Continuum intermediate states

Figure 1. Schematic illustration of two-pathway control in the (a) frequency and (b) time domains. In case (a) the ground state is excited to a coupled continuum by either one photon of frequency CO3 or three photons of frequency C >i. Control is achieved by introducing a phase lag between the two fields. In case (b) a two-pulse sequence has sufficient bandwidth to excite a superposition of two intermediate states. Control is achieved by introducing a delay, At, between the pulses, resulting in a phase difference of to At.

Figure 11.1 Populations of initial (Pi), intermediate (P2), and continuum (Prf) state time, in the zero-order (adiabatic) approximation (upper), and the exact solution, ol iterative corrections to the adiabatic approximation [325] (middle), and by direct nt integration (lower). Pulses, lasting lOOps, were coincident and detuned by A] = (Taken from Fig. 2, Ref. [325].)...

SLiJlio), which is excited by the combined action of two laser pulses, with central uencies co0 and co to a continuum of states associated with two or more srent product channels at energy E. The frequency co0 is assumed to be in ir resonance with the transition to an intermediate bound state [ i), and is near resonance with the transition frequency between ,) and the continuum. P oj0 can ies the system from E0) to [ )), and co carries the system from ) to Lcpntinuum. To avoid confusion, note that the energy levels are labeled somewhat Terentiy from that in the previous section, q addition, as shown in Figure 11.3, the continuum is coupled to a third bound 2) by a laser of central frequency co2. Basically, the three-level two-laser ne described in Section 11.1 is being extended here to a four-level three-4 control scheme. 

Results of the counterintuitive pulse sequence. Shown, as function of time, are ed population of the wave packet of initial continuum states, population of u — 34, intermediate state, and population of v = 0, J = 0 final ground state. Dashed lines are y profiles of two Gaussian pulses whose central frequencies are oji = 18,143.775 cm-1 12,277.042 cm J (i.e, Aj = Ae = 0). The maximum intensity of the dump pulse jiflO8 W/cm2 and that of the pump pulse is 3.1 x 109 W/cm2. Both pulses last 8.5ns. i pHse peaks at t0 = 20 ns, the peak time of Na + Na wave packet. Dump pulse peaks f re that time. Initial kinetic energy of Na atoms is 0.0695 cm-1 (or 0.1 IC). (Taken g. 4, Ref. [345],)... 

D[J N]) denotes the contribution from the discrete spectrum in the length (velocity) gauge up to the N-th intermediate state. The total contribution from the discrete and continuum spectrum are denoted by DD and DC, respectively... 

A typical overtone pumping experiment, with double-resonance excitation, is illustrated in the left-hand panel of Fig. 10 [126]. It involves three different laser pulses of 5-8 ns duration. The first laser promotes molecules to a specific vibrational-rotational intermediate state with two quanta in the OH stretch vibration. After a delay time the second pulse promotes only the preselected molecules to a higher OH stretch overtone level, that lies in the continuum and dissociates to OH and Cl. Finally, the OH fragments are detected by LIF using a third laser. 

Another unusual type of highly excited doubly charged ion has one hole in a deep inner shell and one in the valence shell. Such states are of interest theoretically because of (a) the presence of distinct electron vacancies and (b) several possible pathways for their production [86]. The first observation of direct formation of such states in Ne and N2 was recently achieved by the magnetic bottle TOF method [87]. The cross section for their formation is very low, and the electron distribution in the Ne case shows both a direct contribution and an indirect pathway. For the final states from s l2p l ionization the intermediate states are satellites of the Islsi1 3S)ns series whose lines show asymmetric Fano profiles. The presence of these profiles demonstrates that the matrix elements representing direct double ionization, formation of the intermediate states, and interaction of those states with the continuum are of similar magnitudes. 

Rule B. When the direct coupling from only the initial state to the continuum vanishes, but the coupling V 0 for the transition can then occur via the intermediate states n at the rate... 

In such an experiment, therefore, we have the opportunity to study the mechanism of MPI, the nature and lifetimes of the intermediate states, and the competition between vibronic relaxation and excitation into the continuum, by varying the absorption steps (simultaneous or sequential) and polarization of the photons, as Fig. 1 shows. Since electron trapping in all liquids proves to be exceedingly fast, the sudden appearance of a localized electron spectrum, will signify the onset of photoionization of the molecule in that liquid and the location of the conduction band. This quantitative information can then be used to refine models of excess electron states in liquids, since for most liquids Vq is an empirically determined parameter. " Furthermore, by tuning the energy of the third... 

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