D. J. Smith

Time-of-flight mass spectrometry of aromatic molecules subjected to high intensity laser beams

The recent introduction of femtosecond technology to pulsed lasers has led to the development of femtosecond laser mass spectrometry (FLMS). The present paper describes an FLMS investigation of the aromatic molecules, benzene, toluene and naphthalene. Wavelengths of 750 and 375u2005nm were used with beam intensities up to 4u2005×u20051014u2005Wu2005cm−2. Pulse widths were of the order of 50–90u2005fs. The laser system was coupled to a linear time-of-flight mass spectrometer. This experimental method of chemical analysis is gaining momentum, often replacing its nanosecond forerunner, resonant enhanced multiphoton ionization. For the said molecules, predominant parent ion production is found, making identification unambiguous. In fact this characteristic is being consistently attained in small to medium mass molecules irradiated under similar conditions, leading to the conclusion that a universal chemical detection system is a possibility. Such soft ionization is particularly evident at longer wavelengths (∼750 nm) with less relative fragmentation, daughter ion formation, compared to results at shorter wavelengths (∼375 nm). In terms of parent ion formation, similar numbers are produced with laser intensities around 1014u2005Wu2005cm−2 for both wavelengths. It has also been shown that at a threshold of about 5u2005×u20051013u2005Wu2005cm−2, double ionized molecules appear for the 750u2005nm wavelength. These interesting new mass spectra display intense single, double and even triple ionized peaks without significantly increased dissociation. Such effects are less pronounced at 375u2005nm. ©u20041998 John Wiley & Sons, Ltd.

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.

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.

UNIFORM MOLECULAR ANALYSIS USING FEMTOSECOND LASER MASS SPECTROMETRY

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.

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.

Dissociative ionization and angular distributions of CS2 and its ions

The dissociative ionization of CS2 has been investigated at several different wavelengths (375, 750, 395, and 790 nm), for pulse-widths as short as 50fs, and laser intensities in the range of (2.2×1013−3×1016)u200aW/cm2. It is found from the various mass spectra that fragmentation is relatively more pronounced at shorter wavelengths, whilst at longer wavelengths, the parent dominates. Another interesting feature, occurring in the data and literature, is the presence of an S2+ ion at wavelengths less than about 532 nm, and near total absence at wavelengths longer than this. Angular distributions are presented for both 395 and 790 nm. Sn+-ions fragment along the polarization direction, while the Cn+-ions fragment perpendicularly to it, providing a way of distinguishing the different fragments produced in the coulomb explosion process. Finally, distribution widths decrease with higher charge states of the ions, implying that they are more aligned with the field.

The Photo‐Dissociative Pathways of Nitromethane Using Femtosecond Laser Pulses at 375 nm

The dissociative pathways of the nitromethane molecule have been studied using 10 ns, 7 ps and 90 fs laser pulses. In this paper, the differences between the dissociation channels opened using different laser pulse widths are discussed to investigate the potential for femtosecond laser mass spectrometry (FLMS). Using 90 fs laser pulses, a very large parent ion has been observed but the 10 ns and 7 ps laser pulse widths cannot defeat the lifetimes of the dissociative excited states of nitromethane.