Decay of high Rydberg states: A paradigm for intramolecular dynamics in a congested bound level structure coupled to a continuum

Abstract

The dynamics of very high molecular Rydberg states, with special attention to the extreme long time behavior, is discussed using a quantum mechanical multichannel scattering theoretic formulation. Detailed computational results are provided for an electron revolving about a dipolar core in the presence of an external dc field. Two distinct effects are highlighted, trapping and dilution where the former is an origin of long time stability when there are very many bound states but much fewer states that are directly coupled to the continuum. Both trapping and dilution act to elongate the intermediate time decay of the high Rydberg states. The extent of dilution can be varied by changing the magnitude of the external dc electrical field. The formalism and specific results are discussed also towards the implications to other types of unimolecular processes. In particular it is argued that the study of molecular Rydberg states does suggest a possible route to mode selective chemistry.

Electric Field Effects on Long Living ZEKE States

“…The reason is the dilution effect of the electrical field. 29 As noted earlier, it is customary 6,9,10,[21][22][23][24]27,29,40 to expect the dilution effect of the dc field to elongate the lifetime of high Rydberg states. This is certainly observed in the computation, as can be seen in Figure 2 from the effect of the field on the shorter lifetimes.…”

The Stark effect in the predissociating Rydberg states of NO

“…In our previous study of the v þ ¼ 1 autoionising states of nitric oxide we used both approaches [14]. In the former approach, first used for alkali-metal atomic systems by Zimmerman et al [36] and subsequently applied to rare-gas atoms and small molecules, [30,31,[37][38][39][40] a basis set of wavefunctions is constructed which is considered to be a good representation of the zero-field wavefunctions, e.g. a Hund's case d basis set and then the Hamiltonian matrix, using H ¼ H 0 þ Fz (where H 0 is the zero-field Hamiltonian and F the field), is diagonalised.…”

“…Photodissociation imaging studies of NO have also been performed by REMPI detection of the atomic fragments [27,28]. Vrakking and Lee measured the predissociation lifetimes of the v ¼ 0, np and nf states n ¼ 40-122 using a very high resolution laser system and delayed pulsed field ionisation detection [29,30], stimulating detailed studies of the decay dynamics and effects of fields [31][32][33]. Pratt [34] also studied predissociation of very high n states in the presence of electric fields using excitation from the same intermediate as our work.…”

Electron impact cross sections of vibrationally and electronically excited molecules

“…Super-excited states, in fact, are electronically excited states of the neutral molecules lying above the ionization threshold. In molecules, there 256 are a few other bound states, which are iso-energic with the ionization continuum, that is, each ro-vibrational state of the ion-core can support its own series of high Rydberg states, generally called core-excited Rydberg states [24]. 258 If energetically allowed, the excitation to super-excited states can take place in two ways; one way would be by direct ionization and another decay process is the internal conversion also known as intra-molecular relaxation.…”