Showing papers on "Photoionization published in 2005"

Empirical formula for static field ionization rates of atoms and molecules by lasers in the barrier-suppression regime

Abstract: We propose an empirical formula for the static field ionization rates of atoms and molecules by extending the well-known analytical tunnelling ionization rates to the barrier-suppression regime. The validity of this formula is checked against ionization rates calculated from solving the Schrodinger equation for a number of atoms and ions. The empirical formula retains the simplicity of the original tunnelling ionization rate expression but can be used to calculate the ionization rates of atoms and molecules by lasers at high intensities.

Intense-field many-body S-matrix theory

Abstract: Intense-field many-body S-matrix theory (IMST) provides a systematic ab initio approach to investigate the dynamics of atoms and molecules interacting with intense laser radiation. We review the derivation of IMST as well as its diagrammatic representation and point out its advantage over the conventional 'prior' and 'post' expansions which are shown to be special cases of IMST. The practicality and usefulness of the theory is illustrated by its application to a number of current problems of atomic and molecular ionization in intense fields. We also present a consistent S-matrix formulation of the quantum amplitude for high harmonic generation (HHG) and point out some of the most general properties of HHG radiation emitted by a single atom as well as its relation to coherent emission from many atoms. Experimental results for single and double (multiple) ionization of atoms and the observed distributions of coincidence measurements are analysed and the dominant mechanisms behind them are discussed. Ionization of more complex systems such as diatomic and polyatomic molecules in intense laser fields is analysed as well using IMST and the results are discussed with special attention to the role of molecular orbital symmetry and molecular orientation in space. The review ends with a summary and a brief outlook.

Photoionization mass spectrometer for studies of flame chemistry with a synchrotron light source

Abstract: A flame-sampling molecular-beam photoionization mass spectrometer, recently designed and constructed for use with a synchrotron-radiation light source, provides significant improvements over previous molecular-beam mass spectrometers that have employed either electron-impact ionization or vacuum ultraviolet laser photoionization. These include superior signal-to-noise ratio, soft ionization, and photon energies easily and precisely tunable [E∕ΔE(FWHM)≈250–400] over the 7.8–17-eV range required for quantitative measurements of the concentrations and isomeric compositions of flame species. Mass resolution of the time-of-flight mass spectrometer is m∕Δm=400 and sensitivity reaches ppm levels. The design of the instrument and its advantages for studies of flame chemistry are discussed.

Production of Doubly Charged Helium Ions by Two-Photon Absorption of an Intense Sub-10-fs Soft X-Ray Pulse at 42 eV Photon Energy

Abstract: We report on the observation of doubly charged helium ions produced by a nonlinear interaction between a helium atom and photons with a photon energy of 42 eV which are generated with the 27th harmonic of a femtosecond pulse from a Ti:sapphire laser. The number of ions is proportional to the square of the intensity of the 27th harmonic pulse, and thus two-photon double ionization should be dominantly induced as compared with other nonlinear processes accompanying sequential ionization via a singly charged ion. This phenomenon is utilized to measure the pulse duration of the 27th harmonic pulse by using an autocorrelation technique, for the first time to our knowledge, and as a result a duration of 8 fs is found.

Studies of a fuel-rich propane flame with photoionization mass spectrometry

Abstract: Flame-sampling photoionization mass spectrometry, using continuously tunable synchrotron radiation, offers important advantages for studies of flame chemistry. Mole fraction profile measurements for 24 flame species are presented for a fuel-rich low-pressure premixed laminar C 3 H 8 /O 2 /Ar flame. Near-threshold photoionization efficiency measurements provide selective detection of individual isomers and unambiguous identifications of other flame species of near-equal mass. The absolute molar composition of the allene and propyne isomers of C 3 H 4 was determined. Absolute cross-sections for photoionization of C 2 H 2 , C 2 H 4 , CH 3 OH, C 3 H 4 (propyne), CH 3 CHO, (CH 3 ) 2 CO, and C 6 H 6 were used to determine molecular beam mass discrimination factors valid for ion masses ranging from 26 to 78 amu.

Role of electronegative gas admixtures in streamer start, propagation and branching phenomena

Abstract: A critical analysis of photoionization as the major process for seed electron production ahead of the cathode-directed streamer has been made. The accumulation of ions between pulses and fast electron detachment in an electric field could be an effective source of seed electrons for repetitively pulsed discharges in electronegative gases. Measurements and two-dimensional calculations in the hydrodynamic approximation of streamer parameters in air (anode current, propagation velocity and radiative channel diameter) can only agree if one assumes uniform background pre-ionization and not by taking into account only the photoionization process generally adopted for pulsed discharges in air. The results of analytical estimates and full three-dimensional simulations confirm that the seed charge distribution in the undisturbed gap has an effect on the streamer head formation and leads to the streamer branching phenomena.

Nonsequential Double Ionization as a Completely Classical Photoelectric Effect

Abstract: We introduce a unified and simplified theory of atomic double ionization. Our results show that at high laser intensities ($I\ensuremath{\ge}{10}^{14}\text{ }\mathrm{W}/{\mathrm{c}\mathrm{m}}^{2}$) purely classical correlation is strong enough to account for all of the main features observed in experiments to date.

Laser ablation of Cu and plume expansion into 1 atm ambient gas

Abstract: A one-dimensional gas-dynamic model is presented for the laser ablation of Cu and the expansion of the Cu vapor in a background gas (He) at 1atm. The ionization of Cu and He, the inverse bremsstrahlung absorption processes and photoionization process, and the back flux onto the target are considered simultaneously. The binary diffusion, the viscosity, and the thermal conduction including the electron thermal conduction are considered as well. Numerical results show that the consideration of ionization and laser absorption in the plume greatly influences the gas dynamics. The ionization of Cu enables the recondensation at the target surface to happen even during the laser pulse. The ionization degree of Cu and He may change greatly with the location in the plume. For laser irradiances ranging from 2to9×1012W∕m2, the simulations show that the second-order ionization of Cu competes with the first-order ionization. In the region close to the target surface, the first-order ionization of Cu dominates. In the c...

Photoion mass spectrometry of adenine, thymine and uracil in the 6–22 eV photon energy range

Abstract: Using synchrotron radiation as excitation source in the 6–22 eV photon energy region, a photoionization mass spectrometry study of three nucleic acid bases, adenine, thymine and uracil, revealed VUV-induced degradation pathways of these important biological molecules. The fragmentation patterns, ionization energies and ion appearance energies (AE) are reported, many for the first time, and are compared with results of electron impact and other studies. AE values enabled heats of formation of parent and some fragment ions to be revised or determined for the first time. Thermochemical data, coupled with the observed AEs, were also useful in clarifying dissociative photoionization pathways. The main neutral loss species are HCN for adenine, HNCO and CO for thymine and uracil, but many subsequent and other fragmentation pathways, including some not suggested previously, are observed and discussed. The hyperconjugation properties of the methyl group make CO loss easier in thymine than in uracil. The astrophysically important fragment ion HCNH + is shown to be formed by several fragmentation pathways in all three nucleobases. The relative importance of competitive fragmentation processes was determined in some cases. Some astrophysical implications concerning the prospects for observation and survival of these nucleic acid bases in the interstellar medium and in meteorites are briefly discussed.

A comparative analysis of empirical calibrators for nebular metallicity

Abstract: We present a new analysis of the main empirical calibrators of oxygen abundance for ionized gas nebulae. With that aim we have compiled an extensive sample of objects with emissionline data including the near-infrared [S III] lines and the weak auroral lines which allow for the determination of the gas electron temperature. For all the objects the oxygen abundances have been derived in a homogenous way, using the most recent sets of atomic coefficients and taking into the account the effect of particle density on the temperature of O + . The residuals between directly and empirically derived abundances as a function of abundance have been studied. A grid of photoionization models, covering the range of physical properties of the gas, has been used to explain the origin of the uncertainties affecting each abundance calibrator. The range of validity for each abundance parameter has been identified and its average uncertainty has been quantified. Ke yw ords: ISM: abundances ‐ H II regions.

Ultracold electron source

Abstract: We propose a technique for producing electron bunches that has the potential for advancing the state-of-the-art in brightness of pulsed electron sources by orders of magnitude. In addition, this method leads to femtosecond bunch lengths without the use of ultrafast lasers or magnetic compression. The electron source we propose is an ultracold plasma with electron temperatures down to 10 K, which can be fashioned from a cloud of laser-cooled atoms by photoionization just above threshold. Here we present results of simulations in a realistic setting, showing that an ultracold plasma has an enormous potential as a bright electron source.

Measurement of the phase of few-cycle laser pulses

Abstract: Attosecond science can take enormous advantage of intense laser pulses consisting of two optical cycles or less (few-cycle pulses). The temporal variation of the field, which directly governs strong-field interactions, therefore depends on whether the maximum of the pulse amplitude coincides with that of the wave cycle or not, i.e. on the phase of the field with respect to the pulse envelope. It is demonstrated that the direction of electron emission from photoionized atoms can be controlled by varying the phase of the field, providing a tool for its accurate determination. Different phase-dependent effects in strong-field photoionization and their suitability for phase measurement are discussed as well as the limitations of the f-to-2f scheme.

Femtosecond laser absorption in fused silica: Numerical and experimental investigation

Abstract: Single pulse transmissivity and reflectivity of fused silica irradiated by tightly focused 90 fs laser pulses at a center wavelength of 800 nm are numerically and experimentally investigated to study the role of nonlinear photoionization and avalanche ionization processes in free electron generation. The laser beam inside fused silica is modeled with a $(2+1)$-dimensional propagation equation which considers the effects of laser beam diffraction, group velocity dispersion, self-focusing, defocusing, and absorption due to the free electrons and nonlinear photoionization of the valence electrons. Comparison of our simulation to the experimental data reveals that the avalanche ionization coefficients are much smaller than some previously reported results and that avalanche ionization is of minor importance in generating free electrons in fused silica at the laser fluence levels considered in this study.

Laser-induced fluorescence measurements of absolute atomic densities: concepts and limitations

Abstract: The potential of laser-induced fluorescence spectroscopy of atoms is reviewed with emphasis on the determination of absolute densities. Examples of experiments with single-photon and two-photon excitation are presented. Calibration methods applicable with the different schemes are discussed. A new method is presented that has the potential to allow absolute measurement in plasmas of elevated pressure where collisional depletion of the excited state is present.

The Ionized Gas and Nuclear Environment in NGC 3783

Abstract: We present a detailed analysis of the 900 ks spectrum of NGC3783 obtained by Chandra in 2000-2001 (Kaspi et al. 2002). We split the data in various ways to look for time dependent and luminosity dependent spectral variations. This analysis, the measured equivalent widths of a large number of X-ray lines, and our photoionization calculations, lead us to the following conclusions: 1) NGC 3783 fluctuated in luminosity, by a factor N 1.5, during individual 170 ks observations. The fluctuations were not associated with significant spectral variations. 2) On a longer time scale, of 20-120 days, we discovered two very different spectral shapes that are noted the high state and the low state spectra. The observed changes between the two can be described as the appearance and disappearance of a soft continuum component. The spectral variations are not related, in a simple way, to the brightening or the fading of the short wavelength continuum, as observed in other objects. NGC3783 seems to be the first AGN to show this unusual behavior. 3) The appearance of the soft continuum component is consistent with beeing the only spectral variation and there is no need to invoke changes in the absorber s opacity. In particular, all absorption lines with reliable measurements show the same equivalent width, within the observational uncertainties, during high and low states. 4) Photoionization model calculations show that a combination of three ionization components, each split into two kinematic components, explain very well the intensity of almost all absorption lines and the bound-free absorption. The components span a large range of ionization and a total column of about 3 x 10(exp 22) per square centimeter Moreover, all components are thermally stable and are situated on the vertical branch of the stability curve.. This means that they are in pressure equilibrium and perhaps occupy the same volume of space. This is the first detection of such a multi-component equilibrium gas in AGN. 5) The only real discrepancy between the model and the observations is the wavelength of the iron M-shell UTA feature. This is most likely due to an underestimation of the dielectronic recombination O VI and discuss its possible origin. 6) The lower limit on the distance of the absorbing gas in NGC3783 is between 0.2 and 3.2 pc, depending of the specific ionization component. The constant pressure assumption imposes an upper limit of about 25 pc on the distance of the least ionized gas from the central sourec.

Synchrotron photoionization measurements of combustion intermediates: Photoionization efficiency and identification of C3H2 isomers

Abstract: Photoionization mass spectrometry using tunable vacuum-ultraviolet synchrotron radiation is applied to the study of C3H2 sampled from a rich cyclopentene flame. The photoionization efficiency has been measured between 8.5 eV and 11.0 eV. Franck–Condon factors for photoionization are calculated from B3LYP/6-311++G(d,p) characterizations of the neutral and cation of the two lowest-energy C3H2 isomers, triplet propargylene (HCCCH, prop-2-ynylidene) and singlet cyclopropenylidene (cyclo-HCCCH). Comparison of the calculated Franck–Condon envelopes with the experimental photoionization efficiency spectrum determines the adiabatic ionization energy of triplet propargylene to be (8.96 ± 0.04) eV. Ionization energies for cyclopropenylidene, propargylene and propadienylidene (H2CCC) calculated using QCISD(T) with triple-ζ and quadruple-ζ basis sets extrapolated to the infinite basis set limit are in excellent agreement with the present determination of the ionization energy for propargylene and with literature values for cyclopropenylidene and propadienylidene. The results suggest the presence of both propargylene and cyclopropenylidene in the cyclopentene flame and allow reanalysis of electron ionization measurements of C3H2 in other flames. Possible chemical pathways for C3H2 formation in these flames are briefly discussed.

Simultaneous Ultraviolet and X-Ray Observations of Seyfert Galaxy NGC 4151. I. Physical Conditions in the X-Ray Absorbers

Abstract: We present a detailed analysis of the intrinsic X-ray absorption in the Seyfert 1 galaxy NGC 4151 using Chandra High Energy Transmission Grating Spectrometer data obtained in 2002 May as part of a program that included simultaneous ultraviolet (UV) spectra using the Hubble Space Telescope Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectrographic Explorer. Previous studies, most recently using Advanced Satellite for Cosmology and Astrophysics (ASCA) spectra, revealed a large (>1022 cm-2) column of intervening gas, which has varied both in ionization state and total column density. NGC 4151 was in a relatively low flux state during the observations reported here (~25% of its historic maximum), although roughly 2.5 times as bright in the 2-10 keV band as during a Chandra observation in 2000. At both epochs, the soft X-ray band was dominated by emission lines, which show no discernible variation in flux between the two observations. The 2002 Chandra data show the presence of a very highly ionized absorber, in the form of H-like and He-like Mg, Si, and S lines, as well as lower ionization gas via the presence of inner-shell absorption lines from lower ionization species of these elements. The latter accounts for both the bulk of the soft X-ray absorption and the high covering factor UV absorption lines of O VI, C IV, and N V with outflow velocities ≈500 km s-1. The presence of high-ionization gas, which is not easily detected at low resolution (e.g., with ASCA), appears common among Seyfert galaxies. Since this gas is too highly ionized to be radiatively accelerated in sources such as NGC 4151, which is radiating at a small fraction of its Eddington Luminosity, it may be key to understanding the dynamics of mass outflow. We find that the deeper broadband absorption detected in the 2000 Chandra data is the result of both (1) lower ionization of the intervening gas due to the lower ionizing flux and (2) a factor of ~3 higher column density of the lower ionization component. To account for this bulk motion, we estimate that this component must have a velocity 1250 km s-1 transverse to our line of sight. This is consistent with the rotational velocity of gas arising from the putative accretion disk. While both thermal wind and magnetohydrodynamic models predict large nonradial motions, we suggest that the latter mechanism is more consistent with the results of the photoionization models of the absorbers

Photoexcitation of a volume plasmon in C60 ions.

Abstract: Neutral C60 is well known to exhibit a giant resonance in its photon absorption spectrum near 20 eV. This is associated with a surface plasmon, where delocalized electrons oscillate as a whole relative to the ionic cage. Absolute photoionization cross-section measurements for C+60, C2+60, and C3+60 ions in the 17-75 eV energy range show an additional resonance near 40 eV. Time-dependent density functional calculations confirm the collective nature of this feature, which is characterized as a dipole-excited volume plasmon made possible by the special fullerene geometry.

Multiphoton ionization of He by using intense high-order harmonics in the soft-x-ray region

Abstract: We report on the multiphoton ionization processes in the soft-x-ray region $(\ensuremath{\lambda}\ensuremath{\leqslant}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. On the basis of the measured time-of-flight spectra for both ions and electrons obtained using intense soft-x-ray pulses produced by high-order harmonics, the cross sections of the two-photon double ionization and above-threshold ionization of $\mathrm{He}$ are estimated. The high-intensity soft-x-ray radiation achieved by phase-matched high-order harmonics enables the investigation of these nonlinear optical processes, which were beyond the reach of conventional light sources.

Controlling attosecond double ionization dynamics via molecular alignment.

Abstract: We investigate the dynamics of double ionization in aligned nitrogen molecules. An ultrashort, weak laser pulse creates an aligned ensemble of molecules that is ionized with a subsequent, strong probe pulse. We find that the two electrons involved in nonsequential double ionization more likely exit the molecule in the same direction if it is parallel to the probe laser polarization, indicating that they are ejected within a few hundred attoseconds of each other. Double ionization is less probable and takes longer for perpendicular molecules.

Violation of the Franck-Condon principle due to recoil effects in high energy molecular core-level photoionization.

Abstract: Carbon 1s photoelectron spectra of methane are measured over a photon energy range between 480 eV and 1200 eV. Additional components appear between the individual symmetric stretching vibrational components and are attributed to the excitations of asymmetric stretching and bending vibrations due to recoil of the high-energy photoelectron emission. This recoil effect is the evidence for the violation of the Franck-Condon principle which states that neither the positions nor the momenta of the nuclei change during the ionization event.

The Ionized Gas and Nuclear Environment in NGC 3783 V. Variability and Modeling of the Intrinsic Ultraviolet Absorption

Abstract: We present results on the location, physical conditions, and geometry of the outflow in the Seyfert 1 galaxy NGC 3783 from a study of the variable intrinsic UV absorption. Based on analysis of 18 observations with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope and 6 observations with the Far Ultraviolet Spectroscopic Explorer obtained between 2000 February and 2002 January, we obtain the following results: 1) The lowest-ionization species detected in each of the three strong kinematic components (components 1 – 3 at radial velocities −1350, −550, and −725 km s −1 , respectively) varied, with equivalent widths inversely correlated with the continuum flux. This indicates the ionization structure in the absorbers responded to changes in the photoionizing flux, with variations occurring over the weekly timescales sampled by our observations. 2) A multi-component model of the line-of-sight absorption covering factors, which includes an unocculted narrow emission-line region (NLR) and separate covering factors derived for the broad line region and continuum emission sources, predicts saturation in several lines, consistent with the lack of observed variability in these lines. Differences in covering factors and kinematic structure imply component 1 is comprised of two physically distinct regions (1a and 1b). 3) We obtain column densities for the individual metastable levels from the resolved C III* λ1175 absorption complex in component 1a. Based on our computed metastable level populations, the electron density of this absorber is ∼ 3×10 4 cm −3 . Combined with photoionization modeling results, this places component 1a at ∼ 25 pc from the central source. 5) Using time-dependent calculations, we are able to reproduce the detailed variability observed in component 1 and derive upper limits on the distances for components 2 and 3 of ≤ 25 and ≤ 50 pc, respectively. 6) The ionization parameters derived for the higher ionization UV absorbers (components 1b, 2, and 3 with log(U)≈ −0.5) are consistent with the modeling results for the lowest-ionization X-ray component, but with smaller total column density. The high-ionization UV components are found to have similar pressures as the three X-ray ionization components. These results are consistent with an inhomogeneous wind model for the outflow in NGC 3783, with denser, colder, lower-ionization regions embedded in more highly-ionized gas. 7) Based on the predicted emission-line luminosities, global covering factor constraints, and distances derived for the UV absorbers, they may be identified with emission-line gas observed in the inner NLR of AGNs. We explore constraints for dynamical models of AGN outflows implied by these results. Subject headings: galaxies: individual (NGC 3783) — galaxies: active — galaxies: Seyfert — ultraviolet: galaxies

What controls the [O iii] λ5007 line strength in active galactic nuclei?

Abstract: Active galactic nuclei (AGN) display an extreme range in the narrow emission-line equivalent widths. Specifically, in the Palomar-Green (PG) quasar sample, the equivalent width of the narrow [O III] λ5007 line has a range of >300 (<0.5 to 157 A), while the broad Hβ line, for example, has a range of 10 only (23 to 230 A). The strength of [O III] λ5007 is modulated by the covering factor (CF) of the narrow-line region (NLR) gas, its density (n e ) and ionization parameter (U). To explore which of these factors produces the observed large range in [O III] λ5007 strength, we measure the strength of the matching narrow Hβ and [O III] λ4363 lines, detected in 40 out of the 87 PG quasars with z < 0.5 in the Boroson & Green sample. The photoionization code CLOUDY is then used to infer CF, n e and U in each object, assuming a single uniform emitting zone. We find that the range of CF (∼0.02-0.2) contributes about twice as much as the range in both n e and U towards modulating the strength of the [O III] λ5007 line. The CF is inversely correlated with luminosity, but it is not correlated with L/L Edd as previously speculated. The single-zone [O III] λ5007 emitting region is rather compact, having R NLR = 40L 0.45 44 pc. These emission lines can also be fitted with an extreme two-zone model, where [O III] λ4363 is mostly emitted by a dense (n e = 10 7 cm -3 ) inner zone at R in NLR = L 0.5 44 pc, and [O III] λ5007 by a low-density (n e = 10 3 cm -3 ) extended outer zone at R out NLR = 750L 0.34 44 pc. Such an extended [O III] λ5007 emission should be well resolved by Hubble Space Telescope imaging of luminous AGN. Further constraints on the radial gas distribution in the NLR can be obtained from the spectral shape of the infrared continuum emitted by the associated dust.

The Ionized Gas and Nuclear Environment in NGC 3783 V. Variability and Modeling of the Intrinsic Ultraviolet Absorption

Abstract: We present results on the location, physical conditions, and geometry of the outflow in the Seyfert 1 galaxy NGC 3783 from a study of the variable intrinsic UV absorption. Based on 18 observations with HST/STIS and 6 observations with FUSE, we find: 1) The absorption from the lowest-ionization species in each of the three strong kinematic components varied inversely with the continuum flux, indicating the ionization structure responded to changes in the photoionizing flux over the weekly timescales sampled by our observations. 2) A multi- component model with an unocculted NLR and separate BLR and continuum line-of-sight covering factors predicts saturation in several lines, consistent with the lack of observed variability. 3) Column densities for the individual metastable levels are measured from the resolved C III *1175 absorption complex observed in one component. Based on our computed metastable level populations, the electron density of this absorber is ~3x10^4 cm^-3. Photoionization modeling results place it at ~25 pc from the central source. 4) Using time-dependent calculations, we are able to reproduce the detailed variability observed in this absorber, and derive upper limits on the distances for the other components of 25-50 pc. 5) The ionization parameters derived for the higher ionization UV absorbers are consistent with the modeling results for the lowest-ionization X-ray component, but with smaller total column density. They have similar pressures as the three X-ray ionization components. These results are consistent with an inhomogeneous wind model for the outflow in NGC 3783. 6) Based on the predicted emission-line luminosities, global covering factor constraints, and distances derived for the UV absorbers, they may be identified with emission- line gas observed in the inner NLR of AGNs. (abridged)

A photoionization-modelling study of 30 Doradus: the case for small-scale chemical inhomogeneity

Abstract: Photoionization models of the giant H II region (GHIIR) 30 Doradus are built and confronted to available ultraviolet (UV), optical, infrared (IR) [Infrared Space Observatory (ISO)] and radio spectra, under blackbody or CoStar spectral energy distributions for the primary source and various density distributions for the nebular gas. Chemically homogeneous models show very small rms electron-temperature fluctuations and fail to reproduce the heavy-element optical recombination line (ORL) spectrum of the nebula. Dual-abundance models incorporating small-scale chemical inhomogeneities in the form of hydrogen-deficient inclusions which are in pressure balance with the normal-composition ambient gas, provide a better fit to the observed heavy-element ORLs and other nebular lines, while most spectral features are satisfactorily accounted for. The inclusions, whose mass is ∼2 per cent of the total gaseous mass, are 2‐ 3 times cooler and denser than the ambient nebula. Their O/H abundance ratio is ∼0.9 dex larger than in the normal-composition gas and have typical mass fractions of X = 0.687, Y = 0.273 and Z = 0.040. Helium is found to be about as deficient as hydrogen in the inclusions, while elements heavier than neon, such as sulphur and argon, are quite possibly enhanced in proportions similar to oxygen, as indicated by the most satisfactory dual-abundance model obtained. This suggests that the posited hydrogen-deficient inclusions may have arisen from partial mixing of matter which was nucleosynthetically processed in a supernova event with gas of normal Large Magellanic Cloud composition. The average gaseous abundances of the chemically inhomogeneous models are ∼0.08 dex higher than those of the homogeneous models, yet they are lower by a similar ∼0.08 dex than those derived from standard empirical methods (fully corrected for inaccuracies in ionization correction factors and differences in atomic data) which postulate temperature fluctuations in a chemically homogeneous medium. Attention is drawn to a bias in the determination of H II region (HIIR) helium abundances in the presence of hydrogen-deficient inclusions. It is argued that these results provide evidence for incomplete small-scale mixing of the interstellar medium (ISM). The case for the existence of abundance inhomogeneities in HIIRs is examined in the light of current theoretical considerations regarding the process of chemical homogenization in the ISM. Ke yw ords: ISM: abundances ‐ ISM: evolution ‐ H II regions ‐ ISM: individual: 30 Doradus ‐ galaxies: individual: LMC.

Complete photo-induced breakup of the H2 molecule as a probe of molecular electron correlation.

Abstract: Despite decades of progress in quantum mechanics, electron correlation effects are still only partially understood. Experiments in which both electrons are ejected from an oriented hydrogen molecule by absorption of a single photon have recently demonstrated a puzzling phenomenon: The ejection pattern of the electrons depends sensitively on the bond distance between the two nuclei as they vibrate in their ground state. Here, we report a complete numerical solution of the Schrodinger equation for the double photoionization of H2. The results suggest that the distribution of photoelectrons emitted from aligned molecules reflects electron correlation effects that are purely molecular in origin.

Application of time-of-flight mass spectrometry with laser-based photoionization methods for time-resolved on-line analysis of mainstream cigarette smoke.

Abstract: The application of soft photoionization mass spectrometry methods (PIMS) for cigarette mainstream smoke analysis is demonstrated. Resonance-enhanced multiphoton ionization (REMPI) at 260 nm and vac...