Ionic desorption in valence- and core- excited polymers: poly(vinyl chloride) and poly(vinylidene chloride)

Brazilian Journal of Physics, vol. 36, no. 2B, June, 2006 534 Ionic Desorption in Valence- and Core- Excited Polymers: Poly(vinyl Chloride) and Poly(vinylidene Chloride) M. L. M. Rocco, G. S. Faraudo, F. C. Pontes, G. G. B. de Souza, Instituto de Quı́mica, Universidade Federal do Rio de Janeiro, Cidade Universitária, Ilha do Fundão, 21949-900, Rio de Janeiro, RJ, Brazil R. R. Pinho, Departamento de Fı́sica-ICE, Universidade Federal de Juiz de Fora, Campus Universitário, 36036-330, Juiz de Fora, MG, Brazil and M. Ferreira PEMM/COPPE, Universidade Federal do Rio de Janeiro, Cidade Universitária, Ilha do Fundão, 21941-972, Rio de Janeiro, RJ, Brazil Received on 29 July, 2005 Photon stimulated ion desorption (PSID) studies have been performed in poly(vinyl chloride) (PVC) and poly(vinylidene chloride) (PVDC) using synchrotron radiation, covering from valence to core electron (Cl 2p and C 1s) energy ranges. Data acquisition was performed at the TGM beam line from the Brazilian Synchrotron Light Source (LNLS), operating in a multi-bunch mode and using a time-of-flight mass spectrometer (TOFMS). A new pulsed system developed uses as a trigger for the TOF-MS experiments the pulsed extraction high voltage applied to the sample. Ionic desorption from PVC and PVDC shows strong selectivity in the formation of chlorine ions around the Cl 2p-edge while very similar fragmentation patterns are observed for the other energies studied. Keywords: Photon stimulated ion desorption (PSID); Polymers; Synchrotron radiation I. INTRODUCTION The availability of intense and tunable photon sources (synchrotron radiation) has made possible the analysis of the ionic emission from surfaces as a continuous function of the photon energy. The study of the interaction of high-energy photons with polymers is a topic of increasing interest mainly due to their use in many technological applications, like for instance in areas such as microfabrication in which polymers are routinely used as resists [1, 2]. In addition, there is a special interest in the study of surface photochemistry by core level photoexcitation experiments using synchrotron radiation because inner-shell processes show element and site selectivity. One attractive topic in this context is the capability of selectively breaking a chemical bond [3]. It is believed that the main process governing ion desorption is the Auger process, which can be described in a simplified form as a relaxation process following excitation (resonant Auger) or ionization (normal Auger) of a core electron. Several examples are now known in which, following the ionization or excitation of an inner-shell electron and the corresponding relaxation through the Auger process, selective fragmentation is observed around the atom to which the inner-shell electron is closely associated [3-9]. In this paper we present photon stimulated ion desorption (PSID) spectra for poly(vinyl chloride) (PVC) and poly(vinylidene chloride) (PVDC), irradiated in the 21.21 to 310 eV photon energy range. This broad energy interval allows for a general observation of the photon energy dependence of the fragmentation pattern associated with single and double (multiple) ionization processes as well as Auger-assisted double (or even triple) ionization channels. The measurements were performed at the Brazilian Synchrotron Light Source (LNLS), using a time-of-flight mass spectrometer (TOF-MS). The mass scale calibration procedure used in ESID studies of PMMA [10] was also successfully employed in the present PSID studies. The results show new evidence for site-specificity in the ionic desorption from PVC and PVDC, irradiated at the chlorine 2p-edge. II. EXPERIMENTAL Ionic desorption experiments were performed at the TGM beamline (12 - 310 eV) from the Brazilian Synchrotron Light Source (LNLS), located at the city of Campinas, São Paulo. The experimental setup includes a sample manipulator and a time-of-flight mass spectrometer (TOF-MS), which are housed in an ultra-high vacuum chamber (UHV) with a base pressure of about 10−9 Torr. The TOF-MS employed in the present work has been described in detail elsewhere [11]. Basically, it consists of an electrostatic ion extraction system, a drift tube and a pair of microchannel plate (MCP) detectors, disposed in the chevron configuration. After extraction, positive ions travel through three metallic grids (each of which with a nominal transmission of 90%), before reaching the MCP. Ion optics simulation using the SIMION program [12] shows that the use of a collimating electrostatic lens, which was incorporated in the TOF-MS, allows for an efficiency close to 100% in the extraction of ions with up to 40 eV kinetic energy. In order to perform PSID studies using Brazilian Journal of Physics, vol. 36, no. 2B, June, 2006 III. PVC 400 250eV 200 0 40 Intensity (a.u.) synchrotron radiation from the Brazilian storage ring, which is normally operated in a multi-bunch mode, a pulsed high voltage applied to the sample was used as a trigger for the TOF-MS experiments [13]. In this pulsed system, the start signals to the TDC (time-to-digital converter) were provided by a signal associated to the rising edge of the extraction pulse, usually of + 250V, applied to the solid sample. Pulse widths from 300 ns to 1.5 µs with a repetition interval of 150µs, provided by a homemade pulse generator, were employed. The output signal of the detector was processed by a standard pulse counting system and used to provide a stop signal to the TDC. The incident beam makes a 60◦ angle with respect to the normal to the sample, which in its turn was about 25 mm away from the first grid of the TOF spectrometer. Thin films of PVC and PVDC were prepared by the spin casting technique. No additional cleaning procedure was adopted. The samples were attached to an aluminum support, which was mounted onto the X-Y-Z sample manipulator. For a precise calibration of the mass spectra we have used a new calibration procedure previously employed in ESID studies of PMMA [10]. Basically, it consists of introducing a small amount of a noble gas into the vacuum chamber, raising the pressure from 10−9 to about 10−7 Torr and recording a TOF spectrum of the desorbing ions emanating simultaneously from the atomic gas and from the polymer surface. In our case, argon gas was used, thus allowing for an accurate mass scale calibration. 535 60 80 100 120 400 140 210 eV 200 0 40 60 80 100 120 140 400 200 eV 200 0 40 60 80 100 120 800 600 400 200 0 140 110 eV 40 60 80 100 120 140 m/q FIG. 1: Ionic desorption mass spectra of PVC induced by 110, 200, 210 and 250 eV photon energies. RESULTS AND DISCUSSION 0,2 0,1 + + C2HnCl /C4Hn 0,0 100 150 200 250 300 350 0,4 Branching Ratio Photon stimulated ion desorption (PSID) spectra for PVC and PVDC were measured at different photon energies covering the valence region as well as the chlorine 2p and carbon 1s absorpti (...truncated)