Multiphoton photoelectron circular dichroism of limonene with independent polarization state control of the bound-bound and bound-continuum transitions

Abstract

Photoionization of randomly oriented chiral molecules with circularly polarized light leads to a strong forward/backward asymmetry in the photoelectron angular distribution. This chiroptical effect, referred to as Photoelectron Circular Dichroism (PECD), was shown to take place in all ionization regimes, from single photon to tunnel ionization. In the Resonance Enhanced Multiphoton Ionisation (REMPI) regime, where most of the table-top PECD experiments have been performed, understanding the role of the intermediate resonances is currently the subject of experimental and theoretical investigations. In an attempt to decouple the role of bound-bound and bound-continuum transitions in REMPI-PECD, we photoionized the (+)-limonene enantiomer using two-color laser fields in [1+1'] and [2+2'] ionization schemes, where the polarization state of each color can be controlled independently. We demonstrate that the main effect of the bound-bound transition is to break the sample isotropy by orientation-dependent photoexcitation, in agreement with recent theoretical predictions. We show that the angular distribution of PECD strongly depends on the anisotropy of photoexcitation to the intermediate state, which is different for circularly and linearly polarized laser pulses. On the contrary, the helicity of the pulse that drives the bound-bound transition is shown to have a negligible effect on the PECD. 1 arXiv:1801.08394v2 [physics.chem-ph] 31 Aug 2018Photoelectron circular dichroism (PECD) is a chiroptical effect which was theoretically predicted in the 1970's 1 . Unlike most chiroptical phenomena, which rely on magnetic dipole or electric quadrupole effects, PECD can be fully described within the framework of the electric dipole approximation. As a consequence, PECD is several orders of magnitude larger than conventional chiroptical effects, which are in general extremely weak and thus difficult to use for studies in gas-phase samples. The first experimental observation of PECD from an ensemble of randomly oriented chiral molecules was performed in 2001, using Vacuum Ultra Violet (VUV) synchrotron radiation 2 . Since then, many studies relying on single photon ionization have been conducted, showing that PECD is a powerful probe of molecular chirality (for a review see 3 ), sensitive to electronic structure 4 , vibrational excitation 5,6 , molecular conformation 7-9 , structural isomerism 10,11 , clustering 12 as well as chemical substitution 13,14 .Lately, it was also demonstrated that single photon PECD experiments could be performed using table-top femtosecond elliptically polarized VUV sources from resonant high-order harmonic generation 15 . Recently, a whole new field of PECD studies emerged, relying on the use of UV, visible or IR table-top femtosecond laser sources. In two pioneering experiments, circularly polarized femtosecond UV pulses were used to photoionize enantiopure chiral molecules in a [2+1]Resonant Enhanced Multiphoton Ionisation (REMPI) scheme, and led to the observation of large PECDs, in the 10% range ...

Observation of Photoelectron Circular Dichroism Using a Nanosecond Laser

“…[25,26] The influence of polarization state used for excitation and ionization on observed PECD was modeled [26] and investigated experimentally. [27] Using a fs pump-probe setup, dynamics in the 3s intermediate state were investigated for fenchone. [28,29] When the duration of the laser pulses is increased to longer than ps, intra-molecular dynamics in addition to rotation of the molecules can evolve during ionization, where it is not clear whether PECD can be observed.…”

“…For REMPI studies, changing the laser wave-length allows to study the dependence of PECD on intermediate state and photoelectron kinetic energy [17,[22][23][24] in comparison to theoretical descriptions [25,26]. The influence of polarization state used for excitation and ionization on observed PECD was modeled [26] and investigated experimentally [27]. Using a fs pump-probe setup, dynamics in the 3s intermediate state were investigated for fenchone [28,29].…”

Real-time determination of enantiomeric and isomeric content using photoelectron elliptical dichroism

“…In the three molecular species considered here, the resonant absorption of three photons at 515 nm (2.4 eV) promotes the molecules to high-lying Rydberg states, from which they are photoionized by further absorption of one or several photons. By decoupling the excitation and ionization steps through a two-color experiment, we recently demonstrated that photoexciting molecules with linearly or circularly polarized photons led to strong changes in the PECD, with possible sign inversions 31 . This is the result of the anisotropy of excitation introduced by the resonance.…”

Chiral discrimination by recollision enhanced femtosecond laser mass spectrometry

“…Currently, understanding the role of the intermediate resonances is subject of experimental and theoretical investigations. Recently it was demonstrated, by decoupling bound-bound and bound-continuum transitions, that the helicity of the light pulse that drives the bound-bound transitions has a negligible effect on the PECD 51,52 . However, circularly and linearly polarized laser pulses still lead to different PECDs because of the angular dependence of the excitation process, which induces different anisotropies.…”