Laser-induced Thermal Desorption Research Articles

Decomposition of 1,1-Dichloroethene on Pd(111)

The decomposition of 1,1-dichloroethene on Pd(111) is investigated using conventional thermal desorption, laser-induced thermal desorption (LITD), and FT reflection absorption infrared spectroscopy (FT-RAIRS). The decomposition mechanism produces at least three hydrocarbon surface intermediates, including ethylidyne. Thermal desorption results differ between high and low coverages because of relative surface concentrations of Cl and H in combination with kinetic effects.

水素終端Ｓｉ（１１１）面からの紫外光照射による水素脱離の光第二高調波顕微像観察

We have observed SH intensity images of the hydrogen terminated Si(111) surface partly irradiated by UV light (hω=3.5 eV) in ultra-high vacuum using a SH microscope. The SH signal intensity of the surface covered with hydrogen was weak. Hydrogen desorption due to the UV light irradiation caused the enhancement of SH intensity. The observed SH intensity images of the H-Si(111) surface represent the spatial distribution of the surface electronic states created by the hydrogen deficiency. We have also investigated dependence on the UV light power of the hydrogen desorption from H-Si(111) surface using the SH microscope. These experimental results suggest that the observed hydrogen desorption results from the laser induced thermal desorption (LITD).

Thermal Chemistry of cis-1,2-Dichloroethene on Pd(111)

The reaction of cis-1,2-dichloroethene (cis-DCE) on Pd(111) has been investigated by temperature-programmed desorption, laser-induced thermal desorption, Auger electron spectroscopy, and Fourier transform reflection absorption infrared spectroscopy. Below 130 K, molecularcis-DCE aggregates, resulting in only about 30% of the molecules from exposures below saturation significantly interacting with the palladium surface. The decomposition of cis-DCE generates the observable species H2, HCl, and ethylidyne. A fraction of cis-DCE molecules lose both chlorine atoms and add hydrogen to form ethylidyne, which is stable on the surface between 250 and 370 K. Hydrogen is liberated at about 420 K from cis-DCE surface fragments that immediately combine with surface chlorine and desorb as HCl. The most intense HCl desorption occurs at about 575 K and is due to surface chlorine reacting with either subsurface hydrogen or hydrogen from the remaining surface alkyl fragments. No carbon-containing species desorb from the decomposition of cis-DCE.

Investigation of the local structure variance of water molecules in laser-induced thermal desorption process

Investigation of the local structure variance of water molecules in laser-induced thermal desorption process

Investigation of the local structure variance of water molecules in laser-induced thermal desorption process

This paper presents the use of molecular dynamics simulation in the study of laser-induced thermal desorption (LITD) of water molecules adjacent to a laser-heated Au substrate. The local structure of the water molecules is investigated by considering the densities of the oxygen and hydrogen atoms, the average number of neighbors, n NN, and the average number of H-bonds, n HB. At an equilibrium temperature of 300 K, the simulation results show that three adsorption water layers are formed in the immediate vicinity of the Au surface, and that each four-fold hollow site on the uppermost Au(0 0 1) surface is occupied by a single water molecule. Following laser-induced heating of the Au substrate with a sub-picosecond laser pulse of 350 fs, the substrate temperature increases to 1000 K. This causes a gradual heating of the adjacent water film, which is accompanied by a decrease in the values of n NN and n HB. Hence, it can be concluded that an increase in the water film temperature destroys the hydrogen-bonding network throughout the water film. Although the maximum local temperature of the water film occurs in the region immediately adjacent to the Au substrate, it is determined that the attractive energy between the Au atoms and the water molecules in this region causes the water molecules to aggregate together to form three-dimensional water clusters. Furthermore, this energy prevents the hydrogen bonds in this region from breaking apart as violently as those within the phase explosion region. Finally, it is observed that the phase explosion phenomenon occurs in the region of the water film where the values of n NN and n HB are at a minimum.

Laser patterning of metallic films via buffer layer

A novel method is presented for a single pulse laser-patterning of variable width conducting lines on solid surfaces. Employing laser induced thermal desorption (LITD), potassium coverage grating on top of multilayer Xe is formed on Ru(0 0 1) at 20 K. Optical second harmonic-diffraction measurements were performed to characterize the grating as a function of Xe layer thickness and laser power density. Upon annealing to 80 K, the Xe layer desorbs and the metallic pattern softly lands on the substrate. The potential application of this procedure for the parallel writing of conducting wires millimeters long and less than 50 nm wide is discussed.

Supramolecular surface layer: coumarin/thiolated cyclodextrin/gold

A formation of supramolecular host–guest inclusion surface layer is reported. A nanostructure of this layer consists of an inclusion complex, where a guest molecule – coumarin-6 (3-(2-benzothiazolyl)-7-(diethylamino) coumarin, C 20H 18N 2O 2S) is placed into a ‘bucket’ of a host molecule – thiolated cyclodextrin (6-monodeoxy-6-monothio-β-cyclodextrin, C 42H 70O 34S) which is attached on Au surface. The inclusion and the layer formation are determined by using time-resolved fluorescence anisotropy, second-harmonic generation (SHG), and fluorescence in combination with laser-induced thermal desorption from the surface.

Adsorption and Laser-Induced Thermal Desorption of 1,3-Butadiene on HOPG(0001)

The adsorption and UV photochemistry of 1,3-butadiene (CH 2 =CHCH=CH 2 ) on highly oriented pyrolytic graphite (HOPG(0001)) was studied using temperature-programmed desorption (TPD) and electron energy loss spectroscopy (EELS). Butadiene physisorbs on HOPG with the monolayer exhibiting first-order desorption kinetics and a corresponding desorption energy of 32 kJ/mol. The multilayer exhibits zero order desorption kinetics with a desorption energy of 28.5 kJ/mol. Irradiation of the C 4 H 6 /HOPG(0001) adlayer (one and three monolayers (ML) of C 4 H 6 ) with UV photons at 193, 248, and 351 nm and fluences of up to 100 mJ/cm 2 _ pulse resulted in molecular photodesorption. Photodesorption was described by a laser-induced thermal desorption process as supported by simple calculations of the surface temperature rise. The photoprocesses occurring in both 1 and 3 ML C 4 H 6 adlayers on HOPG(0001) were also investigated at incident powers of ≤ 10 mJ/cm 2 _pulse. Under these conditions, there was no detectable photodesorption or condensed phase photochemistry at any of the wavelengths investigated.

An Experimental and Computational Study of the Gas-Phase Structures of Five-Carbon Monosaccharides

The gas-phase structures of five five-carbon monosaccharides (d-ribose, d-lyxose, 2-deoxy-d-ribose, d-xylose, and d-arabinose) were studied via ion−molecule reactions with dimethoxyphosphenium ion and 1,3-dioxolane-2-phosphenium ion in a Fourier transform ion cyclotron resonance mass spectrometer. These reagent ions have been earlier demonstrated to be sensitive to the three-dimensional structures of diastereomeric vicinal diols. They were found to display unique reactivity toward each monosaccharide. The results indicate that the gaseous monosaccharides are cyclic molecules. On the basis of a comparison of the reactions of monosaccharides introduced into the gas phase via two different methods, laser-induced acoustic desorption (LIAD) and thermal desorption, the monosaccharides are concluded to maintain their crystalline structure, a pyranose form, throughout the evaporation procedure. For all the monosaccharides in this study except for d-lyxose, the lowest-energy structure found computationally using d...

H 2O and HCl adsorption on single crystal α-Al 2O 3(0001) at stratospheric temperatures

The adsorption of H 2O and HCl on single crystal α-Al 2O 3(0001) at stratospheric temperatures was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The α-Al 2O 3(0001) surface was used to model stratospheric Al 2O 3 exhaust particles from solid rocket motors. H 2O and HCl coverages may influence heterogeneous chemistry on Al 2O 3 particles at stratospheric temperatures of ∼200–270 K. Compared with earlier measurements at 298–300 K, the saturation coverages of both H 2O and HCl on α-Al 2O 3(0001) were approximately two times higher at 210 K. The saturation coverages of H 2O and HCl at 210 K were Θ H 2O =4.4×10 14 molecules/cm 2 and Θ HCl=2.0×10 14 molecules/cm 2. The sticking coefficients for both H 2O and HCl on α-Al 2O 3(0001) at 210 K were also higher than the corresponding sticking coefficients at 298–300 K. The sticking coefficients (S) for both H 2O and HCl at 210 K were ∼0.08 at the lowest coverages. The TPD spectra revealed that H 2O and HCl desorption occurred over a broad range of temperatures. These results indicate that H 2O and HCl adsorb onto a wide distribution of binding sites on the α-Al 2O 3(0001) surface. The LITD and TPD results predict that α-Al 2O 3 rocket exhaust particles will be covered with chemisorbed H 2O and HCl under stratospheric conditions. Additional H 2O and HCl exposure can also replace the adsorbed HCl and H 2O species, respectively, by exchange reactions.

HBr Uptake on Ice: Uptake Coefficient, H2O/HBr Hydrate Formation, and H2O Desorption Kinetics

HBr uptake on thin ice films was examined using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD) and Fourier transform infrared (FTIR) spectroscopy techniques. LITD was used to determine the uptake coefficient in situ during low HBr exposures as a function of HBr pressure and ice film temperature. The HBr uptake coefficient on ice was γ = 0.61 ± 0.06 at 140 K and γ = 0.24 ± 0.05 at 100 K at low HBr coverages for HBr pressures ranging from 3 × 10-8 to 1.4 × 10-7 Torr. TPD and FTIR were used to examine HBr uptake on ice at higher HBr exposures. TPD studies observed that thin ice films exposed to HBr at 140 K saturated with HBr after large exposures and formed an HBr hydrate with an H2O:HBr ratio of ∼3.6:1. FTIR measurements monitored the development of the H3O+ bending vibration versus HBr exposure on ice. Saturation of the integrated absorbance for this H3O+ bending vibration was observed after large HBr exposures. The uptake of HBr on ice also had a profound effect on H2O desorption from ice. H2O desorbed at higher temperatures in the presence of HBr. The zero-order kinetics for H2O desorption from pure ice were Ed = 13.4 kcal mol-1 and ν0 = 1.9 × 1032 cm-2 s-1. The zero-order kinetics for H2O desorption from the HBr hydrate formed after saturation HBr exposures were Ed = 10.6 kcal mol-1 and ν0 = 6.3 × 1027 cm-2 s-1.

Interaction and diffusion of potassium onCr2O3(0001)/Cr(110)

The interaction of potassium atoms on top of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)/\mathrm{C}\mathrm{r}(110)$ has been studied using work-function (\ensuremath{\Delta}\ensuremath{\Phi}), temperature programmed desorption (TPD), and optical second-harmonic generation (SHG) measurements. Potassium grows via the completion of a first layer, followed by a second layer in the form of two-dimensional (2D) islands, and at higher coverage 3D clusters are formed. This growth model is supported by and consistent with the results obtained from all three methods. Work-function data suggest that annealing at temperatures above 350 K results in the formation of a surface potassium oxide compound, provided the potassium coverage is higher than 0.5 monolayers (ML). Diffusion of alkali-metal atoms on an oxide surface is reported here over distances of several micrometers. This was measured using optical SH diffraction from coverage gratings that were generated by laser-induced thermal desorption. The activation energy for surface diffusion of potassium on ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)/\mathrm{C}\mathrm{r}(110)$ has been determined to be $11\ifmmode\pm\else\textpm\fi{}0.5\mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}/\mathrm{m}\mathrm{o}\mathrm{l}$ with a preexponential factor ${D}_{0}{=10}^{5}{\mathrm{cm}}^{2}/\mathrm{s}\mathrm{e}\mathrm{c}$ in the coverage range of 1.5--2.5 ML, dropping to 9 kcal/mol and ${D}_{0}=3\ifmmode\times\else\texttimes\fi{}{10}^{3}{\mathrm{cm}}^{2}/\mathrm{s}\mathrm{e}\mathrm{c}$ at a coverage of 3.0 ML. These results are consistent with the diffusion of atoms in the third layer, on top of two-dimensional potassium islands in the second layer, the activation energy represent the barrier for descending from the 2D islands.

Adsorption and desorption of HCl on a single-crystal α-Al 2O 3(0001) surface

The adsorption and desorption of HCl on a single-crystal α-Al 2O 3(0001) surface were examined using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) techniques. α-Al 2O 3(0001) is a model for the surface of Al 2O 3 exhaust particles generated by solid rocket motors. HCl adsorption on these Al 2O 3 particles may alter the surface reactivity and affect heterogeneous atmospheric chemistry. Absolute HCl surface coverages on α-Al 2O 3(0001) were determined using calibrated HCl LITD signals. The reactive sticking coefficient for HCl adsorption on α-Al 2O 3(0001) at 298 K was S∼10 −3 at the lowest HCl coverages. The HCl reactive sticking coefficient decreased nearly exponentially with HCl coverage. The HCl coverage saturated at Θ HCl=1.0×10 14 molecules cm −2 after HCl exposures of >2×10 9 L. HCl desorption from the α-Al 2O 3(0001) surface occurred over a wide temperature range from 300 to 650 K. This broad temperature range suggests a distribution of surface sites with different binding energies. HCl desorption was investigated versus HCl coverage by annealing an α-Al 2O 3(0001) surface that was previously saturated by a large HCl exposure. These results confirmed a wide range of binding energies and suggested that surface diffusion between the adsorption sites must be negligible. Additional HCl desorption results versus HCl coverage, prepared by varying the HCl exposure, revealed that HCl adsorption randomly fills the adsorption sites independent of their binding energies. Modeling of the HCl desorption results was consistent with surface binding energies that ranged from 19 to 36 kcal mol −1. These adsorption and desorption results predict that α-Al 2O 3 rocket exhaust particles will be partially covered with a stable HCl adlayer at stratospheric temperatures and pressures.

Photo-stimulated desorption of rare gas atoms induced by UV–NIR photons at a semiconductor surface

We present a study of photo-stimulated desorption of rare gas atoms adsorbed on a Si(100) surface. Rare gas atoms (Xe and Kr) are desorbed by the irradiation of photons in the range from 1.16 to 6.43 eV. The translational energy distribution of desorbed species is well represented by the Maxwell–Boltzmann distribution with a temperature higher than that expected in normal laser-induced thermal desorption. The translational temperature is independent of excitation fluence and photon energy. The desorption is understood as a result of direct energy flow to the desorption channel from hot surface phonons generated by charge recombination via surface states before they decay into bulk.

Iodobenzene on Pd(111) studied by thermal desorption spectroscopy and laser-induced thermal desorption–Fourier transform mass spectrometry

Iodobenzene decomposes on Pd(111) and forms benzene, iodine, adsorbed carbon, and hydrogen. At low initial iodobenzene exposures no iodobenzene desorbs, benzene desorbs at 500 K, and hydrogen desorbs between 500 and 700 K, indicative of decomposition of some of the benzene. At high initial exposures iodobenzene desorbs at about 200 K and benzene desorbs at about 160 K; however, no hydrogen desorbs. Iodine desorbs from Pd(111) around 1000 K. Adsorbed iodine atoms passivate the Pd(111) surface toward the decomposition of iodobenzene and the dehydrogenation of benzene. Experiments with perdeuterated iodobenzene show that the benzene forms from reaction of phenyl with sub-surface hydrogen by 143 K.

Kinetics of Propene Desorption from Pd(111) Studied by Thermal Desorption Spectroscopy and Laser-Induced Thermal Desorption with Fourier Transform Mass Spectrometry

Thermal desorption spectroscopy (TDS) and laser-induced thermal desorption coupled with Fourier transform mass spectrometry (LITD/FTMS) are used to study the interaction of propene on Pd(111). TDS yields the exposure versus coverage dependence for propene adsorbed to Pd(111). A linear uptake from 0.1 to 1 L (corrected for ion gauge sensitivity) implies precursor mediated adsorption. Significant broadening and shifts to lower temperature occur in the TDS above about 0.26 L, which is assumed to be near saturation. Only propene (320 K) and hydrogen (300 and 500 K) desorb after heating a near-saturation exposure of propene. Propene mostly desorbs intact from the Pd(111) surface. TDS shows that no more than 10% of the near-saturation exposure of propene may decompose to surface bound carbon. Isothermal desorption data are acquired using LITD/FTMS at temperatures between 270 and 390 K. Rate constants are extracted from the data, and an Arrhenius plot yielded kinetic parameters (Ea and A): Ea = 79.2 ± 0.9 kJ/mo...

In Situ Kinetics and Mechanism of Furan Decomposition and Desorption with CO Formation on Pd(111)

The work presented here uses kinetic studies under conditions of varying temperature, coverage, and isotopic substitution to elucidate the energetics and mechanism of furan decomposition and desorption on Pd(111). Laser-induced thermal desorption with Fourier transform mass spectrometry detection is used as a sensitive tool for measuring the in situ kinetics of furan decomposition simultaneously with the formation of CO. The kinetics of both furan loss and CO formation have been studied for 0.17 and 0.32 Langmuir exposures of furan on Pd(111) at 100 K. This corresponds to about 2 and 4% of a monolayer and approximately 15 and 30% of a saturation monolayer, respectively. Total observed furan loss follows two processes involving a competition between desorption and decomposition. The kinetic factors for each process are extracted yielding an activation barrier of approximately 60 kJ/mol and preexponential factor of 107 s-1 for decomposition, and an activation barrier of approximately 95 kJ/mol and preexpone...

Thermal surface chemistry of Fe(CO) 5 on Pd(111) studied by FT-TPD and LITD-FTMS

Iron pentacarbonyl desorbs from Pd(111) in ultra-high vacuum at 153 and 170 K, corresponding to multilayer and monolayer desorption. Approximately 30% of the first saturation layer decomposes during the temperature-programmed desorption (TPD) experiment. Laser-induced thermal desorption shows submonolayer coverages to react under 150 K and then again near 200 K. TPD shows evolution of CO by a reaction-limited process near 260 K. The decomposition process is likely to involve a stepwise decarbonylation.

Coverage grating template for the study of surface diffusion: K coadsorbed with CO on Re(001)

The coverage grating method as a template for coadsorbates is introduced for the study of surface diffusion. The effect of coadsorbed CO inside coverage troughs formed by laser induced thermal desorption (LITD) on potassium surface diffusion on Re(001) has been investigated using the coverage grating optical second harmonic diffraction method. Enhancement of the first-order diffraction peak at a certain CO coadsorption coverage, observed for the first time, demonstrates the very strong electronic interaction between these coadsorbates. The activation energy for K surface diffusion and the pre-exponential factor significantly increase with CO coverage. At initial potassium coverage of 1.0 ML, the activation energy for potassium diffusion increases from 5.0 kcal/mol on the clean rhenium surface to 15.0 kcal/mol in the presence of 0.065 ML CO, and the pre-exponential factor increases from 5.6×10 −3 to 2.0×10 2 cm 2/s. The activation energy doubles for potassium coverages of 0.9 ML and 0.8 ML as a result of the same CO coverage. TPD measurements indicate that strong attractive interactions exist between CO and K on Re(001). Coadsorbed CO and K stabilize each other while forming K x –CO surface complexes which slow down the potassium surface diffusion predominantly by site-blocking effect. This site-blocking effect increases as the number of potassium atoms ( x) interacting with a single CO coadsorbate increases at higher initial K coverages. The nature of this site-blocking is discussed.

Effect of HNO3 and HCl on HDO Diffusion on Crystalline D2O Ice Multilayers

HDO diffusion kinetics on ultrathin (25−200 BL, 90−730 Å) HNO3- and HCl-dosed crystalline D2O ice multilayers were investigated using a combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth profiling. Crystalline hexagonal D2O ice multilayers were grown epitaxially on a Ru(001) single-crystal substrate. HDO diffusion into the ice bulk was measured along the c-axis of crystalline ice by monitoring the HDO and D2O LITD signals during depth profiling by isothermal multilayer desorption. The HDO diffusion rate into HNO3-dosed D2O ice films was ∼30−70 times slower than that of HDO diffusion into pure D2O ice. The measured HDO diffusion coefficients at initial HDO and DNO3 coverages of 0.5−3.0 bilayers (BLs) ranged from D = 2.5(±0.3) × 10-18 cm2/s at T = 150 K to D = 1.5(±0.2) × 10-15 cm2/s at T = 173 K. Arrhenius analysis yielded diffusion kinetic parameters of Do = 71.9 ± 9.2 cm2/s and EA = 13.2 ± 1.4 kcal/mol. In contrast, the HDO diffusion rate into HCl-dosed D2O ice films was ∼10−20 times faster than that of HDO diffusion into pure D2O ice. The measured HDO diffusion coefficients at initial HDO and DCl coverages of 0.3−5.0 BL varied from D = 9.8(±0.7) × 10-17 cm2/s at T = 146 K to D = 3.5(±0.2) × 10-14 cm2/s at T = 161 K. Arrhenius analysis yielded diffusion kinetic parameters of Do = 2.4(±0.1) × 1012 cm2/s and EA = 19.0 ± 0.3 kcal/mol. Measurements of HDO surface diffusion were also conducted using LITD probing to monitor the relaxation of a HDO coverage gradient on the basal (001) face of the ice multilayer. HDO surface diffusion on HNO3- and HCl-dosed ice multilayers was not observed within the resolution of the LITD experiment at 140 K. The lack of measurable HDO surface diffusion is consistent with HDO diffusion into the ice bulk at the surface diffusion temperatures. The diffusion kinetics predicted at stratospheric temperatures of T = 180−210 K indicate that H2O molecules readily diffuse into pure crystalline ice on the millisecond to microsecond time scale. In the presence of HNO3 and HCl, the H2O surface residence times are considerably increased and decreased, respectively. The effect of HNO3 and HCl on the H2O surface residence time may influence heterogeneous atmospheric chemistry by altering absorption rates into ice cloud particles.