Nonresonant Multiphoton Ionization Research Articles

Pressure Measurement in XHV Region Using Nonresonant Multiphoton Ionization by Picosecond Pulsed Laser.

By means of nonresonant multiphoton ionization using a picosecond pulsed YAG laser, pressure measurement for H2, CO and CO2 gases was carried out in the XHV pressure range. It was experimentally verified that pressure measurement was possible in the range of 10-11 Pa, and this pressure measurement method had pressure sensitivity of the same order of magnitude for the three molecules.

Photoion detection with a wide dynamic range using a quadrupole mass spectrometer for nonresonant multiphoton ionization of sputtered neutrals

A sputtered neutral mass spectrometer using nonresonant multiphoton ionization by an excimer laser has been developed. The composition of this instrument is similar to the dynamic secondary ion mass spectrometer. Depth profiling with a wide dynamic range can easily be performed by the combination of a quadrupole mass spectrometer and pulse-counting ion detection. The mass spectrum of photoionized neutrals of GaAs shows that the dynamic range of detection is more than six orders of magnitude and that the product of ionization efficiency and mass spectrometer transmission is about 2.1×10−3. The detection limit for Zn impurity in GaAs is under 1 ppm at a laser repetition rate of 50 Hz.

Energy Distributions of Atoms Sputtered from Cu-Pt Alloy Measured by Nonresonant Multiphoton Ionization Sputtered Neutrals Mass Spectrometry

Energy distributions of sputtered Cu and Pt atoms from monoatomic and Cu-Pt (55 at.%) alloy samples under Ar+ ion bombardment were measured using an sputtered neutrals mass spectrometry (SNMS) instrument in nonresonant multiphoton ionization. The distributions for the pure Cu and Pt samples agreed very well with the theoretical distribution derived by Thompson [Philos. Mag. 18 (1968) 377], while considerable deviation was found in the distributions of Pt atoms from the Cu-Pt alloy, particularly at lower energy. The results have also revealed that the peak position, which reflects the surface binding energy according to Thompson's theoretical expression, of the energy distribution of Pt atoms from the alloy is ∼0.6 eV lower than that of the Pt atom energy distribution from the pure Pt sample.

Quasistationary state decay theory of non-resonant multiphoton ionization

A non-perturbative theory which is based on the three-dimensional gauge-invariant quasistationary state decay approach is developed to describe the non-resonant multiphoton ionization of real atoms. Using this approach, we are able to (a) solve a time-dependent Schrodinger equation for an atom-field system in a controllable approximation. Thus this leads us to (b) the possibility of describing non-resonant multiphoton processes which are straightforwardly treated in our formalism with none of the gauge difficulties sometimes encountered in previous approaches. Low-order low-frequency moderate-field approximations of the theory eventually reduce to results of earlier efforts, namely, the pioneering non-perturbative description of multiphoton ionization by Keldysh and refinements to it through partial consideration of atom-field interaction in initial state (Faisal, Reiss). Besides, (c) we can analyse the exceptional stabilization effect in a high-frequency superintense laser field. Our analytic high-frequency results provide a simple yet instructive extension to the weak-bound regime of ionization processes.

Non-resonant multophoton ionization of cyclopentane and its heterocyclic analogues

We report non-resonant multiphoton ionization time-of-flight mass spectra for cyclopentane, tetrahydrofuran, tetrahydrothiophene and pyrrolidine for 355 nm excitation over a wide range of laser intensities. Unlike the corresponding three-membered ring system, there is little dependence of the fragmentation pathways upon the laser intensity. The results also indicate that the mechanisms for the molecules studied cannot be generalized and are not necessarily related to those of the three-membered rings.

High-angular-momentum states as population traps in multiphoton ionization

Resonant and nonresonant multiphoton ionization of xenon is studied using short, circularly polarized light pulses (100 fs, 597 nm, 22 TW/${\mathrm{cm}}^{2}$). A pump-probe measurement shows that, although bound states are substantially populated, they do not enhance the ionization signal. The bound states do not ionize because their high angular momentum repels the wave functions from the nucleus. Ionization does occur through intermediate states in the continuum, in spite of a large energy mismatch, because these states have more energy and therefore suffer less from the centrifugal barrier.

Quantum electrodynamics of multiphoton ionization

A nonperturbative quantum-electrodynamical approach to strong-field nonresonant multiphoton ionization is described and compared with other scattering models. This approach which is based on a work by Guo, Åberg, and Crasemann makes use of formal time-independent scattering theory and of availability of Volkov states for the construction of Green's operators. The semiclassical laser-field approximation is derived from the Dirac equation by considering the large-photon-density limit of a multimode electromagnetic field. A recent interpretation of the strong-field Kapitza-Dirac effect by Guo and Drake is treated as an example.

Space charge effects in the ion time-of-flight spectra following non-resonant multiphoton ionization

The authors present a kinetic model which describes the evolution of charged particles produced by high-intensity, ultrashort laser pulses, in non-resonant multiphoton ionization (MPI) processes. The model is based on the Vlasov-Maxwell equation and on the approximation of a linear profile for the space charge field. It has been used for predicting waveforms of the ion detector signal obtained in non-resonant MPI of Xe atoms with 30 ps, 1013 W cm-2, 1.064 mu m laser pulses. The comparison of computed and experimental results is satisfactory, thus underlying the relevance space charge effects in typical non-resonant MPI conditions, and showing the effectiveness of the model in accounting for them.

Influence of laser parameters on the detection efficiency of sputtered neutrals mass spectrometry based on non-resonant multiphoton ionization

Non-resonant multiphoton ionization experiments on sputtered neutral particles have been performed with nanosecond and picosecond laser pulses. The ionization yields for particles removed from copper and silicon samples have been measured with respect to the laser intensity, the focused laser beam profile and the laser pulse duration. An experimental set-up is described for measuring the focused beam profile of the high power laser and to correlate it directly with the ionization yield. The results suggest that there are two competing channels of ionization: namely, non-resonant ionization of neutral atoms and dissociative ionization of sputtered molecules. This is further evidenced by experiments with copper atoms evaporated from a heated wire. If the laser beam profile and the ion extraction optics of the mass spectrometer are not optimized, post-ionization originates dominantly from the dissociative ionization process which makes calibration difficult. Calculations show that the nature of the detection process depends on laser intensity. At low laser intensities the system works like a particle density detector whereas at intensities beyond the saturation intensity the system can become a particle flux detector. Within this approximation we give the optimum pulse durations and decay times for the ion and laser pulses to make the post-ionization yield quasi-independent of the sputtered particle velocities.

Non-resonant multiphoton ionization of small cyclic molecules: a comparison of fragmentation pathways for ethylene sulfide, ethylene oxide, propylene oxide and cyclopropane

We report non-resonant multiphoton ionization time-of-flight mass spectra for ethylene sulfide (thiirane), ethylene oxide, propylene oxide and cyclopropane for 355 nm excitation over a wide range of laser pulse energies. The fragmentation pathways are laser energy independent for energies greater than approximately 5 mJ pulse−1. The results indicate that the major fragmentation patterns are molecule specific and cannot be generalized for the analogous ring systems. The fragmentation mechanism is discussed separately for each molecule.

Quantitative analysis of steel samples with laser ionization mass spectrometry

An analytical method based on sputtering, laser ionization and time-of-flight mass spectrometry is used to determine the composition of stainless-steel samples. Ionization is performed as non-resonant multi-photon ionization, which yields a multi-element mass spectrum. The aim of the method is to investigate the prospects for quantitative analysis. Instrumental sensitivity factors for the various elements are measured by analysing certified reference samples. The concentrations of the most abundant elements in other samples are then measured, using these sensitivity factors, and are found to be within ± 10 to 15% of the specified compositions. These deviations seem to depend mainly on statistical fluctuations, rather than on any matrix effects. This shows that, to a high degree, sensitivity factors are transferable from one sample to another, provided that the same class of material is studied. This surface analytical method is also useful for the analysis of bulk compositions when the surface of the sample is properly cleaned.

Ionization suppression of Stark states in intense laser fields.

We have observed suppression of ionization in Rydberg atoms irradiated by an intense short-pulse laser field. We find that single-photon ionization of the outer electron in a Ba 6snk Stark state can be severely suppressed relative to high-order (\ensuremath{\ge}3 photon) nonresonant multiphoton ionization of the tightly bound 6s core electron. This population trapping occurs in intense short laser pulses, where the bandwidth of the ionizing laser is greater than the spacings between adjacent Stark states, and may be due to a substantial modification of the initial Stark state via stimulated Raman processes through the continuum. The ${\mathrm{Ba}}^{+\mathrm{*}}$ ion should be left in a higly excited superposition state, or wave packet.

Surface analysis by photoionization of sputtered species with intense picosecond laser radiation

Intense ps laser radiation is used to photoionize sputtered atoms and molecules ∼ 1 mm above solid surfaces by nonresonant multiphoton ionization (NRMPI), followed by time-of-flight mass spectrometry. At intensities ≈ 1 x 10 14 W/cm 2 even red light at 650 nm can completely photoionize (saturate) elements having a high ionization potential (IP), such as atomic O and Ar, as well as elements of lower ionization potential. The ability to efficiently ionize all these elements translate into uniform and high detection sensitivities, and the ability to quantify elements over a great range of ionization potentials.

High spatial resolution chemical imaging of surfaces by combination of a field-emission ion gun and intense laser radiation

Scanning surface analysis by laser ionization (SALI) has been developed to provide chemical images of surfaces. SALI is a recently developed technique that uses nonselective photoionization of sputtered or desorbed neutral atoms and molecules emitted from a sample close to but above (~ 1 mm) the surface, followed by time-of-flight mass spectrometry (TOF-MS). In this new development, specifically nonresonant multiphoton ionization (NRMPI) and sputtering using a field-emission liquid-metal ion gun are employed. The reason for decoupling the sputtering and ionization is that in nearly all situations, the neutral particles constitute the dominant channel for the sputtered material; therefore, SALI is much less sensitive than scanning secondary ion mass spectrometry (SIMS) to changes in the chemical matrix, and also SALI delivers a much more uniform sensitivity between different species than SIMS. Furthermore, the mass multiplex advantage of TOF-MS allows an entire mass spectrum of the sample to be mapped out in one scan of the surface. Initial results with a transmission electron microscopy (TEM) grid and an electronic device are presented.

Laser post-ionization improves surface analysis

Laser post-ionization (PI), a technique that overcomes the greatest handicap of secondary-ion mass spectroscopy (SIMS), namely, its susceptibility to matrix effects is discussed. The SIMS matrix effect is the often unknown relationship between the chemical composition of a solid and the secondary ion yield of an analyte in the sample, which is unpredictable and can vary as much as seven orders of magnitude. PI, which involves passing a laser beam parallel to, and just above, a surface undergoing stimulated desorption, solves the problem by decoupling the ionization and desorption steps. This allows the detection of secondary neutral species, which typically constitute more than 95% of the material removed from the surface during ion beam sputtering. This largely eliminates matrix effects because the chemical matrix generally has much less influence on the emission of charged particles. To compete with SIMS on sensitivity, PI methods efficiently ionize sputtered neutrals and detect those positionized particles. The nonresonant method of PI, which comes in two variations, nonresonant multiphoton and nonresonant single-photon ionization and is called SALI (surface analysis by laser ionization), is described, and its application to semiconductors is examined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of Sputtered Neutrals by Nonresonant Multiphoton Ionization. III. Change of the Velocity Distribution with Time after Sputtering

The nonresonant multiphoton ionization technique was applied to the composition analysis of Cu-Ni (52 at.%) alloy to verify the capability for quantitative surface characterization. It has been revealed that the intensities of Cu- and Ni-sputtered neutrals from the pure and alloy samples represented a rapid change after high-current ion bombardment was stopped. This suggests that the Ar atoms implanted in the surface during Ar+ ion bombardment caused a considerable change of the surface binding energy, resulting in the change of the velocity distributions of the photoionized neutrals.

Analysis of Sputtered Neutrals by Nonresonant Multiphoton Ionization. II. A Quantitative Composition Analysis of Cu-Al Alloy

A sputtered neutral mass analysis by multiphoton ionization was applied to the composition analysis of Cu-Al alloy. The reliability of the quantitative estimation of the signal intensity was improved by using an ion-counting system with a simple flight tube. Measured intensities of Cu and Al from a Cu-Al alloy sample represented atomic compositions with high accuracy. Errors in the estimation of the surface chemical composition are within several percent.

Analysis of Sputtered Neutrals by Nonresonant Multiphoton Ionization I. Development of Laser Ion Counting System with a Simple Time-of-Flight Measurement

A simple time-of-flight, pulse-counting system is introduced to a sputtered neutral analysis by a nonresonant multiphoton ionization technique. The system consists of an ion acceleration assembly, a flight tube, a microchannel plate detector and pulse-counting electronics. A direct counting of detected photoions on each laser shot provided a new capability of comparing different elements with no detection efficiency difference.

Characterization of polycyclic aromatic compounds on surfaces using ion-beam-induced desorption and multiphoton resonance ionization

The results show that it is possible to obtain extremely high-quality mass spectra of these compounds with subfemtomole sensitivity limits. Although the degree of ionization selectivity of atomic species is much lower than for MPRI of molecules, we show that it is possible to control the degree of photofragmentation by varying the laser intensity. Spectra obtained by MPRI, by nonresonant multiphoton ionization (MPI), and directly by secondary ion mass spectrometry (SIMS) are quantitatively compared. Finally, we demonstrate that mixtures of PAC's are readily detected by examining a cocktail of codeposited benzo [a] pyrene, dibenz [ac]-anthracene, triphenylene, and pyrene. In general, we propose that this approach is a powerful new method for detecting low concentrations of complex molecules adsorbed on surfaces

On the use of single-photon ionization for inorganic surface analysis

A general surface analysis method has been developed based on non-selective photoionization of sputtered or desorbed neutral atoms and molecules above the surface, followed by time-of-flight mass spectrometry. The approach, currently utilizes two main types of ionizing radiation and a variety of desorption probes. For photoionization, coherent untuned sources are used; an intense focused pulsed UV laser beam is used for non-resonant multiphoton ionization to give elemental and limited chemical information, usually used for inorganic analysis; a coherent VUV source is used for single-photon ionization at 118 nm (10.5 eV) produced by frequency tripling of 355 nm light from a Nd:YAG laser. This paper focuses on single-photon ionization for inorganic systems. The desorption probes used are ion, electron, and laser beams as well as thermal desorption. For depth profiling, ion beams are specifically used. Any focused desorption probe beam can provide lateral spatial resolution.