Liam O'Toole

Radiofrequency-induced plasma polymerisation of propenoic acid and propanoic acid

Inductively coupled, radiofrequency-induced plasmas of propenoic (‘acrylic’) acid and propanoic acid, operated at a low electrical power (1–10 W), have been investigated using a combination of mass spectrometry (MS) and deposition-rate measurements. Thin films of plasma polymers of both compounds were deposited onto silicon substrates and analysed by X-ray photoelectron spectroscopy (XPS). The positive-ion MS data obtained from both compounds indicate the presence of species of the form (M+ H)+, (2M+ H)+ and (3M+ H)+, where M represents the molecular weight of the starting material. No neutral oligomeric species were detected. XPS analysis reveals an inverse relationship between the electrical power supplied to the plasma and the degree of retention of the carboxylic acid functionality in the solid product. Comparison of the MS and XPS results suggests that the above cationic species are responsible for the carboxylic-acid functionalisation of the plasma-polymer product, whereas fragmentation processes lead to the introduction of other functional groups such as alcohol and ketone. The thin film which featured the highest degree of retention of carboxylic acid (65%) was obtained from a plasma of propanoic acid operated at 1 W, and was deposited at the lowest rate (0.90 ng s–1).

Mass spectrometry of and deposition-rate measurements from radiofrequency-induced plasmas of methyl isobutyrate, methyl methacrylate and n-butyl methacrylate

Radiofrequency-induced plasmas of methyl isobutyrate, methyl methacrylate and n-butyl methacrylate have been investigated using mass spectrometry. Both neutral and positively charged species were detected and their relative abundances were monitored as a function of the characteristic power : flow-rate ratio of the plasma. This enabled two regimes to be distinguished, characterised by the different roles of uniragmented molecules and fragments of the starting material in the process of plasma polymerisation. The mechanism of gas-phase polymerisation of the α,β-saturated esters, methyl methacrylate and n-butyl methacrylate, is shown to differ from that of the saturated monomer, methyl methacrylate. In a second experiment, the depostion rates of plasma polymers of methyl isobutyrate and methyl methacrylate were monitored. The deposition rates of both plasma polymers were found to increase with increasing power and to have very similar values at a given power and flow rate, a typical value being 0.1, µg s–1 at 2 W and ϕ= 0.11 cm3STP min–1. The combined results are discussed in the light of previously reported SIMS data acquired from plasma polymers of the three molecules studied.

An investigation of the mechanisms of plasma polymerisationof allyl alcohol

Inductively coupled, radiofrequency-induced plasmas of allyl alcohol, operated at low electrical power (1–8 W), have been investigated using mass spectrometry (MS) and deposition rate measurements. The solid, plasma polymer product was collected on silicon substrates and analysed by X-ray photoelectron spectroscopy (XPS). The positive-ion MS data indicate that species of the form (M-H) + , (2M-H) + and (3M-H) + , where M represents a unit of the starting material, are present in the plasma. The relative abundance of these was monitored as a function of the electrical power supplied to the plasma. No neutral oligomeric species were detected. XPS analysis reveals an inverse relationship between power and retention of alcohol functionality in the solid product. Comparison of the MS and XPS results suggests that the above cationic oligomers are responsible for alcohol retention, whereas fragmentation events lead to the introduction of new functional groups. The thin film which featured the highest degree of retention of alcohol (65%) was obtained from a plasma operated at 1 W, but was deposited at the lowest rate (1.81 ng s -1 ).

Vacuum-ultraviolet absorption spectra of propanone, butanone and the cyclic ketones CnH2n–2O (n= 4, 5, 6, 7)

The high-resolution vacuum-ultraviolet absorption spectrum of propanone is reported and analysed in terms of Rydberg series of s, p and d type. The longest series is the s-series observed to n= 17 allowing the ionisation potential to be accurately determined at 78 295 ± 5 cm–1. A number of extensions and corrections to previous work are reported. The spectra of butanone and the cyclic ketones are similarly analysed and extensive Rydberg structure is identified in all spectra except that of cyclobutanone. For cyclopentanone and cyclohexanone long d-series allow accurate determination of the ionisation potentials at 74 800 and 73 794 ± 5 cm–1, respectively. Comparison of the spectra exposes systematic trends in the number of series of p- and d-types and in their relative intensities. The trends are related to the symmetry of the molecules and the location of the geometric origin of the Rydberg orbital set at the centre of charge of the molecular cation. The n= 3 transitions are distinct from all others in showing extensive vibronic structure. The absolute intensities of these transitions are discussed and the vibronic structure analysed. The spectra are discussed in terms of a general description of molecular Rydberg states.

Multiphoton ionisation spectroscopy of the 3s(2 + 2, 1) and 4s(3 + 1) Rydberg states of acetone: evidence for a molecular valence state at 153 nm

The resonance-enhanced multiphoton ionisation spectra of a molecular beam of acetone seeded in helium in the region of the 3s Rydberg state shows clear evidence of rotational and vibrational cooling of the compound. The very large spectroscopic width of the 3s adsorption band in photoabsorption spectroscopy is entirely due to thermal effects and not to interaction with an underlying molecular valence state. The vibrational modes active in the spectrum are all localised on the methyl groups of the molecule, which is explained in terms of the location of the 3s Rydberg orbital of the molecule. The REMPl spectra of the 4s Rydberg state shows no enhancement of resolution when the sample is cooled and the presence of a valence state centred at 153 nm is established. This is the first valence state found at energies higher than the UV (noπ*) state. Very sharp structure at 185 nm is attributed to a triplet state, probably associated with one of the 3p Rydberg states.

On the plasma polymerisation of allyl alcohol: an investigation of ion–molecule reactions using a selected ion flow tube

Low power plasmas of allyl alcohol monomer (M) have been previously probed by mass spectrometry. Mechanisms describing the formation of solid-phase product, based on ion–molecule reactions, were proposed. Major mechanisms proposed initially involve the reaction of (M-H) + ion with M, followed by a sequence of secondary ion–molecule reactions. In this paper the feasibility of these reactions has been tested using a selected ion flow tube. Parent and product ions have been monitored as a function of M concentration, providing information on the reaction rate coefficient and branching ratios, respectively. Primary and secondary products have been distinguished. The new data support the mechanism proposed for the formation of dimers and trimers in the plasma phase. However, the identity of an m/z 81 cation, previously attributed to a purely hydrocarbon containing fragment, has been revised. This ion is a primary product arising directly from reaction of (M-H) + and M, and is most likely the pyrylium cation. This new information has resulted in a significant modification to the previously proposed scheme.

Vacuum ultraviolet absorption spectra of methyl-substituted cyclopentanones and cyclohexanones

The vacuum ultraviolet absorption spectra of methyl-substituted cyclohexanones and cyclopentanones are reported and analysed in terms of Rydberg series. Apart from an anomaly in the n= 4 region, which is attributed to interference by a molecular valence state, long series are assigned in all cyclohexanone spectra allowing accurate determination of the ionization energies. The cyclopentanone spectra are similare but less extensive. The Rydberg structure differs with substituent position in the length and the type of series that dominates the spectra, and in the width of the individual bands. Two distinct substituent effects are identified; an ‘internal pressure’ effect causing increased bandwidth and increased Rydberg–valence state interaction; an orbital mixing effect causing transfer of transition intensity from a d-series in the parent molecules to a p-series of the substituted compounds as the substituent approaches the chromophore. The absolute intensities of the spectra are reported and discussed, and molecular orbital calculations on the ground states of the molecules and their ions used to interpret aspects of the spectra. The vibronic structure of the lowest energy, n= 3s, transitions of selected compounds both from photoabsorption and resonance-enhanced multiphoton ionization (REMPI) spectra are compared with an REMPI study of cooled molecules.

Vacuum-ultraviolet absorption spectrum of acetaldehyde, CH3CHO, and the related deuterides CH3CDO and CD3CDO

The vacuum-ultraviolet absorption spectrum of acetaldehyde in the region 180–120 nm has been measured at high resolution. Analysis of the Rydberg series determines a value of the ionisation potential to be 82 526.45 ± 0.05 cm–1, the high accuracy resulting from one series that is observed to n= 45. Rydberg series of s, p and d type are resolved over a large region of the spectrum, and this is the most extensive large-molecule spectrum ever reported. The spectra of two deuteride derivatives of acetaldehyde are also reported and analysed. These are similar except for significant shifts in the ionisation potential which are shown to be consistent with zero-point energy effects. The very long series is of p-type and is not predissociated throughout the entire spectrum. Explaining this requires consideration of the location of the origin of the Rydberg orbitals and their orientation with respect to the bonds in the molecule. Such consideration of location, distribution and orientation of molecular Rydberg orbitals is novel and may be of general applicability.