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Öğe Behavior of Polyatomic Molecules in Intense Infrared Laser Beams(American Chemical Society, 1998) Ledingham, K. W. D.; Singhal, R. P.; Smith, D. J.; McCanny, T.; Graham, P.; Kılıç, H. Ş.; Peng, W. X.; Wang, S. L.; Langley, A. J.; Taday, P. F.; Kosmidis, C.In the present Letter we report that a number of polyatomic molecules (M) when irradiated with short pulse lasers <90 fs at 750-790 nm and intensities up to 1015 W cir-2 produce multiply charged parent ions and do not fragment to any great degree. This surprising observation is found in both linear and ring structured molecules and is very similar to the behavior of inert atoms such as xenon under the same irradiation conditions. This is a very different behavior from irradiating with nanosecond pulses at 109 W cm-2 where low-mass fragments dominate the spectrum. For the hydrocarbon molecules presented in this work, there exists an envelope of 2+ ionized peaks, which corresponds to the parent and a number of (M -nH) satellites. This feature is characteristic of these molecules in the intensity region 1014-15 W cm-2 and is interpreted as evidence for tunneling or barrier suppression. Coulomb explosion leading to multiply charged atoms, which is evident for CS2, does not seem to be operating for the larger hydrocarbon molecules.Öğe Ionization and Dissociation of Benzaldehyde Using Short Intense Laser Pulses(AMER CHEMICAL SOC, 1998) Smith, D. J.; Ledingham, K. W. D.; Kılıç, H. S.; McCanny, T.; Peng, W. X.; Singhal, R. P.; Langley, A. J.; Kosmidis, C.In a recent series of experiments, femtosecond laser mass spectrometry (FLMS) was applied to benzaldehyde utilizing laser pulse widths in the range 90 fs to 2.7 ps. Beam intensities up to 2 x 10(14) W cm(-2) were used with wavelengths of 750 and 375 nm. Different ionization-dissociation channels were found compared with previous studies by other authors based in the nanosecond regime. The general theme emerging is one of predominant above-threshold ionization-dissociation (ID) in which predissociative states are largely bypassed via rapid optical up-pumping to the molecular ionization continuum. Above the parent ionization threshold, ladder-switching is seen to be a function of laser pulse width, intensity, and wavelength with exclusive parent ion formation being achievable in the lower intensity regions at all pulse widths. Increasing fragmentation occurs as the laser intensity increases, although parent supremacy remains. At 750 nm however, the increase of fragmentation with intensity is greatly reduced compared to 375 nm, leading to the conclusion that FLMS at longer wavelengths is preferred for chemical analysis. Moreover at 750 nm and at laser intensities close to 10(14) W cm(-2) C7H6O2+ becomes evident. It is also shown that the C7H6O+/C7H5O+ ratio is strongly dependent on the pulse width, suggesting that a hydrogen loss pathway has a dissociation time of about a picosecond.Öğe Molecular Fragmentation Induced by Femtosecond Laser(SPIE, 1998) Kosmidis, C.; Ledingham, K. W. D.; Kılıç, H. S.; McCanny, T.; Singhal, R. P.; Smith, D.; Langley, A. J.The 90 femtosecond laser induced fragmentation at 375 nm for a number of different nitro-molecules is compared to that induced by a nanosecond laser at the same wavelength by means of time-of-flight mass spectrometry. The potential of femtosecond laser mass spectrometry for analytical purposes is discussed.Öğe Multiphoton Ionization and Dissociation of Nitromethane Using Femtosecond Laser Pulses at 375 and 750 Nm(American Chemical Society, 1997) Kılıç, H. S.; Ledingham, K. W. D.; Kosmidis, C.; McCanny, T.; Singhal, R. P.; Wang, S. L.; Smith, D. J.; Langley, A. J.; Shaikh, W.The photochemistry of nitromethane has been studied extensively for many years. Although it is generally agreed that the principal photodissociative process is cleavage of the C-N bond to yield the methyl radical and nitrogen dioxide, there is some evidence of minor competing dissociation channels. A number of different groups have used lasers of different wavelengths, but the results of these studies vary considerably and no clear picture of the minor dissociative channels has yet emerged. The use of femtosecond (fs) duration laser pulses for photoionization of molecules is currently an area of considerable interest, since the process can lead to the efficient production of intact molecular ions. It was felt that femtosecond laser mass spectrometry (FLMS) could provide added information on the dissociation pathways of nitromethane. Laser pulses of 90 fs time duration at wavelengths of 375 and 750 nm, coupled to a time-of-flight mass spectrometer, have been used in this study, and contrary to photoexcitation using nanosecond (ns) pulses, a large parent ion, 61 (CH3-NC2+), is detected together with strong peaks at m/e = 15 (CH3+), 30 (NO+), 46 (NO2+) as well as a number of other minor peaks. This fragmentation pattern can be explained by a predominantly ID (ionization followed by dissociation) route.Öğe On the Fragmentation of Nitrobenzene and Nitrotoluenes Induced by A Femtosecond Laser at 375 Nm(AMER CHEMICAL SOC, 1997) Kosmidis, C.; Ledingham, K. W. D.; Kılıç, H. S.; McCanny, T.; Singhal, R. P.; Langley, A. J.; Shaikh, W.The photodissociation of nitrobenzene and the nitrotoluene isomers at 375 nm, induced by a 90 femtosecond laser, is analyzed and compared with the fragmentation by a 10 nanosecond laser at the same wavelength, The molecular ionization is attributed to a nonresonant multiphoton process, and the observed fragmentation can be explained predominantly by an above ionization mechanism (ladder climbing). The mass spectra of the three nitrotoluene isomers show differences which can be used for analytical purposes. The molecular rearrangement taking place prior to the dissociation is also discussed. For nitrobenzene, it is suggested that most of the dissociation occurs from the nitrobenzene structure rather than that of phenyl nitrite. In the case of o-nitrotoluene, it seems that the hydrogen transfer from the -CH3 to the NO2 group (ortho effect) is favored in ionic states, while the rearrangement to a nitrite structure is possible in the excited electronic states.Öğe The Onset of Coulomb Explosions in Polyatomic Molecules(1999) Smith, D.J.; Ledingham, K. W. D.; Singhal, R. P.; McCanny, T.; Graham, P.; Kilic, H. S.; Tzallas, P.; Kosmidis, C.; Langley, A. J.; Taday, P. F.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 1015 and 1016W 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 ~1016 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.Öğe Time of Flight Mass Spectrometry of Aromatic Molecules Subjected to High Intensity Laser Beams(JOHN WILEY & SONS LTD, 1998) Smith, D. J.; Ledingham, K. W. D.; Singhal, R. P.; Kılıç, H. S.; McCanny, T.; Langley, A. J.; Taday, P. F.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 375 nm were used with beam intensities up to 4 x 10(14) W cm(-2). Pulse widths were of the order of 50-90 fs, 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 (similar to 750 nm) with less relative fragmentation, daughter ion formation, compared to results at shorter wavelengths (similar to 375 nm), In terms of parent ion formation, similar numbers are produced with laser intensities around 10(14) W cm(-2) for both wavelengths. It has also been shown that at a threshold of about 5 x 10(13) W cm(-2), double ionized molecules appear for the 750 nm 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 375 nm..
