Volume 42, number 3 OPTICS COMMUNICATIONS 1 July 1982 LASER4NDUCED CONTINUUM STRUCTURE IN MULTIPHOTON IONISATION P.E. COLEMAN and P.L. KNIGHT Optics Section, Blackett Laboratory, lmperial College, London SW7 2BZ, UK and K. BURNETT Joint Institute for Laboratory Astrophysics and Department of Physics, University of Colorado, Boulder: Colorado 80309, USA Received 9 March 1982 Dressed-state resonancescan be induced in photoionisation continua by radiative coupling from energeticallyaccessible bound states. Such resonanceslead to enhanced photoionisation rates and interference minima. We present results on the effects of saturation and Rabi oscillations on ion yields and asymmetries in AC Stark-split photoelectron spectra. 1. Introduction Autoionising (AI) resonances have proved useful in the enhancement of multiphoton processes involving the continuum such as multiphoton ionisation [1 ] and VUV generation by 4-wave mixing [2]. Such resonant enhance- ment is limited to those processes involving the structured region of the continuum, although these can be tuned by applying static electric or magnetic fields to Stark and Zeeman tune the AI state [3]. The possibility of inducing structure in the continuum using a radiation field [4] rather than the Coulomb interaction responsible for AI reso- nances has the advantage of tunability of position and variability of width of the "pseudo-autoionising" states. Such induced resonances have attracted theoretical [5] and experimental attention [6]. The close-coupling responsible for AI resonances and of induced continuum structure (ICS) requires a non-per- turbative treatment using stationary states of the hamfltonian describing the discrete state embedded into the con- tinuum. Following the method developed by Fano [7] to describe AI states, the structured continuum is described by the stationary (dressed) states of the diagonalised atom-plus-field hamiltonian analogous to the bound-bound dressed states used frequently in multiphoton physics. Early work involving transitions to both AI resonances and to ICS resonances used lowest-order perturbation theory to describe the probing of the resonance [4, 5,7], although the embedding interaction can be arbitrarily strong. More recently, processes involving "probe" fields comparable in strength to the embedded-st'ate coupling, have received attention. Intense field excitation of AI resoancnes has been investigated theoretically by Armstrong et al. [8], Lambropoulos and Zoller [9], and Rz~.ewski and Eberly [10]. Intense field effects include saturation, Rabi oscillations and AC Stark splittings, and can be described in at least three ways: (i) the undressed states are used with the time-dependent Schr6dinger equation to generate equations of motion for state amplitudes and solved by standard quantum damping theory methods; (ii) the continuum resonance is described by the sta- tionary close-coupled embedded state and continuum, and equations of motion generated for the continuum reso- nance and the initial state coupled by the "probe" field, and (iii) the entire hamiltonian including dressing and probe is diagonalised to give dressed states, recognising that both "probe" and "embedding" have identical effects on the continuum describable by a single dressing. We present here an analysis of intense field effects on ICS using method (iii) and emphasising Rabi oscillations and asymmetric Stark splittings. 171