We report on the lasing in air and pure nitrogen gas pumped by a single 800 nm femtosecond laser pulse. Depending on gas pressure, incident laser power and beam convergence, different lasing lines are observed in the forward direction with rapid change of their relative intensities. The lines are attributed to transitions between vibrational and rotational levels of the first negative band of the singly charged nitrogen molecule-ion. We show that self-seeding plays an important role in the observed intensity changes.

Self-seeded lasing in ionized air pumped by 800 nm femtosecond laser pulses

An overview of both experimental and theoretical results in the field of resonant scattering of tunable soft and hard x-ray radiation is presented, with a main focus on the closely related processe ...

Dynamics of resonant x-ray and Auger scattering

Studies of photoemission processes induced by hard X-rays including production of energetic electrons have become feasible due to recent substantial improvement of instrumentation. Novel dynamical phenomena have become possible to investigate in this new regime. Here we show a significant change in Auger emission following 1s photoionization of neon, which we attribute to the recoil of the Ne ion induced by the emission of a fast photoelectron. Because of the preferential motion of the ionized Ne atoms along two opposite directions, an Auger Doppler shift is revealed, which manifests itself as a gradual broadening and doubling of the Auger spectral features. This Auger Doppler effect should be a general phenomenon in high-energy photoemission of both isolated atoms and molecules, which will have to be taken into account in studies of other recoil effects such as vibrational or rotational recoil in molecules, and may also have consequences in measurements in solids.

/pdf/atomic-auger-doppler-effects-upon-emission-of-fast-19c6wotork.pdf

Atomic Auger Doppler effects upon emission of fast photoelectrons

The angular distribution of electrons ionized from an atom by a chirped few-cycle attosecond pulse is analyzed using perturbation theory (PT), keeping terms in the transition amplitude up to second order in the pulse electric field. The dependence of the asymmetry in the ionized electron distributions on both the chirp and the carrier-envelope phase (CEP) of the pulse are explained using a simple analytical formula that approximates the exact PT result. This approximate formula (in which the chirp dependence is explicit) reproduces reasonably well the chirp-dependent oscillations of the electron angular distribution asymmetries found numerically by Peng et al. [Phys. Rev. A 80, 013407 (2009)]. It can also be used to determine the chirp rate of the attosecond pulse from the measured electron angular distribution asymmetry.

/pdf/perturbation-theory-analysis-of-ionization-by-a-chirped-few-1w814ovzjz.pdf

Perturbation-theory analysis of ionization by a chirped few-cycle attosecond pulse

The subject of this theoretical study is the role ofelectronic structure as well as of rotational and vibrational motionson interactions between molecules and electromagnetic radiation,ranging from optical to x-ray. The thesis concerns both linear and nonlinear regimes of the light-matter interaction. The first part of the thesis is devoted to propagation of opticalpulses with different time-structure through various nonlinear absorbers.First we explain the double-exponential decay of fluorescence caused by photobleaching of pyrylium salt irradiated by a train of short (100 fs) optical pulses. The main reason for this effect is the transversal inhomogeneity of the light beam which makes the dynamics of the photobleaching differ in the core of the pulse and on its periphery. We also explore the optical power limitingof C60 fullerene irradiated by either microsecond optical pulses or a picosecond pulse trains. Enhancement of nonlinear absorption is caused by strong triplet-triplet absorption that becomes important due toelongation of the interaction time.Here we show the importance of the repetitionrate for the optical power limiting performance.The second part of the thesis addresses the interaction of optical and x-rayfields with rotational degrees of freedom of molecules. In this part the main attention is paid to the rotational heating caused by the recoil, experienced by molecules due to the ejection of photoelectrons. We have quantitatively explained two qualitatively different experiments with the N2 molecule.We predict the interference modulation of the recoil-induced shift,which is a shift of the photoelectron line caused by the rotational recoil effect, as a function of the photon energy.The developed theory also explains the rotational heating ofmolecules observed in the optical fluorescence induced by x-ray radiation.Based on this explanation, we suggest a new scheme of the optical fluorescence induced by x-rays that allows to detect the recoil effect via the recoil-inducedsplitting of the optical resonance.The last part of the thesis focuses on multi-mode nuclear dynamics of the resonant Auger scattering from the C2H2 molecule, that was the subject of a recent experimental study.Here we develop a theory that explains the observed vibrationalscattering anisotropy. We have found that three qualitatively different mechanisms are responsible for this phenomenon. The first mechanism is the interference of the direct and resonance scattering channels. The second mechanismis the interference of the resonant scattering channels through core excitedstate with the orthogonal orientation of the vibrational modes of core excitedstate. The Young's double slit like interference of the quantum pathways through the double-well potential of the bending motion of core excited state is the third mechanism of the vibrational scattering anisotropy.

Photoelectron-recoil-induced rotational excitation of the B 2Σ+u state in N2+

We report on the coherent control of the ultrafast ionization and fragmentation dynamics of the bromochloroalkanes C2H4BrCl and C3H6BrCl using shaped femtosecond laser pulses. In closed-loop control experiments on bromochloropropane (C3H6BrCl) the fragment ion yields of CH2Cl+, CH2Br+, and C3H3+ are optimized with respect to that of the parent cation C3H6BrCl+. The fragment ion yields are recorded in additional experiments in order to reveal the energetics of cation fragmentation, where laser-produced plasma radiation is used as a tunable pulsed nanosecond vacuum ultraviolet radiation source along with photoionization mass spectrometry. The time structure of the optimized femtosecond laser pulses leads to a depletion of the parent ion and an enhancement of the fragment ions, where a characteristic sequence of pulses is required. Specifically, an intense pump pulse is followed by a less intense probe pulse where the delay is 0.5 ps. Similarly optimized pulse shapes are obtained from closed-loop control experiments on bromochloroethane (C2H4BrCl), where the fragment ion yield of CH2Br+ is optimized with respect to that of C2H4BrCl+ as well as the fragment ion ratios C2H2+/CH2Br+ and C2H3+/C2H4Cl+. The assignment of the underlying control mechanism is derived from one-color 804 nm pump–probe experiments, where the yields of the parent cation and several fragments show broad dynamic resonances with a maximum at Δt = 0.5 ps. The experimental findings are rationalized in terms of dynamic ionic resonances leading to an enhanced dissociation of the parent cation and some primary fragment ions.

Coherent control of the ultrafast dissociative ionization dynamics of bromochloroalkanes.

Photoionization and autoionization of electronically excited atomic oxygen O(D1) are investigated in the energy range between 12 and 26eV using tunable laser-produced plasma radiation in combination with time-of-flight mass spectrometry. A broad, asymmetric, and intense feature is observed that is peaking at 20.53±0.05eV. It is assigned to the 2s22p4(D1)→2s12p5(P1) transition, which subsequently autoionizes by a Coster-Kronig transition, as predicted by the previous theoretical work [K. L. Bell et al., J. Phys. B 22, 3197 (1989)]. Specifically, the energy of the unperturbed transition occurs at 20.35±0.07eV. Its shape is described by a Fano profile revealing a q parameter of 4.25±0.8 and a width of γ=2.2±0.15eV. Absolute photoionization cross section σ is derived, yielding σ=22.5±2.3Mb at the maximum of the resonance. In addition, weak contributions to the O(D1) yield from dissociative ionization originating from molecular singlet oxygen [O2(Δg1)] are identified as well. Possible applications of the 2s22p...

Inner-valence photoionization of O(D1): Experimental evidence for the 2s22p4(D1)→2s12p5(P1) transition

We report on fluorescence spectra of N2+(B Σ2u+)→N2+(X Σ2g+) obtained from multiphoton ionization of molecular nitrogen by 804 nm femtosecond laser pulses. The analysis of the fluorescence spectra reveals that the vibrational levels v=0 and v=4 in the B Σ2u+-state of N2+ are primarily populated. The rotational state distribution of N2+(B Σ2u+, v=0) is determined from the rotationally resolved fluorescence spectra. It is demonstrated that the linear chirp of the 804 nm femtosecond laser pulse has a strong influence on the rotational state distribution of the vibrational ground state of the molecular cation N2+(B Σ2u+, v=0). Possible mechanisms leading to the experimental results are discussed. The particular population of the vibrational levels as well as the linear chirp dependence of the fluorescence signal gives evidence for the importance of a resonant intermediate state. The N2 a Π1-state is likely involved in a resonant multiphoton excitation process. This permits to selectively control the rotationa...

Chirped pulse multiphoton ionization of nitrogen: Control of selective rotational excitation in N2+(B Σ2u+)

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Photoelectron-recoil-induced rotational excitation of the B 2Σ+u state in N2+

Coherent control of the ultrafast dissociative ionization dynamics of bromochloroalkanes.

Inner-valence photoionization of O(D1): Experimental evidence for the 2s22p4(D1)→2s12p5(P1) transition

Chirped pulse multiphoton ionization of nitrogen: Control of selective rotational excitation in N2+(B Σ2u+)