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The photoelectron spectrum of ozone for binding energies in the range 12.5-14.2 eV, recorded using the University of Liège PE spectrometer, is shown in figure 4.6.1. The ground state o f the molecular ion was observed to have a mean vibrational spacing o f ~70 meV (peaks B-G, in the notation o f [D6]), and the magnitude of this spacing varied across the band as observed previously [B9, D6, F4, K3]. The vibrational progression was in the bending mode (1)2), having a fundamental frequency o f 692 cm^ (86 meV) in the ground state of the neutral molecule^ [B12].

^ Fundamental frequencies o f vibrational modes in the ground state o f a molecular ion are often diSerent to those o f the neutral species in the ground state. This is due to the removal o f an electron from an orbital which contributes to the bonding of the molecule; see section 1.3.4.

The irregularity of the vibrational spacing has been ascribed to an anharmonicity inherent in the potential energy surface of the cation fK3],

14 -

12.5

13.0

13.5

Binding e n e rg y (eV)

14.0

Figure 4.6.1. Photoelectron spectrum of ozone.

The first band has been well established as the (6ar’)^Ai ionisation due to the vibrational progression in U] [B9, D6, F4, K3], However, the energy positions of the vibrational peaks differed slightly between each experimental study (table 4.6.1). This effect could be explored further by studying the photoelectron spectrum as a function of ionising photon energ> , e.g. using a synchrotron radiation facilit) as the light source.

Vibronic peak

Binding energy (eV)

Tbis work [B9] [D61 [F4] IK3]

B 12.572+0.002 12.56 ±0.01 12.53 ±0.01 12.53 ±0.01 12.51 ±0.01 C 12.611 ±0.002 12.61 ±0.02 12.61 ±0.01 12.59 ±0.01 12.58 ±0.01 D 12.674+0.002 12.68 ±0.02 12.67 ±0.01 — 12.65 ±0.01 E 12.752 ±0.002 12.75 ±0.01 12.75 ±0.01 12.75 ±0.01 12.73 ±0.01 F 12.830 ±0.003 12.85 ±0.02 12.83 ±0.01 — 12.81 ±0.01 G 12.926 ±0.004 — 12.92 ±0.01 — 12.91 ±0.01

The second band (peaks H and I, figure 4.6.1) was observed to overlap with the first band, as seen in the previous studies. The vibrational separation o f the two peaks observed in the present spectrum was -170 meV, concurrent with the literature values of 174, 185 and 171 meV ([B9], [D6] and [K3] respectively). This vibration involved the symmetric stretch mode (ui), the energy spacing having increased from -139 meV in the ground state of the neutral molecule [D7], The assignment of this band has long been a subject o f controversy, since calculations using Koopman's theorem (section 1.6.6) predict that both the (4b2^)^B2 and ( la 2^)^A2 ionisations lie in this energy region [D6, F4]. However, experimental observations o f the angular distribution of photoelectrons have suggested that this band represents the (4b2*’)^B2 ionisation [K3]. This assignment was fully supported by the Rydberg state analysis of the photoabsorption spectrum presented in this work (section 4.5), giving strong evidence that this is indeed the correct interpretation for this band. The recorded energy positions of the vibronic peaks associated with this band are shown in table 4.6.2.

Vibronic peak

Binding energy (eV)

This work [B9] [D61 [F4] [K3]

H 13.031 ±0.002 13.02 ±0.01 13.02 ±0.01 13.03 ±0.02 13.00 ±0.01

I 13.20 ±0.01 13.20 ±0.01 13.20 ±0.01 13.20 ±0.02 13.17 ±0.01

Table 4.6.2. Vibronic peak positions of the 8% band of ozone.

The third ionisation band of O3 was observed above 13.5 eV (figure 4.6.1). The band consists of a large peak at 13.656 eV with unresolved vibrational structure apparent as two shoulders on the high energy side. Attempts to resolve this structure have failed in all previous studies [B9, D6, F4, K3]; indeed it has been suggested that the vibrational peaks are broadened by predissociation [W8]. The vibrational progression had an apparent spacing of -125 meV, hence it was also attributed to the symmetric stretch (ui). The historical assignment of this band has also been uncertain, with both the ^B2 and states being likely candidates. Angular photoelectron distribution measurements have suggested that this band is associated with the (la 2'^)^A2 state of the molecular ion [K3]. Again, this assignment has been reinforced by the conclusions gained from the Rydberg state analysis presented in this work.

It is therefore believed that, from our PE and VUV experiments, the energy order of the first three anionic states of ozone is (in order of vertical ionisation energy): (12.75 eV), (13.03 eV) and (13.56 eV). To date, theoretical calculations using Koopman's theorem cannot satisfactorily distinguish these states [D6, F4j; it remains for more intricate calculations to predict the correct energ} ordering of the first three ionisations of ozone.

4.7 Near-Threshold Electron Energy-Loss Spectroscopy

4.7.1 Data analysis

Near-threshold EEL spectra were recorded for a mixture of O3 and O2 (roughly 50% O3) using the Heriot-Watt University trapped-electron (TE) spectrometer (section 2.7). Matching experiments demonstrated that the cross-sections for pure molecular oxygen were much smaller than those for ozone, hence all the pronounced features observed in the EEL data could be ascribed to O3.

Figure 4.7.1 shows the total negative ion yield spectrum obtained using a well depth. W=0 V. The large peak observed around 1.3 eV had been previously reported in the literature, the major ion product of the DEA resonance being 0 [Cl 1]. However, the present data also revealed negative ion resonances centred around 3.5 eV and 7.4 eV for the first time. O' ions from DEA to molecular oxygen were observed to have a maximum intensity at 6.5 eV, hence the contribution of

O2 to the spectrum observed for the O3/O2 mixture was found to be small (figure 4.7.1). Hence

the resonances observed at 3.5 eV and 7.4 eV were confidently identified as DEA processes of ozone. These resonances were explored further using the DEA spectrometer (see section 4.8).

0 .7 ITT 0 .6 W = 0 V 0 .3 ^ 0.2

4

D) 0.0 E n ergy (eV)

The near-threshold EEL spectrum of ozone recorded with a well depth of 0.2 V (i.e. residual energies, Er, of the collected electrons were <0.2 eV) is shown in figure 4.7.2. Negative ion contributions to the signal were removed by subtraction of the zero well depth spectrum (figure 4.7.1), hence the observed peaks were all due to excitations of ozone.

c 3 f c O) œ 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 W = 0 . 2 V 9 10 11 12 7 8 5 6

3

4 2 0 1

E n erg y -lo ss (eV)

Figure 4.7.2. Near-threshold EEL spectrum of O3, Er<0.2 eV, with ion signal removed.

Figure 4.7.3 shows the spectra recorded using a well depth of 1.0 V. The spectrometer was used in the modulated well depth mode for these data to remove signal contributions due to electrons with higher energy losses (i.e. only electrons with c(W-AW) < Er < eW were detected, where W=1.0 V, AW«0.1 V), negative ion signals were also eliminated by this process.

6 -q g 5 4 3 4 4

f

34

i 2 4

D) m 1 ^ 0

A

W = 1 V

V

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 0 1 2 3 4 5 6 7 8 9 10 11 12

E n erg y -lo ss (eV)

The vertical lines in figures 4.7.2 and 4.7.3 mark the energies of optically forbidden transitions. Below 3 eV these energies were determined in other experimental studies [A4], and above 3 eV the energies were calculated [T7]; broken lines indicate triplet states (^A,, ^A:, ^B] and ^B]), full lines singlet states ('A 2).

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