Modulation of resonant multiphoton ionization of CH3I by laser phase variation

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Abstract
The phase control of molecular absorption, first proposed by Shapiro, Hepburn, and Brumer was accomplished with CH3I using the technique of Chen, Yin, and Elliott. Red light (λ near 603 nm) was focused in a cell containing CH3I gas at a pressure around 1 Torr. The emerging light, a coherent mixture of the fundamental and the third harmonic (λ near 201 nm), was refocused on a molecular beam of CH3I and multiphoton ionization was detected. Ionization was by two simultaneous processes: a uv photon absorption followed by the absorption of two red photons (1+2 process) and an absorption of three red photons followed by the absorption of two more (3+2 process). Because of the sharp resonant Rydberg transition at 201 nm, the one and three photon matrix elements dominate the transition. Interference between the amplitudes of these two paths was demonstrated by varying the pressure and, hence, the index of refraction of the Ar gas in a tuning cell where the two light beams were refocused by two spherical mirrors. The modulation depth varied with wavelength but had a maximum of 75%, the largest modulation so far observed in this kind of experiment.