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Control of Molecular Motion (Molecular Alignment)

Femtosecond pump–probe experiments have extensively been used to follow atomic and molecular motion in time [1]. Alignment at an FEL has been demonstrated previously using several approaches [2] but for very fast rotational dynamics the jitter of SASE (Self Amplified Spontaneous Emission) FELs can be problematic, although advanced data analysis has been demonstrated to recover the details in silico. In this respect, the advantage of using a jitter-free FEL as FERMI is immediately evident with the final goal of recording molecular movies with femtosecond, sub-Ångstrom resolution.
Recently the experimental realization of impulsive alignment of carbonyl sulfide (OCS) molecules at LDM/FERMI has been performed [3]. OCS molecules in a molecular beam were aligned using 200 fs pulses from a near-infrared laser. The alignment was probed through time-delayed ionization above the S 2p edge, resulting in multiple ionization via Auger decay and subsequent Coulomb explosion of the molecules. The ionic fragments were collected using a time-of-flight mass spectrometer and the analysis of ion–ion covariance maps confirmed the correlation between fragments after Coulomb explosion (see Fig.5).
This result opens the way for a new class of experiments at LDM within the field of coherent control of molecular motion. Stronger alignment of molecules (than what was demonstrated for OCS) will be required to perform experiments using photoelectron holography techniques able to image the molecules from within, and allowing the recording of molecular movies on a femtosecond scale with picometer resolution.
Figure 5. Rotational revival alignment structure measured by monitoring the integrated signal for OC+ and S+ fragments while changing the time delay of the FEL pulse with respect to the alignment NIR pulse (left). The region of the OC+ and S+ channel in the partial covariance map at two different delays between the pump and probe pulses (right) [3].
[1] A.H. Zewail. J. Phys. Chem. A. 104,  5660 (2000).
[2] T. Kierspel et al., J. Phys. B 48, 204002 (2015).
[3] M. Di Fraia et al.,Phys. Chem. Chem. Phys. 19, 19733 (2017).
Last Updated on Wednesday, 25 March 2020 12:17