Nuestro Coloquio de Hoy

[Guillermo Hinojosa <hinojosa@xxxxxxxxxxx>]

 Estimados Colegas,
 
 En nuestro Coloquio de hoy contaremos con la presencia del Dr. Roland
 Lefebvre del Laboratorio de Fotofísica Molecular del CNRS en Orsay,
 Francia. Su platica se intitula:
 
           "Zero-Width Resonances and Exceptional Points 
                  in Molecular Photodissociation"
                  
 Nos dará mucho gusto contar con su amable presencia, el día de hoy
 miércoles 24 de febrero a las 5:30 PM, en el auditorio del Instituto
 de Ciencias Físicas de la UNAM- Campus Morelos.
 
 Atentamente:
 
 Maximino Aldana y Guillermo Hinojosa.
                  
                                ~ Abstract ~
                                
 The interaction of a molecule with a cw laser field is described by a 
 time-periodic Hamiltonian. 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[1] 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 vibrational states of a 
 diatomic molecule. The example is the molecular ion H2+. 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 [2]. 
 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.
 
 [1] O. Atabek, R. Lefebvre, C. Lefebvre and T.T. Ngyugen-Dang, 
 Phys. Rev. A 74, 063412(2008)
 [2] R. Lefebvre, O. Atabek, M. ¢ Sindelka and N. Moiseyev, Phys. Rev. Lett. 
 103, 123003(2009).