Pump-Probe XAFS

◆Equipment overview

Microspectrometry measurement equipment at BL39XU and BL25SU are used for measurements with a time-resolution of up to 40 ps. This is performed by adding a function to synchronize the sample stimulation and X-ray irradiation timing for electric devices. In addition, the measurement equipment can be adapted to many SPring-8 BL as necessary. Pictured is an X-ray chopper (left) that cuts out an X-ray single bunch, then the timing circuit (right) stimulates the sample which is synchronized with the X-rays.

◆Features of the Equipment

Measurement techniques developed in each beamline can be integrated with synchronized time-resolved measurements of X-ray irradiation and sample stimulation. The X-ray chopper selects and extracts a single bunch, then by using a dialing method the signal time is collected at the gate circuit, allowing for X-ray pulse time-resolution measurements up to 60 ps (H mode) and 40 ps (A mode). This is not limited to BL39XU and BL25XU, and the microspectrometry equipment of BL17SU can be adapted to macro measurements as well. Similar time-resolved measurements are performed on the BL02B1 and BL40LXU X-ray diffraction measurement equipment and the BL19XU and BL47XU HAX-PES measurement equipment.

◆Equipment accessories

In the Pump-Probe time-resolved measurements, the sample is stimulated (pump) to react, then after a period of time, it is irradiated with X-rays (probe) to observe the state of change of the sample.

・Pump:
To stimulate, a pulse laser or electric stimulation is used as the stimuli. Femtosecond titanium sapphire lasers are installed in beamlines BL17SU, BL19LXU, BL25SU and BL40XU, which are measured at repeated frequencies 1kHz (BL17SU, BL40XU), 5kHz (BL25SU) and 10kHz(BL19LXU). These pulse lasers have pulse widths and wavelengths of 170fs and 800nm respectively (coupling is also available). In the case of electric simulation, the degree of freedom of sample simulation, such as repeat frequency and voltage is high. This can be completed over a wide range according to the measurement objective, from RF applications with a maximum frequency of 6GHz to high voltage applications from 1kHz and 10kV. In addition, user provided stimulation sources are also available to be used.

・Probe：
X-rays are irradiated in sync with the sample stimulus to observe the state at any time. An X-ray chopper is used to extract X-rays at any time. The BL which have femtosecond lasers installed have X-ray choppers according to the necessary frequencies. In addition, small, portable choppers are available in BL39XU and BL13XU, which can perform time-resolved measurements by electrical stimulation, and these correspond to a wide cutting frequency range from 1kHz to 208.8kHz, which is the orbital frequency of SPring-8. The chopper can also be selected according to the measurement objective.

◆Experiment / sample preparation

It is difficult to measure insulators since the electrons emitted from the sample by X-ray irradiation are measured from soft X-ray BLs at BL17SU and BL25SU, as well as the HAX-PES measurements of BL19LXU. This is because the insulator continues to emit electrons during X-ray irradiation, but since there is no conductivity, electrons are not supplied to the X-ray irradiation elements and it becomes positively charged, causing a phenomenon (charge-up) in which electrons are not emitted. In some cases, this phenomenon can be avoided by making the sample surface conductive by depositing a very thin conductor, such as AI, on the surface of the sample.
For electrical stimulation time-resolution measurements, please prepare a large number of samples that have been wired in advance. A common issue is that the wiring comes off during the measurement, and there is a possibility that the experiment must be cancelled because it cannot be rewired at the BL site. For this reason, it is recommended that you prepare a large number of pre-wired samples. We are also considering building a prober system.
In the case of time-resolution measurements of electron devices using BL39XU microscopic spectrum measurement devices, fluorescent X-rays are often used as probes since the target sample is as thin as several hundred nm or less. In the case of hard X-rays, it passes through the sample and X-rays also excite the substrate and circuits as well, generating X-ray fluorescence. Please examine the sample structure in advance to ensure it does not interfere with the detection of the X-ray fluorescence of the target element. In addition, electronic devices often use a silicon wafer as the base of the sample, but X-ray diffraction from this silicon wafer may also interfere with X-ray fluorescent detection. Please consider using an amorphous base such as glass.

◆Experimental procedure / precautions

In pump-probe time-resolved measurements, except for fully synchronized measurements of all X-ray pulses and pumps in A-mode, X-rays at the required timing are cut out and measured. Because of this, the intensity of the observed X-rays are very weak, from a few percent to tens of thousands of times compared to the normal measurements, requiring a lot of measurement time.
It is necessary to keep this in mind when applying for an assignment, and plan to apply for machine time several times longer than usual.
For microspectrometry measurements, such as the ones done with BL25SU and BL39XU, a very small beam of several hundred nm or less are used.
In addition, the measurement target is also very small, of microns or less.
XMCD and XAFS spectral measurements require that the two must match.
Although these beamlines allow for faster-than-usual image measurements, on the fly, adjusting the measurement position generally requires a long time.
Please consider that the adjustment operation of the measurement position may be shortened by installing a marker or something similar, to indicate the measurement position of the sample in advance.
The BL39XU is also equipped with a kinematic mount sample holder and an offline sample position adjuster which uses an optical microscope.
This makes it possible to set the position offline with an accuracy of several microns.

◆Contact

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