Mayur Manoj Jhamnani (New York University Abu Dhabi, United Arab Emirates)
LinkedIn: @Mayur Jhamnani; X: @mayur_jhamnani
Abstract: DNP is transforming NMR and MRI by significantly enhancing sensitivity through the transfer of polarization from electron spins to nuclear spins via microwave irradiation. However, the use of monochromatic continuous-wave irradiation limits the efficiency of DNP for systems with heterogeneous broad EPR lines. Broad-band techniques such as chirp irradiation offer a potential solution, particularly for Solid Effect (SE) DNP in such cases. Despite its widespread use, the role of quantum coherence generated during chirp irradiation remains unclear, even though it is a key factor in determining the maximum achievable DNP efficiency. In this work, we use density matrix formalism to provide a comprehensive understanding of the quantum coherence generated during non-adiabatic passages through electron-nucleus double-quantum (DQ) and zero-quantum (ZQ) SE transitions and their impact on Integrated Solid Effect DNP under chirp irradiation. Our analysis employs fictitious product-operator bases to trace the evolution of electron-nucleus coherence leading to integrated or differentiated SE. We also explore the role of decoherence in maximizing chirped DNP in microwave power or nutation frequency limited scenario. These findings provide an understanding of the role of coherence generated during pulsed-DNP and MAS-DNP at different temperature ranges. Our results reveal that quantum coherences generated during non-adiabatic passages critically determine whether the chirped DNP process yields Integrated Solid Effect (ISE), or Differential Solid Effect (DSE). By analyzing the evolution of the density matrix in DQ and ZQ subspaces, we show how coherence generation and its decay through decoherence play a decisive role in shaping the net DNP enhancement.
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Hello,
Could you clarify what you mean when you state that adiabatic pulses do not generate coherence, while non-adiabatic pulses generate coherence?
What would you say was the most surprising finding of your study? Is there a plan to confirm any of your theoretical findings experimentally?-
Hello,
Thanks for your question.
1. When the chirped MW irradiation is adiabatic (i.e., satisfies the Landau Zener condition for adiabaticity), an initial density matrix that is a polarization, Sz, undergoes complete inversion to -Sz. However, if the chirp is non-adiabatic, an initial density matrix that is a polarization, Sz, does not undergo complete inversion, rather, some coherence is generated.
2. Most surprising/important finding: We found that when the initial density matrix is a pure coherence (mSx + nSy)- an adiabatic pulse would lead to a generation of only coherence however, a non-adiabatic pulse would give both coherence and +/-polarization (sometimes +ve polarization is generated while other times -ve polarization is generated). This helped us explain the caveat between ISE and DSE in the paper: https://arxiv.org/html/2410.19170v1.
3. Regarding experiments – our findings are experimentally hard to validate as there are several effects, powder averaging and B1 field inhomogeneity.
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Hello Mayur, thank you for the presentation. It is a very interesting study.
Could you comment more about why a chirp pulse is never adiabatic between ZQ and DQ transitions? Does it depend on the available mw power (omega1) and how?
Why the decoherence process favours the ISE over the DSE?
Thank you. -
Hello,
Thanks for your question.
1. The MW chirp pulse can result in an adiabatic or non-adiabatic transition depending on the Landau-Zener (LZ) condition. The condition is that (pi*p^2) / 2k >> 1 for the chirp pulse to be adiabatic across a transition. Here p is the perturbation and k is the sweep rate.
For a single quantum transition, perturbation is just MW power (in the rotating frame). As a result, it is much easier to ensure adiabaticity. However, the perturbation for DQ and ZQ transition have the pseudo-secular hyperfine coupling term (B) in the denominator. This means, higher MW power (beyond the available MW power) is required to excite these resonances adiabatically.
2. The role of quantum coherence was causing the caveat between ISE and DSE. If the coherence is decayed, we can easily look at it from just a polarization standpoint. Since the SQ transition inverts the electron spin polarization from Sz to -Sz, the enhancement from the following ZQ transition will add to that from the DQ transition (leading to ISE). A detailed explanation is provided in https://arxiv.org/html/2410.19170v1.
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Hi Mayur, interesting talk! I am curious, what would you expect the results to look like if you assume relaxation and coupling parameters from various known radicals (e.g. Trityl, AMUPOL, P1 centers). Would the resulting DNP mechanism be strongly dependent on the couplings/relaxations expected within such radicals?
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Thank you for your reply Mayur.
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