William R. Creasy

Large carbon cluster ion formation by laser ablation of polyimide and graphite

Abstract Polyimide is studied by laser ablation using 266 nm radiation. Ions formed in the ablation are detected using a Fourier transform mass spectrometer. Mass spectra of graphite are also obtained under the same experimental conditions. The mass spectra from polyimide show large intensities of high-mass carbon cluster ions ( m/e > 800 amu), similar in composition to those observed by laser ablation of graphite under some conditions. For ablation of polyimide using higher laser fluence, lower-mass distributions of ions are also observed. The craters formed by the laser ablation of both polyimide and graphite are measured in order to find the volume of material ablated per pulse, which is used to estimate the particle density in the laser-generated plasma. The results are shown to be consistent with the growth of the high-mass ions by gas phase reactions.

Formation of high mass carbon cluster ions from laser ablation of polymers and thin carbon films

Three materials were studied by laser ablation/Fourier transform mass spectrometry, using 266 nm laser radiation: a copolymer of ethylene and tetrafluoroethylene (ETFE), polyphenylene sulfide (PPS), and a diamond‐like carbon film (DLC). In each case, positive ion mass spectra exhibit primarily even‐numbered, high mass carbon clusters (‘‘fullerenes’’) of the type previously reported for graphite ablation. In the case of ETFE, a large C+60 peak (‘‘buckminsterfullerene’’) was observed. The polymer spectra showed a strong dependence on the number of laser pulses on one spot and the laser power density. For ETFE, the fullerene ion relative intensity first increases and then decreases as a function of the number of laser pulses. For the DLC film, fullerenes are observed with a single laser pulse on a fresh spot of the sample. The results are interpreted in terms of a gas phase growth model for the fullerene ion formation.

Some model calculations of carbon cluster growth kinetics

Simple kinetic mechanisms are used to model the features of ionic carbon clusters that have been observed by laser vaporization mass spectrometry. The basis of the mechanism involves only stepwise addition of small species (C to C3) followed by collisional quenching. The model is used to examine experimental observations including the variation of cluster size distributions with plasma density, the formation of ‘‘magic’’ numbers, the formation of predominantly even clusters, the loss of hydrogen from the clusters in the presence of a hydrogen‐containing plasma, and the effect of isomerization of n>31 on the high mass distribution. Substantial agreement between experimental observations and the kinetic model are obtained, and the effect of more complicated processes are discussed.

Epoxy resin analysis by Fourier transform mass spectrometry: a comparison of pyrolysis and laser ablation

Abstract The pyrolysis or decomposition products of cured, fire retardant epoxy resin (diglycidyl ether of tetrabrominated bisphenol A) were compared under several different conditions: thermal pyrolysis, laser ablation and cw laser pyrolysis. Volatile neutral products were ionized by electron impact, except in the case of direct laser ionization, and detected by Fourier transform mass spectrometry. Pyrolysis gave high molecular weight fragments and a considerable amount of information about the monomer composition. Detection of directly ionized, laser-ablated products gave some molecular fragments from microprobe sample sizes, although the spectra are complex. Laser ablation/electron impact ionization produced mostly low molecular weight fragments that gave limited information about the material. Laser pyrolysis using a cw argon ion laser provided some large fragments and consumed small amounts of sample. The relationships and analytical value of these techniques are discussed.

Experimental evaluation of apodization functions for quantitative fourier transform mass spectrometry

Abstract A selection of apodization functions for Fourier transform mass spectrometry have been evaluated for peak height and isotope ratio reproducibility using an experimentally acquired series of transients. The Noest—Kort function consistently gives relative standard deviations (RSDs) 20–80% smaller than the other functions considered. The limited dynamic range of this function, due to the slow roll-off auxiliary maxima, gives rise to systematic interference. This is true even in the case of a target peak adjacent to an interference but resolved from its central maximum. We recommend the Noest—Kort function for repetitive quantitative measurements in cases in which interferences are either absent or can be systematically taken into account.

Carbon cluster ion (C+n, 11≤n≤23) reactivity investigated via reaction with naphthalene: Kinetic energy dependence and evidence for long‐lived complexes

Reactions of carbon cluster ions (C+n, 11≤n≤23) with naphthalene have been investigated in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR‐MS). Reactive channels include the formation of adduct ions stabilized by radiative association, three‐body collisions and loss of a hydrogen atom. Radiative association competes favorably with the dissociation of the ion/neutral complex and estimates of the average complex lifetime derived via kinetic modeling range from 28 to 59 (±50%) ms. The reactions have been found to be extremely dependent on the kinetic energy of the cluster ion. This dependency has been exploited to determine the average kinetic energy of the desorbed ions by comparison of reaction product distributions obtained as a function of kinetic energies. Calculated values were on the order of the trapping potential to twice that value (2–4 eV).

Pyrolysis and laser ablation of plasma‐polymerized fluorocarbon films: Effects of gold particles

Plasma‐polymerized fluorocarbon (PPFC) films were analyzed by thermogravimetric analysis (TGA), direct pyrolysis/mass spectrometry, and laser‐ablation/electron‐impact mass spectrometry. Fourier transform mass spectrometry was used to detect products. The films were made by plasma polymerizing tetrafluoroethylene in an argon plasma. Two types of films were studied: with and without fine gold particles incorporated in the PPFC films. TGA showed that gold‐containing films decompose more rapidly and at lower temperature with increasing gold content. Pyrolysis products were determined as a function of temperature. The predominant positive product ions, using 20 eV electron‐impact ionization, were C2F4+, CF3+ and a distribution of higher‐mass unsaturated fluorocarbon species, CnFm+, up to at least n=14 and m≥n+1. The predominant negative ions, formed by electron attachment, were also unsaturated fluorocarbon ions which extended up to 1145 u in mass. These species are different from those observed from polytetra...

Elemental composition of ions associated with low mass carbon clusters generated by UV laser ablation of polyimide

Abstract Previously, it has been shown that three types of C clusters, characterized by mass range and relative intensities, are produced upon laser ablation of polyimide. The low mass clusters, C n 9 n n H 2 , rather than C n -1 N, which is allowed since polyimide contains 5 at% N. These results establish the positive ion spectrum of polyimide to be composed of C and H with little contribution from heteroatoms, and show that low mass C clusters preferentially add 2 H atoms in the high energy, multi-element plasma induced by the laser.

The formation of large polyaromatic hydrocarbons via carbon cluster ion reactions

Reactions of carbon cluster ions (C+n, 10<n<25) with polyaromatic hydrocarbons (PAHs) and substituted benzenes, studied using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry, provide evidence for the formation of large, highly conjugated PAHs. Product ions consist of adducts formed by radiative association/collisional stabilization and adducts accompanied by the loss of a hydrogen. These two reaction pathways alternate between the even‐ and odd‐numbered cluster ions dependent on reactant neutral employed. Adduct formation continues up to a maximum of five associations and to molecular weights in excess of 900 amu. Collisionally induced dissociation (CID) of product ions produced fragmentation indicative of species with multiply bonded carbon atoms and possibly fullerene‐type structure. Complete dehydrogenation of the adduct ions was possible.

Laser ablation/Fourier transform mass spectrometry of polymers

Laser ablation/ionization followed by Fourier transform mass spectrometry is used to identify and characterize polymers. The mass spectra of several polymers are discussed, including polyimide, polyamic acid, Dupont Tefzel, and polyphenylene sulfide.