Quasi-Coherent Molecular Vibrations with Energies above the Dissociation Threshold in the Ground Electronic State

Abstract

The purpose of this paper is to present a new type of molecular vibration that may be prepared and monitored by IR and UV laser pulses in the femtosecond time domain: quasicoherent vibrations in the ground electronic state but with energies well above the dissociation threshold and even above any potential barriers. (The word “quasi-coherent” indicates certain deviations from perfect coherent vibrations, which exist only in ideal model systems, e.g., harmonic oscillators. 1 ) More specifically, we show that molecules may absorb vibrational energy in both dissociative and nondissocative “spectator” bonds such that the dissociative bond oscillates in a quasi-periodic fashion and with rather long lifetimes (.10 ps), that is, without any abrupt cleavage, which is blocked, even though the total amount of vibrational energy is sufficient to break the dissociative bond immediately. We also suggest how to excite these quasi-coherent vibrations above the dissociation threshold, that is, by means of a series of selective femtosecond IR laser pulses (the pumping process) and how to discover them, namely, by femtosecond UV laser pulses (the probing process). The new type of quasi-coherent vibrations in the continuum of the ground electronic state should be distinguished from other well-known quasi-coherent states or resonances with energies above the dissociation threshold. First, the molecules may be prepared in electronically excited states where they may vibrate quasicoherently before they eventually dissociate, that is, by tunneling through a potential barrier (predissociation) or by electronic transitions to lower, dissociative electronic states (dissociative internal conversion, IC). These types of quasi-coherent molecular vibrations in electronically excited states have been prepared and monitored since the early days of femtosecond chemistry (see ref 2 and the books and reviews 3-5 ). In contrast, we consider quasi-coherent molecular vibrations in the ground electronic state that do not decay by tunneling through any potential barriers or by electronic transitions. Second, molecules may possess vibrational resonances with energies Er above the dissociation thresholds and with finite lifetimes Ur, both in the electronic ground and excited states. In ideal cases of isolated, that is, nonoverlapping, resonances, they give rise to corresponding peaks in the dissociative absorption spectra, with specific energies Er and widths ir such that