Berlin 2001 – wissenschaftliches Programm
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AMPD: EPS AMPD
AMPD 3: Sitzung 3
AMPD 3.4: Vortrag
Dienstag, 3. April 2001, 11:55–12:20, H105
Controlling the orientation of molecules by strong laser pulses — •H. Stapelfeldt1, J. J. Larsen1, H. Sakai2, and M. D. Poulsen1 — 1Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Århus C, Denmark — 2University of Tokyo, Japan
An important goal in chemistry is to obtain control of the spatial orientation of molecules because it allows for studies of orientational effects in chemical reactions and possibly control of the outcome of the reaction. In this talk we present a new method to control the orientation of molecules based on a strong laser field. The technique, termed laser induced alignment, exploits the anisotropic polarizability interaction of an intense nonresonant laser field with the induced dipole moment of molecules. The interaction creates a potential minimum for the molecules along the polarization of the field, forcing them to librate over a limited angular range instead of rotating freely with random spatial orientations.
1) Linear Alignment We use a 4-nanosecond long laser pulse to align rotationally cold iodine molecules.To measure the spatial orientation of the molecules we dissociate them with a femtosecond laser pulse, synchronized to the alignment pulse, and record the direction of the photofragments by ion imaging techniques. Without the alignment pulse the I fragments come out in a circularly symmetric pattern as expected for a sample of randomly oriented molecules. By contrast, when the alignment laser is on, the fragments are ejected along the direction of the polarization of the alignment laser showing that the molecules are aligned along this axis. The degree of alignment can be increased by increasing the intensity of the alignment pulse or by lowering the rotational temperature of the molecules.
2) 3-Dimensional Alignment To fully exploit the natural anisotropy of asymmetric top molecules the concept of alignment needs to be generalized from one to three dimensions. This is done by using an elliptically polarized alignment laser pulse. The 3-dimensional alignment can be understood by describing the elliptically polarized field as the sum of a circularly polarized field and a linearly polarized field, with the former confining the molecule to the polarization plane and the latter restricting the rotation within that plane.
3) Control of photochemical reactions Since absorption of light depends on the relative orientation between the polarization of the light and the molecular geometry it should be possible to achieve selectivity in the photoabsorption process by using aligned molecules. Next step is to investigate if the selective photoexcitation of molecules will allow certain bonds to be broken while others survive. So far, we have demonstrated that the yield of two equally likely photodissociation channels in molecular iodine can be modulated by a factor of 26 upon using aligned I2 molecules. We are now directing investigations towards larger molecules to study if control of photochemical reactions using aligned molecules is possible for larger systems.
[1] H. Sakai, C. P. Safvan, J. J. Larsen, K. M. Hilligsøe, K. Hald, and H. Stapelfeldt, J. Chem. Phys. 110, 10235 (1999); J. J. Larsen, H. Sakai, C. P. Safvan, I. Wendt-Larsen, and H. Stapelfeldt, J. Chem. Phys. 111, 7774 (1999).
[2] J. J. Larsen, I. Wendt-Larsen, and H. Stapelfeldt, Phys. Rev. Lett. 83, 1123 (1999).
[3] J. J. Larsen, K. Hald, N. Bjerre, H. Stapelfeldt, Phys. Rev. Lett. 85, 2470 (2000).