On the control of product yields in the photofragmentation of deuteriumchlorid ions (DCl+)

Femtosecond interferometry of molecular dynamics – the role of relative and absolute phase of two individual laser pulses

Dynamical implications of the dissociation of a molecular ion under the influence of interferometrically generated light fields have been investigated by numerical solution of time dependent Schrödinger equations. As an example the dissociation of DCl+ ions by means of two interfering 7 fs laser pulses at 800 nm has been chosen. We demonstrate that product branching ratios D+:Cl+ can be manipulated from 10:1 to 1:10 not only by adjusting the appropriate delay time in the time-shifted two-pulse approach, but also by choosing the proper carrier envelope phase (CEP) of the two partial light fields. The effects of the phase shift related to the time shift and that of the CEP can be clearly distinguished.

Dissociation of H2NCH dication in a strong laser field

Ab initio classical molecular dynamics calculations have been used to simulate the dissociation of H(2)NCH(2+) in a strong laser field. The frequencies of the continuous oscillating electric field were chosen to be ω = 0.02, 0.06, and 0.18 au (2280, 760, and 253 nm, respectively). The field had a maximum strength of 0.03 au (3.2 × 10(13) W cm(-2)) and was aligned with the CN bond. Trajectories were started with 100 kcal/mol of vibrational energy above zero point and were integrated for up to 600 fs at the B3LYP/6-311G(d,p) level of theory. A total of 200 trajectories were calculated for each of the three different frequencies and without a field. Two dissociation channels are observed: HNCH(+) + H(+) and H(2)NC(+) + H(+). About one-half to two-thirds of the H(+) dissociations occurred directly, while the remaining indirect dissociations occurred at a slower rate with extensive migration of H(+) between C and N. The laser field increased the initial dissociation rate by a factor of ca. 1.4 and decreased the half-life by a factor of ca. 0.75. The effects were similar at each of the three frequencies. The HNCH(+) to H(2)NC(+) branching ratio decreased from 10.6:1 in the absence of the field to an average of 8.4:1 in the laser field. The changes in the rates and branching ratios can be attributed to the laser field lowering the reaction barriers as a result of a difference in polarizability of the reactant and transition states.

Symmetry-adapted-cluster configuration interaction study of the doublet states of HCl+: Potential energy curves, dipole moments, and transition dipole moments

The electronic structure of the HCl(+) molecular ion has been calculated using the general-R symmetry-adapted-cluster configuration interaction (SAC-CI) method. The authors present the potential energy curves, dipole moments, and transition dipole moments for a series of doublet states. The data are compared with the previous CASSCF and MCSCF calculations. The SAC-CI results reproduce quite well the data available in literature and extend the knowledge on the HCl(+) electronic structure for several higher states. The calculated R-dependent behavior of both dipole moments and transition dipole moments for a series of bound and unbound states reveals an intricate dissociation process at intermediate distances (R>R(e)). The pronounced maxima in transition dipole moment (TDM) describing transitions into high electronic states (X (2)Pi-->3 (2)Pi, X (2)Pi-->3 (2)Sigma, 2 (2)Pi-->3 (2)Pi, 3 (2)Pi-->4 (2)Pi) occur at different interatomic separations. Such TDM features are promising for selection of excitation pathways and, consequently, for an optimal control of the dissociation products.

The Morse oscillator under time-dependent external fields

A method to solve the equations for the Morse oscillator under intense time-dependent external fields is presented. Exact analytical formulas for the dipole matrix elements are calculated by the use of the hypergeometric algebra. The continuum is described by an expansion using Laguerre functions. The full algorithm for the calculation of wave functions can be controlled by the convergence of series and by the errors of a first order integration method. We apply our technique to the selective preparation of high overtones by femtosecond laser pulses. The population of the target state is optimized as a function of the intensity and frequency. Introducing a second simultaneous laser, we study the effects of relative frequency and phase over the target state population and dissociation channels. The calculations exhibit a rich interference pattern showing the enhancement and the suppression of the target population by varying the laser parameters.

Control of Branching Ratios in the Dissociative Ionization of Deuterium Chloride

The dissociative ionization of deuterium chloride (DCl) has been investigated by employing femtosecond laser pulses at 805 nm. The product branching ratio D(+)/Cl(+) of the fragments D(+) and Cl(+) is strongly affected by the chirp alpha of the laser pulses. The ratio can be controlled by a factor of 3 ranging from D(+)/Cl(+) = 0.7 at alpha = -800 fs(2) to D(+)/Cl(+) = 1.9 at alpha = +150 fs(2). The observation can be rationalized by a model where negative chirp favors intra-electronic state excitation, and positive chirp favors inter-electronic state excitation in the dissociation of the molecular ion. Complementary experiments on hydrogen chloride (HCl) are discussed.