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Register a new userUsing circular dichroism to control energy transfer in multi-photon ionization
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Chirality causes symmetry breaks in a large variety of natural phenomena ranging from particle physics to biochemistry. We investigate one of the simplest conceivable chiral systems, a laser-excited, oriented, effective one-electron Li target. Prepared in a polarized p state with |m|=1 in an optical trap, the atoms are exposed to co- and counter-rotating circularly polarized femtosecond laser pulses. For a field frequency near the excitation energy of the oriented initial state, a strong circular dichroism is observed and the photoelectron energies are significantly affected by the helicity-dependent Autler-Townes splitting. Besides its fundamental relevance, this system is suited to create spin-polarized electron pulses with a reversible switch on a femtosecond timescale at an energy resolution of a few meV.
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We investigate few-photon ionization of lithium atoms prepared in the polarized 2$p$($m_ell=!+1$) state when subjected to femtosecond light pulses with left- or right-handed circular polarization at wavelengths between 665 nm and 920 nm. We consider whether ionization proceeds more favorably for the electric field co- or counter-rotating with the initial electronic current density. Strong asymmetries are found and quantitatively analyzed in terms of circular dichroism ($CD$). While the intensity dependence of the measured $CD$ values is rather weak throughout the investigated regime, a very strong sensitivity on the center wavelength of the incoming radiation is observed. While the co-rotating situation overall prevails, the counter-rotating geometry is strongly favored around 800 nm due to the 2$p$-3$s$ resonant transition, which can only be driven by counter-rotating fields. The observed features provide insights into the helicity dependence of light-atom interactions, and on the possible control of electron emission in atomic few-photon ionization by polarization-selective resonance enhancement.
We consider few-photon ionization of atomic lithium by linearly polarized femtosecond laser pulses, and demonstrate that asymmetries of the electron angular distribution can occur for initially polarized (2p, m=+1) target atoms. The dependence of the main photoelectron emission angle relative to the electric field direction is investigated at different laser intensities and wavelengths. The experimental spectra show excellent agreement with numerical solutions of the time-dependent Schroedinger equation. In the perturbative picture, the angular shift is traced back to interferences between partial waves with mean magnetic quantum number <m>$ e$0. This observation allows us to obtain quantum mechanical information on the electronic final state.
We investigate the differential ionization probability of chiral molecules in the strong field regime as a function of the helicity of the incident light. To this end, we analyze the fourfold ionization of bromochlorofluoromethane (CHBrClF) with subsequent fragmentation into four charged fragments and different dissociation channels of the singly ionized methyloxirane. We observe a variation of the differential ionization probability in a range of several percent. Accordingly, we conclude that the helicity of light is a quantity that should be considered for the theoretical description of the strong field ionization rate of chiral molecules.
The intermediate state dependence of photoelectron circular dichroism (PECD) in resonance-enhanced multi-photon ionization of fenchone in the gas phase is experimentally studied. By scanning the excitation wavelength from 359 to 431 nm we simultaneously excite up to three electronically distinct resonances. In the PECD experiment performed with a broadband femtosecond laser their respective contributions to the photoelectron spectrum can be resolved. High-resolution spectroscopy allows us to identify two of the resonances as belonging to the B- and C-bands, which involve excitation to states with 3s and 3p Rydberg character, respectively. We observe a sign change in the PECD signal depending on which electronic state is used as an intermediate. Additionally, scanning the laser wavelength reveals a decrease of PECD magnitude with increasing photoelectron energy for the 3s state.
Ionization of atoms and molecules by absorption of a light pulse results in electron wavepackets carrying information on the atomic or molecular structure as well as on the dynamics of the ionization process. These wavepackets can be described as a coherent sum of waves of given angular momentum, called partial waves, each characterized by an amplitude and a phase. The complete characterization of the individual angular momentum components is experimentally challenging, requiring the analysis of the interference between partial waves both in energy and angle. Using a two-photon interferometry technique based on extreme ultraviolet attosecond and infrared femtosecond pulses, we characterize the individual partial wave components in the photoionization of the 2p shell in neon. The study of the phases of the angular momentum channels allows us to unravel the influence of short-range, correlation and centrifugal effects. This approach enables the complete reconstruction of photoionization electron wavepackets in time and space, providing insight into the photoionization dynamics.
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