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Neon gas excited by 800-nm laser pulses (15 mJ, 125 fsec) at an intensity near 10^15 W/cm^2 generates harmonics up to 109th order. The appearance of successively higher harmonics as the laser intensity is increased is compared to recent calculations of the strong-field atomic response. Blueshifting of the laser and harmonic wavelengths indicates a small degree of ionization until the threshold for the highest harmonic (>91st) is reached.
Macklin et al. (Mon,) studied this question.