Press Releases
Jun. 20, 2008

Gaseous molecules can be oriented in the laser-field-free condition

Presenters
  • Hirofumi Sakai (Associate Professor, Department of Physics, Graduate School of Science, The University of Tokyo)
  • Akihisa Goban (Graduate student, Department of Physics, Graduate School of Science, The University of Tokyo)
  • Shinichirou Minemoto (Assistant Professor, Department of Physics, Graduate School of Science, The University of Tokyo)

Abstract

Figure 1

Figure: (a) Molecular alignment is probed by the two-dimensional ion imaging technique. The polarizations of the shaped pump pulse and the probe pulse are parallel and perpendicular to the detector plane, respectively.

(b) An ion image of CO+ produced from OCS2+ at the peak of the pump pulse. The degree of alignment is evaluated by ‹cos2θ2D›, where θ2D is the projection of θ (the polar angle between the pump pulse polarization and the molecular axis) onto the detector plane.

(c) Molecular orientation is probed by time-of-flight (TOF) mass spectrometry. The polarizations of both the shaped pump pulse and the probe pulse are parallel to the TOF axis.

(d) TOF spectra of S3+ fragment ions observed with (red curve) and without (blue curve) the pump pulse.

Laser-field-free molecular orientation has been demonstrated with the combination of a moderate electrostatic field and an intense nonresonant rapidly-turned-off laser field, meaning that a new class of molecular sample has become available for various applications.

Since a molecule is generally an anisotropic quantum system, alignment or orientation dependence called steric effects is ubiquitous nature in various phenomena where molecules are involved. Therefore, not only in stereodynamics of chemical reactions but also in electronic stereodynamics in molecules, alignment or orientation dependence is always a matter of central concern and the importance of molecular alignment and orientation techniques has been more and more rising.

In recent years, molecular manipulation techniques based on laser technologies are primary methods to achieve molecular alignment and orientation. The realization of molecular orientation to arrange polar molecules in a “head-versus-tail” order has been thought to be much more difficult than that of molecular alignment. Although both one- and three-dimensional orientation have already been demonstrated in the presence of an intense laser field by Sakai and his co-workers, an intense laser field can modify the physics and/or chemistry involved. Therefore, the realization of laser-field-free orientation has been desired for a long time.

Here we demonstrate laser-field-free molecular orientation with the combination of a moderate electrostatic field and an intense nonresonant rapidly-turned-off laser field. The rapidly-turned-off pulse can be shaped with the plasma shutter technique. We employ OCS molecules as a sample. Molecular alignment and orientation are probed by the two-dimensional ion imaging technique and time-of-flight (TOF) mass spectrometry, respectively (see Figure). It is found that molecular alignment and orientation are adiabatically created in the rising part of the laser pulse and they revive at around the rotational period of an OCS molecule with the same degrees of alignment and orientation as those at the peak of the laser pulse in the virtually laser-field-free condition. This accomplishment means that a new class of molecular sample is now available for various applications in stereodynamics in chemical reactions, electronic stereodynamics, ultrafast molecular imaging, attosecond science, surface science, and so on.

Information about publication

This work has been published in Physical Review Letters and also been selected as an Editors’ Suggestion with the goal of promoting reading across fields.

Akihisa Goban, Shinichirou Minemoto, and Hirofumi Sakai, “Laser-Field-Free Molecular Orientation,” Phys. Rev. Lett. 101, 013001 (2008).

The above paper has been picked up in RESEARCH HIGHLIGHTS of Nature. Nature (London) 454, 257 (2008).