pr3+, which is available for comparing with our results for Pr@C82. In contrast to the smooth profile of the resonance peak of the bare Pr in Fig. 2c, the yield curve of Pr@C2+ 82 from Pr@C82 in Fig. 2a has an oscillatory structure above 120 eV. This characteristic structure reflects directly the variation of the photoabsorption cross section of Pr@C82. In the Pr@C+ 82 curve the corresponding structure seems to be ambiguous due to the poor statistics of the data points. An analogous structure in the ion yield spectra of Ce@C2+ 82 was reported in Ref. [7] and was understood on the basis of an interference effect of photoelectrons by the fullerene cage that was first proposed in the theoretical study of Ba@C60 [4]. The oscillatory structure of the Pr@C2+ 82 spectrum is attributable to an interference of the same kind. The detailed 2+ shapes of the peaks in Pr@C2+ 82 and Ce@C82 are, however, not identical. This leads to the possibility that the magnitude of the interference depends on the properties of the encapsulated atom, that is, on its electronic structure. Moreover, the difference in the interference effect might imply that the site of the metal atom inside the fullerene cage differs for different species. 3.3. Evaluation of photofragmentation cross sections We will explain the procedure for calculating an absolute partial photofragmentation cross section, s(Pr@Cz+ 82 ). The procedure is essentially the same as that employed in our previous paper for the evaluation of the cross sections of C60 [26]. In the present paper we improved this procedure to obtain more accurate cross sections by introducing a correction for the relative detection efficiencies of the MCPs. For singly and doubly charged states, the cross section can be expressed as zþ zþ sðPr@Czþ 82 Þ ¼ RðPr@C82 Þ=FnLZabs ðPr@C82 ÞF t,
R(Pr@Cz+ 82 )
(1) Pr@Cz+ 82 ,
where is the count rate of the ion signals of F is the photon flux of synchrotron radiation, n is the number density of Pr@C82 in the ionization region, L is the length of the ionization volume along the light path, Zabs( Pr@Cz+ 82 ) is the ‘‘absolute’’ overall efficiency of the apparatus, F is the repetition rate of the pulsed electric field applied to the ionization region, and t is the average residence time of the ions in the ionization volume. The value of R/F can be obtained directly from Figs. 2a and b. Taking account of the experimental geometry, we evaluated n from the averaged sublimation rate and t from the temperature of the sample holder, following the procedure described in Ref. [26]. To avoid difficulties in determining precise values of Zabs(Pr@Cz+ 82 ), we utilized C60 as a standard sample for which absolute photofragmentation cross sections are known. Eq. (1) can be rewritten with relative detection efficiencies Z given in Section 2.3 as sðPr@Czþ 82 Þ ¼
zþ ðRðPr@Czþ 82 Þ=ZðPr@C82 ÞÞ FC60 nC60 tC60 stot ðC60 Þ. P 0 0 zþ zþ F n t z0 ðRðC60 Þ=ZðC60 ÞÞ
(2)
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Table 1 2+ Partial cross sections for the formation of Pr@C+ 82 and Pr@C82 from Pr@C82 and the total photoabsorption cross section of Pr@C82 at photon energies of 110 and 130 eV Partial cross sectiona,b
Photon energy
110 eV (off-resonance) 130 eV (on-resonance) a
s(Pr@C+ 82)
s(Pr@C2+ 82 )
Total photoabsorption cross sectiona,b s tot
22 (11) 28 (11)
15 (4) 24 (7)
37 (12) 52 (13)
All cross sections are in Mb. Numbers in parentheses represent errors estimated in Section 3.3.
b
0
0
zþ zþ Here, RðC60 Þ designates the count rate for C60 ions produced from C60, and stot(C60) is the total photoionization cross section of C60. The physical quantities with a subscript C60 have the same meanings as those for Pr@C82 but are applied to C60. It should be noted that the L values of the two samples cancel out, as 0 2+ do the F values. The relative detection efficiencies ZðCz60þ Þ for C+ 60 and C60 were estimated to be 0.074 and 0.23, respectively, from Twerenbold’s formula [22]. We supposed that stot(C60) is equal to 60 times the absorption cross section of a carbon atom or 60 s(C) and used the value of 24.5 Mb at 108.5 eV reported by Henke et al. [27,28]. 2+ Photofragmentation cross sections for producing Pr@C+ 82 and Pr@C82 were calculated from Eq. (2) and are listed in Table 1. The total photoionization cross section stot of Pr@C82 is assumed to be equal to the sum of these cross sections. Errors associated with the cross sections in Table 1 are derived from (i) the standard deviation of the mean of the number density of the sample (see Section 2.3) and (ii) the error bars in Fig. 2. 3+ 3+ The contribution of s(Pr@C3+ 82 ) was disregarded, since the corrected count rate R(Pr@C82 )/Z(Pr@C82 ) of 3+ + 2+ Pr@C82 was one order of magnitude smaller than those of Pr@C82 and Pr@C82 . The value of stot at the on-resonance position of 130 eV is larger for Pr@C82 (52 Mb) than for Ce@C82 (19:6þ6:5 3:9 Mb) [7]. Nevertheless, introduction of the relative detection efficiency into the data of Ref. [7] makes stot for Ce@C82 closer to that for Pr@C82. The value of stot at the off-resonance position of 110 eV was evaluated to be 37 Mb. An enhancement of the photoabsorption due to the giant resonance was roughly estimated to be 15 Mb. This value agrees well with that of Ce@C82 (14.3Mb) within the experimental uncertainties. Since typical values of cross sections for the lanthanide atoms are 40–50 Mb [6,8–10], it is likely that the giant resonance was weakened by the fullerene cage in Pr@C82 as well as in Ce@C82. If stot at the off-resonance position originates exclusively from the fullerene cage, the values of stot of Pr@C82 and Ce@C82 should be equal to or larger than that of C82. The latter value is estimated to be 33.5 Mb from 82 times the total photoabsorption cross section of a carbon atom or 82 s(C) at 108.5 eV [27]. The value of stot for Pr@C82 obtained experimentally was 37 Mb. This value agrees well with the calculated value of 82 s(C). This confirms the validity of our estimation of the cross sections. Moreover, we have measured the ion yield spectra of Xe and calculated s(Pr@Cz+ 82 ) by applying Eq. (2) replacing C60 by Xe as another standard sample. A reliable total photoionization cross section of Xe has been reported in a paper by Holland et al. [29]. The s(Pr@Cz+ 82 ) values thus obtained agreed with those in Table 1 within the estimated uncertainties. Although the photoionization cross section of Xe has been measured with a higher degree of accuracy than that of C60, we chose C60 rather than Xe as the standard sample in Eq. (2) for the following two reasons. First, the extrapolation of Twerenbold’s formula to the Xez+ ions gives rise to considerable uncertainties in their relative detection efficiencies, since his formula has been empirically determined for ions with m/z larger than 1500. Second, the molecular beam of C60 was produced in a similar manner to that of Pr@C82, and the number densities of both the samples in the ionization region were evaluated similarly from their sublimation rates.
4. Conclusions We have detected a giant dipole resonance of the encapsulated praseodymium atoms in the photoion yield of the endohedral metallofullerenes, Pr@C82. The present results of Pr@C82, together with the previous
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observation on Ce@C82, clearly showed that the resonance due to the 4d–4f transition does not collapse even within the fullerene cages. We evaluated the absolute photoabsorption cross sections of Pr@C82 at photon energies of 110 and 130 eV and found out that the resonance was reduced by encapsulation. These results agreed qualitatively with the theoretical prediction. Moreover, the present study gives the first evidence asserting that the existence and suppression of the giant resonance is not unique to Ce@C82 but almost universal among metallofullerenes. Acknowledgments We are grateful to the members of the UVSOR for their help during the course of the experiments. This work has been supported by the Joint Studies Program (2004–2006) of the Institute for Molecular Science, by Grants-in-Aid for Scientific Research (Grant Nos. 14340188, 18045031, 18350016, and 17750023) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by a grant for scientific research from Research Foundation for Opto-Science and Technology.
References [1] Chai Y, Guo T, Jin C, Haufler RE, Chibante LPF, Fure J, et al. Fullerenes with metals inside. J Phys Chem 1991;95:7564–8. [2] Shinohara H. Endohedral metallofullerenes. Rep Prog Phys 2000;63:843–92. [3] Heath JR, O’Brien SC, Zhang Q, Liu Y, Curl RF, Kroto HW, et al. Lanthanum complexes of spheroidal carbon shells. J Am Chem Soc 1985;107:7779–80. [4] Wendin G, Wa¨stberg B. Many-electron effects in BaC60-collective response and molecular effects in optical conductivity and photoionization. Phys Rev B 1993;48:14764–7. [5] Wa¨stberg B, Wendin G. Theoretical investigations of relaxation and screening in X-ray-absorption and photoionization of C60 and La, Ba-C60. Nucl Instrum Methods Phys Res B 1994;87:162–6. [6] Connerade JP, Esteva JM, Karnatak RC, editors. Giant resonances in atoms, molecules, and solid, NATO ASI series. New York: Plenum; 1987. [7] Mitsuke K, Mori T, Kou J, Haruyama Y, Kubozono Y. 4d-4f dipole resonance of the metal atom encapsulated in a fullerene cage: Ce@C82. J Chem Phys 2005;122:064304-1–5. [8] Dehmer JL, Starace AF. Potential-barrier effects in photoabsorption. 3. Application to 4d-shell photoabsorption in lanthanum. Phys Rev B 1972;5:1792–6. [9] Libermani DA, Zangwill A. A relativistic program for optical-response in atoms using a time-dependent local density approximation. Comput Phys Commun 1984;32:75–82. [10] Tong XM, Kato D, Watanabe T, Ohtani S. Sharp and window resonances in the 4d photoabsorption spectrum of Eu+ ions. Phys Rev A 2001;64:032716-1-6. [11] Mitsuke K, Mori T, Kou J, Haruyama Y, Takabayashi Y, Kubozono Y. Photoion yield curves of Dy@C82 in the vacuum UV region. Int J Mass Spectrom 2005;243:121–5. [12] Shibata K, Rikiishi Y, Hosokawa T, Haruyama Y, Kubozono Y, Kashino S, et al. Structural and electronic properties of Ce@C82. Phys Rev B 2003;68:094104-1–7. [13] Hosokawa T, Fujiki S, Kuwahara E, Kubozono Y, Kitagawa H, Fujiwara A, et al. Electronic properties for the C2v and Cs isomers of Pr@C82 studied by Raman, resistivity and scanning tunneling microscopy/spectroscopy. Chem Phys Lett 2004;395:78–81. [14] Ding J, Yang S. Isolation and characterization of Pr@C82 and Pr2@C80. J Am Chem Soc 1996;118:11254–7. [15] Akasaka T, Okubo S, Kondo M, Maeda Y, Wakahara T, Kato T, et al. Isolation and characterization of two Pr@C82 isomers. Chem Phys Lett 2000;319:153–6. [16] Rikiishi Y, Kubozono Y, Hosokawa T, Shibata K, Haruyama Y, Takabayashi Y, et al. Structural and electronic characterizations of two isomers of Ce@C82. J Phys Chem B 2004;108:7580–5. [17] Mori T, Kou J, Ono M, Haruyama Y, Kubozono Y, Mitsuke K. Development of a photoionization spectrometer for accurate ion yield measurements from gaseous fullerenes. Rev Sci Instrum 2003;74:3769–73. [18] Kou J, Mori T, Kumar SVK, Haruyama Y, Kubozono Y, Mitsuke K. Double photoionization of C60 and C70 in the valence region. J Chem Phys 2004;120:6005–9. [19] Twerenbold D, Gerber D, Gritti D, Gonin Y, Netuschill A, Rossel F, et al. Single molecule detector for mass spectrometry with mass independent detection efficiency. Proteomics 2001;1:66–9. [20] Krems M, Zirbel J, Thomason M, DuBois RD. Channel electron multiplier and channelplate efficiencies for detecting positive ions. Rev Sci Instrum 2005;76:093305-1–7. [21] Fraser GW. The ion detection efficiency of microchannel plates (MCPs). Int J Mass Spectrom 2002;215:13–30. [22] Mitsuke K, Katayanagi H, Kafle BP, Huang C, Yagi H, Prodhan MSI, et al. Relative partial cross sections for single, double, and triple photoionization of C60 and C70. J Phys Chem A 2007;111:8336–43.
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[23] Mitsuke K, Katayanagi H, Kou J, Mori T, Kubozono Y. Fragmentation mechanism of highly excited C70 cations in the extreme ultroviolet. AIP Conf Proc 2006;811:161–6. [24] Reinko¨ster A, Korica S, Pru¨mper G, Viefhaus J, Godehusen K, Schwarzkopf O, et al. The photoionization and fragmentation of C60 in the energy range 26–130 eV. J Phys B 2004;37:2135–44. [25] Richter M, Meyer M, Pahler M, Prescher T, Raven Ev, Sonntag B, et al. Experimental-study of atomic 4d giant-resonances by photoabsorption and photoelectron-spectroscopy—Sm, Eu, and Gd. Phys Rev A 1989;40:7007–19. [26] Mori T, Kou J, Haruyama Y, Kubozono Y, Mitsuke K. Absolute photoabsorption cross section of C60 in the extreme ultraviolet. J Electron Spectrosc Relat Phenom 2005;144–147:243–6. [27] Henke BL, Gullikson EM, Davis JC. X-ray interactions—photoabsorption, scattering, transmission, and reflection at E ¼ 50–30,000 eV, z ¼ 1–92. At Data Nucl Data Tables 1993;54:181–342. [28] Kafle BP, Katayanagi H, Huang C, Yagi H, Prodhan MSI, Mitsuke K. Absolute total photoionization cross section of C60 in the range of 25–120 eV, revisited. J Phys Soc Jpn 2008;77:xxxx-1-5. [29] Holland DMP, Codling K, West JB, Marr GV. Multiple photoionization in the rare-gases from threshold to 280 eV. J Phys B 1979;12:2465–84.