Investigations of ultrafast dynamics in electronically excited alkylbenzenes

We investigate ultrafast dynamics in electronically excited states of some typical alkylbenzenes by time-resolved two-colour four wave mixing and velocity map imaging as complementary methods. In this context an upgraded double-sided timeresolved velocity map imaging setup is also proposed.


Introduction
Ultrafast electronic relaxation processes play a central role in photochemistry.As the prototypical aromatic species benzene molecules have been the subject of a great number of investigations.Compared with benzene, the non-degeneracy of states in alkyl-benzenes leads to a higher density of the vibrational levels and to a reduction of symmetry forbidden interactions [1].Hence, higher coupling rates for non-adiabatic transitions are expected.
The absorption of alkyl-substituted benzenes between 190-270 nm is assigned to the excitation of the phenyl ring [2].The corresponding excited states are stable with respect to dissociation that consequently occurs indirectly after conversion to lower electronic states.The photo excitation of the S 1 state of e.g.ethylbenzene subsequently results in different relaxation pathways with distinct time constants and different translational energies of the photo fragments [3].At wavelength below 266 nm the lifetime of the S 1 state is determined by internal conversion (IC) S 1 AES 0 and by intersystem crossing (ISC) S 1 AET 1 .The T 1 state decay can occur either barrierless via T 1 AE S 0 ISC or by direct dissociation over a barrier.The latter process is expected to dominate the T 1 relaxation at 248 nm.We apply time-resolved two-colour four wave mixing spectroscopy (TCFWM) [4] and time-resolved photoelectron imaging (TRPEI) [5] to monitor the electronic relaxation of some typical alkylbenzenes in real time.For further improvement an upgraded double-sided time-resolved velocity map imaging (VMI) setup for enhanced investigation of the complicated dissociation dynamics is proposed.

Dispersed time-resolved TCFWM
Dispersed time-resolved TCFWM spectroscopy has been used to investigate excited states of alkylbenzenes, including ethylbenzene and propylbenzene.Time-resolved FWM methods implicate the coherent interaction of three light pulses with a medium to generate a signal pulse.Two pump pulses at 265 nm (~37735 cm-1) simultaneously cross the sample, interfere, and generate a polarization that comprises information about the excited state dynamics.The delayed probe pulse is set to ~790 nm in resonance with S 1 AES 2 transitions as depicted in Fig. 1

(a). The bimodal spectral distribution (∆~27
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A double-sided time-resolved VMI setup
In earlier experiments we have applied TRPEI to investigate o-xylene [7].The angular distributions of the ejected electrons and their kinetic energy can be obtained from the photoelectron images.Photoelectron images have been detected after a 400 nm two-photon absorption to the o-xylene S 2 state (origin at ~45400 cm -1 ).Fig. 2 (a) depicts the angular distribution of the three measured electron bands.The 1 st band relates to the S 1 state while the others reflect the population in S 2 .The rapid decay of the 2 nd and 3 rd bands with increasing delay time (Fig. 2 (b)), reveals that population in S 2 undergoes an ultrafast IC into vibrationally excited lower states.The relaxation time due to the IC process has been determined to be ~60 fs.XVIIIth International Conference on Ultrafast Phenomena as e.g. is shown in Fig 2 (c).The given limitations confine a more detailed investigation of alkylbenzenes.To overcome these restrictions, we assemble a double-sided time-resolved VMI setup, simultaneously collecting energy and momentum information about electrons and coincident ions, which cannot be achieved by photoelectron imaging alone [8].Moreover, we intend to preserve the longitudinal focus condition for TOF mass resolution on our setup while maintaining the lateral focus for optimal VMI energy resolution simultaneously, which can be achieved by the implementation of additional, especially optimized, electrical lenses.A sketch of the new design is shown in Fig. 3.By simulations (Simion 8.1) we can demonstrate that it is possible to achieve velocity-resolving capabilities of <1% (full range regime) in conjunction with an optimal TOF mass resolution.Fig. 4 compares the mass resolving capability of our new design (a) with a conventional VMI setup (b) [7].The resolving power of the new design exceeds the conventional one by almost ten times.

Fig. 3 .
Fig. 3.A sketch of the upgraded double-sided time-resolved VMI setup.

Fig. 4 .
Fig. 4. TOF focus profile of ions with m/q=1001-1006 for new design (a) and a conventional setup (b).