Femtosecond time-resolved X-ray absorption spectroscopy of liquid using a hard X-ray free electron laser in a dual-beam dispersive detection method

Yuki Obara,Toshinori Suzuki,Tetsuo Katayama,Naoya Kurahashi,Kazuhiko Misawa,Yusuke Isokawa,Makina Yabashi,Yuichi Inubushi,Yoshihiro Ogi,Tadashi Togashi,Takayuki Suzuki,Yuhei Chiba,Shutaro Karashima

doi:10.1364/oe.22.001105

Abstract

We present femtosecond time-resolved X-ray absorption spectroscopy of aqueous solution using a hard x-ray free electron laser (SACLA) and a synchronized Ti:sapphire laser. The instrumental response time is 200 fs, and the repetition rate of measurement is 10 Hz. A cylindrical liquid beam 100 μm in diameter of aqueous ammonium iron(III) oxalate solution is photoexcited at 400 nm, and the transient X-ray absorption spectra are measured in the K-edge region of iron, 7.10 - 7.26 keV, using a dual X-ray beam dispersive detection method. Each of the dual beams has the pulse energy of 1.4 μJ, and pump-induced absorbance change on the order of 10(-3) is successfully detected. The photoexcited iron complex exhibits a red shifted iron K-edge with the appearance time constant of 260 fs. The X-ray absorption difference spectra, with and without the pump pulses, are independent of time delay after 1.5 ps up to 100 ps, indicating that the photoexcited species is long-lived.

Highlights

Time-resolved X-ray absorption spectroscopy (TRXAS) is useful for the studies of ultrafast electronic and structural dynamics of materials, as it enables element-selective observation of electronic states and the local geometrical structures in real time [1,2,3]
We present femtosecond time-resolved X-ray absorption spectroscopy of aqueous solution using a hard x-ray free electron laser
The method is expected to be even more useful in the soft X-ray region, where the X-ray absorption cross section is considerably larger than in the hard X-ray region

Summary

Introduction

Time-resolved X-ray absorption spectroscopy (TRXAS) is useful for the studies of ultrafast electronic and structural dynamics of materials, as it enables element-selective observation of electronic states and the local geometrical structures in real time [1,2,3]. X-ray absorption nearedge spectroscopy measures the absorption spectrum near the ionization threshold, providing information on the local electronic state of the ionized atom [4]. Femtosecond TRXAS experiments have been performed with tabletop X-ray sources using laser-induced plasma on a metal wire [12–15]. The plasma emits X-ray radiation over large solid angles, so that only a small fraction can be utilized for TRXAS measurements; when the same driving laser is employed for generating both of the pump and probe pulses, accurate time delay is ensured. We selected the iron oxalate complex [22,23], because its photo-induced dynamics is not well understood despite TRXAS studied by Rentzepis and associates using laser-plasma X-ray source [14]. The data acquisition, storage, and analysis are fully optimized and the pump-induced absorbance change on the order of 10−3 has been successfully detected

Experimental setup

Data acquisition

Sample description

Results

Estimation of detection limit

Retrieval of excited state spectrum