Integrated Calendar

We have studied strong-field laser-induced fragmentation of the benchmark hydrogen molecule in sub 5 fs near infrared (IR) pulses. As the pulse duration approaches a single cycle, the molecular response depends on the waveform of the pulse. For a particularly simple example of the waveform which is characterized by the carrier-envelope phase (CEP) – the CEP can be used as a control knob for the ensuing molecular dynamics.
In this kind of experiments a “CEP-locking” or “CEP-tagging” technique is typically employed. The latter method was used in our studies as demonstrated in this talk by two examples. The first involves the study of CEP control over pathway interference in strong-field dissociation of H2+ [1,2], where a molecular-ion beam was used as the target. In the second example we explore the formation of long-lived excited H fragments from the fragmentation of an H2 target. As predicted by theory [3,4], the measurements show that the CEP steers protons of H2+ (or excited-hydrogen fragments H2) one way or the other along the laser polarization. Moreover, the fragmentation yield itself depends on the CEP.

Others contributing to this work:
N.G. Johnson, M. Zohrabi, B. Berry, U. Ablikim, B. Jochim, T. Severt, K. J. Betsch, K.D. Carnes,
Shuo Zeng, F. Anis, J. Hernández, Yujun Wang, and B.D. Esry
1Supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy

Critical transitions have attracted a great deal of attention due to their relevance to many natural and
social systems. Much research has been devoted to the characterization and identification of imminent
critical transitions. In spatially extended systems, the dynamics (close to and away from the critical
point) is more complicated due to the expansion, shrinking and coalescence of alternative-state
domains. Pattern-forming systems introduce additional complexity due to the patterned nature of one of
the stable states. In this talk, I will present several works in which we used the context of drylands
vegetation dynamics to study various aspects of this additional complexity: (i) Using a minimal model,
we showed that in systems exhibiting a bistability of a patterned state with a uniform state, a multitude
of intermediate stable localized states may appear, giving rise to step-like gradual shifts with extended
pauses at these states. This result suggests that a combination of abrupt-shift indicators and
gradual-shift indicators might be needed to unambiguously identify regime shifts. (ii) The existence of
these localized states in models for the dynamics of drylands vegetation and the response of the
systems described by these models to local perturbations will be discussed. (iii) We show how a
simplified version of a model for drylands vegetation dynamics can explain the emergence and the
observed dynamics of the spectacular phenomenon of “fairy circles” in southern Africa. If time
permits, I will present recent results demonstrating the effects of heterogeneity on the pattern
formation, survivability and resilience of water-limited vegetation.