Thermal desorption of large molecules from solid surfaces

We use molecular-dynamics simulations and importance sampling to obtain transition-state-theory rate constants for thermal desorption of an n-alkane series from Au(111). We find that the binding of a large molecule to a solid surface involves different types of local minima. The preexponential factors increase with increasing chain length and can be substantially larger than typical estimates for small molecules. Our results match recent experimental studies and indicate that a proper treatment of conformational isomerism and entropy, heretofore not found in coarse-grained models, is essential to quantitatively describe the thermal desorption of large molecules from solid surfaces.     

Cross sections for electron-stimulated desorption of alkali-metal atoms from adlayers on oxidized tungsten

Earlier cross-section measurements of electron-stimulated desorption (ESD) of Li, Na, K, and Cs atoms from adlayers on oxidized tungsten are analyzed with respect to substrate temperature and degree of oxidation. It is conjectured that the ESD cross sections are determined by the ratio of the rates of neutral alkali-metal re-ionization on the surface and of relaxation of the O⁺ charge in the substrate. A comparison with experiment revealed the dependence of the re-ionization rate of an alkali-metal adatom on its size and mass, as well as the dependence of the O⁺ charge relaxation rate on substrate temperature and degree of oxidation. The relation of the charge relaxation time to the substrate band structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1491–1497 (August 1997)     

Multi-dimensional mixed quantum–classical description of the laser-induced desorption of molecules

A mixed quantum–classical method for the simulation of laser-induced desorption processes at surfaces is implemented. In this method, the nuclear motion is described classically, while the electrons are treated quantum mechanically. The feedback between nuclei and electrons is taken into account self-consistently. The computational efficiency of this method allows a more realistic multi-dimensional treatment of desorption processes. We apply this method to the laser-induced desorption of NO from NiO(100) using a two-state two-dimensional potential energy surface derived from ab initio quantum chemical calculations; we extend this potential energy surface to seven dimensions employing a physically reasonable model potential. By comparing our method to jumping wave-packet calculations on exactly the same potential energy surface we verify the validity of our method. We focus on the velocity, rotational, and vibrational distributions of the desorbing NO molecules. Furthermore, we model the energy transfer to the substrate by a surface oscillator. Including recoil processes in the simulation has a decisive influence on the desorption dynamics, as far as the velocity and rotational distribution is concerned. In particular, the bimodality in the velocity distribution observed in low dimensions and in the experiment disappears in a high-dimensional treatment. PACS  68.43.Tj; 68.43.Rs; 82.20.Gk; 82.20.Wt     

Nonequilibrium desorption of atoms and molecules from surfaces

A model, based on concepts from the theory of atom-surface accommodation coefficients, for nonequilibrium desorption of atoms and molecules from surfaces is proposed. We contend that the nonequilibrium effects should be expected to be general properties of not only polyatomic molecule-surface systems, but of all desorbing systems. The model predicts large deviations from the equilibrium dependences, on desorption angle, of (a) the desorbing atomic flux, (b) the average energy of desorbing atoms and (c) the desorbing atomic speed ratio. The deviations are larger when the accommodation coefficient is smaller.     

Thermal desorption of dysprosium from tungsten microcrystal

Activation energy for thermal desorption of dysprosium from a tungsten microcrystal of about 300 nm diameter was determined by means of the field-emission method. The desorption was detected from the whole W emitter surface in the temperature range 1490-1665 K for dysprosium average coverage θ < 0.06 monolayer. The average activation energy was determined to be 4.09 ± 0.06 eV/atom and the frequency factor to be about 10¹¹ s⁻¹. The energy may mainly concern the desorption from the atomically rough regions of the microcrystal.     

Thermal desorption of Ag from oxidized W

Thermal desorption spectra (TDS) of Ag condensates deposited at two substrate temperatures T_s = 300 K and T_s = 779 K have been obtained. A shift of the temperature T_m of the maximum of the desorption flux R_e was observed. It was established that the shift of the maximum depends on the value of the initial coverage σ₀. A significant difference was found to exist between TDS of silver condensates deposited on oxidized and clean W substrates due to differences in the mechanism of condensation. Silver condensates were deposited on oxidized W at different initial conditions (T_s, impingement rates R_i, etc.) but equal σ₀. The corresponding TDS were compared and a conclusion has been drawn that the shift of T_m is due to the different structure of Ag condensates. TDS of Ag condensates deposited at room temperature (T_s = 300 K) were interpreted using the method of Bauer et al. [J. Appl. Phys. 45 (1974) 5164: Surface Sci. 53 (1975) 87]. The dependence of the desorption flux R_e on the substrate temperature T_s and coverage σ was treated on the basis of the Polanyi-Wigner equation and some parameters of the condensation process were evaluated.     

Thermal desorption spectra of chemisorbed atoms and molecules with lateral interactions and finite mobility: Nondissociatively adsorbed molecules

A model of thermal flash desorption of interacting chemisorbed atoms and molecules from a crystal surface is developed. The possibility of adatom diffusion along the surface is taken into account, but thermal equilibrium of the adatom system during the desorption process is not assumed. The whole range of adsorbate mobilities, including absolutely immobile species as well as adsorbates in thermal equilibrium, is investigated. The result is obtained in the form of a chain of differential equations for correlation functions of occupation numbers which is solved in various approximations. The numerical results differ from the results of previous calculations and are in better agreement with the experiments. The experimental thermal desorption spectrum of CO on W(210) is satisfactorily reproduced.     

Thermal desorption of silicon from polycrystalline tungsten surfaces

The adsorption of Si on polycrystalline tungsten surfaces was studied for the first time by means of thermal desorption. Instead of the main peak of Si ($\text{m}\text{e}\text{=28}$), the isotope ($\text{m}\text{e}\text{=30}$) contained naturally (～3%) was monitored by mass spectrometry. This method can reduce the contribution from CO to the mass signal of Si. Two desorption peaks were observed at 1480 and 1820 K. The activation energies for the desorption were estimated to be about 90 and 110 kcal/mol, respectively.     

Thermal desorption from metal surfaces: A review

In this review an attempt is made to draw correlations between thermal desorption and structural studies of chemisorption on metal surfaces. Alternative models are discussed for the appearance of multiple peaks and for lineshape analysis in desorption, the first involving multiple binding states and the second lateral interactions within a homogeneous chemisorbed layer. Criteria are discussed for distinguishing between the various possibilities for a particular system, in particular with relation to the adsorption of hydrogen and carbon monoxide on tungsten.     

Thermal desorption of metals from tungsten single crystal surfaces

After a short review of the experimental methods used to determine desorption energies E and frequencies v the assumptions underlying the theoretical analysis of the experimental data are discussed. Then recent experimental results on the flash desorption of Cu, Ag and Au from clean, well characterized W {110} and {100} surfaces are presented and analysed in detail, in particular with respect to the coverage dependence. The results obtained this way clearly reveal the limitations of previous analysis methods and of the experimental technique per se (such as structure and phase changes below and in the temperature region in which desorption occurs). Furthermore the need for more theoretical work to understand the large changes of v and E with coverage and the so-called “compensation effect”, i.e. the relation between In v and E, becomes evident.     

Thermal desorption of cyanogen from Pt(100)

Thermal desorption of cyanogen adsorbed on Pt(100) was studied by flash desorption mass spectrometry. By investigating the parent ion and all possible fragmentation products in the mass spectrometer during desorption it was concluded, that desorption takes place exclusively as molecular C₂N₂. Three desorption peaks were observed at 140, 410 and 480°C denoted as α, β₁ and β₂. The respective surface coverages at saturation were determined by quantitative evaluation of the flash desorption curves to be 2.0 ± 0.2 × 10¹⁴ and 5.5 ± 1.0 × 10¹⁴$\text{molecules}\text{cm}{}^2$ for the α and the β states, respectively. First order desorption kinetics was suggested by the coverage dependencé of the desorption spectra for both α and β states with desorption energies of 12 and 38–42 $\text{kcal}\text{mole}$, respectively. A large difference in the sticking probabilities of α and β states was observed with initial values of 0.06 (α) and 0.9 (β). Adsorption experiments at elevated temperatures led to the assumption, that α and β states coexist on the surface with no or very little interactions between them. The results are discussed in terms of different models for the adsorption states.     

Laser desorption jet-cooling of organic molecules

Laser desorption followed by jet-cooling allows wavelength-selective as well as mass-selective detection of molecules desorbed from a surface without fragmentation. The cooling characteristics and detection sensitivity of laser desorption jet-cooling of organic molecules are investigated. From the rotational contour of the electronic origin of the S ₁ S ₀ transition of laser-desorbed anthracene, rotational cooling to 5–10 K is demonstrated. Vibrational cooling is studied for laser-desorbed diphenylamine, a molecule with low-energy vibrations, and a vibrational temperature below 15 K is found. The absolute detection sensitivity is determined for the perylene molecule. Using two-color (1+1) resonance enhanced multi-photon ionization (with a measured ionization efficiency of 0.25) for detection, it is found that one ion is produced in the detection region for every 2×10⁵ perylene molecules evaporated from the desorption laser spot. A two-color (1+1) REMPI spectrum (400 points) of perylene is recorded using only 30 picogram of material.     

Thermal desorption of carbon monoxide from Mo(110)

Adsorption and desorption of carbon monoxide (CO) from Mo(110) have been investigated by temperature programmed desorption (TPD). The TPD spectra exhibit peaks in two temperature regions: 250-400 and 850-1100 K. The first region correspond to adsorption in molecular form, whereas the latter region corresponds to desorption of CO that has been dissociatively adsorbed. When the surface was saturated by CO, about 35% of the desorption intensity originates from the high-temperature region (recombinative desorption) and the remaining desorption signal is from the low-temperature region (molecular adsorption). Desorption parameters for molecular adsorbed CO were obtained using several different heating rates. Desorption energies were estimated to range from 0.9 eV for low coverages of CO to about 0.65 eV for high coverages. Corresponding prefactors were estimated to be in the range from 1 210¹¹ to 1 210⁹ s^{m 1}. The experimental data have been compared with Monte Carlo simulations of first-order TPD from a bcc (110) lattice, in which partial poisoning of adsorption sites by dissociated carbon and oxygen was modelled. Repulsive near-neighbour interactions were used.     

铒、钪膜中离子注入氦的热解析行为

采用热解析法初步研究了铒、钪膜中离子注入氦的热解析行为。研究结果表明:同种元素铒中离子注入氦的热释放峰位相同,但膜的致密性将影响氦的释放量,结构疏松的膜中存在的孔洞是氦的快速释放通道;在相同注入剂量和能量条件下,铒、钪膜中注入氦的热释放峰位不同,这可能与氦在铒、钪膜中的深度分布及膜的致密性有关,利用质子增强背散射法测量出能量为60 keV的4He+在铒、钪膜中的注入深度分别为210,308 nm。     

Thermal desorption spectroscopy of he from Ni at and below saturation

Trapping of helium after implantation at energies of 8 to 150 keV and fluences up to 10¹⁹ He-ions/cm² in nickel at room temperature is studied by measuring the thermal desorption spectra during linear heating up to 1000°C. At several annealing stages the trapped helium is measured by means of the nuclear reaction ³He(d, α)H and the target surface is observed by laser scattering and with the scanning electron microscope.

The thermal desorption spectra depend strongly on the implantation fluence but only slightly on the implantation energy, indicating a similar trapping of He in the lattice for the implantation energies used here, The temperature at which desorption starts decreases with increasing fluence. Above the critical fluence for blistering an additional low temperature (150°C) desorption maximum is found.

The desorption peak at 150°C can be approximated theoretically with a single jump desorption process of first order and a Gaussian distribution of activation energies around 1 eV. The measurements indicate that at higher temperatures (>300°C) helium desorption is partly due to the opening of helium bubbles at the target surface.     

铒、钪膜中离子注入氦的热解析行为

采用热解析法初步研究了铒、钪膜中离子注入氦的热解析行为。研究结果表明：同种元素铒中离子注入氦的热释放峰位相同,但膜的致密性将影响氦的释放量,结构疏松的膜中存在的孔洞是氦的快速释放通道;在相同注入剂量和能量条件下,铒、钪膜中注入氦的热释放峰位不同,这可能与氦在铒、钪膜中的深度分布及膜的致密性有关,利用质子增强背散射法测量出能量为60 keV的4He+在铒、钪膜中的注入深度分别为210,308 nm。     

Theory of laser-stimulated desorption of molecules via electronic excitation of adsorbed molecules

In this paper, we treat the laser-stimulated desorption (LSD) of adsorbed molecules induced by using an UV-visible laser which excites electronic states of the adsorbed molecule. We shall show that for the case in which the desorption takes place in the excited electronic state of the adsorbed molecule, the Beutler-Fano type of asymmetric band shapes can be observed for the plot of the LSD yield against laser-frequency. We have also studied the case in which electronic relaxation is responsible for LSD; in this case, the Beutler-Fano band shape will not be observed. To demonstrate our theoretical results, some numerical calculations have been performed.     

Dipole moments induced in three interacting molecules

Fluids composed of infrared inactive molecules, such as homonuclear diatomics, absorb infrared radiation by virtue of dipole moments induced by molecular interactions. The induced dipoles of two colliding molecules and their associated absorption spectra are generally well known, both from theory and experiment. More recently, empirical models of the most significant dipole components of three interacting molecules have been obtained which permit semi-quantitative predictions of the intensities of the various spectroscopic observations of the ternary spectra, e.g., third virial coefficients of collision-induced absorption spectra, triple transitions, and intercollisional dips. Both the pairwise-additive and the irreducible dipole components are considered. These ternary components should affect the spectra and optical properties also of liquids. It is suggested that the accounting for these effects gives novel information on the structure of liquids through a better understanding of the triple distribution function.