J. Phys. Chem. 1994, 98, 10671-10673
10671
Laser Study on the Resonance-Enhanced Multiphoton Electron Detachment (REMPED) Processes for c60- and c70Takeshi Kodama Department of Functional Molecular Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444, Japan
Tatsuhisa Kato' Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
Taro Moriwaki, Haruo Shiromaru, and Yohji Achiba* Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan Received: July 11, 1994; In Final Form: August 25, 1994@
The resonance-enhanced multiphoton electron detachment (REMPED) spectra for Cm- and c70- have been obtained. They are regarded as the absorption spectra for these fullerene anions in the gas phase. The REMPED spectrum of a fragment anion produced in the fragmentation process of C70-, having a mass of 720, has been measured. This spectrum shows a threshold identical to that of c60- at ca. 8200 cm-'. Thus the fragment anion is assigned to Cm- with the icosahedral structure. The REMPED spectrum of c 7 0 - shows a broad band in the near-infrared region. The nature of the appearance of this band is discussed in consideration of the ground electronic state for C70-.
Introduction In 1991, we reported an electronic absorption spectrum of Cm- in a y-irradiated glassy polyatomic matrix at 77 K.' The spectrum showed an intense band with vibrational structure in the near-infrared (NIR) region (peak maximum at 1076 nm or 1.15 eV). This band was assigned to the first optically allowed transition of Cm- by calculations',2 and fluorescence experim e n t ~ Smalley . ~ ~ ~ and co-workers also determined the electron affinity (EA) of 2.650 zk 0.050 eV for C W . ~The EA is more than twice the transition energy of exciting Cm- to the first optically allowed electronic excited state. So we pointed out the possibility of an electron detachment process enhanced via the resonant excitation.' Recently, the measurement of this process became possible because an optical parametric oscillator, which is tunable from the NIR to the visible region, has become commercially available. In this report, we present a resonanceenhanced multiphoton electron detachment (REWED) spectrum of Cm-. On the basis of a certain approximation, it is regarded as an absorption spectrum of Cm- in the gas phase. It is generally known that many fragment ion species are detected in the laser time-of-flight (TOF) mass measurement with the high intensity of the ionization laser. The fragment ions have been usually characterized only by their mass number, but the molecular structures were not specified because of lack of spectroscopic information. Thus it is intriguing to carry out a spectroscopic characterization of the ions produced by the fragmentation process. The production of an icosahedral c60in the fragmentation of C70- is examined by the REMPED method in this report. The NIR absorption band around 1400 nm for C70- has been reported by several groups6-* although this band was missed in our measurement using a y-irradiated glassy polyatomic matrix at 77 K.' For the assignment of this NIR band, Lawson and co-workers6 supposed that the ground electronic state of @
Abstract published in Advance ACS Abstracts, October 1, 1994.
0022-365419412098-10671$04.5010
c70- was ?-A'' on the basis of the result of a Hiickel MO calculation for c 7 0 with the D5h stru~ture.~If the ground electronic state of C70- is 2A1", there is no allowed electronic transition in the NIR region. Therefore they assumed that the structure of C70- was distorted from D5h to Cs,, but the driving force for this distortion was not explicitly discussed. On the other hand, Fulara et aL7 presented an alternative assignment based on the result of an ab initio calculation for C70 with the D5h structurelo in which the ground electronic state of c70- was 2El''. To settle this contradiction and to assign this transition, the absorption spectrum of C70- in the gas phase is indispensable. Thus, the REMPED method was applied to c70- and the NIR band was detected in the gas phase. On the basis of this result, a consistent explanation of this NIR band is discussed.
Experimental Section Cm and C70 were prepared by a method similar to that described by Kratschmer et al." Pure c 6 0 or c 7 0 was vacuum vapor-deposited on a copper rod, and the c60- (or c70-) beam was produced by the laser desorption (SHG of Nd:YAG output/ defocused) in a He gas flow. The fragment anion beam was produced by laser vaporization (THG/focused) of C70. The anions passed through an isolated skimmer and entered the acceleration region, where they were given 1 keV kinetic energy in the direction of the beam expansion. In the time-of-flight region, the anions were separated according to their mass and only the selected anions (the selection was accomplished by use of a mass gate) were exposed to the NIR light (Spectra Physics MOPO, pumped by a Q-switched Nd:YAG laser of Spectra Physics) to detach an electron. The typical line width of MOPO's light was about 20 wavenumbers. Its wavelength was monitored by a monochromator with a photodiode. The neutral molecules produced by the electron detachment from the anions were passed through a reflectron and detected with a channeltron electron multiplier. The remaining parent anions were reflected and detected with a microchannel plate. The laser power was monitored with a photodiode. 0 1994 American Chemical Society
Letters
10672 J. Phys. Chem., Vol. 98, No. 42, 1994
Results and Discussion The obtained REMPED spectra are shown in Figures 1-3. The horizontal axis shows the wavenumber of the laser light, and the vertical axis, the relative efficiency of electron detachment from the anion. The latter is estimated from the intensity of the neutral molecules produced by electron detachment normalized by both the intensity of the parent anion and the laser power. As a first approximation, it is assumed that the electron detachment efficiency from the resonant intermediate excited state is almost constant in the investigated region. With this approximation, a REMPED spectrum can be regarded as a gas phase absorption spectrum. As the REMPED spectrum for c 6 0 - (or c 7 0 - ) resembles the absorption spectrum for c 6 0 - (or C70-) in the condensed phase (see below), this approximation is reasonable. The obtained REMPED spectra are rather noisy, which is attributed to two reasons. One is the fluctuation of the parent anion intensity, which is governed by the desorption laser power, the pressure gas flow rate, and the thickness of deposited c 6 0 on a copper rod. The other is the fluctuation of the electron detachment laser power. Cm-. Figure 1 shows the REMPED spectrum of c60-. The absorption spectrum of Cm- in solution reported before12 is also shown for comparison. As expected, the REMPED spectrum of c 6 0 - exhibits a threshold coincident with that of the absorption spectrum in solution. Thus this band is assigned to the first optically allowed electronic transition for Ca- in the gas phase. However, its width is much broader than expected for a gas phase experiment and its origin is seen at a lower energy than reported in the matrix experiments.'~'~These two features may be due to a high temperature of the c 6 0 - beam. In order to get more detailed information about the excited states of Cm-, cooling of the c 6 0 - beam and the two color experiment are indispensable. Taking the EA of c 6 0 into account, the border for two and three photon processes should be located between 10 484 and 10 887 cm-'. No such border is seen in the REMPED spectrum of c 6 0 - (Figure 1). Presumably the broad width because of the high temperature of the c 6 0 - beam hides the border. (260- Produced by the Fragmentation of C70-m Figure 2 shows the REMPED spectrum of the fragment anion with the mass of 720 produced by the fragmentation of C70-, together with the REMPED spectrum of c 6 0 - (Figure 1). Although the two spectra show slightly different thresholds, they can be regarded as essentially identical, since the threshold variation is due to the different temperature of the anions (see below). Thus it is suggested that the fragment anion produced by the fragmentation of C70- is c 6 0 - with the icosahedral structure. This result means that the fragment anion with the mass of 720, the hot C60-, may have a broken network at the beginning of the process due to the five successive C 2 losses from c 7 0 - , but the structure of the fragment anion is relaxed to the icosahedral one during the flight time because of its high stability. It was noticed that the threshold of the REMPED spectrum of c 6 0 - shifted to some extent by the c 6 0 - beam condition, that is by the change of He backing pressure or of the distance between the nozzle and skimmer. This means that the threshold position depends on the temperature of the c 6 0 - beam. As the fragment anion produced by the fragmentation of c 7 0 - should have a much higher temperature than Cm- obtained by the beam expansion after laser desorption, it is reasonable that the threshold of the spectrum for the c 6 0 - fragment shifts to lower energy. c70-. The REMPED spectrum of C70- exhibits a broad band around 7700 cm-' (1300 nm), as shown in Figure 3. The spike around 7200 cm-l originated from the sudden drop of detach-
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Figure 1. REMPED spectrum of Cm- (squares) and absorption spectrum of Ca- in solution (solid line). a .
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Figure 2. REMPED spectrum of the fragment anion with the mass of 720 produced by the fragmentation of C~O(squares) with the REMPED spectrum of Cm- shown in Figure 1 (circles).
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Figure 3. REMPED spectrum of C~O-. The spike around 7200 cm-' originated from the sudden drop of detachment laser power. ment laser power, Le. a result of the division by very small laser power. Several groups have reported the absorption spectra with the band around 1400 nm for c 7 0 - in matrices7%* and in solution.6 Because the band position of the REMPED spectrum coincides with the reported ones, the spectrum is regarded as the absorption spectrum of C70- in the gas phase. From a Hiickel MO calculation for neutral c 7 0 ? the ground electronic state of c 7 0 - is expected to be 2A1''. On the other hand, from a SCF-HF calculation with basis sets of double-c plus polarization for neutral C7O,l0 it is predicted as *El". Kobayashi and Nagase showed that the electronic ground state was 2A1" at the optimized geometry in the AM1 calculation but changed to 2E1'' in the HF STO 3-21G calculation and that the energy difference between 2El'' and 2A1" was very ~mal1.I~ These results mean the ground electronic state symmetry depends upon the calculation method adopted. Thus two assignments for the NIR band could be possible for each ground state. First, in the case of the 2El" electronic ground state, there is an allowed transition 2E1' *El'' in the NIR region, as Fulara et al. pointed Furthermore, as the ground electronic state is degenerate, the structure of C70- must be distorted from D5h to C2" or C, by the Jahn-Teller effect, and this symmetry lowering of the structure increases the number of allowed transitions. In the case of the 2A1" electronic ground state, there
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Letters is no allowed transition in the NIR region and the intensityborrowing mechanism is the only way to explain the band. In general, the absorption intensity is small in the intensityborrowing mechanism. However, judging from the rather large value 4000 M-' cm-' of the reported molar absorption coefficient, the first explanation, that the electronic ground state has the 2El'' symmetry, is reasonable.
Acknowledgment. The authors thank Professor S . Nagase for valuable discussions. T.K. is indebted to the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. This work was supported by the Grantsin-Aid for Scientific Research on Priority areas (No. 05233108 and No. 05233 110) from the Ministry of Education Science and Culture. References and Notes (1) Kato, T.; Kodama, T.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromm, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991,180, 446.
J. Phys. Chem., Vol. 98, No. 42, 1994 10673 (2) Heath, G . A.; McGrady, J. E.; Martin, R. L. J. Chem. Soc., Chem. Commun. 1992,1272. (3) Kato, T.;Kodama, T.; Shida, T. Chem. Phys. Lett. 1993,205,405. (4) Kato, T.Laser Chem. 1994,14, 155. (5) Wang, L.-S.; Conceicao, J.; Jin, C.; Smalley, R. E. Chem. Phys. Lett. 1991,182, 5. (6) Lawson, D. R.; Feldheim, D. L.; Foss, C. A,; Dorhout, P. K.; Elliott, C. M.; Martin, C. R.; Parkinson, B. J. Phys. Chem. 1992,96, 7175. (7) Fulara, J.; Jakobi, M.; Maier, J. P. Chem. Phys. Lett. 1993,206, 203. ( 8 ) Hase, H.; Miyatake, Y. Chem. Phys. Lett. 1993,215, 141. (9) Fowler, P. W.; Woolrich, J. Chem. Phys. Lett. 1986,127, 78. (10) Scuseria, G. E. Chem. Phys. Lett. 1991,180,451. (11) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990,347, 354. (12) Kato, T.; Kodama, T.; Oyama, M.; Okazaki, S.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromm, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991,186,35. (13)Fulara, J.; Jakobi, M.; Maier, J. P. Chem. Phys. Lett. 1993,211, 227. (14) Kobayashi, K.; Nagase, S. Personal communication.
