Pulsed Laser Desorption Research Articles

Pulsed 266 nm Laser Desorption of Aluminum, Hydrogen, and Water from H-Y Zeolites Studied with Time-of-Flight Mass Spectrometric Analysis

The pulsed laser desorption of monoatomic, Al, H2, and H2O was studied at 266 nm with the time-of-flight (TOF) method at laser fluences between 63.3 and 90.2 mJ cm-2. TOF data were analyzed with sh...

Laser desorption study of deuterium implanted in beryllium

Pulsed laser desorption was used to investigate the reemission of deuterium implanted in beryllium. The amount of D 2 molecules released as a function of the laser energy density was measured on samples with different implantation energies and doses. Numerical modelling of our results indicates that detrapping and diffusion are the limiting processes. The data are best fitted with second order detrapping with a trap energy of 1.7 ± 0.1 eV and the effective diffusion coefficient is described by the equation D = (8 ± 1) × 10 −8 exp(− 0.32 ± 0.02 eV/ kT) m 2 s −1.

Mechanistic aspects of fullerene coalescence upon ultraviolet laser desorption from thin films

Positively and negatively charged coalescence products of C60 and C70 were characterized following pulsed laser desorption from fullerene thin films. Coalescence occurs by gas-phase reactions. Positive ions are generated by thermoionization of hot neutrals, while negative ions likely form by electron attachment to neutral species in the outermost region of the desorption plume. Thermoionization rate and surface-induced dissociation (SID) measurements are consistent with fullerene-like structures for coalescence products detected as positive ions.

Edge ion fluxes and recycling studied in TdeV by use of biasable Be, C and Si collector probes and laser desorption

A pulsed laser desorption diagnostic was used to analyse, between two tokamak shots, the retention of hydrogen in a collector probe exposed in the scrape-off layer (SOL) of the TdeV tokamak. The probe can be rotated during a shot to obtain temporally resolved exposure, or biased electrically relative to the vacuum chamber. Hydrogen recycling was studied by measuring the ion saturation current drawn from the plasma by the probe and retention of H in the Be and C collectors. It is found that, at low incident energy, the H recycling coefficient is close to unity. The codeposition of carbon plays an important role in the retention of H. The main release mechanism for the collected hydrogen is thermal desorption. The effects of the conditions of the edge plasma, such as those produced by divertor plate biasing and boronization, were also studied. The former has a weak influence on the retention of H. A slight increase in the retention with positive biasing is believed to be due to the increase in the codeposition with the carbon flux. However, boronization shows a strong influence on the retention: the hydrogen retained in the probe is found to increase by 80%, compared with a tokamak shot with the same plasma density and current before boronization.

Effect of gas-phase collisions in pulsed-laser desorption: A three-dimensional Monte Carlo simulation study

The gas flow of particles laser desorbed from an elemental target into a vacuum is studied by Monte Carlo simulation. Pulsed desorption off a finite area is modeled; this is possible by using a three-dimensional simulation algorithm. We monitor the temporal evolution of the desorption cloud and global features of the flow, such as the number of collisions occurring in the gas, and the fraction of particles backscattered to the surface. The angle and energy distribution of the desorbed particles is investigated as a function of the number of monolayers desorbed, and the laser spot width. Our results show the formation of a desorption jet, in which fast particles are focused towards the jet axis, while slow particles leave the jet at oblique angles. Many features of the particle flux may be fitted by so-called elliptical distributions. However, these represent the velocity distribution of particles at oblique angles only poorly. Finally, we demonstrate the differences which exist between our three-dimensional simulation and previous, one-dimensional treatments. These studies are of fundamental interest for the characteristics of thin films deposited from the desorbed material.

Carbon cluster ions from low- and high-temperature glassy carbon, highly oriented pyrolytic graphite, and polycrystalline graphite

Carbon cluster ions have been produced by pulsed Nd:YAG laser desorption of four sp[sup 2]-hybridized carbon solids: glassy carbon-6 (GC-6), glassy carbons-25 (GC-25), highly oriented pyrolytic graphite (HOPG), polycrystalline graphite, and the oligomeric precursor to GC-6, poly(phenylenediacetylene). The desorbed ions were detected by Fourier transform ion cyclotron resonance mass spectrometry. Under similar laser and mass spectrometry conditions, polycrystalline graphite yielded abundant fullerene ions (C[sub 44][sup +] to C[sub 124][sup +]). GC-6, synthesized by a low-temperature route, gave increased ion populations from C[sub 14][minus] to C[sub 25][minus] relative to conventional GC-25. Laser desorption of the organic precursor, poly(phenylenediacetylene), yielded fullerenes from C[sub 60][sup +] to C[sub 15-][sup +] in positive-ion mode, and C[sub 3][minus] to C[sub 10][minus] in negative-ion mode. The present results show that (a) pulsed laser desorption of polycrystalline graphite produces a higher yield of fullerence cations compared to other sp[sup 2]-type carbon solids, (b) low-mass carbon cluster ions from C[sub 14] to C[sub 30] are consistently produced from all four carbon solids as positive ions but not as negative ions, and (c) no fullerene anions are produced from any of the carbon solids under these experimental conditions, implying different formation mechanisms for positive and negative carbon cluster ions.more » 59 refs., 3 figs., 1 tab.« less

Gas-phase segregation effects in pulsed laser desorption from binary targets.

Monte Carlo simulation is used to study the gas flow of particles laser desorbed from a binary target into a vacuum. Pulsed desorption off of a finite area is modeled by using a three-dimensional simulation algorithm. Our results show the formation of a desorption jet. For a thermal desorption mechanism, the light species is desorbed with a higher velocity than the heavy species, but accelerates the latter and focuses it toward the jet axis. These phenomena influence the composition of the desorption jet, and the angle and energy distribution of the desorbed particles

Instantaneous measurement of hydrogen isotope retention in collector probes in the scrape-off layer of a tokamak by pulsed laser desorption

The pulsed laser desorption technique is employed for the first time for the instantaneous analysis of hydrogen isotope fluxes in the scrape-off layer (SOL) of a tokamak. The laser desorption system developed for the TdeV tokamak is described. It has been used recently for the investigation of hydrogen isotope behavior in the SOL of TdeV during an isotope exchange experiment. A silicon probe is utilized for the collection of deuterium and hydrogen fluxes in both ion and electron drift directions. After exposure to a tokamak discharge, the hydrogen isotopes retained in the probe are analyzed in situ between two tokamak discharges by means of laser desorption. The measurements of the isotopic ratio D/(H+D) from hydrogen and deuterium retention in the Si collector probes show good agreement with neutral particle analysis, optical spectroscopy, mass spectrometry, and nuclear microanalysis. At the beginning of the experiment, the isotopic ratio decreases radially from the separatrix due to hydrogen recycling from the wall. As the changeover proceeds, the radial profile of the isotopic ratio becomes flat, showing evidence of the replacement of H with D in the wall. Hydrogen isotope retention is larger on the ion side than on the electron side. Measurements done during a plasma density scan show that the radial profile of retention changed with density due to the variation of the incident fluxes.

Laser desorption sources and time-of-flight injection for RFQ traps

Pulsed laser desorption sources are well suited to the production of ions to be stored in Paul radio-frequency quadrupole traps. With external injection, ions of different masses may be cleanly separated by time-of-flight and efficiently captured. Resonant ionization techniques may be used at the source for element selection.

Multiphoton ionization study of water and ammonia elimination from dipeptides with pulsed rapid heating and laser desorption for sample vaporization

The origins of the peaks at 17 and 18 mass units lower than the molecular ion peak, often found in the multiphoton ionization (MPI) mass spectra of dipeptides, have been investigated. Further experimental results confirm earlier reports that thermal decomposition takes place during laser desorption (LD) as well as in a pulsed rapid heating process in a time-of-flight mass spectrometer with or without the use of a supersonic jet. The M — 18 peak is generated from the MPI of the thermally decomposed product, cyclodipeptide. The other process is the fragmentation of the molecular ion which results in the formation of the M — 17 peak along with the M — 16 peak, corresponding to the elimination of ammonia and amino radical respectively. However, the M — 17 and M — 16 peaks are only observed for the dipeptides having an N-terminal aromatic group. It is noted that these results are different from those obtained using LD-MPI with a Fourier transform mass spectrometer. A possible explanation for the discrepancy is given.

Quantitative analysis of deuterium implanted in crystalline silicon and pyrolytic graphite by pulsed XeCl laser desorption

We have investigated the pulsed XeCl laser induced thermal desorption of deuterium implanted in crystalline silicon (c-Si) and highly oriented pyrolytic graphite (HOPG). Calibrations and experimental procedures for the quantitative analysis of deuterium are described. The desorption thresholds are 0.55±0.09 and 0.43±0.07 J/cm2 for c-Si and c-axis HOPG, respectively. However, for a-axis HOPG, deuterium is difficult to desorb because of the higher thermal conductivity. As a desorption product from HOPG, CD4 has also been observed. The deuterium evolution in both c-Si and c-axis HOPG has been modeled with the code dtrlas. It appears to be mainly limited by the detrapping processes with the effective activation energies, EB= 1.2 ± 0.3 eV and EB = 2.35 ± 0.55 eV for c-Si and c-axis HOPG, respectively.

The laser desorption of organic molecules in ion mobility spectrometry

An application of pulsed ultraviolet laser desorption as a source of ions in ion mobility spectrometry is described. Examples of ion mobility spectra obtained for polyaromatic hydrocarbons and indole derivatives are given. A feature of laser desorption under atmospheric pressure conditions is found to be an apparent absence of ion fragmentation for most molecules studied.

On-line laser spectroscopy by resonance ionization of laser-desorbed, refractory elements

Resonance ionization mass spectroscopy was applied at the on-line isotope separator ISOLDE-3/CERN, Geneva to the refractory elements Au and Pt. The hyperfine structures and isotope shifts of gold and platinum isotopes were studied in long isotopic chains in the range from the stable isotopes down to the neutron-deficient ones with A = 183. Spins, magnetic moments, quadrupole moments and the differences in mean-square nuclear charge radii were determined. A mass-selected Hg ion beam was implanted into a graphite target. After the decay to Au or Pt, a pulsed atomic beam was formed by pulsed laser desorption. The atoms were ionized by three-colour, three-step resonant excitation. The photoions were detected mass-selectively by a time-of-flight mass spectrometer. An overall efficiency of ϵ ⋍ 10 −5 was achieved defined as the number of atoms detected in the optical resonance to the number of Hg atoms implanted into the target. The background was as low as 1 event per 1000 laser shots. The publication gives a detailed description of the applied method, the experimental setup and its performance.

Ion/molecule reactions of arsenic and phosphorus cluster ions: ionization potentials and novel reaction pathways

Abstract : Ionization potentials (IP's) for arsenic and phosphorus clusters (Asn, n = 1-5; Pn, n = 1-4) have been determined by gas phase charge transfer reactions. Arsenic and phosphorus cluster ions were generated by pulsed CO2 laser desorption from GaAs and InP substrates, mass selected, thermalized, and allowed to react with compounds of known ionization potential in a Fourier transform ion cyclotron resonance mass spectrometer. The IP's for As3 and As5, previously unreported, as well as more accurate IP values for several of the other clusters, have been determined. Products and rate coefficients for some interesting reactions of the cluster ions with the charge transfer agents are also reported. Keywords: Arsenic clusters, Phosphorus clusters, Ionization potentials, Charge transfer reactions, Fourier transform ion cyclotron resonance mass spectrometry.

Picosecond time-resolved laser desorption

We have developed a pump-and-probe scheme for measuring the flux of desorbing particles with ultrafast time resolution. Neutral particles released during pulsed laser desorption are photoionized with the help of a delayed probe pulse and detected with a mass spectrometer. By changing the time delay of the probe pulse the temporal evolution of the flux of the various desorbing surface species can be measured. We have studied picosecond thermal desorption from GaAs(100) surfaces. Desorption of Ga is measured using a fixed frequency probe pulse for non-resonant two-photon ionization or a two-color tunable probe pulse for resonant two-photon ionization. A detailed study was made of the effect of ion sputtering and thermal annealing, which lead to interesting changes of the desorption flux and its time dependence.

Applications of multiphoton ionization mass spectrometry: small protected nucleosides and nucleotides

Pulsed laser desorption with a 90 ns pulse-width IR laser is used to evaporate protected nucleosides and nucleotides as neutrals into an expanding supersonic beam. The addition of the IR-absorbing matrix material Na 3PO 4 increases the desorption yield of neutral deoxynucleotides. The neutral molecules are then ionized by tunable multiphoton absorption and mass analysed in a high resolution reflectron time-of-light mass spectrometer. Since all investigated compounds contain heterocyclic aromatic bases and aromatic protecting groups, resonant multiphoton ionization can occur readily by exciting the S 1 ← S 0 transition of the aromatic π-electron system. As found previously for other systems, the molecular ion is clearly detected for each nucleotide at low photon fluxes. Increased photon densities lead to the formation of extensive fragment ions, yielding information about the nucleotide's primary structure. The general fragmentation path via charging of the aromatic protecting function reflects the fact that photoionization is taking place in these protecting groups as well as in the heterocyclic aromatic nucleobases.

Interference effects in laser desorption mass spectrometry

A study is made of the temperatures which can be produced within samples in pulsed laser desorption mass spectrometry. Calculations are carried out to estimate the effects of optical interference on the temperatures attained within a sample film which is irradiated with a pulsed laser. In general, the effects predicted for samples which absorb at the laser wavelength can be substantial. The involvement of interference in laser desorption is likely to be seen as undesirable as it will lead to behaviour which is difficult to reproduce. The factors that determine the magnitude of interference effects are discussed.

Resonance enhanced multiphoton ionization of nucleosides by using pulsed-laser desorption in supersonic beam mass spectrometry

Pulsed-laser desorption is used as a means of volatilizing nucleosides into the gas phase for entrainment into a supersonic jet expansion. These species are then studied in a time-of-flight mass spectrometer by resonance enhanced multiphoton ionization. Using this combination of techniques either relatively soft ionization or controlled fragmentation can be produced. The degree of molecule ion formation is shown to be relatively strong compared to other soft ionization methods such as FAB/MS, SIMS, FD or FI. In addition, the resulting fragmentation produces ions that are characteristic of the structure of the nucleoside. It is further demonstrated that such fragmentation can be used to discrimination between isomeric nucleosides and also between ribose and deoxyribose sugar groups.

Resonance-Enhanced Multiphoton Ionization Jet Spectroscopy and Mass Spectrometry of Tyrosine-Containing Dipeptides Using a Pulsed Laser Desorption/Volatilization Method

Pulsed laser desorption is used as a means of volatilizing thermally labile tyrosine-containing dipeptides for entrainment into a supersonic jet expansion. The jet expansion provides ultracold molecules whose spectral features are probed by resonant two-photon ionization (R2PI) spectroscopy in a time-of-flight mass spectrometer. In the dipeptides studied, spectral contours ∼1.5 nm FWHM are observed. Although no sharp spectral features were obtained, the shape of the contour is quite distinctive, and isomeric dipeptides can be distinguished. In addition, the ions produced are monitored in a time-of-flight mass spectrometer. It is shown that soft ionization can be obtained for each of these dipeptides by optimizing the jet cooling and exciting at the maximum intensity of the specific origin transition. By an increase in the laser energy, fragmentation is produced which can be used for structural identification of each of these dipeptides. Other factors that may affect these results, such as thermal decomposition, are also investigated.

Resonant two-photon ionization spectroscopic analysis of indole and catechol derivatives using pulsed laser desorption with entrainment into supersonic jet expansions

Pulsed laser desorption is used as a means of volatilizing thermally labile indole and catechol derivatives for entrainment into a supersonic jet expansion. The jet expansion provides ultracold molecules whose sharp spectral features are probed by resonant two-photon ionization (R2PI) spectroscopy in a time-of-flight mass spectrometer.