Samir Saha

Above-threshold multiphoton dissociation of and in intense laser fields

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.

Resonant and inelastic transitions in collision between HCl molecules

Transition probabilities and cross sections for rotational transitions from the initial state j01=0, j02=1 have been calculated for an initial relative kinetic energy E0=0.025 eV. The semiclassical exponential approximation has been used. All possible single‐molecule and simultaneous transitions as well as intermultiplet transitions have been included with all energetically accessible states. These transitions are caused by the anisotropic short‐range, dispersive, inductive, and electrostatic interactions. The influence of pairs of resonating channels on the various inelastic transitions has been discussed. The limitations of the first order perturbation theory calculations are clearly brought out. The importance of various factors determining the relative magnitudes of the cross sections are shown.

Rotational transitions in collisions between polar molecules: an application of the semiclassical strong-coupling method to HCl-HCl collisions

Probabilities and cross sections for simultaneous rotational transitions in collision between HCl molecules have been calculated for a relative kinetic energy of 0.025 eV. The calculations have been made for two sets of initial states, j10=0, j20=1 and j10=0, j20=0. All energetically accessible rotational channels were included in the semiclassical strong-coupling formalism. The special features of the resonant transition have been shown. The oscillatory behaviour of various transition probabilities with impact parameters have been physically interpreted as the consequence of interference between different intermediate transitions leading to the same final state, as well as between different terms of the coupling potential.

Alignment in angular distribution of photofragments in multiphoton above threshold dissociation of HD+ by linearly and circularly polarized intense laser fields

We have investigated the angular distribution of photofragments resulting from multiphoton above threshold dissociation (ATD) of HD+ from initial bound levels νi=0, 1, 2 and Ji=0 induced by linearly (↕) and circularly (↻) polarized laser fields of intensities I=1, 5, and 10 TW/cm2 and frequency ω=30 333 cm−1 (λ=329.7 nm). The time-independent close coupling (CC) method has been used. Molecular rotation has been taken into account by including 72 channels with J=0–7 and number of photons n=−1–7, for ↕ polarization. In case of ↻ polarization, the channel number reduced to 48. All the radiative couplings including those due to the intrinsic dipole moments of HD+ have been considered in a truncated length gauge form of interaction. For (net) one-photon dissociation (from νi=1, 2) by bond softening, the alignment of the angular distribution of photofragments increases with intensity (1–10 TW/cm2), for both ↕ and ↻ polarizations. The angular distributions for (net) two-photon fragmentation via the adiabatic pat...

Two-photon dissociation of HD+ by the 1sσg→2pπu electronic transition via resonance in the continuum

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.

Resonant two‐photon dissociation of HD+ via Fano formalism

We have studied the resonant two‐photon dissociation (TPD) of vibrationally excited HD+ by both 1sσg→1sσg vibrational and 1sσg→2pσu electronic transitions. Following Armstrong et al., a quantum mechanical description of the laser field enabled us to define the intermediate resonant state as a ‘‘pseudodissociating state’’ of the molecule–radiation system. This makes our theory formally similar to that of single‐transition (photon) autoionization problem as formulated by Fano. This similarity has been exploited to obtain the intensity dependent line shapes of the resonant TPD of HD+. Resonant TPD cross sections from the vi=6, Ji=0 level of the ground 1sσg state of HD+ are calculated with a linearly polarized infrared laser radiation in the wavelength range 16 530–16 565 A and at different intensities ranging from 1.53×108–3.83×1013 W/cm2. Effects of laser intensity on the linewidth, line shift, and cross sections are found to be very different for the two types of transitions. Our previous results of nonres...

Two-photon dissociation of HD+ by the 1sσg → 2pσu electronic transition

Two-photon dissociation (TPD) of HD+ is studied for the 1sσg → 2pσu transition using a direct perturbation technique. Non-resonant TPD cross sections from the ν = 6, J = 0 level of the ground 1sσg state are calculated at different wavelengths of linearly polarised infrared photons, considering all possible bound and free intermediate states. The minimal coupling form (A · p) of the interaction Hamiltonian is used. The cross sections are found to have an oscillatory behaviour in the wavelength range (λ = 16000–17000 A) considered.

Molecule–molecule collisions: Semiclassical calculations of rotational excitation for the H2–CO2 system using the exponential approximation

The formalism of the semiclassical exponential approximation is developed for rotational transitions in collisions between two linear molecules. The H2 and CO2 molecules are treated as rigid rotors and their interaction potential includes anisotropic contributions from the short‐range repulsive forces as well as from the long‐range dispersion, induction, and electrostatic interactions. The calculations are made over a wide range of energy below the rotational threshold of H2 and for several initial rotational states of both molecules. Neglecting the small probability of de‐excitation of H2 when possible, the rotational inelasticity in CO2 is found to depend significantly on the rotational state of its collision partner. The effect of varying the repulsive anisotropy parameter on the inelastic cross sections is also investigated.

A time-independent Hermitian Floquet approach for high harmonic generation in H2+ and HD+: Effect of nonadiabatic interaction in HD+

We have theoretically investigated the high harmonic generation (HHG) in H(2)(+) and HD(+) using a time-independent Hermitian nonperturbative three-dimensional Floquet approach for continuous wave monochromatic lasers of intensities of 2.59x10(13), 4.0x10(13), and 5.6x10(13) W/cm(2), and wavelengths of 1064, 532, and 355 nm. For the moderate intensities and the wavelengths used, the Keldysh parameter gamma > 1 and no tunnel ionization occurs. We have endeavored to explain the dynamics of HHG in H(2)(+) and HD(+) within the framework of transitions due to electronic as well as intrinsic (for HD(+)) dipole moments and the nuclear motion on the field coupled ground and the first excited electronic states of these single-electron molecular ions, without considering any ionization. To evaluate the HHG spectra, the resonance Floquet quasienergy and the Fourier components of the Floquet state corresponding to the initial vibrational-rotational level v = 0, J = 0 have been calculated by solving the time-independent close-coupled Schrödinger equation following the Floquet ansatz. For HD(+), we have taken into account the symmetry breaking effect of the nonadiabatic interaction or coupling [beyond the Born-Oppenheimer (BO) dynamics] in our calculations of the HHG spectra and have compared the results with the HHG spectra of HD(+) obtained with the BO approximation.

Rotational branching in population transfer in H2 by chirped adiabatic Raman passage

We have theoretically investigated the branching in population transfer in H2 by chirped adiabatic Raman passage from the ground vg=0, Jg=0 level to the final rotational levels Jf=0 (Q-branch) and Jf=2 (S-branch) of the fundamental transition (vf=1) in the ground X 1Σg+ state via the nonresonant intermediate B 1Σu+ and C 1Πu+ states. The density matrix technique has been employed to describe the time evolution of the process. We have evaluated the rotational branching of transferred population in the final levels using linearly chirped pump and Stokes laser pulses with different chirp rates and chirp widths. Both the pulses are considered to have the same temporal shape, duration, peak intensity, and linear parallel polarizations. It has been observed that for negative chirping of the pump laser and positive chirping of the Stokes laser, branching in population transfer to the Q- and S-branches occur for fast chirping and short pulses while for slow chirping and long pulses, more and more population build...