Stark deceleration and trapping of hydrogen Rydberg atoms

Hydrogen atoms in supersonic expansions with velocities in the range from $700\text{to}800\mathrm{m}∕\mathrm{s}$ have been excited to Rydberg-Stark states with principal quantum number $n$ between 20 and 40, decelerated to zero velocity in the lab frame using time-dependent inhomogeneous electric fields, and trapped in a two-dimensional electrostatic trap with an initial density of $\approx 5\times {10}^6{\mathrm{cm}}^{-3}$. The motion of the atomic cloud in the trap was observed by measuring the times of flight and images of the ${\mathrm{H}}^{+}$ ions produced by pulsed field ionization. The velocity distribution of the trapped atoms can be described by an effective temperature of $350\mathrm{mK}$. The decay of the population of trapped atoms does not follow a single-exponential behavior and has contributions from radiative and collisional processes.

Figure 1

(a) Voltage pulse sequence applied to the four electrodes. Photoexcitation takes place at $t=0$ and pulsed field ionization takes place at $t_{\mathrm{PFI}}$. (b) Schematic view of the electrode setup during deceleration. The particles, which move along the $z$ axis, are excited at the position indicated by the black dot and are decelerated because of the large electric field inhomogeneity. (c) Schematic view of the electrode setup during trapping. The solid lines in (b) and (c) are lines of constant electric-field strength in steps of $100\mathrm{V}∕\mathrm{cm}$ (b) and $10\mathrm{V}∕\mathrm{cm}$ (c). (d) Electric-field strength along the $z$ axis (solid line) and along the $y$ axis (dashed line) about the center of the trap.

Figure 2

(Color online) (a) Integrated ${\mathrm{H}}^{+}$ ion intensity measured as a function of the delay time between photoexcitation and pulsed field ionization. The initial velocity of the H atoms amounted to $800\mathrm{m}∕\mathrm{s}$. The nozzle stagnation pressure in the measurements was $1.5\text{bar}$ (trace A), $2.0\text{bar}$ (trace B), and $2.5\text{bar}$ (trace C). The traces have been normalized so that the signal strengths are the same for all traces at $250\mu \mathrm{s}$. (b) and (c) CCD camera images of an undecelerated beam of Rydberg hydrogen atoms hitting the MCP. The nozzle stagnation pressure in these two measurements was $1.5\text{bar}$ (b) and $2.5\text{bar}$ (c).

Figure 3

(Color online) (a) Instrumental function describing the signal losses originating purely from geometrical constraints imposed by the size of the MCP detector and the ion optics for nozzle stagnation pressures of $1.5\text{bar}$ (trace A), $2.0\text{bar}$ (trace B), and $2.5\text{bars}$ (trace C). The steps in the instrumental function are a consequence of the pixels of the CCD camera. (b) Best estimates of the trap losses obtained by dividing the observed signal decay curves with the instrumental function. The solid lines represent single-exponential fits of the data points beyond $75\mu \mathrm{s}$.

Figure 4

(a) Ion time of flight for an ionization delay of $13.1\mu \mathrm{s}$. (b), and (c) Average ion time of flight as a function of the ionization delay taken from the experimental and simulated traces, respectively. The squares, circles, and diamonds correspond to measurements carried out with $V_{\mathrm{decel}}=1375$, 1155, and $935\mathrm{V}$, respectively. The relative uncertainty in the average times of flight amounts to $\sim 5\mathrm{ns}$.

Figure 5

Spectra of the deceleration and trapping efficiency during the first $50\mu \mathrm{s}$ for $n=24–34$. The potentials that were used are $V_{\text{trap}}^{12}=45\mathrm{V}$, $V_{\text{trap}}^{34}=25\mathrm{V}$, and $V_{\mathrm{decel}}=1155\mathrm{V}$. The intensity of the strongest line $(n=28)$ has been normalized to 1.