Photodetachment resonance structures

Quite often one will find there are multiple resonant states lying in the continua under consideration. Some of them may interfere with each other through their mutual interaction with the same continua. The interference may dramatically alter the profile of the resonance structure from what they d look like if no interference occurred. These resonant states are said to overlap. On the other hand, there is usually more than one available channel within the range of photon energy under study. The formalism for photodetachment calculations should then be able to treat the case of multiple discrete states embedded in multiple continua. The configuration interaction in the continuum (CIC) theory is our theory of choice. 

Fig. 8.24. Interaction between fine structure in negative ion photodetachment induced by a strong magnetic field and a broad Feshbach resonance (after C.H. Greene [412]).

The small dip at 4.456 eV is a Wigner cusp that arises from the opening of the Li(32p)+e (ks) photodetachment channel (hereafter labeled 32Pks, for brevity). The other two structures are resonances associated with the decay of doubly excited states. These resonances overlap one another as well as the Wigner cusp, making it... 

Experimental evidence for the vibrational structure of XHX transition states has been provided by photoelectron spectroscopy of XHX- anions with X = Cl, Br, and I (134,160-163). This technique, by inducing photodetachment of an electron from the XHX" anions, probes the Franck-Condon region, which is believed for these systems to include geometries in the vicinity of the transition state region for the neutral systems. Spectral bands have been interpreted as evidence for trapped-state resonances associated with asymmetric stretch-excited levels of the transition state (160-163), and they are in general agreement with synthetic photoelectron spectra calculated from the scattering computations of Schatz (17-19). In recent experimental spectra (158,162), more closely spaced oscillations have been observed these are apparently related to rotational thresholds as described by Schatz. 

We have already mentioned the interpretation of photodetachment spectra of FH2 and XHX (X = Cl, Br, I), in terms of quantized transition state resonances. Similar experiments have been carried out for IDF, OHF , OHC1, OHOH , and HOHOH (174-177), and these experiments have been interpreted in terms of resonances and other types of vibration-rotation energy level structure associated with the transition state species of the neutral product (10,11,17-19,162,163,174-178). The FH2 and FD " photodetachment experiments provide a particularly striking example of the observation of quantized transition states in experimental spectra (133-135). In theoretical work carried out to analyze recent experimental work on photodetachment, in particular for OHC1 (176), the calculated cumulative reaction probability for the O + HC1 reaction showed steps at quantized hindered rotor energies (as well as sharper resonances due to trapped states), but the steps had transmission coefficients considerably smaller than unity. 

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]. 

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