Possible dose-rate effects in a plasmid DNA exposed to pulsed extreme ultraviolet (XUV) and soft x-ray (SXR) water window radiation from two different table-top plasma-based sources was studied. Dose delivered to the target molecule was controlled by attenuating the incident photon flux with aluminum thin foils as well as varying the DNA/buffer-salt ratio in the irradiated sample. Irradiated samples were analyzed using the agarose gel electrophoresis. Some additional bands were identified in gel electrophoretograms as results of a DNA cross-linking. They were inspected by atomic force microscopy (AFM). Yields of single- and double-strand breaks (Gy-1 Da-1) were determined as a function of incident dose rate. Both yields decreased with a dose rate increasing. The ratio of single- and double-strand breaks exhibited only a slight increase at elevated dose rates. In conclusion, complex and/or clustered damages do not seem to be initiated under these irradiation conditions.
Ionizing radiation induces a variety of DNA damages including single-strand breaks (SSBs), double-strand breaks
(DSBs), abasic sites, modified sugar and bases. Most theoretical and experimental studies have been focused on DNA
strand scissions, in particular production of DNA double-strand breaks. DSBs have been proven to be a key damage at a
molecular level responsible for the formation of chromosomal aberrations, leading often to cell death. The complexity of
lesions produced in DNA by ionizing radiations is thought to depend on the amount of energy deposited at the site of
each lesion. We have studied the nature of DNA damage induced directly by the pulsed 46.9 nm radiation provided by a
capillary-discharge Ne-like Ar laser (CDL). Different surface doses were delivered with a repetition rate of a few Hz and
an average pulse energy ~ 1 μJ. A simple model DNA molecule, i.e., dried closed-circular plasmid DNA (pBR322), was
irradiated. The agarose gel electrophoresis method was used for determination of both SSB and DSB yields. Results are
compared with a previous study of plasmid DNA irradiated with a single sub-nanosecond 1-keV X-ray pulse produced
by a large-scale, double-stream gas puff target, illuminated by sub-kJ, near-infrared (NIR) focused laser pulses at the
PALS facility (Prague Asterix Laser System).
Neutral AlmCn clusters are investigated both experimentally and theoretically for the first time. Single photon ionization through 193 nm, and 46.9 nm lasers is used to detected neutral cluster distributions. AlmCn clusters are generated through laser ablation of a mixture of Al and C powders pressed into a disk. An oscillation of the VIEs of AlmCn clusters is observed in the experiments. The ionization energies of AlmCn clusters change as a function of the numbers of Al and C atoms in the clusters. Theoretical density functional theory and ab initio calculations are carried out to explore the structures, ionization energies, and molecular orbitals of the AlmCn clusters. C=C bonds are favored for the lowest energy structures for AlmCn clusters. Based on calculations of HOMOs (highest occupied molecular orbitals) of AlmCn clusters we find that the ionization energies of these clusters do not simply depend on open or closed shell valence electron configurations, but also depend on the electronic structure details of the clusters. The calculational results provide good
and consistent explanations of the experimental observations, and are consistent with them.
The desktop capillary-discharge Ne-like Ar laser (CDL) providing 10-μJ nanosecond pulses of coherent 46.9-nm
radiation with a repetition rate up to 12 Hz was developed and built at the Colorado State University in Fort Collins and
then installed in Prague. The beam of the laser was focused by a spherical mirror covered with Si/Sc multilayer coating
onto the surface of poly(methyl methacrylate) - PMMA. Interaction parameters vary by changing the distance between
sample surface and beam focus. The samples were exposed to various numbers of shots. Analysis of damaged PMMA by
atomic force (AFM) and Nomarski (DIC - differential interference contrast) microscopes allows not only to determine
the key characteristics of the focused beam (e.g. Rayleigh's parameter, focal spot diameter, tight focus position, etc.) but
also to investigate mechanisms of the radiation-induced erosion processes.
The reactivity of prospective capping layer extreme ultraviolet (EUV) mirror materials with hydrocarbons, alcohols, and
water is studied in the gas phase using mass spectroscopy of metal-oxide clusters. We report the results of chemistry
studies for Si, Ti, Hf, Zr, and Ru metal oxide nanoclusters in which the reaction products were ionized with little or no
fragmentation by 26.5 eV photons from a desk-top size 46.9 nm Ne-like Ar laser. Hf and Zr oxides are found to be much
less reactive than Si or Ti oxides in the presence EUV light. Ru oxides show different chemistry and are inactive with
water/alcohols, and active with saturated/unsaturated hydrocarbons. The results are relevant to the design of EUV mirror
capping layers that are resistant to carbon contamination.
Metal oxide clusters are employed in studies to help understand an important, specific, type of surface chemical
problem: the contamination of soft x-ray mirrors by carbon deposits. Herein we report nanocluster chemistry studies that
are relevant to the use of silicon oxide and titanium oxide capping layers. Systems involving SimOn, and TimOn metal
oxide nanoclusters are generated in a pulsed supersonic expansion/ablation source and passed through a reactor
containing any reactant desired. The reaction products of these gas phase clusters are ionized using single photon
ionization from a desk-top sized 46.9 nm Ne-like Ar laser providing the advantage of little or no fragmentation of
desired nanoclusters. The ionized products are analyzed by a time of flight mass spectrometer and experimental results
supply useful information related to condensed phase soft x-ray optical surfaces. The results illustrate the great potential
of the use of very compact soft x-ray lasers in photochemistry and photophysics studies.
We report the demonstration of a high repetition rate desktop-size capillary discharge laser emitting at (= 46.9 nm (26.5 eV) used for experiments in photochemistry. Laser pulses with energy ~ 13 uJ were generated at 12 Hz repetition rate by single pass amplification in a 21 cm long Ne-like Ar capillary discharge plasma column. The capillary lifetime is 2-3 104 shots. This new type of portable laser is of interest for numerous applications requiring a compact intense source of short wavelength laser light. One such application that we are currently pursuing is the study of small molecules using time of flight mass spectroscopy. Molecules include ammonia, NO, oxygen, hydrogen bonded nanoclusters, and metal oxide nanoclusters. Through single photon ionization the reactivity and catalytic behavior of these molecules is studied.
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