VOLUME 70, NUMBER 8 PHYSICAL REVIEW LETTERS 22 FEBRUARY 1993 Light-Induced Vibrational Structure in H2+ and D2+ in Intense Laser Fields A. Zavriyev ' and P. H. Bucksbaum Physics Department, Uni c ersi ty of Michigan, Ann Arbor, Michigan 48I09 3. Squier and F. Saline Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48I09 (Received 14 May 1992) Molecular bond-softening and above-threshold dissociation of H2+ and 02+ in intense laser fields is reexamined using 160 fs pulses of 769 nm light. At intensities greater than —10'3 W/cm new features appear in the dissociation spectrum. These are consistent with the formation of trapped population in light-induced vibrational states, formed in the adiabatic potential near the three-photon resonance be- tween the 1sag and 2po„states. Rovibrational structure of such light-induced states appears in the ki- netic energy spectra of the ion fragments detected after multiphoton ionization. PACS numbers: 33.80. Wz The behavior of molecules in strong laser fields is an area of active investigation. Multiphoton ionization (MPI) and above-threshold ionization (ATI) occur in molecules just as in atoms [1-6]. Additional degrees of freedom in molecules lead to some new phenomena as well. An example is above-threshold dissociation (ATD), in which the internuclear electronic potentials are mixed by the laser field at points of multiphoton resonance, causing the molecule to dissociate via several possible channels corresponding to the absorption of one, two, or more photons. A closely related phenomenon is bond softening, where the potential curves flatten, or "soften" in the vicinity of a multiphoton resonance [6-9]. These effects were observed first in dissociation spectra of' H2+ and D2+ [6-8]. The initial experiments em- ployed laser pulses that were much longer than the vibra- tional or rotational oscillation periods of the molecule. Recently, high intensity laser pulses with much shorter widths have become widely available [10,11]. Strickland et al. have used sub-100-fs pulses to essentially "freeze" the motion of molecular iodine during multiple ionization [4]. We report the results of 160 fs multiphoton ioniza- tion and dissociation of hydrogen and deuterium molecu- lar ions. In addition to above-threshold dissociation, we find evidence for a new phenomenon: population trapping in light-induced vibrational states of the molecular ions. Our experiments used 160 fs pulses of 769 nm light from a mode-locked and amplified Ti:sapphire laser. The focal spot diameter was estimated to be 15 pm, based on previous measurements in this apparatus, and corroborat- ed by measurements of multiple ionization in xenon, where the relation between peak power and ionization threshold has been studied previously [12]. Typical ion kinetic energy spectra are shown in Fig. 1. The most prominent feature is a narrow peak that can be attributed to two photon above--threshold dissociation [6]. The prominence of this peak is dramatically different than in the long-pulse regime, where most of the ions are in the lowest energy peak, corresponding to one-photon absorption from the bound vibrational level. The second ATD peak (actually one-half photon higher in energy since the two fragments share the excess photon energy) comes from three-photon absorption from the 1sag lower state to the 2pa„upper state, followed by one-photon emission during dissociation. The observed pulse-width dependence is consistent with the formation of a vibrational wave packet, i. e. , a con- structive superposition state of two or more vibrational levels produced during rapid ionization of the neutral parent molecule. Since this initial ionization of a neutral molecule is a high order process (at least ten photons are required at X =765 nm), its rate increases rapidly with in- tensity, and may approach or exceed the ion's vibrational period when ultrashort pulses are used. Thus, a vibra- tional wave packet may form in the vicinity of the ionic potential where there is overlap with the ground state of the parent molecule. Figure 2 shows the results of a classical model designed to illustrate the wave-packet hypothesis. It plots the tra- jectory of a particle representing the deuterium molecular ion, executing bound motion in the "dressed" ground state internuclear potential. Dressed in this case means that the shape of the potential includes shifts due to the interaction of the ion with the intense laser pulse. The dominant shifts occur because the ground state (lsog) and the repulsive first excited electronic state (2pcr„) can be resonantly coupled by an odd number of photons. When an ion passes through such a resonance, it can adi- abatically switch states via the emission or absorption of the requisite photons. A Floquet analysis was performed to calculate the shape of the gaps, or avoided crossings, which appear in the potential at internuclear separations R where there are odd-photon resonances. With an in- crease of the laser field, these gaps become wider, forcing the ion to take the adiabatic path. The ab initio calcula- tion of' this potential curve is described more fully in Refs. [5] and [8]. The model [13] assumed an initial ionization at an in- tensity of 1. 4&10' W/cm, on the rising edge of'a 160 fs F W H M Gaussian pulse with peak intensity of 10' 1993 The American Physical Society 1077