High-resolution spectroscopy of neutral Yb atoms in a solid Ne matrix

We present an experimental and theoretical study of the absorption and emission spectra of Yb atoms in a solid Ne matrix at a resolution of 0.025 nm. Five absorption bands were identified as due to transitions from the $4f^{14}5d^{0}6s^2{}^1{S}_0$ ground-state configuration to $4f^{14}5d^{0}6s6p$ and $4f^{13}5d^{1}6s^2$ configurations. The two lowest-energy bands were assigned to outer-shell transitions to $6s6p{}^3{P}_1$ and ${}^1{P}_1$ atomic states and displayed the structure of a broad doublet and an asymmetric triplet, respectively. The remaining three higher-frequency bands were assigned to inner-shell transitions to distinct $J=1$ states arising from the $4f^{13}5d^{1}6s^2$ configuration and were highly structured with narrow linewidths. A classical simulation was performed to identify the stability and symmetry of possible trapping sites in the Ne crystal. It showed that the overarching $1+2$ structure of the high-frequency bands could be predominantly ascribed to crystal-field splitting in the axial field of a 10-atom vacancy of $C_{4v}$ symmetry. Their prominent substructures were shown to be manifestations of phonon sidebands associated with the zero-phonon lines on each crystal-field state. Unprecedented for a metal–rare-gas system, resolution of individual phonon states on an allowed electronic transition was possible under excitation spectroscopy which reflects the semiquantum nature of solid Ne. In contrast to the absorption spectra, emission spectra produced by steady-state excitation into the ${}^1{P}_1$ absorption band consisted of simple, unstructured fluorescence bands.

Figure 1

Low-lying energy levels of the Yb atom with the fine structures omitted. The arrows represent five transitions observed in solid Ne labeled from (a)–(e) following Tables 1 and 2. They correspond to $E1$ transitions from the $6s^2{}^1{S}_0$ ground state to $J=1$ states in five fine structure manifolds.

Figure 2

A sketch of the cold surface inside the liquid-helium cryostat together with the effusion oven and Yb beam line. A, B, and C are three view ports covered with fused silica windows. D is a six-way cross where the Yb beam intensity is measured.

Figure 3

The absorption spectrum of Yb atoms in solid Ne taken by the Ocean Optics spectrometer with 1.5-nm resolution. All five absorption peaks are identified as Yb atomic transitions from the ground state. See Table 1 for details.

Figure 4

The detailed line shapes of the absorption bands shown in Fig. 3, registered using the McPherson spectrometer at 0.025-nm resolution. The labels (a)–(e) are the same as those used in Table 1. All the panels use the same wave-number scale and the absorption strengths are separately normalized. The positions of the peaks discussed in the text are indicated by dashed vertical lines.

Figure 5

Ratio of the absorbed to incident light intensity (i.e., the absorptance $\alpha$) for the $6s^2{}^1{S}_0\to 4f^{13}5d^{1}6s^2{(5/2,5/2)}_1$ transition using the frequency-doubled OPO as the probe light. The spectrum has a resolution of 2 GHz. The parameters obtained by using Eq. (3) to fit the first four features of the vibronic band, indicated by the black line, are given in the inset.

Figure 6

The steady-state emission spectrum of Yb atoms in solid Ne induced by the 385-nm LED and recorded using the McPherson spectrometer at 0.025 nm resolution. The extent of each color represents the extent of the camera frame. The labels on the identified emission lines are the same as those used in Table 2.

Figure 7

Above, the accommodation energy of the Yb atom in the Ne fcc crystal as a function of $N$, the number of Ne atoms removed from the lattice (solid line). Red circles indicate the points lying on the convex hull of the $E\left(N\right)$ diagram. Below, the schematic structures of the Yb/Ne stable trapping sites: 6V (a), 8V (b), 10V (c), and 13V (d). Yb and Ne atoms are represented by large and small spheres, respectively; light gray spheres indicate the lattice positions of Ne atoms removed from the system; dark gray spheres indicate the lattice positions of the remaining Ne atoms.

Figure 8

(a) Yb-Ne interaction potentials for the ${}^3{P}_1$ and ${}^1{P}_1$ atomic states as functions of the internuclear distance $R$ with $\mathrm{\Omega }$ values indicated. The solid vertical line corresponds to the ground-state equilibrium distance, and the dashed vertical line to the Ne-Ne distance in the fcc crystal. The green horizontal bars on the right indicate the maxima of the corresponding absorption bands in Fig. 4. (b) Vertical $6s^2{}^1{S}_0\to 6s6p{}^1{P}_1$ transition frequency shifts for the lowest-energy structures with different $N$. The shifts for each of the excited-state adiabatic PESs relative to the transition frequency in vacuum are represented by black squares, red circles, and blue triangles in ascending order. The vertical dashed lines indicate thermodynamically stable sites. (c) For the $6s^2{}^1{S}_0\to 6s6p{}^3{P}_1$ transition the same is shown as for the $6s^2{}^1{S}_0\to 6s6p{}^1{P}_1$ transition in (b).

Figure 9

Simulated and observed absorption intensities of the ${}^3{P}_1$ (a) and ${}^1{P}_1$ (b) transitions. Frequency is presented in terms of the shift relative to the atomic transition in vacuum. Each band is normalized to unit area.