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Response of the ⁴He trimer to a short laser pulse

Kruse, Jan

Published Jan 1, 2021• 0 citations
OpenAlex

Abstract

A combination of the pump-probe technique and Coulomb explosion imaging is used to image the time resolved response of the 4He trimer to the perturbation by a femtosecond laser pulse. In the experiment, the interaction of the helium trimer with a non-ionizing pump-pulse is imaged by an ionizing probe pulse. The momenta, which the charged trimer fragments gain in the Coulomb explosion triggered by the probe pulse, are measured by a COLTRIMS reaction microscope. The response of the helium trimer is first analysed in momentum space. Subsequently, a reconstruction algorithm is used, in order to obtain the structural information about detected trimers from the momenta of the trimer fragments. This allows to analyse the response of the helium trimer in position space. The experiment is primarily motivated by the question, whether a laser pulse can excite the trimer ground state to the Efimov state, which is a huge quantum halo. Although a population of the Efimov state is not observed, in the experiment, the recorded data show the population of an unknown state, which could not yet be identified. Further, the response of the helium trimer shows an alignment wave, which might arise from the interference of two quantum states.

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•Jan 1, 2021•0 citations
A combination of the pump-probe technique and Coulomb explosion imaging is used to image the time resolved response of the 4He trimer to the perturbation by a femtosecond laser pulse. In the experiment, the interaction of the helium trimer with a non-ionizing pump-pulse is imaged by an ionizing probe pulse. The momenta, which the charged trimer fragments gain in the Coulomb explosion triggered by the probe pulse, are measured by a COLTRIMS reaction microscope. The response of the helium trimer is first analysed in momentum space. Subsequently, a reconstruction algorithm is used, in order to obtain the structural information about detected trimers from the momenta of the trimer fragments. This allows to analyse the response of the helium trimer in position space. The experiment is primarily motivated by the question, whether a laser pulse can excite the trimer ground state to the Efimov state, which is a huge quantum halo. Although a population of the Efimov state is not observed, in the experiment, the recorded data show the population of an unknown state, which could not yet be identified. Further, the response of the helium trimer shows an alignment wave, which might arise from the interference of two quantum states.
Abstract Small helium clusters consisting of two and three helium atoms are unique quantum systems in several aspects. The helium dimer has a single weakly bound state and is of huge spatial extent, such that most of its probability distribution resides outside the potential well in the classically forbidden tunnelling region. The helium trimer possesses only two vibrational states, one of which is of Efimov nature. In this chapter, we discuss application of the Coulomb explosion imaging technique for studying geometries and binding energies of these peculiar two- and three-body quantum systems. Irradiation of a helium cluster by a strong laser field allows tuning interactions between helium atoms. Such ultrashort interaction modification induces response dynamics in a cluster that is observed by combination of the imaging technique with the pump-probe approach.
Motivated by ongoing pump-probe spectroscopy experiments, this work develops a theoretical framework for describing the rovibrational wave packet dynamics that ensues when a single weakly-bound van der Waals trimer is exposed to a short, sub-picosecond linearly polarized pump laser pulse. The intensity I of the pump laser is chosen such that excitation and ionization of the electronic degrees of freedom are negligible while excitation of the wavepacket in the nuclear degrees of freedom is non-negligible. The numerical treatment, which takes advantage of the fact that the laser pulse is very short compared to typical molecular time scales, is based on a wave packet decomposition that utilizes hyperspherical coordinates. The framework is applied to the extremely floppy bosonic helium trimer. A convergence analysis of the partial wave decomposition is conducted. The kinetic energy release and orientation dynamics are presented. While the dynamics of more strongly-bound van der Waals trimers such as, e.g., the argon trimer display negligible coupling between vibrational and rotational degrees of freedom, rendering a description within a rigid-body picture appropriate, those of weakly-bound trimers display non-negligible coupling between vibrational and rotational degrees of freedom, rendering a description within a rigid-body picture inappropriate. It is shown that a model that constructs the helium trimer dynamics from the dynamics of the helium dimer captures a number of key characteristics of the alignment signal, including the interference between different angular momentum wave packet components.
Rubidium dimers residing on the surface of He nanodroplets are doubly ionized by an intense femtosecond laser pulse leading to fragmentation into a pair of Rb+{\mathrm{Rb}}^{+}Rb+ ions. We show that the kinetic energy of the Rb+{\mathrm{Rb}}^{+}Rb+ fragment ions can be used to identify dimers formed in either the XXX $^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ ground state or in the lowest-lying triplet state, $a\text{ }^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$. From the experiment, we estimate the abundance ratio of dimers in the aaa and XXX states as a function of the mean droplet size and find values between $4\ensuremath{\mathbin:}1$ and $5\ensuremath{\mathbin:}1$. Our technique applies generally to dimers and trimers of alkali atoms, here also demonstrated for Li2{\mathrm{Li}}_{2}Li2​, Na2{\mathrm{Na}}_{2}Na2​, and K2{\mathrm{K}}_{2}K2​, and will enable femtosecond time-resolved measurements of their rotational and vibrational dynamics, possibly with atomic structural resolution.
Quantum theory dictates that upon weakening the two-body interaction in a three-body system, an infinite number of three-body bound states of a huge spatial extent emerge just before these three-body states become unbound. Three helium (He) atoms have been predicted to form a molecular system that manifests this peculiarity under natural conditions without artificial tuning of the attraction between particles by an external field. Here we report experimental observation of this long-predicted but experimentally elusive Efimov state of (4)He3 by means of Coulomb explosion imaging. We show spatial images of an Efimov state, confirming the predicted size and a typical structure where two atoms are close to each other while the third is far away.
In this work, the strong-field-modified dipole response at the ionization threshold of helium is studied. The dipole response is induced by an attosecond pulse in the extreme ultraviolet spectral range and is manipulated by an ultrashort and strong femtosecond pulse in the near-infrared. To probe the response, the transient absorption spectrum of helium is recorded for different time delays between both pulses and different intensities of the femtosecond pulse. From the spectra, the dipole response of the ionization threshold is reconstructed, which is linked to the dynamics of excited electrons with energies in the transition region from bound to free. To identify the underlying processes of light-matter interaction leading to the observed structures in the time and spectral domain, different quantum-mechanical model simulations are conducted. As a result, the measured dipole response reveals light-induced energy shifts of the photoelectron’s kinetic energy close to the parent ion, signatures for field-driven recollisions of a photoelectron into the parent ion, and a temporal amplitude and phase gating mechanism. With the latter, the build-up dynamics of complex spectral structures are temporally resolved, which are the time-dependent separation and line-shape modification of the doubly excited Rydberg series as well as the temporal build-up of the ionization threshold.