Francesca Calegari

Ultrafast Electron Dynamics in Phenylalanine Initiated by Attosecond Pulses

In the past decade, attosecond technology has opened up the investigation of ultrafast electronic processes in atoms, simple molecules, and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub–4.5-femtosecond temporal scale, which is shorter than the vibrational response of the molecule. The ability to initiate and observe such electronic dynamics in polyatomic molecules represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems. Electronic dynamics in a complex polyatomic molecule are tracked faster than the time scale for vibrational motion. A very quick look at phenylalanine Over the past decade, laser technology has pushed back the fastest directly observable time scale from femtoseconds (quadrillionths of a second) to attoseconds (quintillionths of a second). For the most part, attosecond studies so far have probed very simple molecules such as H2 and O2. Calegari et al. now look at a more elaborate molecule—the amino acid phenylalanine. They tracked changes in the electronic structure of the compound after absorption of an ultrafast pulse, before the onset of conventional vibrational motion. Science, this issue p. 336

Advances in attosecond science

Attosecond science offers formidable tools for the investigation of electronic processes at the heart of important physical processes in atomic, molecular and solid-state physics. In the last 15 years impressive advances have been obtained from both the experimental and theoretical points of view. Attosecond pulses, in the form of isolated pulses or of trains of pulses, are now routinely available in various laboratories. In this review recent advances in attosecond science are reported and important applications are discussed. After a brief presentation of various techniques that can be employed for the generation and diagnosis of sub-femtosecond pulses, various applications are reported in atomic, molecular and condensed-matter physics.

Observation of Ultrafast Charge Migration in an Amino Acid

We present the first direct measurement of ultrafast charge migration in a biomolecular building block - the amino acid phenylalanine. Using an extreme ultraviolet pulse of 1.5 fs duration to ionize molecules isolated in the gas phase, the location of the resulting hole was probed by a 6 fs visible/near-infrared pulse. By measuring the yield of a doubly charged ion as a function of the delay between the two pulses, the positive hole was observed to migrate to one end of the cation within 30 fs. This process is likely to originate from even faster coherent charge oscillations in the molecule being dephased by bond stretching which eventually localizes the final position of the charge. This demonstration offers a clear template for observing and controlling this phenomenon in the future.

Time-delay compensated monochromator for the spectral selection of extreme-ultraviolet high-order laser harmonics.

The design and the characterization of a monochromator for the spectral selection of ultrashort high-order laser harmonics in the extreme ultraviolet are presented. The instrument adopts the double-grating configuration to preserve the length of the optical paths of different diffracted rays, without altering the extremely short duration of the pulse. The gratings are used in the off-plane mount to have high efficiency. The performances of the monochromator have been characterized in terms of spectral response, efficiency, photon flux, imaging properties, and temporal response. In particular, the temporal characterization of the harmonic pulses has been obtained using a cross-correlation method: Pulses as short as 8 fs have been measured at the output of the monochromators, confirming the effectiveness of the time-delay compensated configuration.

Attosecond Electron Dynamics in Molecules

Advances in attosecond science have led to a wealth of important discoveries in atomic, molecular, and solid-state physics and are progressively directing their footsteps toward problems of chemical interest. Relevant technical achievements in the generation and application of extreme-ultraviolet subfemtosecond pulses, the introduction of experimental techniques able to follow in time the electron dynamics in quantum systems, and the development of sophisticated theoretical methods for the interpretation of the outcomes of such experiments have raised a continuous growing interest in attosecond phenomena, as demonstrated by the vast literature on the subject. In this review, after introducing the physical mechanisms at the basis of attosecond pulse generation and attosecond technology and describing the theoretical tools that complement experimental research in this field, we will concentrate on the application of attosecond methods to the investigation of ultrafast processes in molecules, with emphasis in molecules of chemical and biological interest. The measurement and control of electronic motion in complex molecular structures is a formidable challenge, for both theory and experiment, but will indubitably have a tremendous impact on chemistry in the years to come.

Charge migration induced by attosecond pulses in bio-relevant molecules

We acknowledge the support from the European Research Council under the ERC grants no. 637756 STARLIGHT, no. 227355 ELYCHE and no. 290853 XCHEM, LASERLABEUROPE (grant agreement no. 284464, European Commissions Seventh Framework Programme), European COST Action CM1204 XLIC, the Ministerio de Ciencia e Innovacion project FIS2013-42002-R, European grants MC-ITN CORINF and MC-RG ATTOTREND 268284, UKs Science and Technology Facilities Council Laser Loan Scheme, the Engineering and Physical Sciences Research Council (grant EP/J007048/ 1), the Leverhulme Trust (grant RPG-2012-735), and the Northern Ireland Department of Employment and Learning

Interplay between group-delay-dispersion-induced polarization gating and ionization to generate isolated attosecond pulses from multicycle lasers

We implemented a new experimental scheme for the generation of single-shot extreme-UV continua that exploits a combination of transform-limited 15 fs, 800 nm pulses and chirped 35 fs, 800 nm pulses with orthogonal polarizations. Continua are interpreted as the formation of a single attosecond pulse and attributed to the interplay between polarization, ionization gating, and trajectory selection operated by suitable phase-matching conditions.

Attosecond Technology and Science

The generation of attosecond pulses has been the result of tremendous efforts and advances in the field of the interaction of ultrashort intense laser pulses with matter. Nowadays, the electric field waveform of femtosecond pulses can be precisely controlled and used to synthesize and measure the evolution in time of attosecond pulses. The quest for increasing photon fluxes in the extreme-ultraviolet region (XUV) and the requirement of a complete characterization of complex attosecond waveforms are stimulating the development of new technological approaches that should make feasible XUV sources combining attosecond pulse duration, high energy, and high repetition rate. The use of such sources with state-of-the-art techniques for the measurement of the momentum of charged particles will allow a detailed description of photoionization and photodissociation processes. Advances in selection of the initial quantum state and structure of molecules will open the way for the extension of attosecond spectroscopy to complex molecules. The first applications of attosecond pulses allowed the investigation of electron dynamics with a resolution close to the atomic unit of time. Simple systems such as helium and noble gases were investigated using trains and isolated pulses. These experiments represent benchmarks to elucidate the response of atoms on the typical electron timescale.

Phase-matching effects in the generation of high-energy photons by mid-infrared few-cycle laser pulses

We report on our experimental and theoretical investigations on the generation of high-order harmonics driven by 1500 nm few-cycle laser pulses in xenon. In contrast to the common belief, we found experimental evidence suggesting that harmonic generation driven by mid-infrared laser pulses can be realized with high efficiency; in particular, an enhancement of very high harmonic orders can be achieved under suitable conditions of the laser–medium interaction. The experimental results were simulated by a 3D non-adiabatic model. The theoretical outcomes confirm the experimental findings and provide a physical explanation for the counter-intuitive results. In particular, a time-dependent phase-matching analysis threw light on the generation mechanisms at a timescale of half optical cycle of the fundamental pulse.

High order harmonics driven by a self-phase-stabilized IR parametric source

We analyze the emission of coherent XUV radiation in gaseous targets by high-order harmonics of tunable infrared (1.4–1.6 μm) laser pulses generated by a parametric source. As noticeable application, we show that a continuous spectrum spanning the 40–200 eV region can be efficiently generated by mixing the infrared pulses with visible pulses at 800 nm. These results open new perspectives for the extension of attosecond physics towards the soft-X region and for the investigation of atoms and molecules on a broader range of photon energies with respect to the state of the art.