Plasma polymerization chemistry of unsaturated hydrocarbons: neutral species identification by mass spectrometry

Abstract

Radio frequency discharges ignited in low-pressure and pure hydrocarbon gases were investigated by mass spectrometry. The plasma process was applied to four unsaturated monomers: styrene C 8 H 8 , benzene C 6 H 6 , ethylene C 2 H 4 and acetylene C 2 H 2 . The remote mass spectrometer location restricted species identification to neutral closed-shell molecules in their respective plasmas. Among the peaks in the mass spectra, those directly due to neutrals produced in the plasma were determined following a successful two-step methodology. Firstly, the use of low electron impact energy limited the fragmentation and strongly simplified the cracking patterns. Secondly, attribution of peaks directly due to neutrals was confirmed or ruled out by systematically measuring their appearance potential. In the case of styrene, not less than 48 new molecules were detected.The discussion of the observed stable by-products in each discharge suggested several radicals responsible for their production. Comparing the set of species among the four plasmas showed that the repeated addition of intermediates with one or two carbon atoms and with low H content dominated the chemistry. Under our conditions of intermediate to high W/FM (power over mass flow ratio), the gas-phase plasma polymerization then preferentially occurred through significant fragmentation and recombination. Finally, the measured appearance potentials during plasma provided estimation for the threshold ionization energy of several highly unsaturated hydrocarbons, useful for modeling.

The Chemistry of Cosmic Dust Analogs from C, C2, and C2H2 in C-rich Circumstellar Envelopes

“…A different situation has been found in the CRL618 PPN, where methylpolyynes (CH 3 C 2 H and CH 3 C 4 H) have been detected with abundances much larger than those observed in IRC +10216 (Cernicharo et al 2001a) Aromatic compounds with aliphatic moieties are neither produced in acetylene pyrolysis (Shukla & Koshi 2012) nor detected (or only very marginally) as neutral species in the gas phase of acetylene discharges (Deschenaux et al 1999;Consoli et al 2008). However, this is drastically different when other hydrocarbons are used, particularly in ethylene pyrolysis and ethylene plasmas (Jäger et al 2006;Gillon & Houssiau 2014). This is related to the formation of alkyl radicals which enter into the chemistry, thus promoting the formation of alkyl moieties.…”

Nonthermal tetravinylsilane plasma used for thin‐film deposition: Plasma chemistry controls thin‐film chemistry

“…[ 33 ] Sometimes, lower electron energies (12–20 eV) are used to reduce the fragmentation of precursor molecules. [ 14,15,17 ] As indicated above, the average kinetic energy of electrons is only 0.1–2 eV [20.21] in non‐thermal plasma, but due to the shape of the electron‐energy distribution function [ 22 ] (Equation ), there is a sufficient concentration of electrons with energies above 15 eV enabling ionization of neutral molecules. However, we wanted to determine whether the proportion of the most produced cations varies depending on the energy of the electrons.…”

“…Formation of species with a mass greater than that of the precursor molecule can occur through radical recombination in the plasma state and was observed at higher pressures of 60-150 mTorr. [14][15][16][17] Three-body collisions in the bulk plasma are unlikely during the low-pressure process. From the mass spectrum in Figure 1, we can distinguish some significant groups of cations that can be assigned to Si-(vinyl) 3 , Si-(vinyl) 2 , and Si-(vinyl) 1, dominated by m/z 108, 82, and 55, respectively.…”

The chemistry of cosmic dust analogues from C, C$_2$, and C$_2$H$_2$ in C-rich circumstellar envelopes

“…A different situation has been found in the CRL618 PPN, where methylpolyynes (CH 3 C 2 H and CH 3 C 4 H) have been detected with abundances much larger than those observed in IRC+10216 (Cernicharo et al 2001a) Aromatic compounds with aliphatic moieties are neither produced in acetylene pyrolysis (Shukla & Koshi 2012) nor detected (or only very marginally) as neutral species in the gas phase of acetylene discharges (Deschenaux et al 1999;Consoli et al 2008). However, this is drastically different when other hydrocarbons are used, particularly in ethylene (C 2 H 4 ) pyrolysis and ethylene plasmas (Jäger et al 2006;Gillon & Houssiau 2014). This is related to the formation of alkyl radicals which enter into the chemistry, thus promoting the formation of alkyl moieties.…”