Desorption laser stimulated

Fig. 2.5 An ion kinetic energy distribution of field desorbed He ions taken with a pulsed-laser time-of-flight atom-probe. In pulsed-laser stimulated field desorption of field adsorbed atoms, atoms are thermally desorbed from the surface by pulsed-laser heating. When they pass through the field ionization zone, they are field ionized. Therefore the ion energy distribution is in every respect the same as those in ordinary field ionization. Beside the sharp onset, there are also secondary peaks due to a resonance tunneling effect as discussed in the text. The onset flight time is indicated by to, and resonance peak positions are indicated by arrows. Resonance peaks are pronounced only if ions are collected from a flat area of the...

Fig. 2.22 In [UP - 1 )/T] vs. 1 IT plot for field adsorption of He on the W (112) surface, obtained from a pulsed-laser stimulated field desorption experiment. In the high temperature region, the data points fit well into a straight line of H = 0.17 eV, as expected from a theoretical analysis. In the low temperature region, the data points deviate significantly from the linear plot of the solid line. This deviation indicates that there may be another binding state with a much smaller binding energy of 0.08 eV.

The disturbing effects of the applied field can be reduced by using pulsed-laser stimulated field desorption which needs a field strength only about half that needed in regular field desorption and the effects of the field should be reduced to about one-quarter of the original ones since an energy level shift by the polarization effect is proportional to F2 electric field, in fact, is believed to be one of the factors which can be responsible for the enhanced chemical reactivity of the surface. 

In field ionization, hydrogen molecules near the tip region are attracted to the tip surface. They either hop around the tip surface or are field adsorbed on it. As the hopping motion and the field adsorption are dynamical phenomena, some of the ionic species detected may also come from field adsorbed states, not necessarily just from the gas phase. On the other hand, in pulsed-laser stimulated field desorption, where gas pressure is very low, of only 1 X 10-8 Torr, gas molecules are thermally desorbed by laser pulses from their field adsorbed and chemisorbed states. When they pass across the field ionization zone some of them are field ionized. The critical ion energy deficit in pulsed-laser stimulated field desorption of a gas is therefore found to be identical to that found in field ionization. In both pulsed-laser stimulated field desorption and field ionization of hydrogen, the majority of ions detected are H3 and H+. 

Early field ion emission studies of gas-surface interactions use field ionization mass spectrometry. Gas molecules are supplied continuously to the tip surface by a polarization force and by the hopping motion of the molecules on the tip surface and along the tip shank. These molecules are subsequently field ionized. The role of the emitter surface in chemical reactions is not transparent and has not been investigated in detail. Only in recent pulsed-laser stimulated field desorption studies with atom-probes are these questions addressed in detail. We now discuss briefly a preliminary study of reaction intermediates in NH3 formation in pulsed-laser stimulated field desorption of co-adsorbed hydrogen and nitrogen,... 

In pulsed-laser stimulated field desorption, if the field is high enough, the adsorbed species can be thermally field desorbed, most probably within one to a few atomic vibrations. If the activation barrier of evaporation has been reduced by the applied field to much less than the surface diffusion barrier, then the adsorbed species will be desorbed before they have any chance of interacting with other atoms or molecules on the surface. Thus the desorbed species should represent well the... 

Fig. 4.56 In pulsed-laser stimulated field desorption with coadsorption of N2 and H2, if the temperature of the Pt tip is between 120 and 150 K, then reaction intermediates of NH3 can be detected as shown in (a). If the surface temperature of the tip is only —50 K lower, no reaction intermediates of NH3 can be detected. Instead, only field adsorption products, N2 and N2H+ are detected.

Laser stimulated desorption has been also studied as in the work of Freund s group [100], Low energy excitation and desorption dynamics from NO adsorbed on NiO (100) and (111) oxide surfaces is stimulated by a Ar-F-excime laser with 6.4 eV energy. They found total cross sections of 6x10 cm and explained their results on the basis of the MGR model. 

Techniques for bond activation and lattice vibration include the thermal, electron, and laser-stimulated desorption spectroscopy (TDS, EDS, and EDS), the electron-energy-loss spectroscopy (EELS), Raman scattering, and Eourier transform infrared spectroscopy (FTIR) spectroscopies. 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Wattiez R et al. Human bronchoalveolar lavage fluid protein two-dimensional database study of interstitial lung diseases. Electrophoresis 2000 21 2703-2712. Yanagida M et al. Matrix assisted laser desorption/ionization-time of flight-mass spectrometry analysis of proteins detected by anti-phosphotyrosine antibody on two-dimensional-gels of fibrolast cell lysates after tumor necrosis factor-alpha stimulation. Electrophoresis 2000 21 1890-1898. 

The observations of complex dynamics associated with electron-stimulated desorption or desorption driven by resonant excitation to repulsive electronic states are not unexpected. Their similarity to the dynamics observed in the visible and near-infrared LID illustrate the need for a closer investigation of the physical relaxation mechanisms of low energy electron/hole pairs in metals. When the time frame for reaction has been compressed to that of the 10 s laser pulse, many thermal processes will not effectively compete with the effects of transient low energy electrons or nonthermal phonons. It is these relaxation channels which might both play an important role in the physical or chemical processes driven by laser irradiation of surfaces, and provide dramatic insight into subtle details of molecule-surface dynamics. 

Most of the mass spectrometry applications for combinatorial chemistry will be described in the following sections of this chapter. Here we will give a short overview of MS techniques utilized for the characterization of resin-bound molecules. The majority of publications in this field describe applications of matrix-assisted laser desorption ionization (MALDI), combined with time-of-flight (TOF) detection. The major difference of MS application for analysis of resin-bound molecules from the above-described NMR and IR applications is that analyte should not be covalently bound to solid support prior to mass measurement. Detachment of compound molecules from resin can be done chemically (for example, by bead exposure to TFA vapors) [30,31] or photochemically, such that cleavage, desorption, and ionization of molecules occur simultaneously upon stimulation by laser radiation [32], Since the... 

Thus, in this review we present the desorption phenomena focused on the rotational and translational motions of desorbed molecules. That is, we describe the DIET process stimulated by ultraviolet (UV) and visible nanosecond pulsed lasers for adsorbed diatomic molecules of NO and CO from surfaces. Non-thermal laser-induced desorption of NO and CO from metal surfaces occurs via two schemes of DIET and DIMET (desorption induced by multiple electronic transitions). DIET is induced by nanosecond-pulsed lasers and has been observed in the following systems NO from Pt(0 0 1) [4, 5],... 

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