Laser-induced Thermal Desorption Research Articles

Infrared photodesorption of CO from astrophysically relevant ices studied with a free-electron laser.

The infrared excitation and photodesorption of carbon monoxide (CO) and water-containing ices have been investigated using the FEL-2 free-electron laser light source at the FELIX laboratory, Radboud University, The Netherlands. CO-water mixed ices grown on a gold-coated copper substrate at 18 K were investigated. No CO photodesorption was observed, within our detection limits, following irradiation with light resonant with the C-O vibration (4.67 μm). CO photodesorption was seen as a result of irradiation with infrared light resonant with water vibrational modes at 2.9 μm and 12 μm. Changes to the structure of the water ice, which modifies the environment of the CO in the mixed ice, were also seen subsequent to irradiation at these wavelengths. No water desorption was observed at any wavelength of irradiation. Photodesorption at both wavelengths is due to a single-photon process. Photodesorption arises due to a combination of fast and slow processes of indirect resonant photodesorption (fast), and photon-induced desorption resulting from energy accumulation in the librational heat bath of the solid water (slow) and metal-substrate-mediated laser-induced thermal desorption (slow). Estimated cross-sections for the slow processes at 2.9 μm and 12 μm were found to be ∼7.5 × 10-18 cm2 and ∼4.5 × 10-19 cm2, respectively.

Surface Coverage- and Excitation Laser Wavelength-Dependent Luminescence Properties of U(VI) Species Adsorbed on Amorphous SiO2

Time-resolved luminescence spectroscopy is usefully used to identify U(VI) surface species adsorbed on SiO2. However, the cause of the inconsistent luminescence lifetimes and spectral shapes reported previously remains undetermined. In this study, the U(VI) surface coverage (Γ) and excitation laser wavelength (λex) were examined as the predominant factors governing the luminescence properties of U(VI) surface species. At neutral pH, the luminescence lifetimes of U(VI) surface species increased with decreasing Γ. In the low-Γ region, where a relatively large number of adjacent surface sites are involved in the formation of multidentate surface complexes, the displacement of more number of coordinated water molecules in the equatorial plane of U(VI) results in a longer lifetime. The pH-dependent luminescence lifetimes of U(VI) surface species at the same U(VI) to SiO2 concentration ratio in the pH range of 4.5–7.5 also explain the effect of the surface binding sites on the luminescence lifetime. The time-resolved luminescence properties of the U(VI) surface species were also investigated at different excitation wavelengths. Continued irradiation of the SiO2 surface with a UV laser beam at λex = 266 nm considerably reduced the luminescence intensities of the U(VI) surface species. The higher the laser pulse energy, the greater the decrease in luminescence intensity. Laser-induced thermal desorption (LITD) of U(VI) surface species is suggested to be the origin of the decrease in luminescence intensity. LITD effects were not observed at λex = 355 and 422 nm, even at high laser pulse energies.

Forensic investigation of arson residue by infrared and Raman spectroscopy: From conventional to non-destructive techniques.

Arson can result in highly challenging and complicated crime scenes. Much physical evidence undergoes chemical degradation because of the destructive nature of fire, while accelerants either completely burn or evaporate, and may be present in traces within any of the decomposed materials. To identify the original material and the accelerant involved, it is necessary to use advanced analytical techniques. Gas chromatography, with different detectors, is one of the most frequently used instruments in fire debris and accelerant analysis. Among other instruments, capillary electrophoresis and laser-induced thermal desorption Fourier transform mass spectrometry are two major contributors. Vibrational spectroscopy, including infrared absorption and Raman scattering, is one of the major non-destructive tools for the analysis of evidence because of its advantages over other spectroscopic techniques. Most studies involving vibrational spectroscopy (i.e. infrared and Raman spectroscopy) have focused on the identification of commonly found household materials, while very few studies have considered the identification of ignitable liquids. This article reviews studies based on an analysis of fire debris and accelerants by vibrational spectroscopic techniques and considers the limitations and future perspectives of arson investigations in forensic science.

Time-resolved laser-induced desorption spectroscopy (LIDS) for quantified in-situ hydrogen isotope retention measurement and removal from plasma facing materials.

A laboratory scale laser induced thermal desorption spectroscopy system is developed and tested on tungsten-deuterium and titanium-deuterium codeposits, and its feasibility as a hydrogenic inventory measurement diagnostic is demonstrated over a range of retention values from 5 × 1019 m-2 to 7 × 1023 m-2 for absorbed laser power densities as low as 8 MW m-2. Codeposit layer samples are grown by magnetron sputtering and immersed in a weak argon rf plasma. A 1 kW fiber laser (λ = 1100 nm) heats the samples up to a peak surface temperature ranging from 900 to 1500 K using pulse widths of 0.5 and 1 s. Spectral line emission from Balmer series Dα and Hα from thermally desorbed deuterium and hydrogen, as well as line emission from argon, are monitored as a function of time using an optical spectrometer with maximum temporal resolution of 1 ms. To correct for wall recycling and pumping speed, and to accurately measure the time evolution of the laser-induced thermal desorption, the raw Dα signal is deconvolved with the system response function, which is obtained by injecting a short burst of D2 to approximate an impulse. Calibration is done with a standard D2 leak, and laser induced desorption spectroscopy deuterium retention values are found to be in good agreement with companion measurements made using conventional temperature programmed desorption on samples from the same codeposit batch.

An apparatus to manipulate and identify individual Ba ions from bulk liquid Xe.

We describe a system to transport and identify barium ions produced in liquid xenon, as part of R&D towards the second phase of a double beta decay experiment, nEXO. The goal is to identify the Ba ion resulting from an extremely rare nuclear decay of the isotope (136)Xe, hence providing a confirmation of the occurrence of the decay. This is achieved through Resonance Ionization Spectroscopy (RIS). In the test setup described here, Ba ions can be produced in liquid xenon or vacuum and collected on a clean substrate. This substrate is then removed to an analysis chamber under vacuum, where laser-induced thermal desorption and RIS are used with time-of-flight mass spectroscopy for positive identification of the barium decay product.

The Effect of Laser Power Density on the Observed Products of Combustion of Gasoline Using Laser‐Induced Thermal Desorption with Fourier Transform Mass Spectrometry

Layered soot analysis is explored as a potential indicator of gasoline in a fire. Current techniques lack the ability to analyze layered soot samples without the destruction of the layered information. Laser-induced thermal desorption (LITD) coupled with Fourier transform mass spectrometry (FTMS) is developed as a method for analyzing soot samples. Samples of soot on glass from the free combustion of gasoline are collected and analyzed using LITD-FTMS, and the effect of power density on the resulting chemical profile is reported. At higher power densities, a distribution of low m/z products is observed. At lower power densities, the products observed shift to higher molecular weights, with peaks attributable to the tropyllium ion, naphthalene, and pyrene. Results at low power densities suggest that LITD-FTMS is a viable method for the analysis of soot, with potential for use with layered soot samples. Peaks potentially useful for fuel differentiation of gasoline are identified.

Laser-Induced Thermal Desorption Facilitates Postsource Decay of Peptide Ions

We investigated the thermal mechanism involved in laser desorption/ionization (LDI) of thermally labile molecules from the flat surfaces of amorphous Si (a-Si) and crystalline Si (c-Si). a-Si was selected for this study because of its thermal property, such as low thermal conductivity; thus, it was predicted to be highly susceptible to laser-induced surface heating. By virtue of lack of surface nanostructures, the flat surfaces offer a simple model system to focus on the thermal mechanism, avoiding other effects, including possible non-thermal contributions that can arise from the physical existence of surface nanostructures. For the energetics study, the internal energies of substituted benzylpyridinium ions produced by LDI on the bare and coated surfaces of a-Si and c-Si were obtained using the survival yield method. The results, including LDI thresholds, ion yields, and internal energies all suggested that the LDI mechanism would be indeed thermal, which is most likely promoted by thermal desorption caused by laser-induced surface heating. In addition, the LDI process driven by laser-induced thermal desorption (LITD) was also found to be capable of depositing an excessive internal energy in resulting LDI ions, which underwent a dissociation. It exhibited the essentially same features as in postsource decay (PSD) in MALDI-TOF/TOF mass spectrometry. We report that the LDI process by LITD offers not only a way of intact ionization but also a facile means for PSD of peptide ions, which this work demonstrates is well suited to peptide sequencing using TOF/TOF mass spectrometry.

Experimental and numerical analysis of sum frequency microscopic images of a H‐Si(111) 1 × 1 surface after IR light pulse irradiation

AbstractIn this study, we have observed sum frequency (SF) images of a HSi(111) surface after IR light irradiation of pulse width ∼6 µs, and power from 11 to 13 mJ/pulse. After the IR light pulse irradiation, nonresonant SF signals appeared in the irradiated area. The width of the area emitting the nonresonant SF signal was sensitive to the power of IR light pulses. We compared the profile of the SF signal with that of the calculated hydrogen deficiency by using laser‐induced thermal desorption (LITD) model. The profiles of the calculated hydrogen deficiency were consistent with those of the observed nonresonant SF signal qualitatively. Copyright © 2010 John Wiley & Sons, Ltd.

ChemInform Abstract: Laser-Induced Thermal Desorption Studies of Surface Reaction Kinetics

Laser induced thermal desorption (LITD) has proven to be an effective probe of reaction kinetics on single-crystal surfaces. The ability of LITD techniques to measure adsorption, decomposition and desorption kinetics will be illustrated by studies of SiCl4 and (CH3CH2)2 SiH2 on Si(111)7X7. Silicon tetrachloride, SiCl4, is important in silicon epitaxial growth and diethylsilane, (CH3CH2)2 SiH2, is a promising candidate for silicon atomic layer epitaxy. The desorption products, sticking coefficients and decomposition and desorption kinetics measured by these LITD investigations are important for an understanding of silicon epitaxy by chemical vapor deposition.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The sticking of H2(v = 1, J = 1) on Cu(100) measured using laser-induced thermal desorption

The sticking of vibrationally excited H2 on Cu(100) is measured using molecular beam techniques and stimulated Raman scattering for state preparation. Laser Induced Thermal Desorption (LITD) is developed to measure surface hydrogen coverage by calibration with known H-coverages. We find that LITD can be effectively used to measure adsorbed hydrogen concentrations with a sensitivity of better that 1% of a monolayer. Sticking coefficients of H2 are determined from the ratio coverage to exposure. Measurements are made for H2 incident translational energies in the range of 74–189 meV. For the first excited vibrational state, we find and an upper limit of the H2 sticking coefficient to be 0.065 at E i = 74 meV and 0.17 at E i = 189 meV. These limits imply that previous measurements of loss of H2(v = 1) upon scattering from Cu(100) cannot be predominately a result of dissociative adsorption. The results are broadly consistent with results determined from quantum-state resolved measurements for recombinatative desorption and the principle of detailed balance for H2 from Cu(111).

Proton transfer in water–hydroxyl mixed overlayers on Pt(1 1 1): Combined approach of laser desorption and spatially-resolved X-ray photoelectron spectroscopy

Proton transfer in water–hydroxyl mixed overlayers on a Pt(1 1 1) surface was studied by a combination of laser induced thermal desorption (LITD) method and spatially-resolved X-ray photoelectron spectroscopy (micro-XPS). The modulated pattern OH + H 2O/H 2O/OH + H 2O was initially prepared by the LITD method; vacant area with a 400 μm width was first formed in the mixed OH + H 2O overlayer by irradiation of focused laser pulses, and followed by refilling the vacant area with pure H 2O. Spatial distribution changes of OH and H 2O were measured as a function of time with the micro-XPS technique, which indicated that H 2O molecules in the central region flow into the OH + H 2O region. From quantitative analyses using a diffusion equation, we found that the proton transfer in the mixed overlayer consists of at least two pathways: direct proton transfer from H 2O to OH in the nearest site and the proton transfer to the next-nearest site via H 3O + formation. The time scale of first and second path was estimated to be 5.2 ± 0.9 ns and 48 ± 12 ns at 140 K, respectively. In the presence of water capping layer, however, the rate of proton transfer is reduced by an order of magnitude, which would be explained by peripatetic behavior of proton into H 2O capping layer.

Low-energy electron microscopy of CO/Pt(111) surface diffusion by nonequilibrium coverage profile evolution

The adsorption and diffusion of CO molecules on a Pt(111) surface have been studied using low-energy electron microscopy (LEEM). The explicit relationship between LEEM image intensity and CO coverage that is determined during adsorption is used to characterize nonequilibrium CO coverage profiles that are subsequently prepared by laser-induced thermal desorption. Real-time observations of the temporal evolution of these profiles toward equilibrium uniform coverage distributions are analyzed by predictive and inverse solutions of the diffusion equation. These two methods determine consistently the detrimental effect on diffusion of the laser-induced surface damage that is observed directly with LEEM. The inverse method also provides independent information on the coverage dependence of diffusion with high coverage resolution.

Numerical analysis of second harmonic intensity images of a H–Si(1 1 1) surface after UV light pulse irradiation

By using laser induced thermal desorption (LITD) model, we have calculated spatial distributions of hydrogen deficiency and melting on a H–Si(1 1 1) surface after UV light pulse irradiation. We compared the calculated results with the observed second harmonic response from the Si surface after the UV light pulse irradiation with pulse duration of 30 ps and wavelength of 355 nm. The calculated profiles of hydrogen deficiency reproduced the spatial profiles of the observed SH light intensity from the Si surface very well. The calculated profiles of surface melting were interpreted to be consistent with the spatial profiles of the observed SH light intensity from the bulk Si. Furthermore, the simulation clarified that the hydrogen desorption and the surface melting occur nearly simultaneously. The result also indicated that the surface melting did not give rise to the reduction of pre-exponential factor or activation energy of hydrogen desorption from the H–Si(1 1 1) surface. These results were consistent with the study of the spatially integrated H desorption yield from H–Si(1 1 1) by Koehler and George [B.G. Koehler, S.M. George, Surf. Sci. 248 (1991) 158].

Proton Transfer in a Two-Dimensional Hydrogen-Bonding Network: Water and Hydroxyl on a Pt(111) Surface

The time scale of proton transfer between H(2)O and OH adsorbed on a Pt(111) surface was determined by a combination of laser-induced thermal desorption (LITD) and microscale x-ray photoelectron spectroscopy (micro-XPS). The patterned distribution OH+H(2)O/H(2)O/OH + H(2)O was initially prepared on the Pt(111) surface by the LITD method and the time evolution of the spatial distribution of H(2)O and OH was observed by the micro-XPS technique. From quantitative analyses based on a diffusion equation, we found two proton-transfer pathways with different time scales of 5.2+/-0.9 ns and 48+/-12 ns at 140 K, which were attributed to direct proton transfer to the neighbor site and H(3)O(+)-mediated transfer to the next-nearest site, respectively.

Laser induced thermal desorption of hydrogen from Zr(0 0 0 1): Relationship to water dissociation and hydrogen dissolution

On metals such as Zr, during hydrogen exposure, dissolution competes with desorption; this competition can be probed by thermal desorption at different heating rates. In the case of desorption from preadsorbed hydrogen, only ∼1% of the hydrogen can be desorbed even at heating rates of >10 10 K s −1. Recent measurements of the dynamics of hydrogen released by water dissociation on Zr(0 0 0 1) [G. Bussière, M. Musa, P.R. Norton, K. Griffiths, A.G. Brolo, J.W. Hepburn, J. Chem. Phys. 124 (2006) 124704] have shown that the desorbing hydrogen originates from the recombinative desorption of adsorbed H-atoms and that over 25% of the water collisions lead to hydrogen desorption. To gain further insight into the desorption and dissolution of hydrogen and in an attempt to resolve the paradox of the different desorption yields from H 2 vs. H 2O exposures, we report new measurements of the laser induced thermal desorption (LITD) of hydrogen from Zr(0 0 0 1) at initial temperatures down to 90 K. The low temperature was chosen because work function measurements suggested that hydrogen adsorbed into only the outermost (surface site) of the two available adsorption sites (surface and subsurface), from which we postulated much more efficient desorption at high heating rates compared to desorption from the sub-surface sites. However, hydrogen desorption by LITD from Zr(0 0 0 1) at 90 K still only accounts for 1% of the adsorbed species, the remainder dissolving into the bulk at LITD heating rates. The different yields alluded to above remain unexplained (Bussière, 2006).

Laser-induced thermal desorption mass spectrometry of functionalized silicon surfaces

Functionalization of materials and laser patterning of chemisorbed layers play an increasing role in tailoring and structuring surface properties on the nanoscale. An attractive method of investigating organic functionalizations is laser-induced thermal desorption (LITD). The analysis of well-defined H- and D-terminated Si(1 1 1)-(1 × 1):H(D) surfaces was used to quantify the LITD technique. Moreover, oxidized silicon surfaces were functionalized with trimethylsilyl (TMS) and (3,3,3-trifluoropropyl)-dimethylsilyl (TFP) hydrophobic end groups. The samples were irradiated normal to the surface with focused XeCl laser pulses. The desorbed species were monitored at an oblique angle and their time-of-flight (TOF) distributions were measured with a quadrupole mass analyzer. The TOF temperatures of silicon were calibrated for different laser pulse energies by desorption of H 2 and D 2. In the LITD experiments, the desorption of trimethylsilanol groups was observed for TMS terminations, indicating that essentially the whole molecule desorbs from the surface. The TOF data could be fitted to Maxwellian distributions, providing the desorption yield of the emitted species, their mass, and temperature. On the other hand, several characteristic fragments were found for the TFP-terminated surface. The TOF distributions indicate that the fragments detected with the analyzer derived from different desorbed species.

Real-space investigation of fast diffusion of hydrogen on Si(001) by a combination of nanosecond laser heating and STM

The rearrangement of silicon dangling bonds induced by pulsed laser heating of monohydride-covered Si(001) surfaces has been studied by means of scanning tunneling microscopy (STM). The initial configurations, which were created by laser-induced thermal desorption, consist of isolated pairs of dangling bonds at two adjacent dimers and represent an excited state of the surface. Hydrogen diffusion causes this arrangement to change during only a few nanosecond laser pulses into the equilibrium configuration with most of the dangling bonds being paired up at a single dimer. STM images taken after different numbers of heating pulses show snapshots of the surface configuration frozen at various stages of the fast equilibration process. In this way, hydrogen diffusion associated with rates as high as ${10}^{8}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ could be monitored with atomic resolution. Comparison with Monte Carlo simulations of the diffusion processes allows for the precise determination of both the diffusion rates and the pairing energies between dangling bonds at high surface temperature.

Stability and fragmentation of organic silicon functionalizations studied by laser desorption mass spectrometry

A method is introduced to investigate organic functionalizations on silicon by laser-induced thermal desorption (LITD), where well-ordered Si(1 1 1)-(1 × 1):H(D) surfaces are used to determine the desorption temperature as a function of laser fluence. To demonstrate the potential of this technique silicon surfaces with ultrathin oxide layers were functionalized with organic end groups. The species desorbed with focused XeCl laser pulses were monitored at an oblique angle and their time-of-flight (TOF) distributions were measured with a quadrupole mass analyzer after electron impact ionization. By assuming a negligible contribution of the oxide and organic layers to the heating effect, the TOF temperatures measured for Si(1 1 1)-(1 × 1):H(D) could be used to determine the mass of the desorbed species. Detailed results are presented for dimethylsilyl (DMS), bromomethyldimethylsilyl (BMDMS), and chloromethyldimethylsilyl (CMDMS) terminated surfaces which were prepared by silanization with suitable chloro and disilazane compounds. While for the DMS termination dimethylsilanol (76 u) is desorbed as a single species, clearly identifying the terminating group, in the case of BMDMS and CMDMS further fragmentation of the end group occurs at the surface.

Laser desorption of NO from a thick C 60 film

The desorption of NO molecules from a thick C 60 film is reported. A thermal desorption spectrum indicates two adsorption sites with binding energies of E b = 0.30 eV and 0.55 eV. For laser desorption the fullerene surface is exposed to NO and excited by 7 ns UV laser pulses. Desorbing NO molecules are recorded state selectively as well as time resolved. The time-of-flight measurement indicates three different desorption pathways. A fast channel shows rovibronic temperatures of T rot( v″ = 0) = 370 K, T rot( v″ = 1) = 390 K and T vib = 610 K as well as strong rotational–translational coupling. The desorption yield for the fast channel increases linearly with pulse energy with a desorption cross section of σ = (5.1 ± 0.9) × 10 −17 cm 2. Dominating the signal for small J″ values is a slow channel with low rotational and translational temperatures of about 110 K. We assign this peak to a laser-induced thermal desorption. For large pump–probe delays the data deviate from the Maxwellian flux distribution and a third channel appears with extremely late arrival times.

Optical second harmonic intensity images of hydrogen deficiency on H-Si(111) surfaces

In order to obtain the spatial distribution of hydrogen desorption by ultraviolet (UV) laser pulses from a hydrogen terminated Si (H-Si) surface, optical second harmonic (SH) intensity images of the surface have been observed in ultra high vacuum. The observed SH signal included both contributions of the hydrogen deficiency on the H-Si surface and the surface damage. The spatial distribution of the hydrogen deficiency has been obtained from the difference between the SH intensity images before and after the hydrogen re-termination. The hydrogen desorption occurred above the threshold fluence of ∼40 mJ/cm2, and the total amount of the hydrogen desorption increased monotonically as a function of the UV laser pulse energy. It is suggested that the observed hydrogen desorption may have resulted from the laser induced thermal desorption (LITD) mechanism. [DOI: 10.1380/ejssnt.2006.105]