Communication: Transfer of more than half the population to a selected rovibrational state of H2 by Stark-induced adiabatic Raman passage

Abstract

By using Stark-induced adiabatic Raman passage (SARP) with partially overlapping nanosecond pump (532 nm) and Stokes (683 nm) laser pulses, 73% ± 6% of the initial ground vibrational state population of H2 (v = 0, J = 0) is transferred to the single vibrationally excited eigenstate (v = 1, J = 0). In contrast to other Stark chirped Raman adiabatic passage techniques, SARP transfers population from the initial ground state to a vibrationally excited target state of the ground electronic surface without using an intermediate vibronic resonance within an upper electronic state. Parallel linearly polarized, co-propagating pump and Stokes laser pulses of respective durations 6 ns and 4.5 ns, are combined with a relative delay of ∼4 ns before orthogonally intersecting the molecular beam of H2. The pump and Stokes laser pulses have fluences of ∼10 J/mm2 and ∼1 J/mm2, respectively. The intense pump pulse generates the necessary sweeping of the Raman resonance frequency by ac (second-order) Stark shifting the rovibrational levels. As the frequency of the v = 0 → v = 1 Raman transition is swept through resonance in the presence of the strong pump and the weaker delayed Stokes pulses, the population of (v = 0, J = 0) is coherently transferred via an adiabatic passage to (v = 1, J = 0). A quantitative measure of the population transferred to the target state is obtained from the depletion of the ground-state population using 2 + 1 resonance enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The depletion is measured by comparing the REMPI signal of (v = 0, J = 0) at Raman resonance with that obtained when the Stokes pulse is detuned from the Stark-shifted Raman resonance. No depletion is observed with either the pump or the Stokes pulses alone, confirming that the measured depletion is indeed caused by the SARP-induced population transfer from the ground to the target state and not by the loss of molecules from photoionization or photodissociation. The two-photon resonant UV pulse used for REMPI detection is delayed by 20 ns with respect to the pump pulse to avoid the ac Stark shift originating from the pump and Stokes laser pulses. This experiment demonstrates the feasibility of preparing a large ensemble of isolated molecules in a preselected single quantum state without requiring an intermediate vibronic resonance.

Optical preparation of H2 rovibrational levels with almost complete population transfer

“…Recently, we provided the first demonstration of SARP to pump H 2 in a molecular beam, but we were only able to achieve approximately 73% transfer. 20 This paper is an extension of this study in which we present new experimental data showing that SARP is able to achieve almost complete population transfer from the ground H 2 (v = 0, J = 0) to an excited rovibrational state H 2 (v = 1, J = 0), as predicted by the theory. Figure 1 shows a schematic of the SARP experiment.…”

Perspective: Ultracold molecules and the dawn of cold controlled chemistry

“…Moreover, efforts are underway, in the group of Zare and coworkers, for selective preparation of H 2 molecules in specific vibrational, rotational, and magnetic projection quantum numbers. [161][162][163] Unlike atom-diatom systems, molecule-molecule collisions allow simultaneous changes in rotational and vibrational quantum numbers of both molecules. Since the H 2 molecule exists in both ortho and para-modifications, ortho-para conversion does not occur in non-reactive H 2 + H 2 collisions.…”

Coherent superposition of M-states in a single rovibrational level of H2 by Stark-induced adiabatic Raman passage

“…To prepare hydrogen molecules using these resonant techniques, the requirement of appropriate VUV laser sources and ionization loss poses a practical challenge. To overcome these limitations and to transfer significant population to a rovibrational (v, J, M) eigenstate of the H 2 molecule we use a coherent optical technique based on Stark-induced adiabatic Raman passage (SARP), [10][11][12] which does not require an intermedi-ate resonance. Using SARP, we demonstrated nearly complete population transfer from an initial H 2 (v = 0, J = 0) ground state to a vibrationally excited H 2 (v = 1, J = 0) state within the ground electronic surface.…”