Observation of Photoelectron Circular Dichroism Using a Nanosecond Laser

Abstract

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 multi‐photon 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.

Experimental and Theoretical Investigation of the 3sp(d) Rydberg States of Fenchone by Polarized Laser Resonance‐Enhanced‐Multiphoton‐Ionization and Fourier Transform VUV Absorption Spectroscopy

“…These include cavity ring‐down polarimetry, [10] and photoelectron circular dichroism (PECD) measured with synchrotron radiation in both soft X‐Ray C 1s ionization [11] and vacuum ultraviolet (VUV) valence band ionization [12,13] . Even more recently, fenchone has been a primary molecule of choice in fundamental investigations of laser multiphoton ionization PECD [14–20] including pump‐probe time‐resolved measurements [21,22] . Complementing the fundamental laser‐based studies, multiphoton ion yield [23] and enantiomeric excess determination [24,25] experiments have addressed more analytical applications using fenchone as a testcase (Scheme 1).…”

“…Older measurements of the He I photoelectron spectroscopy of fenchone [26–28] mapped out the valence band ionization profile at low resolution. Very much higher resolution was achieved in resonance enhanced multiphoton ionization (REMPI) spectra obtained using nanosecond pulse lasers for two photon excitation via the 3 s [29,30] and, more recently, an extended scan to include the 3 p region, [18] but with only limited analysis. Much of the understanding of the n =3 Rydberg states that putatively feature as intermediates in studies using REMPI ionization [14–16,18,20–25] is founded upon the VUV absorption spectrum and accompanying calculations reported in 1997 by Pulm et al [2] …”

Assessing the performance of rotational spectroscopy in chiral analysis

“…9 Recent instrumental developments led to a number of groundbreaking new experiments in the gas phase, which developed into alternative routes for distinguishing enantiomers and measuring the ee. Among these are microwave threewave mixing [10][11][12][13] (M3WM), photoelectron circular dichroism (PECD) using intense circularly polarised laser light [14][15][16] or synchrotron sources; 17 and Coulomb explosion with coincidence imaging. [18][19][20] Chirality research is now even extending to the atto-second regime, which will allow the study of the electron dynamics of chiral systems.…”

High-resolution resonance-enhanced multiphoton photoelectron circular dichroism

“…Earlier we observed nanosecond-pulse PECD without vibrational resolution with the help of a frequency tripled Nd:YAG laser. 57 Experiments are supported by theoretical modeling of the electronically excited states' vibrational level structure. In the low-energy region of the 3s state, the features observed in a previous 2+1 REMPI spectrum 50 can be assigned to single vibrational eigenstates.…”