Theoretical Study of Two-Photon above Threshold Dissociation and Related Processes in Na₂⁺ and Li₂⁺

Abstract

We present wavepacket dynamics simulations of above threshold dissociation (ATD) in Na2 + and Li2 +. We consider resonant two-photon processes in which both the intermediate and the final state belong to the dissociative continuum. We show that in both molecular ions ATD experiments can be planned with moderately high laser intensities. If a vibrational initial state other than v = 0 is selected and/or a two-color excitation is envisaged, a wider range of processes can be observed, such as interference phenomena and Rabi oscillations between continuum states.

Theoretical study of the photodissociation of Li2+ in one-color intense laser fields

“…2,3 Increasing attention has also been paid to pseudo-one-electron alkali molecular ions. 24,[38][39][40][41][42][43][44] Due to the heavier masses of the nuclei, the vibrational periods of alkali molecular ions are larger than in H 2 + , and as such relatively larger pulse durations are needed for the exploration, which can more easily be accessed by experimental techniques. Besides, unlike the above-mentioned investigations of H 2 + , which usually involve only two states (i.e., 1sσ g and 2pσ u ) and the bound-free transition between them, more electronic states need to be taken into account for alkali molecular ions because the energy separations between the ground and several excited states are relatively small and both bound-free and free-free transitions between them play important roles in their dissociations.…”

“…Besides, unlike the above-mentioned investigations of H 2 + , which usually involve only two states (i.e., 1sσ g and 2pσ u ) and the bound-free transition between them, more electronic states need to be taken into account for alkali molecular ions because the energy separations between the ground and several excited states are relatively small and both bound-free and free-free transitions between them play important roles in their dissociations. 24,[38][39][40][41][42][43][44] In particular, Li 2 + has been proven to be more complicated than Na 2 + . In the Na 2 + system, the vertical energy separation between the υ = 0 vibrational level of 1 2 g + and the 1 2 u + excited state is close to the one between 1 2 g and 1 2 u + , so both of the transitions can take place with the same photon energy.…”

“…In the Na 2 + system, the vertical energy separation between the υ = 0 vibrational level of 1 2 g + and the 1 2 u + excited state is close to the one between 1 2 g and 1 2 u + , so both of the transitions can take place with the same photon energy. 40 However, for Li 2 + the excitation energies of the bound-free 1 transitions are substantially different, which implies that, starting from the υ = 0 level (or other low lying vibrational levels), these two transitions cannot be induced at the same frequency. Since all these states (especially in laser fields with nonparallel polarization) could contribute to the richness of phenomena that Li 2 + could exhibit and should be considered in the treatment of the dissociation of Li 2 + , more complex ways need to be employed to circumvent this problem and to increase the populations on such excited states, e.g., pumping the molecular ion with different frequencies (i.e., two-color experiment) or starting the excitation at a higher vibrational level of the ground state 1 2 g + (i.e., one-color experiment).…”

“…Since all these states (especially in laser fields with nonparallel polarization) could contribute to the richness of phenomena that Li 2 + could exhibit and should be considered in the treatment of the dissociation of Li 2 + , more complex ways need to be employed to circumvent this problem and to increase the populations on such excited states, e.g., pumping the molecular ion with different frequencies (i.e., two-color experiment) or starting the excitation at a higher vibrational level of the ground state 1 2 g + (i.e., one-color experiment). 40,43 Both one-color 24,38,[42][43][44] and two-color [39][40][41] schemes have been applied to Li 2 + and Na 2 + . In the theoretical studies of Li 2 + , Magnier et al 40 presented a wave packet simulation of ATD involving the resonant two-photon processes in which both the intermediate and the final states belong to the dissociative continuum and analyzed the interference phenomena and Rabi oscillations between the continuum states through one-color and/or two-color excitations; Khait et al 43 reported the field-free potentials and dipole transition moments of Li 43 dissociations starting from levels energetically close to this region are expected to produce remarkable distributions and reveal interesting behaviors for different channels with changing laser conditions.…”

“…40,43 Both one-color 24,38,[42][43][44] and two-color [39][40][41] schemes have been applied to Li 2 + and Na 2 + . In the theoretical studies of Li 2 + , Magnier et al 40 presented a wave packet simulation of ATD involving the resonant two-photon processes in which both the intermediate and the final states belong to the dissociative continuum and analyzed the interference phenomena and Rabi oscillations between the continuum states through one-color and/or two-color excitations; Khait et al 43 reported the field-free potentials and dipole transition moments of Li 43 dissociations starting from levels energetically close to this region are expected to produce remarkable distributions and reveal interesting behaviors for different channels with changing laser conditions. Analysis of the results from previous theoretical treatments 46 shows that if one starts with a rovibrational level near υ = 13−14 of the ground state, with a relative energy of about 3400−3500 cm −1 , several transitions can take place with almost the same photon energy.…”