| Summary: | <p>Laser-induced magnetic dipole (LaserIMD) spectroscopy and
light-induced double electron–electron resonance (LiDEER) spectroscopy are
important techniques in the emerging field of light-induced pulsed dipolar
electron paramagnetic resonance (EPR) spectroscopy (light-induced PDS). These techniques use the
photoexcitation of a chromophore to the triplet state and measure its
dipolar coupling to a neighboring electron spin, which allows the
determination of distance restraints. To date, LaserIMD and LiDEER have been
analyzed with software tools that were developed for a pair of two <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>S</mi><mo>=</mo><mn mathvariant="normal">1</mn><mo>/</mo><mn mathvariant="normal">2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="6e8f49899d165602582d3610581c1fbd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="mr-4-27-2023-ie00001.svg" width="41pt" height="14pt" src="mr-4-27-2023-ie00001.png"/></svg:svg></span></span>
spins and that neglected the zero-field splitting (ZFS) of the excited triplet.
Here, we explore the limits of this assumption and show that the ZFS can
have a significant effect on the shape of the dipolar trace. For a detailed
understanding of the effect of the ZFS, a theoretical description for
LaserIMD and LiDEER is derived, taking into account the non-secular terms of
the ZFS. Simulations based on this model show that the effect of the ZFS is
not that pronounced in LiDEER for experimentally relevant conditions. However,
the ZFS leads to an additional decay in the dipolar trace in LaserIMD. This
decay is not that pronounced in Q-band but can be quite noticeable for lower
magnetic field strengths in X-band. Experimentally recorded LiDEER and
LaserIMD data confirm these findings. It is shown that ignoring the ZFS in
the data analysis of LaserIMD traces can lead to errors in the obtained
modulation depths and background decays. In X-band, it is additionally
possible that the obtained distance distribution is plagued by long distance
artifacts.</p>
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