Photoelectron spectra vibrational levels

The photoelectron spectrum of nitrogen (N2) has several peaks, a pattern indicating that electrons can be found in several energy levels in the molecule. Each main group of lines corresponds to the energy of a molecular orbital. The additional "fine structure" on some of the groups of lines is due to the excitation of molecular vibration when an electron is expelled. 

Remarkably, the photoelectron spectrum provides more than just the energy of the transition state. As can be seen in Figure 5.5, the spectrum also contains peaks corresponding to the transition state in excited vibrational levels, where the activated vibrations are orthogonal to the reaction coordinate. Therefore, NIPES can even be used to carry out vibrational spectroscopy of reaction transition states. 

Fig. 10. Photoelectron spectrum of oxygen using the helium resonance line (21-21 e.v.) obtained with a magnetic electron energy analyser (May and Turner, unpublished work). Ionization energy increasing from left to right. The spectrum reveals four levels of ionization and the vibrational structure associated with each state of the ion can be clearly distinguished. This spectrum may be compared with that obtained using an electrostatic retarding field analyser (Al-Joboury U al., 1965).

Since the development of photoelectron spectroscopy by Turner and Al-Jabory, (1962), continual efforts have been invested to improve the spectral resolution. In the beginning, the resolution was sufficient to resolve vibrational structure and, by 1970, resolution of 80 cm-1 has been obtained by Edqvist et al, (1970). The first photoelectron spectrum in which partial rotational structure was revealed was reported by Asbrink for H2 (1970). But 80 cm-1 resolution only permits observation of the rotational structure of H2 (Pollard et al., 1982) and high rotational levels of heavier molecules, such as NO, because the rotational spacing increasing as 2B+J+, where B+ is the rotational constant of the ion (Wilson et al, 1984). 

There have been numerous photodetachment studies of small cluster anions, and we now give some examples. Noble metal clusters (Cu, Ag,7, Au , n = 1-10) have been studied by Ho et al. [23], who resolved vibrations in all three dimers. Studies of alkali metal cluster anions have included those of Na ( = 2-5), K (n = 2-19), RbJ 3, and CS2-3 [24,25]. Carbon cluster anions C,T have photoelectron spectra that are consistent with linear chains for n = 2-9 and monocyclic rings for n = 10-29 [26]. Photoelectron spectra of Sb and Bi to n = 4 [27] show rich vibrational structure for the dimers, and the spectra of the larger clusters could be interpreted in terms of ab initio calculations. The threshold photodetachment (zero electron kinetic energy, ZEKE) spectrum of Si4 [28] shows a progression of well-resolved transitions between the ground state of the rhombic anion (Dzh, and vibrational levels of the first excited... 

The photoelectron spectrum shows the 7t lower than a-g in N2 (Figure 5.10). In addition to the ionization energies of the orbitals, the spectrum provides evidence of the quantized electronic and vibrational energy levels of the molecule. Because vibrational energy levels... 

From the vibrational structure in the photoelectron spectrum of the PH3 frequencies of the symmetric out-of-plane (inversion) vibration of the ion, V2=450 [16], 500 20 [17, 18], and 530 80 cm [5], were obtained. The so-called frequency halving (compared to V2 900 cm" for PH3) can be explained by a double minimum potential with a low inversion barrier which allows the left and right vibrational energy levels to interact and to split into equally spaced doublets see e.g. [18]. Ab initio calculated harmonic vibrational frequencies were reported [7]. 

Figure 18.5 ZEKE photoelectron spectrum of 3-(trifluor-omethyl)-aniline. The assignment of the torsional states is shown as a horizontal comb above the top spectrum, and the vibrational levels of the intermediate state Si are indicated in a vertical column on the left of each spectrum. Reprinted with permission from Macleod et al, J. Phys. Chem. A 105 5646. Copyright 2001 American Chemical Society...

Rotational constants and centrifugal distortion constants of the upper vibrational state 2 vg of H2B-NH2 have also been determined by microwave spectroscopy for details, see [3]. Also, the He(I) photoelectron spectrum of H2B-NH2 (produced by controlled thermal decomposition of H3N-BH3) has been measured [4]. The five ionization potentials observed up to 21.2 eV have been correlated with those of ethene. A good correspondence of the observed values was obtained with data from Koopmans theorem calculations for the ground state molecule (semiempirical MNDO and SCF ab initio calculations with 3-21G and 6-31G bases). Experimental ionization potentials (IP) and calculated orbital energies are given in Table 4/24, p. 222 [4]. A correlation of the IP data of H2B-NH2 and H2CCH2 is given for the five uppermost filled levels in Fig. 4-47, p. 222. 

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