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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.