The H(3)(+) ion is the simplest polyatomic molecule and is destined to play a central role in understanding such molecules in intense ultrashort laser pulses. We present the first measurements of the intense field dissociation and ionization of D(3)(+) using coincidence three-dimensional momentum imaging. Our results show features that are a consequence of this molecule's unique equilateral triangular geometry, providing a fundamentally new system for theoretical development.

We reveal surprisingly high kinetic energy release in the intense-field fragmentation of D + 3 to D + + D + + D with 10 16 W cm −2 , 790 nm, 40 fs (and 7 fs) laser pulses. This feature strongly mimics the behaviour of the D + + D + + D + channel. From the experimental evidence, we conclude that the origin of the feature is due to frustrated tunnelling ionization, the first observation of this mechanism in a polyatomic system. Furthermore, we unravel evidence of frustrated tunnelling ionization in dissociation, both two-body breakup to D + D + 2 and D + + D 2 , and three-body breakup to D + + D + D. Gesellschaft processes involving either elastic scattering [6][7][8], inelastic scattering [9, 10], or electron-ion recombination [11]. These phenomena have led to the birth of new areas of research such as high-harmonic generation and attosecond science [12][13][14][15], laser-driven electron diffraction imaging [5][6][7][8], molecular orbital tomography [16,17] and electron wavepacket probing of molecular dynamics [18-21]-naming only a few.Related to the electron recollision process, recently Nubbemeyer et al [22] reported a new phenomenon dubbed frustrated tunnelling ionization (FTI). Demonstrated originally in strong-field ionization of helium, Nubbemeyer et al showed that an electron wavepacket that starts to tunnel away from the core in an intense laser field, but fails to acquire sufficient drift momentum to escape the attractive potential of the remaining He + ion, can be captured into an excited Rydberg orbital of the He atom-in effect 'frustrating' the tunnel ionization process. This process must occur during the laser pulse to conserve energy and momentum, most likely during the trailing edge, as the electron is gently decelerated over many laser cycles before being pulled into orbit.The same mechanism has been observed in the dissociative ionization of a few diatomic molecules (H 2 [23], D 2 [24], O 2 [25] and Ar 2 [26][27][28]). For such molecules, following ionization, an electron that is excited to the continuum and driven by the laser field tends to be captured to a Rydberg orbital of one of the two 'Coulomb-exploding' fragment ions. The signature of frustrated tunnelling in molecules is that, counterintuitively, the final kinetic energy release (KER) is similar to that of a Coulomb explosion event even though only one product fragment is charged while the other fragment is neutral [23].This description of FTI uses language, such as electron capture, that is usually reserved for discussions involving ionization. Throughout the paper we use this language for convenience. However, it does pose an interesting question in relation to the actual mechanism for FTI, that is,

High-order (three-photon or more) above-threshold dissociation (ATD) of H(2)(+) has generally not been observed using 800 nm light. We demonstrate a strong enhancement of its probability using intense 7 fs laser pulses interacting with beams of H(2)(+), HD(+), and D(2)(+) ions. The mechanism invokes a dynamic control of the dissociation pathway. These measurements are supported by theory that additionally reveals, for the first time, an unexpectedly large contribution to ATD from highly excited electronic states.

We present kinematically complete measurements of the photo double ionization of ethylene (double CC bond) and acetylene (triple CC bond) hydrocarbons just above the double ionization threshold. We discuss the results in terms of the coincident kinetic energy of the photo electrons and the nuclear kinetic energy release of the recoiling ions. We have incorporated quantum chemistry calculations to interpret which of the electronic states of the dication have been populated and trace the various subsequent fragmentation channels. We suggest pathways that involve the electronic ground and excited states of the precursor ethylene dication and explore the strong influence of the conical intersections between the different electronic states. The nondissociative ionization yield is small in ethylene and high in acetylene when compared with the dissociative ionization channels.The reason for such a striking difference is explained in part on the basis of a propensity rule which influences the population of states in the photo double ionization of a centrosymmetric closed shell molecule by favoring singlet ungerade and triplet gerade final states. This propensity rule and the calculated potential energy surfaces clarify a picture of the dynamics leading to the observed dication dissociation products.

In a joint experimental and theoretical endeavor, we explore the laser-induced dissociation and ionization dynamics of H 2 + beams using sub-10-fs, 800 nm laser pulses. Our theory predicts considerable control over the branching ratio of two-photon and three-photon above-threshold dissociation (ATD) by gating the dissociation pathway on a few-femtosecond timescale. We are able to experimentally demonstrate this control. Moreover, our theory also shows the importance of the highly excited H(2l) states of H 2 + that contribute to ATD structure in dissociation. As is the case for dissociation, we find that ionization is also sensitive to the effective laser interaction time.