Photoion imaging

The ultimate desirable outcome in any chemical reaction dynamic experiment is the measurement of flux-velocity contour maps for quantum-state-selected products from photofragmentation, or inelastic and reactive collisions processes for which the initial state is also well defined. From such contour maps, complete information on the chemical process can be deduced in favourable cases. 

In addition to direct (TOF) measurements of the scattered particles, spectroscopic techniques based on the Doppler shift have been developed however, those are normally limited to rather light particles that move with high velocities. One of the most elegant developments, which is based on a combination of TOF and laser spectroscopic methods, has been the introduction of ion imaging techniques. 

Photoion imaging is a method that allows for final-state resolved analysis, and simultaneous detection of all scattering angles and velocity distributions, based on a single experimental geometry. Chandler and Houston (1987) carried out the pioneering experi- 

The nature of the data that can be obtained provides (qualitatively) a direct visualization method into the core of state-resolved chemical reaction processes increasingly, the technique has become quantitative and thus promises to deliver real insight into the exact nature of chemical change. 

The velocity vectors of the fragments wiU be correlated with the laser polarization vector the polarization vector of the laser beam also defines a laboratory-fixed coordinate frame. But one also has to remember that, in the molecular reference frame, the angular recoil distribution depends on the symmetries of the electronic (initial and final) states involved in the absorption process, namely that the relative orientation of the transition dipole moment of a molecule fi and the direction of the laserpolarization vector (see Chapter 16 for details). 

---

Figure 1. Photoelectron circular dichroism angular distribution [/icp(0) - /rcp(6)] for the hv = 10.3-eV photoionization of (/ )-camphor, as imaged with the photon beam propagating along the X axis. The x,y axis scales are the physical pixel coordinates of the detector.

Reactions (19)-(21) represent the dissociation of benzene and reactions (22)-(26) represent the detection of fragments by VUV laser photoionization. The line-shape images resulted from these reactions. 

In conclusion I would like to emphasize that the suggested approach (femtosecond laser spectromicroscopy) is not a simple modification of the Muller microscope [6], for the electric field here is not the decisive factor but serves solely to form the image. Table I lists the comparative characteristics of the Muller projection field-ion microscope (FIM) and proposed laser resonance photoelectron (photoion) spectromicroscope (LRFSM). 

We compare this intensity stripe to the photoelectron spectra reported in previous studies. While the intensity of the TOF spectra for H2 is not directly comparable to this intensity stripe down the center of our image for D2, comparison is appropriate for the discussion of trends in the data. To the extent that electrons associated with different channels of the photoionization have different angular distributions, as they clearly do from Figure 8, traditional TOF photoelectron spectra do not reflect the true intensity for the various channels either. By inspection of the images, trends associated with vibrational distributions of the photoelectrons (and photoions) can be observed. 

Sofar the imaging results of Fig. 3.1 were discussed in very classical terms, using the notion of a set of trajectories that take the electron from the atom to the detector. However, this description does not do justice to the fact that atomic photoionization is a quantum mechanical proces. Similar to the interference between light beams that is observed in Young s double slit experiment, we may expect to see the effects of interference if many different quantum paths exist that connect the atom to a particular point on the detector. Indeed this interference was previously observed in photodetachment experiments by Blondel and co-workers, which revealed the interference between two trajectories by means of which a photo-detached electron can be transported between the atom and the detector [33]. The current case of atomic photoionization is more complicated, since classical theory predicts that there are an infinite number of trajectories along which the electron can move from the atom to a particular point on the detector [32,34], Nevertheless, as Fig. 3.2 shows, the interference between trajectories is observable [35] when the resolution of the experiment is improved [36], The number of interference fringes smoothly increases with the photoelectron energy. 

XPS (x-ray photoelectron spectroscopy) utilizes photoionization and energy-disperse analysis of the emitted photoelectrons to study the composition and electronic state of a region of the surface of a zeolite. However, aU these techniques are destructive ones, and for that reason other methods such as isotopic-transient experiments or reflectance [16] and fluorescence [17] imaging can be used to estimate the effective membrane thickness. 

Tailored modelling taking explicitly into account departure from spherical symmetry is still in its infancy. One may mention the work of Monteiro et al. (2000) who constructed a 3D photoionization model to reproduce the narrow band HST images and velocity profiles of the PN NGC 3132 and concluded that this nebula has a diabolo shape despite its elliptical appearance. For the abundance determination however, which is the topic of this review, their finding has actually no real incidence. 

---