Ablation Laser Fluence Research Articles

Electrical and optical characteristics of organic thin films fabricated by laser ablation

Non-polymer organic thin films, copper-phthalocyanine (CuPc) and 4-dialkylamino-4′-nitrostilbene (DANS), were fabricated by KrF laser ablation. Crystalline thin films with almost the same composition as the target material were deposited when reducing the laser ablation fluence close to the ablation threshold of ∼ 20 mJ/cm2. Higher laser fluences deplete some weakly bound atoms in the thin film, such as nitrogen in CuPc. Semiconductor characteristics of CuPc thin film were examined in a Schottky diode. DANS-doped amorphous fluoropolymer was also fabricated by combining two ablation deposition processes. A crystalline feature of DANS disappears at low DANS concentrations in a fluoropolymer matrix.

Diagnostic and analytical study of post ablation ionization of neutral atoms of major and minor constituents from a low alloy steel

Post ablation ionization (PAI) of neutral atoms from a low alloy steel has been investigated using non-resonant laser ionization in a time-of-flight mass spectrometer. By varying the delay between the ablation and ionization lasers, the velocity distributions of the Ti, V, Cr, Mn and Fe atoms have been determined simultaneously. These distributions have been recorded as a function of ablation laser fluence. The half-range Maxwell-Boltzmann velocity distribution has been used to fit the data and different characteristic temperatures have been determined for the various elements in the sample. The quantitative capability of this method for bulk and surface analysis has been evaluated by calculating the relative sensitivity factors (RSFs) for the various constituent elements. The RSFs for all of the elements are seen to be highly dependent on the delay between the ablating and ionizing lasers. This dependence was reduced by integrating the temporal dependent ion yield, leading to a significant improvement in the calculated RSF values. It was also found that the RSFs were not highly dependent on the power density of the ablation laser beam.

Multiphoton Ionization-Mass Spectrometric Study on Laser Ablation of Polymethylmethacrylate and Polystyrene at 308 nm

A multiphoton ionization-mass spectrometric study has been made on laser ablation of polymethylmethacrylate and polystyrene films by an excimer laser at 308 nm. “Unzipping” reactions occurred, i.e., neutral fragments ablating from the polymer surface were predominantly monomer and dimer methylmethacrylate and monomer and dimer styrene. Exponential dependence of fragment yields on ablating laser fluence and the Maxwell-Boltzmann-type velocity distributions of ablating species were found. They are due to an essentially thermal or photothermal ablation mechanism of these polymers in the experimental conditions.

Laser ablation of organic molecules from frozen matrices

AbstractThe technique of frozen matrix‐assisted laser ablation coupled with resonance‐enhanced multiphoton ionization and reflectron time‐of‐flight mass spectrometry was used to detect intact organic molecules directly from solutions. When frozen at the temperature of liquid nitrogen, the matrices of interest were ablated by a pulsed CO2 single‐mode laser. The analyte molecules emerging from the ablated plume were then ionized by a tunable XeCl excimer laser‐pumped dye laser and analysed with a gridless reflectron time‐of‐flight mass spectrometer. The ablation process from an ice matrix was studied with the amino acids tryptophan and tyrosine dissolved in an aqueous ethanol solution to a concentration level of 5 × 10−4 M. It was found that fragmentation of the analyte molecules is strongly dependent on the ablating laser fluence and that there is a laser fluence range just above the ablation threshold where the decomposition is negligible. The different fragmentation mechanisms are discussed and a cavitation under the liquid surface, causing the sonoluminescence signal from an ice matrix, was shown to be responsible for the decomposition of the analyte molecules ablated by a low photon energy IR laser. Under the appropriate conditions, the analyte molecules were found to have a low rotational temperature of about 150 K resulting from the jet‐like cooling in multiple collisions with matrix molecules.

Characteristics of organic thin films fabricated by laser ablation deposition.

Non-polymer organic thin films such as copper-phthalocyanine (CuPc) and 4-dialkylamino-4′-nitrostilbene (DANS) were fabricated by KrF laser ablation. Changes in the chemical composition and structure, and in the crystallinity were studied for various laser ablation conditions. Crystalline thin films with accurate composition transfer from the target material were deposited when reducing the laser ablation fluence close to the ablation threshold fluence of -20mJ/cm2. Semiconductor characteristics of CuPc thin film was examined in a Schottky diode.

Laser ablation of polymethylmethacrylate and polystyrene at 308 nm: Demonstration of thermal and photothermal mechanisms by a time-of-flight mass spectroscopic study

Laser ablation of polymethylmethacrylate and polystyrene films by an excimer laser at 308 nm was studied by detecting the fragments by time-of-flight mass spectroscopy after multiphoton ionization. Neutral fragments ablating from the polymer surface were predominantly monomer and dimer methylmethacrylate and monomer styrene, as deduced from the mass spectra. Thus, ‘‘unzipping’’ of these polymers occurs. The dependence of fragment yields on ablating laser fluence and kinetic energy distributions of ablating species demonstrates thermal and photothermal dissociation of these polymers under the experimental conditions.

Ablation of high-temperature superconductor studied by resonance ionization mass spectrometry (RIMS)

Laser ablation of bulk high-temperature superconductor (HTS) material provides a useful means of producing high-quality HTS thin films. Mass spectrometric probes of the ablation plume provide a microscopic understanding of the ablation event and plume development as well as providing a process monitor for the thin-film production. Resonance ionization mass spectrometry (RIMS) detection of the ablated neutral species supplies valuable physical information about the ablation event necessary for developing models of the process. Time-of-flight/RIMS detection of Cu, Y, and BaO ablated from YBa2Cu3O7−x indicates the ablation involves post-desorption gas phase collisions, which influence the ablation chemistry and dynamics (e.g., angular and velocity distributions). Approximately equal velocities are observed for all neutral species at constant ablation laser fluence. In addition, BaO exhibits a rotational temperature far less than the directed translational temperature.

High-resolution multiphoton laser-induced fluorescence spectroscopy of zinc atoms ejected from laser-irradiated ZnS crystals

Time-of-flight (TOF) measurements employing high-resolution multiphoton laser-induced fluorescence spectroscopy (LFS) have been used as a probe to determine the yield and velocity distributions of Zn atoms ejected from a ZnS single crystal under irradiation by 308-nm photons. For fluences between 20 and 80 mJ/cm2 (irradiated area 2 mm2), the velocity distributions could be fitted by Maxwell–Boltzmann distributions where the characteristic temperature increased from 2000 to 9000 K, respectively. The absolute neutral Zn yield also increased from 108 to 1012 atoms per pulse over this same fluence range. Plots of temperature (T) as a function of fluence (F) and yield as a function of 1/T and 1/F suggest thermal evaporation as the mechanism for the Zn emission.The results indicate that with increasing fluence a critical combination of particle density and laser intensity is reached for formation of a plasma which interacts with the surface to cause catastrophic failure. For consecutive laser shots at constant ablation laser fluences, a nearly exponential increase of the Zn particle density was observed, although the measured kinetic temperature remains approximately constant even beyond the onset of visible damage to the ZnS surface. Doppler-shift techniques have been combined with TOF measurements for the first time in order to separate prompt from delayed emission of ablated atoms as well as to probe possible molecular or cluster ejection and fragmentation. Evidence for the latter phenomena was obtained.