Perhaps, the most important PES development over the past two decades is the introduction of pulsed field ionization-zero electron kinetic energy (PFI-ZEKE) technique.31–34 Other names in use for this technique include ZEKE-PFI, PFI-PE, or simply ZEKE. PFI-ZEKE involves the detection of electrons produced by delayed, pulsed, electric field ionization of very high-lying
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Figure 2 Left: PE spectra of TiCl4recorded with photon energies at (a) 24 and (b) 40 eV; right: comparison of experimental (dashed line) and theoretical (solid line) branching ratios for the PE bands (CþD) and E with two possible assignments (see text) (reproduced by permission of the American Chemical Society from
Inorg. chem.1994,33, 5086–5093).
Photoelectron Spectroscopy 191
Rydberg states with a principle quantum number n>100. These Rydberg states are formed by laser excitation and are located a few cm1(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 ()
ẳ CE1=2cm1 ð6ị
whereEis the magnitude of the pulsed electric field in the unit of V/cm1andCis a proportional constant that can be determined experimentally. Experiments at various electric fields show that the value ofCis4 for metal-containing molecules.35On the basis of the continuity of ionization oscillator strengths, the relative intensities of the photoelectron bands in a PFI-ZEKE spectrum are expected to be identical to those in a direct threshold photoionization PE spectrum, provided that perturbations by nearby autoionizing states are small. Indeed, PFI-ZEKE spectra of many metal compounds have shown Franck–Condon intensity profiles.6,7,36,37The delay of pulsed field ionization from laser excitation is typically a few microseconds, which allows the ZEKE detection to be free from background electrons produced by direct photoionization and autoionization.
Because of the pulsed nature of the experiment, a time-of-flight electron analyzer is used for the ZEKE detection. The longevity of the ZEKE states is currently contributed to the angular moment l mixing due to the presence of the weak d.c. field in the excitation volume and to the magnetic momentum mlmixing due to the inhomogeneous electric field formed by nearby prompt ions.
The major advantage of PFI-ZEKE is its energy resolution, which allows the resolution of rotational and vibrational structures of metal compounds.6,7,34–51For example, the ZEKE spectra of vanadium dimers (V2) have a line width of 1.5 cm1 (0.19 meV), from which the rotational
Figure 3 ZEKE spectra of MNH3 (MẳAl, Ga, In) and spectral simulations for indium adduct and insertion isomers (reproduced by permission of the American Chemical Society from J. Phys. Chem.
A2000,104, 8178–8182).
constant and bond length of the corresponding cation have been accurately determined for the first time.51Figure 3shows the vibrationally resolved PFI-ZEKE spectra of MNH3(MẳAl, Ga, In), with spectral line widths of 5.0 cm1.42–44 Each of the spectra displays a short vibrational progression and a small peak (a) at the left side of the strongest peak. Additionally, the AlNH3
spectrum exhibits a doublet in the first three strong bands. The doublet is separated by 58 cm1 and is due to the spin–orbit interaction in AlNH3. The splitting is not observed for the Ga and In species due to large energy separations between the two spin components of the heavy metal atoms, which prevent a significant population of the upper spin level. The vibrational progressions yield the MþN stretching frequencies of 339 (AlþN), 270 (GaþN), and 234 (InþN) cm1. The strongest peaks determine adiabatic IEs, 39,746, 40,135, and 39,689 cm1, for the Al, Ga, and In complexes, respectively. The small peaks (a) arise from the transitions of the first vibrational levels of MNH3 to the vibrational ground states of MNH3 and yield the M–N stretching frequencies of 227, 161, and 141 cm1for the three neutral radicals. Converting the frequencies into stretching force constants shows the strength of the metal–nitrogen bonding in the order of Al>Ga>In, and the trend is explained by involving electrostatic and orbital interactions. To determine the geometric conformations of the complexes, density functional calculations were used to obtain the minimum energy structures and harmonic vibrational analyses for both the neutral and the ion; Franck–Condon factors were calculated using the theoretical geometries and vibrational analysis, and spectral simulations were performed with the experimental line widths and at various temperatures to obtain the best fit. A comparison of the 100 K simulations from the two indium isomers (Figures 4dand4e) and the experimental spectrum of InNH3(Figure 3(c)) clearly identifies that the electron carrier is a simple adduct rather than an insertion compound.
An alternative to PFI-ZEKE would be mass-analyzed threshold ionization (MATI), where cations rather than ZEKE electrons are detected.52In principle, MATI is attractive because of the inherent mass selection. However, MATI experiments are difficult to implement because of ion separation fields affecting the high Rydberg levels required for delayed pulsed field ionizations.
The technique has so far only been used for the smallest metal molecules.53
Figure 4 Left: PE spectra of Re2Cl82taken with 157, 193, 266, and 355 nm lasers; right: schematic drawing of potential energy curves showing the repulsive Coulomb barriers (RCB, eV) and vertical detachment energies (eV) of PE bands. The relative positions of the five laser wavelengths are also indicated (reproduced
by permission of the American Chemical Society fromJ. Am. Chem. Soc.2000,122, 2096–2100).
Photoelectron Spectroscopy 193