Ponente: Prof. Roland R. Lefebvre
Institución: Laboratoire de Photophysique Moléculaire du CNRS, Orsay, France.
Hora de inicio: 17:30:00
Lugar: Auditorio ICF
The interaction of a molecule with a cw laser field is described by a time-periodic Hamil-tonian. The wave equation has solutions given by the Floquet formalism, with eigenvalues called the quasi-energies. If the field can lead to photodissociation of the molecule, these quasi-energies are complex, with an imaginary part yielding the photofragmention rate. In the case of intense fields, there is a richness of new processes. We are here interested  by the possibility, with an appropriate choice of laser intensity, to reduce to zero the imaginary part of the energies of some of the resonances associated with field-free vi-
brational states of a diatomic molecule. The example is the molecular ion H+
2 . Not all
resonances present this property for a given wavelength. The Floquet formalism is applicable even for a pulsed laser if the amplitude of the field is varying smoothly enough. It is then possible to build filtration scenarios where only the resonances passing through the zero-width phenomenon during the pulse will survive. Another situation which is possible with an appropriate choice of both wavelength and intensity is to provoke the coalescence of two quasi-energies. The corresponding point in parameter plane is called exceptional. Such points have recently been studied in many areas of physics, either classical or quantum. They have a number of very important consequences which will be presented for the case of molecular photodissociation . A condition for two resonances to yield such a point is that they correspond to respectively a shape-like resonance and a Feshbach-like one. At an exceptional point the two resonance wave functions merge into
a single one, which is said to be ”self-orthogonal”. With an adiabatic variation of the parameters along a closed contour around an exceptional point, it is possible to go from one non-degenerate resonance pole to another. In order to realize such a transfer, it is
necessary to reach a compromise between two conflicting conditions: the laser pulse must vary slowly enough for an adiabatic transfer to take place, but fast enough to keep a fair amount of undissociated molecules. This can be checked through the adiabatic Floquet theory.
 O. Atabek, R. Lefebvre, C. Lefebvre and T.T. Ngyugen-Dang, Phys. Rev. A 74,
 R. Lefebvre, O. Atabek, M. ¢ Sindelka and N. Moiseyev, Phys. Rev. Lett. 103,