Atomiclike ionization and fragmentation of a series of CH3-X (X: H, F, Cl, Br, I, and CN) by an intense femtosecond laser

Methane derivatives of CH(3)-X (X: H, F, Cl, Br, I, and CN) were ionized and fragmented by an intense femtosecond laser with a 40 fs pulse at 0.8 microm in intensities of 10(13)-10(15) W cm(-2). The curves of the ionization yields of CH(3)-X versus laser intensities have been found to be fitted with an atomic ionization theory (the theory of Perelomov, Popov, and Terent'ev) that has been established to reproduce experimental results well for rare gas atoms. The saturation intensities have been reproduced within a factor of 1.6 of the calculated ones. For molecules with low ionization potentials such as amines, another atomic ionization theory (the theory of Ammosov, Delone, and Krainov) reproduced the saturation intensities. The atomiclike ionization behavior of molecules indicates that the fragmentation occurs after the ionization. The fragmentation mechanisms after the ionization of some molecular ions are discussed.     

On the signal collection in laser enhanced ionization spectrometry

A theoretical model is developed which relates the experimentally observed signal to the charged species produced in laser enhanced ionization. The model is based on a general equation, which describes the electric field in a flame confined between electrodes at fixed potentials, and a relation for the charge induced on these electrodes. A complete analytical solution is found in a one-dimensional geometry. The signal corresponding to the laser induced electrons is described accurately by the model. The ionic part of the signal, although in most cases negligible, is influenced significantly by dynamic screening in the flame, and requires a more complete model. The theory is applied to an experimental situation, and relations for optimal signal collection are derived. Then the predictions from the theory in a cylindrical geometry are compared to results from a graphite furnace experiment. The temporal behaviour of the signal is further used for a determination of the electron mobility in a flame.     

Wavelength dependence of the suppressed ionization of molecules in strong laser fields

We study ionization of molecules by an intense laser field over a broad wavelength regime, ranging from 0.8 to 1.5 μm experimentally and from 0.6 to 10 μm theoretically. A reaction microscope is combined with an optical parametric amplifier to achieve ionization yields in the near-infrared wavelength regime. Calculations are done using the strong-field S-matrix theory and agreement is found between experiment and theory, showing that ionization of many molecules is suppressed compared to the ionization of atoms with identical ionization potentials at near-infrared wavelengths at around 0.8 μm, but not at longest wavelengths (10 μm). This is due to interference effects in the electron emission that are effective at low photoelectron energies but tend to average out at higher energies. We observe the transition between suppression and nonsuppression of molecular ionization in the near-infrared wavelength regime (1-5 μm).     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. I. Extreme multielectron ionization

In this paper we present a theoretical and computational study of extreme multielectron ionization (involving the stripping of all the electrons from light, first-row atoms, and the production of heavily charged ions, e.g., Xe(+q) (q< or =36) from heavy atoms) in elemental and molecular clusters of Xe(n),(D(2))(n), and (CD(4))(n) (n=55-1061) in ultraintense (intensity I=10(15)-10(19) W cm(-2)) laser fields. Single atom or molecule multielectron ionization can be adequately described by the semiclassical barrier suppression ionization (BSI) mechanism. Extreme cluster multielectron ionization is distinct from that of a single atomic or molecular species in terms of the mechanisms, the ionization level and the time scales for electron dynamics and for nuclear motion. The novel compound mechanism of cluster multielectron ionization, which applies when the cluster size (radius R(0)) considerably exceeds the barrier distance for the BSI of a single constituent, involves a sequential-parallel, inner-outer ionization. The cluster inner ionization driven by the BSI for the constituents is induced by a composite field consisting of the laser field and inner fields. The energetics and dynamics of the system consisting of high energy (< or =3 keV) electrons and of less, similar 100 keV ions in the laser field was treated by molecular dynamics simulations, which incorporate electron-electron, electron-ion, ion-ion, and charge-laser interactions. High-energy electron dynamics also incorporates relativistic effects and includes magnetic field effects. We treat inner ionization considering inner field ignition, screening and fluctuation contributions as well as small [(< or =13%)] impact ionization contributions. Subsequent to inner ionization a charged nanoplasma is contained within the cluster, whose response to the composite (laser+inner) field results in outer ionization, which can be approximately described by an entire cluster barrier suppression ionization mechanism.     

Ionization and Coulomb explosion of small group 10 transition metal oxide clusters in strong light fields

The ionization properties of small group 10 metal oxide clusters are explored using ultrafast pulses centered at 624 nm. Maximum atomic charge states resulting from Coulomb explosion were observed to be Ni(3+), Pd(3+), Pt(5+), and O(2+) species with similar ionization potentials ～30-35 eV. Ion signal as a function of laser intensity of each charge state of Ni, Pd, Pt, and O resulting from Coulomb explosion was mapped and compared to that predicted from semi-classical tunneling theory using sequential ionization potentials to quantify observed enhancements in ionization. The saturation intensity (I(sat)) of each charge state is measured and compared to previous studies on group 5 transition metal oxides. The atomic charge states of nickel showed a large enhancement in ionization compared to palladium and platinum, reflective of the differing bonding properties of each metal with oxygen. Results indicate that nickel oxide clusters undergo a greater extent of ionization enhancement as a result of multiple ionization mechanisms. The ionization enhancement behavior of each metal oxide species is explored herein.     

Extreme ionization of Xe clusters driven by ultraintense laser fields

We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xe(n) clusters (n=2-2171, initial cluster radius R(0)=2.16-31.0 A) driven by ultraintense infrared Gaussian laser fields (peak intensity I(M)=10(15)-10(20) W cm(-2), temporal pulse length tau=10-100 fs, and frequency nu=0.35 fs(-1)). Cluster compound ionization was described by three processes of inner ionization, nanoplasma formation, and outer ionization. Inner ionization gives rise to high ionization levels (with the formation of [Xe(q+)](n) with q=2-36), which are amenable to experimental observation. The cluster size and laser intensity dependence of the inner ionization levels are induced by a superposition of barrier suppression ionization (BSI) and electron impact ionization (EII). The BSI was induced by a composite field involving the laser field and an inner field of the ions and electrons, which manifests ignition enhancement and screening retardation effects. EII was treated using experimental cross sections, with a proper account of sequential impact ionization. At the highest intensities (I(M)=10(18)-10(20) W cm(-2)) inner ionization is dominated by BSI. At lower intensities (I(M)=10(15)-10(16) W cm(-2)), where the nanoplasma is persistent, the EII contribution to the inner ionization yield is substantial. It increases with increasing the cluster size, exerts a marked effect on the increase of the [Xe(q+)](n) ionization level, is most pronounced in the cluster center, and manifests a marked increase with increasing the pulse length (i.e., becoming the dominant ionization channel (56%) for Xe(2171) at tau=100 fs). The EII yield and the ionization level enhancement decrease with increasing the laser intensity. The pulse length dependence of the EII yield at I(M)=10(15)-10(16) W cm(-2) establishes an ultraintense laser pulse length control mechanism of extreme ionization products.     

Characterization of the atmospheric pressure ionization mass spectrometric process obtained using a fused-silica emitter with the high voltage applied upstream

The atmospheric pressure ionization process obtained when a mixture of methanol and water (90:10, v/v) also containing 50 microM sodium hydroxide is dispersed from a fused-silica emitter was studied. A combination of a high electric field and a nebulizer gas with the high voltage applied upstream in the liquid flow was utilized to facilitate the spray process. By comparing the dependences of the spray current and ion signals on the spray potential, it was found that electrical corona discharges were obtained for potentials higher than about 2.6 kV, which resulted in a mixed electrospray and chemical ionization process. By introducing vapour from a solvent, such as benzene or toluene, with a low ionization energy into the nebulizing gas, it was found that the appearance of the corresponding molecular ion was correlated with a change in the slope of the spray current-potential curve. This indicates that the breakpoints in the spray current-potential curves observed were correlated with the onsets of corona discharges. It was shown that the mixed ionization process gives rise to increased amounts of protonated solvent molecules and assists in the formation of sodiated adduct ions from an uncharged fatty acid methyl ester.     

Influence of clustering and molecular orbital shapes on the ionization enhancement in ammonia

The Coulomb explosion of clusters is known to be an efficient source for producing multiply charged ions through an enhanced ionization process. However, the factors responsible for obtaining these high charge states have not been previously explored in detail and remain poorly understood. By comparing intensity-resolved visible laser excitation experiments with semi-classical theory over a range spanning both multiphoton and tunneling ionization regimes, we reveal the mechanism in which extreme ionization proceeds. Under laser conditions that can only singly ionize individual molecules, ammonia clusters generate ions depleted of all valence electrons. The geometries of the molecular orbitals are revealed to be important in driving the ionization, and can be entirely emptied at the energy requirement for removal of the first electron in the orbital. The results are in accord with non-sequential ionization arising from electrons tunneling from three separate molecular orbitals aided through the ionization ignition mechanism.     

Real-time monitoring of monochlorinated benzene and polychlorinated biphenyls in the gas phase with on-line detection: High chemical selectivity of resonance-enhanced two-photon ionization.

The development of molecular spectroscopy has enabled us to select chlorinated aromatic hydrocarbons very rapidly. In particular, the laser ionization TOFMS (time-of-flight mass spectrometry) method is expected to be useful as an on-line, selective, and sensitive method. In the present work, real-time laser ionization TOFMS measurements were carried out on gaseous chlorinated aromatic hydrocarbons. The laser ionization method used resonance-enhanced two-photon ionization with the direct introduction of gas into the vacuum chamber. This method for analyzing aromatic hydrocarbons was developed using a pulsed supersonic molecular beam method. In the context of developing a highly selective and sensitive method, excitation of monochlorinated benzene at lambda = 263.07 nm was found to be effective in the wavelength region from 263 nm to 265 nm. Also the excitation of polychlorinated biphenyls at lambda = 266 nm was found to be substantially more effective than at lambda = 280, 300 or 320 nm. The achievable sensitivity for real-time (1 min) measurements using the laser ionization TOFMS technique was found to be in the ppbV range.     

Theoretical calculation of ionization potentials for disubstituted benzenes: additivity vs non-additivity of substituent effects

The ionization potentials of 55 para- and 55 meta-disubstituted benzenes, consisting of all binary combinations of electron-withdrawing groups (-NO2, -CF3, -CHO, -COOH) and electron-donating groups (-Cl, -CH3, -OH, -OCH3, -NH2, and -N(CH3)2) have been calculated using density functional theory with the B3LYP functional and a 6-31G(d) basis set. Relative ionization potentials (delta IP), referred to benzene, are compared with experimental values and shown to be in good agreement. The disubstituted data were correlated with monosubstituted delta IP data and shown to require quadratic terms in order to achieve a good fit; the validity of this conclusion was possible due to the low scatter in the calculated data. A simple MO analysis gives a semiquantitative interpretation of the observed trends in substitutent effects, including a discussion of combinations of substituents for which nonadditivity should be expected.     

Measurement of laser activated electron tunneling from semiconductor zinc oxide to adsorbed organic molecules by a matrix assisted laser desorption ionization mass spectrometer

Measurement of light induced heterogeneous electron transfer is important for understanding of fundamental processes involved in chemistry, physics and biology, which is still challenging by current techniques. Laser activated electron tunneling (LAET) from semiconductor metal oxides was observed and characterized by a MALDI (matrix assisted laser desorption ionization) mass spectrometer in this work. Nanoparticles of ZnO were placed on a MALDI sample plate. Free fatty acids and derivatives were used as models of organic compounds and directly deposited on the surface of ZnO nanoparticles. Irradiation of UV laser (λ = 355 nm) with energy more than the band gap of ZnO produces ions that can be detected in negative mode. When TiO₂ nanoparticles with similar band gap but much lower electron mobility were used, these ions were not observed unless the voltage on the sample plate was increased. The experimental results indicate that laser induced electron tunneling is dependent on the electron mobility and the strength of the electric field. Capture of low energy electrons by charge-deficient atoms of adsorbed organic molecules causes unpaired electron-directed cleavages of chemical bonds in a nonergodic pathway. In positive detection mode, electron tunneling cannot be observed due to the reverse moving direction of electrons. It should be able to expect that laser desorption ionization mass spectrometry is a new technique capable of probing the dynamics of electron tunneling. LAET offers advantages as a new ionization dissociation method for mass spectrometry.     

Production and fragmentation of multiply charged ions in 'electron-free' matrix-assisted laser desorption/ionization

An unusually large fraction of multiply charged ions is observed in 'electron-free' matrix-assisted laser desorption/ionization (MALDI). Here we investigate how the yield of multiply charged ions depends on experimental parameters in MALDI. It is found to increase if measures are taken to limit the number of electrons in the plume, for example, by using non-metallic MALDI targets or low laser pulse energies. The ionization energy of the matrix is another important parameter that affects the yield of multiply charged ions: matrices with high ionization energies lead to greater intensities of multiply charged ions. It is furthermore proposed that some of the fragment ions observed in MALDI are due to reactions of analyte with electrons in the plume. The possibility of electron capture dissociation of multiply charged ions produced by MALDI is shown.     

Differences between signal currents for both polarities of applied voltages on cavity ionization chambers

A difference between the surface potential of the charge collecting electrode and that of the guard electrode of an ionization chamber changes the charge collecting volume depending on the applied voltage. If the difference is large, the saturation curve of the signal current shows a maximum at a low applied voltage. Even when there is no electrical or mechanical defect, the signal current from a parallel plate ionization chamber irradiated with ⁶⁰Co γ-rays increases or decreases with the applied voltage beyond the recombination region depending on the polarity of the applied voltage. The variation in the signal current is explained as a result of the change in the stopping power of air due to the acceleration or deceleration of secondary electrons. These electrons are emitted from the polarizing electrode towards the collector as a result of Compton scattering. In a range of low applied voltages, the signal current from a cylindrical ionization chamber is expected to be smaller for a negative applied voltage than for a positive applied voltage. This is because epithermal electrons are expected to have a higher probability of being lost by back diffusion than positive ions which are originally produced in a thermal equilibrium condition. An experimental result, however, showed no difference in the polarities of the applied voltage. The result may be explained as a consequence of the fact that epithemal electrons do not drift for long distances and maintain their energies.     

Empirical regularities of atomic ionization potentials

Regularities of the averaged ionization potentials for atoms and ions containing up to 18 electrons are studied in detail. It is shown that a two-variable function constructed from the averaged ionization potentials for each subshell is linear with respect to the degree of ionization q and the occupancy k of the s^k or p^k subshell. One linearity includes previous findings as a special case, and the other introduces a new regularity for atomic ionization potentials. Existing atomic ionization potentials and electron affinities are analyzed employing the regularities, and improved values of these quantities as well as term and fine structure separations in negative ions are derived.     

Competitive consecutive electron transfer in determination of ionization potentials: ketene derivatives

Kinetics of competitive consecutive electron transfer was used to determine ionization potentials of transient species. Kinetics of two-stage electron transfer reactions in aprotic solvent was studied using 355 nm laser flash photolysis. The concentrations of transients produced by the laser flash photolysis were monitored by their light absorption. Triplet-excited tetrachloro-p-benzo-quinone (p-chloranil) generated by a 355 nm laser flash oxidized diethyl ketene, diphenyl ketene, or phenyl ethyl ketene to form radical cations. The ketene radical cations, in turn, oxidized tertiary amine, forming ground state ketene and ammonium radical cation. The kinetics of the disappearance of ketene radical cations (and/or appearance of ammonium radical cations) due to consecutive, competitive electron transfer to ketene and p-chloranil radical cations was monitored. By monitoring kinetics in the presence of tertiary amines with different oxidation potentials, it was established that in acetonitrile the oxidation potential of diethyl ketene was 5.4 eV; for phenyl ethyl ketene, it was approximately 4.8 eV; and for diphenyl ketene, it was 4.6 eV. The results were in agreement with the oxidation potentials of ketenes computed using published data.     

Reduction of Cu(II) in matrix-assisted laser desorption/ionization mass spectrometry

The mechanisms of the reduction of Cu(II) in matrix-assisted laser desorption/ionization mass spectrometry (MALDI) are studied. In MALDI mass spectra, ions cationized by copper mostly contain Cu(I) even if Cu(II) salts are added to the sample. It was found that Cu(II) was reduced to Cu(I) by gas-phase charge exchange with matrix molecules, which is a thermodynamically favorable process. Under some conditions, large amounts of free electrons are present in the plume. Cu(II) can be even more efficiently reduced to Cu(I) by free electron capture in the gas phase. The matrices studied in this work are nicotinic acid, dithranol, and 2,5-dihydroxybenzoic acid.     

Velocity map imaging of dissociative ionization and coulomb explosion of CH3I induced by a femtosecond laser

The dissociative ionization and the Coulomb explosion of CH3I irradiated by a 35 fs 800 nm laser with a laser intensity of 4 x 10(13) to 6 x 10(14) W/cm2 was studied. In a relatively weak laser field (about 10(13) W/cm2), the dissociative ionization of CH3I took place. The speed distributions of the CH3+ and I+ fragments were measured and fitted using multiple Gaussian functions. Different product channels were found for CH3+ and I+, respectively. In a strong laser field (about 10(14) W/cm2), the multiply ionized fragment ions of Iq+ (q 相似文献   

Matrix-free infrared soft laser desorption/ionization

Infrared soft laser desorption/ionization was performed using a 2.94 µm Er : YAG laser and a commercial reflectron time-of-flight mass spectrometer. The instrument was modified so that a 337 nm nitrogen laser could be used concurrently with the IR laser to interrogate samples. Matrix-assisted laser desorption/ionization (MALDI), laser desorption/ionization and desorption/ionization on silicon with UV and IR lasers were compared. Various target materials were tested for IR soft desorption ionization, including stainless steel, aluminum, copper, silicon, porous silicon and polyethylene. Silicon surfaces gave the best performance in terms of signal level and low-mass interference. The internal energy resultant of the desorption/ionization was assessed using the easily fragmented vitamin B₁₂ molecule. IR ionization produced more analyte fragmentation than UV-MALDI analysis. Fragmentation from matrix-free IR desorption from silicon was comparable to that from IR-MALDI. The results are interpreted as soft laser desorption and ionization resulting from the absorption of the IR laser energy by the analyte and associated solvent molecules. Copyright © 2004 John Wiley & Sons, Ltd.     

Long-range energy transfer and ionization in extended quantum systems driven by ultrashort spatially shaped laser pulses

The processes of ionization and energy transfer in a quantum system composed of two distant H atoms with an initial internuclear separation of 100 atomic units (5.29 nm) have been studied by the numerical solution of the time-dependent Schr?dinger equation beyond the Born-Oppenheimer approximation. Thereby it has been assumed that only one of the two H atoms was excited by temporally and spatially shaped laser pulses at various laser carrier frequencies. The quantum dynamics of the extended H-H system, which was taken to be initially either in an unentangled or an entangled ground state, has been explored within a linear three-dimensional model, including the two z coordinates of the electrons and the internuclear distance R. An efficient energy transfer from the laser-excited H atom (atom A) to the other H atom (atom B) and the ionization of the latter have been found. It has been shown that the physical mechanisms of the energy transfer as well as of the ionization of atom B are the Coulomb attraction of the laser driven electron of atom A by the proton of atom B and a short-range Coulomb repulsion of the two electrons when their wave functions strongly overlap in the domain of atom B.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. II. Electron dynamics and outer ionization of the nanoplasma

We explore electron dynamics in molecular (CD4)(1061) clusters and elemental Xen (n=249-2171) clusters, responding to ultraintense (intensity I=10(16)-10(19) W cm(-2)) laser fields. Molecular dynamics simulations (including magnetic field and relativistic effects) and analyses of high-energy electron dynamics and nuclear ion dynamics in a cluster interacting with a Gaussian shaped laser field (frequency 0.35 fs(-1), photon energy 1.44 eV, phase 0, temporal width 25 fs) elucidated the time dependence of inner ionization, the formation of a nanoplasma of unbound electrons within the cluster or its vicinity, and of outer ionization. We determined the cluster size and the laser intensity dependence of these three sequential-parallel electronic processes. The characteristic times for cluster inner ionization (tau(ii)) and for outer ionization (tau(oi)) fall in the femtosecond time domain, i.e., tau(ii)=2-9 fs and tau(oi)=4-15 fs for (CD4)(1061), tau(ii)=7-30 fs and tau(oi)=5-13 fs for Xe(n) (n=479,1061), with both tau(ii) and tau(oi) decreasing with increasing I, in accord with the barrier suppression ionization mechanism for inner ionization of the constituents and the cluster barrier suppression ionization mechanism for outer ionization. The positive delay times Deltatau(OI) between outer and inner ionization (e.g., Deltatau(OI)=6.5 fs for Xen at I=10(16) W cm(-2) and Deltatau(OI)=0.2 fs for (CD4)(1061) at I=10(19) W cm(-2)) demonstrate that the outer/inner ionization processes are sequential. For (CD4)(1061), tau(ii)tau(oi), reflecting on the energetic hierarchy in the ionization of the Xe atoms. Quasiresonance contributions to the outer ionization of the nanoplasma were established, as manifested in the temporal oscillations in the inner/outer ionization levels, and in the center of mass of the nanoplasma electrons. The formation characteristics, dynamics, and response of the nanoplasma in molecular or elemental clusters were addressed. The nanoplasma is positively charged, with a high-average electron density [rho(P)=(2-3)10(22) cm(-3)], being characterized by high-average electron energies epsilon(av) (e.g., in Xe(1061) clusters epsilon(av)=54 eV at I=10(16) W cm(-2) and epsilon(av)=0.56-0.37 keV at I=10(18) W cm(-2), with epsilon(av) proportional, variant I(1/2)). Beyond the cluster boundary the average electron energy markedly increases, reaching electron energies in the range of 1.2-40 keV for outer ionization of Xe(n) (n=249-2171) clusters. The nanoplasma exhibits spatial inhomogeneity and angular anisotropy induced by the laser field. Femtosecond time scales are predicted for the nanoplasma production (rise times 7-3 fs), for the decay (decay times approximately 5 fs), and for the persistence time (30-10 fs) of a transient nanoplasma at I=10(17)-10(18) W cm(-2). At lower intensities of I=10(16) W cm(-2) a persistent nanoplasma with a "long" lifetime of > 50 fs will prevail.