Search

We have studied multiphoton above-threshold dissociation (ATD) of and by a time-independent close-coupling (CC) method over a wide range of intensities for three initial bound states (v = 0, 1, 2 and J = 0) at a laser frequency of . Molecular rotation is taken into account by including 30 channels with J = 0 - 5 for and 20 channels with J = 0 - 7 for and number of absorbed photons n = 1 - 4. All the radiative couplings (including those due to the permanent dipole moment of ) have been considered in a truncated length-gauge form of interaction. At lower intensities the total dissociative decay is exponential, but at high intensity several dressed states with different linewidths contribute to the decay and no single decay rate can be defined. Use of the proper potential surfaces for , including intramolecular nonadiabatic coupling, enables us to obtain the branching ratios between the production of and fragments. The branching ratios for different numbers of photon absorption (n = 1, 2, 3) change dramatically with intensity. For , branching to channels, forbidden in by symmetry, may become dominant over a certain range of intensity. We have tried to interpret these intensity dependences in terms of the motion of the nuclei on the adiabatic field + molecule potential surfaces.

We present a theoretical study of resonant two-photon dissociation (TPD) of the molecule in the presence of pulsed two-colour laser fields using the density matrix formalism. The study includes the calculation of dissociation probabilities and decay rates as functions of time and the variation of total dissociation probabilities with detuning when both fields are assumed to have the same temporal shape and duration. Results are reported for two simultaneous laser pulses as well as for two pulses with a time delay. The investigations were done with different values of peak intensities , for the initial and intermediate levels as v = 0, J = 0 and v'=17, J'=1, respectively, of the ground electronic state and the final continuum energy reached by photons was taken to be . The time variation and delay between pulses lead to interesting features in the time-dependent decay rates and spectral shapes of total dissociation probabilities.

The time-dependent dissociation probabilities, lineshapes of dissociation rates and branching ratios of photofragments between the neutral hydrogen and deuterium channels have been investigated for resonant two-photon dissociation of HD+ from the ground level in the presence of two laser fields of different intensities I1, I2 and frequencies omega 1, omega 2 using the resolvent operator formalism. The symmetry breaking due to non-adiabatic (NA) interactions causes splitting between the two asymptotic channels, unlike the Born-Oppenheimer (BO) approximation. Simultaneous dissociation to the continua of the two electronic states at two different energies occurs through the absorption of either omega 1+ omega 2 or 2 omega 1 photons. Only the field of frequency omega 1 was considered to resonantly couple the near-resonant level nu '=17, J'=1 or nu '=18, J'=1 to the ground level nu =0, J=0. Non-resonant discrete and continuum states significantly affect the dissociation process in the case of high intensity fields. The electric field gauge (E.d) form of interaction Hamiltonian has been used in the computation of various laser-dependent parameters. The results with NA coupling included are found to deviate appreciably from those obtained with the BO approximation.

The two-photon dissociation (TPD) cross section of HD+ by the 1s sigma g to 2p pi u electronic transition is calculated from the nu i=6, Ji=0 level of the ground 1s sigma g state in the wavelength range 1100-2100 AA. All possible bound and free intermediate states have been taken into account. Both the 1s sigma g to 2p pi u to 2p pi u and the 1s sigma g to 1s sigma g (bound) to 2p pi u channels are found to contribute significantly, though with opposite phases. The 1s sigma g to 1s sigma g (free) to 2p pi u channel in which one-photon continuum resonance TPD occurs makes a negligible contribution. The suitability of the radiation gauge form (A.p) of the interaction Hamiltonian is discussed from the point of view of failure of the electric field gauge form (E.d) in the calculation of free-free matrix elements. The cross sections are found to oscillate in the wavelength range considered.