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Chirality-Sensitive Ultrafast Spectroscopy


Chirality-Sensitive Ultrafast Spectroscopy

A. Steinbacher1, S. Schott1, F. Kanal1, C. Schwarz1, P. Nuernberger2, T. Brixner1*

1Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany, brixner@phys-chemie.uni-wuerzburg.de

2Physikalische Chemie II, Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany

Ultrafast structural dynamics can be investigated by a variety of methods. For example, we have shown the applicability of coherent multidimensional spectroscopy for photochemical reactions on the example of cis-trans isomerization [1]. A different approach – the topic of this presentation – is to employ chirality as a signature of (time-dependent) structure. In general, chirality arises from symmetry considerations: A chiral object cannot be superimposed with its mirror image. Progress will be shown toward chirally selective time-resolved spectroscopy of molecules in the condensed phase. For this purpose, a variety of fundamental and practical issues have to be addressed. Solutions to some of the problems will be discussed. As a means to introduce and probe chirality via light in the most flexible fashion, we have developed vector-field shaping with independent ultrafast control over amplitude, phase, and polarization of an ultrashort pulse as a function of time [2]. In a second project, we have developed shot-to-shot detection of full spectra at 100 kHz repetition rate with a synchronized 50 kHz chopper [3]. This is helpful since chiral signals are small and it is thus desirable to increase the signal-to-noise ratio for a given measurement time. In time-resolved spectroscopy of population dynamics one also has to take care that anisotropic contributions to the signal are avoided. With linearly polarized pump and probe pulses in transient absorption spectroscopy, the “magic angle” configuration is commonly employed. However, for pulses with other polarizations or more than two laser beams the situation is more complicated. We have derived conditions for anisotropy-free measurements with arbitrary polarizations and geometry [4]. Another necessary ingredient for chiral spectroscopy is a detection method that provides chiral sensitivity. We have constructed a highly sensitive polarimeter and combined it with accumulative spectroscopy to measure the optical rotation change upon a chirality-modifying photochemical reaction [5]. With this setup we further achieved all-optical discrimination between racemic and achiral molecular solutions [6]. A second option for chiral detection is to measure photoinduced changes in circular dichroism (CD). Femtosecond time-resolved CD spectroscopy is challenging and prone to artefacts, thus in the literature single-wavelength detection is mostly employed. We have developed broadband time-resolved CD spectroscopy [to be published]. It is based on a “light-pulse enantiomer” setup that can create a copy, as well as its precise polarization-mirrored image, of any (polarization-shaped) input laser pulse. Thus, we can switch between opposite chiralities of the (probing) laser field on a shot-to-shot basis and measure broadband time-resolved CD spectra. As an example, we investigated the ultrafast dynamics of hemoglobin upon oxygen release.


[1] S. Ruetzel, M. Diekmann, P. Nuernberger, C. Walter, B. Engels, and T. Brixner, PNAS 111, 4764 (2014).

[2] C. Schwarz, O. Hüter, and T. Brixner, J. Opt. Soc. Am. B 32, 933 (2015).

[3] F. Kanal, S. Keiber, R. Eck, and T. Brixner, Opt. Express 22, 16965 (2014).

[4] S. Schott, A. Steinbacher, J. Buback, P. Nuernberger, and T. Brixner, J. Phys. B 47, 124014 (2014).

[5] A. Steinbacher, J. Buback, P. Nuernberger, and T. Brixner, Opt. Express 20, 11838 (2012).

[6] A. Steinbacher, P. Nuernberger, and T. Brixner, Phys. Chem. Chem. Phys. 17, 6340 (2015).