Pulsed Field Ionization

Because it can be efficient and selective, field ionization of Rydberg atoms has become a widely used tool.1 Often the field is applied as a pulse, with rise times of nanoseconds to microseconds,2 4 and to realize the potential of field ionization we need to understand what happens to the atoms as the pulsed field rises from zero to the ionizing field. In the previous chapter we discussed the ionization rates of Stark states in static fields. In this chapter we consider how atoms evolve from zero field states to the high field Stark states during the pulse. Since the evolution depends on the risetime of the pulse, it is impossible to describe all possible outcomes. Instead, we describe a few practically important limiting cases. 

In Fig. 7.2 the ionization fields shown correspond to an ionization rate of 106 s 1. The fields for the extreme blue and red states are taken from the calculations of Bailey et al,5 and the fields for intermediate states are simply interpolated. We have also not shown any other n states since the Stark states of different n, which cross each other, have different values of nu and do not interact, as described in Chapter 6. 

Usually the H Rydberg states are not prepared in the zero field parabolic states, but in the spherical states by, for example, optical excitation. In this case, when the field is applied we would expect it to project the single n(m state onto the degenerate nn m states, each of which would then follow its own path to 

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In this chapter, we first present a brief overview of the experimental techniques that we and others have used to study torsional motion in S, and D0 (Section II). These are resonant two-photon ionization (R2PI) for S,-S0 spectroscopy and pulsed-field ionization (commonly known as ZEKE-PFI) for D0-S, spectroscopy. In Section HI, we summarize what is known about sixfold methyl rotor barriers in S0, S, and D0, including a brief description of how the absolute conformational preference can be inferred from spectral intensities. Section IV describes the threefold example of o-cholorotoluene in some detail and summarizes what is known about threefold barriers more generally. The sequence of molecules o-fluorotoluene, o-chlorotoluene, and 2-fluoro-6-chlorotoluene shows the effects of ort/io-fluoro and ortho-chloro substituents on the rotor potential. These are approximately additive in S0, S, and D0. Finally, in Section V, we present our ideas about the underlying causes of these diverse barrier heights and conformational preferences, based on analysis of the optimized geometries and electronic wavefunctions from ab initio calculations. 

B. Zero Kinetic Energy Threshold Photoelectron Spectroscopy and Pulsed Field Ionization (ZEKE-PFI)... 

When the pulsed-field ionization signal of a single excited Rydberg peak is measured as a function of the delay time between the extraction field pulse... 

T. P. Softley The aims of ZEKE spectroscopy are conceptually different from the atomic pulsed-field ionization experiments that predate ZEKE. In the latter, the aim is always to observe and study the individual Rydberg states. In ZEKE spectroscopy the aim is to detect small batches of Rydberg states lying below successive vibration-rotation thresholds of the ion without specific interest in the individual Ryd-... 

Figure 3. Pulsed-field ionization spectra from the 1II ( / — 0) state of CO with the initial level J = 2 pumped via (a) S (0) and (b) R (l).

Measurement of the survival probability of Rydberg molecules as a function of delay time for pulsed-field ionization. 

B. Kohler My question to T. Softley ties in to one of the major themes of the meeting, namely coherence. In your presentation you briefly mentioned that it may be important to consider an initially coherent superposition of states in the preparation step of experiments on highly excited Rydberg states. Several groups have now prepared coherent electronic wavepackets using picosecond (and shorter) pulses. Would this kind of an initial state be useful for any of the classes of pulsed-field ionization experiments that you have described ... 

T. P. Softley It has been demonstrated by Hepburn et al. that H can be formed in v = 13 by a pulsed-field ionization technique from a... 

Pulsed field ionization of an alkali atom differs from the description just given for H because of the finite sized ionic core, or equivalently, the nonzero quantum defects. There are three important effects. First, the zero field levels can only be spherical nim levels, not parabolic levels. Second, in the E > l/3n5 regime there are avoided crossings of states of different n. Third, ionization can occur at lower fields than in H. Specifically, in H blue states have higher ionization fields than red states, but in an alkali atom this is not the case due to nx changing ionization. 

As we shall see, microwave ionization can be thought of as a multiphoton absorption or as a process driven by a time varying field. We first discuss microwave ionization of alkali atoms, which can be described by the notions used to describe pulsed field ionization. To show the connection between the time varying field point of view and the photon absorption point of view we then discuss... 

Fig. 13.8 Overall perspective view of the charge exchange apparatus. A thermal Na beam is laser excited in the interaction region to produce a target for Rydberg-to-Rydberg charge transfer experiments. The number of target Rydberg atoms is determined by pulsed field ionization between the parallel plates and by collection of the resultant ions in the electron...

D. -S. Yang, M. Z. Zgierski, D. M. Rayner, P. A. Hackett, A. Martinez, D. R. Salahub, P.-N. Roy, and T. Carrington Jr., J. Chem. Phys., 103, 5335 (1995). The Structure of NbjO and Nb30 Determined by Pulsed Field Ionization-Zero Electron Kinetic Energy Photoelectron Spectroscopy and Density Functional Theory. 

A somewhat more advantageous ZEKE electron detection mechanism consists of pulsed-field ionization (PFI) of Rydberg states. In this schema, the photons do not populate the ion state directly, but rather very high Rydberg states that are convergent on... 

ADE = adiabatic detachment energies ESC A = electron spectroscopy for chemical analysis HOMO = highest occupied molecular orbitals MAES = metastable atom electron spectroscopy MIES = metastable ionization electron spectroscopy OAT = oxygen atom transfer PES = photoelectron spectra PEI = pulsed field ionization PIES = Penning ionization electron spectroscopy QM = quantum-mechanical REMPI = resonantly enhanced multiphoton ionization SC = semiclassical VDE = vertical detachment energies XPS = x-ray photoelectron spectroscopy ZEKE = zero electron kinetic energy Cp = cyclopentadienyl, Ph = phenyl, CeHs Tp =... 

Several methods exist that allow the determination of the standard enthalpies of formation of the ionic species. The reader is referred to two recent rigorous and detailed chapters by Lias and Bartmess and Ervin. The vast majority of the experimental data reported here are obtained by means of Fourier transform ion cyclotron resonance spectroscopy (FT ICR), high-pressure mass spectrometry (HPMS), selected ion flow tube (SIFT), and pulsed-field ionization (PFI) techniques, particularly pulsed-field ionization photoelectron photoion coincidence (PFI-PEPICO). All these experimental techniques have been examined quite recently, respectively, by Marshall, Kebarle, B6hme," ° Ng" and Baer. These chapters appear in a single (remarkable) issue of the International Journal of Mass Spectrometry. An excellent independent discussion of the thermochemical data of ions, with a careful survey of these and other experimental methods, is given in Ref. 37. 

To our knowledge, 46 has never been observed in solution under stable conditions, even at low temperature. Pulse radiolysis " of benzyl chloride as well as flash photolysis ° of several derivatives in HHP have allowed the observation of the electronic absorption spectra of benzyl and its 4-methyl and 4-methoxy derivatives. The and NMR spectra of the 2,4,6-trimethylbenzyl cation and other more heavily substituted benzyl cations, however, have been studied at low temperature in superacid media. In the gas phase, cold benzyl radical has been probed by two-color, resonant two-photon ionization techniques, thus providing very accurate vibrational frequencies below 650 cm for the benzyl cation. Furthermore, the adiabatic ionization energy of benzyl radical and several isotopomers in the ground state were determined from their threshold photoionization spectra using resonant two-photon excitation and detection of electrons by pulsed field ionization. This information, combined with Af//° (CgH5CH2) from Ref. 212 leads to the value of Af//°m(46) reported in Table 9. 

The improved numerical stability of the new deMon2K version also opened the possibility for accurate harmonic Franck-Condon factor calculations. Based on the combination of such calculations with experimental data from pulsed-field ionization zero-electron-kinetic energy (PFl-ZEKE) photoelectron spectroscopy, the ground state stmcture of V3 could be determined [272]. Very recently, this work has been extended to the simulation of vibrationaUy resolved negative ion photoelectron spectra [273]. In both works the use of newly developed basis sets for gradient corrected functionals was the key to success for the ground state stmcture determination. These basis sets have now been developed for aU 3d transition metal elements. With the simulation of vibrationaUy resolved photoelectron spectra of small transition metal clusters reliable stmcture and... 

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... 

PFI-PEPICO pulsed field ionization photoelectron-photoion coincidence technique... 

ZEKE-PEl zero kinetic energy pulsed field ionization photoelectron spectroscopy... 

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