The Astrophysical Journal, 548:L225–L228, 2001 February 20 q 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
VIBRATIONAL SPECTRA OF HYDROGENATED BUCKMINSTERFULLERENE: A CANDIDATE FOR THE UNIDENTIFIED INFRARED EMISSION C. R. Stoldt,1 R. Maboudian,2 and C. Carraro3 Department of Chemical Engineering, 201 Gilman Hall, University of California, Berkeley, CA 94720-1462 Received 2000 October 27; accepted 2000 December 4; published 2001 February 16
ABSTRACT A mixture of hydrides of buckminsterfullerene made by the interaction of atomic hydrogen with C60 is investigated using high-resolution electron energy loss spectroscopy (HREELS) in ultrahigh vacuum. The energy-loss spectra of partially hydrogenated C60 multilayers reveal vibrational features previously observed in infrared emission from interstellar and circumstellar dust clouds, including a broad loss envelope between 1150 and 1310 cm21, followed by a band at 1620 cm21 in remarkable agreement with the canonical interstellar spacing of 300 cm21. Additionally, a major C i H stretching band near 2900 cm21 is observed and compared with the Galactic center absorption spectrum. Subject headings: dust, extinction — infrared: ISM: lines and bands — methods: laboratory Strictly speaking, fullerenes cannot be considered aromatic molecules. With PAHs, they share a polycyclic structure with predominant sp2 bonding, yet the curvature of the C60 cage forces a small admixture of sp3 character, similar to amorphous grains. Interestingly, their molecular size falls in a range in which PAH molecules have not been probed (to our knowledge, IR spectra are available only for PAHs having less than 50 carbon atoms). Webster (1993b) calculated the IR-active normal modes, nF1u (n p 1…9), for fully hydrogenated C60, the frequencies of which are shown in Table 2. Interestingly, these calculations indicate that C60H60 has multiple C i H stretching bands near 2900 cm21, an important requirement for any diffuse medium candidate. Other theoretical studies (Dunlap et al. 1991; Bakowies & Thiel 1992) of C60H36 and C60H60 also predict multiple C i H stretching modes in this frequency range. Additionally, laboratory IR spectra yield multiple bands near 2900 cm21 for C60H36 (Haufler et al. 1990; Bensasson et al. 1997) and C60Hn (n ≤ 17; Howard 1993). These spectra, however, yield only limited information, especially in regard to the lower frequency part of the IR spectrum. Furthermore, fullerene hydrogenation is achieved by synthetic means, implying that the hydrogen atoms are presumably constrained to the outside of the carbon cage. It is interesting to ask whether hydrogenation by energetic H atom bombardment (which is an astrophysically relevant mechanism) can lead to additional endohedral configurations, which may result in added, possibly blueshifted C i H stretching bands.
1. INTRODUCTION
It has long been known (Gillett, Forrest, & Merrill 1973; Russell & Soifer 1977) that interstellar and circumstellar dusts show emission features ranging from 3300 cm21 to below 800 cm21. Sharp IR bands are observed at 2920, 1610, and 880 cm21, with a broad, intense emission envelope near 1300 cm21. Additional features include a recurrent mode at 3050 cm21, a weak mode near 1450 cm21, and distinct shoulder near 1150 cm21. More recently, a small but increasing number of carbon-rich astronomical objects reveal an unidentified emission feature in the far-IR at 490 cm21 (Kwok, Volk, & Hrivnak 1989). It is now generally accepted that most of these features are due to IR fluorescence from large, carbon-rich molecular species. Typical assignments (Allamandola 1990) for these vibrational modes are summarized in Table 1. Nearly all research (Williams 1996) has focused on polycyclic aromatic hydrocarbons (PAHs), hydrogenated amorphous carbon, and carbon grains as potential carriers. These species all reproduce various interstellar absorption features in the mid IR. Soon after the initial discovery of C60 (Kroto et al. 1985) and the development of laboratory methods for synthesizing it in bulk quantities (Kra¨tschmer et al. 1990), theoretical work (Webster 1991, 1993b) suggested hydrogenated C60 as a potential candidate for unidentified interstellar emission features. Yet, few experimental efforts (Webster 1993a, 1997) have been undertaken to substantiate this claim. Here, we present the first comprehensive experimental effort to characterize the vibrational modes of partially hydrogenated C60. Any hydrogenated candidate for the unknown IR emission lines must be stable to the harsh conditions of the diffuse medium. Prolonged interaction with UV photons and atomic hydrogen requires a species stable to ionization as well as hydrogenation. A large, symmetric molecule such as C60 is an ideal candidate, with these characteristics motivating early theoretical and experimental work. Although one can question whether fullerenes and fulleranes are present in sufficient abundance to account for the observed IR emission features, the chemical properties of these molecules make them especially interesting case studies for hydrogenated carbon clusters. 1 2 3
2. EXPERIMENTAL METHODS
The experiments reported here are performed in an ultra–high vacuum chamber with a base pressure below 2 # 10210 torr. C60 multilayer films are deposited on (2 # 1) reconstructed Si(100) crystals held at 300 K. C60 is evaporated from a home-built Knudsen cell–type source operated at approximately 665 K; deposition times ranged from 5 to 8 minutes with the base pressure remaining below 2 # 1029 torr. Subsequent to deposition, C60 films are annealed to 450 K for 5 minutes, then cooled to 300 K for hydrogen exposure. Atomic hydrogen is generated by the dissociation of H2 at a spiral tungsten filament heated to 1850 K. The sample is positioned 4 cm from the hot filament. All hydrogen exposures are given in terms of molecular hydrogen exposures and reported in lang-
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TABLE 1 Unidentified Astronomical IR Emission Frequencies and Their Typical Assignments Astronomical n (cm21) 3050 . . . . . . . . . . . . . 2800–3000 . . . . . . 1620 . . . . . . . . . . . . . 1450 . . . . . . . . . . . . . 1250–1300 . . . . . . 1150 . . . . . . . . . . . . . 1050–1810 . . . . . . 885 . . . . . . . . . . . . . . 490 . . . . . . . . . . . . . .
Typical Assignment of Vibrational Mode 2
sp aromatic C i H stretching sp3 aliphatic C i H2,3 stretching Aromatic C i C stretching Aromatic C i C stretch, aliphatic C i H deformation Blending of several strong aromatic C i C stretching bands Aromatic C i H in-plane bend Blending of several weak aromatic C i C stretching bands Aromatic C i H out-of-plane bend Unknown
muirs [1 langmuir (L) { 1026 torr s]. High-resolution electron energy loss spectroscopy (HREEL) spectra are obtained using an incident electron energy of 6 eV at an incident angle of 607 toward the surface normal and with the sample temperature held at 120 K. Off-specular data are taken at 87 and 107 from the specular direction. 3. RESULTS AND DISCUSSION
Before proceeding, it is important to distinguish HREELS from IR and other optical spectroscopic methods. In a HREELS experiment, a highly monoenergetic beam of electrons is directed toward a surface. The energy spectrum of the scattered electrons is detected at a given detection angle, with the energy loss being characteristic of electron-stimulated vibrations of surface bound species. Generally, two inelastic scattering mechanisms are operative in any HREEL spectra. The long-range electric dipole scattering mechanism contributes to the energy loss in an angular lobe centered about the specular deflection angle. In contrast, the short-range impact scattering mechanism gives rise to losses that depend weakly on the scattering angle, and therefore this mechanism dominates the HREEL spectrum in the off-specular directions. Each mechanism is equally important, and both must be considered to fully characterize the surface vibrational spectrum. It must therefore be kept in mind that the method of excitation in HREELS is inherently different from optical spectroscopies where impact scattering is not operative, and the resulting vibrational spectra, while similar, cannot be compared directly. We now turn to the normalized HREEL spectra of clean and hydrogenated C60 multilayer films shown in Figure 1, plots (a)–(d). Looking first at the spectrum of the clean C60 multilayer (Fig. l, plot [a]), four first-order IR-active vibrational modes are resolved at approximately 1425, 1180, 575, and 520 cm21. These modes are used as a positive identification of the C60 molecule (Dresselhaus, Dresselhaus, & Eklund 1996). Eight
Present Experiment … Observed Observed Observed Observed at 1310 cm21 Observed Observed between 1060 and 1500 cm21 Observed Observed
first-order Raman-active vibrational modes are resolved near 1570, 1425, 1250, 1100, 765, 710, 430, and 265 cm21, with additional higher order IR and Raman modes at 1330, 960, and 335 cm21. Further discussion on the spectroscopic characteristics of unhydrogenated C60 is detailed by Dresselhaus et al. (1996, and references therein). Hydrogen exposure dramatically changes the vibrational spectrum of C60, as seen in Figure 1, plots (b)–(d). New loss features grow in intensity with increasing hydrogen exposure, most readily observed as an intense peak centered at 2900 cm21, a broad band between 1425 and 1060 cm21, and two smaller bands near 2490 and 1620 cm21. Additional loss peaks grow in intensity at 880 and 480 cm21, while the peaks at 765 and 525 cm21 slightly decrease in intensity with increasing hydrogen exposure. We begin by identifying the source of the loss peaks not associated with observed astronomical spectral features, specifically the peaks seen at 2490, 1220, 765, 575, and
TABLE 2 Calculated C60H60 IR Normal Mode Frequencies, Assignments and Mode Types, and Position of Closest Loss Feature Observed in the Present Experiment Theoretical C60H60 n (cm21)
C60H60 Assignment, Mode Type
Present Experiment (cm21)
2912 . . . . . . . . . . . . . . . . 2907 . . . . . . . . . . . . . . . . 1612 . . . . . . . . . . . . . . . . 1450 . . . . . . . . . . . . . . . . 1299 . . . . . . . . . . . . . . . . 1163 . . . . . . . . . . . . . . . . 885 . . . . . . . . . . . . . . . . . 487 . . . . . . . . . . . . . . . . . 435 . . . . . . . . . . . . . . . . .
9F1u, C i H stretching 8F1u, C i H stretching 7F1u, complex 6F1u, complex 5F1u, C i H bending 4F1u, complex 3F1u, complex 2F1u, complex 1F1u, structural
2900 2900 1620 1425 1310 1150 880 480 …
Fig. 1.—HREEL spectra of C60 multilayer films following (a) no hydrogen exposure, FWHM p 36.5 cm21; (b) a 45 L hydrogen exposure, FWHM p 34.8 cm21; (c) a 180 L hydrogen exposure, FWHM p 40.4 cm21; and (d) a 1000 L hydrogen exposure, FWHM p 60.4 cm21.
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525 cm21. The peaks at 765, 575, and 525 cm21 are attributed to vibrational modes originating from the unexposed C60 multilayer lying beneath the uppermost hydrogenated layer(s). This is supported by off-specular HREEL data, which indicate that these modes are mainly dipole active and will radiate at long distances perpendicular to the surface. The off-specular data also shows that the intense mode at 1220 cm21 is primarily impact and not dipole excited; thus, it should not be prominent in IR emission spectra. Finally, the mode centered at 2490 cm21 is due to a multiple loss of the modes between 1450 and 1150 cm21. Multiple scattering is an electron-stimulated process and, consequently, is also unobserved in optical spectroscopies. Before comparing our HREEL vibrational spectra with the calculated spectrum for C60H60 and astronomical spectral features, it is important to note that the data shown in Figure 1, plots (b)–(d) likely represent only partially hydrogenated C60. Because of the random nature of hydrogen bombardment, we expect a surface composed of C60 molecules with varying degrees of hydrogenation. Similarly, a mixture of hydrides is expected to be present locally in the interstellar medium because of the competition between processes that attach and remove hydrogen atoms. Furthermore, the average degree of hydrogenation in interstellar and Galactic clouds will vary as well, depending on their distance from the exciting star (Geballe et al. 1989). In Figure 1, plots (b)–(d), loss features measured at 2900, 1620, 1425, 1310, 1150, and 880 cm21 compare well with the nF1u IR-active normal modes calculated for C60H60, shown in Table 2. Somewhat obstructed by the IR-active C60 mode at 520 cm21, a shoulder grows in intensity with hydrogen exposure near 480 cm21, which is similar in frequency to the calculated 2F1u normal mode of C60H60. Interestingly, this mode arises at nearly the same frequency as the unidentified mode observed in carbon-rich astronomical objects mentioned previously. In fact, the experimental loss features attributed to hydrogenated C60 are comparable in wavelength to the unidentified IR emission features listed in Table 1. Candidate experimental spectra should also reproduce two important characteristics found in most diffuse medium spectra. The 1300 cm21 band should be more intense than the 880 cm21 band (Williams 1996), and the spacing between it and the next main emission feature should approximate the “canonical interstellar spacing” of 300 cm21 (Hudgins & Allamandola 1999). Modeling of astronomical spectra with combinations of neutral and ionized PAHs, for example, fails to reproduce the latter requirement using small-sized PAHs, suggesting that molecules composed of 50–80 carbon atoms are the dominant diffuse medium emitters (Hudgins & Allamandola 1999). The HREEL spectra shown in Figure 1, plots (b)–(d) satisfy both the aforementioned spectral prerequisites. Note also that the position of the 1310 cm21 band does not shift with increasing hydrogen exposure, lending support to a conjecture that this band may be considered a signature for a wide range of buckminsterfulleranes (Webster 1993b). Next, we examine the HREEL peak centered at 2900 cm21, attributed to the C i H stretching modes of hydrogenated C60. In Figure 2, a close-up of this band following a 1000 L hydrogen exposure is shown superimposed upon the C i H stretching band observed toward the Galactic center source IRS 6E (Pendleton et al. 1994). Two important characteristics of these spectra must be noted. First, the HREELS peak maximum is centered near 2900 cm21, while the IR emission spectrum from IRS 6E shows a maximum near 2925 cm21. Second, although the peak maxima differ, the C i H stretching band in
Fig. 2.—HREEL spectrum of the C i H stretching band of C60 after a 1000 L hydrogen exposure (solid line) compared with the C i H stretching band observed toward Galactic center source IRS 6E. Open and filled circles refer to high- and low-resolution observations, respectively. The FWHM of the HREEL experiment is 60.4 cm21, about twice that of the astronomical spectrum.
both spans approximately the same frequencies, between 2800 and 3000 cm21. Quantitatively, many factors can contribute to the discrepancy between experimental peak positions and those observed in interstellar and circumstellar clouds, including instrumental resolution and the temperature at which our experiments are carried out (which is significantly higher than the typical temperature of interstellar dust). Qualitatively, the discrepancy can be understood when the off-specular HREEL spectrum is analyzed. Here, we find that the total integrated intensity of the C i H stretching band becomes redshifted toward the peak maximum at 2900 cm21, indicating that these lower frequency modes are primarily excited by the impact scattering mechanism and will emit less strongly under IR radiation. The higher frequency modes lying above 2900 cm21 lose intensity at off-specular scattering angles, indicating that they are mainly dipole active and will emit more strongly under IR radiation. Therefore, the IR radiation emitted from hydrogenated C60 should be most intense at higher frequencies near 2950 cm21, with a slight decrease in intensity at frequencies approaching 2900 cm21. This is indeed the trend seen in the astronomical data shown in Figure 2. Last, we comment briefly on the identity of the carrier of the IR emission line at 3050 cm21. Webster (1991) proposed that a lightly hydrogenated fullerane such as C60H or C60H1 could carry the 3050 cm21 line, since the predominant bonding is aromatic throughout. For low hydrogen exposures, however, our HREEL spectra in Figure 1 show only a broad band centered at 2900 cm21 and extending only up to 3000 cm21. For exposures greater than 2000 L, we detect the formation of aliphatic CH2 and CH3 bending modes in our HREEL spectra, possibly indicating that bond cleavage occurs with extended hydrogen bombardment. Therefore, a possible source of the 3050 cm21 mode may be aromatic C60 fragments liberated during extensive atomic hydrogen exposure in the diffuse medium. Financial support of the National Science Foundation, the Arnold and Mabel Beckman Foundation, DARPA, and the Hitachi Advanced Research Laboratories is gratefully acknowledged.
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