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Register a new userPhotoelectron circular dichroism with two overlapping laser pulses of carrier frequencies $omega$ and $2omega$ linearly polarized in two mutually-orthogonal directions
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Using a model methane-like chiral system, we theoretically demonstrate a possibility to access photoelectron circular dichroism (PECD) by a single experiment with two overlapping laser pulses of carrier frequencies $omega$ and $2omega$, which are linearly polarized in two mutually-orthogonal directions. Depending on the relative phase, the resulting electric field can be tailored to have two different rotational directions in the upper and lower hemispheres along the polarization of the $omega$-pulse. We predict a strong forward/backward asymmetry in the emission of photoelectrons from randomly oriented samples, which has an opposite sign in the upper and lower hemispheres. The predicted PECD effect is phase- and enantiomer-sensitive, providing new insight in this fascinating fundamental phenomenon. The effect can be optimized by varying relative intensities of the pulses.
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We report on a joint experimental and theoretical study of photoelectron circular dichroism (PECD) in methyloxirane. By detecting O 1s-photoelectrons in coincidence with fragment ions, we deduce the molecules orientation and photoelectron emission di
rection in the laboratory frame. Thereby, we retrieve a fourfold differential PECD clearly beyond 50%. This strong chiral asymmetry is reproduced by ab initio electronic structure calculations. Providing such a pronounced contrast makes PECD of fixed-in-space chiral molecules an even more sensitive tool for chiral recognition in the gas phase.
It is commonly accepted that the magnitude of a photoelectron circular dichroism (PECD) is governed by the ability of an outgoing photoelectron wave packet to probe the chiral asymmetry of a molecule. To be able to accumulate this characteristic asymmetry while escaping the chiral ion, photoelectrons need to have relatively small kinetic energies of up to a few tens of electron volts. Here, we demonstrate a substantial PECD for very fast photoelectrons above 500 eV kinetic energy released from methyloxirane by a participator resonant Auger decay of its lowermost O $1s$-excitation. This effect emerges as a result of the Fano interference between the direct and resonant photoionization pathways, notwithstanding that their individual effects are negligibly small. The resulting dichroic parameter has an anomalous dispersion, i.e. it changes its sign across the resonance, which can be considered as an analogue of the Cotton effect in the X-ray regime.
Chirality is ubiquitous in nature and fundamental in science, from particle physics to metamaterials.The most established technique of chiral discrimination - photoabsorption circular dichroism - relies on the magnetic properties of a chiral medium and yields an extremely weak chiral response. We propose and demonstrate a new, orders of magnitude more sensitive type of circular dichroism in neutral molecules: photoexitation circular dichroism. It does not rely on weak magnetic effects, but takes advantage of the coherent helical motion of bound electrons excited by ultrashort circularly polarized light. It results in an ultrafast chiral response and the efficient excitation of a macroscopic chiral density in an initially isotropic ensemble of randomly oriented chiral molecules. We probe this excitation without the aid of further chiral interactions using linearly polarized laser pulses. Our time-resolved study of vibronic chiral dynamics opens a way to the efficient initiation, control and monitoring of chiral chemical change in neutral molecules at the level of electrons.
We describe a setup to study ultrafast dynamics in gas-phase molecules using time-resolved photoelectron and photoion spectroscopy. The vacuum ultraviolet (VUV) probe pulses are generated via strong field high-order harmonic generation from infrared femtosecond laser pulses. The band pass characteristic in transmission of thin indium (In) metal foil is exploited to isolate the $9^{text{th}}$ harmonic of the 800 nm fundamental (H9, 14 eV, 89 nm) from all other high harmonics. The $9^{text{th}}$ harmonic is obtained with high conversion efficiencies and has sufficient photon energy to access the complete set of valence electron levels in most molecules. The setup also allows for direct comparison of VUV single-photon probe with 800 nm multi-photon probe without influencing the delay of excitation and probe pulse or the beam geometry. We use a magnetic bottle spectrometer with high collection efficiency for electrons, serving at the same time as a time of flight spectrometer for ions. Characterization measurements on Xe reveal the spectral width of H9 to be $190pm60$ meV and a photon flux of $sim1cdot10^{7}$ photons/pulse after spectral filtering. As a first application, we investigate the S$_1$ excitation of perylene using time-resolved ion spectra obtained with multi-photon probing and time-resolved electron spectra from VUV single-photon probing. The time resolution extracted from cross-correlation measurements is $65pm10$ fs for both probing schemes and the pulse duration of H9 is found to be $35pm8$ fs.
Photoelectron circular dichroism (PECD) is a fascinating phenomenon both from a fundamental science aspect but also due to its emerging role as a highly sensitive analytic tool for chiral recognition in the gas phase. PECD has been studied with single-photon as well as multi-photon ionization. The latter has been investigated in the short pulse limit with femtosecond laser pulses, where ionization can be thought of as an instantaneous process. In this contribution, we demonstrate that multiphoton PECD still can be observed when using an ultra-violet nanosecond pulse to ionize chiral showcase fenchone molecules. Compared to femtosecond ionization, the magnitude of PECD is similar, but the lifetime of intermediate molecular states imprints itself in the photoelectron spectra. Being able to use an industrial nanosecond laser to investigate PECD furthermore reduces the technical requirements to apply PECD in analytical chemistry.
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