1. P. B. Corkum, C. Rolland, and T. Srinivasan Rao, Phys. Rev. Lett.,57, 286 (1986).
2. C. Rolland and P. B. Corkum, J. Opt. Soc. Am. B5,641 (1988)
3. S. A. Akhmanov, V. A. Vysloukh, and A. S. Chirkin, Optics of Femtosecond Pulses [in Russian], Nauka, Moscow (1988).
4. M. Gavrila and J. Kaminski, Phys. Rev. Lett.,52, 613 (1984); M. Pont, N. R. Walet, M. Gavrila, and C. R. McCurdy, Phys. Rev. Lett.,61, 939 (1988); M. Pont and M. Gavrila, ibid.,65, 2362 (1990).
5. J. Javanainen, J. H. Eberly, and Q. Su, Phys. Rev.,A38, 3430 (1968); Phys. Rev. Lett.,64, 862 (1990); Q. Su and J. H. Eberly, J. Opt. Soc. Am.,B7, 564 (1990).