Photoelectron Circular Dichroism in the Spin-Polarized Spectra of Chiral Molecules

We studied strong-field multiphoton ionization of 1-iodo-2-methylbutane enantiomers with 395 nm circularly polarized laser pulses experimentally and theoretically. For randomly oriented molecules, we observe spin polarization up to about 15%, which is independent of the molecular enantiomer. Our experimental findings are explained theoretically as an intricate interplay between three contributions from HOMO, HOMO-1, and HOMO-2, which are formed of 5p-electrons of the iodine atom. For uniaxially oriented molecules, our theory demonstrates even larger spin polarization. Moreover, we predict a sizable enantiosensitive photoelectron circular dichroism of about 10%, which is different for different spin states of photoelectrons.

Figure 1

Results of our experiment performed for randomly oriented $R(-)$ and $S(+)$ enantiomers of IMB with 395 nm circularly polarized laser pulses of $3\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ peak intensity and positive helicity. Panel (a): the total photoelectron spectra for both enantiomers. The spectra are distorted by the instrumental response function; see text for details. The $R(-)$-IMB enantiomer is shown in the inset. Panel (b): spin polarization for both enantiomers. The error bars represent statistical uncertainties.

Figure 2

Results of our calculations for randomly oriented $\mathrm{R}(-)$-IMB enantiomer ionized by 395 nm circularly polarized laser pulses of ${10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ peak intensity and positive helicity. Panel (a): the total spectrum for all photoelectrons and its break down into the contributions from different molecular orbitals. The number of photons absorbed to produce each peak in the spectra is indicated by colored numbers. Panel (b): the break down of the total photoelectron spectrum into the contributions from spin-$\uparrow$ and spin-$\downarrow$ photoelectrons. Panel (c): individual spin polarization for each molecular orbital and the total result.

Figure 3

The same as in Fig. 2–2 but for the uniaxially oriented $R(-)$-IMB molecules, such that the $I\text{−}C$ bond is aligned along the light propagation direction.

Figure 4

Calculated multiphoton PECD of randomly oriented $R(-)$-IMB enantiomers ionized by 395 nm circularly polarized laser pulses of ${10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ peak intensity. The most intense low-energy part of the photoelectron spectrum in Fig. 2 with the photoelectron kinetic energies below 2 eV is shown. Two panels represent the chiral asymmetries separately for the spin-$\uparrow$ and spin-$\downarrow$ photoelectrons, each in percent relatively to the maximal pixel intensity in their own momentum distributions obtained for positive light helicity (shown in insets to each panel). Those maxima yield for spin-$\uparrow$ about 55% and for spin-$\downarrow$ 45% of a maximum in the total spectrum of all photoelectrons (correspond to the threshold spin polarization of about $+10\%$ in Fig. 2). The pulse propagates along $k_{||}$ from the left to the right. Note that the photoelectron momentum $k$ is set on the linear scale of the kinetic energy $ϵ$, as shown by the downward inclined arrows with dotted concentric circles.