Hannover 2003 – scientific programme
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MO: Molekülphysik
MO 6: Poster I
MO 6.4: Poster
Tuesday, March 25, 2003, 16:00–18:30, Lichthof
Field-free ion imaging and the velocity redistribution of excited atoms by radiative excitation transfer in the photodissociation of state selected Na2 — •Oliver Kaufmann1, Aigars Ekers1, Nikolai Bezuglov2, and Klaas Bergmann1 — 1University of Kaiserslautern, Dept. of Physics, Erwin-Schrödinger Str., D-67663 Kaiserslautern, Germany — 2St. Petersburg State University, Fock Institute of Physics, 198904 St. Petersburg, Russia
The photodissociation process Na2(X1Σg+, v”) +hν → Na2*(B1Πu) → Na*(3p3/2) + Na(3s1/2) has been studied using a novel No-dqfield-freeNo-dq ion imaging. The experiment uses a supersonic Na/Na2 beam and the STIRAP technique to prepare the molecules in selected rovibronic levels of the electronic ground state. The Na(3p3/2) photofragments are photoionised (or excited to high Rydberg states) in a permanently field-free reaction zone. The measured photofragment angular distributions are well explained by the anisotropic nature of the photodissociation by polarised laser light. However, besides the relatively fast Na(3p3/2) photofragments, the measured images show an unexpectedly efficient formation of Na(3p3/2) atoms with very small momentum transverse to the molecular beam axis. The ratio of numbers of slow and fast excited atoms is about 0.4. This dramatic velocity redistribution is explained by radiative excitation transfer between atoms. Using the theory of radiation trapping, we derived an analytical formula discribing this mechanism. In good agreement with the experiment, it yields a ratio of slow to fast Na(3p3/2) fragments of 0.41 and 0.32 for photofragmentation from vibrational levels v” = 17 and v” = 23, respectively.