Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.
J Chem Phys. 2013 Feb 7;138(5):051101. doi: 10.1063/1.4790402.
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 H(2) (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 H(2). The pump and Stokes laser pulses have fluences of ~10 J/mm(2) and ~1 J/mm(2), 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.
通过使用部分重叠的纳秒泵浦（532nm）和斯托克斯（683nm）激光脉冲的 Stark 诱导绝热 Raman 通道（SARP），73%±6%的初始 H2（v=0，J=0）基态振动能级的布居转移到单个振动激发本征态（v=1，J=0）。与其他 Stark 啁啾 Raman 绝热通道技术相比，SARP 无需在较高电子态中使用中间振动态共振，即可将布居从初始基态转移到基电子表面的振动激发目标态。平行线偏振，共传播的泵浦和斯托克斯激光脉冲的持续时间分别为 6ns 和 4.5ns，在正交交叉 H2 分子束之前组合了约 4ns 的相对延迟。泵浦和斯托克斯激光脉冲的强度分别为约 10J/mm2 和约 1J/mm2。强泵浦脉冲通过 ac（二阶）Stark 移频来产生 Raman 共振频率的必要扫描，从而使振动态能级发生偏移。当在强泵浦和较弱延迟的斯托克斯脉冲的存在下，v=0→v=1 Raman 跃迁的频率通过共振扫过，（v=0，J=0）的布居通过绝热通道相干地转移到（v=1，J=0）。通过在飞行时间质谱仪中使用 2+1 共振增强多光子电离（REMPI）来测量地面状态的布居，从（v=0，J=0）的布居转移到目标状态的定量测量。通过将 Raman 共振时的（v=0，J=0）的 REMPI 信号与 Stokes 脉冲失谐时的 REMPI 信号进行比较来测量损耗。仅使用泵浦或斯托克斯脉冲时都不会观察到损耗，这证实了所测量的损耗确实是由于 SARP 诱导的从基态到目标态的布居转移引起的，而不是由于光致电离或光解吸而导致分子损失引起的。用于 REMPI 检测的双光子共振 UV 脉冲相对于泵浦脉冲延迟 20ns，以避免源自泵浦和斯托克斯激光脉冲的 ac Stark 位移。该实验证明了在不要求中间振动态共振的情况下，在预选择的单个量子态中制备大量孤立分子的可行性。
