Laser multiphoton ionization of aromatic molecules in nonpolar liquids

The laser multiphoton ionization (MPI) of fluoranthene in tetramethylsilane (TMS) and of azulene in n-tridecane, n-pentane, 2,2,4,4-tetramethylpentane, TMS and tetramethyltin is reported. Three distinct types of MPI mechanisms have been identified: two-photon ionization, stepwise three-photon ionization and mixed two- and three-photon ionization. The stepwise three-photon process consists of two-photon excitation, relaxation to a lower lying excited state with a lifetime comparable to the laser pulse duration (for azulene this state is the S₂ while for fluoranthene both the S₁ and S₂ states) and subsequent ionization with the absorption of a third photon. The ionization threshold of azulene in each liquid has been determined and found to vary linearly with the V₀ of the liquid.     

Two- and three-wavelength laser multiphoton ionization for highly sensitive detection in solution

Novel two- and three-wavelength laser multiphoton ionization techniques for highly sensitive detection in solution have been established. The photocurrent signal obtained for benzo[a]pyrene by irradiation at 355 nm in n-heptane was effectively enhanced by additional simultaneous irradiation at 532 and/or 1064 nm. The additional irradiation at 532 nm (5 mJ) doubled the signal-to-noise ratio, while that at 1064 nm (30 mJ) increased it 5.5-fold relative to that obtained when only the 355 nm radiation was used. The simultaneous action of 355, 532 (5 mJ) and 1064 (25 mJ) nm radiation further improved the S/N ratio; the detection limit was as low as 1.9 x 10(-10)M. The 532 nm radiation enhanced the photocurrent signal more effectively than did the 1064 nm radiation.     

Competition between multiphoton fluorescence and multiphoton ionization in NO

A nitrogen laser pumped tunable dye laser has been used to observe the three-photon ionization of NO through a two-photon resonance with the C²II state. Fluorescence is also observed from this state. The wavelength dependence of both signals have been measured. A reaction mechanism is postulated, which includes the initial two-photon excitation of the C²II state as the rate-limiting step. This mechanism predicts the observed second-order intensity dependence of the ionization signal and shows that the simple rate equation treatment is valid in this system.     

Coulometric determination of neptunium

A coulometric titration method with controlled current and amperometric end-point detection with two indicator electrodes was worked out to determine neptunium in solutions which were obtained while treating the spent nuclear fuel. This method allows to determine in presence of 1000-fold quantities and more of Pu, Ce, Cr, Fe, Mn, 1000-fold quantities and more of U. The error of determination is 0.2% for pure Np solutions and increases up to some precent in presence of large amounts of impurity.     

High-order multiphoton fluorescence of organic molecules in solution by intense femtosecond laser pulses

The five and possibly seven-photon fluorescence was observed for organic molecules in solution for the first time. A high-intensity laser enabled us to measure the properties of the high and any-order processes, and the emission could be directly visualized by the eye. These results showed that the common two-photon microscope could be upgraded to the higher order multiphoton microscope by choosing suitable excitation wavelengths. The multiphoton absorption cross sections differed by a factor of 10(33) as the order of the multiphoton process increased.     

Femtosecond multiphoton ionization of pyrrole

The photoionization dynamics of pyrrole are investigated by using a photoelectron imaging method and a tunable femtosecond laser. Two-photon nonresonant ionization experiments in the wavelength range from 261 to 298 nm indicate that the cation and neutral ground states have similar structures. The main vibrational excitation in the cation ground state is the v(8) mode. Two-photon absorption at 406 nm projects neutral pyrrole into a mixed state comprising the 1B(2) valence and 3p Rydberg states. Ionization from this mixed state mainly results in the overtone excitation of vibrational mode v(8) and v(9) of the cation state. In the wavelength range from 336 to 364 nm, a mixed state comprising the 3d/4s Rydberg and the 4A(1) valence states are populated by the absorption of two photons through vibronic coupling. The partition ratio among these states varies with the excitation wavelength, resulting in dramatic changes in both kinetic energy distributions and angular distributions. As the laser wavelength becomes shorter, from 336 to 314 nm, higher excited states, 3B(2), 5A(1), 6A(1), 7B(1) and 4B(2), can be populated. Photoelectron angular distributions provide supplementary verification of assignments. Our experiments indicate that femtosecond multiphoton ionization and photoelectron imaging methods are powerful tools for investigating short-lived intermediated excited states, which cannot be detected in nanosecond experiments.     

On Metal-Atom Clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules

The investigation of electronic and structural parameters of metal molecules as a function of size may be decisive for understanding and control of heterogeneous catalysis with finely divided metals. Metal-atom clusters can be prepared by several methods, most of which yield a molecular mixture only. Expansion of an atomic vapour into vacuum through a supersonic nozzle creates a complex cluster-spectrum which has been investigated by photoionization using a mass-spectrometer as selective detector. Broad band single photoionization ( PI. ) yields the abundances and ionization thresholds for Na_x(x?16), K_x(x?12) and Na_xK_y(x+y?6). Rough indications about other deactivation channels of excited alkali-molecules are obtained from an analysis of the photoionization efficiency (PIE.) curves. Two-photon PIE. curves with narrow-band (laser) light sources give an accurate photoionization threshold value and detailed information on ionization processes. Two-photon ionization spectra via a real intermediate state reached by laser excitation are equivalent to normal absorption spectra, if the exciteation step is controlled by the true transition probabilities. By investigation of the power dependence of the ion current of Na as a function of the wavelength of the tunable cw dye laser (excitation) and of the ionizing Ar⁺- or Kr⁺-laser, conditions under which true spectra are obtained have been clarified, in good agreement with a photon-kinetic model of the processes involved. Vibronic and rovibronic spectra of several transitions in Na₂ and K₂ have been measured. Similarly a spectrum of Na₃ has been determined mass-selectively. The scope of the new method for an absorption spectroscopy in molecular beams is discussed.     

New reactions for neptunium isolation and determination

The possibility of using photochemical reactions of neptunium oxidation and reduction tor analytical and preparation purposes is discussed. Results of Np isolation from alkaline solutions of heptavalent neptunium by sorption, extraction and coprecipitation are given. The colour reaction of Np(IV) with Arsenazo M is described. On this basis a spectrophotometric method of neptunium determination in the presence of a 20-fold uranium amount is suggested.     

Spectrophotometric determination of traces of water in organic solvents (oxygen-free molecules)

Spectrophotometry and potentiometry have been used to determine the self-ionization product of formic acid and the acidity constants of hydrobromic, hydrochloric, methanesulfonic, p-toluenesulfonic, sulfuric, trifluoroacetic, pieric and benzoic acids in formic acid.     

Influence of the vibrational quantum number of the resonant state in resonant multiphoton ionization/dissociation of hydrogen molecules

We have studied the resonant multiphoton ionization of hydrogen in a three-photon excitation, one-photon ionization scheme. Superimposed on the ionization process we find a dissociation mechanism which manifests itself in a strong H⁺ signal. The ratio of H⁺ to H⁺₂ signals depends on the vibrational quantum number v' of the intermediate state and on the laser intensity. We present a simple model which qualitatively reflects this dependence.     

Model for atomic multiphoton ionization via Rydberg states

A model to treat multiphoton ionization through Rydberg states, taking into account blackbody radiation and other processes which lead to ionization with transfer of excitation among the closely-spaced highly excited Rydberg states is presented. The basic idea is to make the rate equations tractable by compacting all of the Rydberg states that experience physically similar processes into a single “bath” in the equations. The model is applied to sodium Rydberg atoms and the predictions show excellent agreement with experiments.     

Luminescent determination of neptunium and plutonium in the environment

The preparation of^99mTc-complexes with nine derivatives of (4R)-1,3-thiazolidine-4-carboxylic acid over a broad pH range is described. The labeling efficiency of all compounds was checked by paper- and thin layer chromatography. Paper electrophoresis and gel chromatography indicated the presence of two types of^99mTc-complexes differing in charge and molecular size. Preliminary results of the biodistribution of anionic complexes in mice are given. The structure of both types of the Tc-complexes is also discussed.     

Electrolytic determination of traces of iodide in solution with a piezoelectric quartz crystal

The frequency of a piezoelectric quartz crystal is decreased when iodide is electrodeposited on the silver electrode of the crystal at—0.05 V vs. Ag/AgCl. From 3 × lO^-7 M to 1 × 10^-5 M iodide can be determined with few interferences, and a procedure for removal of interfering species is given. Iodide is removed from the electrode by electrolysis at —0.4 V after each determination.     

Trace determination of some aromatic molecules by laser two-photon ionization

A simplified yet sensitiive system is described for the detection of the two-photon ionization signal in solution. The photo-ionization cell consists of a quartz cuvette and a pair of stainless steel electrodes. Several aromatic compounds, including quinones, can be detected in the ng l^?1–mg l^?1 range both in hexane and in methanol; detection limits of pyrene in hexane and in methanol are 0.02 μg l^?1 and 4 μg l^?1, respectively. The detectability in hexane is much better than that in methanol. The detection limits and the molar absorptivities at the excitation wavelength are shown to be correlated.     

Evolution of a system undergoing multiphoton ionization

We describe an efficient method for solving the time-dependent Schroedinger equation for an atomic system subject to an oscillating radiation field. The method is based on solving the Schroedinger equation in momentum space, which obviates the need to impose artificial boundaries, and facilitates the extraction of the rapidly varying part of the wavefunction. We present results of a test application to a one-dimensional system with atomic potential ?exp(?|x|/a ₀).     

Infrared multiphoton excitation of small molecules

The collisionless infrared excitation by short CO₂ laser pulses of the molecules SO₂, OCS, NO₂, NH₃ and DN₃ is compared with that of larger molecules. The average number of photons absorbed per molecule and the fraction of molecules dissociated depends predominantly on the laser intensity, while for larger molecules with higher densities of vibrational states the excitation is primarily determined by the laser fluence.     

Laser-excited molecular flourescence spectrometry for the determination of traces of nonmetals : Part 1. determination of traces of fluoride,chloride and bromide based on diatomic molecules in a graphite furnace

For the determination of flouride, chloride and bromide, a suitable metal ion is added in a modified graphite tube atomizer. After drying and ashing, the substances are vaporized and diatomic molecules between the metal and nonmetal are formed. These molecules are excited by a pulsed dye laser and the resulting fluorescence is measured. At constant metal concentration, the fluorescence is proportional to the halide concentration. The diatomic molecules used are MgF, InCl and AlBr. The detection limits are 11 pg of fluoride, 15 pg of chloride and 70 pg of bromide in the 10-μl injections, corresponding to 6 x 10^?8 mol l^?1 fluoride, 4 x 10^?8 mol l^?1 chloride and 9 x 10^?8 mol l^?1 bromides.     

Investigation of charge localization and charge delocalization in model molecules by multiphoton ionization photoelectron spectroscopy and DFT calculations

In this work we focus on the question to which degree a surplus charge is localized or delocalized in extended molecular systems. Molecules consisting of a flexible tail and the benzene chromophore, such as n-propylbenzene, 2-phenylethyl alcohol and 2-phenylethylamine, are used as model molecules. Their S0-S1 resonance enhanced multiphoton ionization (MPI) spectra containing origin transitions of different conformers appear at similar wavelengths. This shows, that in the neutral the electronic excitation is localized at the benzene chromophore. Geometry differences between the neutral and the cation can be qualitatively derived from intensities of vibrational transitions or the onset behavior in MPI high-resolution photoelectron (MPI-PE) spectra. We identify two possible reasons for structural changes: Charge-dipole interaction and charge delocalization. Whereas both effects can be active for the folded gauche conformers, the charge-dipole interaction is expected to be small for the extended anti conformers and geometry changes are attributed to charge delocalization. Density functional calculations of structures and energies qualitatively confirm the experimental results for all molecules and their conformers. They predict charge delocalization into the end group of below 20% for n-propylbenzene and 2-phenylethyl alcohol. In the case of 2-phenylethylamine the charge is equally shared by the near-isoenergetic charge sites of the benzene chromophore and the amine group.     

吡啶气相分子的共振多光子电离谱

本工作观测了双光子能量在35180-36002cm^-^1范围内吡啶分子的共振多光子电离谱。对电子跃迁S~1(^1B~1)→S~0(^1A~l)的振动带做了归属, 首次在^1B~1态观察到7个新的振动模。讨论了N原子取代对分子结构及振动模的影响。