Session W27: Focus Session: Attosecond Science and Strong Field Chemical Physics II

Revealing molecular structure and dynamics through high harmonic generation driven by mid-IR fields

High harmonic generation (HHG) from molecules has recently been shown to be a promising tool for measuring instantaneous molecular structure, sub-femtosecond domain structural rearrangements in molecules and even hole dynamics initiated by laser field ionisation. To fully exploit this promise it is essential that we can; (1) systematically decouple structural and dynamic effects so that both may simultaneously be determined in the measurement, (2) can extend the method of molecular HHG imaging to a wide range of molecules. Here we demonstrate important steps towards both these objectives. Up until now HHG imaging measurements have been restricted to drive laser wavelengths close to 800nm, due to the availability of CPA titanium sapphire lasers, which dictates the use of relatively high intensities ($>$ 2.5 x 10$^{14}$ Wcm$^{-2})$ if a harmonic spectrum spanning to $\sim $70 eV is to be observed which is required for extracting structural data from most small molecules. By using a mid-IR laser (at 1300 nm) we show that with an intensity $\sim $ 1 x 10$^{14}$ W cm$^{-2}$ we can observe a wide molecular harmonic spectrum spanning to $\sim $ 70 eV even in molecules where ionization saturation would clamp the cut-off to much lower energies if an 800nm field were used. Thus we have been able to observe evidence for two-centre interference in two new molecules, N$_{2}$O and C$_{2}$H$_{2}$ for the first time. Moreover we can use the ability to observe a broad harmonic spectrum over a large range of intensities to reveal the subtle interplay between structural and dynamic effects in CO$_{2}$ and so provide a new window into hole dynamics. \\[4pt] In collaboration with R. Torres, Blackett Laboratory, Imperial College London; O. Smirnova, Max-Born-Institute, Berlin; T. Siegel and L. Brugnera, Blackett Laboratory, Imperial College London; I. Procino and Jonathan G. Underwood, Department of Physics and Astronomy, University College London; C. Altucci and R. Velotta, CNSIM and Dipartimento di Scienze Fisiche, Universita di Napoli ``Federico II'', Naples, Italy; E. Springate, C. Froud, and I. C. E. Turcu, Central Laser Facility, STFC Rutherford Appleton Laboratory; and M. Yu. Ivanov and J. P. Marangos, Blackett Laboratory, Imperial College London.   

Isolated attosecond pulses from ionization-gated high harmonics for molecular spectroscopy

Ionization gating of high harmonic emission on the leading edge of the driver pulse affords a convenient route to isolated attosecond pulses. The gating technique is based on a sub-femtosecond loss of phase matching for the high-harmonic generation process due to the rising plasma density during the driver pulse. Several techniques of attosecond spectroscopy are used to characterize the harmonic emission, including half-cycle cutoff analysis (Haworth et al., \textit{Nat. Phys.} \textbf{3}, 52), CEP-scanning (Pfeifer et al., \textit{Opt. Lett.}, \textbf{34},1819), and time-resolved photoelectron streaking (Kienberger et al., \textit{Nature}, \textbf{427}, 817). Ionization gating can generate a cleanly isolated attosecond pulse with $430\pm15$ as duration, limited here by the bandwidth of the reflective x-ray optic employed. We discuss the advantages of ionization gating, including driver pulse duration scalability, and wavelength tunability, and increased x-ray bandwidth over the traditional technique. Further, the ionization gated harmonic radiation can be used to initiate ultrafast molecular electronic dynamics.   

Ultrafast/Attosecond Transient Absorption with High Order Harmonics

Laser-produced high order harmonics are used to probe the chemical dynamics of atoms and molecules on femtosecond and attosecond timescales. The high order harmonics are produced with an 800 nm Ti:sapphire laser by focusing into a rare gas, and these pulses are used as the soft X-ray probe in wavelength-dispersed transient absorption. Inner shell core-level spectroscopic transitions are thus used to analyze the chemical and electronic environment of specific atomic states as a function of time following ionization and dissociation. High field ionization processes, using 800 nm pulses, result in spin-orbit electronic state populations, alignment, wave packet superposition states, and dissociative ionization events, which are investigated with the spectrally-resolved X-ray probe. By using isolated attosecond pulses as the probe, high field ionization events on a subfemtosecond timescale are probed. The generality of the transient absorption method for attosecond dyamics is described, as well as the complications during the pump-probe pulse overlap time period. The results are compared to theoretical calculations by collaborators.   

Attosecond Control of Molecular Photoionization

We will report on experiments on molecular photoionization by a train of attosecond pulses synchronized to an infrared (IR) laser field. It is shown that photoionization of a molecule by an attosecond pulse is sensitive to the instantaneous electric field at the moment of ionization. In the simplest case this can be understood by considering the coupling of two ionic states by the laser field, which effectively polarizes the molecule. Ionization takes place into these coupled states rather than the field free states. We demonstrate this in a prototype experiment by monitoring the yield and angular distributions of fragments produced upon dissociative ionization of hydrogen molecules as a function of delay between the attosecond pulses and the infrared field. Both the yield and the angular distributions, resulting from specific dissociation channels, oscillate as a function of the delay with twice the laser period. Furthermore we show that this effect can be generalized to other more complex molecules.   

Direct optimization of isolated attosecond pulse contrast

Isolated attosecond pulses are a powerful tool for studying electron dynamics. However, the pulse isolation is strongly dependent on the carrier-envelope phase (CEP) of a short pulse in the high-harmonic generation process. We introduce a direct optimization method of the isolated attosecond pulse contrast. By the attosecond streak-camera principle, the photoelectrons produced by the attosecond pulse acquire momentum impulses according to the vector potential of the streaking laser field. By scanning the CEP and measuring the photoelectron spectrum produced by the combined attosecond pulses and the harmonic driver pulse at zero relative time delay, the energy ratio between the main pulse and the neighbor satellite pulses can be measured. An isolated pulse of contrast 3.3:1 with 430 as duration was optimized in experiments here. The method allows quick control over the crucial contrast parameter in experimental applications.   

Double Optical Gating: an easy method for generating isolated attosecond pulses

Isolated attosecond pulses are powerful tools for exploring electron dynamics in matter. So far, such extreme ultraviolet pulses have only been generated using high power, few-cycle lasers, which are very difficult to construct and operate. We propose and demonstrate a technique called double optical gating for generating isolated attosecond pulses with lasers pulses as long as 28 fs that was directly from a chirped pulse amplifier. These XUV pulses, generated from argon gas, are measured to be 148 as by reconstructing the streaked photoelectron spectrograms. This new gating scheme, with a relaxed requirement on laser pulse duration, makes attophysics more accessible to many laboratories that are capable of producing such multi-cycle laser pulses. The double optical gating also works with sub-10 fs driving lasers, which generated supercontinuum spectrum extends from 28 eV to 620eV including the ``water window'' region and supports single 16 as pulses, below one atomic unit of time (24 as). We have used the isolated 140 attosecond pulses in several applications, which include a demonstration that the two-electron dynamics in helium could be observed and controlled.   

Augmented collisional ionization in XUV-cluster interaction

High charge states have been observed in Xenon cluster interaction with extreme ultraviolet (XUV) radition from an intense HHG source [1]. Mechanisms underlying the observed high charge states from clusters exposed to the VUV regime -- such as enhanced Inverse Bremsstrahlung heating -- do not play a role due to the small XUV wavelength. Our many body molecular dynamics simulations show that high charge states result from augmented collisional ionization processes, and not enhanced photoionization. These arise from additional microscopic effects that have been not been implemented previously. We have included the process of electron-ion recombination, and have calculated the dynamics for long times after the XUV pulse. We find that many of the high charge states that exist in the core would not survive to the detector. \\[4pt] [1] B. F. Murphy, K. Hoffmann, A. Belolipetski, J. Keto, and T. Ditmire, PRL 101, 203401 (2008).