P F Taday

DYNAMIC AND GEOMETRIC LASER-INDUCED ALIGNMENT OF MOLECULES IN INTENSE LASER FIELDS

The angular distribution of the ionic fragments from multiphoton dissociative ionization of diatomic molecules shows a pronounced anisotropy. Calculations within the field-ionization, Coulomb explosion model show that the experimental results (pulses of 50 fs at an intensity of ) can be explained without invoking reorientation of the molecular axis. The fragments of the lighter and molecules, on the other hand, show clear signs of being forced into alignment.

One and two-colour studies of the dissociative ionization and Coulomb explosion of with intense Ti:sapphire laser pulses

Phase control of the spatial asymmetry of the dissociative ionization channel, , of molecular hydrogen has been achieved by superposing interferometrically sub-100 fs, Ti:sapphire laser pulses at the fundamental and second harmonic frequency. The counterintuitive asymmetry previously observed with longer, less intense Nd:YLF laser pulses and HD molecules has been reproduced. However, phase modulation of total ionization and dissociation rates was not discernible. In order to further understand the process, a comprehensive study of appearance intensities, kinetic energy releases and angular distributions has been undertaken at each frequency.

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.

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.

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.

Two independently tunable and synchronised femtosecond pulses generated in the visible at the repetition rate 40 kHz using optical parametric amplifiers

Abstract We report on a 40 kHz repetition rate travelling wave optical parametric amplifier (OPA) system designed for ultrafast spectroscopy. Two independently tunable OPAs seeded from a single white light continuum source each generate 40–75 nJ per pulse over a 480–730 nm spectral range when pumped by 1 μJ, 130 fs, 400 nm pulses derived from a Ti:sapphire regenerative amplifier. After removal of chirp, the cross-correlation between the two OPAs was measured to be 150 fs. Bandwidth was 205 ± 85 cm −1 , consistent with the transform limit of the compressed pulses. A single OPA gave up to 280 nJ per pulse signal energy when pumped with 3 μJ per pulse.