J . Phys. Chem. 1993,97, 2500-2504
2500
Multiphoton Ionization of C ~ O D. Ding,+ R. N. Compton,' R. E. Haufler, and C. E. Klots Chemical Physics Section, Oak Ridge National Laboratory, Post Office Box 2008, Oak Ridge, Tennessee 37831 -61 2S Received: December 22, 1992
The multiphoton ionization photoelectron spectrum (MPI-PES) of C ~using O pulsed lasersof nanosecond duration shows prompt electrons resulting from direct two-photon ionization, as well as a dominant low-energy delayed electron signal which follows an approximately thermal energy distribution with average energy: = 0.30 f 0.05 eV. The MPI-PES together with the mass spectrum a t X = 193 nm suggests an important role of intersystem crossing from the singlet to triplet manifold of C ~ OMPI . a t X = 193 nm is believed to result from such intersystem crossing followed by one-photon ionization of triplet Coo. Under this assumption, the photoionization cross section for C ~ inO the lowest triplet state is estimated to be (5.1 f 2.0) X IO-'' cm-?.
Multiphoton ionization (MPI) of certain clusters in a nanosecond pulsed laser field has been shown to occur as a result of multiple-photon absorption leading to the ejection of electrons which are "delayed" (greater than microseconds) relative to typical ionization times ( s). "Delayed" ionization was first reported for transition-metal oxide clusters1and transition-metal cluster^.^,^ Nieman et a1.I have proposed a model of "thermionic emission" driven by multiphoton absorption. Ionization of the icosohedral C6O molecule by laser photons has also been shown to exhibit both direct and delayed ionization processes. The 308nm time-of-flight (TOF) mass spectrum for c 6 0 of Campbell et aLJ showed asymmetric peaks with long tails extending to higher mass which corresponded to longer flight times. The tail was shown to originate from molecules losing their electrons long after the laser pulse, Le., "delayed ionization". Campbell et aL4 ascribed the delayed ionization to "thermionic emission", by analogy with electron emission from heated metals or semiconductors.' Maruyama et a1.h had earlier reported thermionic emission from carbon cluster positive ions, producing multiply charged ions in the size range from 150 to 600 atoms. It is generally believed that delayed ionization for large molecules is related to a process involving the coupling between electronic and vibrational degrees of freedom, akin to vibrational autoionization for small molecules. Interesting questions arise when one considers the photoionization of large clusters: at which cluster size can one consider vibrational autoionization as thermionic emission and under what conditions does such delayed ionization dominate? Smalley and co-workers5haveargued that, for spherical carbon clusters of 300 atoms, the molecular orbitals begin to approximate the surface band structure of the basal plane ofgraphiteand that a thermionic emission model might be appropriate. On the other hand, details of the photoionization dynamics of the delayed ionization remain ambiguous. Under certain conditions, photoionization of C60 does not lead to delayed emission. For example, delayed ionization has been found to be absent when using 118-nm (hv = 10.5 eV) laser pulses of IO-ns duration to ionize C60.7 Zhang et aL8 have also found that there is no delayed ionization of C60using subpicosecond (7 15 fs) laser excitation/ionization at 248 nm. In this study, we report the MPI of a thermal effusive beam ( T = 350-500 " c ) of C6O using a Nd:YAG pumped dye laser system employing a WEX wavelength extender (266 and 355 nm and tunable UV laser around 287 nm). The pulse duration was 20r 8 ns. An ArFlaserwasalsoused togenerateshort wavelength
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' On leave from Jilin University. Changchun 130023, P. R. China. * To whom correspondence should be addressed. Also Department of Chemistry, The University of Tennessee at Knoxville.
0022-365419312097-2500$04.00/0
(193 nm) IO-ns laser pulses whose photon energy (6.42 eV) is below the ionization potential (IP) of C60 (7.6 eV)9 but above the IP of the lowest triplet state of C ~ (-5.9 O eV).l0 The kinetic energies of the electrons resulting from direct and delayed ionization processes were obtained by photoelectron spectroscopy (PES). The advantages of using PES in MPI studies have been pointed out by Kimura' I andothers.12 We employ herea simplevariation of our previous studies (see ref 12) in which photoelectrons can be analyzed under field-free conditions either by collection in a small solid angle ( 1 X 1E4rad) of a spherical sector electrostatic energy analyzer or by using an electric field to gently 'push" the electrons into the energy analyzer during the laser pulse to resolve the prompt electrons or with a controllable delay following the laser pulse to resolve the delayed electrons. Separation of the direct and delayed ionization processes is accomplished by first applying a small dc field (-2.7 V/cm) biased to repel the direct electrons away from the entrance of the analyzer. A small (-4 V/cm) delayed (0-20 ps) pulse of I - p s duration is then used to draw the delayed electrons into the energy analyzer with a small ( - 1 eV) energy in addition to their own intrinsic energy. The positive ion mass spectra are also measured with a miniaturized reflectron TOFMS, giving additional information on the size distribution of the ionic fragments from c 6 0 as well as providing the characteristic delayed ionization signature in the TOF mass spectrum. The mass spectra were similar for various UV wavelengths produced by the Nd:YAG and dye laser system. At low laser fluence, C ~ Zto+ C60+ ions predominated, with the characteristic TOF-delayed ionization tail occurring on the C6o+ TOF signal. At higher laser fluence, fragment ions in the region of C+ to C12+ as well as C3:+ to CS8+were observed, and the Ch0+peak again exhibited a long tail. The tail of the c60+ TOF peak indicates the occurrence of delayed ionization of c 6 0 as observed previ0usly.4.~ Similar mass spectra have been reported earlier." It is important to note that when the 193-nm laser was used, a TOFMS spectrum very different from that described above was obtained: At low laser fluence, only the C ~ O peak ' was produced with almost no fragment ions (c70 impurity ions appeared at low abundance), and at higher laser fluence, minute amounts of small carbon cluster ions were produced. Surprisingly, neither ChO+nor C70+peaks showed tails toward higher TOF, even under very high 193-nm laser fluence (for example >300 mJ/cm*), indicating that no delayed ionization was occurring. Since the photon energy (hv = 6.42 eV) was not large enough to reach the IP from the ground-state 'A, of ChO,multiple-photon absorption is still necessary for ionization of the ground state, just as at the longer wavelengths. Figure 1 shows the remarkable
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0 1993 American Chemical Society
Letters
The Journal oJPhysical Chemistry, Vol. 97, No. 11, 1993 2501
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Mass (amu) Figure 1. TOF mass spectra obtained by (top) 355 nm, 800 mJ/cm2, and (bottom) 193 nm, 2 3 0 0 mJ/cm2 The fragment distributions C,, (n = 30-60) are similar for both wavelengths For 193 nm, the delayed ionization is absent for C,,Oand C70 even under the condition of tight focusing TheappearanceoftheC711f peakin thec'iseof 193 nm indicates the ionization yield of C711(which comes from the sample of ChO/C70 mixture) is little bigger at this wavelength - __
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Figure 2. Observation of (2 + I) R E M P I of fragment C atoms resulting from Chllvia the two-photon allowed transitions 2p3p -'DJ,, 2p? 'DJ (.I"-.I?.
difference in the mass spectra obtained by 355 and 193 nm. Both spectra exhibit similar fragmentation patterns in the region from Ch0+to C32+,except that there is no evidence for delayed ionization of ChOor C70 at 193 nm. C70is present in the beam at a low level (- 2% at the temperature of 550 "C), and perhaps its higher ionization efficiency at 193 nm overrepresents its abundance relative to Ch0. Using a tunable UV laser in the region of 287 nm (hv = 4.32 eV), (2 + 1) resonance-enhanced multiphoton ionization (MPI) of carbon atoms occurs and was observed through one of the two-photon allowed transitions 2p3p 3D~9, 2p2 -'PJ,as shown in Figure 2. There is no enhancement for the ionization observed via the two-photon resonance 2p4p ' P I from the metastable level 2p2 ID2 (1.3 eV above the ground level 2p? 3Po),although this
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Figure 3. MPI-PES of Ch0 under field-free condition with a tunable UV laser. The laser wavelength is on (top) and off (bottom) resonance with the two-photon transition 2p3p ID1 2p2 3Pz of atomic carbon. The peak at 1.69 eV in (top) is from the (2 + I ) R E M P I of atomic carbon.
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two-photon transition is well established in multiphoton dissociation/multiphoton ionization of different carbon-containing molecules (for example, aromatic molecule^,'^ C302,isCC14,'6 and CH3COCH317).The carbon atoms were also determined to possess an approximately thermal kinetic energy distribution by measuring the energy of the mass-analyzed C+ ions (TOF) produced from the (2 1) REMPI of C atoms using the electrostatic energy analyzer. These observations would suggest that the carbon atoms result from dissociation or "boiling off" from carbon clusters, perhaps from Cbo or smaller fragment clusters which no longer possess the closed fullerene structure. The abundance of carbon atoms produced when using the Nd: YAG laser wavelengths for ionization clearly shows that all dissociation mechanisms do not produce even number fragments, although C,+ ( n odd) ions are very weak in the c 6 0 to c32 range. The C2 loss mechanism applies only to closed fullerenes.I8 Structures which are not closed fullerenes will have numerous fragmentation pathways available, including the loss of carbon atoms, C3, etc. Additionally, carbon atoms could be produced upon dissociation of C32 since there are no fullerene structures available through Cz loss fragmentation mechanisms for C32. Another possibility for C atom production has been presented by Gaber et aLi9 These authors provide evidence for laser-induced fission of Cbointo carbon clusters with a bimodal mass distribution similar to that for nuclear fission. Their observation of small clusters C, with odd n is believed to involve the elimination of a C or C3 unit. Figure 3 shows the MPI-PES of C hunder ~ field-free conditions with the UV laser wavelength tuned on and off the resonance corresponding to the (2 + 1) REMPI of carbon atoms. The peak at 1.7 eV results from the ( 2 + 1 ) REMPI of carbon atoms and serves as an accurate calibration for the energy scale of the PES. The primary peak at -0.8 eV is believed to result from direct two-photon ionization of Cho. Since the IP of Ch0is known to be 7.6 eV,' direct two-photon ionization of Ch0 leaving no change in ro-vibrational state of the ion would produce photoelectrons of 0.95 eV. The PES spectrum also shows a series of peaks starting
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Letters
2502 The Journal of Physical Chemistry, Vol. 97, No. 11, I993 100
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Equation 2 represents the electrron energy distribution for thermionic emission from a bulk material, which can be derived from the free-electron theory for both metals and semiconductor^.^ 2kT. In The average kinetic energy in this case should be order to fit the theoretical energy distributions in eqs 1 and 2 to the experimental measurements, it is necessary to mathematically fold in the instrumental resolution of the electron energy analyzer.23 The energy resolution function was determined from the width of the 1.7-eV peak from carbon atoms in Figure 3 (-0.08 eV). The solid and dashed lines in Figure 4 represent fits to eq 1 with and without including the instrumental factor, respectively. Using eq 1 or 2, the best fit of the experimental data gives kT = 0.32 f 0.05 or 0.14 f 0.04 eV, respectively. According to the same development22which leads to eq 1, this temperature is given by
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p(e) ( e 2 / b e) exp(-c/kT) where e is the charge on an electron and b is the classical collision radius for the cluster. In the case of c60, e 2 / b 4.0 eV, and so this distribution gives the average kinetic energy to be kT. When the size of the cluster approaches infinity, eq 1 becomes
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E l e c t r o n Kinetic Energy (eV) Figure 4. MPI-PES of C ~obtained O with a small draw-out electric field (2.7 V/cm), using a I-ps collection window beginning with the laser pulse. The best fit of the kinetic energy distribution for thermionic emission from a large molecule (dash line) is given by using eq 1. A calculated distribution of electron including the instrumental factor is also shown by a solid line. The peak marked by an asterisk is from (2 + 1) REMPI of carbon atoms and is used to calibrate the energy scale of the PES.
at -2.6 eV extending down to the direct two-photon ionization peak. These photoelectrons could result from photoionization of neutral fragments; however, it is possible that this series of peaks results from two-photon ionization of c60 in the lowest triplet state (-1.7 eV above the ground state 'A,)Io produced by intersystem crossing. Note that the progression begins at -2.6 eV, which is 1.8 eV above the direct ionization peak at -0.8 eV. The progression of peaks together with the peak a t 1.1 eV is consistent with leaving the c60+ ion in various states of rovibrational or possibly electronic excitation. Very slow (
