Neon gas excited by 800-nm laser pulses (15 mJ, 125 fsec) at an intensity near ${10}^{15}$ W/${\mathrm{cm}}^2$ 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.

• Received 21 September 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.766