Radiative cooling dynamics of isolated ${\mathrm{N}}_2{\mathrm{O}}^{+}$ ions in a cryogenic electrostatic ion storage ring

We study radiative cooling processes of ${\mathrm{N}}_2{\mathrm{O}}^{+}$ ions isolated in vacuum by taking advantage of the extended storage times possible with a cryogenic electrostatic ion storage ring. The temporal evolution of the internal state populations of the ${\mathrm{N}}_2{\mathrm{O}}^{+}$ ions after being stored in the ring was state-selectively measured by action spectroscopy via a predissociation process using a wavelength-tunable pulsed laser. The rotational level population remained unchanged up to about 200 s after ion storage, which is consistent with theoretical predictions and the small permanent dipole moment. On the other hand, we found that depletion of the vibrational excited states in the NN-O stretching mode proceeded within 5 s, while a noticeable increase of the ground-state population was not observed. This implies that the initial populations of the vibrational excited states are relatively small. We discuss such behavior in the context of the initial population distribution of ions extracted from the ion source with a simulation based on simple cascade modeling.

Figure 1

The potential-energy curves of ${\mathrm{N}}_2{\mathrm{O}}^{+}$ as a function of the N-NO distance $R_{\mathrm{N}-\mathrm{NO}}$ in the N-N-O collinear dimension. Note that the vibrational transitions which we observe correspond to the NN-O stretching mode. The blue curves indicate the electronic states involved in the predissociation path shown by the arrows. Reproduced and modified from relevant literature [45].

Figure 2

Schematic overview of the present experimental setup.

Figure 3

$A^2{\mathrm{\Sigma }}^{+}-X^2\mathrm{\Pi }$ photodissociation spectrum of ${\mathrm{N}}_2{\mathrm{O}}^{+}$ measured at a storage time of 90 ms. The wavelength region corresponds to the $\mathrm{\Delta }{v}_1=2$ sequence, with the main features attributed to the rotational bandheads of the ($v_1^{\prime }0$ 0)-($v_1^{\prime \prime }0$ 0) transitions. The filled yellow area represents the measured region of Fig. 4. By the circle, diamond, and cross symbols, we annotate the bandhead positions considered in Figs. 5 and 6.

Figure 4

Storage time dependence of the photodissociation spectrum in the main bandhead of $A^2{\mathrm{\Sigma }}^{+}$(2 0 0)-$X^2{\mathrm{\Pi }}_{3/2}$(0 0 0). (a) The simulated spectrum assuming a rotational temperature of 320 K. The experimental spectra (b)–(d) were measured at storage times of 9 ms, 9.5–9.9 s, and 195–200 s, respectively. The assignment of the rotational structure is indicated in panel (a).

Figure 5

(a) Neutral yields produced via laser-induced predissociation at each transition as a function of ion storage time before laser irradiation. The error bars represent the statistical error. (b) Storage time dependence of the neutral yields produced by ion-residual gas collisions, corresponding to the relative stored ion beam intensity.

Figure 6

Observed and simulated population evolutions for each vibrational state as a function of ion storage time. The dotted and solid lines represent simulation results under initial distributions in thermal equilibrium and nonequilibrium, respectively. The errors in the experimental values were determined by the uncertainty in the beam intensity, the statistical errors of the neutral yield and the subtracted background, and the propagation of the normalization error. The colors used correspond to the observed levels as shown in Fig. 5.

Figure 7

The initial populations of ground, $v_1$-, $v_2$- and $v_3$-excited, and multimode- excited states of ${\mathrm{N}}_2{\mathrm{O}}^{+}$ for the thermal-equilibrium and nonequilibrium models. Note that a sum of the populations in the excited states ($v_{\mathrm{i}}=1,2,3,...$) for each mode is shown.

Figure 8

Results of the rotational RC simulation with an initial rotational temperature of 400 K. The spectra (filled with pink) were constructed from the simulated rotational population at storage times of (a) 200 s, and (b) ${10}^5$ s. The dashed curves in panels (a) and (b) represent the spectra using Boltzmann distributions at 400 and 120 K, respectively.

Figure 9

Schematic energy levels of three vibrational modes of ${\mathrm{N}}_2{\mathrm{O}}^{+}$.