4d – 4f photoabsorption in laser-produced thulium plasmas
The dual laser plasma (DLP) technique exploits the continuum emission from one laser produced plasma to probe the ions present in a second plasms (Costello et al, Phys. Scr., 1991, T34, 77). By varying the time delay between the formation of the plasma containing the ions to be observed and the creation of the continuum, it is possible to probe successively lower stages of ionisation. Typically the absorbing plasma is formed as a column in which the plasma electron temperature is below 5 eV. Within laser produced plasmas of this kind, successive ionisation occurs by electron impact ionisation (Colombant and Tonan, J. App. Phys., 1973, 44, 3524)
The output of a Q-switched laser (EKSPLA 512, 5 Hz, 450 mJ at 1064 nm) was focused tightly onto a planar hafnium target to produce a backlighting continuum source, hafnium yields an essentially line-free continuum at these wavelengths. This is due to the extremely high numbers of weak lines which are buried in plasma continuum and bremsstrahlung radiation.The output of a second Nd:YAG laser (Continuum Surelite II, 10 Hz, 600 mJ at 1064 nm) was focused to form a line plasma containing thulium of width 1.4 mm via a cylindrical lens.
In this work, spectra revealing photoabsorption due to Tm, Tm+ and Tm2+ ions has been observed in the spectral region between 6 and 8 nm where 4d to 4f photoabsorption is the dominant process. These ions are of particular interest as they have a 4d104f136sn (n = 2, 1 & 0 for Tm, Tm+ and Tm2+, respectively). Thus the 4d – 4f transitions are of the form 4d104f13 - 4d94f14 with the 6s electrons effectively playing a spectator role where present. Only photoabsorption from the lower 2F7/2 term in neutral thulium has been observed prior to this work (Radtke, J. Phys. B: Atom. Molec. Phys 1979 12 L71]. For each ion stage the absorption profile can be explained in terms of two spin-orbit components based on the was observed in this region, peaking at ≈ 7 nm which is maintained through all ion stages.
Last modified: Tue Jun 6 16:40:02 AWST 2017