Tunable Femtosecond Vacuum Ultraviolet Light Generation

1. Introduction

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

Time resolved photoelectron imaging (TRPEI) is a powerful tool for dynamical studies but often presents some limitation mainly due to the Franck-Condon (FC) factor that limits 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 presents a rapid decay from the initial S2 state. In principle, the probe energy is enough to ionise the S1 but due do the poor Franck-Condon factor between S2 and D0, the spectrum does not show 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 induces a two-photon ionisation (1+2') process. In this case the photoelectron spectrum reported in figure 3 presents better detailed dynamics with the population that decays 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 a 2+1' process gives complete information, the spectral features are weak due the reduced cross-section relative to the 1+1' ionisation process. 1,3-butadiene is a perfect molecule, because there is present a large absorption cross-section that allows the 1+2' process to be observed, but in general this process has a weak signal and is not fully spectroscopically resolved from the 1+1' process. Another approach to 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 it is possible to enhance the view along the reaction coordinate without single-photon ionization from unexcited S0 which produces high levels of background signal.

 

 

 

2. Experimental Set up

Fs-VUV generation has been demonstrated by Noak and co-workers using four wave difference frequency mixing in an argon filled gas cell1-4. A 160 nm VUV pulse is created by mixing the fundamental frequency (FF) 800 nm from a fs Ti:sapphire laser with a third harmonic (TH) 267 nm in an argon-filled gas cell that acts as the non-linear medium. The VUV pulse created has an energy of 2 mJ and a 50 fs temporal duration and 0.1 eV bandwidth. This pulse can be used for the single photon probe step in the 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 spans the VUV region between 164 to 132 nm. This corresponds to the photon energy from 7.6 to 11.1 eV under single-photon ionisation potential of many molecules.

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Figure 4:  Cut-through CAD drawing of the VUV generation arm, VUV characterisation cube and photoelectron spectrometer.

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Figure 5: 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.