Philip John

Identification of large molecular mass material in high temperature coal tars and pitches by laser desorption mass spectroscopy

Abstract Molecular masses of up to 12 000 Da have been identified by laser desorption mass spectroscopy (l.d.-m.s.) in a number of coal tars and extracts. Observed molecular masses were greater than any hitherto detected in coal-derived liquids; fractions of high temperature coke oven tars have been used for verification of initial findings. Low molecular mass materials have been analysed using similar experimental procedures, to confirm the absence of laser or other apparatus derived high molecular mass clusters. Spectra extending to 12 000 Da show an envelope between 1000 and 3000 Da and a low molecular mass envelope between 12 and 400 Da, the latter apparently indicating the presence of carbon clusters. At high laser power densities, the 1000 to 3000 Da envelope was observed to lose intensity, with parallel gain in intensity of the low mass carbon cluster envelope, suggesting increased breakdown of substrate molecules under increasing laser power density. Comparisons with results from g.c.-m.s. and probe m.s. are presented, showing masses up to 600 Da. Limited structural information about the large molecular mass materials can be inferred from the present results, suggesting the presence of small aromatic groups with mass differences corresponding to methyl groups, ethylene bridges and benzo groups.

Molecular masses up to 270 000 u in coal and coal-derived products by matrix assisted laser desorption ionization mass spectrometry (MALDI-m.s.)

Abstract This paper describes the application of matrix assisted laser desorption ionization (MALDI) to coals and coal-derived materials using sinapinic acid as the matrix. The mass range of molecules in coal and coal-derived materials has been extended by a factor 100 compared with prelaser desorption mass spectrometric measurements. A peak of intensity is observed for coals and coal-derived materials in the mass range 1000–5000 u which is sample dependent. The upper mass ranges of the spectra vary according to sample, ranging from over 260 000 u for Point of Ayr coal and 200 000 u for a coal tar pitch, to 20 000 u for a maceral concentrate liquefaction extract. These results confirm earlier results using laser desorption mass spectrometry and are in broad qualitative agreement with size exclusion chromatography results. Detailed quantitative agreement, however, requires further work. The implications of this work for the debate on coal structure and models of coal conversion are considerable.

Chemical thermometers in megawatt infrared laser chemistry: The decomposition of cyclobutanone sensitized by ammonia

Abstract Ammonia excited by CO 2 laser radiation may be used to sensitize unimolecular reactions of compounds which do not absorb the radiation directly at the chosen frequency. If cyclobutanone is present as a minor component then analysis of the products formed yields information as to the effective temperature reached by the system and the time the latter remains at this temperature.

A reassessment of the vibrational spectrum of hydrogenated amorphous silicon

Abstract Paul [1] has cogently argued the case against the exclusive assignment of the 2100 cm -1 band in the vibrational spectrum of hydrogenated amorphous silicon to =SiH 2 complexes. Data are presented in the present paper which make the grounds for dissent from the dihydride model less securely based. We assert that, for as-prepared r.f. a -Si films, the contribution to the 2100 cm -1 band intensity from alternative structures is not as significant as hitherto proposed.

Thermal dehydrogenation of glow discharge a-Si

Abstract Infra-red spectroscopic studies have been made of glow-discharge hydrogenated amorphous silicon films prepared under a variety of experimental conditions. Other studies have been made of the thermally induced dehydrogenation of these films and a model is suggested to account for their mode of decomposition.

Photo-oxidation of amorphous silicon-carbon alloys and its use in field effect transistors

Abstract Hydrogenated amorphous silicon-carbon thin-films deposited by rf plasma deposition from silane and propane are photo-oxidised by UV illumination at room temperature in air. The process has high spatial resolution and reduces the electrical conductivity of the alloys to the extent that they can be used as gate insulators in field effect transistors.

An EELS and EXELFS Study of Amorphous Hydrogenated Silicon Carbide

A series of thin films of amorphous hydrogenated silicon carbide (a-Si∶C∶H) produced by RF plasma decomposition of propane and silane has been studied by electron energy-loss spectroscopy (EELS) and extended energy-loss fine structure (EXELFS) studies. The composition of the films has been determined by EELS and the nearest neighbour spacings have been determined by EXELFS. These results, along with the energy of the plasmon loss peaks, have been compared with the deposition conditions for each film. The results show that for a large gas ratio (C3H8/(C3H8+SiH4)) the films have a high proportion of carbon and are similar to a-C∶H in structure, whereas those films prepared with Y = 0.4 or 0.5 have nearest neighbour spacings consistent with those for tetrahedrally bonded carbon. The films prepared with lowest Y have nearest neighbour spacings similar to those for amorphous silicon carbide. The results for a-Si∶C∶H have been compared with the results of EELS and EXELFS of CVD diamond films, amorphous carbon and amorphous silicon.

Laser-desorption mass spectrometry of standard polynuclear aromatic hydrocarbons and fullerenes

Laser-desorption mass spectrometry (LD–MS) has been used to investigate the mass spectra of a suite of standard polynuclear aromatic hydrocarbons and a sample of fullerenes (mixed C60 and C70), as a part of a study of high molecular mass ions in tars and liquid extracts derived from coal. Positive- and negative-ion LD–MS spectra of these standards were compared with electron impact spectra to elucidate the mechanism of laser desorption. The data indicate that ion–molecule reactions occur during the desorption step giving hydrogenated molecular ions via mechanisms analogous to self-chemical ionization; fullerenes gave no hydrogenated molecular ions. Carbon cluster ions (both positive and negative ions) are formed by the laser energy with fewer carbon atoms than the original molecule, suggesting their formation to result from the thermal destruction of parent molecules. However, none of the standard compounds gave cluster ions greater than the hydrogenated molecular ions. This finding provides confirmation that large molecular mass materials identified in coal-derived liquids originated from the sample itself and did not form from smaller molecular mass compounds under the power of the laser.

High mass compounds (up to 12000 u) in coal tars

The presence of high mass molecules (up to 12000 u) in coal-derived material where previously the highest mass components detected did not exceed 1 or 2000 u is demonstrated for the first time by the use of laser desorption mass spectrometry.