Tunable Femtosecond Vacuum Ultraviolet Light Generation

1. Introduction

Molecular structure is the features that give to the molecule a specific function. 
The knowledge of molecular structure allows to hypothesize the possible molecule reaction and the products formation. Nowadays a more complete picture of the molecular behaviour can be highlighted through time resolved techniques that allow following evolution of the molecule from the initial state to the final state. The dynamical studie s become a fundamental part that gives a deeper relationship between structure and molecular functions.
By using femtosecond laser, dynamical studies can be carried out by create an initial electronic excited state and follow the time evolution using a pump probe techniques. By acquiring photoelectron images at different time delay, spectroscopic information of the dynamical process can be extracted and used to fully establish the molecular evolution long the reaction coordinate from the reactant to the products.

Time resolved photoelectron imaging (TRPEI) is a powerful tool for dynamical studies but often present some limitation mainly due to the Franck-Condon (FC) factor that limit the access to the complete reaction coordinate from the reactant to the product.

This point can be shown considering the ionization potential of 1,3-butadiene. Using a pump pulse at 200 nm with 20 nJ the S2 state of the molecules is populated and the ionisation is carried out by using a probe pulse at 267 nm with 350 nJ (1+1' process shown in figure 1 scheme a).

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Figure 1: Frank-Condon picture in photoelectron spectroscopy
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Figure 2:(1+1') photoelectron spectrum
Figure 3:(1+2') photoelectron spectrum

The photoelectron spectrum of 1+1' process shown in figure 2 present rapid decay from the initial S2 state. In principle, the probe energy is enough for ionise the S1 but due do the poor Franck-Condon factor between S2 and D0, the spectrum does not shows any signal from the S1 state.
In order to populate the S1 state it is possible to increase the probe energy up to 2.5 mJ this induce a two-photon ionisation (1+2') process. In this case the photoelectron spectrum reported in figure 3 present a better detailed dynamics with the population that decay from S1 state 1.7 and 3.4 eV and an additional long-lived signal below 0.5 eV due to the decay from S1 to S0. By using this approach a more complete dynamical picture can be revealed.

Although 2+1' process give a complete information, the spectral features are weak due the reduced cross section of relative to the 1+1' ionisation process. 1,3-butadiene is perfect molecule, because present a large cross section absorption that allow to the 1+2' process to be observed, but in general this process have weak signal and is not fully spectroscopic resolved from 1+1' process. Another approach for enhance the capability of the TRPEI techniques is to use a femtosecond vacuum ultraviolet (VUV) pulse (scheme c figure 1) as a probe. By using a tuneable probe across the VUV region is it possible to enhance the view along the reaction coordinate without single-photon ionization from unexcited S0 that produce high levels of background signal.




2. Experimental Set up

Fs-VUV generation has been demonstrated by Noak and co-worker using four wave difference frequency mixing in Argon filled gas cell1-4. 160nm VUV pulse is created by mixing fundamental frequency (FF) 800nm from a fs Ti:sapphire laser with a Third Harmonic 267 nm in a Argon filled gas cell that act as non linear crystal. The VUV pulse created have energy of 2mJ and 50 fs temporal duration and 0.1 eV of Bandwidth. This pulse can be used for the single photon probe step in TRPEI experiment. An additional enhancement can be done by performing a mixing using either idler (1.6 to 2.6 mm) or signal (1.1 to 1.6 mm) from on optical parametric amplifier (OPA) and the third Harmonic from the Ti:Sapphire laser system, this generates, under the correct phase matching condition,  fs-VUV pulse that span the VUV region between 164 to 132 nm. This correspond to the photon energy from 7.6 to 11.1 eV under single-photon ionisation potential of many molecules.

At the moment fs-VUV setup is under development. Once VUV radiation is created need to be spectrally and temporally characterize and then implement to the current photoelectron imaging spectrometer present in the laboratory. Figures 4-6 shown the 3D technical draws of the VUV generation chamber

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Figure 4:  VUV chamber part. In this unit is energy and spectral bandwidth is measured and the overlap between VUV and the pump pulse is performed.

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Figure 5: VUV part top view. Mirror and translation stage allow to the VUV beam to access the spectrometer, energy probe and webcam (placed inside the manipulator) respectively.

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Figure 6: VUV chamber part implemented in the time resolved photon electron spectrometer.


3. Further Reading

M. Beutler, M. Ghotbi, F.Noak, I.V. Hertel, Optics letters  35, 1491 2010.
M.Ghotbi, M. Beutler, F. Noak, Optics letters 35, 3492 2010.
M. Beutler, M. Ghotbi, F.Noak, Optics letters 36, 3726 2011.
M.Ghotbi, P. Trabs, M. Beutler, F. Noak, Optics letters 38, 486 2013.