3.2. Design and test using photodissociation of Na2(v")
Our design of the field-free ion imaging setup is shown in Fig. 4 . To test the setup, we used photodissociation of selectively vibrationally excited molecules. This process yields dissociation fragments with well defined kinetic energies dependent on the vibrational excitation of the molecules and the energy of the photodissociating photons (see Fig. 5):
Na2 (X 1Sg+ ,v??) + hnAr+ Na2*(B 1Pu) Na(3p3/2) + Na(3s1/2) + D E(v??). (4)
The Na(3p3/2 ) dissociation fragments are photoionised and detected by the ion imaging detector. Given that the wavelength of the photodissociating laser is fixed, the kinetic energies of these fragments are dependent on the vibrational excitation of the molecules only. Figure 6 shows 2D images of photodissociation fragments when molecules are selectively excited to v" = 10 and v" = 17 levels. The increase in the radius of the 2D image for higher vibrational excitation is clearly seen and demonstrates the potential of the field free imaging technique. Further efforts are directed towards elimination of distortions of the images and improvement of the energy resolution to better than 10 meV.
Fig. 4: Arrangement of the experiment with field-free ion imaging. Stokes and pump lasers (or pump laser alone) serve for selective vibrational excitation of the molecules via STIRAP. Probe laser allows monitoring of the vibrational excitation. Excitation laser 1 photodissociates the molecules from vibrationally excited levels to the continuum of the B1Pu electronic state, yielding atomic fragments in the 3s1/2 and 3p3/2 states. The atoms in the 3p3/2 state are photoionised by excitation laser 2 and the resulting ions are focussed by ion optics onto PSD (channel plates in Chevron arrangement followed by a phosphor screen and a CCD camera). The zone upstream from the entrance mesh of ion optics is kept free from electric fields. The ion optics is operated in the velocity mapping mode  which, in comparison with the conventional space mapping mode, yields an improved energy resolution. The PSD is displaced from the molecular beam axis to avoid the atoms and molecules from primary beam which have passed by the beam blocker from hitting the channel plates. The ions are correspondingly deflected away from beam axis by a homogeneous field of the electrostatic image shifter.