The relative phase of first ({\omega _1}${\omega _1}$) and third harmonics ({\omega _3}${\omega _3}$) extreme ultraviolet light pulses was varied to control the population of the 2{s^2}$2{s^2}$ state in helium through the interference of {\omega _1} + {\omega _1}${\omega _1} + {\omega _1}$ and {\omega _3} - {\omega _1}${\omega _3} - {\omega _1}$ two-photon excitation paths. The population was monitored by observing the total electron yield due to the 2{s^2}$2{s^2}$ autoionization decay. Maximum yield occurs when the relative phase of the two harmonics matches the phase difference of complex atomic amplitudes governing the two excitation paths. The calculated trend of atomic phase differences agrees well with the measured data in the spectral region of the resonance, provided that time-reversed {-}{\omega _1} + {\omega _3}${-}{\omega _1} + {\omega _3}$ path is also taken into account. These results open the way to accessing phase differences of two-photon ionization paths involving energetically distant intermediate states and to perform interferometry in the extreme ultraviolet range by monitoring final state populations.

中文翻译：

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XUV光诱导的双光子跃迁的干扰

通过改变一阶（{ \omega _1}${\omega _1}$）和三次谐波（{\omega_3}${\omega _3}$）极紫外光脉冲的相对相位，通过{\omega的干涉来控制氦中2{s^2}态的总体$2{s^2}$_1} + {\omega _1}${\omega _1} + {\omega _1}$和{\omega _3} - {\omega _1}${\omega _3} - {\omega _1}$双光子激发路径。通过观察由于2{s^2}引起的总电子产率来监测种群$2{s^2}$自电离衰变。当两个谐波的相对相位与控制两个激发路径的复杂原子幅度的相位差相匹配时，就会出现最大产量。如果还考虑了时间反转的{-}{\omega _1} + {\omega _3}${-}{\omega _1} + {\omega _3}$路径，则计算出的原子相位差趋势与共振光谱区域中的测量数据非常吻合。这些结果为获取涉及能量遥远的中间状态的双光子电离路径的相位差开辟了道路，并通过监测最终状态种群在极紫外范围内进行干涉测量。