John Costello

Dublin City University | DCU · School of Physical Sciences

The expansion dynamics of laser produced plasma plumes in gaseous atmospheres exhibit information on plasma-ambient gas interactions which result in plume splitting, shock formation, sharpening and confinement. We investigate laser spot size variation on shock wave, plume, and ion dynamics from laser produced YBa2Cu3O7 (YBCO) plasmas using fast pho...

New information demonstrating the importance of both sequential and simultaneous (or direct) multiphoton ionization of inner shell electrons from neon is discussed in this paper. Ne was irradiated with intense 93 eV free electron laser (FEL) pulses at FLASH and studied with the aid of photoelectron spectrometry. This resulted in two and three photo...

It has been well established in the literature that surface contamination can adversely affect the aerodynamic performance of aerofoils and hence the efficiency with which turbines can convert wind to electrical power. Hence it is critical to ensure that turbine blades are kept as free as possible of contaminants. In this manuscript, we discuss LIB...

Time and polarization resolved optical emission spectroscopy of laser-produced plasmas formed on Al alloys has been studied in the visible spectral range at a background air pressure of 10 −4 mbar. A comparative experimental investigation has been made between the results obtained from the polarization-resolved laser-induced breakdown spectroscopy...

An open-loop control scheme of molecular fragmentation based on transient molecular alignment combined with single-photon ionization induced by a short-wavelength free electron laser (FEL) is demonstrated for the acetylene cation. Photoelectron spectra are recorded, complementing the ion yield measurements, to demonstrate that such control is the c...

The purpose of this study is to investigate the possible improvement of the limit of detection of laser induced breakdown spectroscopy using the stagnation layer formed at the centre of an annular plasma. An axicon was used to form an annular plasma on various certified reference targets, the spectra from which were used to construct calibration cu...

Steel impacts a variety of industries such as construction, automotive and ship building. The physical properties of steel (e.g., hardness, weldability) are significantly affected by the concentration of carbon. Hence, accurate determination of carbon content in steel has been, and still is, an active field of research. Laser induced breakdown spec...

We report on measurements of resonant three-step, two-colour ionization of atomic molybdenum, using a hollow cathode lamp (HCL) with optogalvanic detection. Wavelength scans were made for two specific transitions involved in the ionization pathways under investigation, namely 4d5(6S)5s 7S3 - 4d5(6S)5p 7P4 and 4d5(6S)5p 7P4 - 4d5(6S)6d 7D5.So-called...

The interaction of ionizing photons with atoms or ions is a fundamental process in nature, with laboratory, atmospheric and astrophysical implications [...]

Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. The relaxation occurs primarily via Auger emission, so excited-state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained...

The effect of laser wavelength on the dissociation mechanisms in methane is examined over a broad range of intensities for both 800 nm and 400 nm laser fields. It is found that, at lower laser intensities, the dissociation pathways identified with the aid of theoretical calculations for the methane cation can account for most of the experimental fi...

In this reply, we address the comments made by A N Ryabtsev on our paper (H Lu et al 2020 J. Phys. B: At. Mol. Opt. Phys. 53 115001). Specifically, we emphasize that the observed spectra depend critically on the plasma source type and conditions that pertain in the source. Our source of ions and experimental setup/method are quite different to the...

We report angle-resolved measurements on photoelectrons emitted upon near-threshold two-photon double ionization (TPDI) of Kr irradiated by free-electron laser (FEL) pulses. These photoelectron angular distributions (PADs) are compared with the results of semirelativistic R-matrix calculations. As reported by Augustin et al. [Phys. Rev. A 98, 03340...

In this work we investigate the effect of the confinement angle on self-colliding aluminium laser produced plasmas. More specifically, we apply V-shaped channel targets of different angles (90 • , 60 • and 30 •) and report both broadband and filtered time-resolved fast imaging measurements on the formation of such plasmas in ambient air. Based on t...

The photoabsorption spectrum of Bi + was measured in the wavelength range between 37 and 60 nm, using the dual laser plasma technique in which one plasma is used as the source of vacuum ultraviolet continuum radiation and the other plasma is used as the sample of atoms and/or ions to be probed. A number of features in the Bi + spectrum was identifi...

The photoabsorption spectrum of Bi + was measured in the wavelength range between 37 and 60 nm, using the dual laser plasma technique in which one plasma is used as the source of vacuum ultraviolet continuum radiation and the other plasma is used as the sample of atoms and/or ions to be probed. A number of features in the Bi + spectrum was identifi...

We have measured the photoabsorption spectra of Pb II and Bi III in the wavelength range between 37 and 70 nm. A number of features in the spectra have been identified with the aid of the Cowan suite of atomic codes. 5d → 6p transitions from the ground configuration (5d¹⁰6s²6p) and three excited configurations (5d¹⁰6s²6d, 5d¹⁰6s²7s, 5d¹⁰6s6p²) give...

We report oxygen K-shell photoabsorption measurements in CO2 plasmas, using a table-top laser produced plasma soft x-ray (SXR) and extreme ultraviolet source. Significant differences are observed between the photoabsorption spectra of the neutral CO2 molecule and photoionized CO2 plasmas, as new spectral features emerge in the case of the latter. T...

We report here the results of a study of polarisation resolved emission from a laser produced copper plasma. We found that in the case of such a plasma, the degree of polarisation was higher for a background pressure of 1 × 10-2 mbar than for 1 × 10-5 mbar, but the time evolution of this parameter was similar for both. Optical diagnostics revealed...

Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. As the relaxation occurs primarily via Auger emission, excited state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained...

In this work we report soft x-ray (SXR) photoabsorption measurements in neutral methane and carbon dioxide molecules and their corresponding photoionized plasmas. The SXR radiation was generated by a table-top laser produced plasma source based on a double stream gas puff target. At low SXR intensities only features related to neutral molecules are...

The past decade has witnessed the emergence of a new X-ray light source technology that has been under development for some decades, the so-called Self Amplified Spontaneous Emission (SASE) based Free Electron Laser (FEL). Such lasers provide pulses with durations from just below one femtosecond (10-15 seconds) to over 100 femtoseconds with energie...

Results of angle-resolved electron spectroscopy of near-threshold photoionization of Ne atoms by combined femtosecond extreme ultraviolet and near infrared fields are presented. The dressed-electron spectra show an energetic distribution into so-called sidebands, being separated by the photon energy of the dressing laser. Surprisingly, for the low...

Spatially resolved and temporally integrated vacuum ultraviolet laser induced breakdown spectroscopy (VUV LIBS) was used for contaminant analysis in thermally aged oil impregnated pressboard (OIP) insulation material taken from high voltage transformers. The signature of copper sulphide contamination was obtained through detection of copper and sul...

List of Scope, Invited Speakers, International Programme, Local Organising Committee are available in this pdf. 24 th International Conference on Spectral Line Shapes (ICSLS 2018) It is commonly accepted that the Europhysics Study Conference on Spectral Line Broadening and Related Topics which was held in Meudon in 1973 was the first in the current...

We present time resolved, spatially integrated spectra of aluminium II oxide formed from the condensation of an aluminium laser plasma in air. The spectra show a revival of the emission from aluminium oxide molecules at 35 µ s after the plasma ignition. Thermodynamic calculations, together with temperature measurements, indicates that as the plasma...

Initial results from time resolved imaging on the formation of a plasma in ambient air, with properties akin to those of the stagnation layer that can be formed at the collision front between two expanding plasmas in vacuo, are reported. Our proposed method for creating stagnation in air at atmospheric pressure is to take a single laser pulse focus...

We report the development of an electrostatic sector analyser (ESA) combined with a time-of-flight (TOF) spectrometer along with some preliminary results from its use in measuring the energies of ions emitted from colliding laser produced plasmas.

Laser Induced Breakdown Spectroscopy (LIBS) is an analytical technique used to classify and potentially quantify elements in complex hosts (or matrices) [1,2]. In this study, silicon based aluminium thin films were developed to study the depth profile and ablation rate of the material. Five films with different thicknesses from 1mm to 1.5 micron we...

We report the development of a portable single-shot Fourier transform spectrometer for application in laser induced breakdown spectroscopy (LIBS) with a particular emphasis on remote or standoff LIBS. The device, based on a design by Harvey and Padgett[1], has a number of inherent features that give it performance advantages over a portable grating...

In this work, we present measurements of the intensity-dependent photoelectron spectrum of Kr irradiated by the FLASH FEL tuned to a photon energy of 25.8 eV. Intensity dependent photoelectron spectra were obtained with the aid of a Velocity Map Imaging (VMI) spectrometer. As the FEL photon energy is close to threshold, two photon sequential double...

Results of angle-resolved electron spectroscopy of near-threshold photoionization of Ne atoms by combined femtosecond extreme ultraviolet and near infrared fields are presented. The dressed-electron spectra show an energetic distribution into so-called sidebands, being separated by the photon energy of the dressing laser. Surprisingly, for the low...

Arbitrary manipulation of the temporal and spectral properties of X-ray pulses at free-electron lasers (FELs) would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the FEL driving electron bunch can be tuned to emit a pair of X-ray pulses with...

The formation of CuTiO2 nanocomposites has been observed in an experiment in which laser plasma plumes of Cu and Ti collide and stagnate in an oxygen atmosphere. The inherent advantage of this technique lies in its simplicity and flexibility where laser, target composition and geometry along with ambient atmosphere are all controllable parameters t...

For the investigation of processes happening on the time scale of the motion of bound electrons, well-controlled X-ray pulses with durations in the few-femtosecond and even sub-femtosecond range are a necessary prerequisite. Novel free-electron lasers sources provide these ultrashort, high-brightness X-ray pulses, but their unique aspects open up c...

We report on the findings of double-pulse studies performed on an aluminium target submerged in water using Nd:YAG laser pulses. Shadowgraphy measurements were performed to examine the dynamic behaviour of the cavitation bubble that eventually forms some considerable time post-plasma ignition. These measurements were used to inform subsequent inves...

Anisotropic emission from a laser-produced aluminium plasma emission has been studied using time and polarisation resolved spectroscopy. The time-resolved degree of polarisation of emission from a bound – bound transition in Al + (466.3 nm) was compared to the time-resolved degree of polarisation from nearby continuum emission. The results are inte...

In this work, we present the photon energy dependence of the two- and three-photon cross sections of the two-electron Li+, Ne8+ and Ar16+ ions, following photoionization from their ground state. The expressions for the cross sections are based on the lowest-order (non-vanishing) perturbation theory for the electric field, while the calculations are...

The polarisation anisotropy of the emission from a laser produced aluminium plasma has been studied using time and polarisation resolved spectroscopy at various background pressures of air. A Wollaston prism was used to resolve the emission from the plasma into polarisation components that are parallel and orthogonal to the plasma expansion axis. S...

The design and implementation of a low cost interferometric autocorrelator with no moving parts is discussed. It is found that the device is optically simple, uses low cost components and self aligns. The device is used to measure the interferometric autocorrelation of an 800nm Ti:Sapphire laser pulse produced from an 80MHz oscillator. The theory a...

Laser ablated copper-titanium colliding plasma plume expansion at 10⁻⁴, 10⁻², and 10⁰ mbar of oxygen ambient pressures has been investigated with a view to understanding heterogeneous colliding plasma plume dynamics and the formation of multi-element stagnation layers using optical emission spectroscopy and fast imaging of plume. The observation of...

Electron emission caused by extreme ultraviolet (XUV) radiation in the presence of a strong near infrared (NIR) field leads to multiphoton interactions that depend on several parameters. Here, a comprehensive study of the influence of the angle between the polarization directions of the NIR and XUV fields on the two-color angle-resolved photoelectr...

In a combined experimental and theoretical study we have investigated the ionization of atomic argon upon irradiation with intense soft-x-ray pulses of 105 eV photon energy from the free-electron laser FLASH. The measured ion yields show charge states up to Ar7+. The comparison with the theoretical study of the underlying photoionization dynamics h...

The photoelectron angular distribution and the circular dichroism in two-colour XUV+NIR above-threshold ionization of helium atoms have been investigated both experimentally and theoretically. Circularly polarized XUV pulses from the free electron laser FERMI have been spatially and temporally overlapped with circularly polarized optical pulses in...

In a combined experimental and theoretical study we have investigated the ionization of atomic argon upon irradiation with intense soft-x-ray pulses of 105 eV photon energy from the free-electron laser FLASH. The measured ion yields show charge states up to Ar$^{7+}$. The comparison with the theoretical study of the underlying photoionization dynam...

In this paper, we report results on two-colour above-threshold ionisation, where extreme ultraviolet pulses of femtosecond duration were synchronised to intense infrared laser pulses of picosecond duration, in order to study the laser-assisted photoelectric effect of atomic helium, neon, krypton and xenon which leads to the appearance of characteri...

The bi-modal nature of charge integrated ion kinetic energy distributions, which result from ultrafast laser produced plasmas, is discussed in this paper. A negatively biased Faraday cup was used as a charge collector to measure ion distributions from three different solid targets that had been irradiated with an ultrafast laser in the fluence rang...

The Pauli-blocking range can be modified by silver nanoparticles grafting of a few layer graphene. The open aperture Z-scan experiment was performed to examine the nonlinear response of the materials. The excitation was done with second harmonics of the Ti sapphire fs amplifier. The results were explained based on the Pauli-blocking, two photon abs...

Nonlinear optical properties of gold nanoparticles (AuNPs) embedded ZnO thin films have been studied at 400 nm. While the AuNPs thin film shows the effect of saturable absorption, AuNPs embedded ZnO film exhibits the RSA behaviour.

We present the results of a time and space resolved optical-spectroscopic study of colliding plasmas formed at the front surfaces of flat and inclined Cu slab targets as a function of both the distance and the wedge angle between them for angles ranging from 100° to 180° (laterally colliding plasmas). The key parameters studied are stagnation layer...

The galvanic manifestation of two transformations of degenerate magnetic states of the neon atom, namely self-alignment non-alignment and alignment orientation is studied by monitoring the voltage across the discharge in a range of hollow cathode discharge (HCD) lamps. The application of a magnetic field to the discharge disorders the self-alignmen...

Deploying the so-called 'Streaking Spectroscopy' technique at LCLS, we demonstrate a non-invasive scheme for temporal characterization of X-ray pulses with sub-femtosecond resolution. Analyzing the substructure indicates pulse durations on the order of hundreds of attoseconds.

Photoionization cross sections and dipole angular distribution asymmetry parameters, β, of 5s and 4d shells of the closed-shell ions (La3+,La9+, and La11+) in the La isonuclear sequence have been studied using the relativistic random phase approximation. The positions of the 5s Cooper minima in La3+ and La9+ ions are found to be extremely sensitive...

Many organic molecules have strong absorption bands which can be accessed by ultraviolet short pulse lasers to produce efficient ionization. This resonant multiphoton ionization scheme has already been exploited as an ionization source in time-of-flight mass spectrometers used for environmental trace analysis. In the present work we quantify the ul...

Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body syst...

In this work, anisotropic emission from a laser produced plasma has been studied using time integrated emission spectroscopy and the results have been interpreted in the framework of radiatively re-combining plasmas. In a laser produced plasma, dichroic anisotropies in the emission can occur [1]. In some cases the background broadband emission is f...

Many advanced applications of X-ray free-electron lasers require pulse durations and time resolutions of only a few femtoseconds. To generate these pulses and to apply them in time-resolved experiments, synchronization techniques that can simultaneously lock all independent components, including all accelerator modules and all external optical lase...

Short-wavelength free-electron lasers are now well established as essential and unrivalled sources of ultrabright coherent X-ray radiation. One of the key characteristics of these intense X-ray pulses is their expected few-femtosecond duration. No measurement has succeeded so far in directly determining the temporal structure or even the duration o...