Abstract
The measurement of chirality and its temporal evolution are crucial for the understanding of a large range of biological functions and chemical reactions. Steady-state circular dichroism (CD) is a standard analytical tool for measuring chirality in chemistry and biology. Nevertheless, its push into the ultrafast time domain and in the deep-ultraviolet has remained a challenge, with only some isolated reports of subnanosecond CD. Here, we present a broadband time-resolved CD spectrometer in the deep ultraviolet (UV) spectral range with femtosecond time resolution. The setup employs a photoelastic modulator to achieve shot-to-shot polarization switching of a 20 kHz pulse train of broadband femtosecond deep-UV pulses (250–370 nm). The resulting sequence of alternating left- and right-circularly polarized probe pulses is employed in a pump-probe scheme with shot-to-shot dispersive detection and thus allows for the acquisition of broadband CD spectra of ground- and excited-state species. Through polarization scrambling of the probe pulses prior to detection, artifact-free static and transient CD spectra of enantiopure [Ru(bpy)3]2+ are successfully recorded with a sensitivity of <2×10−5 OD (≈0.7 mdeg). Due to its broadband deep-UV detection with unprecedented sensitivity, the measurement of ultrafast chirality changes in biological systems with amino-acid residues and peptides and of DNA oligomers is now feasible.
Highlights
Circular dichroism (CD) denotes the difference in absorption of left- and right-handed circularly polarized light in chiral molecular systems
Time-resolved CD spectroscopy (TRCD) is a promising experimental technique that is sensitive to changes in biomolecular configuration as a function of time, combining the time-dependent electronic information provided by traditional transient absorption (TA) spectroscopy with the structural information encoded in the chirality of molecular systems
We address all of the above challenges by combining a femtosecond broadband deep-UV source at a high repetition rate with broadband shot-to-shot polarization state switching via a photoelastic modulator (PEM)
Summary
Circular dichroism (CD) denotes the difference in absorption of left- and right-handed circularly polarized light in chiral molecular systems. TRCD measurements require a sensitivity on the order of 1 mdeg in the commonly employed units of ellipticity, which corresponds to an absorptivity of ≈3 × 10−5 OD Such sensitivities have only recently been achieved in the visible (>400 nm) spectral regime [4,5]. Broadband detection is extremely difficult due to the polarization sensitivity of dispersive and reflective optics, which distorts static CD spectra [6,7]. This has limited commercial static CD spectropolarimeters and many TRCD setups to narrowband detection schemes [3,5,8], requiring probe wavelength scanning with increased DAQ. Our setup minimizes polarization artifacts through polarization scrambling prior to broadband dispersive detection
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