title: Limits on tunneling theories of strong-field ionization.
abstract: It is shown that tunneling theories of ionization by lasers are subject to upper and lower bounds on the Keldysh parameter gamma. The tunneling limit, gamma-->0, applies to ionization by quasistatic electric fields, but not by laser fields. For lasers, the gamma-->0 limit requires a relativistic treatment. Bounds on the applicability of tunneling theories depend on parameters other than gamma, confirming the rule that strong-field phenomena require more than one dimensionless parameter for scaling.
authors.affiliation: Max Born Institute, 12489 Berlin, Germany.
authors.forename: H R
authors.initials: HR
authors.lastname: Reiss