Abstract

Quantitative measurements of the optical conductivity of iron under earth core conditions are important in modelling geomagnetism[1]. We approximate such conditions transiently [2] by exciting an Fe, and a control Al, surface in a vacuum or helium environment with 620 nm, 100 fs FWHM laser pulses with 10⁵ peak-background contrast ratio at .6 ps focussed to peak intensities 10¹²< I < 10¹⁵ W/cm² on target. Figs. 1 and 2 present p- and s-polarized self-reflectivity R_p,s (θ, I) for constant incident intensity I and angle θ, respectively. Geometric corrections for the dependence of spot shape on θ and for Gaussian transverse intensity profile have been made in plotting the data. Using very linear pulse energy reference monitors, reproducibility to within Δ_R/R ≤ 0.1 % was achieved. Fig. 1 shows that the reflectivities of Fe and Al in the solid to plasma transition region (10¹³ - 10¹⁵ W/cm²) approach each other as I increases, consistent with their similar total conduction electron densities (n ~1.8 × 10²³ cm⁻³) when the Fe d-electrons are included. This suggets qualitatively that progressive unbinding of the d-electrnos dominates the changes in optical properties of Fe in this regime.

© 1996 Optical Society of America