Plume Fluorescence Research Articles

ArF laser-induced plume fluorescence – normalization of the fluorescence spectra

Laser-induced plume fluorescence is linear and amenable to intensity normalization.

Soft classification of single samples based on multi-analyte spectra

Plume fluorescence spectrum of an unknown can be mapped to a point (black) in class space of partial-least-square discriminant analysis. Its proximity to known classes (red, green and blue) gives its membership probability. The accuracy and confidence of prediction is excellent.

Chemometric sorting based on laser-induced plume fluorescence: characterization of spectral noise for effective preprocessing

Single-shot laser-induced plume fluorescence spectra can be effectively denoised for clean unsupervised PCA sorting.

Elemental Analysis of Chinese Black Inks on Xuan Paper by ArF Laser-Excited Plume Fluorescence.

Chemical analysis of Chinese black ink on xuan paper is useful for the authentication of Asian artwork. The analysis has to be nondestructive and has to accommodate artworks of all sizes. We apply three analytical techniques, ArF laser-induced plume fluorescence, Fourier transform infrared (FTIR) spectroscopy, and portable X-ray fluorescence (pXRF) to analyze five commercial Chinese black inks on two kinds of xuan paper. The FTIR signal is found to be interfered by the substrate which is inevitable because the pigments diffuse extensively into the xuan fiber network. The XRF signal is shown to be feeble and no signal can be registered until the samples are stacked and when the analytes are present at tens of percent. In contrast, the plume fluorescence technique can detect the minor and trace signature elements. The method is based on a two-laser-pulse scheme performed on a high precision optical setup: the first 355 nm laser pulse ablates a thin layer of the ink to create a plume; the second 193 nm laser pulse induces multi analytes in the plume to fluoresce. Partial-least-squares discriminant analysis of the fluorescence spectra unambiguously sorts the ink-xuan combinations while the sampled area is not visibly damaged even under the microscope. The laser probe can handle samples of arbitrary size and shape, is air compatible, and no sample pretreatment is necessary.

Forensic Analysis of Laser Printed Ink by X-ray Fluorescence and Laser-Excited Plume Fluorescence

We demonstrated a minimally destructive two-tier approach for multielement forensic analysis of laser-printed ink. The printed document was first screened using a portable-X-ray fluorescence (XRF) probe. If the results were not conclusive, a laser microprobe was then deployed. The laser probe was based on a two-pulse scheme: the first laser pulse ablated a thin layer of the printed ink; the second laser pulse at 193 nm induced multianalytes in the desorbed ink to fluoresce. We analyzed four brands of black toners. The toners were printed on paper in the form of patches or letters or overprinted on another ink. The XRF probe could sort the four brands if the printed letters were larger than font 20. It could not tell the printing sequence in the case of overprints. The laser probe was more discriminatory; it could sort the toner brands and reveal the overprint sequence regardless of font size while the sampled area was not visibly different from neighboring areas even under the microscope. In terms of general analytical performance, the laser probe featured tens of micrometer lateral resolution and tens to hundreds of nm depth resolution and atto-mole mass detection limits. It could handle samples of arbitrary size and shape and was air compatible, and no sample pretreatment was necessary. It will prove useful whenever high-resolution and high sensitivity 3D elemental mapping is required.

Reaction between nitrogen gas and silicon species during pulsed laser ablation

The reaction processes occurring during the formation of silicon nitride films were investigated in this study. Films were deposited by pulsed laser ablation (PLA) of a silicon target in nitrogen gas. To reveal reaction processes during the film deposition, both the nitrogen gas pressure and the applied laser fluence were varied. The size of the plume fluorescence and the properties of the resulting film were measured as functions of gas pressure and laser fluence. It was found that the reaction between nitrogen gas and silicon clusters in the plume region effectively takes place under limited conditions. The decomposition of the background gas and multiple collisions of the silicon species in the plume region are essential for the reactive PLA method. Confinement of the species in the plume is one of the key factors to determine the composition and morphology of the film.

Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VII. Localized plasma formation on NaCl single crystal surfaces

Wide band gap insulators containing defects exposed to nanosecond pulses of UV laser radiation at fluences close to the damage threshold often display highly localized flashes of light. In this work, we show that flashes observed during irradiation of cleaved, single crystal NaCl at relatively low fluences are due to localized plume fluorescence. By comparing time-resolved optical images of this fluorescence with subsequent scanning electron microscope images of surface topography, we show that these flashes are often associated with micron-dimension surface and near-surface damage, typically associated with cleavage steps. With continued laser irradiation, plume fluorescence at previously damaged regions usually grows stronger from pulse to pulse. In some cases, weak plume fluorescence disappears after one laser pulse, and may or may not reappear with continued irradiation. We interpret these results in terms of localized laser absorption by deformation-induced defects generated during cleavage. Deliberately deformed material, produced by indentation, is damaged at considerably lower laser fluences, consistent with this interpretation. We suggest that mobile excitations produced by laser absorption preferentially decay along dislocation cores, which strongly localizes laser-induced thermal stresses and damage.

Interaction of wide band gap single crystals with 248 nm excimer laser radiation. V. The role of photoelectronic processes in the formation of a fluorescent plume from MgO

We examine the formation of a fluorescent plume during pulsed laser irradiation of wide band gap materials at sub-band gap photon energies. We show that fluences near the threshold for plume fluorescence in MgO do not produce adequate electron densities for significant laser-plume interactions via inverse bremsstrahlung processes. At a wavelength of 248 nm, 30 ns pulse width, and fluences well below the onset of detected plume fluorescence, we observe intense Mg+ emissions and electron temperatures in excess of 1 eV. The onset and growth of plume fluorescence closely parallels the neutral emission intensities, suggesting that the lack of plume fluorescence below the fluence threshold is due to the lack of gas phase neutral Mg. Time-resolved measurements of the atomic line emissions show peaks well after the peak laser intensity, arguing against direct laser-plume interactions. We propose that electron acceleration involves electrostatic interactions between photoelectrons and photoelectronically emitted ions. The unusual temporal evolution of the atomic line emissions can be attributed to the time dependence of the spatial overlap between neutral particles and the accelerated electrons that collisionally excite the atomic lines.

In situ diagnostics of pulsed laser deposition of ferroelectric Pb(Ti0.48Zr0.52)O3 on Si

The transport of material from target to substrate has been monitored in the pulsed laser deposition of Pb(Ti0.48Zr0.52)O3 films on p-doped Si(100) using two in situ diagnostics, namely plume fluorescence spectroscopy and plume ion mass spectrometry. The as-grown PZT films are specular, with thicknesses which decrease from 350 to 250 nm on a radius 5 mm from the center and show a dielectric hysteresis with typical remanent polarizations and coercive fields of 0.2 μC/cm2 and 124 kV/cm, respectively. The analysis of the plume fluorescence emission and ionic yield indicates that oxidation of the ablated material occurs during transport from target to substrate and that, in order to grow ferroelectric thin films, the substrate should be located in the region of the plume where the relative concentrations of metal oxides and clusters increase.