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Photoelectron-photofragment coincidence spectroscopy in a cryogenically cooled linear electrostatic ion beam trap
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Image of FIG. 1.
FIG. 1.

Overview of the cryo-PPC instrument. Labeled components are as follows: (1). Pulsed valve/discharge assembly. (2). Acceleration stack and potential switch. (3). Electrostatic chopper. (4). Pre-trap ion detector. (5). Electron detector. (6). Post-trap ion detector. (7). Neutral particle detector.

Image of FIG. 2.
FIG. 2.

Detailed diagram of the trap assembly. Labeled components are as follows: (1). Entrance mirror. (2). Exit mirror. (3). Bunching electrode. (4). Pickup electrode. (5). Laser-ion interaction point. (6). Coaxial blackbody radiation baffles. Elements in dark blue are nominally 20 K, elements in light blue are nominally 50 K, and red is nominally room temperature. The magnetic shields span the 20–50 K range, and are shown in purple. Mirror electrode voltages are ordered in the same manner described in Ref. 2.

Image of FIG. 3.
FIG. 3.

Detail of the interaction region and electron detector. The interaction point is shown with a gray circle, and example electron (e) and neutral fragment (N) trajectories are shown. Labeled components are as follows: (1). Ion correction plate (positive bias). (2). Repeller plate and central grid (negative bias). (3). Electron background reducer plate (positive bias). (4). Extraction lens (grounded). (5). Focusing lens (positive bias). (6). Microchannel plates for electron detector. All parts have cylindrical symmetry. The ion trajectory through this region is exaggerated for clarity.

Image of FIG. 4.
FIG. 4.

Timing diagram showing the propagation of an ion pulse through the instrument and the relevant pulse times (long dashed lines), where t0 is the time at which the beam potential is switched, tMG is the mass gate opening and closing, and ttrap is the trap closing. The phases important to proper synchronization, the RF-injection phase (Δtinj) and RF-laser phase (Δtlas), are denoted with short dashed lines. The rate of ion bunch evolution is greatly exaggerated.

Image of FIG. 5.
FIG. 5.

Example of unbunched and bunched photoelectron images for photodetachment of DOCO at 775 nm. For each set of conditions, the images in the first column are of all electrons, regardless of neutral signal, while the second column images are from events with one neutral particle detected in coincidence. In these images, a signal present at large radii arising from two-photon absorption by a single ion is intentionally cut by the edge of the detector, allowing enhanced resolution for low-energy electrons.

Image of FIG. 6.
FIG. 6.

Diagram of the phaselocking system. Solid connectors represent electrical signals and dotted connectors represent laser light.

Image of FIG. 7.
FIG. 7.

Decay times for various species and operational pressures. The vertical axis represents the number of photoelectrons detected in a given trapping interval in logarithmic scale. Fast dynamics due to bunching effects on the overall photodetachment effects are seen for the first few hundred milliseconds, and decay times are measured after the first 500 ms of trapping to reduce the influence of this effect. All data is taken at ∼7 keV beam energy except C2H5O which is 4 keV.

Image of FIG. 8.
FIG. 8.

Neutral position distribution width (FWHM) (×) and photodetachment rate (+) as a function of trap lens voltage. Here the beam energy and mirror voltages are 6872 eV and 10.490 kV respectively, and ions are not bunched. Total trapping time is 90 ms.

Image of FIG. 9.
FIG. 9.

Bunch velocity distribution vs. RF amplitude for NO2 photodetachment at 388 nm and 6930 eV beam energy. Bunching phases are optimized at 1.0 Vp-p, and the bunching frequency is the second harmonic of the natural ion oscillation frequency of 153.65 kHz. The total trapping time is 90 ms. For each trace, the bunching voltage and peak FWHM are listed.

Image of FIG. 10.
FIG. 10.

Bunch energy distribution vs. RF-injection phase shift for the same conditions as Figure 9. Phases given are with respect to the optimal injection phase. In this case the bunching amplitude is fixed at 1 Vp-p.

Image of FIG. 11.
FIG. 11.

An example of cooling in HOCO anions. All data is from a single data file and each spectrum is composed of events falling within the specified trapping time window. Significant sharpening of spectral features is apparent at later times.

Image of FIG. 12.
FIG. 12.

An example of multimass operation. The top panel depicts the times-of-flight of the neutral fragments following photodetachment, with the photoelectron image recorded at a photon energy of 3.20 eV for electrons coincident with any neutral particle inset. The bottom left panel is the electron kinetic energy spectrum extracted by enforcing coincidence with only neutrals of m/z = 43, (C2H3O) with the resulting photoelectron image inset. The bottom right panel shows the same results for m/z = 46 (NO2 ). All images are quadrant-symmetrized for clarity.


Generic image for table
Table I.

Selected ion oscillation frequencies for given trapping voltages and beam energies.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Photoelectron-photofragment coincidence spectroscopy in a cryogenically cooled linear electrostatic ion beam trap