High Harmonic Generation from Multiple Orbitals in N2

High harmonic generation was first discovered in the early 1990s. A high electric field is associated with a high intensity laser pulse. The field can get so high, that some of the electrons get pulled out of an atom or a molecule when the laser pulse hits them. The electrons are then accelerated in the electric field of the laser and they recombine with the atom or molecule they were ionized from. Thereby the electrons loose their kinetic energy and transform it into light (see detailed explanation).

simple picture of high harmonic generation
Animation of our experiment:
A strong infrared laser pulse is focused in a nitrogen gas jet (white cone in the middle). In the jet, the strong laser electric field interacts with the N2 molecules. This results in the emission of light in the form of laser harmonics in the extreme ultraviolet spectral range.

The method can be used to learn about the structure of the electrons in the atom or molecule. The electrons are governed by the laws of quantum mechanics and form so called orbitals. The spectral analysis of the harmonics reveal contain information about the molecular orbital involved in HHG. Up to now, we only had indications that the orbital that is highest in energy (the so called HOMO) can contribute to producing electrons.
In our article we present evidence, that the HOMO and a lower bound orbital (HOMO-1) produce high harmonics.
simple picture of high harmonic generation   High harmonics are generated by recombining an electron with an orbital in a strong laser field.
A) Sketch of the nuclear positions in the nitrogen molecule (green) and the internuclear axis (black line in A, B and C). B) Highest occupied molecular orbital (HOMO) with colors indicating the sign of the orbital. The recolliding electron is presented as a de Broglie wave.
C)  Next lower bound orbital (HOMO-1) with the recolliding electron wave. The HOMO-1 participates in harmonic generation. Its contribution is strongest for an electron recombining under 90 degrees to the internuclear axis.

see High Harmonic Generation from Multiple Orbitals in N2

B. K. McFarland, J. P. Farrell, P. H. Bucksbaum and M. Gühr

Science, Published online October 30 2008, 10.1126/science.1162780   

link to article in Science
Link to Perspectives: Interrogating molecules by Gilles Doumy and Louis F. DiMauro