X-ray Irradiation

Irradiation of the sample was done by means of X-ray tubes with either tungsten (Philips PW 2184/00) or rhodium (Philips PW 2182/00) targets connected to a constant (160kV) potential generator (Phillips). The samples were X-irradiated through a 0.05mm Al-filter for three to four hours with the X-ray tube operated at typical voltage and current settings (55kV/40mA for W-target and 65kV/45mA for Rh-target) to achieve the desired radical concentration. (Dose rates for these X-ray tubes operated at a wide range of settings were calibrated using an ionization chamber.) Under these conditions, the

crystals received a total dose of approximately 100kGy (1Gy ≡ 1J/kg) absorbing an energy amount of approximately 1-2 Joules (considering a crystal mass of 10 – 20 mg). After irradiation, the sample was lowered into the cavity for data collection.

4.5 Spectrometer

A home-built K-band (24 GHz) spectrometer was used to take the EPR, ENDOR and EIE spectra. The block diagram for the spectrometer setup is shown in Fig. 4.1. The microwave circuit is a typical reflection bridge design employing a reference arm with a waveguide-based path length equalizer.4 The microwave source is a klystron (OKI 24V10) normally provides about 140mW microwave power (continuously measured with a sensor connected to the main arm through a 10dB coupler) and the frequency was stabilized by an automatic frequency control (AFC) system. The frequency stabilization system locks the klystron frequency to the sample cavity’s resonant frequency. The actual frequency was measured by a Hewlett-Packard 5351A counter. The attenuator for adjusting the microwave power level has the range of 0 – 60 dB. The cylindrical microwave cavity provides a microwave magnetic field pattern ideal for studies in which the sample is rotated about an axis and resonates in the TE011 mode.5 The microwave signal containing the resonance absorption information was demodulated by a single- ended diode detector. The magnetic field is produced by a water-cooled 9.5 inch Magnion electromagnet with 5 kW power supply (HS-1050 B) which is controlled by a Hall-effect field controller (Bruker BH-15). Measurement of the actual magnetic field at the sample is accomplished by means of a nuclear magnetic resonance (NMR) probe (Bruker ER/035M) placed in front of the microwave cavity.

The microwave cavity also contains a “standard” sample of MgO doped with Cr+3 ions (0.02 atom %). In addition to providing a g-value standard (g = 1.9810±0.0006),6 this “standard” sample also provides an intensity standard. From the procedures for calibrating the Cr:MgO sample as an intensity standard, it was found that the minimum number of unpaired spins in the cavity detectable by EPR is of the order of 1014. Typical crystals used in these experiments contain approximately 1020 molecules (based on their average mass of 10-20 mg). Therefore, with this spectrometer, detection of unpaired spin (radical) concentrations of the order of one part per million (1ppm) can be achieved.

The EPR spectra were taken by sweeping the magnetic field through a desired range (typically 200 Gauss). The field was doubly modulated by 50 kHz and 33 Hz signals applied through two small serial-connected coils located on either side of the microwave cavity and within the vacuum shroud. Phase-sensitive detection was accomplished at 50 kHz (Stanford model SR810) and at 33Hz (EG&G model 5209) by lock-in amplifiers. EPR signal was observed as the second derivative of the absorption as a result of the double modulation. (If necessary, it is also possible to obtained EPR signals in the form of the first derivative of the absorption by singly modulating the field.)

Detector Klystron Power Supply Automatic FrequencyControl Path-length equalizer Isolator Attenuator M a g n e t M a g n e t Lock-in Amplifier andControl

Modulation System Reference Signal Circulator Radio Frequency Generator Computer for Data Acquisition and Control Printer Magnet Power Supply Magnet Control Radio Frequency Amplifier Isolator Klystron Power Splitter

Phase Shifter Combiner

Preamplifier Microwave Preamplifier Waveguide Cavity Vacuum Isolation To ENDOR coil (InsideCavity)

Figure 4.1 The block diagram for the EPR and ENDOR spectrometer setup. Only one lock-in amplifier is shown in the configuration used for ENDOR. For the double modulation method used for EPR the output of one lock-in (50kHz) is fed to the input of the second (33Hz).

In the ENDOR experiment, the radio frequency was produced by a frequency generator (Wavetek modal 3001) with the frequency set by computer and amplified by power amplifier (ENI model 3200L). The radio frequency was introduced into the cavity with a two-turn coil in a “parallel-wire” arrangement along the axis of the cavity. The signal was modulated at 25kHz and detected by a lock-in amplifier (Stanford model SR810 DSP). The ENDOR signal observed as a first derivative shape as a result of single modulation.

In ENDOR-induced-EPR (EIE) experiments, the ENDOR frequency was set to the peak of the first derivative ENDOR line and the magnetic field was then swept. The ENDOR frequency varies with the magnetic field value in proportion to the nuclear g- value. It was therefore necessary to adjust this frequency slightly as the field was being swept. The specific relation for controlling the frequency during an EIE scan is

0 (gn n/ )(h H0 H)

ν

ν

β

where ν is the ENDOR frequency at magnetic field H and ν0 and H0 are initial values. When the ENDOR line is well resolved, the EIE spectrum will show the magnetic profile of the ENDOR response. A particularly important point is that the resulting pattern comes from only the radical producing the ENDOR line chosen.