B. Feuerstein

Correlated Multielectron Dynamics in Ultrafast Laser Pulse Interactions with Atoms

We present the results of the detailed experimental study of multiple ionization of Ne and Ar by 25 and 7 fs laser pulses. Whereas in multiple ionization of Ar different mechanisms, involving field ionization steps and recollision-induced excitations, play a role, for Ne only one channel, where the highly correlated instantaneous emission of up to four electrons is triggered by a recollisional electron impact, is found to be important. Using few-cycle pulses we are able to suppress those processes that occur on time scales longer than one laser cycle.

Resonant structures in the low-energy electron continuum for single ionization of atoms in the tunnelling regime

We present high-resolution fully differential experimental data on single ionization of He, Ne and Ar by ultra-short (25 fs, 6 fs) 795 nm laser pulses at intensities 0.15–2.0 × 1015 W cm−2. We show that the ATI-like pattern can survive deep in the tunnelling regime and that the atomic structure plays an important role in the formation of the low-energy photoelectron spectra even at high intensities. The absence of ponderomotive shifts, the splitting of the peaks and their degeneration for few-cycle pulses indicate that the observed structures originate from a resonant process.

Assessing a tornado climatology from global tornado intensity distributions

Recent work demonstrated that the shape of tornado intensity distributions from various regions worldwide is well described by Weibull functions. This statistical modeling revealed a strong correlation between the fit parameters c for shape and b for scale regardless of the data source. In the present work it is shown that the quality of the Weibull fits is optimized if only tornado reports of F1 and higher intensity are used and that the c–b correlation does indeed reflect a universal feature of the observed tornado intensity distributions. For regions with likely supercell tornado dominance, this feature is the number ratio of F4 to F3 tornado reports R(F4/F3) 0.238. The c–b diagram for the Weibull shape and scale parameters is used as a climatological chart, which allows different types of tornado climatology to be distinguished, presumably arising from supercell versus nonsupercell tornadogenesis. Assuming temporal invariance of the climatology and using a detection efficiency function for tornado observations, a stationary climatological probability distribution from large tornado records (U.S. decadal data 195099) is extracted. This can be used for risk assessment, comparative studies on tornado intensity distributions worldwide, and estimates of the degree of underreporting for areas with poor databases. For the 1990s U.S. data, a likely tornado underreporting of the weak events (F0, F1) by a factor of 2 can be diagnosed, as well as asymptotic climatological c,b values of c 1.79 and b 2.13, to which a convergence in the 1950–99 U.S. decadal data is verified.

Atomic structure dependence of nonsequential double ionization of He, Ne and Ar in strong laser pulses

The ion momentum spectra for nonsequential double ionization of rare gases (He, Ne and Ar) in 23 fs 795 nm laser pulses were measured in the intensity range 0.075–1.25 PW cm−2. In the studies published, confusing differences in the shape of momentum distributions among different targets are consistently explained within a recollision scenario: excitation during recollision plus subsequent field ionization, not implemented in most theoretical approaches, evidently plays a decisive role for He and Ar nonsequential double ionization whereas it plays only a minor role for Ne.

Laser-induced non-sequential double ionization investigated at and below the threshold for electron impact ionization

We use correlated electron–ion momentum measurements to investigate laser-induced non-sequential double ionization of Ar and Ne. Light intensities are chosen in a regime at and below the threshold where, within the rescattering model, electron impact ionization of the singly charged ion core is expected to become energetically forbidden. Yet we find Ar2+ ion momentum distributions and an electron–electron momentum correlation indicative of direct impact ionization. Within the quasistatic model this may be understood by assuming that the electric field of the light wave reduces the ionization potential of the singly charged ion core at the instant of scattering. The width of the projection of the ion momentum distribution onto an axis perpendicular to the light beam polarization vector is found to scale with the square root of the peak electric field strength in the light pulse. A scaling like this is not expected from the phase space available after electron impact ionization. It may indicate that the electric field at the instant of scattering is usually different from zero and determines the transverse momentum distribution. A comparison of our experimental results with several theoretical results is given.

Fragmentation dynamics of molecular hydrogen in strong ultrashort laser pulses

We present the results of a systematic experimental study of dissociation and Coulomb explosion of molecular hydrogen induced by intense ultrashort (7–25 fs) laser pulses. Using coincident recoil-ion momentum spectroscopy we can distinguish the contributions from dissociation and double ionization even if they result in the same kinetic energies of the fragments. The dynamics of all fragmentation channels drastically depends on the pulse duration, and for 7 fs pulses becomes extremely sensitive to the pulse shape.

Wavelength dependence of sub-laser-cycle few-electron dynamics in strong-field multiple ionization

Recoil-ion momentum distributions for double and triple ionization of Ne and Ar, as well as for double ionization of N2 molecule by intense (0.3–0.5 PW cm-2), short (~35–40 fs) laser pulses have been recorded in a so far unexplored long laser-wavelength regime at 1300 nm. Compared to earlier results at 800 nm, the direct (e, ne) ionization pathway during recollision is strongly enhanced manifesting itself in a pronounced double-hump structure in the longitudinal ion momentum spectra not only for Ne, but also surprisingly distinct for Ar and, found for the first time, for molecules. Observed wavelength dependence of the sub-laser-cycle correlated few-electron dynamics might be of paramount importance for possible future applications in attosecond science, in particular, for imaging of ultrafast molecular processes via recollision-induced fragmentation.

Comments on “A Simple Model for Simulating Tornado Damage in Forests”

Holland et al. (2006) present a very interesting study on development and evaluation of a simple analytical model of tornado vortex flow and its impact on specified forest configurations. The authors also make reference to earlier work by Johannes Peter Letzmann (1885–1971) on near-surface tornado wind fields, dating back to 1923 and reviewed, for example, by Peterson (1992a). The authors are correct to say that Letzmann did not include information on the physics of tree response (which was unavailable in his time), even though he considered the question of whether and how twisted tree snapping occurred or how the observed tree damage should be interpreted. However, some other statements by Holland et al. (2006) about Letzmann’s work can be misleading. The review by Peterson (1992a) alone is certainly not sufficient to assess fully the analytical model developed by Letzmann (1923) in his Ph.D. thesis and later summarized in a journal article (Letzmann 1925). It appears as if Holland et al. (2006), based on the limited information they had available on Letzmann’s model, reinvented parts of it. Thus it comes as no surprise that some of Holland et al.’s results “are somewhat analogous to the hand-drawn diagrams of Letzmann” (p. 1598)—the underlying model is the same. The fuzzy wording by Holland et al. (2006) may have been influenced by their references: Letzmann (1925) was cited by Hall and Brewer (1959), yet they only referred to “somewhat similar” work by Letzmann, and Peterson (1992a) mentions Letzmann’s “hand calculations.” When Holland et al. (2006) refer to Letzmann’s work as “experimenting” with various model parameters and emphasize several times his “hand drawn” diagrams and “hand calculations,” the reader may get the false impression that Letzmann had received his results merely by chance, instead of by the rigorous analytical calculations he performed in his Ph.D. thesis and that also extend the wind-field description by Holland et al. (2006). Furthermore, hand calculations and handdrawn diagrams were state of the art in the 1920s and 1930s, just as publishing scientific work in the German language was. Nevertheless, the authors must be highly credited for their tying in with Letzmann’s research and augmenting it by the modeling of tree response. The purpose of our comment is to draw attention to Corresponding author address: Dr. Nikolai Dotzek, DLRInstitut fur Physik der Atmosphare, Oberpfaffenhofen, 82234 Wessling, Germany. E-mail: nikolai.dotzek@dlr.de 726 J O U R N A L O F A P P L I E D M E T E O R O L O G Y A N D C L I M A T O L O G Y VOLUME 47

Ultrafast mapping of H2+ (D2+) nuclear wave packets using time-resolved Coulomb explosion imaging

The time evolution of H2+ (D2+) nuclear wave packets is studied exploiting a combination of coincident Coulomb explosion imaging and femtosecond pump–probe techniques. Using two 25 fs laser pulses, we map the motion of the dissociating molecular ion, observe an enhanced ionization rate at an internuclear separation of ~11 au and resolve trajectories due to the one- and two-photon Floquet channels. With two 7 fs pulses, we are able to visualize the vibrational motion of the bound part of the wave packet, which exhibits counterintuitive quantum behaviour and dephases within about 100 fs, in agreement with recent numerical simulations.

State-resolved measurements of single-electron capture in slow Ne7+- and Ne8+-helium collisions

Single-electron capture in collisions of 9 keV × q Ne8+ and Ne7+ ions with He has been studied using cold-target recoil-ion momentum spectroscopy. With an improved apparatus a longitudinal momentum resolution of 0.07 au has been achieved. This momentum component is directly proportional to the difference in the binding energy of the active electron between the final and the initial state. For the first time state-resolved differential cross sections have been determined with respect to the main quantum number, subshell level and spin state of the captured electron. A comparison with recent theoretical results for energy levels in Be-like Ne is given.