Detailed angle and energy resolved measurements of positrons ejected from the back of a gold target that was irradiated with an intense picosecond duration laser pulse reveal that the positrons are ejected in a collimated relativistic jet. The laser-positron energy conversion efficiency is ∼2×10{-4}. The jets have ∼20 degree angular divergence and the energy distributions are quasimonoenergetic with energy of 4 to 20 MeV and a beam temperature of ∼1  MeV. The sheath electric field on the surface of the target is shown to determine the positron energy. The positron angular and energy distribution is controlled by varying the sheath field, through the laser conditions and target geometry.

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The energy spectrum of relativistic electrons is an important characterization of high intensity laser-matter interactions. We present a technique that utilizes Cerenkov radiation to measure the time-resolved energy distribution of electrons. Electrons escaping from targets irradiated by high-intensity laser pulses were measured, demonstrating the feasibility of such a novel diagnostic. Limitations on the time resolution of this diagnostic are also discussed.

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The degree of dynamic alignment ͑the excess alignment beyond that due to geometric alignment͒ induced by linearly polarized, 100 fs pulses in the 10 15 W/cm 2 intensity range, has been investigated. Exploiting circular polarization to turn off dynamic alignment and 4 angular collection to capture all ejected ions, a quantitative measure of the excess alignment was extracted from the relative atomic ion yields subsequent to Coulomb explosion in linearly and circularly polarized fields for several linear molecules ͑H 2 , N 2 , O 2 , and CO 2 ͒. The degree of dynamic alignment was measured to be about 0.90 ͑H 2 ͒, 0.16 ͑N 2 and O 2 ͒, and 0.29 ͑CO 2 ͒. The anomalously large value of CO 2 implies a torque enhancement that we show is consistent with CO 2 interacting with the field longer than N 2 and O 2 prior to enhanced ionization.