Laser-induced Thermal Desorption Research Articles

Évolution de la teneur en composés aromatiques des aérosols urbains au cours d'une journée. Étude par spectrométrie de masse à deux lasers

Laser desorption/laser photoionization mass spectrometry (two-step laser mass spectrometry, L2MS) is a powerful tool for the selective analysis of particle-bound organic compounds. After infrared laser-induced thermal desorption, the analytes are photoionized by an ultraviolet laser pulse. The ions are then mass separated and detected in a time-of-flight mass spectrometer. The main feature of this analytical method is an excellent sensitivity, allowing the direct analysis of very amail quantities of particles collected on a filter. This allowed us to follow the temporal development of selected polycyclic aromatic hydrocarbons on urban aerosol samples with only 15 minutes time resolution. Data obtained over the course of one day in February 1998 is presented. The temporal behavior was found to be quite different for different mass spectral peaks, which may be due to different emission sources.

Origin of non-zero-order H 2O desorption kinetics from crystalline ice multilayers on Ru(001)

Recent studies have reported non-zero-order kinetics for H 2O desorption from crystalline ice multilayers on Ru(001). To understand the origin of the observed non-zero-order kinetics, D 2O desorption from ultrathin (15–135 BL; 55–490 Å) crystalline D 2O ice multilayers on Ru(001) was measured using a novel combination of laser-induced thermal desorption (LITD) spatial probing and isothermal desorption flux analysis. The ice multilayers were grown on a single-crystal Ru(001) metal substrate using either backfill D 2O vapor deposition or multichannel capillary array dosing methods. The ice multilayers grown via backfill vapor deposition were smooth and highly uniform. The LITD and isothermal desorption flux studies demonstrated that D 2O desorption from these uniform ice multilayers exactly followed zero-order kinetics. Slight deviations from zero-order desorption kinetics were observed only at low D 2O coverages of ≤5 BL D 2O and were attributed to enhanced D 2O-Ru(001) substrate interactions or slight ice surface roughening. In contrast, the ice films prepared using capillary array dosing were spatially non-uniform and exhibited a decreasing multilayer coverage versus distance from the center of the substrate. This initial non-uniform D 2O coverage distribution had a dramatic impact on the isothermal desorption flux measurements and produced non-zero-order desorption kinetics. The deviations from zero-order kinetics were directly related to changes in the ice film surface area as the non-uniform initial multilayer coverage was completely desorbed at various positions on the Ru(001) substrate at different times. The desorption kinetics of D 2O from smooth and uniform D 2O ice multilayers on Ru(001) are strictly zero-order. The previous reports of non-zero-order kinetics are assigned to a non-uniform initial D 2O multilayer coverage distribution.

Is Infrared Laser-Induced Desorption a Thermal Process? The Case of Aniline

A complete study of nanosecond laser-induced thermal desorption (LITD) of aniline submonolayers from silica as well as nanosecond and picosecond laser desorption from glassy graphite was undertaken. The measurements include determination of equilibrium desorption kinetic parameters; time-resolved studies on the surface temperature and the surface coverage during laser heating; and investigation of translational, vibrational, and rotational temperatures of laser-desorbed molecules. Our findings are that LITD of aniline up to heating rates of 1010 K/s is consistent with complete thermalization of the molecules during fast desorption. At 1013 K/s, only the kinetic energy of the desorbed molecules agrees well with a thermal desorption model, whereas the internal degrees of freedom are not fully equilibrated, but appear to be somewhat colder.

Dehydrogenation of cyclohexene to benzene on Pd(111)

The dehydrogenation and decomposition of cyclohexene on clean Pd(111) have been investigated using Fourier transform mass spectrometry detection with thermal desorption spectroscopy (FT–TDS) and laser-induced thermal desorption (LITD–FTMS). TDS shows two molecular desorption peaks for cyclohexene centered around 180 K and 230 K in the submonolayer regime, with benzene desorbing at ∼450 K. LITD experiments show that benzene formation occurs at low temperatures (<150 K). The results of TDS experiments with c-C 6H 10 on Pd(111) are similar to those on Pt(111). However, the LITD results differ dramatically. Where c-C 6H 10 reacts away completely in both systems by 180 K, laser-induced desorption of benzene does not occur in significant amounts until ∼280 K on Pt(111).

Adsorption and desorption kinetics of tetrakis(dimethylamino)titanium and dimethylamine on TiN surfaces

Titanium nitride (TiN) can be deposited using the organometallic precursor tetrakis(dimethylamino)titanium (Ti[N(CH 3) 2] 4) (TDMAT). Deviations from conformal TiN film growth have been observed in trench structures using TDMAT. This nonconformal deposition may be associated with readsorption and site blocking by the dimethylamine (HN(CH 3) 2) (DMA) reaction product. To understand the deviations from conformal TiN deposition in trench structures, the adsorption and desorption kinetics for TDMAT and DMA were measured on a sputter-deposited TiN surface using laser induced thermal desorption (LITD) techniques. The LITD measurements revealed that DMA has a higher sticking coefficient than TDMAT. The sticking coefficients for both TDMAT and DMA were also dependent on surface coverage. The initial sticking coefficient for TDMAT is S 0=0.23 with a coverage-dependence approximated by S( θ)=0.25 exp(−4.7 θ) where θ is the normalized surface coverage. The initial sticking coefficient for DMA is S 0=0.70 and the coverage-dependence is approximated by S( θ)=0.86 exp(−3.7 θ). The observed desorption kinetics of DMA following TDMAT and DMA exposures on TiN were also coverage-dependent. The isothermal desorption measurements could be fit using a simple first-order desorption rate expression, k d= ν dexp[− E d( θ)/ RT] where E( θ) is the coverage-dependent desorption energy, E d( θ)= E 0− E 1 θ. Assuming a desorption preexponential of ν d=1×10 13 s −1, the observed isothermal desorption measurements for DMA desorption following both TDMAT and DMA exposures could be fit using E 0=29.1 kcal mol −1 and E 1=8.2 kcal mol −1. These measured adsorption and desorption kinetics are consistent with DMA readsorption as a major contributor to nonconformal TiN growth in trench structures using TDMAT.

Laser-induced molecular desorption and particle ejection from organic films

We present a comparative study on UV matrix-assisted laser desorption (MALD), laser-induced thermal desorption (LITD), IR laser ablation, and desorption via electrical substrate heating. In order to determine to what extent the material is desorbed as individual molecules or ejected as particles, respectively, the ablated material was intercepted by a trapping plate and the particle size distribution was analyzed by atomic force microscopy (AFM). The ratio of ejected particles to free molecules is strongly dependent on the laser volatilization technique used. High energy densities in the sample, for example in UV MALD, favor molecular desorption leading to very smooth films on the trapping plate. In contrast, IR polymer ablation and LITD of micrometer thick organic films is dominated by ejection of clusters and particulates. Volatization by electrically heated flash filaments is not significantly different from substrate mediated thermal laser desorption (LD). It may allow to miniaturize and simplify mass spectrometric instrumentation for applications requiring desorption–ionization techniques.

Adsorption and decomposition of 1,4-disilabutane (SiH 3CH 2CH 2SiH 3) on Si(100) 2×1 and porous silicon surfaces

The adsorption and decomposition of 1,4-disilabutane (SiH 3CH 2CH 2SiH 3) (DSB) was studied on Si(100) 2×1 and porous silicon surfaces. Temperature programmed desorption (TPD) studies revealed that H 2 and ethylene (C 2H 4) were the major reaction products from Si(100) 2×1. These reaction products were also confirmed using laser induced thermal desorption (LITD) techniques. In addition, Auger analysis showed only ∼0.2–1.7% carbon deposition after saturation DSB exposures on Si(100) 2×1. This surprisingly low carbon incorporation may be explained by the efficient C 2H 4 desorption pathway. Fourier transform infrared (FTIR) spectra obtained after DSB adsorption on porous silicon surfaces at 200 K showed the presence of mostly SiH 3 vibrational modes and the absence of CH 3 vibrational features. These spectral characteristics suggest initial dissociative chemisorption of DSB through Si–C bond breakage. The FTIR spectra versus thermal annealing were consistent with a progressive SiH 3→SiH 2→SiH decomposition and a di-σ “ethylene-like” intermediate that produces the C 2H 4 desorption product. LITD studies also tested for the presence of SiH 3 surface species on Si(100) 2×1 following DSB exposures and observed SiH 3 LITD signals. Comparisons with LITD results following disilane saturation exposures suggest different bond breaking pathways for DSB and disilane adsorption on Si(100) 2×1.

Effect of HNO3 and HCl on D2O Desorption Kinetics from Crystalline D2O Ice

The presence of trace species may perturb H2O desorption kinetics from ice surfaces and alter the stability of atmospheric ice particles. To investigate the effects of atmospheric species on H2O desorption kinetics from crystalline ice, the D2O desorption kinetics from pure and HNO3- and HCl-dosed crystalline D2O ice multilayers on Ru(001) were investigated using isothermal laser-induced thermal desorption (LITD) measurements. The D2O desorption kinetics were studied for D2O ice film thicknesses of 25−200 BL (90−730 Å) and initial acid coverages of 0.5−3.0 BL for HNO3 and 0.3−5.0 BL for HCl. Arrhenius analysis of the D2O desorption rates from pure D2O crystalline ice at T = 150−171 K yielded a desorption activation energy of Ed = 13.7 ± 0.5 kcal/mol and a zero-order desorption preexponential of νo = (3.3 ± 0.7) × 1032 molecules/(cm2 s). The absolute D2O desorption rates were ∼3−5 times smaller for D2O ice films exposed to HNO3. The D2O desorption kinetics from HNO3-dosed ice were Ed = 11.3 ± 0.4 kcal/mol and νo = (5.0 ± 0.9) × 1028 molecules/(cm2 s). In contrast, the absolute D2O desorption rates were ∼2 times larger for D2O ice films exposed to HCl. The D2O desorption kinetics from HCl-dosed ice were Ed = 14.2 ± 0.6 kcal/mol and νo = (3.7 ± 0.8) × 1033 molecules/(cm2 s). The changes in the D2O isothermal desorption kinetics were independent of DNO3 and DCl coverages. The adsorbate-induced perturbations are believed to be associated with the formation of stable hydrate cages and reduced D2O mobility in HNO3-dosed ice and the creation of defects and enhanced D2O mobility in HCl-dosed ice. The effects of HNO3 and HCl on the D2O desorption kinetics indicate that the growth, stability, and lifetimes of atmospheric ice particles should be altered by the presence of adsorbates on the ice surface.

Nonthermal photodesorption of N2 from Ag(111)

We have measured translational and rotational energy distributions of N2 molecules following desorption from a Ag(111) surface by infrared (1064 nm) radiation. The observed desorption yields were large even at laser fluences far below that required for laser-induced thermal desorption. State-resolved laser techniques using coherent VUV radiation showed that the rotational and translational energy distributions of the desorbing N2 molecules are not consistent with the predictions of the heat diffusion model governing laser-induced surface heating. These results suggest that physisorbed adsorbates can couple directly to the nascent-phonon distribution or the nascent electron–hole pairs in the photoexcited substrate without heating of the surface.

Laser-induced thermal desorption analysis of the surface during Ge etching in a Cl2 inductively coupled plasma

Laser desorption laser-induced fluorescence (LD-LIF) detection of GeCl was used to determine in situ the surface coverage of chlorine during the etching of germanium by Cl2 in an inductively coupled plasma (ICP) reactor. The ICP operated in the dim mode for radio frequency (rf) power ≲350 W and in the bright mode for higher powers. The etch rate was 3.5 μm/min with 540 W rf power and −40 V substrate bias. The chlorine surface coverage was about 2× that with chlorine flow only and the plasma off, both with dim- and bright-mode operation, and was independent of rf power within each mode for laser repetition rates of 0.2, 5, and 15 Hz. Similarly, the chlorination of the adlayer did not change when the ion energy was increased from 16 to 116 eV by increasing the substrate bias voltage, both with dim- and bright-mode operation. This was confirmed by x-ray photoelectron spectroscopy measurements in a similar high density reactor, where it was found that the surface density of chlorine was ∼2.6×1015 Cl/cm2. As the ion energy increases from 16 to 116 eV, the etch yield of ions increases from 1 to 3 atoms/ion. “Wait and probe” measurements show that the GeClx adlayer is quite stable. Since the same adlayer chlorine content was measured by LD-LIF under high ion current/fast etch conditions (bright mode) and low ion current/slow etch conditions (dim mode), both the adlayer chlorine content and the etch rate seem to be controlled by the ion current to the wafer. Previously reported work in Si etching in this ICP [J. Vac. Sci. Technol. A 15, 3024 (1997)] showed a similar independence of adlayer chlorination with rf power, but much slower chlorination and an increasing chlorination of the adlayer with increasing ion energy.

Desorption of H 2O from a hydroxylated single-crystal α-Al 2O 3(0001) surface

The desorption of H 2O from a hydroxylated single-crystal α-Al 2O 3(0001) surface was studied using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) techniques. α-Al 2O 3(0001) models the surface of exhaust particles from solid rocket motors. H 2O desorption from α-Al 2O 3(0001) will determine hydroxyl coverages that will influence Al 2O 3 surface chemistry. H 2O desorption from the hydroxylated α-Al 2O 3(0001) surface occurred over a temperature range from 300 to 500 K. This broad distribution suggests a variety of hydroxyl surface sites with different binding energies. H 2O desorption results versus hydroxyl coverage prepared by progressively annealing a fully hydroxylated α-Al 2O 3(0001) surface confirmed a wide range of hydroxyl binding energies. Additional H 2O desorption results versus hydroxyl coverage prepared by varying the H 2O exposure revealed that H 2O dissociative adsorption randomly fills the adsorption sites independent of their adsorption energy. These results argued that hydroxyl surface diffusion between the various adsorption sites must be negligible. Modeling of the desorption results was consistent with hydroxyl desorption energies that range from E d=23 to E d=41 kcal mol −1. The H 2O desorption results predict that hydroxyl groups will be stable on α-Al 2O 3 rocket-exhaust particles under stratospheric conditions.

Adsorption of H2O on a Single-Crystal α-Al2O3(0001) Surface

The adsorption of H2O on a single-crystal α-Al2O3(0001) surface was examined using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) techniques. α-Al2O3(0001) mode...

Mass spectra of Fe(CO) 5 using fourier transform mass spectrometry (FTMS) and laser induced thermal desorption FTMS with electron ionization, charge exchange, and proton transfer

Fourier transform mass spectrometry is used with chemical ionization to observe iron pentacarbonyl molecules in an ultra-high vacuum system. Mass spectra are obtained from molecules desorbed from a cold Pd(111) surface using laser-induced thermal desorption (LITD) and directly from Fe(CO) 5 vapors. A variety of ions are formed when using 70 eV electrons, including some molecular ion. Charge exchange using Ar + and Xe + produces fewer fragment peaks but no molecular ion. Very little fragmentation occurs to the organometallic when using CH 5 + for proton-transfer reactions. The proton transfer-LITD spectrum does, however, show an unexpected, nonprotonated iron tricarbonyl cation, presumably due to desorption of a surface-activated species.

Kinetics and mechanism for the H/D exchange between ethylene and deuterium over Pt(111)

The H/D exchange between ethylene and hydrogen (or deuterium) on Pt(111) surfaces was monitored by laser-induced thermal desorption in combination with Fourier-transform mass spectrometry (LITD/FTMS) and by reflection-absorption infrared spectroscopy (RAIRS). In the case of coadsorbed C 2 D 4 +H 2 the LITD experiments show that the H/D exchange reaction takes place at temperatures as low as 215 K, well below those needed for the decomposition of ethylene to ethylidyne. It was also found that exchange on adsorbed C 2 D 4 is significantly easier with H 2 than with C 2 H 4 , indicating that this H/D substitution requires surface hydrogen but not the dissociation of ethylene, and suggesting that ethyl moieties are the most probable intermediates for the reaction. It was also determined that the dosing order of the reactants influences the kinetics of the reaction, which means that specific surface ensembles are required in the exchange; both the rate and the degree of H/D substitution in ethylene are increased by dosing hydrogen (deuterium) beforehand. In addition to the LITD experiments, the rate of formation of ethylidyne that takes place above 250 K was also monitored with RAIRS. It was shown that, when coadsorbed with deuterium, the extent of deuterium substitution in the resultsing ethylidyne is largely determined by H/D exchange between the original ethylene molecules and surface hydrogen, and that direct H/D exchange between ethylidyne and surface hydrogen is measurable only at low ethylidyne coverages and higher temperatures. Finally, it was established that the rate of ethylidyne formation does not depend on the coverage of hydrogen on the surface, a result that points to an ethylidene intermediate in that reaction. The data are discussed on the basis of a mechanism in which the H/D exchange and ethylidyne formation are parallel processes which involve different surface species.

Laser induced desorption of thin molecular films investigated with high time resolution

The temporal evolution of the surface coverage of aniline submonolayers desorbing from graphite under nanosecond laser induced thermal desorption (LITD) conditions was investigated in a novel two laser experiment. The experimental findings reveal that nanosecond LITD of submonolayers can be described by thermal equilibrium kinetics.

UV-induced desorption of CH3X (X=I and Br)/TiO2(110)

The UV photodesorption of methyl halides from TiO2(110) has been investigated using quadrupole mass spectrometry. For low fluence irradiation (&amp;lt;800 μJ/cm2) and ⩽1 ML coverages of methyl iodide, the photodesorption yield decreases exponentially with photon dose, indicating a nonthermal, single-photon process. The wavelength and coverage dependence of the photodesorption yield and the effect of spacer layers support the conclusion that photodesorption of CH3X (X=I and Br) is initiated by substrate excitation, i.e., photogenerated electrons in the conduction band of TiO2. CH3I and CH3Br desorb from TiO2(110) on 257 nm irradiation, while CH3Cl does not photodesorb in the absence of a photoactive coadsorbate, such as CH3I. This indicates that desorption results from a resonant interaction of subvacuum-level electrons with an adsorbate electronic state. Momentum transfer from energetic photofragments to adsorbates can account for only a minor fraction of the total photodesorption yield for coverages of 1 ML or less. For high fluence irradiation (⩾7 mJ/cm2), the nonlinearity of the desorption yield, the calculated transient surface temperature rise, and the desorption of an inert adsorbate (CH3OH at ⩾12 mJ/cm2) indicate that a laser-induced thermal desorption mechanism is dominant.

A Laser-Induced Thermal Desorption/Fourier Transform Mass Spectrometry Study of Acetylene Cyclization on S/Pd(111): The Formation and Kinetics of Benzene, Thiophene, and 1,3-Butadiene

Acetylene cyclization on S/Pd(111) has been studied using laser-induced thermal desorption with Fourier transform mass spectrometry (LITD/FTMS). The formation of benzene, thiophene, and 1,3-butadiene has been observed on the surface at low temperatures. Investigations reveal that benzene and thiophene are formed in similar amounts and that the formation kinetics of each are comparable to one another. Results also show the kinetics of benzene formation on S/Pd(111) do not differ from that on a clean Pd(111) surface, indicating that the addition of S does not significantly alter the nature of the cylization process.

Surface and bulk diffusion of HDO on ultrathin single-crystal ice multilayers on Ru(001)

The kinetics of HDO surface and bulk diffusion on ultrathin (25–192 BL; 90–700 Å) single-crystal H216O ice multilayers were studied using a combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth-profiling. The single-crystal hexagonal ice multilayers were grown epitaxially on a single-crystal Ru(001) metal substrate with the basal (001) facet of ice parallel to the Ru(001) surface. HDO surface diffusion on the single-crystal ice multilayer was not observed within the resolution of the LITD experiment at T=140 K. These LITD surface diffusion experiments yielded an upper limit to the HDO surface diffusion coefficient of Ds⩽1×10−9 cm2/s at T=140 K. The bulk diffusion coefficients were measured along the c axis of the hexagonal ice crystal which is perpendicular to the (001) plane. HDO was observed to diffuse readily into the underlying H216O ice multilayer. The measured HDO bulk diffusion coefficients ranged from D=2.2(±0.3)×10−16 cm2/s to D=3.9(±0.4)×10−14 cm2/s over the temperature range from 153 to 170 K. The HDO bulk diffusion coefficients were measured for H216O thicknesses of 25–192 BL (1 BL=1.06×1015 molecules/cm2) and initial HDO adlayer thicknesses of 2–9 BL. The HDO bulk diffusion was independent of H216O film thickness and initial HDO coverage. Arrhenius analysis of the temperature-dependent bulk diffusion coefficients yielded a diffusion activation energy of EA=17.0±1.0 kcal/mol and a diffusion preexponential of Do=4.2(±0.8)×108 cm2/s. Compared with extrapolations from macroscopic diffusion kinetics obtained earlier at temperatures close to the melting point, these bulk diffusion coefficients are larger and may reflect the perturbation of the ultrathin ice films induced by the nearby interfaces. The differences between these HDO diffusion kinetics and recently measured kinetics for H218O indicate that H/D exchange and molecular transport make comparable contributions to the HDO diffusion coefficient.

Kinetics of ethylene conversion to ethylidyne on Rh{111} from laser-induced thermal desorption

The kinetics of the conversion of ethylene to ethylidyne at the surface of Rh{111} has been studied using laser-induced desorption. Ethylene is desorbed intact into the gas phase when the surface is rapidly (∼ 10 10 K s -1) heated by a laser pulse, which provides a quantitative means of following the slow thermal conversion of ethylene to ethylidyne at the surface. This conversion was found to occur in the temperature range 170–240 K. Temperature-jump isothermal studies of the process were performed, and the results show coverage-dependent kinetics with product inhibition of the reaction due to an activation energy barrier which increases strongly with the amount of product formed.

Surface Sensitive Studies of the Reactive Uptake of Chlorine Nitrate on Ice

The reactive uptake of chlorine nitrate (ClONO2) on ice surfaces (ClONO2 + H2O → HOCl + HNO3) was studied with surface sensitivity using laser-induced thermal desorption (LITD) techniques. Thin fil...