Asymmetric photoelectron emission from chiral molecules using a high repetition rate laser

C M M Bond,J B Greenwood

doi:10.1088/1742-6596/1289/1/012027

Abstract

In the chiro-optical phenomenon known as photoelectron circular dichroism, the angular distribution of electrons ionized from chiral molecules by circularly polarized light pulses has been found to be anti-symmetric with respect to the direction of the light propagation. To study this phenomenon, chiral molecules were multiphoton ionized with a femtosecond laser in our laboratory. Using a simple stereo-detection setup, direct measurements of the asymmetry for the exemplar aromatic chiral molecule 1-phenylethanol were made using 260 nm pulses produced at a rate of 1 MHz. It was found that the asymmetry had a linear dependence on Stokes’ S3 parameter indicating that the photoelectron circular dichroism is due to a single photon process from the excited state of the molecule. This is in contrast to previous and new results for camphor where a more complex dependence on S3 suggested that selective excitation of molecules with certain orientations was influential.

Highlights

For more than 150 years chirality has fascinated scientists, with the two enantiomers being indistinguishable apart from when they interact with another chiral medium
The 4th harmonic at 260 nm is resonant with the π∗ − π absorption band of the molecule and there is an efficient (1+1) resonantly enhanced multiphoton ionization (REMPI) process
This indicates that the asymmetry is solely a result of the single photon ionization out of the intermediate excited state

Summary

Introduction

For more than 150 years chirality has fascinated scientists, with the two enantiomers being indistinguishable apart from when they interact with another chiral medium (either other chiral molecules or circularly polarized light). Since 2000 a new, highly sensitive method for enantiomer identification called the Photoelectron Circular Dichroism (PECD) effect has been investigated[1]. When a chiral molecule is ionized with circularly polarized photons there will be an asymmetry in the number of electrons emitted in the forward hemisphere (F) to the backward hemisphere (B) with respect to the laser propagation direction. This asymmetry reverses when switching between right circular polarized light (RCP) and left circularly polarized light (LCP). The use of femtosecond laser systems to measure this effect allows for the exploitation of resonances inside these aromatic molecules to create efficient ionization on a rapid time scale, allowing a wide range of interesting organic molecules to be investigated

Methods

Results

Conclusion