Photoelectron spectrum ethane

The first ionization potential of ethane was measured by photoionization techniques more than 30 years ago [56-62] and it was found to be between 11.4 and 11.65 eV. Some indication of vibrational fine structure was found by Chupka and Berkowitz [62]. The classic Hel photoelectron spectrum of ethane was recorded by liirner s group (Baker et al. [25, 57] it is reproduced on Figure 3. 

Figure 3. (a) Helium 584 A photoelectron spectrum of ethane, (b) The first band, (c) the fourth band of the spectrum, with expanded electron energy scale. Baker et al. [25, 57]. Reproduced by permission from Elsevier Science-NL. 

Moreover, it is remarkable that at least four bands in the photoelectron spectrum exhibit vibrational fine structure. Thus, the ion possesses as many stable excited states. Only one of them, the first one is due to ionization from a n-orbital. (This makes the fine structure observed in both the photoelectron and electronic spectra of ethane less surprising.) Vy, U2. and V3 are Raman active, but v is both Raman and infrared inactive and its frequency had to be determined by indirect methods (ref. 96). 

The correspondence between the photoelectron spectrum and the ethane breakdown graph in Fig. 8 is obvious. The appearance of C2H4 and C2H5 corresponds to the first minimum in the photoelectron spectrum. Formation of C2H3 and C2H2 corresponds to ionization of a (tc -h n) electron. 

Fig. 8. The charge-exchange mass spectrum of ethane as a function of energy/ together with its photoelectron spectrum. ...

Below is the photoelectron spectrum for ethane. Does this spectrum better agree with the MO diagram given in Figure 1.13, or does it best agree with six equivalent C-H bonds and a single C-C bond Explain your answer. 

The new results by Buenker et al. [87] constitute a challenge to the interpretation of the ethane spectrum what became traditional in the last 30 years. It clearly indicates the need for more theoretical work with an even more extended basis set and a computation of the potential surfaces for all the relevant vibrational motions, implying a closer look at the consequences of the Jahn-Teller effect in the degenerate states as well as possible vibronic couplings between close-lying excited states. Equally needed are experimental works carried out at even higher resolution than has hitherto been possible more information on the vibrational and rotational structures are badly needed. This concerns not only the absorption spectra but also the electron-impact and photoelectron spectra. 

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