Stepwise (electron-laser) H2 excitation

Basics

Laser radiation is used to probe the electron-impact excitation of the metastable c3Piu states. Compared to helium, the electron-impact step is complicated since the metastable state consists of a manifold of ro-vibrational levels. The supersonically expanded ground state beam consists of molecules with vibrational quantum number v=0 and in rotational states J+=0,1. Electron impact creates metastable states with a range of v,J+ which depends on the electron-impact energy, but is limited to the lowest few (up to around 4) levels.

The absorption of a laser photon between around 340 and 380nm further excites the molecules to high nd Rydberg states, conserving the ro-vibrational quantum numbers. With the currently available laser, we have scanned between n=6 to series limits. In the experiment, the production of Rydberg states is measured by detecting the ion-products of either auto-ionisation or pulsed-field ionisation.

The figures below show some portions of the Rydberg spectrum.

An overview of the entire spectrum.
The v=1-1 Rydberg series of transitions.
Overlapping Rydberg series.
transitions to n=8, v=1 states

By scanning the electron-impact energy and monitoring the ion yield due to a particular transition, information can be obtained about the electron-impact step. This technique can also be used as an aid to interpret the spectrum: by using previously-known information about this step (e.g. the thresholds for exciting successive vibrational levels.) For more information...... jamesh@fs3.ph.man.ac.uk