Laser induced molecular desorption which is mediated by optically generated substrate carriers is considered. State-specific diagnostics are combined with desorption-laser wavelength dependence studies to clarify the excitation and desorption dynamics involved. Results from NO desorption from both metallic and semiconductor substrates are presented, along with theoretical models of the desorption processes.

Investigations of III-V semiconductor growth by laser probing of the gas phase constituents represents a potential new technique for monitoring and improving molecular beam epitaxy . Laser-induced fluorescence (LJF) detection methods have now been devised to measure directly the number density of Ga and In atoms and As2 dimer species during the deposition and growth. The studies here are applied to the early stages of growth of 111-V materials on Si(100). Additional techniques may be employed in the future to detect species such as As₄ , P₄ , and other minor dopants in various molecular forms. Laser detection is used here to make measurements on scattering and sticking coefficients, the rates and energetics of desorbing species, and state-resolved accommodation. Important islanding behavior is detected by measurements of the kinetics for certain constituents in the presence of others. These techniques will be of value in devising practical in situ optical diagnostics for molecular beam epitaxy of III-V semiconductor devices.

Resonantly enhanced multiphoton ionization (REMPI) laser spectroscopic and molecular beam-surface scattering techniques are coupled to study inelastic and reactive gas-surface scattering with state-to-state specificity. Rotational, vibrational, translational and ang ular distributions have been measured for the inelastic scattering of HCI and N₂ from Au(1 1 1 ). In both cases the scattering is direct-inelastic in nature and exhibits interesting dynamical features such as rotational rainbow scattering. In an effort to elucidate the dynamics of chemical reactions occurring on surfaces we have extended our quantum-resolved scattering studies to include the reactive scattering of a beam of gas phase H-atoms from a chlorinated metal surface M-Cl. The nascent rotational and vibrational distributions of the HCI product are determined using REMPI. The thermochemistry for this reaction on Au indicates that the product formation proceeding through chemisorbed H-atoms is slightly endothermic while direct reaction of a gas phase H-atom with M-Cl is highly exothermic (ca. 50 kcal/mole). Details of the experimental techniques, results and implications regarding the scattering dynamics are discussed.

The electronic excitations and subsequent dynamics responsible for the stimulated desorption and dissociation of adsorbed molecules on metal surfaces can be characterized in detail by examining the neutral gas-phase products in a quantum specific fashion. Specifically, we have studied the electron-stimulated desorption (ESD) of NO from clean and 0-covered Pt(lll), and the electron-stimulated dissociation of NO2 on clean Pt(lll) through state-selective, time-of-flight laser resonance ionization of the NO product. In these experiments, we can determine the nature of a given electronic excitation by determining the threshold for the stimulated process and correlating it with photoelectron spectra and calculated excitation lifetimes. By determining the translational, vibrational, and rotational energy distributions of the ESD or stimulated dissociation products, a dynamical picture emerges which can be directly correlated with the electronic excitation and the extent of charge-transfer screening from the substrate. The presence of co-adsorbates such as atomic 0 modify screening charge and thus directly affect both the lifetimes of excitations and the dynamics of the stimulated event.

Multiphoton resonance excitation has been employed to ionize neutral atoms and molecules desorbed from surfaces bombarded by 5 keV Ar+ ion beams. By positioning a laser beam above the target it is possible to image the ions onto a microchannelplate detector and to obtain energy and angle-resolved distributions. These distributions may be compared directly to classical dynamics computer simulations of the ion/impact event. Results are presented using the (001), (111) and (331) crystal faces of Rh to illustrate how the distributions contain specific structural information. Moreover, we compare the distributions from (111) to secondary ion angular distributions. This comparison suggests that there are special impact points which lead to ion formation from clean metal surfaces. Finally, we present preliminary measurements for the MPRI of pyrene molecules desorbed from polycrystalline gold surfaces. The results suggest this technique may be valuable for monitoring reaction intermediates present at very low concentration on catalyst surfaces.

This paper examines the analytical capabilities and applications of Laser Ablation Microprobe Mass Spectrometry (LAMMS). The technique and instrumentation will be described along with the application of LAMMS to investigate dopant lateral diffusion problems in insulating and noninsulating materials. This work focuses on two material examples: p⁺ and n⁺ diffusion within 3 μm device lines in Ta and Co silicides; and trace element and dopant migration in optical fibers. The results show the capability of LAMMS to routinely achieve 1-3 μm lateral resolution with high dopant sensitivity on materials with extremely different optical properties.

Deuterated propylene (C₃D₆) chemisorption and decomposition on the Si(100)-(2x1) surface has been studied in ultrahigh vacuum by using laser induced thermal desorption (LITD) and temperature programmed desorption techniques (TPD). Propylene was found to adsorb molecularly at 110 K and to remain as an undecomposed molecular adsorbate up to approximately 500 K. As the surface temperature is increased, the propylene can both thermally desorb and decompose, ultimately producing a SiC thin film. LITD was used to study C₃D₆ and D₂ desorption as a function of surface temperature during temperature programming. Slow heating leads to strongly enhanced C₃D₆ decomposition compared to fast heating by laser irradiation. The decomposition of propylene is apparently a multistep process because deuterium is released from the chemisorbed propylene (and its fragments) over a temperature range from approximately 450 to 850 K. D₂ desorption from the decomposition of C₃D₆ occurs at higher temperatures compared to that observed for chemisorbed deuterium.

Laser induced thermal desorption (LITD) has emerged as a powerful time-dependent probe of surface processes. The ability of LITD to desorb silicon-containing species from Si(l1l)7x7 has been employed to study the decomposition of H₂0 and NH₃ on Si(l11)7x7. These LITD studies on Si(111)7x7 assumed that the silicon-containing species were derived from surface reaction intermediates. This assumption was recently tested by transmission FTIR investigations of H20 and NH₃ decomposition on high surface area porous silicon surfaces. The LITD and FTIR results were compared for the thermal stability of the SiOH surface species during H₂O decomposition and the SiNH₂ surface species during NH₃ decomposition. The good agreement between the LITD and the FTIR investigations indicated that the silicon containing LITD species correspond to silicon surface reaction intermediates.

A Resonant ionisation Mass Spectrometer has been used to characterise the neutrals ablated by a Nd:YAG laser. The neutrals were post ablation ionised both resonantly and non-resonantly and the signals produced in the spectrometer examined. The delay time between ablation and ionisation was varied and experimental energy distributions determined. The shape of the distribution from a calcium sample has been shown to be independent of the wavelength of the ablation laser at low ablation powers. The results were theoretically modelled to a Maxwell-Boltzman distribution and good agreement was obtained indicating that for calcium a thermal process is mainly responsible for the ablated neutrals. Initial data have also been obtained for lead and gold with similar results, although for gold several unexplained sharp spikes in the spectrum were evident. In addition the broad thermal distribution for gold was considerably wider than that for lead.

Experiments involving the adsorption of gas phase molecules onto thin metal films and the subsequent desorption and ionization of these species are described. The observed ion yield displays a rather striking laser pulse repetition rate dependence that can be interpreted using a simple model that quantitatively analyzes the competition between collisional adsorption and laser induced desorption.

Rotationally anisotropic surface second-harmonic generation (SHG) has been measured from a clean, well-ordered Cu(110) single-crystal surface as a function of both surface temperature and Ag coverage. For the clean Cu(110) surface, the temperature dependence of the SH response at a fixed azimuthal angle can be correlated with a surface phase transformation. A large decrease in the rotationally anisotropic SH response as a function of surface temperature can be related to changes in the surface disorder. The results are compared with other studies of Cu(110) surface structure using both x-ray and He-atom scattering. The rotationally anisotropic SH response has also been measured as a function of Ag coverage with the Cu(110) surface temperature fixed at 300 K. The results closely follow the formation of an ordered Ag(111)-like overlayer, the nucleation of three-dimensional Ag nanoclusters (<20 angstroms thick) that enhance the anisotropic SH response, and the subsequent growth of a ~10 monolayer thick Ag film. Variations in the rotationally anisotropic SH response as a function of Ag coverage are used to separate the resonant surface electronic contributions to the nonlinear susceptibility of the interface.

A nonlinear optical technique based on second harmonic generation (SHG) is introduced to probe the transient temperature jump induced by pulsed laser excitation of a metal surface. This technique is surface sensitive, nonintrusive and has time resolution limited only by the probe laser pulsewidth. For the Ag(1 10) surface, a strong temperature dependence of SHG enhanced by interband transitions is observed, and is used to detect a > 10¹⁰ K/sec heating rate induced by a nanosecond IR excitation pulse. A much stronger temperature dependence is also observed in the SHG resonantly enhanced by a surface state transition on Ag(110) and can in principle be used for surface temperature measurements. This kind of surface temperature measurement method can be generally applied to metals, semiconductors and solids that have temperature dependent electronic transitions.

A method of laser-induced desorption mass spectrometry (LIDMS) has been used to examine the dynamics of Cd(CH₃)₂ chemisorption and spontaneous decomposition on n type Si(100) with native surface oxide, the pathways and efficiencies of the adsorbate desorption due to the absorption of the XeCl. laser radiation by silicon. The extremely fast processes in the adlayer re caused by the preceding irradiation of the surface of the sample by intensive (with fluences up to 0.4 J/cm² ) laser pulses . The conet It ion between intact, dissociative, and recombination desorption pathways s responsible for the observed laser fluence dependences of the desorption of Cd(CH₃)₂ and its fragments.

Laser ablation of bulk High Temperature Superconductor (HTS) material promises to provide a useful means of producing high quality HTS thin films. Mass spectrometric probes of the ablation plume provide a microscopic understanding of the ablation event and plume development as well as providing a process monitor for the thin film production. Detection of the nascent ions in the plume provides real time analytical information, e.g., identification of impurities, major and minor ablation species, etc. The common contaminants sodium and strontium have been easily detected by this technique in a variety of different HTS bulk materials. In contrast, detection of the ablated neutral species by Resonance Ionization Mass Spectrometry (RIMS) provides physical information about the ablation process. Time-of-flight/RIMS detection of Cu,Y, and BaO ablated from YBa₂Cu₃O_x indicates the ablation involves post-desorption gas phase collisions, thereby influencing the ablation chemistry and dynamics (e.g., angular and velocity distributions). Approximately equal velocities are observed for all neutral species at constant ablation laser fluence.

Resonance ionization mass spectrometry (RIMS) of neutral atoms sputtered from a solid is an analytical technique that is complementary to secondary ion mass spectrometry (SIMS). Potential advantages of RIMS include: (1) high sensitivity; (2) rejection of mass interferences while retaining high sensitivity; (3) reduced matrix effects. A modified commercial double-focusing, magnetic sector SIMS instrument is used for photoionization-based measurements. Comparisons of SIMS and RIMS are directly possible. Examples of RIMS' ability to overcome matrix effects and interferences are given. Factors affecting relative sensitivity and detection limits of the two techniques examined are in part: duty cycle, secondary ion yield vs. secondary atom yield, photoionization efficiency ion transmission efficiency and detection mode. In RIMS, the geometrical overlap of laser and sputtered atoms must be optimized, and a three-dimensional formalism is discussed. Because of the pulsed nature of RIMS, duty cycle restrictions can limit the sensitivity relative to SIMS. Absolute ion yields for SIMS and RIMS are presented.

A two-step laser vaporization/laser multiphoton ionization method is used to analyze and detect polymers. In this method a pulsed CO₂ laser is used to vaporize the polymer for supersonic jet entrainment and then a second (UV) laser ionizes the desorbed species. The resulting ions are analyzed in a reflection time-of-flight (TOF) mass spectrometer. The challenges here include selectively detecting an aromatic polymer in a polymer blend and studying molecular weight distributions.

Vibrational energy relaxation of the symmetric C-H stretching mode of methyl thiolate on a Ag(l 1 1) surface is measured by picosecond infrared-visible sum frequency generation. Vibrational relaxation lifetime components of ~3 PS and 63 PS are observed at 300 K. The long lifetime component shows a moderate temperature dependence. Both population relaxation components are assigned to intramolecular energy transfer on the basis of comparisons with other measurements and the predicted temperature dependence of intramolecular relaxation rates. Direct energy transfer to electronic or vibrational degrees of freedom in the substrate is not found to be important for this vibrational mode.

The quality-factor dependent enhancement of the field of a weak probe laser beam inside a resonant optical cavity is used to measure the minute optical absorption of liquid films, metallic and high reflectivity dielectric coatings. The measurement is not affected by the presence of the surface reflections and the scattering from the strong pump laser beam, which is focussed onto the sample placed inside the cavity. Laser surface heating calculations were performed and account qualitatively for both the spatial and temporal behavior of the observed signals.