HOME > Research

Research activity (Youtube)


Design and Reconstruction of Molecular Quantum States of Motion

Molecules are vital existence. In a gas-phase ensemble at room temperature, they are, in an average, flying away by a few hundred meters, making turns almost reaching to 1011 times, and shaking themselves more than 1013 times within the duration of only one second. The ultimate goal of this research group has been aiming to is to capture the lively figures of molecules moving in such a dynamic manner and to have a perfect command over the molecular motions. Here lasers with ultimate resolution in time and energy domains are employed complementally and cooperatively for this purpose.

When a gaseous molecular sample is irradiated by an intense nonresonant ultrashort laser pulse, the laser field exerts a torque that aligns the molecular axis along the laser polarization vector, due to the interaction with the molecular anisotropic polarizability. Here the field.matter interaction only remains in much shorter duration than the characteristic time for molecular rotation, and thus the rotation of the molecules is coherently excited to create a rotational quantum wave packet (WP). We have developed a method to explore the nonadiabatic excitation in a quantum-state resolved manner and applied it to diatomic and symmetric-top molecules. It has been shown that the state distribution is a useful experimental source for verifying the excitation process. When a pair of excitation pulses is implemented with appropriate time delay between them, partial control of rotational-state distribution has been achieved. In a favorable case, the double-pulse excitation coupled with the state-selective probe has enabled us to reconstruct experimentally a rotational WP thus created. If the mutual polarization direction and time delay between the two pulses are adjusted, the sense of rotation around the laser propagation direction can also be controlled, yielding to a rotational WP exhibiting angular-momentum orientation

Experimental scheme for creating an oriented rotational wave packet by tilted double-pulse excitation.

Experimental scheme for creating an oriented rotational wave packet by tilted double-pulse excitation.

 

-References

  1. H. Hasegawa and Y. Ohshima, “Decoding the state distribution in a nonadiabatic rotational excitation by a nonresonant intense laser field,” Phys. Rev. A 74, 061401-1-4(R) (2006).
  2. H. Hasegawa and Y. Ohshima, “Quantum state reconstruction of a rotational wave packet created by a nonresonant intense femtosecond laser field,” Phys. Rev. Lett. 101, 053002-1-4 (2008).
  3. K. Kitano, H. Hasegawa, and Y. Ohshima, “Ultrafast angular-momentum orientation by linearly polarized laser fields,” Phys. Rev. Lett. 103, 223003-1-4 (2009).
  4. Y. Ohshima and H. Hasegawa, “Coherent rotational excitation by intense nonresonant laser fields,” Int. Rev. Phys. Chem. 29, 619-663 (2010).

Determination of intermolecular interactions by high-resolution electronic spectroscopy of molecular clusters

Intermolecular (or non-covalent) interactions underlie almost all properties of balk materials, surfaces and interfaces, finite molecular aggregates, and supra-molecular systems, e.g., complexbiomolecules. Quantitatively reliable evaluation of the interactions is indispensable for molecular-level description of many physical and chemical processes in varieties of the molecular systems mentioned above. Van der Waals (vdW) complexes, in which molecules and/or atoms are weakly bound by the intermolecular forces, have been regarded as useful models for exploring the interactions, and vast numbers of experimental and theoretical studies have been accumulated so far.
In the experimental investigation of vdW complexes, high-resolution spectroscopy has been deserved as one of the most informative approaches to study structure and internal dynamics of complexes. Molecular clusters containing benzene are prototypical systems for elucidating the intermolecular interaction pertinent to aromatic rings. The information on the precise cluster geometry and energy-level structure pertinent to the intermolecular vibration are useful experimental input to reconstruct the intermolecular potential energy surface. We are now focusing on clusters of benzene attached by small numbers of atoms and molecules. So far, electronic spectra of C6H6 complexed with one and two He atom(s), up to three H2 molecules have been examined.

-Reference

  1. Hayashi, M. & Ohshima, Y.
    Sub-Doppler electronic spectra of the benzene-(He)n complexes
    Chem. Phys. 2013, 419, 131-137.

Photogragh of our high-resololution ring Ti:Sapphire laser oscillator.

Photogragh of our high-resololution ring Ti:Sapphire laser oscillator.


Development of a novel, intense, narrow-band, and chirped laser and itsapplications in coherent population control

Utilization of coherent field-matter interaction affords us a capability for advanced manipulation of quantum states, such as complete population transfer and creation of coherent superposition of states. The realization of such interaction includes: pi-pulse, rapid adiabatic passage (RAP), stimulated Raman adiabatic passage (STIRAP), and so on. These methods have been frequently used in many researches, yet they deserve their own drawbacks. Chirped adiabatic Raman passage (CARP) is one form of adiabatic passage, relying on non-resonant stimulated Raman transitions. When exploring low-frequency transitions pertinent to, e.g., rotation and intermolecular vibration via direct one-photon optical process, we have to prepare various light sources for each object in totally different wavelength regions, i.e., RF, millimeter-wave, THz, FIR, and IR. In contrast, transition frequency of stimulate Raman process corresponds to the energy difference between two incident photons and thus is independent from the absolute energy of the photons. To realize CARP, we constructed a frequency-chirped nanosecond (ns) optical parametric amplifier (OPA), by fully utilizing rapidly evolved telecommunication techinology, i.e., Ytterbium doped fibers and fast optical modulators.

-Reference

  1. Miyake, S. & Ohshima, Y.
    Injection-seeded optical parametric amplifier for generating chirped nanosecond pulses.
    Opt. Express, 2013. 21(5): p. 5269-5274.

Photogragh of our Yb-doped fiber amplifier. The blight curves show the IR emission from the optical fiber.

Photogragh of our Yb-doped fiber amplifier. The blight curves show the IR emission from the optical fiber.


Development of a new charged particle imaging apparatus for visualization of molecular dynamics

Charged particle imaging technique has been a powerful tool to probe dynamics of molecular systems. With a typical camera-based imaging, only a 2D projection of the 3D particle distribution is generally measurable. When the particle distribution is cylindrically symmetric, the original 3D distribution can be reconstructed from the 2D projection with a mathematical procedure (e.g. inverse Abel transformation). In molecular dynamics studies, however, we sometimes encounter non-Abel-invertable cases. Figure 1 schematically illustrates such a situation, in which 2D imaging cannot distinguish molecular orientation (a) and (b). Although 3D imaging techniques have been successfully applied for such a system, a 2D detector has many advantages over a 3D detector: higher multi-hit capability (higher measurement efficiency), lower cost, and simpler setup. We, therefore, are designing a new imaging apparatus.

Designing a new apparatus.

A new apparatus is getting ready to run!