Direct measurement of high-lying vibrational repumping transitions for molecular laser cooling

Abstract

Molecular laser cooling and trapping requires addressing all spontaneous decays to excited vibrational states that occur at the ≳10⁻⁴–10⁻⁵ level, which is accomplished by driving repumping transitions out of these states. However, the transitions must first be identified spectroscopically at high resolution. A typical approach is to prepare molecules in excited vibrational states via optical cycling and pumping, which requires multiple high-power lasers. Here, we demonstrate a general method to perform this spectroscopy without the need for optical cycling. We produce molecules in excited vibrational states by using optically driven chemical reactions in a cryogenic buffer gas cell, and implement frequency-modulated absorption to perform direct, sensitive, high-resolution spectroscopy. We demonstrate this technique by measuring the spectrum of the ˜A²Π_(1/2)(1,0,0)–˜X²Σ⁺(3,0,0) band in ¹⁷⁴YbOH. We identify the specific vibrational repump transitions needed for photon cycling, and combine our data with previous measurements of the ˜A²Π_(1/2)(1,0,0)–˜X²Σ⁺(0,0,0) band to determine all of the relevant spectral constants of the ˜X²Σ⁺(3,0,0) state. This technique achieves high signal to noise, can be further improved to measure increasingly high-lying vibrational states, and is applicable to other molecular species favorable for laser cooling.

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