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PubMed2005

Core relaxation in atomic ultrastrong laser field ionization.

We have investigated atomic ionization dynamics in Kr in the transition regime from nonrelativistic to relativistic laser intensities (10(16) to 10(18) W/cm2) by measuring yields of highly charged ions stemming from an inner shell. Interpretation of the data is focused on the applicability of the single active electron description, which requires fully relaxed core states between successive ionization steps. In particular, we are concerned with transient core polarization or alignment effects originating from the strong dependence of the ionization rates on the magnetic quantum number. We found that for intense laser pulses with 40 fs pulse width internal m-mixing processes appear to be sufficiently fast to erase any transient core polarization.

Journal Article↗
PubMed2005

Lithium ionization by a strong laser field.

We study ab initio computations of the interaction of lithium with a strong laser field. Numerical solutions of the time-dependent fully correlated three-particle Schrodinger equation restricted to the one-dimensional soft-core approximation are presented. Our results show a clear transition from nonsequential to sequential double ionization for increasing intensities. Nonsequential double ionization is found to be sensitive to the spin configuration of the ionized pair. This asymmetry, also found in experiments of photoionization of Li with synchrotron radiation, shows evidence of the influence of the exclusion principle on the underlying rescattering mechanism.

Journal Article↗
PubMed2005

Theory of tunnel ionization in complex systems.

A quasianalytical theory of tunnel ionization is developed that is applicable to general complex systems, such as large molecules. Our analysis reveals strong deviations from conventional tunnel ionization theories, dependent upon the system's geometry, angular momentum, and polarizability. A comparison of our theory with recent C(60) ionization experiments yields reasonable agreement.

Journal Article↗
PubMed2005

Dissociation and ionization of H+2 by ultrashort intense laser pulses probed by coincidence 3D momentum imaging.

Laser-induced dissociation and ionization of H(+)(2) were simultaneously measured using coincidence 3D momentum imaging, allowing direct separation of the two processes, even where the fragment kinetic energy is the same for both processes. The results for 45 and 135 fs 790 nm pulses with an intensity of approximately 2.5 x 10(14) W/cm(2) differ from each other much more than one would expect from previous measurements with longer pulses. Ionization was negligible for the longer pulse and was strongly aligned along the laser polarization for the shorter pulse, but showed no structure in its kinetic energy distribution. In addition, the ionization to dissociation ratio was found to be much smaller than theoretically predicted for H(+)(2).

Journal Article↗
PubMed2006

Young-type interference in (e, 2e) ionization of H(2).

We have investigated the electron impact single ionization of the hydrogen molecule, with fully determined kinematics. The experimental and theoretical results are compared with He ionization under the same conditions. The results indicate that the ejected electron angular distribution for H(2) is modified due to Young-type interference between ionization amplitudes for scattering from the two centers in the hydrogen molecule. The observable result is a suppression of the backward scattering (recoil) peak compared with the binary peak.

Journal Article↗
PubMed2006

Elliptic and circular dichroism effects in two-photon double ionization of atoms.

An ab initio parametrization of the two-photon double ionization amplitude from an s2 subshell of an atom in a 1S state is presented and used to predict two light polarization effects on photoelectron angular distributions that do not exist in single-photon double ionization: (i) elliptic dichroism and (ii) circular dichroism at equal energy sharing. Estimates for He show large magnitudes for these effects, which provide a means for polarization control of double ionization by vacuum ultraviolet light.

Journal Article↗
PubMed2006

Cold atomic collisions: coherent control of penning and associative ionization.

Coherent control techniques are computationally applied to cold (1 mK<T<1 K) and ultracold (T<1 muK) Ne*(3s,3P2)+Ar(1S0) collisions. We show that by using various initial superpositions of the Ne*(3s,3P2) M={-2,-1,0,1,2} Zeeman sublevels it is possible to reduce the Penning ionization and associative ionization cross sections by as much as 4 orders of magnitude. It is also possible to drastically change the ratio of these two processes. The results are based on combining, within the "rotating atom approximation", empirical and ab initio ionization widths.

Journal Article↗
PubMed1975

Abscission of Phaseolus and Impatiens Explants: Effects of Ionizing Radiation upon Endogenous Growth Regulators and de Novo Enzyme Synthesis.

Stem-petiole explants from the lower pulvinus of the primary leaves of Phaseolus vulgaris L. cv. Red Kidney and from Impatiens sultani Hook cv. Scarlet Baby were exposed to varying dosages of gamma-radiation. With bean, irradiation of 175 to 525 kiloroentgens (kR) significantly accelerated the onset of abscission with a maximum response at 175 to 280 kR. Higher dosages (beginning at 600-700 kR) usually prevented abscission. With Impatiens, 18 to 35 kR significantly accelerated both the onset of abscission and possibly the initial abscission rate; 350 kR cut the time to 100% abscission in half and substantially accelerated the initial abscission rate. Inhibition of abscission in Impatiens was not possible with the available dose rate (35 kR/hour).The accelerating effect of ionizing radiation was apparently due to primarily a reduction in auxin activity, thus accelerating the shift from the relatively ethylene-insensitive stage I to the ethylene-sensitive stage II. In vitro irradiation of exogenous auxin in agar substantially reduced the abscission-delaying effects of the auxin. The low dosage accelerating effects of the ionizing radiation were negated by exogenous auxin application, either prior to or following irradiation. With prior auxin application, a higher concentration of auxin was required to achieve the same delaying effects as with application following irradiation. Apparently due to this reduction in auxin activity, irradiation hastened sensitivity to exogenous ethylene.The accelerating effects of ionizing radiation may be secondarily due to increased levels of endogenous ethylene. With bean tissues, exposures of 262 through 1500 kR of gamma radiation increased the measurable levels of endogenous ethylene released into the ambient air within 2 or 3 days after irradiation.The results of actinomycin D treatment suggested that required de novo RNA synthesis occurred about 4 hours earlier in those Impatiens explants that had received 105 kR gamma radiation than in the nonirradiated controls.An increased mobilization of Ca from the abscission zone may be a contributing factor in the radiation-induced acceleration of abscission. For both bean and Impatiens, exogenous Ca inhibited abscission of irradiated and nonirradiated explants. In addition, a more concentrated Ca solution was required to inhibit abscission of irradiated bean explants, as compared to the nonirradiated controls.

Journal Article↗
PubMed1992

Effects of ionizing radiation on a plant genome: analysis of two Arabidopsis transparent testa mutations.

Ionizing radiation is known to cause chromosomal alterations such as inversions and deletions and has been used extensively for inducing mutations. In Arabidopsis, two methods for the isolation of genes identified on the basis of mutant phenotypes--genomic subtraction and chromosome walking--either rely on or are greatly facilitated by the availability of these types of mutations. This article gives a detailed characterization of ionizing radiation-induced mutations in plants. The Arabidopsis genes encoding chalcone flavanone isomerase (CHI) and dihydroflavonol 4-reductase (DFR) were cloned and found to correspond to two transparent testa loci. A CHI allele, generated by fast-neutron irradiation, consisted of an inversion within the gene. A 272-bp fragment from 38 centimorgans away on the same chromosome was transferred to one end of this inversion. A DFR allele, induced by x-irradiation, contained two deletions and an inversion of the 2.8-centimorgan intervening region. Sequence analysis of the break points in both mutants indicate that repair of radiation-induced damage involves mechanisms similar or identical to those that mediate the integration of foreign sequences into the genome. The chromosome rearrangements found in these mutants have important implications for the use of ionizing radiation-induced alleles in classical and molecular genetic experiments in plants.

Alcohol Oxidoreductases↗
PubMed2000

Atomic scattering factors for K-shell and L-shell ionization by fast electrons

Atomic scattering factors have been calculated for K-shell ionization for elements in the range Z = 6 (carbon) to Z = 50 (tin) and for L-shell ionization in the range Z = 20 (calcium) to Z = 60 (neodymium). The calculations are based on relativistic Hartree-Fock wave functions for the atomic bound states and Hartree-Slater wave functions for the continuum wave functions. The results are presented in tabular form such that accurate values of the scattering factors can be obtained by cubic spline interpolation for incident electron energies between 50 and 400 keV and for scattering vectors with magnitude s = sin straight theta / lambda up to 2.5 A(-1) (2straight theta is the scattering angle and lambda the wavelength of the incident electrons). A separate parameterization of the form factors is given for 2.5 </= s </= 20 A(-1), where they are small. In addition, a simpler but less accurate parameterization of the atomic scattering factors in an exponential form has been obtained by fitting the calculated form factors in the region s </= 2.5 A(-1). The scattering factors are suitable for the calculation of ionization cross sections for use in atom location by channelling-enhanced microanalysis (ALCHEMI).

Journal Article↗
PubMed2001

Multiple ionization upon K-shell photoabsorption of the CO and N2 molecules.

The spectra of multiple ionization of CO and N2 in the gas phase in the region of the K-shell ionization thresholds of C and N are studied theoretically. The processes of additional excitation/ionization from 1pi and 5sigma-shells are included. The effects of many-electron correlations on the extention of multiplet structure for some configurations of multiple excitations is studied. Absolute values of photoabsorption cross sections are calculated in some of the channels of multiple ionization.

Journal Article↗
PubMed2003

Miniature ionization chamber detector developed for X-ray microprobe measurements.

A windowless small ionization chamber detector has been developed for monitoring the intensity of the microbeam at the ID18F microprobe end-station of the European Synchrotron Radiation Facility. The small dimensions of the ionization chamber (10 mm along the beam direction and 5 mm perpendicular to it) make it possible to place it very close to the sample. A pinhole of diameter 50 microm was used for defining the entrance window of the ionization chamber; thus the small counter can be used as an order-selecting aperture while measuring simultaneously the intensity after the aperture. In the present work the technical characteristics, such as the current-voltage curve, stability and linearity, of the small monitor have been tested.

Journal Article↗
PubMed2006

pK values of the ionizable groups of proteins.

We have used potentiometric titrations to measure the pK values of the ionizable groups of proteins in alanine pentapeptides with appropriately blocked termini. These pentapeptides provide an improved model for the pK values of the ionizable groups in proteins. Our pK values determined in 0.1 M KCl at 25 degrees C are: 3.67+/-0.03 (alpha-carboxyl), 3.67+/-0.04 (Asp), 4.25+/-0.05 (Glu), 6.54+/-0.04 (His), 8.00+/-0.03 (alpha-amino), 8.55+/-0.03 (Cys), 9.84+/-0.11 (Tyr), and 10.40+/-0.08 (Lys). The pK values of some groups differ from the Nozaki and Tanford (N & T) pK values often used in the literature: Asp (3.67 this work vs. 4.0 N & T); His (6.54 this work vs. 6.3 N & T); alpha-amino (8.00 this work vs. 7.5 N & T); Cys (8.55 this work vs. 9.5 N & T); and Tyr (9.84 this work vs. 9.6 N & T). Our pK values will be useful to those who study pK perturbations in folded and unfolded proteins, and to those who use theory to gain a better understanding of the factors that determine the pK values of the ionizable groups of proteins.

Hydrogen Bonding↗
PubMed2001

Chronic health risks from aggregate exposures to ionizing radiation and chemicals: scientific basis for an assessment framework.

Very little quantitative analysis is currently available on the cumulative effects of exposure to multiple hazardous agents that have either similar or different mechanisms of action. Over the past several years, efforts have been made to develop the methodologies for risk assessment of chemical mixtures, but mixed exposures to two or more dissimilar agents such as radiation and one or more chemical agents have not yet been addressed in any substantive way. This article reviews the current understanding of the health risks arising from mixed exposures to ionizing radiation and specific chemicals. Specifically discussed is how mixed radiation/chemical exposures, when evaluated in aggregation, were linked to chronic health endpoints such as cancer and intermediate health outcomes such as chromosomal aberrations. Also considered is the extent to which the current practices are consistent with the scientific understanding of the health risks associated with mixed-agent exposures. From this the discussion moves to the research needs for assessing the cumulative health risks from aggregate exposures to ionizing radiation and chemicals. The evaluation indicates that essentially no guidance has been provided for conducting risk assessment for two agents with different mechanisms of action (i.e., energy deposition from ionizing radiation versus DNA interactions with chemicals) but similar biological endpoints (i.e., chromosomal aberrations, mutations, and cancer). The literature review also reveals the problems caused by the absence of both the basic science and an appropriate evaluation framework for the combined effects of mixed-agent exposures. This makes it difficult to determine whether there is truly no interaction or somehow the interaction is masked by the scale of effect observation or inappropriate dose-response assumptions.

Animals↗
PubMed1983

Decreased serum concentration of ionized calcium in insulin-dependent human diabetes mellitus.

Low serum ionized calcium concentrations were observed in twenty-five insulin dependent diabetic outpatients compared with twenty-three age-matched normal subjects: mean 1.16 mmol/l (SEM 0.01) versus 1.20 mmol/l (0.01), P less than 0.002. Despite this, there was no compensatory increase in serum concentrations of immunoreactive parathyroid hormone, nor was serum total calcium decreased in the diabetic patients. Serum magnesium was significantly decreased in the diabetics compared with normals: mean 0.75 mmol/l versus 0.83 mmol/l, P less than 0.001. No significant correlation could be demonstrated between serum magnesium and serum ionized calcium or parathyroid hormone in the diabetic patients. Since no significant correlations were observable between serum ionized calcium and indices of diabetes control, the etiology and pathogenesis of decreased serum calcium ion in insulin-dependent human diabetes mellitus remain unknown.

Adolescent↗
PubMed1995

Effect of acute bile acid pool depletion on total and ionized calcium concentrations in human bile.

Although calcium salts are important components of gallstones, there are few data on the total and ionized calcium content of human bile. Therefore, in 14 fasting T-tube patients studied 7-11 days after cholecystectomy, we measured bile flow, bile acid [BA], total [CaTOT] and free ionized [Ca++] calcium concentrations, in 20-30 min bile collections during acute BA pool depletion induced by 6-8 h of continuous bile drainage. During washout of the BA pool there were parallel falls in bile flow, BA output and total calcium output (correlation coefficients ranging from 0.59 to 0.99; P < 0.02-0.001). In 12 of the 14 patients, [CaTOT] also fell (from 1.84 +/- 0.29 to 1.32 +/- 0.34 mmol L-1) in parallel with [BA] (from 34.0 +/- 14.0 to 8.2 +/- 8.0 mmol L-1; r = 0.75-0.98; P < 0.005). In contrast, biliary [Ca++] remained virtually unchanged. These data suggest that the BAs are linked to the bound, rather than to the free, ionized, fraction of biliary calcium, which is consistent with in vivo calcium binding by BAs. A model is proposed in which BA-induced biliary calcium secretion results from (i) bile acid-induced water flow via solvent drag; and (ii) calcium binding in the bile canaliculus by bile acids, which induces paracellular diffusion of Ca++, thereby maintaining [Ca++] independent of [BA].

Adult↗
PubMed1987

Electrostatic effects of bound NADH and NAD+ on ionizing groups in liver alcohol dehydrogenase.

A simple point charge model has been used to estimate the effect of electrostatic interactions of bound NADH and NAD+ on ionizing groups in liver alcohol dehydrogenase. The results provide clear evidence that the much-discussed effects of pH on coenzyme binding in the pH range 7-10 are predominantly of electrostatic origin and attributable to ionization of the water molecule which is bound at the active-site zinc ion. Ionization of Lys-228 is proposed to account for the proton-dependent destabilization of the enzyme X NADH and enzyme X NAD+ complexes which has been reported to occur above a pKa of 10.4.

Alcohol Dehydrogenase↗
PubMed1987

The uptake of protons by heme-linked ionizable groups on azide binding to methemoglobin.

When azide ion reacts with methemoglobin in unbuffered solution the pH of the solution increases. This phenomenon is associated with increases in the pK values of heme-linked ionizable groups on the protein which give rise to an uptake of protons from solution. We have determined as a functional of pH the proton uptake, delta h+, on azide binding to methemoglobin at 20 degrees C. Data for methemoglobins A (human), guinea pig and pigeon are fitted to a theoretical expression based on the electrostatic effect of these sets of heme-linked ionizable groups on the binding of the ligand. From these fits the pK values of heme-linked ionizable groups are obtained for liganded and unliganded methemoglobins. In unliganded methemoglobin pK1, which is associated with carboxylic acid groups, ranges between 4.0 and 5.5 for the three methemoglobins; pK2, which is associated with histidines and terminal amino groups, ranges from 6.2 to 6.7. In liganded methemoglobin pK1 lies between 5.8 and 6.3 and pK2 varies from 8.1 to 8.5. The pH dependences of the apparent equilibrium constants for azide binding to the three methemoglobins at 20 degrees C are well accounted for with the pK values calculated from the variation of delta h+ with pH.

Azides↗