1. LASER PHOTODETACHMENT SPECTROSCOPY OF THE S2- ION
John Yukich
Wade Morgan
Andy Muhich
Supported by Davidson College and the American Chemical Society

2. Negative Ion Formation+ - - + -
Electron attracted to neutral atom over ~ 1 atomic diameter
Short-range interaction typically supports 1 or 2 bound states
Electron correlation effects – partly responsible for covalent bonds
Binding energy of extra electron is the electron affinity

3. Photodetachment - - - Neg. ion + photon → neutral atom + e-
Similar to the photoelectric effect
Occurs in earth’s ionosphere with sunlight !

4. Energy Levels of S2- → S2 S2 3∑-g S2- 2∏g V’ = 0 V” = 1 V” = 0 EA ≈
13420 cm-1 (?)
V” = 1
S2- 2∏g
570 cm-1
V” = 0

5. Optical Apparatus DPSS pump Ti:S ring laser Ion trap OCS leak
250 mW tunable ~750 nm
Wavemeter
(to 0.02 cm-1)
Spectrum
Analyzer
8 GHz FSR
Ion
trap
OCS leak

6. S2- formation from OCS OCS + e- → S- + CO
Dissociative attachment to carrier gas OCS
Stumbled upon accidentally:
OCS + e- → S- + CO
S- + OCS → S2- + CO
Confirmation of molecular species in trap:
- dissociative electron energy
- ion cyclotron resonance
Pressure dependent

7. Ion cyclotron resonance for S2-

8. Δ = 1420 cm-1

9. Future work Analysis to determine electron affinity
High-resolution spectroscopy
Rotational constants
Future ions: OD- , SH-, OH-

10. Trap electronics
Lock-in amplifier
To ADC

11. Photodetachment in B fields
Departing electron executes cyclotron motion in field
Motion in plane perpendicular to B is quantized to
cyclotron levels
Cyclotron states separated by ω = eB/me
For typical B = 1.0 Tesla, ω ≈ 30 GHz, period = 36 ps
Quantized Landau levels add structure to detachment
cross section.