K. Rai Dastidar

Dissociative Recombination of H 2 + , HD + and D 2 + Molecular Ions

An ab initio calculation of the cross-sections of dissociative recombination (DR) in H + 2 , HD + and D + 2 has been made using the projection operator technique. A single-configuration molecular orbital wavefunction has been used for the molecular resonance state, although the applicability of such a simple wavefunction has not been fully tested. The decay width has been calculated in the “local” approximation. Comparison with experiment shows that for H + 2 the calculated results can fully account for the total DR cross-section in the low and intermediate energy regions, and that the local approximation is sufficiently accurate to account for the undulatory structure in the energy dependence of the DR cross-section. A similar structure is also obtained for D + 2 , although here the agreement with experiment is less marked. For HD + no particular heteronuclearity effect is observed.

Dissociation of H2+ ion by collision‐induced vibrational excitation

Collision‐induced dissociation of hydrogen molecular ion due to the vibrational excitation from ground vibrational state to the continuum of electronic ground state (1sσg) has been studied, using sudden approximation. In close encounters, the interaction between the nucleus of projectile and the two nuclei of molecular ion has been taken as the sum of two screened Coulomb‐type interactions. Exact values for the ground vibrational state and quasiclassical wavefunctions for the continuum have been used. The angular distribution, energy distribution, and total vibrational dissociation cross section for fragments in a given excess energy range have been calculated with He, Ar, and Xe as projectiles. Cross sections for dissociation by Ar impact have been calculated for collision energies ranging from 3 to 50 keV, and for He and Xe at 10 keV. It is concluded that the electronic dissociation plays a dominant role even in the small excess energy region of dissociated fragments.

Broadening of EIT window by incoherent pumping in a three-level Λ system: Effect of homogeneous and inhomogeneous broadening

The interplay between dynamically induced coherence (DIC) and spontaneously generated coherence (SGC) can induce electromagnetically induced transparency (EIT) in a three-level closed Λ system. When the contributions from these two coherences are equal in magnitude and opposite in sign at the resonances of both the coherent and probe fields, transparency of the probe light occurs at the resonance with absorption humps away from the resonance. We have demonstrated here that the broadening of the EIT window can be controlled by changing the strength of incoherent pumping. This is a new approach for achieving wide EIT window for lossless and distortionless transmission of light of large spectral width. The most attractive feature of this new method is that the FWHM (full width at half-maximum) of the EIT window is further increased when the broadening due to homogeneous/inhomogeneous decay is taken into consideration. This scheme can be applied in atoms/molecules where SGC can be invoked by dressing two closely spaced levels (e.g. hyperfine levels in atoms or rovibrational levels in molecules) by an external field. This effect has been demonstrated in a heteronuclear molecule (e.g. LiH) and examined the role of incoherent pumping for controlling the width of the EIT window in the presence and absence of homogeneous/inhomogeneous decay. The experimental realization of the widening of the EIT window in molecules has been discussed.

Use of an approximate diabatic molecular representation in He+ -He charge transfer collision

Abstract A single-configuration “frozen-orbital” molecular basis set has been used as an approximation to the “standard” radial diabatic representation defined by Smith. The usefulness of this approximate representation has been demonstrated in an eight-state close-coupled treatment of the He+ -He resonant transfer collisions in the impact parameter approximation. Apart from the Σ-Σ potential couplings the two Σ-Π rotational couplings at short range have been considered. Generally satisfactory agreement with the experimental data of Nagy ., together with a significant improvement in the estimate of the rotational coupling effects toward the region of large EO (≳ 3-4 keV deg) as compared to the earlier theoretical work of McCarroll and Piacentini is obtained.

Time development of resonant two-photon autoionisation of H2

Abstract Total two-photon autoionisation of H 2 through the lowest autoionising state of 1 Σg (1σ 2 u symmetry has been studied as a function of time for three resonant cases [corresponding to (ν = 0, j = 1), (ν = 1, j = 1) and (ν = 2, j = 1) levels of the B″ 1 Σ u intermediate state]. A structure due to ac Stark splitting has been obtained in the ionisation profile. Effects of intensity and detuning on the ionisation profile have also been studied.

On amplification of radiation emitted from the excited autoionizing state of the H2 molecule in the presence of incoherent pumping to the lowest autoionizing state

SummaryWe have shown that the amplification without population inversion (AWOPI) from the first excited autoionizing (AI) state of the1Σg symmetry of the H2 molecules can be obtained when the lowest AI state of the1Σg symmetry is pumped incoherently. We have considered a resonant two-photon transition scheme from the groundX1Σg(v = 0;j = 1) state to the ionization continuum embracing the two AI states (the lowest and the first excited autoionizing states) of the1Σg symmetry via the resonant intermediateB1Σu(v = 4;j = 2) level. The effect of two-photon near-resonant autoionizing channels via the near-resonant rovibrational levels of theB1Σu state has also been considered. We have found that the gain can be obtained in different spectral regions corresponding to the resonances with the lowest and first excited AI states from the intermediate levels of theB1Σu state. The inherent coherence between two AI states due to the configuration interaction coupling via the common continuum has been found to have a significant effect on the amplification process, which occurs around all the resonance frequencies connecting this AI state with the intermediate levels. This gain can be significantly modified by increasing the coupling strength of this excited AI state with the continuum. Moreover, the gain from the excited AI state remains unaffected by the damping caused by incoherent pumping to the lowest AI state. It has also been shown that the persistence of inversionless gain from the excited AI state is shorter than that from the lowest AI state. The presence of close-lying near-resonant rovibrational levels leads to the increase in gain around the resonances with two AI states from theB1Σu (v = 4;j = 2) level. The advantages of this type of scheme are: i) AWOPI can be obtained from an excited AI state although it has not been pumped and ii) the amplification obtained from the excited AI state is almost unaffected by the distortion caused by incoherent pumping to the lower AI state.

Gauge Invariance in non-relativistic quantum mechanics

SummaryWe show that non-relativistic quantum mechanics is invariant under a local gauge transformation even in the absence of any external electromagnetic field, provided we do not exclude the arbitrary phase factor in the coordinate representation of the wave vector. A generalised, gauge-invariant form of the Schroedinger equation, as well as gauge-invariant canonical momentum and Hamiltonian operators are introduced. In the presence of an electromagnetic field, the new Hamiltonian operator turns out to be identical with the «energy operator» introduced by K. H. Yang. A previously derived result, proving thenon-equivalence of the minimal-coupling and the multipolar forms of matter-radiation interaction, is shown to follow as a corollary.

Identification of single charge transfer channels in –N2 collisions by translational energy spectroscopy

Single charge transfer reactions in the collision of ions at a laboratory energy of 2 keV with N2 were observed using translational energy spectroscopy (TES). Within the limitation of the energy analyser resolution (∼ 8/q eV, q being the ionic charge number), the translational energy spectrum shows a single broad charge transfer peak with a rich substructure. The latter looks like a conglomeration of closely spaced channels clumped together, corresponding mainly to charge transfer into excited states, rather than into the ground state. The reaction channel assignments were made using data available in the literature. Our findings are in accord with a recent experimental result, but there are no theoretical calculations to compare our experimental measurements with.

Electron Cooling through Resonant Collisions with H2+ Molecular Ion

Electron cooling in H 2 + through resonant capture leading to (i) vibrational excitation and (ii) vibrational dissociation of H 2 + has been studied within the scope of Born-Oppenheimer approximation, using Feshbachs projection operator technique. Results are presented for (i) cross-sections for certain specific vibrational transitions ( v i → v f ), and for (ii) total cross-sections for vibrational dissociation. Variations in the rotational inelasticity effects as obtained for different types of vibrational excitations are discussed.

Lasing without population inversion from autoionizing states

SummaryWe show that in case of (1+1)-photon resonant absorption to and emission from an autoionizing state embedded into a continuum, the gain profile can be modified significantly in the presence of a parallel near-resonant channel leading to another autoionizing state embedded into a different continuum. Due to the coupling between two channels via cross-photoionization (i.e. ionization from the intermediate state of one channel to the continuum of another channel) the gain can be obtained around three different frequencies leading to lasing without population inversion. Results of parametric calculations are presented.