• Samir Saha

      Articles written in Pramana – Journal of Physics

    • Effect of polarization on population transfer in H2 by stimulated Raman transition with partially overlapping laser pulses

      Swaralipi Ghosh Sanjay Sen SS Bhattacharyya Samir Saha

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      Polarization effects on population transfer by stimulated Raman transition using overlapping time dependent pump and Stokes laser pulses from the ground X1Σg/+(vg=0, Jg=1) level of H2 to the final X1Σg/+(vf=1, Jf=1) level via the intermediate B1Σu/+(vi=14, Ji=0,2), C1Πu/+(vi=3, Ji=2) and C1Πu/−(vi=3, Ji=1) levels have been theoretically investigated by applying the density matrix formalism. We have studied in detail the dependence of the population transfer on time delay between two pulses for the cases of on-resonance excitations considering linear parallel and same-sense circular polarizations of the fields. The pump and Stokes fields are taken as having Gaussian pulse shapes with peak intensities IP/0(IS/0)=2 × 106 and 1 × 107 W/cm2. Density matrix equations have been solved for each value of the magnetic quantum number Mg(0, ±1) of the initial ground level taking into account the Mg dependence of the Rabi frequencies. Mg — averaged population transfer to the final level has also been calculated. For resonance excitations to the B(14, 0) or C(3, 1) levels, appreciable population transfer is achieved for intuitive pulse order for some particular values of Mg and Mi (magnetic quantum number of the resonant intermediate level) depending on the nature of polarizations. The calculated values of Mg — averaged population transfer for the two cases of polarizations show that for on-resonance excitation to the B(14, 0) or the C(3, 1) level, linear parallel polarization of the laser fields yield more transfer efficiency whereas for resonance excitation to the B(14, 2) level, larger population transfer results from the same-sense circular polarizations. For resonance excitation to the C(3, 2) level, Mg — averaged population is found to be almost polarization independent. The calculations for the six-level H2 system reveal some interesting features of polarization effects on the population transfer efficiency.

    • High harmonic generation in H$_{2}^{+}$ and HD+ by intense femtosecond laser pulses: A wave packet approach with nonadiabatic interaction in HD+

      Farzana Sharmin Samir Saha S S Bhattacharyya

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      We have theoretically investigated the high harmonic generation (HHG) spectra of H$_{2}^{+}$ and HD+ using a time-dependent wave packet approach for the nuclear motion with pulsed lasers of peak intensities $(I_{0})$ of $3.5 \times 10^{14}$ and $4.5 \times 10^{14}$ W/cm2, wavelengths ($\lambda_{L}$) of 800 and 1064 nm, and pulse durations (𝑇) of 40 and 50 fs, for initial vibrational levels $\nu_{0} = 0$ and 1. We have argued that for these conditions the harmonic generation due to the transitions in the electronic continuum by tunnelling or multiphoton ionization will not be important. Thus, the characteristic features of HHG spectra in our model arise only due to the nuclear motions on the two lowest field-coupled electronic states between which both interelectronic and intraelectronic (due to intrinsic dipole moments, for HD+) radiative transitions can take place. For HD+, the effect of nonadiabatic (NA) interaction between the two lowest Born–Oppenheimer (BO) electronic states has been taken into account and comparison has been made with the HHG spectra of HD+ obtained in the BO approximation. Even harmonics and a second plateau in the HHG spectra of HD+ with the NA interaction and hyper-Raman lines in the spectra of both H$_{2}^{+}$ and HD+ for $\nu_{0} = 1$ have been observed for higher value of $I_{0}$ or $\lambda_{L}$. Our calculations indicate reasonable efficiencies of harmonic generation even without involving the electronic continuum.

    • High harmonic generation in H$_2^+$ and HD$^+$ by two-colour femtosecond laser pulses

      Farzana Sharmin Samir Saha S S Bhattacharyya

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      We have theoretically investigated the high harmonic generation (HHG) spectra of H$_2^+$ and HD$^+$ using a time-dependent wave packet approach for the nuclear motion with combined twocolour (1$\omega_L$–3$\omega_L$) pulsed lasers for ωL corresponding to wavelengths 1064 nm and 800 nm. The 1$\omega_L$ and 3$\omega_L$ lasers have peak intensities of $I_1^0 = 5.0 \times 10^{13}$ W/cm$^2$ and $I_2^0 = 2.0 \times 10^{14}$ W/cm$^2$, respectively. We have taken the pulse duration of $T = 50$ fs for both the fields, and the molecular initial vibrational level $v_0 = 0$. We have argued that for these combinations, the harmonic generation due to transitions in the electronic continuum by tunnelling or multiphoton ionization may be neglected and only the electronic transitions within the two lowest electronic states would be important. Thus, the characteristic features of HHG spectra in the two-colour field are determined, in our model, by the nuclear motions on the two lowest field-coupled electronic states between which interelectronic and intraelectronic (due to the intrinsic dipole moments in case of HD$^+$) radiative transitions can take place. We have studied the role of relative phase ($\varphi$) of the two fields on the HHG spectra of the molecular ions. In case of HD$^+$, the effect of nonadiabatic (NA) nonradiative interaction between the two lowest Born–Oppenheimer (BO) electronic states (1$s\sigma_g$, 2$p\sigma_u$) has been taken into account. Our calculations give realistic HHG spectra which are reasonably efficient and extended for both H$_2^+$ and HD$^+$ in the mixed two-colour field without involving the electronic continuum. The use of two-colour (1$\omega_L$–3$\omega_L$) field enables us to generate high harmonics beyond that achievable with a single 1$\omega_L$ or 3$\omega_L$ field of the corresponding intensity, frequency and pulse time.

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