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.

Gustavo A. Garcia,Dhirendra P. Singh,Nelson De Oliveira,Arno Vredenborg,Ivan Powis

doi:10.1002/cphc.202000365

Abstract

The VUV absorption spectrum of fenchone is re-examined using synchrotron radiation Fourier transform spectrometry, revealing new vibrational structure. Picosecond laser (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy complements this, providing an alternative view of the 3spd Rydberg excitation region. These spectra display broadly similar appearance, with minor differences that are largely explained by referring to calculated one- and two-photon electronic excitation cross-sections. Both show good agreement with Franck-Condon simulations of the relevant vibrational structures. Parent ion REMPI ionization yields with both femtosecond and picosecond excitation laser pulses are studied as a function of laser polarization and intensity, the latter providing insight into the relative two-photon excitation and one-photon ionization rates. The experimental circular-linear dichroism observed in the parent ion yields varies strongly between the 3s and 3p Rydberg states, in good overall agreement with the calculated two-photon excitation circular-linear dichroism, while corroborating other evidence that the 3pz sub-state plays no more than a very minor role in the (2+1) REMPI spectrum. Vibrationally resolved photoelectron spectra are recorded with picosecond pulse duration (2+1) REMPI at selected intermediate vibrational excitations. The 3s intermediate state displays a very strong Δv=0 propensity on ionization, but the 3p intermediate evidences more complex vibronic dynamics, and we infer some 3p→3s internal conversion prior to ionization.

Highlights

Fenchone is a naturally occurring chiral terpenoid compound (C10H16O) that contributes its distinctive odour to plants such as fennel
Alongside its many claimed uses as an essential oil extract it is commonly adopted as a Scheme 1 1R-(-)-fenchone benchmark molecule with established chiroptical techniques such as electronic absorption circular dichroism (ECD) [1, 2], vibrational circular dichroism (VCD) [3], Raman optical activity (ROA) [4], and optical rotatory dispersion (ORD)[5]
By recording the Fourier transform (FT) vacuum ultraviolet (VUV) absorption spectrum of the Rydberg region of Fenchone we have revealed new vibrational structure in the lower energy 3spd region

Summary

Introduction

Fenchone is a naturally occurring chiral terpenoid compound (C10H16O) that contributes its distinctive odour to plants such as fennel. O benchmark molecule with established chiroptical techniques such as electronic absorption circular dichroism (ECD) [1, 2], vibrational circular dichroism (VCD) [3], Raman optical activity (ROA) [4], and optical rotatory dispersion (ORD)[5]. In this context, fenchone has a rigid bicyclic structure that prevents it adopting multiple conformations, removing ambiguity about the structural forms present in various media. 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

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Discussion

Conclusion