Vibronic branching ratios for nearly closed rapid photon cycling of SrOH

The vibrational branching ratios of SrOH for radiative decay to the ground electronic state, $X{}^2{\mathrm{\Sigma }}^{+}$, from the first two electronically excited states, $A{}^2\mathrm{\Pi }$ and $B{}^2{\mathrm{\Sigma }}^{+}$, are determined experimentally at the $\sim {10}^{-5}$ level. The observed small branching ratios enable the design of a full, practical laser-cooling scheme, including magnetooptical trapping and sub-Doppler laser cooling, with $>{10}^4$ photon scatters per molecule. Ab initio calculations sensitive to weak vibronic transitions are performed to facilitate the experimental measurement and analysis, and show good agreement with the experiment.

Figure 1

(a) Level diagram showing a single vibronic transition addressed by a laser [e.g., $X\left(000\right)\leftrightarrow A\left(000\right)$], resulting in decay to many vibrational states with sequentially smaller decay strengths. Stimulated absorption and emission from the laser is represented a straight double-headed arrow, while spontaneous emission is represented by wavy single-headed arrows. (b) Schematic of the apparatus viewed from above. A 6-K copper cell in a cryogenic chamber contains a strontium metal target, which is ablated by a pulsed Nd:YAG laser. The ablation products react with water to form SrOH, which is cooled by helium buffer gas and entrained in a beam out of the cell. A vertical retroreflected laser excites molecules, which subsequently decay to numerous vibrational states. The fluorescence is collimated and directed toward the spectrometer outside of the cryogenic chamber.

Figure 2

Dispersed fluorescence spectrum upon laser excitation to $A\left(000\right)$, showing relative decay strengths to vibrational states in the $X$ electronic manifold. Labels above peaks identify the ground-state vibrational level populated. Vertical bars below 0 denote the wavelengths of hypothetical emissions to all vibrational states of SrOH below 1630 ${\mathrm{cm}}^{-1}$. Inset: Additional detail near the noise floor, showing decays with VBRs as small as 0.003%.

Figure 3

Example laser-cooling scheme. On average, more than 15 000 photon scatters occur before decay to an unaddressed vibrational state occurs. Vibrational states are organized by which modes are active. From left to right: none $\left[\right(v)=(000\left)\right]$, Sr-O stretch ($v_1>0$), Sr-O-H bend ($v_2>0$), and hybrid stretch/bend ($v_1,v_2>0$). Double-headed arrows denote the occurrence of both stimulated absorption and stimulated emission; hence, only one vibrational state is coupled to $A{}^2{\mathrm{\Pi }}_{1/2}\left(000\right)$ to achieve the maximum scattering rate. Laser wavelengths, in nm, are shown for each transition.