ZEKE states

Figure 1. Schematic of a two-color resonant two-photon ionization experiment "ZEKE states" lie just below each cation threshold, as indicated by asterisks.

The ZEKE-PFI practitioner relies on the existence of a narrow band of long-lived, high-n Rydberg states ( ZEKE states )15 lying just below each true cation threshold. If the shift of this band of states relative to the true threshold is essentially constant, then spectroscopic information extracted from differences in band frequencies will faithfully reflect the true cation energy levels. This assumption seems to hold to an accuracy of 1 cm-1 or better in the many molecules studied, as judged... 

The physical nature of the ZEKE states has been the subject of intense experimental and theoretical investigation in the past several years. In the well-studied case of NO,14,21 we know from the 3 cm-1 red shift of the ZEKE-PFI threshold band relative to the true adiabatic ionization potential (extrapolated from highly accurate measurements of Rydberg series) that the ZEKE states have principal quantum number n 200 and lifetime of 2 (is or longer. Recent work has found ZEKE states with lifetimes as long as 20 ps.22... 

E. W. Schlag The pickup is from a sea of ions of one species that picks up the ZEKE electron of the other species. Thus it shows up at the wrong energy. When the sea of ions is shut off, the signal disappears since it is at the wrong mass. This demonstrates that ZEKE states have an existence of their own and need not be formed necessarily from optically excited low-/ Rydberg states. 

The other source of congestion is due to the molecular core. It is most readily discussed using the inverse Bom-Oppenheimer point of view to define the zero-order quantum numbers. Here each state of the ionic core has its very own series of high Rydberg states. The physical reality of this approximation is the observation [36,43] of the long-time stable ZEKE states not just below the lowest ionization threshold but also just below the threshold of ionization processes that leave an excited ionic core. Indeed, it is for this very reason that ZEKE spectroscopy is useful for the spectroscopy of ions (or for such neutrals that are produced by ionization of negative ions... 

In dipolar situations the electron will continue to rotate with the core out to a radius such that the ion-dipole anisotropy is small compared with the relevant rotational energy separation for the ion core. Again since the switch-off distance is insensitive to energy, the dipole transit time will also be roughly the same for all n and one again expects an n 3 scaling law, but with a different coefficient. If this reasoning is correct, it is hard to see how the presence of a dipole can substantially enhance the lifetimes of ZEKE states. 

U. Even In a recent series of papers [M. Bixon and J. Jortner], using a model Hamiltonian quantum treatment, it is shown that all multipole contributions to l mixing are negligible when compared with / mixing by low external fields. Thus the long lifetimes associated with ZEKE states are attributed (in atoms and in molecules) to the external fields alone. 

E. W. Schlag The term zero-kinetic-energy electrons was used already by us in Chem. Phys. Lett. 4, 243 (1969). The new mechanism of redistribution of low-/ into high-/ molecular states is the basis of ZEKE states. 

The extraordinary stability of the magic ZEKE states has been demonstrated in experiments in which a molecule is dissociated [39, 40]. For example, one can start by exciting HBr into the magic states, then pulsed-... 

The sensitivity of ZEKE spectroscopy is high because, even though a small fraction of the molecules excited by the PROBE laser have n-values within the range capable of being ionized by the electric field detection pulse, the ions produced from these special ZEKE states are detected with 100% quantum efficiency against a perfectly dark background. 

U. Aigner, L.Y. Baranov, H.L. Selzle, E.W. Schlag, Lifetime enhancement of ZEKE-states in molecular clusters and cluster fragmentation. J. Electron Spectrosc. Relat. Phenom. 112, 175 (2000)... 

Rydberg states with a principle quantum number w> 100. These Rydberg states are formed by laser excitation and are located a few cm (or a fraction of meV) below the ionization threshold. Because the electrons ejected from these Rydberg states carry near zero electron kinetic energy, these states are known as ZEKE states and the ejected electrons are named as ZEKEs. The measured electron peak position is lower than that without the presence of the field by the Stark shift 6)... 

It should be noted that it is also possible to excite ZEKE states and to extract the corresponding ions, rather than the electrons these are then analysed in a mass spectrometer. In this case the method is called MATI (mass-analysed threshold ionization). The advantage of MATI spectroscopy is that mass resolution enables unambiguous identification of the ionized species, as well as allowing for the identification of fragmentation pathways in the ion. 

---