1. For up-to-date experimental and theoretical reviews seeJ. Opt. Am. Soc. B,4, 705 (1987) andMultiphoton Processes, edited byS. J. Smith andP. L. Knight (Cambridge University Press, Cambridge, 1988). A survey of the effects of the field properties may be found inJ. H. Eberly andJ. Javanainen:Eur. J. Phys.,9, 265 (1988).
2. M. Bashkansky, P. H. Bucksbaum andD. W. Schumacher:Phys. Rev. Lett.,60 2458 (1988);S. Basile, F. Trombetta andG. Ferrante:Phys. Rev. Lett.,61, 2435 (1988).
3. See, for instance,J. H. Eberly: inLaser Spectroscopy, edited byH. Walther andW. K. Rothe (Springer-Verlag, Berlin, 1979), p. 80, andA. T. Georges, P. Lambropoulos andP. Zoller: inLaser Spectroscopy, edited byH. Walther andW. K. Rothe (Springer-Verlag, Berlin, 1979), p. 368.
4. See, for instance,P. H. Bucksbaum, R. R. Freeman, M. Bashkansky andT. J. McIlrath:J. Opt. Soc. Am. B,4, 760 (1987);M. Crance:J. Phys. B,21, 1987 (1988);M. D. Perry, O. L. Landen, A. Szoke andE. M. Campbell:Phys. Rev. A,37, 747 (1988).
5. P. Lambropoulos:Phys. Rev. Lett.,55, 2141 (1985).