Many O−photoelectron spectra have been measured during the course of this work, span- ning a large range of detachment wavelengths and experimental parameters. The electron affinity of oxygen is well defined, with a definitive value of EA=1.4611135(12) eV deter- mined from photodetachment microscopy studies[10]. Wavelengths shorter than 850 nm will photodetach O−(2P) to the neutral ground state O(3P),
O−(2P3/2,1/2) +hν→O(3P2,1,0) +e−, (7.14) giving 6 fine-structure transitions. The excited 1D
2 state of neutral oxygen lies 1.969 eV above the 3P
2 state, with wavelengths shorter then 361.6 nm required for detachment to occur.
O−(2P3/2,1/2) +hν→O(1D2) +e−, (7.15) For atomic species, the energy of a fine-structure level is given by,
EJ=
ASO
2 [J(J + 1)−L(L + 1)−S(S + 1)]. (7.16)
Therefore, the effective splitting of two levels J’ and J” is given by, Aeffec = EJ0 −EJ00= ASO
2
J0(J0+ 1)−J00(J00+ 1)
. (7.17)
To obtain a high resolution spectrum of O(3P)←O−(2P) detachment, O− ions were produced in a pulsed discharge of pure O2 gas. The ions were then accelerated, mass sepa- rated, and photodetached using both the Sunlite OPO system and direct Nd:YAG output. While the energy resolution of velocity mapped-imaging is proportional to the kinetic en- ergy of the mapped particle - with the highest resolution achieved close to threshold - a degeneracy between the idler and signal beams of the OPO near 750 nm (see Fig. 3.12) limits the power achievable near this wavelength. Consequently, the best O− measure- ments of this work were taken at 579 nm with the OPO, and 532 nm with the second harmonic of the Nd:YAG. These photoelectron spectra are presented in Fig. 7.5, along with Gaussian fits of the fine-structure transitions.
the image so it would occupy the entire detector. The 579 nm spectrum contains 6,837,244 electrons, more than the 2,779,172 electrons in the 532 nm image. The first measurement has a colder fitted source temperature of T=206(12) K cf. T=276(11) K, with both spectra recording similar energy resolutions of Γ = 34.9−38.5 cm−1. As a function of kinetic energy this correlates to a spectrometer resolution of ∆E/E∼ 0.6% and ∼0.5% respectively, close to the benchmark resolution of 0.4% previously set by the ANU HR- PEI spectrometer[24]. By calibrating the spectrum to the well defined O−electron affinity, the spin-orbit splitting of the 2P
3/2,1/2 anion is determined to be Aeffec = 181(7) cm−1, which is in agreement with the accepted value in the literature of 177.084(14) cm−1 [10]. This corresponds to an anion spin-orbit coupling of ASO = −121(5) cm−1. The fine
structure of the3P
2,1,0 neutral is set to the known splittings ofA2←1 = 158.265 cm−1 and A2←0= 226.977 cm−1. 11000 11250 11500 11750 12000 12250 12500 12750 13000 Binding Energy (cm 1) 0.0 0.2 0.4 0.6 0.8 1.0
Intensity (arb. units) 3PJ =2 1 0
T=206(12) K =38.5(1) cm1 2 1 0 3PJ = Phototelectron Spectrum of O at 532 nm 2P1/2 2P3/2 0 256
(a) O−spectrum, measured at 532 nm with the direct Nd:YAG output.
11000 11250 11500 11750 12000 12250 12500 12750 13000 Binding Energy (cm 1) 0.0 0.2 0.4 0.6 0.8 1.0
Intensity (arb. units) 3PJ =2 1 0
T=276(11) K =34.9(4) cm1 2 1 0 3PJ = Phototelectron Spectrum of O at 579 nm 2P1/2 2P3/2 0 256
(b) O− spectrum, measured at 579 nm with the OPO.
Figure 7.5: Photoelectron spectra of O−, showing the O(3P2
,1,0)←O−(2P1/2,3/2) fine struc- ture transitions. Raw velocity-mapped images are inserted in the top right corner. An anion fine structure splitting of Aeffec = 181(7) cm−1 is determined.
Detachment yielding the O(1D) excited state may also be investigated by shifting to shorter wavelengths. O(1D
2)←O−(2P3/2,1/2) photodetachment was measured using the 3rd (355 nm) and 4th (266 nm) harmonics of the Nd:YAG laser, with the photoelectron spectrum at 355 nm presented in Fig. 7.6. As detachment to the singlet D state is close to threshold, a high resolution of Γ1D = 13.5(2) cm−1 is achieved. But even though the O(3P) transition produces electrons with kinetic energy over 2 eV a respectable resolution of Γ3P = 75.7(6) cm−1 is achieved, with fine structure transitions still resolved. This illustrates one of the key advantages of the HR-PEI spectrometer, with high resolution measurements still achievable for high electron energies.
From the calibrated photoelectron spectrum in Fig. 7.6, an experimental value for the electron affinity of EA = 11,788(4) cm−1 is obtained, which is in agreement with the precise value determined by photoelectron microscopy[10]. From the O(1D
2)←O−(2P1/2) and O(1D
2)←O−(2P3/2) transitions of the upper state, an anion spin orbit splitting of
Aeffec = 182(6) cm−1 is determined. This value agrees with both the anion splitting
measured in the ground O(3P) state, and the literature microscopy value, which confirms an accurate energy calibration may be achieved over the entire range of the detector.
§7.2 A study of Oxygen 111 12000 14000 16000 18000 20000 22000 24000 26000 28000 Binding Energy (cm 1) 0.0 0.2 0.4 0.6 0.8 1.0
Intensity (arb. units)
Te= 15, 868 cm1
O(3P) O (2P)
EA = 11,788(4) cm1 O(1D) O (2P)
Phototelectron Spectrum of O at 355 nm
0 256
(a) O− spectrum at 355 nm, showing detach-
ment to both 3P and 1D neutral electronic
states. 11000 11250 11500 11750 12000 12250 12500 12750 13000 0.0 0.2 0.4 0.6 0.8 1.0 T=192(7) K =75.7(6) cm1 3P
Fine structure transitions of O at 355 nm
26000 26250 26500 26750 27000 27250 27500 27750 28000 Binding Energy (cm 1) 0.0 0.2 0.4 0.6 0.8 1.0 =13.5(2) cm1 ASO ASO=182(6) cm1 1D
(b) Fine structure transitions from O−(2P
1/2,3/2) to the 3P and 1D neutral
states.
Figure 7.6: Photoelectron spectrum of O− at 355 nm showing detachment to both the 3P and 1D neutral states. From the ratio of the 2P
1/2 to 2P3/2 fine structure transition intensities, a source temperature of T=192(7) K is determined, along with an anion spin orbit splitting ofAeffec= 182(6) cm−1.
Angular distributions
From the inset of the raw velocity-mapped image of O− photodetachment in Fig. 7.6, it is clear that at 355 nm O(3P
2,1,0)←O−(2P1/2,3/2) photodetachment displays a strong negative anisotropy parameter, whereas O(1D
2)←O−(2P1/2) detachment appears more isotropic. This is not unexpected, given the1D transition is close to threshold and detach- ment is from a p orbital. However by measuring both detachment channels at different wavelengths, the energy dependence of their anisotropy parameters may be investigated, providing determination of the Hanstorp coefficients and partial wave phase shifts.
The anisotropy parameter for ap-detachment orbital electron follows the Cooper-Zare expression with`= 1, βp= 2A212−4A 1cos ∆`±1 1 + 2A2 12 , (7.18)
where the Hanstorp coefficient A1 is defined as the ratio of radial matrix elements A212∼
χ2 1,2
χ2
1,0. Eq. (7.18) is a result of the electric-dipole selection rule `
0 = `±1, describing the interference between the s and ddetachment partial waves. At low kinetic energies the lower angular momentums wave will dominate the process, resulting in a more isotropic distribution close to threshold, as is observed for detachment yielding 1D
2 in Fig. 7.6. O−anisotropy parameters were measured at a variety of detachment wavelengths, from 266 nm up to 800 nm. These experimental data points were fitted to the Cooper-Zare equation for p−orbital detachment (Eq. 7.18), as shown in Fig. 7.7. Detachment to the ground state, O(3P
2)←O−(2P3/2), is shown in blue, while detachment to the excited state O(1D
2)←O−(2P3/2) is shown in green. It can be seen that photodetachment to both states follow a similar shaped anisotropy curve, however detachment to the 1D
2 state follows a gentler slope, with the minimum in the anisotropy occurring at a larger kinetic energy. This is reflected in the value for the Hanstorp coefficients extracted from the fits. For 3P
2, A1 = 0.49(4) eV−1 with cos ∆`±1 = 0.92(1) while for 1D2, A1 = 0.31(8) eV−1 with cos ∆`±1 = 0.95(3). Qualitatively, this infers that the dpartial wave contributes more at
low energies for 3P
2 detachment as opposed to 1D2 detachment. While the anisotropy of O(3P
2,1,0)←O−(2P1/2,3/2) photodetachment has been well studied by Cavanagh et al.[24], the new 266nm measurement of this work provides useful insight into the behaviour of O(1D
2)←O−(2P3/2) detachment away from threshold.
0.0 0.5 1.0 1.5 2.0 2.5
Kinetic energy (eV) 1.0 0.8 0.6 0.4 0.2 0.0 An iso tro py p ar am et er
O anisotropy measurements
O(3P) O (2P) O(1D) O (2P) O(1D) Domesle et al.Figure 7.7: Anisotropy parameters for O(3P
2)←O−(2P3/2) and O(1D2)←O−(2P3/2) pho- todetachment at various wavelengths. The experimental data points are fitted to the Cooper-Zare equation for p−orbital detachment (Eq. 7.18). The 266 nm measurement of Domesle et al.[153] is also included (4).