Efficient Coherent Population Transfer of D2 Molecules by Stark‐Induced Adiabatic Raman Passage

Vibration to Vibration: Product Energy Distribution of F + HD Crossed Molecular Beam Experiments

We investigated the F + HD(v = 1, j = 0) → HF + D reaction using the crossed molecular beam technique combined with the D atom Rydberg tagging time-of-flight spectroscopy. By detecting the products at various scattering angles for different collision energies in the range of 0.8-1.2 kcal/mol, we observed the forward-scattering products of HF(v' = 4) and determined the threshold energy for the opening of this reaction channel. Similar experiments were conducted for the F + HD(v = 0, j = 0) → HF + D reaction within the range of 1.1-1.6 kcal/mol, where forward-scattering products of HF(v' = 3) were observed, and the threshold energy for this reaction channel was determined as well. Furthermore, we measured the differential cross-sections for the F + HD → HF + D reaction in both the vibrational ground state and the excited state of HD and analyzed the vibrational quantum-state distribution of the HF products. It was found that the population of vibrational quantum states of the HF products increases synchronously with the excitation of the reactant HD vibrationally.

Enhanced Total Vibrational Excitation Yield in a Slow Narrow-Pulsed Hydrogen Molecular Beam

High-efficiency excitation of a molecular beam is critical for investigating state-selected chemistry. However, achieving vibrational excitation of the entire beam for Raman-active molecules such as H2 proves extremely challenging, primarily because laser pulses are much shorter than the molecular beam. In this study, we achieve a total excitation efficiency of over 20% by employing stimulated Raman pumping (SRP) in a slow, narrow-pulsed molecular beam. Through optimizing the intensity and spot shape of the SRP lasers, we attain saturated excitation within the laser crossing region. Furthermore, by reducing the beam velocity and narrowing the beam pulse using a cold valve and a fast chopper, we significantly enhance the total excitation yield. COMSOL simulation and a newly developed model reveal that a critical velocity allows the chopper to block unexcited molecules and reserve most of the excited ones from the beam, resulting in the highest overall excitation yield. This innovative setup opens new possibilities for state-selected experiments in surface science and ion-molecule reaction dynamics, particularly involving weak transitions and pulsed lasers.

Stark-induced adiabatic Raman passage examined through the preparation of D2 (v = 2, j = 0) and D2 (v = 2, j = 2, m = 0)

We study the conditions that must be met for successful preparation of a large ensemble in a specific target quantum state using Stark-induced adiabatic Raman passage (SARP). In particular, we show that the threshold condition depends on the relative magnitudes of the Raman polarizability (r_0v) and the difference of the optical polarizabilities (Δα_00→vj) of the initial (v = 0, j = 0) and the target (v, j) rovibrational levels. Here, v and j are the vibrational and rotational quantum numbers, respectively. To illustrate how the operation of SARP is controlled by these two parameters, we experimentally prepared D₂ (v = 2, j = 0) and D₂ (v = 2, j = 2, m = 0) in a beam of D₂ (v = 0, j = 0) molecules using a sequence of partially overlapping pump and Stokes laser pulses. By comparing theory and experiment, we were able to determine the Raman polarizability r₀₂ ≈ 0.3 × 10^-41 Cm/(V/m) and the difference polarizabilities Δα_00→20 ≈ 1.4 × 10^-41 Cm/(V/m) and Δα_00→22 ≈ 3.4 × 10^-41 Cm/(V/m) for the two Raman transitions. Our experimental data and theoretical calculations show that because the ratio r/Δα is larger for the (0,0) → (2,0) transition than the (0,0) → (2,2) transition, much less optical power is required to transfer a large population to the (v = 2, j = 0) level. Nonetheless, our experiment demonstrates that substantial population transfer to both the D₂ (v = 2, j = 0) and D₂ (v = 2, j = 2, m = 0) is achieved using appropriate laser fluences. Our derived threshold condition demonstrates that with increasing vibrational quantum number, it becomes more difficult to achieve large amounts of population transfer.