Abstract

One of the more conspicuous nonlinear effects associated with atoms and molecules in the presence of intense fields is excess photon absorption in the continuum. Excess photon absorption manifests itself in two very prominent ways, above-threshold ionization (ATI) and above-threshold dissociation (ATD). Similar to its electronic counterpart, ATD corresponds to the situation in which the diatomic system absorbs more photons than the minimum necessary to dissociate. This gives rise to an ion spectrum punctuated with peaks spaced by a fixed fraction of the photon energy, which depends on the relative mass of two departing fragments. Although ATI has been observed in a variety of atomic systems and even in molecular systems, ATD has previously been reported only in Hf . In this paper we describe what we believe to be the first observation of ATD in a system other than HT, i.e., CO²*. ATD is structure sensitive and, hence, wavelength sensitive. Although apparently not seen at lower frequencies, ATD is observable at high frequency, λ = 193 nm (6.4 eV). As is discussed, ATD is supported by a strong asymmetric dissociation channel, and by asymmetric we mean

© 1995 Optical Society of America