Denis Cubaynes

Spectroscopic characterization of vacuum ultraviolet free electron laser pulses

Because of the stochastic nature of self-amplified spontaneous emission (SASE), it is crucial to measure for single pulses the spectral characteristics of ultrashort pulses from the vacuum ultraviolet free electron laser (FLASH) at DESY, Germany. To meet this particular challenge, we have employed both photon and photoelectron spectroscopy. Each FEL pulse is composed of an intense and spectrally complex fundamental, centered at a photon energy of about 38.5 eV, with a bandwidth of 0.5% accompanied by higher harmonics, each carrying an intensity of typically 0.3 to 0.6% of that of the fundamental. The correlation between the harmonics and the fundamental is in remarkable agreement with a simple statistical model of SASE FEL radiation.

Probing the KII 3p54p fine structure by photoelectron spectroscopy of laser-excited potassium

Photoelectron spectra of atomic potassium excited by laser optical pumping into the 3p 6 4p 2 P1/2 and 2 P3/2 states are measured with high-energy resolution. The relative intensities of the 3p 5 4p fine-structure lines depend strongly on the initial excitation to one of the 4p spin–orbit components. Similar to the case of sodium, dynamically and quasiforbidden transitions are observed in the photoelectron spectra of potassium. The theoretical predictions of the generalized geometrical model are in excellent agreement with the experimental data.

Generalized geometrical model for photoionization of polarized atoms: II. Magnetic dichroism in the 3p photoemission from the K 3p64s 2S1/2 ground state

The generalized geometrical model for photoionization from polarized atoms is extended to include mixing of configurations in the initial atomic and/or the final photoion states. The theoretical results for angle-resolved linear and circular magnetic dichroism are in good agreement with new high-resolution photoelectron data for 3p−1 photoionization of potassium atoms polarized in the K 3p64s 2S1/2 ground state by laser optical pumping.

The generalized geometrical model for the photoionization of polarized atoms: application to linear dichroism in the 2p photoemission from Na 32S and Na* 32P initial states

The generalized geometrical model utilized for the interpretation of photoelectron spectra is extended to the photoionization from polarized atoms. Within this model, the linear magnetic and the linear alignment dichroisms in the photoemission are calculated. The theoretical results are compared to those obtained from high-resolution angle-resolved photoelectron spectra, which were recorded upon 2p−1 photoionization of sodium atoms oriented in the Na 2p63s 2S1/2 ground and Na* 2p63p 2P1/2 laser-excited states or aligned in the Na* 2p63p 2P3/2 state. Good overall agreement is found between theory and experiment.

Absolute photoionization cross sections along the Xe isonuclear sequence: Xe{sup 3+} to Xe{sup 6+}

Absolute photoionization cross sections of Xe{sup 3+} to Xe{sup 6+} ions have been measured over an extended photon energy range from the first ionization threshold to 160 eV. Single and double photoionization cross sections have been obtained. In the case of Xe{sup 3+} and Xe{sup 6+} ions, two completely independent setups, both based on the merged beam technique, have been used. Multiconfiguration Dirac-Fock calculations were performed to interpret the spectra in the region of 4d electron excitations. The partition of the oscillator strength between the discrete photoexcitation and the direct photoionization channels in the 4d subshell has allowed us to follow precisely the collapse of the 4f wave function for these ions of the isonuclear series of Xe.

Photoexcitation and decay of hollow lithium states

Photoelectron measurements have been carried out with three different sources of synchrotron radiation for studying photoexcitation and autoionization of hollow lithium atoms. The comparison of the three sets of data demonstrate the need of a third generation storage ring for a better understanding of atomic structures and dynamical photoionization processes of atomic systems. Partial cross sections have been measured with high spectral resolution at ALS, for hollow atoms formed by triple excitation of lithium atoms in the ground- or first optically excited states, providing the first definitive test of advanced ab initio calculations for this highly excited four-body atomic system. The existence of Rydberg series within the many observed hollow lithium states is demonstrated.