Absolute total, partial, and differential cross sections for photodetachment of ${\mathrm{O}}^{-}$

Absolute total, partial, and differential cross sections for the photodetachment of the oxygen anion are reported for photon energies ranging from threshold (1.46 eV) to 5.5 eV. The total cross section was measured using the animated-crossed-beam technique while partial and differential cross sections were obtained using velocity map imaging. The measured values are in good agreement with theoretical results obtained from an $R$-matrix calculation using polarized pseudostates.

Figure 1

Sketch of the experimental animated-crossed-beam setup. $D_1$: circular collimator (diameter 1 mm); $D_2$: rectangular collimator (100 $\mu \mathrm{m}$ $\times$ 1 mm); Q: quadrupolar electrostatic deflector; FC: Faraday cup; CEM: channel electron multiplier; RP: rotating glass plate; EM: energy meter.

Figure 2

Schematic view of the velocity map imaging experiment. $D_1$ and $D_2$: electrostatic deflectors; FC: Faraday cup; Reref : rereferencing tube; VMI: velocity map imaging spectrometer. Electrodes R, D, ${\mathrm{L}}_1$, ${\mathrm{L}}_{12}$, ${\mathrm{L}}_2$, ${\mathrm{L}}_{23}$, and ${\mathrm{L}}_3$ are discussed in the text, S are shielding electrodes. The detector consists of a stack of multichannel plates, a phosphor screen, and a CMOS camera.

Figure 3

Data for the photodetachment of ${\mathrm{O}}^{-}$ at $\lambda =357.14\mathrm{nm}$ (3.47 eV). (a) Circularized raw image ($1024\times 1024$ pixels). The horizontal arrow indicates the direction of the laser polarization. (b) Inverse-Abel transform image computed with meveler. The large outer circle is the edge of the detector. $r_e$ is the radius of the bright outer edge of the image, ${\vec{r}}_{\mathrm{ion}}$ is the shift of the image center with respect to the center of the detector due to the ion beam velocity. (c) Radial distribution of the electrons after Abel inversion.

Figure 4

Sketch of the fine-structure energy levels of ${\mathrm{O}}^{-}$ and of the ground and first excited levels of O. Transitions, depicted by the arrows, are numbered by increasing energy. The arrow labels are the theoretical fine-structure branching ratios [see Eqs. (13) and (14) in Sec. 5b]. The initial and final populations, obtained from experimental transition intensities and theoretical branching ratios, are indicated on the right. The sketch is not to scale.

Figure 5

Total photodetachment cross section of ${\mathrm{O}}^{-}$. Full squares: present experiment; full triangles: absolute measurement of Hlavenka et al. [8]; open inverted triangles: absolute measurement of Génévriez et al. [9]; full circles: absolute measurement of Smith [5]; open circles: relative measurement of Branscomb et al. [6]; crosses: absolute measurement of Lee and Smith [7]; full line: present $R$-matrix calculation in the length form. The vertical dashed line corresponds to the $\mathrm{O}\left({}^{1}D\right)$ threshold. The insets show results from the present experiment around the $\mathrm{O}\left({}^{3}P\right)$ and $\mathrm{O}\left({}^{1}D\right)$ thresholds, from left to right. The vertical dotted lines indicate the position of the various fine-structure thresholds, numbered as in Fig. 4.

Figure 6

Total photodetachment cross section of ${\mathrm{O}}^{-}$. Full squares: present experiment; open triangles: model potential calculation of Robinson and Geltman [12]; full thick line and dashed line: present $R$-matrix calculation in the length and velocity gauges, respectively; dotted line and dotted-dashed line: BSR calculation of Zatsarinny and Bartschat [17] in the length and velocity gauges, respectively.

Figure 7

(a) Branching ratio for the $\mathrm{O}\left({}^{1}D\right)$ final term and (b) total and partial photodetachment cross sections. (a) Full triangles: present measurement; open square: measurement of Domesle et al. [24]; full line: fit of the present data (see text). Uncertainties are $1\sigma$ values coming from least-square fitting procedure. (b) Full squares: total cross section; crosses: partial cross section to the $\mathrm{O}\left({}^{3}P\right)$ term; open squares: partial cross section to the $\mathrm{O}\left({}^{1}D\right)$ term; dashed lines: partial cross sections from the present $R$-matrix calculation in the length gauge.

Figure 8

Asymmetry parameter for photodetachment of ${\mathrm{O}}^{-}$ leaving O in the ${}^{3}P$ ground term. Full squares: present experiment; open triangles: Cavanagh et al. [31, 32]; open circles: Hanstorp et al. [29]; full diamonds: Breyer et al. [28]; cross: Hall and Siegel [27]; open square: Domesle et al. [24]; dotted line: best fit to the present results using the model of Hanstorp et al. [29]; dashed line: Cooper and Zare [26]; full line: present $R$-matrix calculation in the length gauge.

Figure 9

Asymmetry parameter for photodetachment of ${\mathrm{O}}^{-}$ leaving O in the ${}^{1}D$ term. Full squares: present experiment; open square: Domesle et al. [24]; dotted line: best fit to the present results using the model of Hanstorp et al. [29]; full line: present $R$-matrix calculation in the length gauge.