Low-energy Photoionization Research Articles

Processes triggered in guanine quadruplexes by direct absorption of UV radiation: From fundamental studies toward optoelectronic biosensors.

Guanine quadruplexes (GQs) are four-stranded DNA/RNA structures exhibiting an important polymorphism. During the past two decades, their study by time-resolved spectroscopy, from femtoseconds to milliseconds, associated to computational methods, shed light on the primary processes occurring when they absorb UV radiation. Quite recently, their utilization in label-free and dye-free biosensors was explored by a few groups. In view of such developments, this review discusses the outcomes of the fundamental studies that could contribute to the design of future optoelectronic biosensors using fluorescence or charge carriers stemming directly from GQs, without mediation of other molecules, as it is the currently the case. It explains how the excited state relaxation influences both the fluorescence intensity and the efficiency of low-energy photoionization, occurring via a complex mechanism. The corresponding quantum yields, determined with excitation at 266/267 nm, fall in the range of (3.0-9.5) × 10-4 and (3.2-9.2) × 10-3 , respectively. These values, significantly higher than the corresponding values found for duplexes, depend strongly on certain structural factors (molecularity, metal cations, peripheral bases, number of tetrads …) which intervene in the relaxation process. Accordingly, these features can be tuned to optimize the desired signal.

Guanine Radicals Induced in DNA by Low-Energy Photoionization.

Guanine (G) radicals are precursors to DNA oxidative damage, correlated with carcinogenesis and aging. During the past few years, we demonstrated clearly an intriguing effect: G radicals can be generated upon direct absorption of UV radiation with energy significantly lower than the G ionization potential. Using nanosecond transient absorption spectroscopy, we studied the primary species, ejected electrons and guanine radicals, which result from photoionization of various DNA systems in aqueous solution.The DNA propensity to undergo electron detachment at low photon energies greatly depends on its secondary structure. Undetected for monomers or unstacked oligomers, this propensity may be 1 order of magnitude higher for G-quadruplexes than for duplexes. The experimental results suggest nonvertical processes, associated with the relaxation of electronic excited states. Theoretical studies are required to validate the mechanism and determine the factors that come into play. Such a mechanism, which may be operative over a broad excitation wavelength range, explains the occurrence of oxidative damage observed upon UVB and UVA irradiation.Quantification of G radical populations and their time evolution questions some widespread views. It appears that G radicals may be generated with the same probability as pyrimidine dimers, which are considered to be the major lesions induced upon absorption of low-energy UV radiation by DNA. As most radical cations undergo deprotonation, the vast majority of the final reaction products is expected to stem from long-lived deprotonated radicals. Consequently, when G radical cations are involved, the widely used oxidation marker 8-oxodG is not representative of the oxidative damage.Beyond the biological consequences, photogeneration of electron holes in G-quadruplexes may inspire applications in nanoelectronics; although four-stranded structures are currently studied as molecular wires, their behavior as photoconductors has not been explored so far.In the present Account, after highlighting some key experimental issues, we first describe the photoionization process, and then, we focus on radicals. We use as show-cases new results obtained for genomic DNA and Oxytricha G-quadruplexes. Generation and reaction dynamics of G radicals in these systems provide a representative picture of the phenomena reported previously for duplexes and G-quadruplexes, respectively.

Direct observation of photoionization dynamics in solution phase induced by femtosecond two-photon excitation

In solution phase, the solute can be photo-ionized in the lower excitation energy than its ionization potential in gas phase. Therefore, the specific interaction is expected to be exist between the surrounding media and higher excited (Sn) state of the solute. In order to elucidate such polarization effect of solvent on the photoionization process, femtosecond double-pulse excitation was applied to direct detection of low-energy photoionization dynamics of a phenylenediamine derivative in solution phase. From the results of the transient absorption change, in polar solvent, it is clearly indicated that photoionization does not proceed directly from the Sn state, but through specific intermediate state. Moreover,

On the Photoionization Spectrum of Propyne: A Fully ab Initio Simulation of the Low-Energy Spectrum Including the Jahn–Teller Effect and the Spin–Orbit Interaction

The low energy photoionization spectrum of propyne (CH3-CCH), which reveals the vibronic structure of the propyne cation, is simulated using vibronic coupling theory. The spin-orbit interaction is included using an intensity borrowing approach, enabling determination of the (X̃(2)E1/2,3/2, v = 0) splitting and the relative photoionization intensity of these closely spaced levels. The results are compared with recent experimental studies and misstatements are corrected.

Ionization surprise

The common picture of how atoms and molecules are ionized in intense laser fields has had decades of success. However, the observation of an unexpected but apparently universal low-energy photoionization feature suggests this picture is incomplete.

Fast electron and X-ray scattering as a tool to study target's structure

We concentrate on several relatively new aspects of the study of fast electron and X-ray scattering by atoms and atom-like objects, namely endohedral atoms and fullerenes. However, main attention is given to fast charge particle scattering. We show that the corresponding cross-sections, being expressed via so-called generalized oscillator strengths (GOS), give information on the electronic structure of the target and on the role of electron correlations in it. We consider what sort of information became available when analyzing the dependence of GOS upon their multipolarity, transferred momentum q and energy ω. To obtain theoretical results, we employ both the one-electron Hartree–Fock approximation and account for the multi-electron correlation in the target, using the random phase approximation with exchange. We demonstrate the role of non-dipole corrections in the small-angle fast-electron inelastic scattering. There dipole contribution dominates while non-dipole corrections can be considerably and controllably enhanced as compared to the case of low and medium energy photoionization. We show also that analyses of GOS for discrete level excitations permit to clarify their multipolarity. The results of calculations of Compton excitation and ionization cross-sections are presented. Attention is given to cooperative effects in inelastic fast electron-atom scattering that results in directed motion of the secondary electrons, a phenomenon that is similar to “drag currents” in photoionization. We demonstrate how one should derive GOS for endohedral atoms, e.g. A@ C 60 and what is the additional information that can be obtained from corresponding GOS. Most of discussions are illustrated by the results of concrete calculations.

Magnetic circular dichroism of ferromagnetic Co2+-doped ZnO

Cobalt-doped ZnO (Co2+:ZnO) films were studied by magnetic circular dichroism (MCD) spectroscopy. A broad 300K ferromagnetic MCD signal was observed between 1.4 and 4.0eV after exposure of paramagnetic Co2+:ZnO films to zinc metal vapor, attributed to low-energy photoionization transitions originating from a spin-split donor impurity band in ferromagnetic n-type Co2+:ZnO.

Dramatic nondipole effects in low-energy photoionization: experimental and theoretical study of Xe 5s.

The Xe 5s nondipole photoelectron parameter gamma is obtained experimentally and theoretically from threshold to approximately 200 eV photon energy. Significant nondipole effects are seen even in the threshold region of this valence shell photoionization. In addition, contrary to previous understanding, clear evidence of interchannel coupling among quadrupole photoionization channels is found.

Cerium-doped fluorescent and scintillating ionic crystals

Relaxation of electronic excitations created by the absorption of a high energy photon is analyzed in the case of rare earth crystals. it is shown that the energy transfer to cerium ions strongly depends on the band structure of the system, in particular on the position of the Ce 4f level in the bandgap. The probability of the hole capture by Ce3+ is related to the scintillation efficiency; it is in general weak in fluorides and stronger in oxides. The fact that cerium has two stable valence states Ce3+ and Ce4+ can lead to Ce bound exciton formation and Ce3+ low energy photoionization processes which can influence the fluorescence and scintillation efficiencies of the crystals.

Strong nondipole effects in low-energy photoionization of the5sand5psubshells of xenon

Large nondipole effects are predicted in the angular distribution of photoelectrons from the $5s$ and $5p$ subshells of xenon for photon energies below 200 eV. The nondipole parameter ${\ensuremath{\gamma}}_{5s}$ exhibits a dispersion-curve variation near the first minimum of the $5s$ cross section at 35 eV, reaching a minimum value of $\ensuremath{-}0.8$ near 40 eV. Rapid variation of ${\ensuremath{\gamma}}_{5s}$ is also found near the second minimum of the $5s$ cross section at 150 eV, where ${\ensuremath{\gamma}}_{5s}$ reaches a maximum value of 1.2. Smaller, but significant, nondipole effects are also found in the parameter ${\ensuremath{\zeta}}_{5p}={\ensuremath{\gamma}}_{5p}+3{\ensuremath{\delta}}_{5p},$ which has a maximum value of 0.15 near 50 eV, and a second maximum value of 0.18 near 160 eV. The higher energy maxima in ${\ensuremath{\gamma}}_{5s}$ and ${\ensuremath{\zeta}}_{5p}$ arise from correlation enhanced by shape resonances in the $4\stackrel{\ensuremath{\rightarrow}}{p}f$ quadrupole photoionization channels. These predictions are based on relativistic random-phase approximation calculations in which excitations from $5p,$ $5s,$ $4d,$ $4p,$ and $4s$ subshells are coupled.

Mirroring doubly excited resonances in argon.

New features are revealed in the low-energy photoionization spectrum of Ar by critically combining high photon resolution and differential photoelectron spectroscopic techniques. Two LS-forbidden doubly excited resonances are seen in the 3p(-1)(3/2, 1/2) partial cross sections which exhibit mirroring profiles, resulting in complete cancellation in the total photoionization cross section, as was predicted by Liu and Starace [Phys. Rev. A 59, R1731 (1999)]. These results demonstrate that a new class of weakly spin-orbit induced, mirroring resonances should be observable in partial, but not in total, collisional cross sections involving atoms, molecules, and solids in general.

Relativistic effects in the photoionization of Ne-like iron

We demonstrate that much simpler techniques can reproduce the results of the full Breit-Pauli R-matrix method to include relativistic effects in photoionization calculations. To allow for the fine-structure splitting of channels in the photoionization of ${\mathrm{Fe}}^{16+},$ we have performed three sets of calculations. The first combined an $\mathrm{LS}$ R-matrix calculation with an $\mathrm{LS}\ensuremath{-}\mathrm{JK}$ frame transformation, using multichannel quantum defect theory. The second used a relativistic random phase approximation based on the Dirac equation. Both methods give resonant photoionization results nearly identical to those from a third calculation using the full Breit-Pauli R-matrix method. An accurate treatment of fine-structure splitting in ${\mathrm{Fe}}^{16+}$ is necessary to realistically include the ${2p}^{5}{(}^{2}{P}_{1/2})nl$ resonances which dominate the low-energy photoionization cross section; consequently, in the inverse process of photorecombination, the low-temperature rate coefficient is dominated by the ${2p}^{5}{(}^{2}{P}_{1/2})nl$ dielectronic recombination resonances.

Enhanced Nondipole Effects in Low Energy Photoionization

It is found that electric-dipole--electric-quadrupole $E1$- $E2$ interference effects at low photon energies, of the order of tens of eV, in photoelectron angular distributions can be significant, comparable to, or even larger than, effects at energies of hundreds to thousands of eV, owing to quadrupole autoionizing resonances. This is illustrated both for Ne $2p$ photoionization around 51 eV and Ar $3p$ photoionization around 33 eV, where random phase approximation with exchange calculations are performed with allowance for correlations in both dipole and quadrupole channels.

The need to include multipole effects beyond the dipole approximation in the description of photoionization both at nonrelativistic and relativistic energies

We may ask what information characterizes low energy photoionization and what further information is accessible when photoionization is studied at higher energies, as in the X-ray and γ-ray regimes. For a “complete” experiment it is in fact not generally sufficient, even at low energy, to determine the energy dependence of the usual dipole parameters. Rather, such parameters should be determined for each multipole, or alternatively, the energy and angle dependence of angular distributions and polarization correlations. However it is known that in some circumstances, particularly for total cross sections from s-subshells, relativistic and higher multipole contributions tend to cancel well into the γ-ray regime, leaving a non-relativistic dipole description useful. Conversely, it has been shown that in some circumstances quadrupole effects remain of some importance at low energy in angular distributions, even for light elements and even at threshold. In general, and neglecting small relativistic effects, except for bound state normalization the high energy photoeffect matrix element is given by the matrix element in the nuclear point Coulomb potential. Consequently, at all energies the dependence of the photoionization matrix element on atomic properties is given by dipole and quadrupole terms, plus small octapole contributions, within an accuracy of 1%. This result, demonstrated within independent particle approximation, is expected to remain valid when many-electron correlations are included.

Chemical characterization of Hungarian brown coals by curie-point pyrolysis-low-energy electron impact mass spectrometry and multivariate analysis and by curie-point pyrolysis-gas chromatography-photoionization mass spectrometry

A series of Hungarian Miocene brown coals were analysed by pyrolysis-low-energy electron impact mass spectrometry and discriminant analysis. Correlation patterns in the mass peaks of these brown coals coincided with the geothermal history determined from the reflectivity of the macerals. Coal rank and the first discriminant function had a correlation coefficient of 0.96. Discriminant analysis also classified the coals according to their depositional environment. The mass peaks were deconvoluted using pyrolysis-gas chromatography-mass spectrometry data obtained under low-energy photoionization conditions. Several apparently homologous series of mass peaks in the pyrolysis-low-energy electron impact mass spectra were found to represent entirely different types of pyrolysis products. The low-energy fragmentation patterns of isoprenoid and alicyclic hydrocarbons are substantially different from the usual 70 eV electron impact spectra and may cause erroneous interpretation of non-deconvoluted pyrolysis-low-energy electron impact mass spectrometric data. The deconvoluted pyrolysis-low-energy electron impact mass spectrometric data resulted in a more detailed reconstruction of the original environments of deposition. Marine influences were deduced from several types of organic sulphur compounds. Markers for lignified wood, leaf and root remains and resins were present in the pyrolysates.

Low-energy photoionization of alkali-metal atoms: Relativistic random-phase-approximation calculations.

A complete theoretical survey of low-energy photoionization cross sections, angular asymmetry, and spin-polarization parameters for the ground states of the alkali-metal atoms Li, Na, K, Rb, and Cs, including virtual core excitations in a gauge-independent relativistic random-phase approximation (RRPA) is presented and compared with length- and velocity-form Dirac-Fock results and with available experimental data. The ab initio RRPA formulation substantially improves the agreement of cross sections with experiment at energies above the Cooper minimum.

Observation of interference between quadrupole and dipole transitions in low-energy (2-eV) photoionization from a sodium Rydberg state.

A measurement of the azimuthal dependence of the angular distribution of photoelectrons from ${13}^{3}$${\mathrm{D}}_{3/2}$-state sodium atoms aligned transversely to the direction of propagation of, and parallel to the direction of linear polarization of, 532-nm ionizing laser radiation, is described. The measured distribution is I(\ensuremath{\varphi})=1+0.026(6)cos\ensuremath{\varphi} where \ensuremath{\varphi}=0 is the direction of the laser beam. This departure from the symmetry I(\ensuremath{\varphi})=const predicted for a pure electric dipole transition is represented theoretically as due to a dipole-quadrupole interference term.

Multiconfiguration Tamm-Dancoff approximation applied to photoionization of the outer shells of Be and Mg.

A multiconfiguration version of the Tamm-Dancoff approximation (MCTD) is developed to study photoexcitation of atoms in cases where the simultaneous excitation of two atomic electrons is important. Starting from a multiconfiguration Dirac-Fock description of the ground state, the variational principle is applied to give equations for a final-state wave function consisting of both singly excited and doubly excited configurations. To illustrate the method, the MCTD equations are applied to determine the 2pns and 2pnd autoionizing resonances in the low-energy photoionization cross section of Be and the corresponding 3pns and 3pnd resonances in Mg. The predicted cross sections are found to be in good agreement with previous theoretical results as well as with the available experimental data.

Application of the relativistic local-density approximation to photoionization of the outer shells of neon, argon, krypton, and xenon

Parameters describing the low-energy photoionization of the outer shells of the rare gases Ne, Ar, Kr, and Xe have been calculated using a relativistic generalization of the time-dependent local-density approximation. The resulting photoionization cross sections, angular-distribution asymmetry parameters, and spin polarization parameters are presented here. The results are found to be in good agreement with those from the relativistic random-phase approximation (RRPA), even though the present theory is simpler, formally and computationally, than the latter. These results, and those of the RRPA, are generally in good agreement with experiment, illustrating the utility of the present approach to photoionization, especially for heavy atoms in which both relativistic and correlation effects are important.

Photoelectron branching ratios and angular distributions for the valence levels of SF6in the range16≤hν≤30eV

Photoelectron branching ratios and angular distributions have been measured for the six outermost levels of S${\mathrm{F}}_{6}$ in the range $16\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}30$ eV with the use of synchrotron radiation. These results are discussed in the framework of the large variety of experimental and theoretical studies of S${\mathrm{F}}_{6}$ with the dual objective of resolving the long-standing problem of the ordering of the valence shells and of gaining some insight into the role of shape-resonant behavior in the low-energy photoionization continua of S${\mathrm{F}}_{6}$. These objectives are met with partial success in that we tentatively conclude the valence configuration $5{a}_{1g}^{2}4{t}_{1u}^{6}1{t}_{2g}^{6}3{e}_{g}^{4}(1{t}_{2u}^{6}+5{t}_{1u}^{6})1{t}_{1g}^{6}^{1}A_{1g}$ is most consistent with the available body of evidence and that it is plausible to invoke channel interaction near the strong resonant feature at $h\ensuremath{\nu}\ensuremath{\sim}23\ensuremath{-}24$ eV to help account for some of the apparently contradictory evidence. Further experimental and theoretical work is suggested to clarify these issues.