2-Dimensional Phase-sensitive ULtrafast SpEctroScopy: unravelling photo-induced reactions by multi-dimensional Raman

Introduction

We propose the construction and development of a visible/ultraviolet (UV) two-dimensional resonance Raman (2DR) setup with phase-sensitive detection to tackle ultrafast chemical, physical, and biological processes. Light-induced reactions cover a broad range of phenomena, from screening of photo-damage in skin upon UV irradiation to carrier relaxation in opto-electronic devices and energy conversion in proteins.

Background

Their lowest hierarchical level lies in the interplay of nuclear motion and normal mode couplings, such as:

1. Funnelling the absorbed energy to the solvent via molecular oscillations in nucleobases
2. Electron-phonon/phonon-phonon couplings in graphene
3. Vibrational cooling in hemeproteins

Nature has intricately coupled vibrational degrees of freedom to facilitate light-energy conversion into synergistic nuclear motions, ruling femtochemistry and femtophysics.

Limitations of Conventional Methods

Conventional spectroscopic methods project structural information along specific normal coordinates, providing limited insights into these coupled motions.

Advantages of 2DR

2DR combines the structural sensitivity inherent to the Raman process with a multi-dimensional scheme, yielding frequency correlation spectra that encode information on the vibronic mode couplings across the entire vibrational manifold.

Challenges in Development

Critically, the development of 2DR and its application to light-driven processes has been hindered by technical and conceptual hurdles. Among them:

1. 2DR realizations have been confined to restricted visible regions, while most biomolecules require spectral tunability and/or UV excitations.
2. Vibrational signatures recorded by 2DR can be assigned both to vibrational (anharmonic) mode couplings as well as to (harmonic) high-order Raman transitions.

Proposed Solution

The setup of the proposed novel 2DR approach will circumvent these limitations, establishing an interdisciplinary research team toiling over unsolved problems in which the ultrafast and multidimensional facets play a key role.