Photon Factory Activity Report 2022 #40 (2023)

BL-2B/2021G095

Ion Desorption by Resonant Core Excitation of Biphenyl Monolayers
Kakuto YOSHIOKA1, Akinobu NIOZU1, Shogo TENDO1, Jun-ichi ADACHI2,Hirokazu TANAKA2, and Shin-ichi WADA1,*
1
Graduate School of Advanced Science and Engineering, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan2
Photon Factory, Institute of Materials Structure Science,High Energy Research Organization,
1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
1 Introduction
Soft X-rays can be used to selectively excite coreelectrons of specific atoms in molecules; therefore, it has
great potential for site-selective chemical bond breaking,also known as the “molecular scalpel” [1]. While suchselective bond breaking has not been clearly identified ingas phase experiments, it has been reported in ion
desorption from molecules adsorbed on surfaces. Our
previous studies have suggested the key role of interaction
between the excitation site and substrate for achieving siteselective bond breaking [2].
In this study, we investigated the ion desorption
dynamics from self-assembled monolayers (SAMs) ofbiphenyl molecules adsorbed on gold substrates. A
biphenyl molecule is composed of two phenyl rings and its
molecular conductance is dramatically varied dependingon the torsional angle between two rings. Therefore, we
focused on the core-excitation dynamics of biphenyl
SAMs with different torsional angles and degrees of πconjugation [3, 4].
2 Experiment
We employed SAMs of two biphenyl molecules withtorsional angles. The torsion was achieved by the additionof two methyl groups to the upper phenyl ring (Fig. 1). The
molecules have a methyl ester groups (- COOCH3) as thetail group. The SAM samples were prepared by immersionof Au substrates into 1 mM solution of the precursor
molecules.
To examine the core-excitation dynamics, we performed
time-of-flight mass spectrometry (TOF-MS) of ionsdesorbed from core-excited SAMs. The experiment wascarried out at Photon Factory (PF) BL-2B in the hybrid
operation mode. We used a mechanical pulse selector
synchronized to the master clock of the storage ring
operation to utilize only the single-bunch component ofradiation [5]. The measurements were performed at room
temperature under ultrahigh vacuum conditions ofapproximately 2 × 10-6 Pa. The X-ray incidence angle was
set to 20°, and the ions were measured in the normal
emission geometry.
The samples were evaluated by near-edge X-ray
absorption fine structure (NEXAFS) spectroscopy. The
measurements were conducted at BL-13 of HiSOR. The

spectra were obtained at the C and O K-edges by the total
electron yield (TEY) method measuring the sample draincurrent.
3 Results and Discussion
The obtained NEXAFS spectra of the SAM samples
exhibited noticeable polarization dependence in theintensity of C 1s → π* (C = C, C = O) transition peaks,
which suggested nearly upright orientation of themolecules on the Au surface.
In the TOF spectra obtained in the C K-edge region,clear peaks corresponding to CHn+ (n = 0-3) ions wereobserved at C 1s → σ* (O – CH3) transitions [2]. The
spectra for non-substituted biphenyl SAM exhibited
sufficient fragmentation, as shown in Fig. 1. Notably, therelative yields of C2Hm+ (m = 2-5) ions are significantly
increased in the TOF spectrum for SAM with the larger
torsional angle. Furthermore, the spectrum suggested
reduced degree of fragmentation of the CHn+ ions. Fromthe interrelationship between ion desorption and the energy
relaxation process involving charge transfer, it can beinferred that charge transfer between the molecule with alarger torsional angle and the substrate is suppressed inthese observations. Therefore, this suppression of chargetransfer can be attributed to the diminished π-conjugation
caused by the larger torsional angle.

Fig. 1: Ion TOF mass spectra measured for core-excitedbiphenyl SAMs.

Photon Factory Activity Report 2022 #40 (2023)

Acknowledgement
The experiments were performed under the approval of
the Photon Factory Program Advisory Committee(Proposal No. 2021G095). This work was supported by theJapan Society for the Promotion of Science (JSPS)
KAKENHI Grant No. JP21K20538.
References[1] W. Eberhardt et al., Phys. Rev. Lett. 50, 1038 (1983).
[2] S. Wada et al, Phys. Condens. Matter. 18, 1 (2006).
[3] A. Masillamani et al., Chem. Eur. J. 18, 10335 (2012).
[4] L. Venkataraman et al, Nature 442, 904 (2006).
[5] J. Adachi et al, J. Phys.: Conf. Ser. 1412, 152092 (2020).
* wadasin@hiroshima-u.ac.jp