Ultrafast Ionisation Dynamics Induced by Short Proton Pulses
This work presents experimental results obtained using the TARANIS laser at Queen’s University Belfast of transient absorption spectroscopy of dynamics induced by bursts of radiation, predominantly ions, from high-power laser-solid interactions. The absolute measurement of proton bursts as short as 3.7 ± 0.7 psis reported, enabled by the ultra fast decay pathways available to excited electrons in the formation of self-trapped excitonic states.The role of defects in the recovery of an induced opacity is examined in borosilicate glass. Here the recovery time of the opacity is measured to be orders of magnitude greater than the proton pulse duration, with a decay constant of over 550 ps. This long recovery time is attributed to the presence of interstitial energy levels due to defects created by the multi-component structure of the glass. Ultrafast pulsed-ion radiolysis is performed on H2O to investigate the formation of solvated electrons. Significant delay in the formation of these aqueous electrons is observed to occur under proton irradiation compared to X-rays/electrons. Molecular Dynamics simulations suggest that this is due to the formation of nanoscale cavities around the proton track in the Bragg region due to the significantly higher energy deposition in this region that does not occur for X-rays and electrons.The optical streaking technique is finally applied to investigate shifts in polarisation caused by irradiation of lithium niobate crystals. Two different oscillations are observed, firstly high frequency polarisation changes due to the arrival of relativistic electrons from the laser-target interaction followed by much lower frequency oscillations induced by the proton burst.
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