P. Tzallas

On the fragment ion angular distributions arising from the tetrahedral molecule CH3I

The mass spectra for both horizontal and vertical polarizations and the angular distributions of fragment ions arising from Coulomb explosion of tetrahedral methyl iodide (CH3I) ions, obtained at a laser intensity of 1016 W cm-2 are presented. All fragment ion distributions are peaked along the direction corresponding to collinearity of the laser electric field with the time-of-flight mass spectrometer axis. The In + ion (n≤7) angular distributions from the dissociation of the parent ions are all of similar widths, which would imply a geometric, as opposed to dynamic, alignment. Additionally, the lower-charged I ions have an isotropic component that decreases as the charge state increases. Measurements of the CHm+ (m≤3), Cp + (p≤4) and H+ ion distributions show that these are also maximal along the polarization direction. Furthermore, there is also a CH22+ ion peak present in the CHm group, which has a distribution similar to those measured for the other ions. This mass peak is the prominent multi-charged ion in this group. As the CH3I molecule is initially tetrahedral, these results suggest that the molecular structure undergoes a change such that the H-C and C-I bonds tend to lie along the field. Several authors have described work which first aligned CH3I molecules with a nanosecond laser and then photodissociated with a femtosecond laser, to produce fragment ion distributions. This is the first time that the angular distributions from a tetrahedral molecule have been presented using femtosecond laser pulses only and in the case of CH3I, for fragments other than CH3+ and I+. The fragment energetics from the single CH3I molecule have been compared with those from recent work dealing with the Coulomb explosion of CH3I clusters.

AN INVESTIGATION OF THE ANGULAR DISTRIBUTIONS OF FRAGMENT IONS ARISING FROM THE LINEAR CS2 AND CO2 MOLECULES

The nonlinear interaction of the triatomic molecules CS2 and CO2 with the intense field of a linearly polarized laser beam of femtosecond (fs) pulse duration, was used to study the ionization and dissociation of the parent molecule. The fragment ion angular distributions arising from the Coulomb explosion of the parent ions were also measured. For CS2, the angular distributions of CS2+, CS22+, CS23+, CS+, CS2+, Sn+ (n6) and Cm+ (m4) ions are presented for a laser intensity of 1 × 1016 W cm-2 at a wavelength of 790 nm and pulse duration of 50 fs. The angular distributions of the parent molecular ions are all isotropic. The Sn+ fragments are peaked along the time-of-flight (TOF) axis, whereas the Cm+ fragments explode perpendicularly to this. Similar results for CO2 are also presented for comparison. The S ion distributions do not narrow as their ionic charge increases, and it is argued that the angular distributions for CS2 are due mainly to the angular dependence of the ionization probability. On the other hand, the distributions from the lighter CO2 molecule are thought to be at least partly due to alignment via dipole moments induced by the laser, as in this case the On+ angular distributions are seen to narrow as their charge increases. The conclusion of these results is that the laser pulse may be too short for the CS2 molecule to align in the pulse. Angular distributions are also presented for varying laser pulse durations, in the range of 50 fs to 300 ps. The dynamics of the ionization/dissociation mechanism are discussed in the context of the TOF mass spectra and angular distributions recorded for CS2.

Coulomb explosion in aromatic molecules and their deuterated derivatives

Coulomb explosion within some aromatic molecules (furan, pyrrole, pyridine and pyrazine) and their deuterated derivatives induced by strong fs laser fields (4 10 16 W=cm 2 ) is studied at ka 790 nm by means of timeof-flight (TOF) mass spectrometry. It is found that in hydrogenated molecules the Coulomb explosion process begins at internuclear distances about twice larger than the equilibrium distance OReU, while the expansion of the molecular skeleton in the deuterated derivatives is smaller. Based on the estimated kinetic energy values of the fragment ions, the charge distribution in the transient molecular species is also discussed. ” 2000 Elsevier Science B.V. All rights reserved.

Ionization/dissociation of thiazole and thiazolidine induced by strong laser fields

Abstract The interaction of thiazole and thiazolidine with a strong (∼ 2×10 16 W/cm 2 ) fs laser field has been studied at λ=790 nm by means of time-of-flight (TOF) mass spectrometry. The observed relative abundance of the doubly charged intact parent ion in thiazolidine is higher than that of thiazole, while the laser-molecule coupling strength is found to be much more efficient for the aromatic (thiazole) than the nonaromatic (thiazolidine) molecule. The mass spectra of thiazolidine are attributed to a combination of field ionization with subsequent multiphoton processes. It is also concluded, that direct Coulomb explosion within the transient multiply charged parent ions leads to the production of multiply charged atomic ions.

The onset of Coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.

The angular distributions of fragment ions from labelled and unlabelled N2O in intense laser fields

The mass spectra and angular distributions of fragment ions arising from a Coulomb explosion of highly charged parent nitrous oxide ions, obtained in the femtosecond regime (~1016 W cm-2), are investigated. The N ion angular distributions from 14N2O show maxima when the laser polarization is parallel and orthogonal to the time-of-flight axis. Measurements with labelled molecules (15N-14N = 16O) indicate that the maxima arise from the peripheral and central N atoms in the molecular structure. The anisotropic distributions may be explained by assuming increased ionization and fragmentation when the molecular axis is parallel to the laser field. The bond angle prior to explosion is determined to be ~140°, irrespective of the charge state of the precursor, and calculations of the kinetic energies imparted to the fragment ions suggest that dissociation occurs at the equilibrium internuclear distance of the neutral molecule.

Laser induced ionisation/dissociation of cyclopentanone in the 320–370 nm region

The two-photon excitation of the 3p and 3d Rydberg states of cyclopentanone in a jet-cooled molecular beam has been investigated by resonance enhanced multiphoton ionisation in a time-of-flight mass spectrometer. The three 3px,y,z components are clearly resolved while the 3di excitations are obscured by the S1(nπ*) one-photon resonance. The ns laser induced mass spectra are analysed and compared with those induced by fs laser and 70 eV electron impact.

Ultrafast laser analysis of nitro-PAHs using laser desorption/femtosecond ionization mass spectrometry

Analytical interest in nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) is related to their high mutagenicity and potential presence in a variety of environmental media such as diesel exhaust and urban air particulate matter. Furthermore, fundamental interest in these molecular systems stems from the photophysics,of the labile NO2 functional group, which has been investigated using mass spectrometry. The nitro- PAHs, 1-nitronaphthalene, 9-nitroanthracene, and 1-nitropyrene, have been studied using both femtosecond (lambda = 395 and 790 nm) and nanosecond (lambda = 266 nm) lasers coupled to a reflectron time of flight mass spectrometer. Analysis of mass spectra taken over a range of intensities (10(14)-10(15) W/cm(2)) has demonstrated that structurally specific ions can be observed. for each molecule, with little or no fragmentation at lower intensities. It has also been found that an intact parent ion can be detected using femtosecond ionization at 395 mn in each case. This work demonstrates the potential use of laser desorption/femtosecond laser mass spectrometry (LD/FLMS) as an analytical technique for the detection of nitro-PAHs and other environmental pollutants and as a means of studying the photodynamics of labile molecular systems

Unusual fragmentation patterns from the dissociation of some small molecules

The nonlinear interaction of molecular matter with the intense field of a linearly-polarized laser beam, allows the study of not only the ionization dynamics of the parent molecule, but also the angular distribution of the exploding fragments. This is carried out by rotating the polarization vector of the laser with respect to the laboratory reference frame. The angular distributions for the CS2, CO2, N2O, H2S, and CH3I molecular ions, at intensities of about 10(16) W cm(-2), are presented. The distributions seem to be dependent on the molecule under consideration, but common features are that the peripheral atoms of the molecule are ejected along the ToF-axis, and the central atom perpendicularly to it, whenever the polarization vector and ToF-axis are collinear. It would seem that the distributions for the lighter of the molecules are partly due to alignment via dipole moments induced by the laser, as the distributions narrow as their charge-state increases. This is indicative of a larger torque acting on the higher-charged precursor molecular ion, via the interaction of the field with the laser-induced dipole moment. On the other hand, the angular dependence of the heavier molecules studied, are thought to originate from a dependence of the ionization/dissociation probability of the molecular ion on the initial angle made between the molecular axis and the polarization vector, that is, a preferential ionization/dissociation process. Spatial alignment in the laser pulse, in this case, is not thought to occur since the peaks do not narrow as the ionic charge increases. Finally, the results for H2S and also N2O are particularly interesting, since distributions for up to S7+ are presented, while the N-distributions show both a parallel and perpendicular component of the distribution. Neither of these results has, to the authors knowledge, been previously observed.

Ultrafast laser time-of-flight mass analysis of laser-desorbed atoms and molecules

Femtosecond time-of-fight mass spectra of laser-desorbed gallium arsenide (GaAs) and trinitrobenzene (TNT) reveal the characteristic features and differences of femtosecond atomic and molecular ionization. Significant yields of multiply-charged atomic fragments, parent molecular ions and structure-specific fragment ions of labile molecules have been obtained. An optimum desorption laser intensity for the transfer of neutral intact parent molecules in to the gas phase has been observed to be molecule-dependent. This work demonstrates the potential of ultrafast laser ionization and has significant implications for analytical and environmental studies of solid materials.