ZEKE spectroscopy

The ZEKE detection scheme is equivalent to ionization of high-n Rydberg states by Pulsed Field Ionization (PFI). If one assumes that the pulsed field ionization of the Rydberg electron follows a diabatic process (Chupka, 1993), then the ionization threshold is lowered by 

Deviations from predicted rotational intensity distributions are very common in ZEKE spectra. This is due to random near coincidence between extremely numerous rapidly- and slowly-autoionizing resonances (Rydberg series converging to excited rovibronic states of the ion). Since the waiting time between excitation and pulsed field ionization is long, and the very weak DC and stray electric fields present during the ZEKE waiting period can induce weak interactions 

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Reed, R.I., Applications of Mass Spectrometry to Organic Chemistry, Academic Press, New York, 1966. Schlag, E.W., ZEKE Spectroscopy, Cambridge University Press, London, 1998. 

TABLE 5.1. Ionization energies of organic radicals measured by using ZEKE spectroscopy. 

Schlag, E.W. ZEKE Spectroscopy Cambridge University Press Cambridge, 1998. 

My question to Prof. Gerber is the following Could you please explain the different virtues of femtosecond pump-pulse experiments versus ultrashort zero electron kinetic energy (ZEKE) spectroscopy Do they yield complementary information on the molecular dynamics or are there specific domains where one of them should be preferred with respect to the other ... 

Figure 1. Historical development leading to ZEKE spectroscopy.

Threshold electron spectroscopy, or its newest variant ZEKE spectroscopy, represents a new approach to these problems that has already afforded a broad set of new applications, particularly for soft bonds and metastable species, such as is characteristic for metastable reactive intermediates. 

Molecular systems studied with ZEKE spectroscopy. 

Figure 6. Spectroscopy of neutral ground state via anion ZEKE spectroscopy.

Figure 8. Types of systems studied with ZEKE spectroscopy.

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... 

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

The dramatic effect of the dilution of the coupling strength to the continuum is to endow the trapped states with a finite (but long) lifetime. We consider that this is the origin of the extremely long decay times observed in ZEKE spectroscopy. 

GENERAL DISCUSSION ON MOLECULAR RYDBERG STATES AND ZEKE SPECTROSCOPY PART I... 

There is also the question of which zero-order basis is more natural for the experiment. Of course, here, too, simplicity is in the eye of the beholder, but as I already argued, to my mind, ZEKE spectroscopy points out the advantage of the inverse Bom-Oppenheimer zero-order basis. 

ZEKE Spectroscopy, this volume) are obtained by ionizing the Rydberg series which is built on a particular state of the ionic core. The inverse Bom-Oppenheimer picture is thus natural when one begins the discussion with what is actually the raw observation. 

Theory [F. Remade and R. D. Levine, Phys. Lett. A 173,284 (1993)] and simulations suggest that there should also be a prompt decay corresponding to a yet shorter scale. Your statement that you can now begin the time-resolved ZEKE spectroscopy at much earlier times than before makes me suggest that benzene be one of the first molecules that you choose for experimental study. 

Prof. Schlag ( ZEKE Spectroscopy, this volume) has introduced a new sequential technique of ZEKE spectroscopy In the first step, a negative ion M is photoionized, yielding the neutral core M of the excited Rydberg state of the anion M. In the second step, M is further photoionized, yielding the cationic core M+ of the excited Rydberg state of the neutral molecule M. The overall sequence is thus... 

E. W. Schlag On the way up from M - M — M+ it would be often helpful to state select M via ZEKE spectroscopy. It would also be useful for its overtones. 

V. S. Letokhov In his exciting and dynamical report Prof. Schlag mentioned the historical development leading to ZEKE spectroscopy. Let me comment on this point. Laser-induced REMPI and ZEKE spectroscopies belong to the rich family of laser ionization spectroscopy techniques (Fig. 1) [1]. 

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