A. D. Bandrauk

Sub-laser-cycle electron pulses for probing molecular dynamics

Experience shows that the ability to make measurements in any new time regime opens new areas of science. Currently, experimental probes for the attosecond time regime (10-18–10-15 s) are being established. The leading approach is the generation of attosecond optical pulses by ionizing atoms with intense laser pulses. This nonlinear process leads to the production of high harmonics during collisions between electrons and the ionized atoms. The underlying mechanism implies control of energetic electrons with attosecond precision. We propose that the electrons themselves can be exploited for ultrafast measurements. We use a ‘molecular clock’, based on a vibrational wave packet in H2+ to show that distinct bunches of electrons appear during electron–ion collisions with high current densities, and durations of about 1 femtosecond (10-15 s). Furthermore, we use the molecular clock to study the dynamics of non-sequential double ionization.

Laser-induced electron diffraction: a new tool for probing ultrafast molecular dynamics

Abstract Momentum angular distributions of the nonlinear multiphoton photoelectron spectra from intense ultrashort pulse ionization of H2+ molecular ions show signatures of electron diffraction from the two nuclei. The interference patterns depend on the internuclear distance of H2+. Photoelectron angular distributions should serve as a new tool to probe ultrafast molecular dynamics in pump-probe studies with intense, ultrashort laser pulses.

TWO-FREQUENCY IR LASER ORIENTATION OF POLAR MOLECULES. NUMERICAL SIMULATIONS FOR HCN

Abstract Using ab initio nuclear-coordinate-dependent dipole moments and polarizabilities, we study the orientation dynamics of HCN, by numerically solving the time-dependent Schrodinger equation, in the presence of a superposition of intense, linearly-polarized infrared laser pulses of frequency ω and 2ω. We show that polarizability acts in concert with permanent dipole moments to orient polar molecules, as opposed to alignment which occurs alone with a single laser frequency or one moment only (permanent or induced). Optimal orientation occurs for the field configuration E (t)= E 0 (t) cos ωt+0.5 cos 2ωt , where 2ω is resonant with a 0 → 1 vibrational transition and E 0 (t) is a picosecond pulse.

Two-step Coulomb explosions of diatoms in intense laser fields

A two-step model of Coulomb explosions of diatoms in intense laser fields is presented. In this model the molecule loses several electrons when the atoms are at the equilibrium internuclear distance and then fast Coulomb explosions occur until the products reach a critical distance Rc approximately=9 Bohr, at which several additional electrons are lost due to a recently discovered maxima of ionization rates occurring at R=Rc. Then the subsequent Coulomb explosion for the higher-charged ions takes place. The total combined Coulomb explosion energy agrees well with experimental results, showing striking regularities. The origin and intensity dependence of unexpectedly high ionization rates of dissociating nuclei at preferential, large internuclear distances R=Rc is also discussed and an analytic expression for Rc is derived.

Multiphoton ionization of inner-valence electrons and fragmentation of ethylene in an intense Ti:sapphire laser pulse

Abstract Using linearly polarized 200 fs Ti:sapphire laser pulses the multiphoton ionization and fragmentation of ethylene was studied. A model is proposed which is able to satisfactorily predict the abundance of the different fragments, C2H4+, C2H3+ and C2H2+ as a function of laser intensity. It is shown that fragmentation of the molecule occurs as a result of multiphoton ionization of inner-valence electrons followed by radiationless transitions to various dissociation channels of the molecular ion.

S-matrix analysis of non-resonant multiphoton ionisation of inner-valence electrons of the nitrogen molecule

Abstract We have used a strong field S -matrix theory to analyse the ionisation of inner-valence electrons of the nitrogen molecule in an intense laser pulse. The results of calculations show that the laser-induced population distribution in the electronic states of the molecular ion is shifted towards lower vibrational levels as compared to a Franck–Condon distribution. This is found to be due to the strong dependence of the rates of multiphoton ionisation on the ionisation potential. Theoretical results for the intensity dependence of the strongest band head of the first negative band system of N 2 + agree with experimental data.

Preparation and alignment of highly vibrationally excited molecules by CARP - chirped adiabatic Raman passage

Abstract Three-dimensional time-dependent Schrodinger equation simulations of Raman excitation of the Cl 2 molecule are used to demonstrate the feasibility of efficient high vibrational excitation ( v ⩾20) of symmetric non-polar bonds using short (ps) chirped frequency laser pulses at intensities below the ionization threshold ( I=2×10 13 W/cm 2 ). The process called chirped adiabatic Raman passage (CARP) involves sequential Raman excitations Δv =+1 during the pulse accompanied by rotational transitions. It is shown that as a result of CARP, considerable laser alignment of the molecule is achieved in high vibrational levels, thus offering a new tool for the study of dynamics and reactivity of aligned excited molecules.

Circularly polarized molecular high harmonic generation using a bicircular laser

We investigate the process of circularly polarized high harmonic generation in molecules using a bicircular laser field. In this context, we show that molecules offer a very robust framework for the production of circularly polarized harmonics, provided their symmetry is compatible with that of the laser field. Using a discrete time-dependent symmetry analysis, we show how all the features (harmonic order and polarization) of spectra can be explained and predicted. The symmetry analysis is generic and can easily be applied to other target and/or field configurations.

Theoretical study of unimolecular decomposition of allene cations.

Ab initio coupled clusters and multireference perturbation theory calculations with geometry optimization at the density functional or complete active space self-consistent-field levels have been carried out to compute ionization energies and to unravel the dissociation mechanism of allene and propyne cations, C(3)H(4)(n+) (n=1-3). The results indicate that the dominant decomposition channel of the monocation is c-C(3)H(3)(+) + H, endothermic by 37.9 kcal/mol and occurring via a barrier of 43.1 kcal/mol, with possible minor contributions from H(2)CCCH(+) + H and HCCCH(+) + H(2). For the dication, the competing reaction channels are predicted to be c-C(3)H(3)(+) + H(+), H(2)CCCH(+) + H(+), and CCCH(+) + H(3)(+), with dissociation energies of -20.5, 8.5, and 3.0 kcal/mol, respectively. The calculations reveal a H(2)-roaming mechanism for the H(3)(+) loss, where a neutral H(2) fragment is formed first, then roams around and abstracts a proton from the remaining molecular fragment before leaving the dication. According to Rice-Ramsperger-Kassel-Marcus calculations of energy-dependent rate constants for individual reaction steps, relative product yields vary with the available internal energy, with c-C(3)H(3)(+) + H(+) being the major product just above the dissociation threshold of 69.6 kcal/mol, in the energy range of 70-75 kcal/mol, and CCCH(+) + H(3)(+) taking over at higher energies. The C(3)H(4)(3+) trication is found to be not very stable, with dissociation thresholds of 18.5 and 3.7 kcal/mol for allene and propyne, respectively. Various products of Coulomb explosion of C(3)H(4)(3+), H(2)CCCH(2+) + H(+), CHCHCH(2+) + H(+), C(2)H(2)(2+) + CH(2)(+), and CCH(2)(2+) + CH(2)(+) are highly exothermic (by 98-185 kcal/mol). The tetracation of C(3)H(4) is concluded to be unstable and therefore no more than three electrons can be removed from this molecule before it falls apart. The theoretical results are compared to experimental observations of Coulomb explosions of allene and propyne.

Orbital alignment and electron control in photodissociation products by two-color laser interference

Time-dependent coupled equations are used to illustrate the laser control of the spatial orientation of diatomic photodissociation products and the concomitant orbital alignment of the electrons in the atomic fragments along the dissociation orientation. Such asymmetrical photodissociation and its control can be achieved by symmetry-breaking interference between two multiphoton pathways which produce isoenergy states of different symmetry. Conditions for this new laser control scenario are obtained for the simultaneous photodissociation of the 1 1Πu and 1 1Πg states of Cl2 by a two-color laser multiphoton absorption with subpicosecond pulses.