Slow and velocity-tunable beams of metastable ${\mathrm{He}}_2$ by multistage Zeeman deceleration

We report on the use of multistage Zeeman deceleration to generate beams of ${\mathrm{He}}_2$ molecules in the metastable $a{}^3{\Sigma }_u^{+}$ state with velocities tunable down to 100 m/s. The metastable molecules are generated by striking a discharge in a supersonic expansion of pure helium gas from a pulsed valve held at cryogenic temperature. The velocity and internal-state distributions of the metastable ${\mathrm{He}}_2$ molecules are measured for nozzle temperatures of 300, 77, and 10 K by high-resolution photoelectron and photoionization spectroscopy. The deceleration process does not exhibit any rotational state selectivity with rotational levels up to $N^{\prime \prime }=21$ being populated, but eliminates molecules in spin-rotational sublevels with $J^{\prime \prime }=N^{\prime \prime }$ from the beam, where $J^{\prime \prime }$ and $N^{\prime \prime }$ are the total and the rotational angular momentum quantum number, respectively. The lack of rotational state selectivity is attributed to the fact that the Paschen-Back regime of the Zeeman effect in the rotational levels of ${\mathrm{He}}_2$ is already reached at fields of only 0.1 T.

Figure 1

(a) Schematic representation of the experimental setup (not to scale): 1, pulsed-valve body and orifice; 2, discharge electrodes; 3, cryogenic-liquid container; 4, skimmer; 5, multistage Zeeman decelerator; 6, UV laser beam; 7, extraction electrodes; and 8, MCP. The supersonic beam propagation direction, the UV laser beam, and the ion extraction direction are mutually orthogonal. (b) Zeeman effect in the $N^{\prime \prime }=1$ rotational ground state of ${\mathrm{He}}_2$ $\left(a{}^3{\Sigma }_u^{+}\right)$. The low-field-seeking states, for which the deceleration pulse sequence is optimized, are indicated in red. The low-field regime of the Zeeman effect is depicted in the right panel. HV = high-voltage supply for discharge. U = high-voltage supply for ion extraction.

Figure 2

Ion-TOF spectrum obtained following UV-laser photoionization at a wave number of 34 467 ${\mathrm{cm}}^{-1}$ and extracting the ${\mathrm{He}}^{+}$ and $\mathrm{He}{{}_2}^{+}$ ions with a pulsed extraction field applied 1 $\mu$s after the laser pulse. The electronic noise between 1.0–1.25 $\mu$s is an artifact originating from the electric-field switch-on process.

Figure 3

Plot of the He${{}_2}^{*}$ neutral-TOF spectra obtained for temperatures of the nozzle assembly of 300, 77, and 10 K. The red curves correspond to fits to the experimental data that led to the parameters summarized in Table 1. See the text for details.

Figure 4

Plot of the PFI-ZEKE photoelectron spectra of the origin band of the $\text{He}{{}_2}^{+}{X}^{+}{}^2{\Sigma }_u^{+}\leftarrow \text{He}{{}_2}^{*}a{}^3{\Sigma }_u^{+}$ photoionizing transition obtained for nozzle-assembly temperatures of (a) 10 K (blue trace) and (b) 300 K (red trace). The numbers along the upper and lower assignment bars represent the values of the rotational quantum number $N^{\prime \prime }$ of $\mathrm{He}{{}_2}^{*}$ of the dominant $Q$-type-branch (i.e., $\Delta N=N^{+}-N^{\prime \prime }=0$) lines of the $v^{+}=0\leftarrow v^{\prime \prime }=0$ and $v^{+}=1\leftarrow v^{\prime \prime }=1$ bands, respectively. The weak lines designated as $S$(1) and $S$(3) are $\Delta N=N^{+}-N^{\prime \prime }=2$ transitions originating from the rotational levels $N^{\prime \prime }=1$ and 3, respectively. The two strong, sharp lines marked by stars correspond to autoionization resonances, which strongly enhance the intensities of the $O$(3) and $O$(5) lines. The intensities in both spectra were normalized to the intensity of the $Q$(1) transition. See the text for details.

Figure 5

Time-of-flight profiles of $\mathrm{He}{{}_2}^{*}$ detected 60 mm beyond the last stage of the decelerator. The photoionization laser was set to 34 467 cm${}^{-1}$, i.e., above all relevant ionization thresholds, to avoid the artificial enhancement of the photoionization signal of molecules in specific rotational levels of the $a{}^3{\Sigma }_u^{+}$ state. The black trace with a broad TOF distribution centered at 2.05 ms corresponds to the undecelerated beam. The three distributions presented at times of flight between 2.9 and 3.3 ms were obtained using deceleration pulse sequences designed to decelerate molecules with initial velocities of 505, 500, and 495 m/s, respectively, at a phase angle of 35${}^{\circ }$.

Figure 6

Photoionization spectra of ${\mathrm{He}}_2$ in the vicinity of the ionization threshold from the $a{}^3{\Sigma }_u^{+}$ state recorded for (a) an undecelerated beam and (b) a beam decelerated to a final velocity of 135 m/s. The assignment bars indicate the positions of the $N^{\prime \prime }\to np{(N^{+}=N^{\prime \prime })}_{(N=N^{+})}$ Rydberg states for $N^{+}=1–9$. The lines marked by stars belong to $N=N^{+}\pm 1$ Rydberg series that are perturbed by rotational channel interactions. The intensity of trace (b) was multiplied by a factor of 3 and vertically offset for clarity.

Figure 7

Comparison of the Zeeman effect in (a) and (c) the $X{}^3{\Sigma }_g^{-}$ ground state of ${\mathrm{O}}_2$ and (b) and (d) the $a{}^3{\Sigma }_u^{+}$ state of ${\mathrm{He}}_2$ (right column). (a) and (b) Zeeman energy of the rotational states $N^{\prime \prime }=1–5$ in the magnetic-field range between 0 and 20 T. (c) and (d) Zeeman energy in the magnetic-field range where the Zeeman shifts are comparable to the spin-rotation splittings. Note the different scales used in these panels.