Laser produced plasmas as a source of ions, protons and X-rays

The work presented in this thesis is primarily focused on the use of a laserproduced plasma as a source of protons, ions and X-rays. It explores highimpact applications of both high power ultrafast lasers and nanosecond lasers.Section 1 gives a general introduction to the physics governing the experimentsand the lasers in the following sections. The physics of plasmas isdescribed in Section 1.1. The physics of ultrafast lasers is described in detailin Section 1.2 from the theory of mode locking of oscillators to the compressionof high power ultrafast systems. The physics and mechanisms of X-raylasers are then explained in Section 1.3. Both the collisional excitation mechanismand the Linford gain equation are described along with a summery ofthe challenges in pumping X-ray lasers. Next in Section 1.4 laser acceleratedprotons are described in detail. Acceleration mechanisms are described andthe interaction mechanism with insulators is introduced. Finally, a numberof tools for photoabsorption studies are described in Section 1.5. In this Sectionthe dual laser plasma technique for obtaining photoabsorption spectra isdescribed along with the atomic structure codes (the Cowan and RTDLDAcodes) used to simulate the spectra.Section 2 describes all the laser systems used in this thesis. The variouscomponents of the Quantronix ultrafast laser system from UCD includingthe oscillator, the Odin-II amplifier and the Thor amplifier are described inSection 2.2, Section 2.3 and Section 2.4. The operation of the Surelite III andthe Spectron SL805 Q-switched nanosecond lasers are described in Section2.5 and Section 2.6. Finally, the TARANIS multi-terrawatt system and itsdifferent components are described in Section 2.7.Section 3 is covers two different experiments involving the generation ofX-rays from a laser produced plasma. The first technique described in Section3.3 is the generation of coherent X-rays by pumping a preformed laser plasmawith the TARANIS laser system. A Ni-like Mo laser was successfully pumpedyielding energies of 2 x 10-7 J with a gain length product of 3000.The second technique described in Section 3.5, involved inner shell X-rayemission from an indium target irradiated by the Quantronix laser system.The indium K series emission was observed using a lithium-drifted silicondetector. Seven counts of K_ emission was detected under the followingconditions (12000 shots through a 0.5 mm pin hole filtered with a 60_m Alfoil and a laser energy of 30 mJ). Due to the laser reliability this experimentis only a preliminary one which is intended to be continued at a later stage.Section 4 describes a laser accelerated proton experiment conducted in theCenter for Plasma Research in Queens University Belfast, using the TARANISlaser system. In this experiment 13 MeV protons were accelerated from10 μm gold foil targets into a sample of BK-7 glass. The interaction of theprotons with the glass were observed by taking spatially resolved images ofthe transient opacity induced by the protons interacting with the BK-7 Glassand an optical probe beam. These spatially resolved images are presented inSection 4.3.In Section 4.4.2 an optical streaking technique experiment is described tocalculate the lifetime of the transient opacity induced by the proton interaction.This process showed that the opacity in the glass began to occur 62ps after the TARANIS main pulse was _red and reaches 30% transmission88 ps after the firing of the TARANIS main pulse. Due to the limited timewindow available, the exact lifetime of the opacity is not known, howeverglass is seen to de-excite after 138 ps and has returned to 50 % transmissionby 175 ps.Section 5 describes the refurbishment of the 1-m normal incidence VUVspectrometer. It describes the replacement of a photographic plate based detectionsystem with a linear CCD array. The CCD array can detect the VUVradiation through a sodium salicylate phosphor coating which emits at 410nm on interacting with VUV radiation. Different phosphors are compared interms of sensitivity and ease of coating and the grounds for choosing sodiumsalicylate are explained. The adaptations to the spectrometer to use the linearCCD array are described and the details on calibrating the spectrometerare explained.Finally, Section 6 describes a set of spectroscopic experiments which usethe refurbished 1-m normal incidence spectrometer. First Section 6.2 describesa repeat of the photoabsorption of indium and indium plus whichwas conducted previously on the spectrometer. This was designed as a proofof principle of the working of the new phosphor based linear CCD arraysystem.Next, Section 6.3, describes the photoabsorption of thulium in the 23 eVto 40 eV region. The experimental spectrum is compared to the RTDLDAcalculations and to the autoionized Cowan code calculations. The simulationssuccessfully describe the observed absorption structure and 5p → nd and5p → ms transitions are identified.In Section 6.4 an indium laser produced plasma is successfully reheatedusing the Odin-II first amplifier of the Quantronix laser system. An experimentis described which shows emission from an indium plasma fromtransitions which are normally observed in absorption. This reheating wasoptimized with a specific set of target parameters (a delay of Δτ= 500 nsbetween the lasers, the Odin was focused into the center of the absorbing lineplasma, which was set at a height of Δz = -0:2 mm from the optical axis of the spectrometer)

Quantum watch and its intrinsic proof of accuracy

We have investigated the rich dynamics of complex wave packets composed of multiple high-lying Rydbergstates in He. A quantitative agreement is found between theory and time-resolved photoelectron spectroscopyexperiments. We show that the intricate time dependence of such wave packets can be used for investigatingquantum defects and performing artifact-free timekeeping. The latter relies on the unique fingerprint that iscreated by the time-dependent photoionization of these complex wave packets. These fingerprints determinehow much time has passed since the wave packet was formed and provide an assurance that the measured time iscorrect. Unlike any other clock, this quantum watch does not utilize a counter and is fully quantum mechanicalin its nature. The quantum watch has the potential to become an invaluable tool in pump-probe spectroscopy dueto its simplicity, assurance of accuracy, and ability to provide an absolute timestamp, i.e., there is no need to findtime zero

Experimental confirmation of the delayed Ni demagnetization in FeNi alloy

Element-selective techniques are central for the understanding of ultrafast spin dynamics in multi-element materials, such as magnetic alloys. Recently, however, it turned out that the commonly used technique of the transverse magneto-optical Kerr effect (T-MOKE) in the extreme ultraviolet range may have issues with unwanted crosstalk between different elemental signals and energy-dependent non-linear response. This problem can be sizeable, which puts recent observations of ultrafast spin transfer from Fe to Ni sites in FeNi alloys into question. In this study, we investigate the Fe-to-Ni spin transfer in a crosstalk-free time-resolved x-ray magnetic circular dichroism (XMCD) experiment with a reliable time reference. With XMCD near the absorption maxima, we find a very similar Fe and Ni dynamics as with T-MOKE from identical samples. Considering the potential non-linearities of the T-MOKE response, such a good agreement in our findings is remarkable. Our data provide the ongoing discussion about ultrafast spin-transfer mechanisms in FeNi systems with a broader experimental basis

Ultrafast dynamics in Fe65Co35 alloys: Effect of Re doping

Soft magnetic FeCo alloys are of great interest due to their potential spintronics applications. The magnetic damping parameter plays a vital role in the performance of these spintronics devices. The Gilbert damping parameter increase in these alloys with doping of 5d elements. Here, we have investigated the effect of Re doping on the element-specific magnetization dynamics of Ru/Fe65Co35/Ru thin films using the time-resolved magneto-optical Kerr effect. When varying the concentration of Re from 0 to 12.6 %, no change of the demagnetization time constant is observed. However, a gradual change of the remagnetization time is observed with the increase of Re concentration. This remagnetization dynamics can be related to the Gilbert damping parameter of these films. An interesting  time-resolved dynamics at the Ru-edge is observed. A significant increase (40%) of the asymmetry signal is observed for the undoped sample and drops down with the Re doping. This effect is possibly a super diffusive spin current going from the magnetic layer to the non magnetic capping layer

Ultrafast dynamics in Fe65Co35 alloys: Effect of Re doping

Soft magnetic FeCo alloys are of great interest due to their potential spintronics applications. The magnetic damping parameter plays a vital role in the performance of these spintronics devices. The Gilbert damping parameter increase in these alloys with doping of 5d elements. Here, we have investigated the effect of Re doping on the element-specific magnetization dynamics of Ru/Fe65Co35/Ru thin films using the time-resolved magneto-optical Kerr effect. When varying the concentration of Re from 0 to 12.6 %, no change of the demagnetization time constant is observed. However, a gradual change of the remagnetization time is observed with the increase of Re concentration. This remagnetization dynamics can be related to the Gilbert damping parameter of these films. An interesting  time-resolved dynamics at the Ru-edge is observed. A significant increase (40%) of the asymmetry signal is observed for the undoped sample and drops down with the Re doping. This effect is possibly a super diffusive spin current going from the magnetic layer to the non magnetic capping layer

Resonant inelastic x-ray scattering on CO2 : Parity conservation in inversion-symmetric polyatomics

Resonant inelastic x-ray scattering (RIXS) spectra excited at the oxygen K edge of CO2 are presented and discussed. Although excitation from a gerade initial state to the intermediate 1s-1π* state breaks the inversion symmetry due to strong vibronic coupling, RIXS excited at the corresponding resonance exclusively populates gerade vibrations in the gerade electronic ground state. This observation constitutes an experimental confirmation of the prediction that the parity selection rule applies in RIXS on an inversion-symmetric polyatomic system, provided that the total electronic-vibronic wave function is considered. Parity selectivity is used for assigning spectra to the population of electronically excited final states, a procedure hampered only when symmetry-breaking vibronic coupling in the final states is prominent. A RIXS spectrum excited in the Rydberg region is tentatively assigned using a simplified quasi-two-step model in which it is assumed that the electron in the Rydberg orbital excited in the first step remains as a spectator during the second decay step

Element-Specific Magnetization Dynamics in Co-Pt Alloys Induced by Strong Optical Excitation

Ever since its first observation, the microscopic origin of ultrafast magnetization dynamics has been actively debated. Even more questions arise when considering composite materials featuring a combination of intrinsic and proximity-induced magnetic moments. Currently, it is unknown whether the specific ultrafast dynamics of different sublattices in the popular ferromagnets consisting of 3d (Co, Fe) and 4d, 5d (Pd, Pt) transition metals are playing a crucial role in various effects, including all-optical magnetization switching. Here we investigate the element-specific dynamics of Co-Pt alloys on femtosecond and picosecond time scales using magneto-optical spectroscopy in the extended ultraviolet (EUV) region. Our results reveal that despite the proximity-induced nature of the magnetization of Pt atoms, the two sublattices in the alloy can have different responses to the optical excitation featuring distinct demagnetization rates. Additionally we show that it is important to consider the modification of magnetic anisotropy in opto-magnetic experiments as the vast majority of them are sensitive only to a single projection of the magnetic moment on the predefined axis, which may lead to experimental artifacts

Ultrafast magnetization dynamics in half-metallic Co2FeAl Heusler alloy

We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy Co2FeAl,probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to resultsfrom electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we findthat the demagnetization time (τM) in films of Co2FeAl is almost independent of varying structuralorder, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower processof magnetization recovery, specified byτR, is found to occur on picosecond time scales, and isdemonstrated to correlate strongly with the Gilbert damping parameter (α). Our results showthat Co2FeAl is unique, in that it is the first material that clearly demonstrates the importance ofthe damping parameter in the remagnetization process. Based on these results we argue that for Co2FeAl the remagnetization process is dominated by magnon dynamics, something which mighthave general applicabili

Optical and extreme UV studies of spin dynamics in metallic and insulating ferrimagnets

We present all-optical studies of spin dynamics in two classes of ferrimagnets. Both sets of experiments use table-top laser-based pump–probe techniques to examine the ultrafast and longer timescale spin excitations. We use the visible/near infra-red time-resolved magneto-optical Kerr effect (tr-MOKE) to follow the spin dynamics of a series of metallic (FeCo)1−xGdx thin films with varying Gd content. Magnetic compensation in the films occurs at a Gd concentration of ≈26%, and the spin dynamics of the films exhibit a non-monotonic variation in effective magnetization. We also examine spin dynamics in an insulating NiFe2O4 spinel using ultrafast techniques up at extreme ultraviolet energies, which permit element-specific investigations. The element and time-resolved delay scans reveal a non-trivial interaction between spin excitations on the different magnetic sub-lattices of the magnetic insulator