A. Przystawik

Ion induced snowballs as a diagnostic tool to investigate the caging of metal clusters in large helium droplets

Metal clusters embedded in ultracold helium nanodroplets are exposed to femtosecond laser pulses with intensities of 10(13)-10(14) W/cm2. The influence of the matrix on the ionization and fragmentation dynamics is studied by pump-probe time-of-flight mass spectrometry. Special attention is paid to the generation of helium snowballs around positive metal ions (Me(z+)He(N), z=1,2). Closings of the first and second helium shells are found for silver at N(1)=10,12 and N(2)=32,44, as well as for magnesium at N1=19-20. The distinct abundance enhancement of helium snowballs in the presence of isolated atoms and small clusters in the droplets is used as a diagnostics to explore the cage effect. For silver, a reaggregation of the clusters is observed at 30 ps after femtosecond laser excitation.

Thomson scattering from near-solid density plasmas using soft x-ray free electron lasers

We propose a collective Thomson scattering experiment at the VUV free electron laser facility at DESY (FLASH) which aims to diagnose warm dense matter at near-solid density. The plasma region of interest marks the transition from an ideal plasma to a correlated and degenerate many-particle system and is of current interest, e.g. in ICF experiments or laboratory astrophysics. Plasma diagnostic of such plasmas is a longstanding issue. The collective electron plasma mode (plasmon) is revealed in a pump-probe scattering experiment using the high-brilliant radiation to probe the plasma. The distinctive scattering features allow to infer basic plasma properties. For plasmas in thermal equilibrium the electron density and temperature is determined from scattering off the plasmon mode.

Steplike intensity threshold behavior of extreme ionization in laser-driven xenon clusters.

The generation of highly charged Xe(q+) ions up to q=24 is observed in Xe clusters embedded in helium nanodroplets and exposed to intense femtosecond laser pulses (λ=800  nm). Laser intensity resolved measurements show that the high-q ion generation starts at an unexpectedly low threshold intensity of about 10(14)  W/cm2. Above threshold, the Xe ion charge spectrum saturates quickly and changes only weakly for higher laser intensities. Good agreement between these observations and a molecular dynamics analysis allows us to identify the mechanisms responsible for the highly charged ion production and the surprising intensity threshold behavior of the ionization process.

Spectroscopy of silver dimers in triplet states

Silver dimers embedded in ultracold helium nanodroplets are ionized by two-photon excitation via a strong resonance which extends from 3.85 eV up to 4.1 eV. The corresponding photoelectron spectra reveal that the ionization threshold is shifted by more than 1.4 eV towards lower values when compared to the gas phase. This gives strong evidence that weakly bound dimers in the lowest lying triplet state are present, thus enabling convenient spectroscopy of the triplet Ag2. A comparison with predictions from theory allows an assignment of the structure in the spectra. The successful identification of triplet silver dimers embedded in helium droplets shows exemplarily that the formation of such weakly bound systems is not restricted to surface locations as with the alkalis, but represents a general feature of the ultracold helium droplet environment.

High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 1011 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH3I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.

Optimal control of the strong-field ionization of silver clusters in helium droplets

Optimal control techniques combined with femtosecond laser pulse shaping are applied to steer and enhance the strong-field induced emission of highly charged atomic ions from silver clusters embedded in helium nanodroplets. With light fields shaped in amplitude and phase we observe a substantial increase of the Ag{sup q+} yield for q>10 when compared to bandwidth-limited and optimally stretched pulses. A remarkably simple double-pulse structure, containing a low-intensity prepulse and a stronger main pulse, turns out to produce the highest atomic charge states up to Ag{sup 20+}. A negative chirp during the main pulse hints at dynamic frequency locking to the cluster plasmon. A numerical optimal control study on pure silver clusters with a nanoplasma model converges to a similar pulse structure and corroborates that the optimal light field adapts to the resonant excitation of cluster surface plasmons for efficient ionization.

Probing near-solid density plasmas using soft X-ray scattering

X-ray scattering using highly brilliant x-ray free-electron laser (FEL) radiation provides new access to probe free-electron density, temperature and ionization in near-solid density plasmas. First experiments at the soft x-ray FEL FLASH at DESY, Hamburg, show the capabilities of this technique. The ultrashort FEL pulses in particular can probe equilibration phenomena occurring after excitation of the plasma using ultrashort optical laser pumping. We have investigated liquid hydrogen and find that the interaction of very intense soft x-ray FEL radiation alone heats the sample volume. As the plasma establishes, photons from the same pulse undergo scattering, thus probing the transient, warm dense matter state. We find a free-electron density of (2.6 ± 0.2) × 1020 cm−3 and an electron temperature of 14 ± 3.5 eV. In pump–probe experiments, using intense optical laser pulses to generate more extreme states of matter, this interaction of the probe pulse has to be considered in the interpretation of scattering data. In this paper, we present details of the experimental setup at FLASH and the diagnostic methods used to quantitatively analyse the data.

Charging of metal clusters in helium droplets exposed to intense femtosecond laser pulses

We review the strong field (10(13)-10(16) W cm(-2)) laser excitation of metal clusters (Cd(N), Ag(N) and Pb(N)) embedded in He nanodroplets. Plasmon enhanced ionization obtained by stretching the laser pulses to several hundreds of femtoseconds or by using dual pulses with a suitable optical delay leads to a Coulomb explosion of highly charged atomic ions. The charging dynamics can be well described by corresponding semiclassical Vlasov simulations. The influence of the He environment on the ionization process and on the final charge distribution is discussed. Evidence is found that He(2+) is generated in collisions with highly charged metal ions. In contrast, singly and doubly charged ions with low recoil energies induce the formation of He snowballs with a distinct shell structure around the ion. Laser intensity thresholds for snowball formation and for the ionization of clusters are investigated by applying intensity selective scanning.

Photoelectron studies of neutral Ag3 in helium droplets

Photoelectron spectra of neutral silver trimers, grown in ultracold helium nanodroplets, are recorded after ionization with laser pulses via a strong optical resonance of this species. Varying the photon energy reveals that direct vertical two-photon ionization is hindered by a rapid relaxation into the lower edge of a long-living excited state manifold. An analysis of the ionization threshold of the embedded trimer yields an ionization potential of 5.74+/-0.09 eV consistent with the value found in the gas phase. The asymmetrical form of the electron energy spectrum, which is broadened toward lower kinetic energies, is attributed to the influence of the matrix on the photoionization process. The lifetime of the excited state was measured in a two-color pump-probe experiment to be 5.7+/-0.6 ns.

Femtosecond dynamics of correlated many-body states in C60 fullerenes

Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C60 by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of ab initio calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of fs. Energy dissipation towards nuclear degrees of freedom is studied with time-resolved techniques. The evaluation of the nonlinear autocorrelation trace gives a characteristic time constant of fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg–Teller coupling), and their interplay.