Photodissociation of an Internally Cold Beam of ${\mathrm{CH}}^{+}$ Ions in a Cryogenic Storage Ring

We have studied the photodissociation of ${\mathrm{CH}}^{+}$ in the Cryogenic Storage Ring at ambient temperatures below 10 K. Owing to the extremely high vacuum of the cryogenic environment, we were able to store ${\mathrm{CH}}^{+}$ beams with a kinetic energy of $\sim 60\mathrm{keV}$ for several minutes. Using a pulsed laser, we observed Feshbach-type near-threshold photodissociation resonances for the rotational levels $J=0–2$ of ${\mathrm{CH}}^{+}$, exclusively. In comparison to updated, state-of-the-art calculations, we find excellent agreement in the relative intensities of the resonances for a given $J$, and we can extract time-dependent level populations. Thus, we can monitor the spontaneous relaxation of ${\mathrm{CH}}^{+}$ to its lowest rotational states and demonstrate the preparation of an internally cold beam of molecular ions.

Figure 1

Potential energy curves of the lowest electronic states of ${\mathrm{CH}}^{+}$. The plot shows the Born-Oppenheimer potentials taken from Ref. [6] and the spin-orbit splitting at asymptotic distances (not to scale). The UV laser excitation and the subsequent predissociation process are indicated by dashed arrows.

Figure 2

Near-threshold photodissociation spectra of ${\mathrm{CH}}^{+}$. The respective storage time intervals are given in the upper left-hand corner of the plots. The thick gray curve shows a fit of the calculated cross sections to the experimental data. The fit parameters represent the individual level populations in the lowest rotational states, which are given in the upper right-hand corner of each plot. The thin red, blue, and green lines show the cross sections for the individual states ($J=0$, 1, 2) multiplied with their respective populations. All states with $J>2$ are summed up into a single cross section (thin black line), as their continuum contributions in this wavelength range are identical and unspecific.

Figure 3

Time-dependent ${\mathrm{CH}}^{+}$ level populations. The markers with error bars result from the fits to the photodissociation data shown in Fig. 2. The experimental data are averaged over time intervals that vary in length from 20 to 60 s (shown by black bars at the top of the graph). The solid and dotted lines depict the outcome of a rate equation model that assumes a uniform blackbody radiation field at temperatures between 15 and 25 K (given at the individual curves). For fair comparison, the results have been averaged over the same time bins as the experimental data, taking into account the exponential decay of the ion beam. To show the influence of the time binning, the dashed curves represent the same data as the 20 K curves (thick solid lines), but without averaging. The initial rotational temperature of the injected ions is assumed to be $>1000\mathrm{K}$.