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Schematic of the SARP experiment using partially overlapping pump and Stokes laser pulses. The 532 nm pulses (green) are harmonically generated by a seeded single-mode Nd3+: YAG laser. After passing the delay line that is formed using high reflecting mirrors, M, the pump pulses are spatially overlapped with the Stokes pulses (red) from the dye laser at the dichroic beamsplitter, BS2. Then both laser pulses are focused to excite the molecular beam of H2 in the vacuum chamber by lens F1 (focal length ∼40 cm). H2 molecules in the ground and vibrationally excited states are selectively probed using 2 + 1 REMPI with deep UV pulses (∼200 nm) counterpropagated through the vacuum chamber focused by lens F2 (focal length ∼20 cm). The H2 + ions are accelerated and detected by a time-of-flight mass spectrometer located perpendicular to the plane of the molecular beam and the laser pulses.
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Simulation of Stark-induced adiabatic Raman passage of H2 (v = 0) → H2 (v = 1) using temporally shifted but overlapping Gaussian pump ΩP (top panel, dashed green), and Stokes ΩS (dashed red) pulses. The top panel shows the Raman coupling term, r ∝ ΩP ΩS (purple). The middle panel shows the Stark-shifted Raman detuning, δ2 = (δ0 − δac) (blue). The bottom panel shows the fractional population transfer to v = 1 (orange).
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The 2 + 1 REMPI signal (a.u.) from the Q branch transition versus the Stokes laser frequency detuning (GHz) from resonance (438 538.9 GHz) for a relative delay τD = 4.2 ns between the pump and Stokes laser pulses. The REMPI UV light is polarized parallel to the pump and Stokes laser fields. (a) The 2 + 1 REMPI signal from the H2 (v = 1, J = 0) state and (b) the 2 + 1 REMPI signal from the H2 (v = 0, J = 0) ground state.
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SARP-induced population transfer (a) experimental REMPI signal and (b) theoretical fractional population in v = 0, J = 0 state for two different relative delays τD = 0, and 4.2 ns between the 6 ns (intensity FWHM) pump and 4.5 ns Stokes laser pulses. The pulse energies are nearly the same as in Fig. 3 .
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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.
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