Experiments are reported on the detection of slow photoelectrons resulting from the photoionization of Xe atoms in a dc electric field by electron imaging. In the far-field photoelectron velocity distributions we can distinguish between direct and indirect ionization processes (involving long range Coulomb interactions with the Xe+ ion). Also, a new modulation of the velocity distribution is observed which cannot be explained by previously discussed mechanisms. Classical and quantum mechanical calculations are presented to support the interpretations.

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We use a time-dependent multichannel quantum defect theory to calculate wave packets in perturbed predissociated Rydberg states of molecular hydrogen. The calculations are ab initio in that they are based solely on known electronic energy curves U͑R͒ dependent on the internuclear distance R and calculated by quantum chemical methods. The correlated motions of the nuclear and electronic wave packets are treated in a unified way which allows for nonadiabatic exchange of energy between them. We predict that the energy flow from the initially excited perturbed Rydberg levels to the fragmentation channels characteristically reflects the interactions between Rydberg channels.[S0031-9007(98)07635-2] PACS numbers: 31.70. Hq, 33.80.Gj, 33.80.Rv, 34.60. + z With the development of picosecond and femtosecond lasers, increasing attention has been paid to the timedependent evolution of optically excited atomic and molecular quantum systems. The discussion of wave-packet dynamics proceeded largely independently in different scientific communities: Atomic physicists studied radial or angular Rydberg wave packets, typically of one-electron systems, created by bringing an atomic electron into a highly excited state with a short laser pulse [1-3]. Physical chemists discussed time-dependent chemical dynamics of molecular systems in terms of nuclear wave packets evolving on one or several electronic potential energy surfaces [4,5]. Simultaneous nuclear-electronic wave-packet motion has only rarely been considered and, with some notable exceptions [6][7][8], the time evolution of perturbed systems has received little attention. The coupling between electronic and nuclear motions involves complicated nonadiabatic processes. Here we present nonperturbative calculations of combined nuclear-electronic time-dependent motion made for a real system, namely, predissociating molecular hydrogen excited by two-photon absorption to high Rydberg states. The calculations are based on generalized multichannel quantum defect theory (MQDT) and the wave packets are formed by coherent superposition of the time-independent wave functions. Previous applications of MQDT to time-dependent processes were restricted to Coulomb fields and to the evaluation of fluxes [7,8] rather than of the full time-dependent wave function. Figure 1 shows a portion of the multiphoton predissociation spectrum of H 2 observed in a vacuum ultraviolet (VUV)/visible laser experiment with ionization detection of the resulting atomic fragments [9]. Also shown is a spectrum derived from ab initio theory using a new unified formulation of MQDT for competing ionization and dissociation processes [10]. The spectrum shown is an example of a "complex" resonance resulting from the interference of several coupled Rydberg states associated with various rovibrational ionization limits y 1 , N 1 of the ion H 1 2 with the dissociation continua corresponding to fragmentation into H͑1s͒ and H͑n 2͒.The sequential radiative and nonradiative processes considered here are H 2 ͑X 1 S 1 g , y 00 0,...