G. Urbasch

Correlated electron emission in multiphoton double ionization

Electronic correlations govern the dynamics of many phenomena in nature, such as chemical reactions and solid state effects, including superconductivity. Such correlation effects can be most clearly investigated in processes involving single atoms. In particular, the emission of two electrons from an atom—induced by the impact of a single photon, a charged particle or by a short laser pulse—has become the standard process for studies of dynamical electron correlations. Atoms and molecules exposed to laser fields that are comparable in intensity to the nuclear fields have extremely high probabilities for double ionization; this has been attributed to electron–electron interaction. Here we report a strong correlation between the magnitude and the direction of the momentum of two electrons that are emitted from an argon atom, driven by a femtosecond laser pulse (at 38 TW cm-2). Increasing the laser intensity causes the momentum correlation between the electrons to be lost, implying that a transition in the laser–atom coupling mechanism takes place.

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toFthres = 172 mJ/cm2. The ablation rate per pulse is measured as a function of intensity in the range of 5 × 109 to 2 × 1011 W/cm2 and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.

Analysis of Chirality by Femtosecond Laser Ionization Mass Spectrometry

Recent progress in the field of chirality analysis employing laser ionization mass spectrometry is reviewed. Emphasis is given to femtosecond (fs) laser ionization work from the authors group. We begin by reviewing fundamental aspects of determining circular dichroism (CD) in fs-laser ionization mass spectrometry (fs-LIMS) discussing an example from the literature (resonant fs-LIMS of 3-methylcyclopentanone). Second, we present new data indicating CD in non-resonant fs-LIMS of propylene oxide.

Circular Dichroism in Ion Yields of Femtosecond-Laser Mass Spectrometry

Telling the difference quickly: Femtosecond laser pulses are not only suitable to distinguish structural isomers. They also provide access to the distinction of enantiomers by combination of circular dichroism and mass spectrometry [picture: see text].

Sequential and nonsequential contributions to double ionization in strong laser fields

We demonstrate experimentally the difference between a sequential interaction of a femtosecond laser field with two electrons and a nonsequential process of double ionization mediated by electron-electron correlation. This is possible by observing the momentum distribution of doubly charged argon ions created in the laser field. In the regime of laser intensities where the nonsequential process dominates, an increase in laser power leads to an increase in the observed ion momenta. At the onset of the sequential process, however, a higher laser power leads to colder ions. The momentum distributions of the ions from the sequential process can be modelled by convolving the single-ionization distribution with itself.

Unusual mechanism for H3+ formation from ethane as obtained by femtosecond laser pulse ionization and quantum chemical calculations

The formation of H(3)(+) from saturated hydrocarbon molecules represents a prototype of a complex chemical process, involving the breaking and the making of chemical bonds. We present a combined theoretical and experimental investigation providing for the first time an understanding of the mechanism of H(3)(+) formation at the molecular level. The experimental approach involves femtosecond laser pulse ionization of ethane leading to H(3)(+) ions with kinetic energies on the order of 4 to 6.5 eV. The theoretical approach involves high-level quantum chemical calculation of the complete reaction path. The calculations confirm that the process takes place on the potential energy surface of the ethane dication. A surprising result of the theoretical investigation is, that the transition state of the process can be formally regarded as a H(2) molecule attached to a C(2)H(4)(2+) entity but IRC calculations show that it belongs to the reaction channel yielding C(2)H(3)(+) + H(3)(+). Experimentally measured kinetic energies of the correlated H(3)(+) and C(2)H(3)(+) ions confirm the reaction path suggested by theory.

Two-photon fluorescence and femtosecond two-photon absorption studies of MeLPPP, a ladder-type poly(phenylene) with low intra-chain disorder

Abstract MeLPPP is a ladder-type conjugated poly(phenylene) with particularly low intra-chain disorder and inhomogeneous broadening of only 20 meV. The two-photon excited state (mA g ), investigated by two-photon fluorescence excitation spectroscopy, is much broader [ E (mA g )≈3.4±0.2 eV] and without well-resolved vibronic structure. This agrees with a broad underlying feature in the electroabsorption spectra of MeLPPP. Femtosecond pump–probe measurements reveal transient absorption into the mA g state within the duration of the pump pulse (140 fs) and rapid subsequent decay ( τ ≈150 fs) to the 1B u exciton, with photoluminescence clearly visible 600 fs after excitation. A value of the nonlinear absorption coefficient β ≈11 cm 2 /GW is calculated for the two-photon absorption, mA g ←1A g .

Circular Dichroism in Ion Yields in Multiphoton Ionization of (R)-Propylene Oxide Employing Femtosecond Laser Pulses

Abstract The feasibility of measuring circular dichroism (CD) in ion yields is demonstrated in multiphoton ionization employing femtosecond laser radiation for the first time. CD values for (R)-propylene oxide are reported at 738 nm, 810 nm and 878 nm. The data suggest the possibility of resonance contributions at 810 nm. At the other wavelengths the CD appears to originate from non-resonant multiphoton ionization. Possible implications of measuring non-resonant CD are discussed.

Polarized Photoluminescence and Spectral Narrowing in an oriented Polyfluorene Thin Film

The oriented polyfluorene 1 is a promising candidate for a blue polarized emitter in organic optoelectronic devices. By examination of the anisotropic luminescence of thin films of 1, polarization-dependent spectral narrowing was found at high excitation densities, which is of particular interest in reducing the threshold for optically pumped polymer lasers. Femtosecond pump–probe experiments show optical gain at this spectral position.

Phase control of molecular fragmentation with a pair of femtosecond-laser pulses

We demonstrate the control of molecular fragmentation of o-xylene (C(8)H(10)) on a femtosecond time scale in two-pulse measurements with a pair of femtosecond-laser pulses. Parent and fragment-ion yields were recorded as a function of interpulse delays, i.e., different relative phases of the excitation pulses. The experiments revealed different fragmentation mechanisms in the temporal region of direct overlapping pulses and for separated pulses. For overlapping pulses all ion yields followed the excitation intensity which oscillated as a function of interpulse delay due to the change of constructive and destructive interference of the light fields. For larger delays, in particular, the oscillations of the C(+) and CH(3) (+) fragment-ion yield showed a significant deviation from each other. The results are interpreted as a manifestation of optical phase-dependent electronic excitations mapped onto the nuclear fragmentation dynamics.