Photoionization of helium atoms irradiated with intense vacuum ultraviolet free-electron laser light. Part II. Theoretical modeling of multi-photon and single-photon processes

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOWe consider the problem of a helium atom under the radiation field of the DESY vacuum ultraviolet (VUV) free electron laser (FEL) (Phase I, h congruent to 13 eV). We find by solving numerically the time-dependent Schrodinger equation, that there is a large probability for resonant two-photon excitation from the ground state into a low kinetic energy state just above the first He ionization threshold. From this it is possible to go into another quasi-free state higher up, by resonant absorption of an additional photon. There is no double ionization of He. These results are in general agreement with the He photoelectron and time-of-flight (TOF) spectra recorded on March 2002, in the last week of the DESY VUV FEL Phase I operation. A detailed report on the experiments is given in a companion paper.We consider the problem of a helium atom under the radiation field of the DESY vacuum ultraviolet (VUV) free electron laser (FEL) (Phase I, h congruent to 13 eV). We find by solving numerically the time-dependent Schrodinger equation, that there is a large probability for resonant two-photon excitation from the ground state into a low kinetic energy state just above the first He ionization threshold. From this it is possible to go into another quasi-free state higher up, by resonant absorption of an additional photon. There is no double ionization of He. These results are in general agreement with the He photoelectron and time-of-flight (TOF) spectra recorded on March 2002, in the last week of the DESY VUV FEL Phase I operation. A detailed report on the experiments is given in a companion paper.72219CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOSem informaçãoWe acknowledge CNPq/Brasil and DLR/Germany for continued support of a fruitful collaboration on VUV and soft X-ray spectroscopy between the Brazilian synchrotron source LNLS, Instituto de Fisica Gleb Wataghin at UNICAMP, both in Campinas Brazil, and HASYLAB at DESY, Hamburg, Germany. The first author dedicates this work to the memory of his deceased wife Sandra. He is also grateful to Professor David M. Bishop (Dept of Chem., University of Ottawa, Canada) who discussed with him the intricacies of helium states, a few years ago. The extensive help given by Marcelo Juni Ferreira (LNLS) for the installation of Cygwin is gratefully acknowledged. Finally, T.M. wants to thank Hugo van der Hart (Belfast) for helpful calculations and discussions in the early phase of the presente work

Interface Excitons in Krmnen Clusters : The Role of Electron Affinity in the Formation of Electronic Structure

The formation of the electronic structure of small Kr_m clusters (m<150) embedded inside Ne_N clusters (1200<N<7500) has been investigated with the help of fluorescence excitation spectroscopy using synchrotron radiation. Electronically excited states, assigned to excitons at the Ne/Kr interface, 1i and 1'i were observed. The absorption bands, which are related to the lowest spin-orbit split atomic Kr 3P1 and 1P1 states, initially appear and shift towards lower energy when the krypton cluster size m increases. The characteristic bulk 1t and 1't excitons appear in the spectra, when the cluster radius exceeds some critical value, R_cl>Delta_1i . Kr clusters comprising up to 70 atoms do not exhibit bulk absorption bands. We suggest that this is due to the penetration of the interface excitons into the Kr_m cluster volume, because of the negative electron affinity of surrounding Ne atoms. From the energy shift of the interface absorption bands with cluster size an unexpectedly large penetration depth of delta_1i =7.0+/-0.1 A is estimated, which can be explained by the interplay between the electron affinities of the guest and the host cluster

Reply To "comment On 'photoionization Of Helium Atoms Irradiated With Intense Vacuum Ultraviolet Free-electron Laser Light. Part I. Experimental Study Of Multiphoton And Single-photon Processes'"

We do not agree with the conclusion of the Comment by Charalambidis questioning our observation of two-photon ionization of helium by intense radiation with 13 eV photons from a vuv free-electron laser. Two-photon ionization is clearly established by the detection of low-energy photoelectrons at ∼1.7 eV, which agrees very well with the expected energy for a two-photon ionization process. © 2006 The American Physical Society.743Laarmann, T., De Castro, A.R.B., Schulz, J., Wabnitz, H., Möller, T., (2005) Phys. Rev. A, 72, p. 023409. , PLRAAN. 1050-2947. 10.1103/PhysRevA.72.023409Ayvazyan, V., Baboi, N., Bohnet, I., Brinkmann, R., Castellano, M., Castro, P., Catani, L., Zapfe, K., (2002) Phys. Rev. Lett., 88, p. 104802. , PRLTAO. 0031-9007. 10.1103/PhysRevLett.88.104802Ayvazyan, V., Baboi, N., Bähr, J., Balandin, V., Beutner, B., Brandt, A., Bohnet, I., Schreiber H, -J., (2006) Eur. Phys. J. D, 37, p. 297. , EPJDF6. 1434-6060. 10.1140/epjd/e2005-00308-1Wabnitz, H., Bittner, L., De Castro, A.R.B., Döhrmann, R., Gürtler, P., Laarmann, T., Laasch, W., Yurkov, M., (2002) Nature (London), 420, p. 482. , NATUAS. 0028-0836. 10.1038/nature01197Laarmann, T., De Castro, A.R.B., Gürtler, P., Laasch, W., Schulz, J., Wabnitz, H., Möller, T., (2004) Phys. Rev. Lett., 92, p. 143401. , PRLTAO. 0031-9007. 10.1103/PhysRevLett.92.143401Laarmann, T., Rusek, M., Wabnitz, H., Schulz, J., De Castro, A.R.B., Gürtler, P., Laasch, W., Möller, T., (2005) Phys. Rev. Lett., 95, p. 063402. , PRLTAO. 0031-9007. 10.1103/PhysRevLett.95.063402Wabnitz, H., De Castro, A.R.B., Gürtler, P., Laarmann, T., Laasch, W., Schulz, J., Möller, T., (2005) Phys. Rev. Lett., 94, p. 023001. , PRLTAO. 0031-9007. 10.1103/PhysRevLett.94.023001Santra, R., Greene, C.H., (2004) Phys. Rev. A, 70, p. 053401. , PLRAAN 1050-2947 10.1103/PhysRevA.70.053401Tzallas, P., Charalambidis, D., Papadogiannis, N.A., Witte, K., Tsakiris, G.D., (2003) Nature (London), 426, p. 267. , NATUAS 0028-0836 10.1038/nature02091Papadogiannis, N.A., Nikolopoulos, L.A.A., Charalambidis, D., Tsakiris, G.D., Tzallas, P., Witte, K., (2003) Appl. Phys. B, 76, p. 721. , APDOEM 0946-2171Papadogiannis, N.A., Nikolopoulos, L.A.A., Charalambidis, D., Tsakiris, G.D., Tzallas, P., Witte, K., (2003) Phys. Rev. Lett., 90, p. 133902. , PRLTAO 0031-9007 10.1103/PhysRevLett.90.133902De Castro, A.R.B., Laarmann, T., Schulz, J., Wabnitz, H., Möller, T., (2005) Phys. Rev. A, 72, p. 023410. , PLRAAN. 1050-2947. 10.1103/PhysRevA.72.02341

Interaction of argon clusters with intense VUV-laser radiation: the role of electronic structure in the energy-deposition process

Sem informaçãoThe response of Ar clusters to intense vacuum-ultraviolet pulses is investigated with photoion spectroscopy By varying the laser wavelength, the initial excitation was either tuned to absorption bands of surface or bulk atoms of clusters. Multiple ionization is observed, which leads to Coulomb explosion. The efficiency of resonant 2-photon ionization for initial bulk and surface excitation is compared with that of the norresonant process at different laser intensities. The specific electronic structure of clusters plays almost no role in the explosion dynamics at a peak intensity larger than 1.8 X 10(12) W/cm(2). The inner ionization of atoms for resonant and nonresonant excitation is then saturated and the energy deposition is mainly controlled by the plasma heating rate. Molecular dynamics simulations indicate that standard collisional heating cannot fully account for the strong energy absorption.The response of Ar clusters to intense vacuum-ultraviolet pulses is investigated with photoion spectroscopy By varying the laser wavelength, the initial excitation was either tuned to absorption bands of surface or bulk atoms of clusters. Multiple ionization is observed, which leads to Coulomb explosion. The efficiency of resonant 2-photon ionization for initial bulk and surface excitation is compared with that of the norresonant process at different laser intensities. The specific electronic structure of clusters plays almost no role in the explosion dynamics at a peak intensity larger than 1.8 X 10(12) W/cm(2). The inner ionization of atoms for resonant and nonresonant excitation is then saturated and the energy deposition is mainly controlled by the plasma heating rate. Molecular dynamics simulations indicate that standard collisional heating cannot fully account for the strong energy absorption.921414Sem informaçãoSem informaçãoSem informaçãoWe thank A. Swiderski for technical support and the TTF team at DESY for providing the FEL beam and the photon diagnostics. This work was supported by DFG

Photoionization of helium atoms irradiated with intense vacuum ultraviolet free-electron laser light. Part I. Experimental study of multiphoton and single-photon processes

Sem informaçãoThe interaction of He atoms with intense vacuum-ultraviolet light of a free-electron laser is investigated using time-of-flight mass spectroscopy and photoelectron spectroscopy. The atoms were irradiated with 100 fs pulses at 95 nm wavelength, which corresponds to similar to 13 eV photon energy. The ionization of He atoms is observed at a peak intensity of 10(10)-10(13) W/cm(2), which is due both to nonlinear multiphoton ionization with the fundamental wavelength and single-photon ionization with third harmonic radiation of the free-electron laser. The observation of two sharp photoelectron peaks in the kinetic energy spectra, that are separated by the photon energy, is in agreement with the numerical solution of the time-dependent Schrodinger equation. The calculation was done using the fully quantized field and a limited but representative set of basis states. The ionization rate dependence on the laser peak intensity indicates that: (a) The low-energy peak in the photoelectron spectra is mainly due to two-photon absorption of the fundamental, but (b) the high-energy peak at 15.4 eV is probably due to third harmonic FEL radiation. The theoretically predicted contribution from three-photon absorption of the fundamental is of about the same order of magnitude and could not be separated from the third harmonic background signal. Particularly, the photoelectron spectra and He+ time-of-flight data give evidence that the intensity of third harmonic light is high enough to perform single-shot spectroscopy on gas phase samples.The interaction of He atoms with intense vacuum-ultraviolet light of a free-electron laser is investigated using time-of-flight mass spectroscopy and photoelectron spectroscopy. The atoms were irradiated with 100 fs pulses at 95 nm wavelength, which corresponds to similar to 13 eV photon energy. The ionization of He atoms is observed at a peak intensity of 10(10)-10(13) W/cm(2), which is due both to nonlinear multiphoton ionization with the fundamental wavelength and single-photon ionization with third harmonic radiation of the free-electron laser. The observation of two sharp photoelectron peaks in the kinetic energy spectra, that are separated by the photon energy, is in agreement with the numerical solution of the time-dependent Schrodinger equation. The calculation was done using the fully quantized field and a limited but representative set of basis states. The ionization rate dependence on the laser peak intensity indicates that: (a) The low-energy peak in the photoelectron spectra is mainly due to two-photon absorption of the fundamental, but (b) the high-energy peak at 15.4 eV is probably due to third harmonic FEL radiation. The theoretically predicted contribution from three-photon absorption of the fundamental is of about the same order of magnitude and could not be separated from the third harmonic background signal. Particularly, the photoelectron spectra and He+ time-of-flight data give evidence that the intensity of third harmonic light is high enough to perform single-shot spectroscopy on gas phase samples.72218Sem informaçãoSem informaçãoSem informaçãoWe thank A. Swiderski for technical support in the design and construction of the experiment and the TTF team at DESY for running the accelerator, working on the FEL and the photon diagnostics. This work was supported by the DFG

Multiple ionization of rare gas atoms irradiated with intense VUV radiation

Sem informaçãoThe interaction of intense vacuum-ultraviolet radiation from a free-electron laser with rare gas atoms is investigated. The ionization products of xenon and argon atomic beams are analyzed with time-of-flight mass spectroscopy. At 98 nm wavelength and similar to10(13) W/cm(2) multiple charged ions up to Xe6+ (Ar4+) are detected. From the intensity dependence of multiple charged ion yields the mechanisms of multiphoton processes were derived. In the range of similar to10(12)-10(13) W/cm(2) the ionization is attributed to sequential multiphoton processes. The production of multiple charged ions saturates at 5-30 times lower power densities than at 193 and 564 nm wavelength, respectively.The interaction of intense vacuum-ultraviolet radiation from a free-electron laser with rare gas atoms is investigated. The ionization products of xenon and argon atomic beams are analyzed with time-of-flight mass spectroscopy. At 98 nm wavelength and similar to10(13) W/cm(2) multiple charged ions up to Xe6+ (Ar4+) are detected. From the intensity dependence of multiple charged ion yields the mechanisms of multiphoton processes were derived. In the range of similar to10(12)-10(13) W/cm(2) the ionization is attributed to sequential multiphoton processes. The production of multiple charged ions saturates at 5-30 times lower power densities than at 193 and 564 nm wavelength, respectively.94214Sem informaçãoSem informaçãoSem informaçãoWe thank A. Swiderski for technical support in the design and construction of the cluster experiment and the TTF team at DESY for providing the FEL beam and the photon diagnostics. One of the authors (H.W.) is grateful to Peter Lambropoulos for helpful comments. In addition, we thank R. Santra for making calculations available prior to publication

Human primary endothelial cells are impaired in nucleotide excision repair and sensitive to benzo[a]pyrene compared with smooth muscle cells and pericytes

The endothelium represents the inner cell layer of blood vessels and is supported by smooth muscle cells and pericytes, which form the vessel structure. The endothelium is involved in the pathogenesis of many diseases, including the development of atherosclerosis. Due to direct blood contact, the blood vessel endothelium is inevitably exposed to genotoxic substances that are systemically taken up by the body, including benzo[a]pyrene, which is a major genotoxic component in cigarette smoke and a common environmental mutagen and human carcinogen. Here, we evaluated the impact of benzo[a]pyrene diol epoxide (BPDE), which is the reactive metabolite of benzo[a]pyrene, on the three innermost vessel cell types. Primary human endothelial cells (HUVEC), primary human smooth muscle cells (HUASMC) and primary human pericytes (HPC) were treated with BPDE, and analyses of cytotoxicity, cellular senescence and genotoxic effects were then performed. The results showed that HUVEC were more sensitive to the cytotoxic activity of BPDE than HUASMC and HPC. We further show that HUVEC display a detraction in the repair of BPDE-induced adducts, as determined through the comet assay and the quantification of BPDE adducts in post-labelling experiments. A screening for DNA repair factors revealed that the nucleotide excision repair (NER) proteins ERCC1, XPF and ligase I were expressed at lower levels in HUVEC compared with HUASMC and HPC, which corresponds with the impaired NER-mediated removal of BPDE adducts from DNA. Taken together, the data revealed that HUVEC exhibit an unexpected DNA repair-impaired phenotype, which has implications on the response of the endothelium to genotoxicants that induce bulky DNA lesions, including the development of vascular diseases resulting from smoking and environmental pollution

Ultrafast X-ray scattering of xenon nanoparticles: imaging transient states of matter

Sem informaçãoFemtosecond x-ray laser flashes with power densities of up to 10(14) W/cm(2) at 13.7 nm wavelength were scattered by single xenon clusters in the gas phase. Similar to light scattering from atmospheric microparticles, the x-ray diffraction patterns carry information about the optical constants of the objects. However, the high flux of the x-ray laser induces severe transient changes of the electronic configuration, resulting in a tenfold increase of absorption in the developing nanoplasma. The modification in opaqueness can be correlated to strong atomic charging of the particle leading to excitation of Xe4+. It is shown that single-shot single-particle scattering on femtosecond time scales yields insight into ultrafast processes in highly excited systems where conventional spectroscopy techniques are inherently blind.Femtosecond x-ray laser flashes with power densities of up to 10(14) W/cm(2) at 13.7 nm wavelength were scattered by single xenon clusters in the gas phase. Similar to light scattering from atmospheric microparticles, the x-ray diffraction patterns carry information about the optical constants of the objects. However, the high flux of the x-ray laser induces severe transient changes of the electronic configuration, resulting in a tenfold increase of absorption in the developing nanoplasma. The modification in opaqueness can be correlated to strong atomic charging of the particle leading to excitation of Xe4+. It is shown that single-shot single-particle scattering on femtosecond time scales yields insight into ultrafast processes in highly excited systems where conventional spectroscopy techniques are inherently blind.108915Sem informaçãoSem informaçãoBMBF [05KS4KT1, 05KS7KT2]HGF Virtuelles Institut [VH-VI-103, VH-VI-302]Sem informaçãoWe would like to thank all staff at FLASH for their outstanding support. Funding is acknowledged from BMBF 05KS4KT1 and 05KS7KT2, as well as HGF Virtuelles Institut VH-VI-103 and VH-VI-302