Time multiplexing: a new single shot femtosecond pump-probe technique

Received: 26 August 1996/Revised version: 13 November 1996     

Measurements of multiphoton ionization coefficients with ultrashort ultraviolet laser pulses

Multiphoton ionization coefficients of atmospheric gases were measured with ultrashort ultraviolet laser pulses. The values were obtained using two different experimental setups and the pressure range covered four orders of magnitude. The coefficients were pressure-independent and consistent with numbers predicted by the nonresonant Keldysh theory. Received: 13 June 2000 / Revised version: 5 October 2000 / Published online: 7 February 2001     

Hydrogen dissociation on metal surfaces – a model system for reactions on surfaces

Received: 23 March 1998/Accepted: 21 August 1998     

Cavity ring-down spectroscopy of OH radicals in low pressure flame

10 molecules/cm³) and large dynamic range (more than 20 000) are demonstrated. CRDS also provides accurate temperature measurements, with statistical errors less than 2%. Measured concentration profiles are in reasonable agreement with calculated values. It is observed that in the preflame zone (where the temperature is about 1000 K), the OH concentration at the first vibrational excited state is about seven times larger than the equilibrium OH(v”=1) concentration at this temperature. Received: 24 June 1997/Revised version: 12 September 1997     

Photochemical effects of KrF excimer excitation in laser-induced fluorescence measurements of OH in combustion environments

This paper presents experimental evidence that using the KrF excimer laser for quantitative laser-induced fluorescence (LIF) studies of the OH A-X (3,0) system is highly problematic if the effects of both photobleaching and photochemistry are not included for laser spectral irradiances greater than 20 MW/cm² cm^-1. Pump-probe and time-resolved measurements of the OH LIF signal in an atmospheric pressure, premixed CH₄-air flame at low- and high-laser-spectral-irradiance conditions show that a significant amount of OH is produced from photofragments resulting from the simultaneous 2-photon predissociation of H₂O molecules in the C-X system. A 5+2-level rate-equation model that includes the effects of both photobleaching and photochemical OH production is shown to satisfactorily predict the data using a single adjustable parameter given by the effective, spectrally integrated 2-photon cross-section of H₂O near 248 nm. The time-integrated OH LIF signal was found to depend on both the laser spectral irradiance and the local concentration of H₂O. Additionally, use of the KrF excimer laser for 2-line rotational thermometry can produce temperature errors as great as +550 K at high laser-pulse energies. Received: 21 August 2000 / Revised version: 30 October 2000 / Published online: 21 February 2001     

Sum-frequency generation study of reacting species at surfaces

Received: 6 October 1998 / Revised version: 3 November 1998     

A study of dynamics and chemical reactions in laser-ablated PbTiO3 plume by optical-wavelength-sensitive CCD photography

3 (PT) in ambients of oxygen, nitrogen, and argon was studied by using a CCD camera. Two optical filters with different transmittance bands were used to identify the excitation, dissociation, ionization, and, especially, the chemical reactions in the plume. The results show that the atomic processes and chemical reaction mechanisms in the plume formed in active oxygen are very different from those formed in inert argon and nitrogen. It is proposed that the chemical reactions between ablated metal species and ambient oxygen are initiated by the strong shock wave formed during the propagation of the PT plume. Received: 6 October 1997/Accepted: 7 October 1997     

Fabrication of large-scale ultra-smooth metal surfaces by a replica technique

The IR double-resonance techniques IR/R2PI (infrared/resonant 2-photon ionization), IR/PIRI (infrared-photo-induced Rydberg ionization) and IR-photodissociation spectroscopy are valuable tools to investigate structure, vibrations, and dynamical processes of neutral and ionic hydrogen-bonded clusters containing aromatic molecules. In this paper we report on the application of the IR double-resonance techniques to determine the NH and OH stretching vibrations of 4-aminophenol and 4-aminophenol(H₂O)₁, both in the neutral (S₀) and ionic (D₀) ground state. All vibrational frequencies obtained for 4-aminophenol and the cluster are compared with the values obtained from ab initio and DFT calculations. In the S₀ state, a trans-linear arrangement of 4-aminophenol(H₂O)₁ is obtained containing an O-H·· O hydrogen bond. In the D₀ state an overlay of two spectra can be observed resulting from the trans-linear structure and a second structure which contains a N-H·· O hydrogen bond. The observation of these two structures within the ion is an interesting example of a rearrangement reaction in the ionic state. Received: 3 November 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Femtosecond dynamics of chemical reactions at surfaces

One of the major goals in physical chemistry is to obtain a microscopic understanding of chemical reactions. Recent developments in femtosecond laser techniques provide the opportunity to resolve the timescale of elementary steps of chemical reactions at surfaces. This is exemplified for the femtosecond laser-induced oxidation of CO on Ru(001). Among other adsorbate-specific probes vibrational sum-frequency generation spectroscopy offers the possibility to monitor adsorbates or reaction intermediates directly at the surface. Recently, we have employed this technique to investigate the dynamics of the CO-stretch vibration of CO adsorbed on Ru(001) after optical excitation leading to CO desorption. Received: 4 August 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

In situ investigation of the catalytic reaction H2+< ![IGNORE[O2→H2 O with second-harmonic generation

2 +O₂→H₂ O in the pressure range 0.2 Torr≤p_tot≤10 Torr on Pt(111) surface. At a catalyst temperature of T=700 K the equilibrium oxygen coverage θ_o is determined as a function of hydrogen partial pressure α. The experimentally obtained θ_o is modelled in a two step process considering the mass transport in the gas phase as well as the catalytic reaction on the surface. In this pressure range the mass transport in the gas phase changes from molecular flow conditions to laminar flow, inducing a strong modification of the gas phase present at the catalyst through different diffusivities of the reactants as well as through desorbing reaction products from the catalyst. It is shown that these gas phase alterations have to be taken into account for a proper modelling of the surface mechanism. Simulation calculations allow one to identify the sequential hydrogen addition reaction as the main reaction path for water production in this parameter range. Excellent agreement with previous investigations is obtained for the determined activation energies of the water-producing reaction steps equal to E^f _H2O≥0.7 eV. Received: 20 September 1998 / Revised version: 15 December 1998     

Simultaneous interaction of methyl radicals and atomic hydrogen with amorphous hydrogenated carbon films, as investigated with optical in situ diagnostics

Methyl radicals (CH₃) and atomic hydrogen (H) are dominant radicals in low-temperature plasmas from methane. The surface reactions of these radicals are believed to be key steps leading to deposition of amorphous hydrogenated carbon (a-C:H) films or polycrystalline diamond in these discharges. The underlying growth mechanism is studied, by exposing an a-C:H film to quantified radical beams of H and CH₃. The deposition or etching rate is monitored via ellipsometry and the variation of the stoichiometry is monitored via isotope labeling and infrared spectroscopy. It was shown recently that, at 320 K, methyl radicals have a sticking coefficient of 10^-4 on a-C:H films, which rises to 10^-2 if an additional flux of atomic hydrogen is present. This represents a synergistic growth mechanism between H and CH₃. From the interpretation of the infrared data, a reaction scheme for this type of film growth is developed: atomic hydrogen creates dangling bonds by abstraction of bonded hydrogen within a surface layer corresponding to the range of H in a-C:H films. These dangling bonds serve at the physical surface as adsorption sites for incoming methyl radicals and beneath the surface as radicalic centers for polymerization reactions leading to carbon–carbon bonds and to the formation of a dense a-C:H film. Received: 18 July 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Wavelength-resolved REMPI mass spectrometry in a hostile industrial environment, limitations and promises of the method

The complementary REMPI (resonance-enhanced multi-photon ionisation) techniques using either a tuneable laser or a fixed-frequency laser are compared. Our Jet-REMPI apparatus, consisting of a tuneable laser unit (Nd:YAG pump laser and OPO) and a reflectron mass spectrometer, is briefly introduced. This machine has been upgraded to cope with the difficult conditions (such as vibrations, temperature fluctuations and dust) prevailing in an incinerator. On-line measurements of the raw gas have been carried out at the municipal waste incinerator (MWI) in Stuttgart. Although this test series could not be completed the results are encouraging. They show that wavelength resolution can provide valuable information beyond that which can be obtained by fixed-frequency REMPI. Examples discussed include identification of phenanthrene as major constituent of the isomeric pair phenanthrene/anthracene and the structural identification of xylene isomers in the incinerator raw gas. Both examples are also of more general interest, the former with regard to investigation of soot precursors and the latter for quality assessment of refinery products. Spectra of some deuterated aromatics are reported for use of these compounds as standards. In addition, a mass spectrum containing DDT is shown and a chlorobenzene profile measured earlier in a pilot scale incinerator as demonstration of the sensitivity of the instrument. As is to be discussed, significant further sensitivity gains through increase of the beam density are limited because of ion collisions within the sample beam. Received: 23 February 2000 / Revised version: 9 June 2000 / Published online: 5 October 2000     

Sympathetic cooling rate of gas-phase ions in a radio-frequency-quadrupole ion trap

We theoretically investigated the mass dependence of the sympathetic cooling rate of gas-phase ions trapped in a linear radio-frequency-quadrupole ion trap. Using an a priori molecular dynamical calculation, tracing numerically with Newtonian equations of motion, we found that ions with a mass greater than 0.54±0.04 times that of the laser-cooled ions are sympathetically cooled; otherwise, they are heated. To understand the mass dependence obtained using the molecular-dynamical calculation, we made a heat-exchange model of sympathetic cooling, which shows that the factor of 0.54±0.04 is a consequence of absence of micro-motion along the axis of the linear ion trap. Received: 10 December 2001 / Revised version: 28 January 2002 / Published online: 14 March 2002     

DUV scattering measurements as a tool for characterization of UV-optical surfaces

A sensitive total-scattering measurement setup for the DUV spectral range is presented, which allows precise determination of both forward and backward scatter losses from optical components for excimer lasers with a sensitivity below 10 ppm for 248 nm and 50 ppm for 193 nm. Scattering from several different coated and uncoated DUV optical surfaces was monitored. For uncoated samples, the backward scatter losses are in good agreement with the predictions of scalar scattering theory, indicating that in this case scattering is mainly determined by surface effects. Although forward and backward scatter losses are of the same order of magnitude for uncoated samples, they differ by up to two orders of magnitude for high-reflection- and by one order of magnitude for anti-reflection-coated samples. The experimental data demonstrate that the anti-reflection coatings suffer from substantial losses due to forward scattering, whereas backward scattering is the predominant loss channel for high-reflection coatings. In addition, the strong influence of defects and impurities on the total scattering is demonstrated. Received: 5 June 2000 / Accepted: 6 June 2000 / Published online: 13 September 2000     

Femtosecond laser-induced formation of spikes on silicon

We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500-fs laser pulses in SF₆. The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes. Received: 10 September 1999 / Accepted: 7 January 2000 / Published online: 8 March 2000     

Infrared-laser-pulse control of bond- and state-selective excitation, dissociation and space quantization: application to a three-dimensional model of HONO2 in the ground electronic state

Selective control over the vibrational excitation and space quantization of the dissociation fragments by optimally designed linearly polarized and shaped infrared (IR) laser pulses of the picosecond (ps) and subpicosecond duration is demonstrated by means of quantum-dynamical simulations within the Schr?dinger wave-function formalism for a three-dimensional (3-D) model of HONO₂ in the ground electronic state, wherein the OH and the ON single-bond stretches are explicitly treated, together with the bending angle between them, on the basis of the ab initio defined 3-D potential-energy surface and dipole function. The high-lying zeroth-order vibrational states of the OH bond are prepared selectively both below and above the dissociation threshold of the ON single bond, and demonstrate a quasi-periodic oscillatory behaviour, manifesting intramolecular vibrational energy redistribution (IVR) on the picosecond timescale. Selective breakage of the ON single bond in HONO₂ with more than 97% probability is demonstrated, along with control of the space quantization of the dissociation fragments: the OH fragments rotating clockwise, OH(c), and anticlockwise, OH(a), are prepared selectively, with the OH(a)/OH(c) branching ratio being as high as 10.975. The results obtained show that optimally designed strong and short IR-laser pulses can compete against IVR and manipulate vibrational excitation and dissociation of polyatomic molecules. Received: 3 November 1999 / Published online: 13 July 2000     

Applications of scanning force microscopy operating in dynamic modes on self-organized assemblies of nanoclusters and organic amphiphiles

Received: 3 July 1998     

Sputtered neutral mass spectrometry (SNMS) as a tool for chemical surface analysis and depth profiling

An experimental system for mass spectrometry of supttered neutral particles involving a hf plasma operated in Ar at several 10⁻⁴ Torr is described. The potentialities of the method for quantitative surface analysis are reasoned. Depth profiling by sputtered neutral mass spectrometry is demonstrated for anodic oxide layers on Nb and Ta.     

AFM, a tool for single-molecule experiments

Received: 27 March 1998