Takushi Hiroi (ICYS Researcher, National Institute for Materials Science)

Yuya Morimoto (Team Leader, RIKEN Hakubi Research Team, RIKEN)

Reika KANYA (Professor and PRESTO Researcher, Tokyo Metropolitan University)

Kaoru Yamauchi (Professor, Department of Chemistry / Head of Department, Ultrafast Intense Laser Field Science Center)

Key Points of the Presentation

• In order to observe laser-assisted (e,2e), which is an electron impact ionization process ^{(Note 1)} in an intense laser field, we developed an original apparatus in which two electron analyzers detect two generated electrons with high efficiency, and observed laser-assisted (e,2e) for the first time using argon atoms.
• As a result of analyzing the Mie differential scattering cross section ^{(Note 2)} of the obtained laser-assisted (e,2e) signal, we succeeded in capturing the distortion of the electronic wave function ^{(Note 3)} of argon atoms, thus verifying the theoretical prediction reported over 30 years ago.
• By applying the newly developed method, it will be possible to track the distortion of atomic and molecular wavefunctions induced by intense lasers at ultrahigh speed, which is expected to contribute to the design and control of complex chemical reactions using light.

Summary of presentation

Atoms and molecules exposed to intense laser fields are known to be in a "photo-distorted state" ^{(Note 4)} in which the electron density distribution is distorted, which has been experimentally observed by electron scattering in intense laser fields. Since the electron density distribution is described by multiple electron wavefunctions of atoms and molecules, it has been believed that the electron wavefunctions of atoms and molecules in the photo-distorted state are also distorted. Although this phenomenon was reported more than 30 years ago in the form of enhanced scattering cross sections in electron impact ionization in intense laser fields, there has been no successful experimental demonstration of this phenomenon because of the difficulty of conducting experiments.

Professor Yamauchi's research group at the Graduate School of Science, The University of Tokyo, has developed an apparatus to observe electron impact ionization in an intense laser field with high efficiency, and has measured the scattering cross sections of electron impact ionization from the 3p orbitals of argon (Ar) atoms in the presence of a laser field. The obtained scattering cross sections are about two times larger than those calculated by neglecting the formation of the photo-distorted state, and are the first experimental results showing that the electronic wavefunction of Ar is distorted in the photo-distorted state. This technique enables ultrafast tracking of the distortion of electronic wavefunctions of atoms and molecules in the presence of intense laser fields, and is expected to lead to the design and control of complex chemical reactions using light.

Announcements

When an atom or molecule is placed in an intense laser field, the electron density distribution of the atom or molecule is greatly affected by the light field, forming a state called a photodressed state. An experimental indication that atoms and molecules are in a photodressed state is the enhancement of the scattering intensity in the small-angle region of the elastic scattering of electrons in the presence of a laser field, which was actually observed in 2015 [Y. Morimoto, R. Kanya, and K. Yamanouchi, Phys. Rev. Lett. 115, 123201-1-5 (2015)]. What was observed here was the distortion of the electron density distribution of atoms and molecules by light, but the electron density distribution is a physical quantity described by the multiple electronic wave functions of atoms and molecules, and there has been interest in how the electronic wave functions of atoms and molecules themselves are distorted by the light field.

One of the promising experimental techniques to experimentally clarify the change in the electronic wavefunction in a light-distorted state is laser-assisted (e, 2e) (LA(e, 2e)), in which an atom or molecule is ionized by an electron impact in a laser field. In a theoretical study reported in 1988, the scattering cross section of the one-photon absorption process of LA(e, 2e) was found to increase or decrease by an integer multiple of the laser photon energy. Theoretical studies reported in 1988 predicted that the scattering cross section of the LA(e, 2e) one-photon absorption process should be increased by a factor of several times when the photo-dressed state is taken into account. However, the observation of the LA(e, 2e) scattering cross section with the one-photon absorption process distinguished is experimentally difficult and has not been observed for more than 30 years since the theoretical prediction.

In this study, we have independently developed an experimental apparatus that can observe electron impact ionization with high efficiency. The newly developed instrument is designed to detect the two electrons produced by electron impact ionization with high efficiency using two angle-resolved time-of-flight electron analyzers ^{(Note 5)}, achieving an extremely high collection efficiency compared to conventional electron impact ionization instruments. The two electrons are then irradiated to Ar atoms in an intense laser field with an incident energy of 1 keV, and the energy and scattering angles of the two electrons generated after electron impact ionization are measured by coincidence ^{(Note 6)}. (e, 2e) one-photon absorption process by means of coincidence measurements of the electron energy and scattering angle. By comparing the experimental results with numerical simulation results, we succeeded in observing an increase in the scattering cross section due to the formation of a photo-destructive state of Ar.

Figure 1: (a) Summed energy spectra of scattered and emitted electrons detected by coincidence. The solid red line represents the experimental results when the laser field is present, and the dashed black line represents the experimental results when the laser field is absent. (b) Red line: experimental LA(e, 2e) energy spectrum. Green region: energy spectrum of LA(e, 2e) calculated without considering light-dressed states.

The solid red line in Figure 1(a) is the spectrum obtained by coincidence detection of the energies of two electrons produced by electron impact ionization induced in the presence of a laser field and plotting the sum of their energies. The black dotted line is the background signal obtained by performing the same experiment in the absence of the laser field, and shows that the scattering intensity is enhanced in the energy sum region around 985.4 eV, corresponding to the LA(e, 2e) one-photon absorption process from the 3p orbital of Ar.

The solid red line in Figure 1(b) is the LA(e, 2e) spectrum obtained by subtracting the background signal (dotted black line) from the scattered signal (solid red line) in the laser field shown in Figure 1(a). The spectrum filled in green in Figure 1(b) is the LA(e, 2e) spectrum calculated by neglecting the interaction between Ar and light. Comparison of the two spectra reveals that the intensity of the measured LA(e, 2e) spectrum is about two times larger than that of the calculation ignoring the formation of photodestructive states. This indicates that the 3p orbitals of Ar are distorted by the formation of photodressed states of Ar atoms.

This study shows that the photodistorted state, which has been regarded as a distortion of the electron density distribution, can be experimentally observed as a distortion of the individual electron wavefunctions that make up the electron density distribution. This is expected to advance the study of atoms and molecules in multi-electron systems in intense laser fields, which have been complicated and have not been studied before. In addition, since electron impact ionization is also known as a method for experimentally observing the wavefunctions of atoms and molecules, this method is expected to enable direct observation of the wavefunctions of atoms and molecules in intense laser fields. Furthermore, since the time resolution of this method is determined only by the pulse width of the laser pulse, it can be used to track the distortion of atomic and molecular wavefunctions by intense light at ultrahigh speeds (pico- to femtosecond scale) and to design and control complex chemical reactions using light based on the obtained knowledge, which is a great dream for chemists. This is expected to be the basis for achieving the great dream of chemists to design and control complex chemical reactions using light based on the knowledge obtained.

Journals

Journal name	Physical Review A
Title of paper	Observation of Laser-Assisted (e, 2e) in Ultrashort Intense Laser Fields
Authors	Takashi Hiroi, Yuya Morimoto, Reika Kanya, and Kaoru Yamanouchi* (in Japanese)
DOI Number	10.1103/PhysRevA.104.062812
Abstract URL	https://doi.org/10.1103/PhysRevA.104.062812

Terminology

Note 1 Electron impact ionization process

A phenomenon in which electrons in a target sample are emitted as a result of the scattering of incident electrons when electrons collide with the target sample. The energy and momentum of the incident electrons, scattered electrons, and emitted electrons are measured to obtain information on the ionization potential and electron wavefunction of the emitted electrons when they existed in the target sample. Also called (e, 2e), because it is a process in which two electrons are produced as a result of a single electron collision. ↑up

Note 2 Scattering cross section

In scattering phenomena, a physical quantity that expresses the intensity of scattering. In this study, the Mie differential scattering cross section, which is described by the number of electrons observed for each scattering angle and energy, was measured for a given incident electron energy by specifying the scattering angle direction and energy of the two electrons produced by the electron impact ionization process and the final state of the target sample. ↑up

Note 3 Electron wave function

In quantum mechanics, a physical quantity that defines the probability of the existence of electrons. There are five types of electron wave functions for argon atoms in the ground state: 1s, 2s, 2p, 3s, and 3p. ↑up

Note 4: Light-dressed state

A state in which the electron density distribution of atoms and molecules is greatly affected by a strong light field. It is so called because atoms and molecules can be regarded as light-dressed. It is known as an important concept in understanding various atomic and molecular dynamics induced by intense laser fields. ↑up

Note 5 Angle-resolved time-of-flight electron analyzer

An instrument for measuring the kinetic energy and momentum of electrons. It measures the time taken for electrons to pass through the analyzer and reach the detector, and the kinetic energy and momentum of the electrons based on the position on the detector where the electrons arrived. ↑up

Note 6 Coincidence measurement

A method to measure all the multiple signals generated from a single event. In this study, it refers to measuring both the energy and momentum of two electrons generated by a single electron impact ionization process. ↑up