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Register a new userAlignment of the CS$_2$ Dimer Embedded in Helium Droplets Induced by a Circularly Polarized Laser Pulse
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James D Pickering Mr
Publication date
2019
fields
Physics
and research's language is
English
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Dimers of carbon disulfide (CS$_2$) molecules embedded in helium nanodroplets are aligned using a moderately intense, 160ps, non-resonant, circularly polarized laser pulse. It is shown that the intermolecular carbon-carbon (C-C) axis aligns along the axis perpendicular to the polarization plane of the alignment laser pulse. The degree of alignment, quantified by $langle cos^2(theta_text{2D}) rangle$, is determined from the emission directions of recoiling CS$_2$$^+$ fragment ions, created when an intense 40fs probe laser pulse doubly ionizes the dimers. Here, $theta_text{2D}$ is the projection of the angle between the C-C axis on the 2D ion detector and the normal to the polarization plane. $langle cos^2(theta_text{2D}) rangle$ is measured as a function of the alignment laser intensity and the results agree well with $langle cos^2(theta_text{2D}) rangle$ calculated for gas-phase CS$_2$ dimers with a rotational temperature of 0.4K.
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We demonstrate the experimental realization of impulsive alignment of carbonyl sulfide (OCS) molecules at the Low Density Matter Beamline (LDM) at the free-electron laser FERMI. OCS molecules in a molecular beam were impulsively aligned using 200 fs pulses from a near-infrared laser. The alignment was probed through time-delayed ionization above the sulphur 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. The analysis of the CO+ and S+ channels allowed us to extract the rotational dynamics, which is in agreement with our theoretical description as well as with previous experiments. This result opens the way for a new class of experiments at LDM within the field of coherent control of molecules with the possibilities that a precisely synchronized optical-pump XUV-probe laser setup like FERMI can offer.
Here, we report the observation of electron transfer mediated decay (ETMD) involving Mg clusters embedded in helium nanodroplets which is initiated by the ionization of helium followed by removal of two electrons from the Mg clusters of which one is transferred to the He environment neutralizing it while the other electron is emitted into the continuum. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. The photoelectron spectrum reveals a low energy ETMD peak. For Mg clusters larger than 5 atoms we observe stable doubly-ionized clusters. We argue that ETMD provides a new pathway to the formation of doubly-ionized cold species.
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