Laser light statistics and bandwidth effects in resonant multiphoton ionisation of caesium atoms at 1.059 μm

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Abstract
Laser temporal-coherence effects are investigated in four-photon ionisation of caesium atoms using a tunable wavelength Nd-glass laser pulse and tuning the frequency onto the resonant three-photon transition 6S to 6F. The laser can be operated in either single or multimode. In the latter case, the laser bandwidth can be varied between 3*10-2 and 1.5 cm-1. The resonance peaks obtained with incoherent laser pulses are enhanced, shifted and broadened with respect to those induced by coherent pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses is statistically enhanced by a factor of 1.8+or-0.2 compared with the shift induced by coherent pulses, in good agreement with calculations. The resonant multiphonon ionisation rate induced by multimode pulses depends both on intensity and bandwidth effects. It is enhanced by a factor of 2.5 for pulses with bandwidths less than 0.2 cm-1, while it becomes smaller than that induced with a coherent pulse of the same intensity when the bandwidth becomes larger than 1 cm-1. Finally, the resonance width depends both on intensity an bandwidth effects. At low intensity (2*107 W cm-2) it is governed only by the laser bandwidth, while at high intensity (108 W cm-2) it varies linearly with the intensity and is statistically enhanced.