Laser Photodetachment Spectroscopy

Another group of experiments based on the coaxial arrangement deals with photodetachment spectroscopy of the negative molecular ions [9.84]. Negative molecular ions play an important role in the upper atmosphere and in many chemical reactions. Although hundreds of bound molecular negative ions are known, very few have been measured with rotational resolution. 

Since the extra electron generally has a low binding energy, most negative ions can be ionized (photodetachment) by visible or infrared lasers. The remaining ions are separated from the neutral molecules formed in the photodetachment process by a deflecting electric field. An example of sub-Doppler photodetachment spectroscopy of can be found in [9.85]. 

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We present the results of experimental studies of photon-negative ion interactions involving the dynamics of two electrons. Resonances associated with doubly excited states of Li and He" have been observed using laser photodetachment spectroscopy. Total and partial photodetachment cross sections have been investigated. In the former case, the residual atoms are detected irrespective of their excitation state, while in the latter case only those atoms in specific states are detected. This was achieved by the use of a state selective detection scheme based on the resonant ionization of the residual atoms. In addition, in the case of Li-photodetachment, the threshold behavior of the Li(2 P)+e-(ks) partial cross section has been used to accurately measure the electron affinity of Li. 

While one might expect that the techniques developed for photodissociation studies of closed-shell molecules would be readily adaptable to free radicals, this is not the case. A successful photodissociation experiment often requires a very clean source for the radical of interest in order to minimize background problems associated with photodissociating other species in the experiment. Thus, molecular beam photofragment translation spectroscopy, which has been applied to innumerable closed-shell species, has been used successfully on only a handful of free radicals. With this problem in mind, we have developed a conceptually different experiment [4] in which a fast beam of radicals is generated by laser photodetachment of mass-selected negative ions. The radicals are photodissociated with a second laser, and the fragments are detected in coinci-... 

Ion-beam photodetachment spectrometer used to measure high-resolution photodetachment spectra and autodetachment of C2 ions Laser optogalvanic spectroscopy used to determine photodetachment threshold for CN ... 

Abbreviations used in the tables calc = calculated value PT = photodetachment threshold using a lamp as a light source LPT = laser photodetachment threshold LPES = laser photoelectron spectroscopy DA = dissociative attachment attach = electron at-tachment/detachment equilibrium e-scat = electron scattering kinetic = dissociation kinetics Knud=Knudsen cell CT = charge transfer CD = collisional detachment and ZEKE = zero electron kinetic energy spectroscopy. 

The electron affinities listed in the table below agree within the error limits given. They were obtained by laser photoelectron spectroscopy (LPES) on the PHg ion [19] and PH2 photodetachment using a tunable laser (LPD) [20, 21] or an Xe arc lamp (with a grating monochromator) [21] and ion cyclotron resonance (ICR) spectrometry [20, 21]. All values are reported in two reviews on electron affinities [22] and electron photodetachment [23], and may be considered as adiabatic (see the remarks below the table) ... 

Fig. 4.35 A typical laser photodetachment electron spectroscopy setup. Electrons are energy analyzed and guided into a detector. The laser beam can be polarized to examine the angular distribution [492]...

Recommended values from a recent compilation [1] are shown below, together with the atomic (M) and ionic (M ) ground states involved and the determination methods used (LPES = laser photodetachment electron spectroscopy, LPT = (tunable) laser photodetachment threshold, SE=semiempirical extrapolation). For original work and details, see the remarks. 

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... 

Following the above-mentioned spectroscopic study by Johnson and co-workers [55], Neumark and co-workers [56] explored the ultrafast real-time dynamics that occur after excitation into the CTTS precursor states of I (water) [n — 4-6) by applying a recently developed novel method with ultimate time resolution, i.e., femtosecond photoelectron spectroscopy (FPES). In anion FPES, a size-selected anion is electronically excited with a femtosecond laser pulse (the pump), and a second femtosecond laser pulse (the probe) induces photodetachment of the excess electron, the kinetic energy of which is determined. The time-ordered series of the resultant PE spectra represents the time evolution of the anion excited state projected on to the neutral ground state. In the study of 1 -(water), 263 nm (4.71 eV) and 790 nm (1.57 eV) pulses of 100 fs duration were used as pump and probe pulses, respectively. The pump pulse is resonant with the CTTS bands for all the clusters examined. 

Figure 4.8 In (a) the principle oftransition state spectroscopy is shown. A continuous wave UV laser pulse with a frequency exceeding the energy gap between anionic potential (XHXq and neutral potential (XHX) is employed. After photodetachment of the electron, dissociation will occur equally in the two possible channels XH-tX and Xt-HX. In (b)...

Figure 9 Anion photoelectron spectroscopy. Its unique features are (I) Intrinsic mass selectivity and (ii) neutrals as final states. Here, as an example the results for compounds of iron, carbon and hydrogen are shown which exist in catalytic processes, high-temperature terrestrial or low-temperature astrophysical chemistry. Bottom spectrum a primary anion mass spectrum containing anions of the complexes of interest. Top spectra anion photoelectron spectra obtained by electron kinetic energy analysis after laser-induced photodetachment. They reveal the change of molecular structure and electronic energies for increasing numbers of hydrogen atoms in the complex.

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