Spectroscopic Studies of Quantum Solvation in ${}^4\mathrm{H}\mathrm{e}_N\mathrm{\text{−}}{\mathrm{N}}_2\mathrm{O}$ Clusters

High resolution microwave and infrared spectra of ${\mathrm{H}\mathrm{e}}_N\mathrm{\text{−}}{\mathrm{N}}_2\mathrm{O}$ clusters were studied in the range $N=3$ to 12. The apparent cluster moments of inertia increase from $N=3$ to 6, but then decrease, showing oscillatory behavior for $N=7$ to 12. This provides direct experimental evidence for the decoupling of helium atoms from the rotation of the dopant molecule in this size regime, signaling the transition from a molecular complex to a quantum solvated system and directly exploring the microscopic evolution of “molecular superfluidity.”

Figure 1

Part of the observed infrared spectrum of ${\mathrm{H}\mathrm{e}}_N\mathrm{\text{−}}{\mathrm{N}}_2\mathrm{O}$ clusters, showing increasing clustering with higher backing pressure. Two $R(0)$ ($J=1\leftarrow 0$) lines marked “1” for $\mathrm{H}\mathrm{e}\mathrm{\text{−}}{\mathrm{N}}_2\mathrm{O}$ correspond to known $a$- and $b$-type transitions of this asymmetric rotor complex [14]. With increasing pressure and decreasing nozzle temperature, they are joined successively by the lines marked “2” to “9” in bold numerals, assigned as $R(0)$ transitions of ${\mathrm{H}\mathrm{e}}_N\mathrm{\text{−}}{\mathrm{N}}_2\mathrm{O}$ clusters with $N$ equal to the given number. Many additional pressure sensitive lines were assigned to other transitions, for example, the $R(1)$ lines ($J=2\leftarrow 1$) marked by italic numerals. Unmarked lines are due to known transitions with $N=0$ or 1, or to transitions of larger clusters ($N>7$).

Figure 2

Spectra of the $J=1–0$ rotational transitions of ${\mathrm{H}\mathrm{e}}_N\mathrm{\text{−}}{}^{14}\mathrm{N}{}^{15}\mathrm{N}\mathrm{O}$ clusters showing ${}^{14}\mathrm{N}$ nuclear quadrupolar hyperfine structure. (a) $N=5$; (b) $N=7$; (c) $N=12$.

Figure 3

(a) Observed variation with $N$ of the vibrational band origin for ${\mathrm{H}\mathrm{e}}_N\mathrm{\text{−}}{\mathrm{N}}_2\mathrm{O}$ clusters. The origin undergoes a large blueshift for $N=0$ to 5, and then turns around and begins to shift towards the red for $N=6$ to 9. (b) Experimental $B$ rotational constants of ${\mathrm{H}\mathrm{e}}_N\mathrm{\text{−}}{\mathrm{N}}_2\mathrm{O}$ clusters with $N=0$ to 12. The horizontal dashed line indicates the experimental value for ${\mathrm{N}}_2\mathrm{O}$ in a helium nanodroplet [13]. Note the oscillation of $B$ for $N>6$.