Ahmed Alzaidi
Advisor: Assist. Prof. Dr. Deniz Aybaş
Title: Spectral analysis for Nuclear Magnetic Resonance across Ultralow to High Magnetic Fields
Abstract: Nuclear magnetic resonance spectroscopy spans magnetic field strengths from conventional high field (1-23 Tesla) to the emerging zero to ultralow field (ZULF) regime (microtesla range and below). The fundamental physics, detection methods, and spectral analysis techniques differ significantly between these regimes, requiring distinct experimental and computational approaches.
In high-field NMR, the Zeeman interaction dominates, yielding spectra characterized by chemical shifts and J-coupling multiplets. Inductive detection provides sensitivity scaling as the square of field strength. In ZULF NMR, J-coupling interactions govern the spin Hamiltonian, with eigenstates described by total angular momentum quantum numbers in the singlet-triplet basis. Detection requires non-inductive magnetometers such as Superconducting Quantum Interference Devices (SQUIDs), Optically Pumped Magnetometers operating in the Spin-Exchange Relaxation-Free regime, or Nitrogen-Vacancy centers in diamond, providing frequency-independent sensitivity in the audio range.
Experimental characterization of dichloromethane around a Tesla magnetic field validates the theoretical framework. Quadrature detection yields a spectra with which multiple decay models are evaluated: a simple exponential model is compared with a polynomial envelope, where the latter fitting captures the non-exponential character arising from static magnetic field field inhomogeneity. The oscillating signal fits directly \textbf{what does this mean?} extract frequency and phase, confirming pulse calibration
Quantum mechanical simulations using QuTip reproduce experimental Free Induction Decay signals and spectra with high fidelity. Critical corrections include proper transverse lowering operators in collapse operator formalism, and appropriate scaling for averaging with a multi-spin system. Classical Bloch equation simulation and quantum Lindblad master equation methods both achieve spectral peak agreement within experimental resolution. Theoretical molecular relaxation time from dipolar coupling combined with field gradient contribution predicts a spin coherence time within 25% of experimental values, validating the inhomogeneity-dominated relaxation mechanism.
Field-dependent analysis across 50 microTesla to 3 Tesla reveals that molecular relaxation remains constant in the extreme narrowing regime, while effective spin coherence time decreases at higher fields due to increasing gradient-induced dephasing. The validated simulation and analysis framework enables precise characterization of relaxation dynamics, spectral features, and sensitivity optimization across the full range of NMR field strengths.
Date: December 25, Thursday
Time: 10:30
Place: Department of Physics seminar room SA-240