A. J. Langley

The prospects for ultrashort pulse duration and ultrahigh intensity using optical parametric chirped pulse amplifiers

Abstract The properties of optical parametric amplifiers are presented to show that, in addition to their use in providing tuneable pulses, they can form the critical component in systems generating shorter pulse duration with higher power and intensity than is possible with existing systems. Practical designs are evaluated leading to anticipated powers exceeding 10 PW and focused intensities greater than 10 23 W cm −2 .

Characterization of multiterawatt laser-solid interactions for proton acceleration

A comprehensive characterization of experimental parameters in a study of proton acceleration by short-pulse laser–solid interactions at intensities up to 1019 W cm−2 is reported. Laser pulse and prepulse conditions were measured, with a contrast ratio of the order of 10−6 obtained. The focused laser intensity was experimentally calibrated using a time-of-flight spectrometer to resolve the stages of ionization of a target gas. By comparing the measured ion yields with predictions of an atomic tunneling ionization model a factor of 1.5 uncertainty in the focused intensity was determined. Drive mechanisms for mounting solid targets with thickness in the range of 0.2 to 125 μm have been developed for use with high-repetition rate lasers. A retro-focus imaging system has also been implemented to position the target relative to the laser focus. The techniques have been applied to study proton acceleration as a function of various laser and target parameters. Measurements of the energy distribution of protons a...

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 375 nm were used with beam intensities up to 4 × 1014 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 (∼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 1014 W cm−2 for both wavelengths. It has also been shown that at a threshold of about 5 × 1013 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. © 1998 John Wiley & Sons, Ltd.

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.

Multielectron-dissociative-ionization of by intense femtosecond laser pulses

The first detailed observations of the multielectron-dissociative-ionization of sulphur hexafluoride have been made using 60 fs laser pulses at 750 nm and . The fragmentation pattern displays features similar to those found in experiments with diatomic and triatomic molecules, such as a tendency towards symmetrically dissociative channels and fragment kinetic energies which for all channels are consistent with a Coulomb explosion at a single, critical, internuclear separation.

Multiphoton ionisation and dissociation of NO2 by 50 fs laser pulses

Abstract Multiphoton ionisation and dissociation of NO 2 has been studied experimentally at 375 nm for laser pulse widths of 10 ns and 50 fs. The parent NO 2 ion peak is not seen in the ns data. In all spectra, the main peak observed is due to the ionisation of the NO molecule which results from the dissociation of excited NO 2 formed after absorbing a 375 nm photon. The intensity dependencies of both NO and NO 2 ion peaks have also been measured. The data has been analysed within the context of a rate equation model using published cross-sections and dissociation rates except for the two-photon ionisation cross-section for NO 2 which was chosen to reproduce the NO 2 /NO ion signal ratios at 50 fs. The rate equation model provides a good description of the complete set of data. Indirectly, it may be concluded that coherence effects do not play an important role in the multiphoton excitation/ionisation of NO 2 . The data also rules out the importance of above-ionisation dissociation in NO 2 — a conclusion which is consistent with previous data at 496 and 248 nm for laser pulse widths ⩾ 300 fs.

Fast-beam study of H2+ ions in an intense femtosecond laser field

A fast beam of H2+ ions, produced from a low energy ion accelerator, has been used for the first time in intense laser field experiments. The technique has enabled neutral dissociation products to be analysed and detected for the first time in such studies. Energy spectra of neutral and ionized fragments, product yields as a function of focused laser intensity and angular distributions of neutral dissociation products have been measured. Significant differences are observed between the present results and those obtained from experiments involving neutral H2 molecules. These differences are indicative of the precursor H2 molecule playing an important and hitherto neglected formative role in the laser-induced fragmentation processes.

DOUBLE-PULSE MEASUREMENTS OF LASER-INDUCED ALIGNMENT OF MOLECULES

The nonlinear interaction of molecules with intense laser fields is a potential method of preparing states with very high degrees of alignment. The physical rotation of molecules can be confirmed using two pulses of orthogonal polarization and by observing the behaviour of the lowest fragmentation channels. New results show that this dynamic alignment is molecule specific. In the case of , virtually all molecules are depleted by the pump pulse, but we see no evidence for rotation of , in agreement with recent systematic studies of angular distributions. Further analysis indicates that the ubiquity of laser-induced alignment has thus far been overestimated.

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.