Sub-picosecond laser ionization of free C60 and C70 at 248 nm

Delayed ionization is found to be absent for sub-picosecond laser excitation of free C₆₀ and C₇₀ at 248 nm. The autocorrelation trace obtained for C ₆₀ ⁺ in a laser time-of-flight (TOF) mass spectrometer using two time-delayed and collinear 248 nm ultrashort laser pulses has a width of 1.1 ps (715 fs for sech² pulses), in agreement with the laser pulse duration measurement in NO gas. Both above observations can be explained by direct ionization of C₆₀ via coherent two-photon absorption by the high intensity sub-picosecond 248 nm laser excitation avoiding the channel leading to delayed ionization.     

Multiphoton ionization versus dissociation of diatomic molecules irradiated by an intense 40 ps laser pulse

The production of molecular and atomic ions has been measured for CO, N₂ and O₂ with 1064 and 532 nm 40 ps pulses in the 10¹²-10¹⁴ W cm^?2 intensity range. A simultaneous ionization-dissociation process occurs at lower intensities, while a sequential process appears in oxygen at higher intensities.     

Above-threshold ionisation of He at 248 nm: the role of the ionic contribution

Photoelectron spectra of rare gas atoms interacting with 0.5 ps KrF laser pulses at intensities up to 10¹⁶ W/cm² are reported. For intensities higher than the saturation intensity of the atom the envelope of the logarithm of the spectrum exhibits two different slopes with strong evidence that these originate from the superposition of the atomic and ionic above-threshold ionisation (ATI) spectra. At high intensities the ionic part of the spectrum becomes so flat that it appears plateau-like.     

Saturation and fragmentation in non resonant laser postionization of sputtered atoms and molecules

Summary Using non resonant two photon ionization at =248 nm, the emission of neutral atoms and dimers was investigated during ion beam sputtering of polycrystalline silver and copper with 5 keV Ar⁺. Saturation effects were examined by measuring the ion intensities as a function of the photon flux density. It is found that with the maximum available laser power density (1·10⁹ W/cm²) the ionization of Ag atoms can be easily saturated, whereas no complete saturation was achieved for Cu atoms. At the same time the saturation curve measured for Ag differs significantly both from that measured for Cu and from the theoretically expected behaviour for non resonant two photon ionization. For Ag₂ and Cu₂ the ionization is shown to be strongly resonance enhanced already being saturated in the region of 10⁷ W/cm². From the data taken at low laser power densities (<10⁶ W/cm²), the role of fragmentation processes either by dissociative ionization or by neutral dissociation is discussed. As an example, the results using laser postionization are employed to quantitatively determine the abundance of Ag₂ molecules in the flux of neutral particles sputtered from the silver sample.     

A model of energy transfer to noble gas atoms due to forced resonance within a continuum of plasmon excitation by intense laser pulses

We have developed and applied a model of energy transfer to noble gas atoms due to a resonance mechanism within a continuum of plasmon oscillation induced by intense laser pulses. The model is based on a generalization to 3D of the 1D many-body RPA method of Tomonaga. Total cross sections for laser energy absorption, the saturation intensities for ionization, mean energy transfer and degree of ionization of several noble gases, Xe, Kr, Ar, Ne are obtained for λ = 193 nm and λ = 1064 nm for a short Gaussian laser pulse. Probability distribution of absorption of a given number of photons in Xe is also obtained for λ = 193 nm andI ₀ = 10¹⁴ W/cm². The results are consistent with the rapid energy transfer necessary for multiple ionization at these frequencies.     

Ionization Suppression of Heteronuclear Diatomic and Triatomic Molecules in Strong Infrared Laser Fields

Ionization is the fundamental process in interaction of atoms/molecules with femtosecond strong laser fields. Comparing to atoms, molecules exhibit peculiar behaviors in strong-field ionization because of their diverse geometric structures, molecular electronic orbitals as well as extra nuclear degrees of freedom. In this study, we investigate strong field single and double ionization of carbon monoxide (CO) and carbon dioxide (CO₂) in linearly polarized 50-fs, 800-nm laser fields with peak intensity in the range of 2×10¹³ W/cm² to 2×10¹⁴ W/cm² using time-of-flight mass spectrometer. By comparing the ionization yields with that of the companion atom krypton (Kr), which has similar ionization potential to the molecules, we investigate the effect of molecular electronic orbitals on the strong-field ionization. The results show that comparing to Kr, no significant suppression is observed in single ionization of both molecules and in non-sequential double ionization (NSDI) of CO, while the NSDI probability of CO₂ is strongly suppressed. Based on our results and previous studies on homonuclear diatomic molecules (N₂ and O₂), the mechanism of different suppression effect is discussed. It is indicated that the different structure of the highest occupied molecular orbitals of CO and CO₂ leads to distinct behaviors in two-center interference by the electronic wave-packet and angular distributions of the ionized electrons, resulting in different suppression effect in strong-field ionization.     

Nonadiabatic response of molecules to strong fields of picosecond, femtosecond, and subfemtosecond duration: an experimental study of the methane dication

The double ionization of methane has been accomplished using strong optical fields that are generated using moderately intense lasers, and by strong fields that are induced by fast-moving, highly charged ions. In the former case laser intensities in the range 10(14) W cm(-2) generate fields whose durations are of 35 ps and 36 fs while in the latter case equivalent fields last for only 200-300 as. The dynamics of the field-ionized electrons are different in the two temporal regimes, fast (picoseconds), and ultrafast (few tens of femtoseconds and subfemtoseconds). Our experiments show that nonadiabatic effects come into play in the ultrafast regime; we directly monitor such effects by measuring the kinetic energy that is released when a specific bond in the doubly charged methane molecular ion breaks.     

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.     

Ionization of lithium vapor byCW quasiresonant laser light

We report measurements of the excitation and ionization of dense lithium vapor irradiated byCW dye laser light scanning the 2² P?3² D lithium atomic transition at 610.3 nm. Lithium vapor with a density of 8×10¹⁶ cm^?3 was ionized by a focused beam with as little as 1 mW of single-frequency laser power. The ionization mechanism has been studied and found to consist of a three stage process in which both atomic and molecular absorption of the laser power, two distinct collisional processes, and single-photon ionization of excited lithium atoms all play essential roles.     

FLUORESCENCE RELAXATION KINETICS AND QUANTUM YIELD FROM THE ISOLATED PHYCOBILIPROTEINS OF THE BLUE-GREEN ALGA NOSTOC SP. MEASURED AS A FUNCTION OF SINGLE PICOSECOND PULSE INTENSITY, I

Abstract— Phycobilisomes from the blue-green alga Nostoc sp. are known to contain the phycobiliproteins: c-phycoerythrin (c-PE), c-phycocyanin (c-PC) and four forms of allophycocyanin (APC I, II, III, and B). We have made a detailed study of the effects of the intensity of a single 6 ps excitation pulse on the decay kinetics and the yield of fluorescence in the individual isolated phycobiliproteins at pH 7 and 23°C. The risetime of the fluorescence of c-PE, c-PC and APC was > 12 ps. We found that the decay of the fluorescence was exponential at intensities of 10¹⁴ photons/cm² in all the phycobiliproteins; the lifetimes being 1552 ± 31ps for c-PE, 2111 ± 83ps for c-PC, 1932 ± 165ps for APC I, 1870 ± 90ps for APC II, 1816 ± 88ps for APC III, (1869 ± 62ps for the averaged APC's I, II, and III), and 2667 ± 233 ps for APC B. We also found that the fluorescence decay became non-exponential in c-PE at excitation intensities < 10¹⁴ photons/cm², but was exponential for all the other phycobiliproteins even at a pulse intensity of 10¹⁵ photons/cm². The relaxation times of c-PE and c-PC decreased with excitation intensity above 10¹⁴ photons/cm². For c-PE and c-PC the relative fluorescence vs excitation intensity was readily described by a relationship derived for a model in which exciton–exciton annihilation occurs. In APC the fluorescence yield and relaxation time were only slightly dependent on the excitation intensity. The results are interpreted to indicate the occurrence of singlet–singlet annihilation intramolecularly among the several phycobilin chromophores within the individual phycobiliprotein molecules in solution. The s to f transfer time is less than 12ps in c-PC.     

Mass-spectrometry of frozen matrix-assisted water in environmental analysis

Summary CO₂ laser evaporation/KrF-laser ionization combined with reflectron time-of-flight mass-spectrometry have been used for the analysis of drinking water polluted with phenol. A minimum phenol concentration level of 10^–8% was detected. Considerable temperature reduction (up to 50 K) due to laser evaporated molecules from the frozen water surface has been observed.     

Principle and analytical applications of resonance lonization mass spectrometry

Resonance ionization mass spectrometry (RIMS) is a very sensitive analytical technique for the detection of trace elements. This method is based on the excitation and ionization of atoms with resonant laser light followed by mass analysis. It allows element and, in some cases, isotope selective ionization and is applicable to most of the elements of the periodic table. A high selectivity can be achieved by applying three step photoionization of the elements under investigation and an additional mass separation for an unambiguous isotope assignment.An effective facility for resonance ionization mass spectrometry consists of three dye lasers which are pumped by two copper vapor lasers and of a linear time-of-flight spectrometer with a resolution better than 2500. Each copper vapor laser has a pulse repetition rate of 6.5 kHz and an average output power of 30 W.With such an apparatus measurements with lanthanide-, actinide-, and technetium-samples have been performed. By saturating the excitation steps and by using autoionizing states for the ionization step a detection efficiency of 4 × 10^–6 and 2.5 × 10^–6 has been reached for plutonium and technetium, respectively, leading to a detection limit of less than 10⁷ atoms in the sample. Measurements of isotope ratios of plutonium samples were in good agreement with mass-spectrometric data. The high elemental selectivity of the resonance ionization spectrometry could be demonstrated.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Pathway competition of H+ in intense femtosecond laser fields

We experimentally measured the kinetic energy and angular distributions of fragment ion H⁺ of H₂ as a function of 810 nm femtosecond laser intensity by using velocity map imaging technique. The reasonable origination of dissociation channels (1.0) and (1.1) are proposed. The analysis of the angular distribution indicates the net two-photon pathway via the 3ω crossing dominates over the direct one-photon pathway in channel (1.0). The relative yield of fragment peaks indicates that dissociation and ionization of H ₂ ⁺ are competitive. The lower laser intensities emphasize the dissociation probability of H ₂ ⁺ , and the higher laser intensities favor higher ionization stages.     

Multiple ionization and Coulomb explosion of mercury clusters in femtosecond laser fields

We report on studies of multiple ionization and fragmentation of free Hg_n (n ≤ 80) clusters in the femtosecond time domain at wavelengths ranging from 255 nm to 800 nm. After excitation by single laser pulses of an intensity of 5 * 10¹¹ W/cm² we observe prompt formation of multiply charged Hg_n clusters. The Hg_n cluster size distribution observed up to n ≈ 80 shows in additon to singly charged also doubly and triply charged clusters with a surprisingly high amount of doubly charged clusters. The measured cluster size distribution is nearly independent of laser wavelengths. For higher laser intensities (2 * 10¹² W/cm²) we observe multiply charged mercury atoms up to Hg⁵⁺. At 10¹³ W/cm² molecules and clusters eventually disappear due to Coulomb explosion and complete Fragmentation. Only atomic ions, singly and multiply charged, with high kinetic energies are then observed.     

Parametric evaluation of laser ablation and ionization time-of-flight mass spectrometry with ion guide cooling cell

A novel laser ablation and ionization time-of-flight mass spectrometer has been used for direct elemental analysis of alloys. The system was incorporated with an ion guide cooling cell to reduce the kinetic energy distribution for the purpose of better resolution. Parametric studies have been conducted on the system with respect to the buffer gas pressure and the distance from sample to the nozzle to obtain the maximal signal intensities. In order to obtain satisfactory relative sensitivity coefficients (RSC) for different elements, the influence of the laser irradiance, nozzle voltage, rf frequency and voltage of the hexapole were also investigated. Under the optimized conditions, the RSC of different elements were available for direct semi-quantitative analysis. The mass resolving power (FWHM) of the spectrometer was approximately 7000 (m/Δm) and the limit of detection (LOD) was 10^− 6 g/g.     

Dissociative ionization of methane by chirped pulses of intense laser light

Measurements have been made of optical field-induced ionization and fragmentation of methane molecules at laser intensities in the 10(16) W cm(-2) range using near transform limited pulses of 100 fs duration as well as with chirped pulses whose temporal profiles extend up to 1500 fs. Data is taken both in constant-intensity and constant-energy modes. The temporal profile of the chirped laser pulse is found to affect the morphology of the fragmentation pattern that is measured. Besides, the sign of the chirp also affects the yield of fragments like C2+, H+, and H2+ that originate from methane dications that are formed by optical field-induced double ionization.     

Photofragmentation of C60

Photo-ionisation and -fragmentation ofC ₆₀ by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection ofC _n,n>2, as opposed to successiveC ₂ evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy ofC ₆₀ ⁺ prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.     

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.     

Formation of free radicals in water under high-power laser uv irradiation

It has been shown that under high-power laser UV irradiation (I = 10⁸–10¹⁰ W/cm², λ = 266 nm, τ_p = 30 ps) water becomes ionized by a two-photon mechanism to form some free radicals including the hydrated electron e^?_aq.     

Absolute infrared band intensities and air broadening coefficient for spectroscopic measurements of formic acid in air

The absolute infrared intensities of the ν₂, ν₃ and ν₆ bands of formic acid have been evaluated in a 480 L White cell system using FTIR and ion chromatography techniques. The values obtained are, respectively; (4.2 ± 0.2) × 10⁻¹⁷ cm molec⁻¹ for the ν₆ band, (4.8 ± 0.2) × 10⁻¹⁷ cm molec⁻¹ for the ν₃ band and (0.57 ± 0.04) × 10⁻¹⁷ cm molec⁻¹ for the ν₂ band. The air broadening coefficient of transitions in the ν₆ band, has been measured using a tunable diode laser spectrometer, equal to (0.101 ± 0.005) cm⁻¹ atm⁻¹ (half width at half maximum). A computer search has been performed to find absorption lines of formic acid suitable for second derivative tunable diode laser measurement of this gas in ambient air.