5036
J. Phys. Chem. 1993,97, 5036-5039
Laser Desorption Mass Spectrometry of Photopolymerized Cm Films D. S. Cornett, I. J. Amster, and M. A. Duncan’ Department of Chemistry, University of Georgia, Athens, Georgia 30602
A. M. Rao and P. C. Eklund Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 4051 1 Received: December 15, 1992; In Final Form: February 9, 1993
Laser desorption mass spectra are presented for thin films of c 6 0 (pristine films) and photopolymerized c 6 0 (transformed films). At the threshold energy for laser desorption, Cm+ is the only product of desorption/ ionization of either film. At intermediate laser energies, transformed films yield ions with the composition (C6o)N+ (N = 1-8), while pristine films produce only c60+. At higher desorption laser energies, the transformed films are found to form c 6 0 cluster ions with N values of up to 20. At higher resolving power, the mass spectra of the cluster ions show substantial structure, dominated by loss of units of Czx ( x = 0-9) from integer multiples of c60. At high desorption laser energy, pristine films also produce c 6 0 cluster ions, but with a substantially reduced distribution of sizes compared to the transformed films. Comparisons of these data are made to mass spectra of recently reported giant fullerenes.
Introduction Since the initial report on the unusual stability of CSO’and the subsequent isolation of macroscopic quantities of material,2 a major focus of fullerene research has been directed toward the synthesis of new C60-based materials. Because of its spherical structure, c 6 0 can be derivatized inside and outside the cage, creating compounds which exhibit unique physical, chemical, and electronic properties. We have recently reported the first observation and characterization of a new class of fullerene materials, formed by photoinduced polymerization of c 6 0 thin films.3 In this report, we describe laser desorption mass spectrometry measurements on these c 6 0 polymer films, which provide information about the structure and bonding in these materials. Polymerized fullerenes are unique among other Cm materials. Much of the previous research has been directed toward either doping the fullerene lattice with alkali metals or modifying individual c 6 0 molecules by attaching ligands or encapsulation of metal atoms.4 Organic polymers containing c 6 0 have been reported.ss6 However the spheres are linked together by ligand groups or dangle as side chains from a conventional organic polymer backbone. We believe our phototransformed films represent the first c 6 0 polymer composed exclusively of crosslinked fullerene molecules. In our previous paper,3we provided Raman, IR, X-ray, and mass spectral data to support this claim. For example, numerous Raman and IR bands observed for the c 6 0 films shift significantly upon phototransformation,and X-ray diffraction shows that the spacing between adjacent cages is smaller after transformation. The solubility of the phototransformed films in toluene is also markedly reduced compared to pure C60.3We focus here on the mass spectral analysis of several c 6 0 films, both before phototransformation (pristine films) and after (transformed films), using laser desorption (LD) coupled with time-of-flight mass spectrometry. Laser desorption mass spectrometry has been shown to be effective for the analysis of both f~llerenes’-~and organic polymers.Ikl3 A key feature of LD is that it can provide relatively “soft” ionization of large molecules prepared either as neat films or embedded in a matrix.I4 The high mass capability and the intrinsic pulsed nature of LD couple well to time-of-flight mass
* Author to whom correspondence should be addressed. 0022-3654f 93f 2097-5036$04.00f 0
spectrometry, and the two combined techniques are common tools in fullerene research. However, one must be cautious in applying laser desorption for the analysis of new materials. Certain conditions may induce photochemistry in the material to be analyzed. In the present case of phototransformed c 6 0 films, it is especially important to understand to what degree the analysis step might cause further cross-linking of the fullerene cages. We have performed an extensive study of experimental conditions for laser desorption of pristine and polymerized c 6 0 films. The results of these studies indicate that the laser desorption can be performed without causing phototransformationof the c 6 0 films. We observe significant differences between the mass spectra of the phototransformed films and the pristine films, confirming that the proposed photopolymerization process has occurred prior to analysis. Experimental Section The time-of-flight (TOF) mass spectrometer used in these experiments was designed and constructed in our laboratory at the University of Georgia specifically for laser desorption of large molecules. The ion source, shown in Figure 1, consists of a single stage acceleration region with 3.3-cm spacing between the acceleration and extraction plates. A hole drilled in the center of the acceleration plate accommodates a 4.8-mm stainless steel probe which is coated with a 200-nm film of fullerene material. The probe is attached to an insulating rod and inserted into the vacuum chamber via a sliding seal vacuum lock. The face of the sampleprobe rests flush with the surfaceof theacceleration plate. A small motor connected to the protrudingend of the rod provides continuous rotation of the sample at a rate of 4 rpm throughout the experiment. Desorption is accomplished by focusing the attenuated output of a N2 laser (Laser Photonics; 337 nm, 200 pJ/pulse, 9-11s pulse, 10 Hz)to provide a power density of approximately 1 MW/cm2 for a 0.3-mm-diameter spot. Laser fluence is measured by placing a fused silica plate immediately in front of the entrance to the vacuum chamber to direct 8% of the laser radiation onto a joule meter (Laser Precision, Inc.). Ions ejected from the surface of the sample are accelerated directly into the time-of-flight tube at an energy of 25 keV. There is no collisional stabilization of the desorbed ions as has been employed in other recently reported, related experiments.15 Conventional einzel optics, located im0 1993 American Chemical Society
Photopolymerized c 6 0 Films
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5037 A
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Figure 1. Schematic of the LD-TOFmass spectrometer used for these experiments: (a) vacuum lock and sample rod; (b) acceleration region; (c) einzel lens; (d) dl and d2, detector positions for linear and reflectron operation (dl is used for these experiments); (e) X and Y deflection plates; (f) ion mirror, not used in these experiments.
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mediately after the acceleration region, focus the ion beam onto the detector, minimizing the loss of ions from the expanding plume of desorbed material. Two sets of deflection plates are used to steer the ions in both horizontal and vertical planes. The TOF may be operated in the linear or reflectron mode, using detectors at positions d l or d2, respectively. In the present experiment, we use the linear mode. Ions then pass through a 1.5-m field-free region before being detected by an electron multiplier tube (Hamamatsu R-595).Signals from the electron multiplier tube are collected by CAMAC based digitizer/averaging memory modules (DSP Technology) and transferred via an IEEE interface to a PC-compatible computer. Data collection and analysis are performed by software developed in-house. Transformed and pristine films of c 6 0 are prepared at the University of Kentucky as described p r e v i o ~ s l y .These ~ , ~ ~ samples are sealed under argon prior to shipping to the mass spectrometry laboratory at Georgia. In addition to the limited exposure to oxygen, pristine samples are also handled in the dark to prevent possible low-level phototransformation from fluorescent room lighting. A limited number of samples have also been prepared at Georgia for immediate mass spectrometry analysis. These samples are prepared by exposing the c 6 0 films to the 5-Woutput of an Ar+ laser (all lines) for time periods of 15,30, and 60 min, while they are held under argon at atmospheric pressure. All samples are transferred to the mass spectrometer under a nitrogen atmosphere in order to reduce the possibility of oxygenation.I6
Results and Discussion Figure 2a shows the laser desorption mass spectrum we observe for a typical film of c 6 0 which has been vacuum deposited and subsequently phototransformed using a Hg arc lamp. The remarkable pattern which we observe is a mass spectrum containing peaks of the form (C60)N. This mass spectrum is unlike any which has ever been observed for a fullerene sample, or for carbon clusters produced by any other technique. There are masses characteristic of multiples of 0 , but with the exception of ions grouped around the primary multiples (see below), there are no intermediate-sized carbon molecules. It is important to note that this sample is probed by laser desorption of the fullerene film directly into the mass spectrometer. There is no collisional stabilization of the ions formed, such as that described in studies ongoing inother laboratories,"JS whichcouldlead to the formation of weakly bound c 6 0 aggregates. The best explanation for this mass spectrum, as explained below, is that the phototransformed film containspolymerized C a units which are desorbed and ionized by the probe laser. To investigate the laser desorption process giving rise to this unusual mass spectrum, both transformed and pristine samples are analyzed at a variety of laser fluences ranging from the ion production threshold to the threshold for plasma generation. Data of this type provide important information for evaluating and understanding the composition of the films. At low laser power, for which desorption/ionization is first observed, the technique
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probes species weakly bound to the substrate. For example, these conditions are often used for analysis of amorphous fullerene sample^.^-^ Higher laser powers are required for samples in a crystalline film, or in a polymer network, where there are significantbinding interactions with the surroundings. This latter scenario is exactly that expected for a cross-linked fullerene polymer. Thus, as desorption fluence increases, desorption should be more efficient, and polymerized films should yield ions of increasingmass. Unfortunately, the polymerizationprocess under study here can be photoinitiated by a laser in the same wavelength region as the desorption laser. Therefore, at high desorption fluence, some degree of polymerization may be caused, as well as detected, by the probe laser. Using pristine films as experimental controls, multimers of C60 are observed to be formed by photoreactions initiated by the desorption laser, but only under high fluence conditions, as shown in Figure 2b. The interacting effectsof desorption versus photoinitiatedreactions can be isolated by selecting the appropriate desorption laser fluence. The features of the mass spectra of phototransformed and pristine samples are dependent on the desorption laser irradiance. At low desorption fluence (
