3566 J. Phys. Chem. 1992, 96, 3566-3568 Vaporlzation Studies on Buckminsterfullerene C. K. Mathews,* M. Sai Baba, T. S. Lakshmi Narasimhan, R. Balasubramanian, N. Sivaraman, T. G. Srinivasan, and P. R. Vasudeva Rao Radiochemistry Programme, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India (Received: December 30, 1991) A Knudsen cell mass spectrometric study of pure Ca was carried out in the temperature range 600-800 K to obtain the vapor pressure and enthalpy of sublimation. The measured appearance potential for Ca+ (8.1 f 0.5 V) is in good agreement with the recommended value (7.6 * 0.2 V) for the ionization potential of Ca. The enthalpy of sublimation was found to be 181.4 f 2.3 kJ/mol at 700 K by the second law method. The vapor pressure of c60 is given as a function of temperature by the equation log @/Pa) = -9777 * 138/T (K) + 11.582 * 0.126. Introduction A simple method for the preparation of bulk quantities of fullerenes, first reported by Kratschmer et al.' a year ago, has opened up an exciting area of research on the physical and chemical properties of these fascinating molecules. The inter- national interest is reflected in the large number of reports on the spectroscopic (IR, UV-vis, Raman, and NMR), electrochemical, and structural (XRD) properties of Cm as well as on their de- rivatives and compounds. However, very limited attention has been given to thermodynamic properties. There have been two measurements on heats of s~blimation,~.~ but both of them were made on mixtures of fullerenes. Further, no vapor pressure data have been reported. In this paper we report for the first time the vapor pressure of pure Ca as a function of temperature as well as its heat of sublimation. The principal technique used in this study is Knudsen cell mass spectrometry, which has earlier been employed in our laboratory in our studies on selenium and tel- lurium clusters4 and on the vaporization thermodynamics of metal tellurides. 5-9 Experimental Section The procedure used for the preparation of C60 was similar to the contact arc method described by Haufler et a1.I0 The arc was struck between two graphite electrodes in a helium atmosphere of 200 Torr, and the graphite soot collected was subjected to Soxhlet extraction by using toluene or carbon tetrachloride as the solvent. The extract was evaporated using a rotary evaporator to obtain solid samples consisting mainly of Ca and C70. This mixture was separated by column chromatography by using a neutral alumina column, and Cm was eluted with hexane. The samples were characterized by HPLC, UV-vis as well as IR spectroscopy, and XRD. The details of characterization are given (1) Krastchmer, W.; Lamb, L. D.; Forstiropoulos, K.; Hauffman, D. R. Nature 1990, 354, 347. (2) Pan, C.; Sampson, M. P.; Chai, Y.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. 1991, 95, 2944. (3) Mathews, C. K.; Vasudeva Rao, P. R.; Srinivasan, T. G.; Ganesan, V.; Sivaraman, N.; Lakshmi Narasimhan, T. S.; Kaliappan, I.; Chandran, K.; Dhamcdaran, R. Curr. Sci., in press. (4) Viswanathan, R.; Sai Baba, M.; Darwin Albert Raj, D.; Balasubra- manian, R.; Mathews, C. K. In Advances in Mass Spectrometry; Todd, J. F. J., Ed.; John Wiley & Sons: New York, 1985; Vol. 10, p 1087. (5) Saha, B.; Viswanathan, R.; Sai Baba, M.; Darwin Albert Raj, D.; Balasubramanian, R.; Karunasagar, D.; Mathews, C. K. J. Nucl. Mater. 1985, 130, 316. (6) Sai Baba, M.; Viswanathan, R.; Darwin Albert Raj, D.; Balasubra- manian, R.; Saha, B.; Mathews, C. K. J. Chem. Thermodyn. 1988,20, 11 57. (7) Viswanathan, R.; Sai Baba, M.; Darwin Albert Raj, D.; Balasubra- manian, R.; Saha. B.; Mathews, C. K. J. Nucl. Mater. 1987, 149, 302. (8) Viswanathan, R.; Sai Baba, M.; Darwin Albert Raj, D.; Balasubra- manian, R.; Saha, B.; Mathews, C. K. J. Nucl. Mater. 1989, 167, 94. (9) Viswanathan, R.; Balasubramanian, R.; Mathews, C. K. J. Chem. Thermodyn. 1989, 21, 11 83. (10) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byrne, N. E.; Flanagan, S.; Haley, M. M.; OBrien, S. C.; Pan, C.; Xiao, Z.; Billups, W. E.; Caufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.; Smalley, R. E. J. Phys. Chem. 1990, 94, 8634. else~here.~ The purity of the substance was also checked by mass spectrometry. A VG Micromass 30BK mass spectrometer (electron impact ion source, single focusing, 90° sector magnetic analyzer) was used for vapor pressure measurements. The molecular beam effusing out of the Knudsen cell was ionized by electrons of 38-eV energy. The ions were accelerated to 3 kV and measured by a secondary electron multiplier. For determining the appearance potential, ionization efficiency curves were obtained by measuring the ion intensities as a function of electron energy at constant temperature. The electron impact energy scale was calibrated against the first ionization potentials of Ag, In, Hg, Ar, and He.' The relevant data were acquired and processed by an IBM compatible PC. Alumina Knudsen cells (i.d. = 7.5 mm, 0.d. = 10.0 mm, height = 10.0 mm, and orifice diameter = 0.5 1 mm) were used to contain the samples. The Kundsen cell with the sample (normally half to three-fourths of the cell) was kept inside a molybdenum cup which was heated by electron bombardment. Temperatures were measured by a chromel-alumel thermocouple touching the base of the Knudsen cell. The thermocouple was calibrated against the melting point of silver. Two samples from independent preparations were used in these experiments. Sample 1 was annealed at 500 K for about 12 h while sample 2 was preheated at 800 K for about 3 h. In each experiment the ion intensities were measured as a function of time from those temperatures where a detectable signal was obtained, and the sample was taken to the next temperature (either higher or lower) only after ensuring equilibrium conditions for a rea- sonable period of time (normally 3 W min). Such stable reading at each temperature was chosen for obtaining the temperature dependence of the ion intensities. The samples were weighed (along with the Knudsen cell) in a Mettler microbalance (sen- sitivity 10 pg) before and after the experiment to obtain the weight loss during the experiment. Experiments were carried out with samples of different initial weights and for different durations. Prior to each experiment with C, an experiment with silver (NBS standard) was carried out. Results and Discussion In the mass spectrum of the equilibrium vapor, major peaks were observed in the mass ranges 720-722 and 360-361. The peaks in the mass range 720-722 were attributed to Ca+ on the basis isotopic abundance. The peaks in the mass range 360-361 may be due to fragmentation of Ca molecules or doubly positive ions of c60. No other peak with significant ion intensity was detected up to a mass of 1020. Particular care was taken to detect any C70 present. The ratio of Ia+fI,,-,+ at the highest temperature (800 K) was around 4000. Such a large value for the ratio indicates the purity of Ca and the effectivenessof the separation procedure. An appearance potential of 8.1 f 0.5 eV obtained for the ion Ca+ is in good agreement with the ionization energy value of 7.61 f 0.2 eV given in the assessment of Kroto et a1.l' (11) Kroto, H. W.;Allaf, 4. W.; Balm,S. P. Chem. Rev. 1991, 91, 1213. 0022-3654/92/2096-3566$03.00/0 0 1992 American Chemical Society