J. H. Sanderson

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.

Observation of multiple ionization pathways for OCS in an intense laser field resolved by three-dimensional covariance mapping and visualized by hierarchical ionization topology

The two- and three-body Coulomb explosion of carbonyl sulfide (OCS) by 790 nm, 50 fs laser pulses focused to {approx_equal}10{sup 16} W cm{sup -2} has been investigated by the three-dimensional covariance mapping technique. In a triatomic molecule, a single charge state, in this case the trication, has been observed to dissociate into two distinct energy channels. With the aid of a three-dimensional visualization technique to reveal the ionization hierarchy, evidence is presented for the existence of two sets of ionization pathways resulting from these two initial states. While one group of ions can be modeled using a classical enhanced ionization model, the second group, consisting of mainly asymmetric channels, cannot. The results provide clear evidence that an enhanced ionization approach must also be accompanied by an appreciation of the effects of excited ionic states and multielectronic processes.

Ion mobility measurements for O2+ in helium gas at 4.35 K

The mobility of O2+ in He has been measured using a liquid helium cooled, selected ion drift tube at 4.35 K. The dependence of mobility on effective temperature, for O2+, is similar to that measured previously for rare gas ions moving in He. The mobility reaches a maximum at between 500 K and 600 K where it is in reasonable agreement with the existing room temperature measurements. Below 70 K there is a region in which mobility is constant, but lower than the polarization limit, and below 10 K, mobility decreases sharply. Although clustering reactions occur at low effective temperature, it is not certain that they are responsible for these phenomena, and although a quantum mechanical orbiting resonance is expected to occur at low collision energies, there are no theoretical predictions of its importance for the O2+-He system.

Alignment and bending of by intense femtosecond laser pulses

We report on direct measurements of the angular distributions of fragment ions produced in interactions with 60 fs laser pulses at 750 nm. These distributions show dramatically the bent nature of the dissociating molecule, and allow observation of the alignment of the different dissociation channels. It is found that for the lowest channel of multielectron dissociative ionization, alignment parallel to the laser field is preferred, but orientation perpendicular to the laser field direction is allowed. For the higher channels orientation perpendicular to the field is not allowed. Where possible the distributions measured are compared with those deduced from fits to the double-correlation data of Cornaggia.

Sub-pulselength time resolution of bond softening and Coulomb explosion using polarization control of laser-induced alignment

Two 50 fs laser pulses with orthogonal laser E-fields are superposed with a time delay of less than the pulse duration, producing a single peaked pulse with a continuously varying ratio between the horizontal and vertical laser E-fields. Since the ejection angle of fragments in the multiphoton dissociative ionization of is dependent upon the laser polarization, this technique can be used to resolve events on a sub-pulse timescale. Using focused intensities of about , the results show that Coulomb explosion of occurs primarily on the rising edge of the pulse. Bond-softening peaks, however, are associated with the falling edge, in agreement with the earlier notion that only then can enhanced ionization at the critical internuclear separation be avoided. The surprisingly small energy spread of the bond-softening peaks, as observed in a recent high-resolution study, can be explained in a consistent manner.

HIGH-INTENSITY FEMTOSECOND LASER INTERACTIONS WITH VIBRATIONALLY EXCITED CO2

In this work we report the first measurements of the interaction of 60 fs laser pulses at 750 nm with vibrationally excited . Comparison with ground state is made and enhanced kinetic energies for and observed. These observations are consistent with unusually large bends in the molecule, induced by the laser field.

Intense laser field studies of positive ions

There is considerable current interest in the study of atoms and molecules in very strong electric fields. The advent of short pulse high power lasers has enabled new experimental studies in this area leading to the discovery of new effects. We describe an experiment designed to extend these studies to targets of positive ions produced from conventional low energy accelerators. First results with H2+ molecular ions are presented, and noted differences with previous measurements using neutral H2 molecules are discussed.