Photodetachment cross-sections

The photodetachment cross section for a photon of angular frequency u is, in the length formulation,... 

Steiner, B. Photodetachment cross sections and electron affinities, in Case studies in atomic collision physics II. by McDaniel, E. W., McDowell, M. R. C. (eds.). Amsterdam North-HoUand Publishing Company 1972, pp. 483-545 Steinmetz, H., Hettich, A. Z. anorg. Chem. 167, 75 (1927)... 

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. 

The total photodetachment cross section describes the probability that an electron is detached from a negative ion following the absorption of a photon, regardless the excitation state of the residual atom or the energy or direction of the emitted electron. A total cross section is the sum of partial cross sections for detachment into each of the energetically allowed continua. This is illustrated in Fig. 1. Here we show the three possible channels accessible to a doubly excited state of Li" that lies just below the Li(32P) detachment threshold. The total cross section may be determined by... 

Doubly excited states of He of doublet symmetry have been observed in studies of electron impact on He [23]. In contrast, data on quartet states are sparse. Selection mles on photoexcitation from the P° ground state limit excited state production to those of S, P and symmetry. Recently, the He photodetachment cross section... 

In Fig. 8 we show an extended 3 Skp partial photodetachment cross section which includes measurements below the LiCS S) threshold in addition to the data already shown in Fig. 7b. The data in the range 5.29-5.39 eV represents a relatively low statistics survey scan. It is normalized to the calculation of Pan et a/. [28] using the same factor as for the data between 5.39-5.46 eV. It is clear, even from this relatively low quality data, that the two resonances labeled f and g are observed at approximately the calculated energies but their measured strengths appear to be weaker than predicted. Presumably, the lowest lying resonance, labeled f, is the intrashell resonance representing symmetric excitation of the two valence electrons. 

Wigner [30] predicted that in the vicinity of a threshold the photodetachment cross section is represented by... 

However, the absolute photodetachment cross section is difficult to obtain without an exact knowledge of the time the ions spend in the light beam and the geometrical overlap factor. 

T.F. O Malley, Effect of long-range final-state forces on the negative-ion photodetachment cross section near threshold, Phys. Rev. 137 (1965) A1668. 

Additional support for the high value for A(N02) was recently obtained by Warneck from studies of the photodetachment of NO2 . A threshold at 2.74 + 0.03 eV was observed but ascribed to the presence of vibrationally excited NO2 . A sharp break in the photodetachment cross section at 3.10 + 0.05 eV was assigned to the transition between ground states of N02 and NO2 and hence should correspond to the electron affinity of NO2. Because of the different geometries of the ground states of the neutral and negative ion, it is doubtful that a sharp break is to be expected in this cross section. 

Two experimental results are available for the adiabatic electron affinity which were obtained by photodetachment studies on PH ions. A=1.028 0.010 eV was obtained from a laser (488 nm) photoelectron spectrum of PH after rotational corrections had been made to the detachment energy measured from the center of the PH(X v=0) PH (X rij, v = 0) peak [1]. A less precise result, A=1.00 0.06 eV, has been obtained from the lowest threshold energy of the photodetachment cross section measured between 0.8 and 2.8 eV (1.5 to 0.4 xm a second threshold at -1.9 eV obviously corresponds to the PH(a A) PH (X rij) transition) [2]. 

The geometrical structure of gaseous PH2 in its X Ai ground state appears to be similar to that of ground-state PH2 (with an internuclear distance of r=1.42 A and an interbond angle of a = 92° see p. 72). This was inferred from a sharp increase of the photodetachment cross section at threshold, measured by ion cyclotron resonance [2, 3] and from the predominance of the (0, 0, 0)<-(0, 0, 0) transition in the PH2, X Bi PH, X A photoelectron spectrum [4]. r=1.34 0.05 A and a = 92 5 were taken from the isoelectronic H2S molecule (and used to calculate the thermodynamic functions of PH, see p. 109) [5]. r and a have also been theoretically calculated by several ab initio MO methods, i.e., at an MP2 [6, 7], a CEPA (coupled electron pair approximation) [8], and an HF level [9 to 15]. r was also obtained from a united-atom approximation [16] a was also calculated by a semiempirical (CNDO/2) method [17] and estimated by extended Huckel calculations [18]. 

Relative photodetachment cross sections have been measured in the wavelength range 725 to 1020 nm (1.71 to 1.22 eV) by ion cyclotron resonance (ICR) spectroscopy [1]. 

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... 

Absolute values of the photodetachment cross section of C F " in TMSi are shown in Figure 30. From a comparison of the magnitude of the photodetachment cross section in the gas phase and in the liquid, an estimate of the effective mass of the electron in the liquid can be obtained (Baird, 1983). Values estimated by Baird... 

Figure 30 Photodetachment cross section of C F", O data plotted according to the threshold law (Equation 62), . (Redrawn from the data of Faidas, H., Christophorou, L.G., and Mc-Corkle, Chem. Phys. Lett., 193, 487,1992.)...

Photodetachment cross sections were also theoretically calculated for Rh and lr using a zero-core-contribution model The anion was described by a single electron loosely bound to a frozen core consisting of the neutral atom. Good agreement with the experimental spectra [5] was obtained [9]. A many-body calculation for Pd confirmed its existence with the 4d °5s configuration [10]. 

The Pt ion was produced in another sputter ion source, using positive ions from a discharge [6]. The source was earlier described in [11]. The photodetachment cross section (a) was measured in a crossed-beam experiment using a pulsed tunable dye laser. A threshold (at 17160 + 16 cm or 2.128+0.002 eV, see p. 254) was ascribed to the transition... 

The application of the theory shown above to computation of photodetachment cross sections of lanthanide negative ion Ce is summarized below. After that, the limitations and approximations in the calculation will be discussed. The work combined with experiment has identified bound-to-bound transitions in Ce for the first time [5]. 

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