Summary of "Лаба по изучению электронного парамагнитного резонанса (ЭПР) и определению g-фактора электрона"

Summary of the Video: Laboratory Work on Electron Paramagnetic Resonance (EPR) and Measurement of Electron g-Factor

This video presents a detailed walkthrough of a laboratory experiment focused on studying Electron Paramagnetic Resonance (EPR) and determining the electron g-factor using a radio spectrometer setup. The content covers the theoretical basis, experimental setup, measurement techniques, data acquisition, and calibration procedures.

Main Ideas and Concepts

Electron Paramagnetic Resonance (EPR): EPR is a phenomenon where unpaired electrons in a substance absorb electromagnetic energy in the presence of a magnetic field, causing transitions between spin energy levels split by the Zeeman effect.
Substance Under Study: Diphenyl-picryl-hydrazyl (DPPH), a paramagnetic crystal with unpaired electrons, is used as the sample. Its unpaired electrons interact with the magnetic field, enabling resonance absorption.
Resonance Condition: Resonance occurs when the energy of the electromagnetic radiation matches the energy difference between electron spin states in a magnetic field. This energy difference depends on the magnetic field strength and can be tuned by changing the field or the frequency.
Radio Spectrometer Components:
- High-frequency signal generator: Around 70–140 MHz to excite the LC circuit.
- LC circuit (face contour): Comprises a coil and variable capacitor plates; its resonant frequency can be adjusted by changing capacitor plate distance.
- Modulation coils: Apply an alternating magnetic field superimposed on a constant magnetic field to modulate resonance conditions.
- Permanent magnet coils: Generate a constant magnetic field.
- Oscilloscope: Displays signals proportional to the amplitude of oscillations in the LC circuit, reflecting resonance conditions.
- Voltmeter and ammeter: Measure current and voltage related to magnetic field strength and circuit parameters.
- Probe coil: Used for calibration by measuring induced voltages related to magnetic field strength.
Quality Factor (Q) Measurement: The presence of the paramagnetic sample affects the quality factor of the LC circuit because the sample absorbs energy at resonance, reducing the circuit’s Q-factor. This change is detected by measuring amplitude variations in the oscillations.
Modulation Technique: A small alternating magnetic field is applied to enhance signal detection by modulating the absorption, enabling detection of small changes in resonance conditions.
Calibration: Calibration involves correlating the measured voltage across a known resistor (linked to coil current) with the magnetic field strength. A probe coil with known parameters is used to measure the effective magnetic field amplitude, allowing conversion of voltages to magnetic field values (in millitesla).
Data Acquisition and Analysis:
- The resonance frequency is scanned by adjusting the generator frequency and capacitor plates to find the resonance condition where the amplitude signal is minimized or maximized (depending on inversion settings).
- The magnetic field is varied by changing current through the permanent magnet coils, and resonance peaks are tracked on the oscilloscope.
- Measurements are taken at multiple frequencies (e.g., 100 MHz to 144 MHz) to establish the linear relationship between resonance frequency and magnetic field strength.
- Symmetry and position of resonance peaks on the oscilloscope are adjusted using phase shifters and current controls to ensure accurate readings.
- The width of resonance peaks is analyzed to estimate energy level widths and electron spin relaxation times.
Final Outcome: The experiment yields data correlating resonance frequency with magnetic field strength, enabling calculation of the electron g-factor. Calibration curves are constructed to translate measured voltages into magnetic field units.

Methodology / Step-by-Step Instructions

Setup and Initialization:
- Assemble the radio spectrometer including the LC circuit, modulation coils, permanent magnet coils, and connect the sample (DPPH) in the flask.
- Turn on the high-frequency generator and set an initial frequency range (~70–140 MHz).
- Adjust capacitor plates to tune the LC circuit resonant frequency close to the generator frequency.
Signal Detection and Oscilloscope Configuration:
- Connect the oscilloscope to monitor the amplitude of oscillations in the LC circuit.
- Set oscilloscope to DC mode to observe baseline signals and switch to AC mode to detect modulated signals.
- Adjust sensitivity and position controls to center the signal and maximize visibility of resonance peaks.
Magnetic Field Application and Modulation:
- Use the power supply to run current through permanent magnet coils to create a constant magnetic field.
- Adjust current to vary magnetic field strength.
- Apply an alternating current through modulation coils to superimpose a small variable magnetic field.
Finding Resonance:
- Slowly vary generator frequency and capacitor plate spacing to find resonance, indicated by changes in amplitude on the oscilloscope.
- Adjust current in permanent magnet coils to shift magnetic field and observe corresponding shifts in resonance peaks.
- Use inversion and phase adjustment controls to symmetrize and center resonance peaks on the oscilloscope screen.
Calibration:
- Insert the probe coil near the spectrometer coil to measure induced voltages proportional to the magnetic field amplitude.
- Record voltages from the probe coil and voltmeter across the resistor in the magnet coil circuit.
- Calculate calibration curve correlating voltage readings to magnetic field strength (in millitesla).
Data Recording:
- For multiple frequencies, record the resonance peak positions and corresponding current/voltage values.
- Measure shifts in resonance peaks by moving them one division to the left and right on the oscilloscope grid to estimate calibration accuracy.
- Analyze resonance peak widths to estimate electron spin relaxation properties.
Analysis and Calculation:
- Use the calibration curve to convert measured voltages into magnetic field strengths.
- Plot resonance frequency versus magnetic field strength to verify linear dependence and extract the electron g-factor.
- Use peak widths to estimate energy level widths and related quantum properties.

Key Lessons and Concepts

EPR allows measurement of electron spin properties by detecting resonance absorption of electromagnetic energy in a magnetic field.
Quality factor changes in an LC circuit can be used to detect resonance absorption by a paramagnetic sample.
Modulation of the magnetic field enhances detection sensitivity by converting small absorption changes into measurable oscillations.
Calibration with a probe coil is essential for converting electrical measurements into magnetic field units.
Careful tuning and phase adjustment are critical for obtaining clear, symmetrical resonance signals.
The linear relationship between resonance frequency and magnetic field strength enables determination of fundamental electron properties like the g-factor.

Speakers / Sources Featured

Primary Speaker / Narrator: An instructor or laboratory demonstrator explaining the setup, theory, and procedures throughout the video.
No other distinct speakers or external sources are identified; the entire content appears to be a continuous demonstration and explanation by a single presenter.

This summary captures the essence of the video, detailing the experimental setup, theoretical background, measurement techniques, and calibration processes involved in studying EPR and determining the electron g-factor using a radio spectrometer.

Share this summary

Summarize another video