Photodetachment

The photodetachment energy, lion(liq)/ can be measured for instance as a photo conductivity threshold. Such measurements have been described for the system 02/liquid hydrocarbons (Lukin and Yakovlev, 1976 Sowada and Holroyd, 1979). Negative ions of O2 were produced by irradiation of a solution with a pulse of high energy electrons or X-rays. Ionization of the molecules of the liquid yields positive ions and electrons. 

Variation of the photon energy with a tunable laser allows the determination of the photoconductivity threshold. In the gas phase, the photodetachment cross section was found to follow a dependence on the photon energy, ep o, given as 

The same law was found to hold for the detachment in the liquid solution, too. In the gas phase the electron affinity of O2 was determined to be 0.44 eV. The results obtained in some hydrocarbons are summarized in Table 9. 

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The above fomuilae for the absorption spectrum can be applied, with minor modifications, to other one-photon spectroscopies, for example, emission spectroscopy, photoionization spectroscopy and photodetachment spectroscopy (photoionization of a negative ion). For stimulated emission spectroscopy, the factor of fflj is simply replaced by cOg, the stimulated light frequency however, for spontaneous emission... 

ZEKE (zero kinetic energy) photoelectron spectroscopy has also been applied to negative ions [M]. In ZEKE work, the laser wavelengdi is swept tlirough photodetachment thresholds and only electrons with near-zero kinetic energy are... 

Pegg D J 1996 Photodetachment Atomic, Molecular, and Optical Physics Handbook ed G W F Drake (Woodbury, NY AiP)... 

Mead R D, Stevens A E and Lineberger W C 1984 Photodetachment in negative ion beams Gas Phase Ion Chemistry ed M T Bowers (New York Academic)... 

Miller T M 1981 Photodetachment and photodissociation of ions Advances In Electronics and Electron Physics ed L Marten and C Marton (New York Academic)... 

The transition-state spectroscopy experiment based on negative-ion photodetachment described above is well suited to the study of the F + FI2 reaction. The experiment is carried out tln-ough measurement of the photoelectron spectrum of the anion FH,. This species is calculated to be stable with a binding energy of... 

Gantefor G, Meiwes-Broer K H and Lutz H O 1988 Photodetachment spectroscopy of cold aluminum cluster anions Phys. Rev. A 37 2716... 

Kitsopoulos T N, Chick C J, Zhao Y and Neumark D M 1991 Study of the low-lying electronic states of Sij and Si. using negative ion photodetachment techniques J. Chem. Phys. 95 1441... 

Wang L S 2000 Photodetachment photoelectron spectroscopy of transition metal oxide species Photoionization and Photodetaohment Advanced Series in Physical Chemistry 10, ed C Y Ng (Singapore World Scientific)... 

Anion photoelectron spectroscopy [37, 38] amd photodetachment techniques [39] provide accurate information on electron detachment energies of negative ions. Ten closed-shell ainions considered here exhibit sharp peaks, indicative of minor or vanishing final-state nuclear rearrangements, in their photoelectron spectra. Comparisons between theory and experiment are straiightforward, for differences between vertical and adiabatic electron detachment energies (VEDEs and AEDEs, respectively) are small. 

Recently, we [61] applied this method to model the (HOCO) photodetachment experiments of Continetti and co-workers [62]. We are currently using the method to study the dissociation dynamics other four-atom systems, including hydrogen peroxide and formaldehyde. 

A second role for mass spectrometry in the investigation of reactive intermediates involves the nse of spectroscopy. Althongh an important nse of ion spectroscopy is the determination of thermochemical properties, including ionization energies (addition or removal of an electron), as in photoelectron or photodetachment spectroscopy, and bond dissociation energies in ions, as in photodissociation methods, additional spectroscopic data can also often be obtained, inclnding structural parameters such as frequencies and geometries. 

Photodetachment/Photoionization Photodetachment and photoionization refer to the processes of removing an electron from either a negative ion or a neutral molecule. 

An important feature of photodetachment methods is that the selection rule for electronic transition is A5 = +1. Therefore, photodetachment of (doublet) negative ions... 

If the geometry of the lower (ionic) state is very similar to that of the transition state and is very different from that of the stable neutral states, then the transition state can be generated by vertical photodetachment. For example, as shown in Figure... 

Figure 5.4. Potential energy surfaces for out-of-plane bending of cyclooctatetraene and its negative ion. The arrow indicates a vertical transition upon photodetachment to the transition state of the neutral.

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

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