A. Clark

Laser-induced dissociation, ionization and fragmentation processes in nitroaromatic molecules

Abstract Laser multiphoton ionisation and fragmentation processes have been studied in many molecular species, including a large number of substituted aromatic compounds. Recent studies in this laboratory on nitrobenzene have yielded significant data on the wavelength dependence of laser-induced ionisation and fragmentation processes: in particular, the very striking behaviour of the NO fragment. In this paper we extend the wavelength range to study nitrobenzene at shorter wavelengths and also investigate another nitroaromatic compound, o -nitrotoluene, where similar behaviour is observed. We also report laser ionisation of nitric oxide (NO) and nitrogen dioxide (NO 2 ) over the same wavelength region. Comparison of the various spectra suggests that the nitroaromatic parent molecules predissociate releasing NO 2 , which in turn predissociates to form NO before going through a multiphoton ionisation process. The hydrocarbon components of the o -nitrotoluene molecule also ionise, but there is no evidence of distinctive resonance enhancement of any of these fragment ions in the wavelength range studied.

On the dissociation pathways of nitrobenzene

Abstract The fragmentation of nitrobenzene has been studied in the wavelength range 225–275 nm using a single dye laser, frequency doubled, in conjunction with a time-of-flight (TOF) mass spectrometer. The parent (C6H5NO2+), nitrosobenzene (C6H5NO+), phenoxy (C6H5O+) and phenyl (C6H5+) ions were all observed in addition to many other lighter daughter fragments. The formation of the nitrobenzene, phenoxy and phenyl ions are all explained invoking pathways where dissociation of the parent molecule from an excited state takes place first, followed by ionization after the absorption of further photons (DI) by the fragmented neutrals. Ionization of the parent molecule to states which are dissociative (ID) can explain the increase in the production of phenyl ions at wavelengths shorter than 230 nm.

Laser ionisation studies of nitroaromatic and NOx(x=1 or 2) molecules in the region 224-238 nm

Abstract Wavelength dependent ionisation studies have been carried out on the nitroaromatic molecules nitrobenzene and o -nitrotoluene as well as NO and NO 2 gases in the wavelength region 224–238 nm. Mass spectra from the nitroaromatic molecules show extensive fragmentation with a particularly prominent NO + ion peak. This particular fragment ion has been of great importance, not only as a marker to identify the presence of a nitro-type compound, but also to distinguish from NO x ( x = 1 or 2) molecules from which it may have originated and to provide information about possible energy pathways in the fragmentation processes. The particular studies described in this paper were carried out to identify wavelength regions where greater ionisation efficiency of the NO molecule arising from the nitroaromatic molecules could be achieved. In the course of these studies an important difference in the NO + wavelength dependent ionisation spectra of nitrobenzene and o -nitrotoluene was observed. The observation of atomic oxygen resonances in the nitrobenzene and NO 2 spectra suggest that similar pathways are being followed in these two dissociating molecules.

Wavelength-dependent laser-induced fragmentation of nitrobenzene

Abstract This paper describes the wavelength-dependent yield of some of the positively charged ions resulting from the interaction of a UV laser beam with nitrobenzene molecules in a high vacuum chamber. In the wavelength region 245–250 nm, which spans the two-photon ionisation threshold (249.2 nm), the CnH+m fragments display a similar wavelength dependence, whereas the behaviour of the mass 30 fragment (NO+) is markedly different. The wavelength dependence of the fragment-ion yields suggests that the parent molecule dissociates into C6H5 plus NO2. Additionally, at high pulse energies, ionisation of atomic carbon demonstrates that the dissociation process generates carbon atoms in both ground and excited states.

Attomole detection of nitroaromatic vapours using resonance enhanced multiphoton ionization mass spectrometry

A very sensitive and selective procedure has been developed for the detection of nitrobenzene (C6H5NO2) and o-nitrotoluene (C6H4CH3NO2) vapours using resonance enhanced multiphoton ionization mass spectrometry. The time-of-flight mass spectra of these two nitroaromatic molecules are characterized by a prominent NO+ ion signal (m/z 30) together with a characteristic pattern of hydrocarbon fragment ions. The intense NO+ ion signal arises via efficient two-photon resonant ionization of neutral nitrogen monoxide (NO) molecules produced by dissociation of the nitroaromatic species. In the wavelength range studied to date, 224–260 nm, NO+ ion generation is observed to be strongly dependent on laser wavelength, with an intensity maximum occurring at 226.3 nm. At this particular wavelength, NO+ ion signals have been detected with less than 1 amol (<10–18 mol) of nitrobenzene vapour in the laser beam. The two aromatics can be distinguished by observing differences in the laser induced mass spectra and in the wavelength dependence of fragment ion production. Furthermore, it is possible to distinguish NO+ ion formation from NO and NO2 gases and NO+ ion formation from nitroaromatic molecules in vacuum by studying the wavelength dependence of the NO+ ion signal in the range 245–250 nm and it is hoped that this procedure can be used to make similar distinctions with atmospheric samples.

Resonant laser ablation (RLA)

An enhancement of several orders of magnitude in the laser ablation signal of Al, Ga and Ca was observed in a time-of-flight mass spectrometer by tuning the ablation laser to known resonant transitions. At low fluences the enhancement at resonance occurs at the known transition wavelengths and is less than 0.05 nm wide.

Resonant ionization spectroscopy of carbon atoms following laser-induced fragmentation of nitro-aromatic molecules

Abstract During the course of studies on resonance enhanced multiphoton ionization (REMPI) spectra of nitro-aromatic molecules, a number of sharp atomic transitions were observed among the broad molecular peaks of the 12 C + spectrum in the wavelength region 245–260 nm. These have been unambiguously identified as transitions in atomic carbon from both ground and excited states. In addition, laser fluence measurements on two of these transitions exhibit a sixth order power dependence on resonance and a cubic dependence off resonance. This can be explained by a model requiring three photons to produce a neutral carbon atom from the parent molecule together with a further three photons to ionize.

Sensitive atmospheric pressure detection of nitroaromatic compounds and NOx(x= 1,2) molecules in an ionization chamber using resonance-enhanced multi-photon ionization

Nitrobenzene and o-nitrotoluene were detected in trace concentrations in gas mixtures at atmospheric pressure in a simple unity-gain ionization chamber. The detection procedure relies on tunable ultraviolet laser radiation that identifies the characteristic wavelength dependence of the NO+ ion fragment. Sensitivity levels are 0.25 ± 0.05 ppm for nitrobenzene and 0.75 ± 0.1 ppm for o-nitrotoluene, with the current limitations being background ionization, laser power and small sampling volume. Additionally, nominally 1 ppm NO and NO2 gas samples were analysed and the saturation fluence (via a resonant two-photon process at 226.3 nm) for NO+ production from NO gas was determined. Sensitivity levels of 120 and 50 ppb were estimated from the data on NO2 and NO, respectively. It is shown that an increase in sensitivity of an order of magnitude is possible in all instances if the experimental parameters are optimized.

Resonant ionization of oxygen and hydrogen atoms following laser-induced photodissociation of nitrobenzene vapour

The resonant ionization of oxygen and hydrogen atoms has been observed in a linear time-of-flight (TDF) mass spectrometer during studies of the resonance enhanced multiphoton ionization (REMPI) of nitrobenzene vapour in the wavelength region 225-245 nm. Oxygen ions have been observed in the mass spectrum in the region of 226 nm. and have been identified as arising from the 2p4 3 P to 2p33p 3P two-photon transition in atomic oxygen. A large enhancement in the hydrogen ion yield from nitrobenzene was also recorded when the laser wavelength was tuned through the 1s 2S to 2s 2S two-photon transition of atomic hydrogen at 243.13 nm. The large on-resonance enhancements in both signals indicate that oxygen and hydrogen atoms are produced in abundance during the interaction between ultraviolet laser light and nitrobenzene vapour.

The characteristics of Q‐switched laser ablation for several elements from a low alloy steel

The laser ablation of low alloy steel by a Q‐switched laser beam has been investigated by employing non‐resonant laser ionization of the neutral species in a time‐of‐flight mass spectrometer. By varying the delay between the ablation and ionization lasers it is possible to determine the velocity distribution of the Ti, V, Cr, Mn, and Fe atoms simultaneously. These distributions have been recorded as a function of ablation laser fluence. The half‐range Maxwell‐Boltzmann velocity distribution has been used to fit the data and different characteristic temperatures have been determined for the various elements in the sample. The quantitative capability of this method for bulk and surface analysis has been evaluated by calculating the relative sensitivity factors (RSFs) for the various constituent elements. The RSFs for all of the elements are seen to be highly dependent on the delay between the ablating and ionizing lasers. This dependence was removed by integrating the temporal dependence ion yield, which le...