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Conclusions

Acknowledgements

Measured Zeeman Photodetachment Transition StrengthsA. K. Langworthy, D. M. Pendergrast, J. N. Yukich, Davidson College, Davidson, North Carolina

Abstract

We have probed the relative weight of the first Zeeman transition in photodetachment from O- and S- at the 2P3/2 3P2 detachment threshold, using laser light polarized perpendicular to a 1-T field. We find a non-zero transition strength at the first threshold, a clear discrepancy with previously published theory based on LS coupling in the ion and the atom. Our results agree, however, with other work published on detachment from Se -.

Active Layer

 

Motivation

This work has been supported by:• Research Corporation• Davidson College• ACS Petroleum Research Fund

We would like to thank R.C. Stoneman for providing some of the S- data for this work.

References

Detachment cross section in B field

• X- + photon → X + e-

• Considered as ½ of an electron-atom collision.• Minimum energy needed to detach is called the “electron affinity”, analogous to photoelectric effect.• Electron detaches as plane wave into continuum.

Detachment in Magnetic Fields

• Departing electron executes cyclotron motion in field.• Motion in plane perpendicular to B is quantized to Landau levels separated by cyclotron ω = eB/me.• For typical B = 1.0 Tesla, ω ≈ 30 GHz, period = 36 ps.• Electron revisits atomic core once every cyclotron period.• Motion along axis of field is continuous, non-quantized.• Quantized Landau levels add structure to detachment cross section. Structure results from electron wave function interfering with itself as it revisits core.

Background

• Previous results, notably by Elmquist et al 4, have shown a clear departure from the conventionally accepted theory of Blumberg, Itano, and Larson1-2 [hereafter referred to as BIL]. While BIL theory has produced good agreements with a number of experimental results, in certain cases it does not.

• As O- and S- are isoelectronic with the Se - species used for the results of Elmquist et al, we want to know how well the first Zeeman threshold agrees with BIL theory for O - and S- detachment.

• The experiments done by Elmquist et al were done at a very high magnetic field. Our experiment is partially an attempt to determine if the disagreement with BIL theory is manifested at a lower field strength.

• Spectroscopic measurements are influenced by knowledge of Zeeman transition strengths. Therefore, knowledge of how the Zeeman levels behave experimentally for O - and S- will aid in properly analyzing future experiments.

Experimental Technique

• Ions produced by dissociative attachment from a carrier gas, using hot tungsten filament.

• Ions trapped and stored in Penning ion trap (see figures below), with B = 1.0 Tesla. 3

• Relative detachment cross section probed with highly-tunable, single-mode laser . For O-, an amplified diode laser at 850nm is used. For S-, a ring dye laser tuned to 598nm with a birefringent filter and solid etalons is used.

• Least-squares fitting of the BIL theory to the data, using adjustable parameters, determines the strength of Zeeman transitions.

Apparatus

Penning ion trap system• Trap consists of three hyperbolic electrodes coaxial with B field.• Biased trap endcaps form nearly-harmonic axial potential well.• Heterodyne detection system measures relative trapped ion population before and after laser illumination.

Overall equipment layout• Single-mode tunable laser used in experiments.• Beam output from laser split to Fabry-Perot spectrum analyzer and traveling Michelson-interferometer wavemeter.• Computer controlled shutter gives precise beam control into trap, while a photodiode measures light flux to compensate for beam variation.

Example Data

Future Work

Ion trap apparatus, showing UHV vacuum, 2.0 Teslaelectromagnet and magnet power supply.

Optical apparatus, showing diode laser MOPA inforeground and wavemeter electro-optics.

Detachment scan showing ratio of S- ions surviving laser illumination near the 2P3/2 → 3P2

threshold (electron affinity). The first Zeeman threshold is responsible for the initial sharpincrease in detachment probability.

• By fitting BIL theory to the data with adjustable parameters, we find for both ion species a non-zero strength for the first Zeeman transition, consistent with that of Ref. [4]. BIL theory predicts zero transition strength for this threshold.

• Although the first Zeeman threshold is not visually resolvable in our data, our results show that the discrepancy with the BIL theory is numerically resolvable even at the lower magnetic fields used in our experiment.

• Our results strongly suggest that the discrepancy discovered by Elmquist et al 4 for Se- was not somehow an artifact of the high magnetic field used, or of the ion trap used, or unique to the Se - ion.

• The observed discrepancies suggest an underlying failure of the BIL theory with regard to relative strengths of the Zeeman transitions.

• Evaporative cooling of trapped ion population: by precise control of the cooling of the ion sample, theory dictates that we can improve the spectroscopic resolution of Landau and Zeeman levels. This work is already underway at the time of this writing.

• Replace hot tungsten filament with cold field-emission electron source to reduce further the trapped ion population temperature.

• Possible analysis of other ion species.

Magnetic Structure of S & S-

• To the left we see the magnetic structure of S and S-at a magnetic field of roughly 1 Tesla.

• The S and S- states are split by the Zeeman effect. Thefirst Zeeman transition is 2P3/2 [mJ = -3/2] 3P2 [mJ=-2]