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Tuesday, 12 November 2013

Zolt: DM: How to "count the number" of electrons on a molecular orbital

One of the main goals of attosecond science is to see how the electrons are "behaving" in a molecule and to have a control over the behavior.

Two-color HHG may provide a way to achieve both of the above listed aims at the same time. In N2 the energy separation of the HOMO and HOMO-1 corresponds to 1.4eV. Removing an electron from HOMO by tunnel ionization leaves an electronic hole there. Subsequently, irradiating the target with a pump beam, one may start a Rabi kind of population transfer between HOMO and HOMO-1. Depending on the excursion time of the electrons the different harmonic orders may find the HOMO orbital populated differently.

We expect that due to the population evolution of the HOMO electronic state the spectrum of the emitted harmonics will be altered.

Tuesday, 29 October 2013

Felicity: JC: High Harmonic Spectroscopy of the Cooper Minimum in Molecules (click to see details)

The Cooper minimum (CM) has been studied using high harmonic generation solely in atoms. Here, we present detailed experimental and theoretical studies on the CM in molecules probed by high harmonic generation using a range of near-infrared light pulses from λ=1.3 to 1.8  μm. We demonstrate the CM to occur in CS2 and CCl4 at ∼42 and ∼40  eV, respectively, by comparing the high harmonic spectra with the known partial photoionization cross sections of different molecular orbitals, confirmed by theoretical calculations of harmonic spectra. We use CM to probe electron localization in Cl-containing molecules (CCl4, CH2Cl2, and trans-C2H2Cl2) and show that the position of the minimum is influenced by the molecular environment.
Download the paper here:
or here:

Thursday, 24 October 2013


To make it easier for me to add events and also easier for you I will now use a new website and a google calendar. You can link individual events and the whole calendar to your own calendar.

HERE it is:

we are back here on the original...

Tuesday, 22 October 2013

Thoma: DM: The Water Project

I will present two new types of targets which could get implemented in the red dragon lab over the next few months. The novelty of these targets lie in the fact that they make the liquid phase of matter available in our lab.

The 1st project looks into using a thin liquid water sheet as the generation medium for high harmonics. Gravity driven aqueous thin film technology has been demonstrated at atmospheric pressure [1] but I believe that with adequate design considerations we should be able to move it inside the vacuum tin. "non perturbative" harmonic generation has been demonstrated converting a mid-IR laser to visible wavelengths [2].

The 2nd project will be looking at 100nm sized silicon nitride cells to hold water in the transient absorption beam-line. Once the targets exists, a range of experiments can be thought of: one of the 1st ones I would like to try would be the time resolved generation of H3O+ following the 2 to 4 photon ionisation of H2O by a 400nm field by looking at absorption by the 3rd IP of H3O+ around 32eV [3]. This has been theorised to happen in 10's of fs but to my knowledge has never been measured.

Our collaborators from epfl would be keen to look at some vibrational excitation induced changes [4] in the absorption spectrum to use alongside some photo-electron spectra data acquired at ral last year.

During this presentation I will discuss both the science behind these experiments and the mechanical designs we hope to use to surpass the technical difficulties associated with combining XUV radiation and liquid phase samples.

[1] Rev. Sci. Instrum. 74, 4958 (2003); doi: 10.1063/1.1614874

[2] Vol. 17, No. 23 / OPTICS EXPRESS 20959

[3] J. Am. Chem. Soc. 128, 3864 (2006)

[4] science 297, 587 (2002)

Tuesday, 15 October 2013

Jon L: JC: Probing Time-Dependent Molecular Dipoles on the Attosecond Time Scale

Photoinduced molecular processes start with the interaction of the instantaneous electric field of the incident light with the electronic degrees of freedom. This early attosecond electronic motion impacts the fate of the photoinduced reactions. We report the first observation of attosecond time scale electron dynamics in a series of small- and medium-sized neutral molecules (N2, CO2, and C2H4), monitoring time-dependent variations of the parent molecular ion yield in the ionization by an attosecond pulse, and thereby probing the time-dependent dipole induced by a moderately strong near-infrared laser field. This approach can be generalized to other molecular species and may be regarded as a first example of molecular attosecond Stark spectroscopy.

Tuesday, 8 October 2013

Thoma: JC: LIAD-fs scheme for studies of ultrafast laser interactions with gas phase biomolecules

Laser induced acoustic desorption (LIAD) has been used for the first time to study the parent ion production and fragmentation mechanisms of a biological molecule in an intense femtosecond (fs) laser field. The photoacoustic shock wave generated in the analyte substrate (thin Ta foil) has been simulated using the hydrodynamic HYADES code, and the full LIAD process has been experimentally characterised as a function of the desorption UV-laser pulse parameters. Observed neutral plumes of densities >109 cm−3 which are free from solvent or matrix contamination demonstrate the suitability and potential of the source for studying ultrafast dynamics in the gas phase using fs laser pulses. Results obtained with phenylalanine show that through manipulation of fundamental femtosecond laser parameters (such as pulse length, intensity and wavelength), energy deposition within the molecule can be controlled to allow enhancement of parent ion production or generation of characteristic fragmentation patterns. In particular by reducing the pulse length to a timescale equivalent to the fastest vibrational periods in the molecule, we demonstrate how fragmentation of the molecule can be minimised whilst maintaining a high ionisation efficiency.


Tuesday, 10 September 2013

Simon: JC: Timing analysis of infrared-driven high-order harmonic generation with above-ionization attosecond pulses

We theoretically study infrared (IR)-driven high-order harmonic generation (HHG) assisted by attosecond pulses with a central energy above the atomic ionization threshold. We provide a clear physical picture for controlling HHG using the time delay between the attosecond pulses and the IR laser reported by Faria et al. [Phys. Rev. A 74, 053416 (2006)]. This physical picture also indicates that the combined attosecond pulses and IR laser can help resolve the dynamics of ionized electrons from time-dependent harmonic spectra. We present the quantum effect on HHG as an example. While leaving parent ions, ionized electrons can still emit harmonics in the semi-classically forbidden situation. The two-color excitation provides a practical method to observe the quantum effect experimentally. Furthermore, in our work, attosecond pulses and an IR field are considered with a realistic pulse shape, which shows a quantitatively important effect in controlling harmonic spectra. Accordingly, a guide to optimize the control capability for HHG is presented, and a method to determine the IR carrier-envelope phase based on the pulse-shape effect on the HHG is also proposed.

Tuesday, 6 August 2013

Johannes Floß: DM: Quantum resonance, Anderson localisation and selective rotational excitation in periodically kicked molecules

guest speaker: Johannes Floß, Department of Chemical Physics, Weizmann Institute of Science, Israel

The periodically kicked rotor has attracted much attention in the recent decades. In the classical regime the kicked rotor can exhibit truly chaotic motion, whereas in the quantum mechanical analogue, this chaotic motion is suppressed by Anderson-like localisation of the wave function in momentum space [1]. On the other hand, when the kicking period is equal to the rotational revival time, the kicked quantum rotor exhibits quantum resonance, an effect which leads to a linear increase of the angular momentum with the number of kicks [2]. Until today these phenomena were observed experimentally mainly in an atom optics analogue of a kicked rotor [3].

In this talk, I will provide an introduction to the two phenomena of quantum resonance and Anderson localisation in periodically kicked rotors. Then I will show how standard laser techniques used for molecular alignment provide a new way for observing these phenomena in a real rotor system, a diatomic molecule [4, 5]. I will also show that these phenomena provide a new toolbox for selective laser manipulations in molecular mixtures.

[1] S. Fishman, D. R. Grempel, and R. E. Prange, Phys. Rev. Lett. 49, 509 (1982).
[2] F. M. Izrailev and D. L. Shepelyanskii, Theor. Math. Phys. 43, 553 (1980).
[3] F. L. Moore, J. C. Robinson, C. Bharucha, B. Sundaraman, and M. G. Raizen, Phys. Rev.
Lett. 75, 4598 (1995).
[4] J. Floß and Ilya Sh. Averbukh, Phys. Rev. A 86, 021401 (R) (2012).
[5] J. Floß, S. Fishman, and Ilya Sh. Averbukh, arXiv 1305.5995 (2013).

Tuesday, 30 July 2013

Felicity: Extending HHG Spectroscopy to New Molecular Species

18 month transfer presentation. This presentation details the objectives of this PhD project and the current status of experiments. A review of the first apparatus which did yield Benzene HHG at 1500nm, albeit with limited success is presented along with the developments taken to construct the current apparatus. The acquisition of HHG spectra in a variety of molecules and ellipticity scans are presented along with impulsive alignment of the benzene molecule. The intended experiments which include the Jahn Teller Effect and PACER will be presnted and the possibility of working with other targets and spectroscopic methods will also be discussed.

Tuesday, 23 July 2013

William: DM: CEP stability of hollow-fibers used for high-energy, few-cycle pulse generation

We investigated the carrier-envelope phase (CEP) stability of a hollow-fiber setup used for high-energy, fewcycle pulse generation. Saturation of the output pulse energy is observed at 0:6 mJ for a 260µm inner-diameter,
1 m long fiber, statically filled with neon, with the pressure adjusted to achieve an output spectrum capable
of supporting sub-4 fs pulses. The maximum output pulse energy can be increased to 0:8 mJ by using either
differential pumping, or circularly polarized input pulses. We observe the onset of an ionization-induced CEP
instability, which does not increase beyond an input pulse energy of 1:25 mJ due to losses in the fiber caused
by ionization. There is no significant difference in the CEP stability with differential pumping compared to
static-fill, demonstrating that gas flow in differentially pumped fibers does not degrade the CEP stabilization

Tuesday, 28 May 2013

Emma: JC: Controlling dielectrics with the electric field of light

Modern-day computing and optical communications are limited by the speed at which signals can be turned on and off. This paper demonstrates that frequencies of the order of Petahertz could be achieved by exploiting the change in behavior from insulator to conductor of a dielectric when exposed to strong, few-cycle laser pulses. Transient absorption, streaking and reflectivity techniques are used to probe these sub-femtosecond dynamics and observe the reversibility of the changes in the dielectric.


supplementary information:

Tuesday, 26 February 2013

Davide: VUV double whammy

Extreme-ultraviolet pump–probe studies of one-femtosecond-scale electron dynamics

Observation of Ultrafast Charge Migration in an Amino Acid

and maybe a discussion of some first results?

Tuesday, 19 February 2013

Thomas: Manipulation of quantum paths for space–time characterization of attosecond pulses

Attosecond extreme-ultraviolet pulses1 have a complex space–time structure2. However, at present, there is no method to observe this intricate detail; all measurements of the duration of attosecond pulses are, to some extent, spatially averaged1, 3, 4, 5. A technique for determining the full space–time structure would enable a detailed study of the highly nonlinear processes that generate these pulses as a function of intensity without averaging6, 7. Here, we introduce and demonstrate an all-optical method to measure the space–time characteristics of an isolated attosecond pulse. Our measurements show that intensity-dependent phase and quantum-path interference both play a key role in determining the pulse structure. In the generating medium, the attosecond pulse is strongly modulated in space and time. Propagation modifies but does not erase this modulation. Quantum-path interference of the single-atom response, previously obscured by spatial and temporal averaging, may enable measuring the laser-field-driven ion dynamics with sub-cycle resolution.

Tuesday, 12 February 2013

Han: Generation of EUV radiation by plasmonic field enhancement using nano-structured bowties and funnel-waveguides

Generation of EUV radiation by plasmonic field enhancement using nano-structured bowties and funnel-waveguides

Recent experimental data of high-order harmonic generation (HHG), obtained by use of the plasmonic field enhancement of nanostructure bowties and funnel-waveguides, are presented. Emphasis is laid on reproduction of previous experimental results and also elucidation of the fundamental limitations associated with the nanostructure thermal damage, small laser-gas interaction volume, and atomic line emission in the plasmon-driven HHG process. In addition, the dominance of coherent harmonics is quantitatively verified by implementing a two-beam interference experiment using a pair of funnel-waveguides. This study proves that funnel-waveguides are a superior plasmonic device capable of providing not only high thermal immunity but also sufficient atom emitters to produce practically usable extreme-ultraviolet (EUV) radiation in a reproducible manner.

Tuesday, 5 February 2013

Yusuke: HHG in aligned molecules with CEP-controlled pulses

High-order harmonics generated aligned molecules with CEP-controlled pulses are investigated. With short duration pulses harmonics are spectrally broadened due to non-adiabatic effect, which give rise to interferences between adjacent odd harmonics. The overlapped harmonics create interferences that become visible by locking the CEP. The interference contains the information of the phase of harmonics. We compare spectra generated with 25 fs pulses and spectra generated with 10 fs pulses. We have found some characteristics of the interference observing order dependence and molecular alignment dependence of the interference.