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Welcome to the online documentation of the Bridge12 QLP probe for pulsed dipolar spectroscopy. Bridge12 does not distribute paper manuals with their probes to keep the documentation always up to date. If you have questions, or suggestions for edits please contact us at info@bridge12.com.
Bridge12 QLP TOP
The Bridge12 QLP EPR probe is shipped in a sturdy case to protect the probe from damages during transportation (see picture below).
Bridge12 QLP Overview
If the probe is not used, we recommend storing the probe in the original shipping case.
The Bridge12 QLP probe (B12TQLP or QLP) is a probe head for pulsed Q-Band (34 - 36 GHz) EPR spectroscopy. The probe is optimized for pulsed dipolar experiments such as Double Electron-Electron Resonance (DEER/PELDOR) experiments, Relaxation Induced Dipolar Modulation Enhancement (RIDME) experiments, etc.
Pulsed Electron Paramagnetic Resonance (EPR) spectroscopy using high-power microwave pulses or arbitrary waveform generated (AWG) broadband pulses require a large resonator bandwidth (low resonator Q) to avoid distortion of the pulse shape. However, large resonator bandwidth often comes at the cost of a reduced microwave conversion factor. Loop-Gap Resonators (LGR) combine all these desired features together with an excellent field homogeneity across the sample. This is especially important when using AWG generated broadband pulses.
At the heart of the probe is a Loop-Gap Resonator (LGR), with a large conversion factor, and an homogenous B1e microwave field across the entire sample volume to improve the performance of pulsed EPR experiments. The resonator is the ideal choice when using pulses generated by an Arbitrary Waveform Generator (AWG).
Bridge12 QLP Overview
An overview of the Bridge12 QLP probe is given in the figure shown above. For low-temperature measurements, the probe can be operated inside a cryostat. The Bridge12 QLP is compatible with commonly used cyrostats, such as the Oxford Instruments cryostat model CF935.
The main features of the probe are:
Optical access to the sample location for e.g. light/laser irradiation is possible in several different ways:
If you like to use an optical fiber to irradiate the sample using the sample holder/stick (option 1), please contact Bridge12 at info@bridge12.com for further information.
The Bridge12 QLP probe has a built-in, calibrated Cernox temperature sensor. The connector is located on the top of the probe. Please use the cable that came with the probe to connect the probe to the temperature controller.
More information about the pin-out can be found in the section Temperature Sensor.
A rectangular waveguide port (WR-28) is located at the top of the probe. The waveguide flange is a UG-599/U with 4 tapped holes, screw size #4-40. This waveguide port is used to connect the QLP probe to the EPR spectrometer. The probe waveguide has an integrated vacuum window to seal the probe to the atmosphere, when operating the probe at cryogenic temperatures.
Do not drop any screws or washers into the waveguide port. This could potentially damage the vacuum window located inside the waveguide.
If parts are accidentally dropped into the waveguide, do not use any sharp objects to retrieve the part. Instead, flip the probe over and gently shake the probe to remove the dropped item.
If a part got stuck inside the waveguide and you are not able to retrieve it, do not connect the probe to the spectrometer, and contact Bridge12 at info@bridge12.com.
Please make yourself familiar with the instructions below before attempting to install the probe. At no point should you use any force during the installation process. In general, all threads, screws, etc. are imperial size, not metric. Please use the appropriate tools to fasten all screws.
Please follow these steps to connect the probe to the spectrometer:
First time installation: Remove Kapton tape from waveguide flange (for shipping the waveguide port of the probe is protected by a strip of Kapton tape).
Place probe inside cryostat. If you don’t use a cryostat, place probe in the appropriate support structure.
Turn/orient the probe so the waveguide port is located to the left. The waveguide port should be on the side that provides the shortest distance to the EPR bridge. Typically, the EPR bridge is located to the left of the probe.
To connect the probe to your EPR instrument, the probe comes with additional waveguide components:
In addition to these parts, please also use the flexible WR-28 waveguide section provided with the EPR instrument to connect the probe to the spectrometer.
Every EPR instrument is slightly different. Here are different scenarios to connect the probe.
When connecting two waveguide sections, always orient the waveguides so that the long section of both waveguides are parallel to each other.
For the Bridge12 QLP-1.6 mm please use a sample capillary with a maximum OD of 1.6 mm. A link to the sample tube vendor can be found on the Cosumables Page.
Please follow the instructions for using the sample stick to mount the EPR sample to the Sample Stick. In general, the distance between the center of the sample and the bottom of the sample holder should be about 1 inch (25.4 mm) (see figure below).
Recommended distance between the center of the sample and the bottom of the sample holder
To insert the sample stick into the probe follow these instructions:
Please keep in mind that these instructions can differ, depending on the type and make of the low-temperature cryostat.
The frequency of the empty resonator is typically between 35.5 and 36 GHz. This may be outside of the normal operating frequency of a typical EPR spectrometer, however, inserting the sample into the resonator, this will shift the frequency to lower values (dielectric loading) as shown in the figure below. The amount of the frequency shift depends on the size and material of the sample capillary and the nature of the sample (e.g. polymer at room temperature, frozen aqueous solution, etc.). Please see the section Consumables for the recommended sample size.
QLP Frequency Shift
Due to the nature of the resonator, the resonance frequency will not noticeably change when cooling down to cryogenic temperatures.
If you have already samples that are loaded into a smaller capillaries, we recommend placing the existing capillary into the WG-221T-RB sample tube to shift the resonator frequency to the correct regime.
The microwave coupling to the resonator is adjusted using an iris. The iris can be mechanically controlled using a micrometer screw head, located at the top of the probe (Iris Adjustment, see figure above). By turning the micrometer screw, the iris is moved up and down and the resonator coupling can be changed from undercoupled to critically coupled to overcoupled as shown in the figure below.
QLP Iris Adjustment
Typical Q values and conversion factors for the overcoupled and critically coupled resonator:
| Parameter | Value |
|---|---|
| Microwave Conversion Factor (critically coupled) | > 12 G/sqrt(W) |
| Bandwidth (critcially coupled) | Q ~ 300 - 400 (85 - 113 MHz) |
| Microwave Conversion Factor (overcoupled) | > 5 G/sqrt(W) |
| Bandwidth (critcially coupled) | Q < 85 (> 400 MHz) |
To achieve the largest resonator bandwidth, the iris should be at its highest position. However, in some circumstances it may be beneficial to lower the iris to increase the B1e field strength, at the expense of the resonator bandwidth.
Critically coupling the resonator can lead to significant probe ring-down, with power reflected back to the EPR spectrometer exceeding safe levels. When attempting to critically couple the resonator, make sure to start a low and safe level of excitation power.
To adjust the coupling of the resonator, follow these steps:
Raising the iris (turning the micrometer head clockwise) will lower the Q-value and increase the resonator bandwidth (see figure below) without changing the resonator frequency.
General probe specifications:
| Parameter | Value |
|---|---|
| Resonator Frequency (empty) | 36 GHz |
| Resoantor Frequency (with sample capillary) | 34 GHz |
| Microwave Conversion Factor (critically coupled) | > 12 G/sqrt(W) |
| Bandwidth (critcially coupled) | Q ~ 300 - 400 (85 - 113 MHz) |
| Microwave Conversion Factor (overcoupled) | > 5 G/sqrt(W) |
| Bandwidth (critcially coupled) | Q < 85 (> 400 MHz) |
| Operating Temperature | 4 K to RT |
Currently, the Bridge12 QLP is only available for samples with an OD of up to 1.6 mm.
| Resonator Model | Description |
|---|---|
| QLP-1.6 | Resonator height: 4 mm Maximum Sample OD: 1.6 mm |
| QLP-2.5 * | Resonator height: 4 mm Maximum Sample OD: 2.5 mm |
* Resonator currently under development. Please contact info@bridge12.com if you are interested in this specific resonator model.
List of links to scientific publications in which the Bridge12 QLP resonator was used:
The field of pulsed dipolar spectroscopy is rapidly evolving. Below, find some general literature references for dipolar spectroscopy and the required instrumentation.