Resolving the nature of the carotenoid S* state via Femtosecond Stimulated Raman scattering

Miroslav Kloza, Joern Weisenbornb, Tomas Polivkac, Harry A. Frankd, John Kennisb
aELI-Beamlines, Na Slovance 2, 182 21 Praha 8, Czech Republic; bDepartment of Physics and Astronomy, VU University Amsterdam, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands; cUniversity of South Bohemia, Branišovská 1645/31a, 370 05 České Budějovice; dDepartment of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, CT 06269 USA

A new method for recording femtosecond stimulated Raman spectra was developed that dramatically improves and automatizes baseline problems. Instead of using a narrowband Raman source, the experiment is performed using shaping of a broadband source. This allows locking the signal into carefully crafted watermarks that can be recovered from measured data with high fidelity. The approach uses unique properties of Raman scattering, thus allowing a direct recording of stimulated Raman signals with robust rejection of baselines and fixed-pattern-noise. Low cost technology for generating required pulse-shapes was developed and demonstrated. The methodology is applicable to any Raman experiment but primarily targets. Femtosecond Stimulated Raman spectroscopy (FSRS) where a lack of robust methods for parasitic signal rejection has been a major obstacle in the practical development of the field in the last decade. The delivered improvement in FSRS experiments was demonstrated by recording evidence that the so-called S* state of carotenoids in solution corresponds to the optically forbidden S1 state of a sparsely populated carotenoid conformation.

Figure 1: Femtosecond transient absorption spectra of β-carotene (left panel) and spirilloxanthin (right panel) in the S1 state fingerprint region at the time delays indicated. We can observe the spirilloxanthin peak to manifest a higher energy shoulder that decays at a distinctly slower rate making the S1 peak to effectively shift its maximum from 1743 cm-1 to 1771 cm-1 in less than 3 ps. These observations are in agreement with known S1 and S* dynamics in spirilloxanthin suggesting that 1771 cm-1 is indeed an S* state signature.