Electron Paramagnetic Resonance PH425 System outline Michael Kabatek & Jessica Lovewell Version 1.1 This outline is intended to introduce you to the PH425 EPR system. The system consists of a field controlled lock-in detection scheme which you will be using to study EPR of a solid black powered sample of the free radical diphenyl picryl hydrazil (DPPH). Understanding the system will be essential in order to perform EPR experiments in this laboratory. Understanding this system will allow one to perform a complete power calibrated measurement of EPR. It will allow for measurement of not only the resonance conditions of black powered DPPH but also the energy and power absorption associated with the resonance. The system schematic is shown in figure 1. Figure 1. The field controlled lock-in detection scheme used to measure EPR in solid black powder sampled of DPPH. The system consists of a microwave source, electromagnet, helical cavity, lock-in amplifier, and computer with interface cards.
Measurement techniques are covered in an additional theory handout (ELECTRON PARAMAGNETIC RESOSANCE), be sure you read and understand the experiment description before continuing. The components which comprise the system are the microwave source, electromagnet, frequency counter, lock-in amplifier, diode detector, helical cavity, 60Hz reference, and computer with interface cards. Measurements are preformed by sweeping the applied magnetic field, and recording the output voltage from the lock-in. This can be automated in a number of ways using the computer and labview program. The microwave source will allow you to set the power and frequency of microwaves which are pumped into the sample. The microwave source is not computer controlled so you will have to manually adjust the frequency and power of microwaves pumped into the sample. The microwave source is shown below: The microwave source is picked off, sent through a 20 db attenuator, and input into a digital frequency counter so that one can accurately read the pumping frequency. The microwave source should be connected into the helical cavity which contains the DPPH sample. The output lead of the helical cavity will then go through a diode detector and into the lock-in amplifier. Note that the 60Hz modulation is also connected to both the helical cavity and lock-in amplifier and neither are computer controlled.
The lock-in amplifier is shown below (left) and the 60Hz reference/field modulator is shown below (right): The lock-in amplifier output is connected to one of the lab jack data acquisition unit (DAQ) inputs (AI0 - analog input 0), which is connected to the computer via USB. This will allow you to read the voltage output from the lock-in on the computer. The DAQ also has a second function which is to control the current supplied to electromagnet which will control the applied magnetic field. One of the DAQ s outputs (AO0 - analog output 0), is connected to a high current electronic measurements inc. (EMI) power supply. The positive lead of AO0 is connected to V AMP IN (3) on the TBIO1 bus on the back of the EMI power supply. The GND lead of AO0 is connected to the GND (8) on the TBI bus on the back of the EMI power supply The voltage applied by the DAQ to the back of the EMI power supply will control the current supplied to the magnet.
Because the DAQ cannot sink the current required to drive the magnet the current supplied to the magnet is recorded using a Keithly A175 digital multimeter (DMM) connect to the computer via a GPIB card in the computer. (shown below): A conversion will have to be done by the experimentalist in order to convert the current read by the DMM to magnetic field. Data relating the magnetic field to the current driving the magnet is located on the desktop of the computer. A labview program is provided to automate sweeping the magnetic field, recording the current supplied to the magnet, and recording the voltage output from the lock-in. Using the labview program one can change the starting current, current sweep resolution, time delay between measurements, and the filename to which the data is output as well as the number of data points to be taken. The program is located on the desktop (EPR1.vi). The front panel of the program is shown above, and the block diagram of the program can be found by click Window from the file menu and then show block diagram
It would be wise to become familiar with using and programming in labview. Feel free to modify and improve the program to your liking but be sure you leave the original program intact. One idea would be to add a graph to the program so that the program makes a graph of lock-in output versus current supplied to the magnet in real time We will give a brief overview of the functionality of the program, by looking at the block diagram. The program consists of a for loop in which the number of data points can be set on the front panel. Inside the for loop is the main function of the program which executes in a sequence called a stacked sequence frame. The first frame of the sequence is shown below. The first frame tells the lab jack to output a voltage on AO0 depending on the values of Magnet offset and Magnet Sweep inc. set by the user on the front panel.
The next frame in the sequence is a simple delay. This delay is a delay set by the user in order to allow the system to reach a steady state before recording data. The delay should be set in accordance with the time constant on the lock-in amplifier. The next frame in the sequence records the current supplied to the magnet from the Keithly multimeter via a GPIB card installed in the computer.
The final frame in the sequence records the voltage output from the lock-in amplifier into the AI0 input of the lab jack and writes the data into a spreadsheet file specified by the user. For each data point recorded the program averages over 100 voltage values from the lock-in and outputs a mean and standard deviation of each point. The standard deviation read from the multimeter is close to negligible thus one can use the instrumental uncertainty associated with this specific multimeter. These four sequences are run each iteration of the outer for loop, which is set by the user on the front panel. The program will output a file containing three columns [current, lock-in voltage, lock-in standard deviation]. You will need to configure the program s options based on your lock-in settings; you will want to configure the program to optimally record data based on the time constant of the lock-in. Before running the program you should ensure all the required equipment is turned on, and you know at what current/magnetic field value the resonance occurs. You will need to set the magnet offset and Magnet Sweep Inc setting to a reasonable value such that when you run the program it will sweep the magnetic field through the resonance of the sample. You will need to adjust all the parameters to your liking in order to get adequate data.