Molecules ZEKE states

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... 

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. 

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. 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Figure 9.50 Processes involved in obtaining (a) an ultraviolet photoelectron spectrum, (b) a zero kinetic energy photoelectron (ZEKE-PE) spectrum by a one-photon process and (c) a ZEKE-PE spectrum by a two-photon process in which the first photon is resonant with an excited electronic state of the molecule...

Figure 9.51 A zero kinetic energy photoelectron (ZEKE-PE) resonant two-photon spectrum of 1,4-difluorobenzene in which the first photon excites the molecule of the zero-point level of the S-[ excited electronic state of the molecule. (Reproduced, with permission, from Reiser, G., Rieger, D., Wright, T.G., Muller-Dethlefs, K. and Schlag, E.W., J. Phys. Chem., 97, 4335, 1993)...

The resolution of the ZEKE-PE spectmm of 1,4-difluorobenzene can be compared with, for example, that of the ultraviolet photoelectron spectmm of benzene in Figure 8.12. The greatly increased resolution in the ZEKE-PE spectmm is attributable mostly to the fact that only photoelectrons with zero kinetic energy are being detected. It is also partly attributable to the molecules being in a supersonic jet this has the effect of sharpening the bands because of the restricted rotational populations in the ground state of the molecule. 

Photoelectron spectroscopy (PES, a non-mass spectral technique) [87] has proven to be very useful in providing information not only about ionization potentials, but also about the electronic and vibrational structure of atoms and molecules. Energy resolutions reported from PES are in the order of 10-15 meV. The resolution of PES still prevents the observation of rotational transitions, [79] and to overcome these limitations, PES has been further improved. In brief, the principle of zero kinetic energy photoelectron spectroscopy (ZEKE-PES or just ZEKE, also a nonmass spectral technique) [89-91] is based on distinguishing excited ions from ground state 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. 

V. Engel Let me come back to the distribution of lifetimes of the ZEKE Rydberg states. I wonder if there is a simple picture behind. Consider a much simpler molecule, namely the Nal molecule Prof. Zewail told us about. There you have a bound state coupled to a continuum. It can be shown that in such a system the lifetimes of the quasibound states oscillate as a function of energy. In fact, Prof. Child showed with the help of semiclassical methods that there are lifetimes ranging from almost infinity to zero [1]. That can be understood by the two series (neglecting rotation) of vibrational levels obtained from the adiabatic and diabatic picture. If two energy levels of different series are degen-... 

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... 

L. Woste In stationary spectroscopy ZEKE certainly provides spectroscopic results at an impressive resolution. Using femtosecond pulses one can certainly not excite specific states as compared to ZEKE. The Fourier transform of the wavepacket evolution, however, exhibits also spectral resolution that easily reaches and even exceeds what we see in ZEKE spectra. For this reason, I do not see any disadvantage in using femtosecond NeNePo to probe states of a prepared molecule. 

FROM RYDBERG STATE DYNAMICS TO ION-MOLECULE REACTIONS USING ZEKE SPECTROSCOPY... 

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