1：Experimental Design and Method Selection:

The study employed a pump-probe technique using a wavelength-tunable infrared free electron laser (FEL) to investigate relaxation dynamics. The method involved resonant excitation of boron acceptor states and measurement of transmission changes with variable delays between pump and probe pulses. Theoretical models, including cascade relaxation with multiple levels, were used for data analysis.

2：Sample Selection and Data Sources:

Samples were IIb type boron-doped diamond single crystals grown by the high-pressure high-temperature (HPHT) method, with low nitrogen content. Two samples were used: one doped with natural boron (80% 11B + 20% 10B) and another with isotopically enriched 11B (99%). Samples were cut, polished, and wedged to avoid optical feedback.

3：List of Experimental Equipment and Materials:

Equipment included a Bruker VERTEX 80v infrared Fourier transform spectrometer (FTS) for absorption spectra, a free electron laser (FEL) at FELIX Laboratory for pump-probe measurements, a liquid helium flow cryostat for temperature control, a mercury cadmium telluride (MCT) infrared detector, beam splitters (BaF2), delay stages, power attenuators, polarization rotators, parabolic mirrors, and a boxcar integrator. Materials included boron-doped diamond samples with specified boron concentrations.

4：Experimental Procedures and Operational Workflow:

The FEL generated micropulses at 25 MHz repetition rate, split into pump, probe, and reference beams. Pump and probe beams were focused on the sample, with variable delays introduced via a delay stage. Transmission signals were measured at different temperatures (5 K to room temperature), pump wavelengths (3.4-4.1 μm), and intensities. Data were normalized and analyzed to extract relaxation times.

5：4-1 μm), and intensities. Data were normalized and analyzed to extract relaxation times. Data Analysis Methods:

5. Data Analysis Methods: Data were fitted using multi-level cascade relaxation models (two-, three-, and four-level) to derive time constants. The analysis involved solving balance equations for level populations and minimizing root mean square deviations. Radiative recombination rates were estimated based on absorption cross-sections, and nonradiative processes were emphasized.