Oct 8, 2021

Ultrahigh-resolution spectroscopy using ultrashort intense laser pulses: Determination of spin-orbit splitting of rare gas atomic cations with 10-7 precision


Overview of the press release

Dr. Toshiaki Ando, Prof. Atsushi Iwasaki, and Prof. Kaoru Yamanouchi at School of Science, the University of Tokyo, have successfully determined the spin-orbit splitting energy of the rare gas atom cations, Ar+, Kr+, and Kr2+, with a precision of 10-7 by strong-field ultrahigh-resolution Fourier transform (SURF) spectroscopy, which they have been developing for several years.

In conventional high-resolution spectroscopy, the energies of eigenstates of atoms and molecules are determined with high precision based on the spectroscopic measurements of optical transitions in the frequency domain. However, because such optical transitions do not occur between the spin-orbit sublevels studied here, determining the spin-orbit splitting energies of rare gas atom cations with high precision using conventional spectroscopic methods has been a difficult task.

In SURF spectroscopy, a superposition of eigenstates of atomic and molecular systems is generated by an ultrashort intense laser pulse (a pump pulse). This “superposition” state, called a wave packet, evolves in time at the frequencies corresponding to the energy differences among the eigenstates of the system. Therefore, if the motion of the wave packet in the time-domain can be probed for a long period of time by irradiating the system with another ultrashort intense laser pulse (a probe pulse), the energy differences among the eigenstates can be determined with high precision.

In this study, a wave packet was created in the electronic ground states of Ar+, Kr+, and Kr2+ ions by a few-cycle near-IR pump laser pulse, and the temporal evolution of the wave packet was monitored using the yields of Ar2+ and Kr2+ created by the irradiation of a few-cycle near-IR probe laser pulse in the pump-probe delay time range up to 500 ps (1 ps = 10-12 seconds). By the Fourier transform of the yields, the energy separations between the spin-orbit sublevels were determined with a precision of 10-7, which is 6 times higher for Ar+ and 50 times higher for Kr+ than the precision achieved before by conventional high-resolution spectroscopic measurements.

SURF spectroscopy can be universally applicable to a wide range of atomic and molecular systems as no optical transitions among the eigenstates are required. Therefore, it is not necessary to prepare light sources whose frequencies are tuned respectively to specific energy differences among the eigenstate of atoms and molecules.

Furthermore, the resolution of the SURF measurement can be easily increased by the extension of the pump-probe delay time range. By SURF measurements, extremely small energy separations among the eigenstates of atomic and molecular systems that have never been resolved before can be determined with ultra-high precision, expanding the frontiers of ultrahigh resolution spectroscopy.

Figure : Determination of spin-orbit splitting energies of rare gas atom cations by SURF spectroscopy.


This paper was selected as an Editors' Suggestion in Physical Review A, directing readers to interesting, important, and well-written articles in areas of research beyond their usual interests.


Publication details


Physical Review A

Spin-orbit splitting of Ar+, Kr+, andKr2+ determined by strong-field ultrahigh-resolution Fourier-transform spectroscopy
Authors Toshiaki Ando, Alex Liu , Naoki Negishi , Atsushi Iwasaki , and Kaoru Yamanouchi*


Paper link



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