Resonance-enhanced multiphoton ionization spectrometry (REMPI), using time-of-flight mass spectrometers and tuned lasers, has proved an important ultra-sensitive analytical technique. Nevertheless, conventional nanosecond REMPI suffers from a number of shortcomings: most importantly, REMPI often fails through rapidly (pre)dissociating states. In the case of thermally labile molecules, which include the nitro-molecules, either no or very small parent or high/mass fragment ion peaks exist, making the interpretation of the mass spectra ambiguous at best and often impossible. Femtosecond laser mass spectrometry (FLMS) can often 'defeat' these dissociative states, resulting in large parent or high-mass fragment ion peaks which make the interpretation less ambiguous. In the present paper, nanosecond and femtosecond multiphoton ionization and fragmentation are compared using time-of-flight mass spectrometry for NOz gas and a number of different nitro-molecules: nitromethane, nitrobenzene, rn-nitrotoluene, dinitrotoluene and trinitrotoluene.

The photofragmentation of the nitrotoluene isomers in the gas phase is studied in the wavelength region 210-270 nm using a pulsed UV laser in conjunction with a time-of-flight mass spectrometer. Laser-induced mass spectra are analysed and compared with those produced by the electron impact technique. The generation of the observed fragment ions is explained by invoking different fragmentation pathways followed by these molecules. Observed differences in the mass spectra of the ortho-meta-, and para-nitrotoluene isomers and in the wavelength dependence of the NO fragment released from these molecules are discussed as a possible way of providing a laser-based method for their identification.Recently, it has been shown by Kosmidis et al.' that the mass spectrum from the multiphoton ionization and dissociation of nitrobenzene, using UV laser light, can be explained by both the dissociation ionization (DIdissociation followed by ionization) and the ionization dissociation (ID-ionization followed by dissociation) decomposition mechanisms. For these two fragmentation routes, different dissociation pathways have been proposed.Due to the similarities which exist between nitrobenzene and the nitrotoluene isomers it is believed that many common characteristics could appear in their photodissociation processes. Thus, while the knowledge of the photochemistry of nitrobenzene is essential for understanding that of the nitrotoluene isomers, it is expected that the latter could contribute to the understanding of the entire group of nitro-explosive compounds which includes dinitrotoluene and trinitrotoluene. Moreover, this knowledge is a prerequisite for the sensitive and selective photodetection of these materials.The photodecomposition of the nitrobenzene molecule has been extensively studied using laser light, but the case is different for the nitrotoluenes. Although the fragmentation of their molecular ions has been well by other techniques, little work has been presented for molecular fragmentation using lasers.'"'' These laser studies do not, however, cover all the isomers and have mainly been carried out using a small number of discrete wavelengths.In this paper, the UV-laser-induced dissociation of ortho-, meta-and para-nitrotoluene in the gas phase is presented in the wavelength range 210-270 nm. The electron impact (EI) mass spectra of these molecules have also been recorded for reasons of completeness, and are compared with the laser-induced mass spectra.The study is carried out in this wavelength range for two reasons. Firstly, there is the possibility to study the molecular fragmentation with respect to the ionization thresholds. For aromatic molecules, it has been proposed that the extensive fragmentation observed after a

Resonance-enhanced multiphoton ionization has been used to detect the presence of vapour-phase explosive-type samples in a linear time-of-flight (TOF) mass spectrometer. In particular, nitrobenzene, 2-nitrotoluene, 2,4-dinitrotoluene, 2,4,64rinitrotoluene, ethylene glycol dinitrate, pentaerythritol tetranitrate (PETN), 1,3,5-trinitro-l,3,5-triazacyclohexane (RDX) and SEMTEX (PETN plus RDX plus plasticizer) have been analysed at a laser wavelength of 226.3 nm corresponding to a strong resonant transition of the neutral NO molecule. TOF mass spectra have been recorded and the intensity of the NO+ ion monitored as a function of temperature for comparison with the temperature dependence of the various vapour pressures.The detection and identification of explosives and explosive residues is an area of analytical science which has continued to be of great importance for society. A wide variety of techniques have been investigated, with chromatographic and mass spectrometric methods being the most widely used. ' The use of laser technologies for the detection and identification of organic molecules has been assessed by a number of workers using a variety of approaches. Generally, a pulsed laser is used to effect multiphoton ionization/fragmentation of the parent molecule' with the resulting parent and/or fragment ions being analysed using a mass spectrometer. The degree of fragmentation can be controlled via the laser fluence, and different species can be identified by their fragmentation patterns. Laser ionization can also be used to detect the presence of vapours at atmospheric pressure3 by virtue of known wavelength-dependent ionization fingerprints. Application of some of these approaches to the realm of explosives detection and identification have allowed progress to be made in both atmospheric pressure4 and vacuum',6 environments.The use of tunable lasers has demonstrated that characteristic ionization yields are possible if the wavelength can be ~c a n n e d ,~ due to resonance enhancements in the initial absorption step. In the case of nitrotype explosive molecules which generally have broad, featureless absorption bands occurring at similar wavelength regions, this might not be expected to lead to an obvious fingerprint with which to identify a particular species. However, since these molecules have a tendency to photodissociate on exposure to ultraviolet radiation, the photodissociation products can be probed to yield important information relating to wavelength-dependent fingerprints.', '-I3A series of studies"'-I2 has previously been carried out by this group on some nitroaromatic molecules (nitrobenzene and 2-nitrotoluene) with the intention of identifying a procedure for the detection of explosive vapours using tunable, pulsed lasers. These studies have demonstrated the importance of the NO+ fragAuthor for correspondence. + Permanent address: Department of Physics, University of Ioannina, Ioannina, GR 451 10, Greece. ment ion which provides a prominent signal and which has a very characteristic wavelengt...

A comparison of the NQR parameters of the monoclinic and orthorhombic phases of TNT and their relation to the twist or dihedral angle between the plane of the NO 2 substituents and that of the benzene ring as determined in the X-Ray crystal structure analysis enables an assignment of different frequencies to specific sites in the two independent molecules in the unit cell of both forms to be made. The slow transformation of the metastable orthorhombic phase to monoclinic can then be followed by monitoring the NQR spectrum in which specific lines can be assigned to molecular sites in the two phases. NQR spectra of TNT referred to in the literature often differ; this could be due partly to the TNT often being a mixture of monoclinic and orthorhombic phases and partly to changes in the spectral line width, factors which must be taken into account when NQR is used to detect landmines.

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