The photoassociation of alkali metal molecules (Rb2, Cs2, and RbCs) at short range is investigated in theory. Based on ab initio calculations to rationalize Franck–Condon filtering, the target states of photoassociation have been obtained. They correspond to vibrational transition levels from excited state (Rb2: $${v}{\kern 1pt} '$$ = 17–21, Cs2: $${v}{\kern 1pt} '$$ = 21–25, and RbCs: $${v}{\kern 1pt} '$$ = 18–22) to ground state. By using quantum wave-packet dynamic methods, the yields with time evaluation of the selected states are calculated interacting with a resonant laser pulse. Using gaussian pulse leads to the yield of Rb2 up to 82% at 860 fs. After a laser pulse, the positive chirp promotes the yield of vibrational states to increase, but the negative chirp inhibits it. That is to say, by changing the laser parameters and pulse shapes, it is much easier to control the photochemical process along desired direction. For photoassociation of alkali-metal dimers, homonuclear molecules have a better response to laser pulse than heteronuclear molecules. The efficiency of photochemical reaction of homonuclear molecules is much better and the photoassociative yield is much higher. Those conditions will provide an important reference and suggest a scheme for a feasible photoassociation for further experimental and theoretical research.
The photoassociation of alkali metal molecules (Rb2, Cs2, and RbCs) at short range is investigated in theory. Based on ab initio calculations to rationalize Franck–Condon filtering, the target states of photoassociation have been obtained. They correspond to vibrational transition levels from excited state (Rb2: v′ = 17–21, Cs2: v′ = 21–25, and RbCs: v′ = 18–22) to ground state. By using quantum wave-packet dynamic methods, the yields with time evaluation of the selected states are calculated interacting with a resonant laser pulse. Using gaussian pulse leads to the yield of Rb2 up to 82% at 860 fs. After a laser pulse, the positive chirp promotes the yield of vibrational states to increase, but the negative chirp inhibits it. That is to say, by changing the laser parameters and pulse shapes, it is much easier to control the photochemical process along desired direction. For photoassociation of alkali-metal dimers, homonuclear molecules have a better response to laser pulse than heteronuclear molecules. The efficiency of photochemical reaction of homonuclear molecules is much better and the photoassociative yield is much higher. Those conditions will provide an important reference and suggest a scheme for a feasible photoassociation for further experimental and theoretical research.
The selective control of photoassociation of Yb2 molecule is investigated in theory. Based on ab initio to rationalize Franck-Condon filtering, the optimal target states of photoassociation have been obtained. That correspond to vibrational transition levels from X1Σ+g to excited state (A1Σu+, B1Πu , C1Σu+, D1Πu) are v’=23, 50, 55, and 0, respectively. The proposed laser drive transitions to target states for association process at wavelength A1Σu+: 464.7nm, B1Πu: 373.1nm, C1Σu+: 361.8nm, D1Πu: 339.1nm are determined. By using quantum wave-packet dynamic methods, we calculated the yields with time evaluation for the selected target states. Interacting with a resonant laser pulse, the projections of time-dependent wave functions of initial states on the target vibrational eigenstates reflected the photoassociation yields of Yb2. For target A1Σu+, using gaussian pulse made the value of v’=23 up to 97% at 725fs. After a laser pulse, that the positive chirp promotes the yield of projection of vibrational states to increase, but the negative chirp inhibits it decrease. For D1Πu state, when laser intensity is 1.0×1014 W/cm2, not only the purity is high but also the yield is high reached 99% of target state v’=0 at 1570fs. That is to say, by changing the laser parameters and pulse shapes, it could much easier control the photochemical along our desired direction. These conditions will provide an important reference and suggest a scheme for a feasible photoassociation for further experimental and theoretical researches.
The selective control of photoassociation of Yb2 is investigated in theory. Based on ab initio to rationalize Franck–Condon filtering, the optimal target states of photoassociation have been obtained. The corresponding vibrational transitions from X1Σ+g to the excited state (A1Σu+, B1Πu, C1Σu+, and D1Πu) are v ′ = 23, 50, 55, and 0, respectively. By using quantum wave packet dynamic methods, we calculated the yields with time evaluation for the selected target states. The projections of time-dependent wave functions of initial states on the target vibrational eigenstates reflected the synthetic yields of Yb2. For target A1Σu+, we used Gaussian pulse to make the yield of v ′ = 23 up to 97% at 725 fs. After a laser pulse, the positive chirp promoted the yield of vibrational states to increase, but the negative chirp inhibited its decrease. For the D1Πu state, when laser intensity is 1.0 × 1014 W/cm2, the purity and yield of target state v ′ = 0 reached the maximum at 1350 fs. That is to say, changing the laser parameters and pulse shapes could control the photochemical reaction along our desired direction. These conditions will provide an important reference and suggest a scheme for a feasible photoassociation of further experimental and theoretical research studies. Current study may promote an important step toward the realization of highly accurate quantum manipulation and material synthesis.
Abstract The laser synthesis of alkaline earth dimers (Be2, Mg2, Ca2, Sr2, and Ba2) is investigated by theoretical calculation. Photoassociation based on ab initio calculations is rationalized by Franck‐Condon filtering. The optimal target states of photoassociations have been obtained, which are A1∑+u state for Be2, Mg2, and Ca2, and are B1∑+u state for Sr2 and Ba2, corresponding to vibrational transition bands from X1∑+g to excited state are 0‐1, 0‐7, 0‐10, 0‐13, and 0‐3, respectively. The proposed laser drive transitions to target states are also investigated that range from 350 to 740 nm. By using quantum wave‐packet dynamic methods, we calculated the absorption cross sections of photon energy and yields for the selected states of photoassociation. It show that the most sensitive molecules for quantum selective control of photoassociation are Be2 and Ba2 for alkaline earth dimers. These important parameters will provide a feasible scheme for further experimental researches.
Promising pathways for photoassociative formation of ultracold heteronuclear alkali metal dimers in their lowest rovibronic levels (denoted X(0,0)) are examined using high quality ab initio calculations of potential energy curves currently available. A promising pathway for KRb, involving the resonant coupling of the 2 1 Π and 1 1 Π states just below the lowest excited asymptote (K(4s)+Rb(5p1/2)), is found to occur also for RbCs and less promisingly for KCs as well. The resonant coupling of the 3 1 Σ + and 1 1 Π states, also just below the lowest excited asymptote, is found to be promising for LiNa, LiK, LiRb, and less promising for LiCs and KCs. Direct photoassociation to the 11Π state near dissociation appears promising in the final dimers, NaK, NaRb, and NaCs, although detuning more than 100 cm−1 below the Research with ultracold atoms has been an amazingly fruitful area of frontier
Promising pathways for photoassociative formation of ultracold heteronuclear alkali metal dimers in their lowest rovibronic levels (denoted X(0,0)) are examined using high-quality ab initio calculations of potential energy curves currently available. A promising pathway for KRb, involving the resonant coupling of the 2(1)Pi and 1(1)Pi states just below the lowest excited asymptote (K(4s) + Rb(5p(1/2))), is found to occur also for RbCs and less promisingly for KCs also. The resonant coupling of the 3(1)Sigma(+) and 1(1)Pi states, also just below the lowest excited asymptote, is found to be promising for LiNa, LiK, and LiRb and less promising for LiCs and KCs. Direct photoassociation to the 1(1)Pi state near dissociation appears promising in the final dimers NaK, NaRb, and NaCs, although detuning more than 100 cm(-1) below the lowest excited asymptote may be required.
This paper aims at studying the time-dependent effects involved in the photoassociation reaction for a sample of cold alkali-metal atoms, within a two-channel model where the vibrational motion in the excited state is coupled by laser light to the continuum state describing two colliding atoms in the lowest triplet electronic state $(a{}^{3}{\ensuremath{\Sigma}}_{u}^{+}).$ Both photodissociation and photoassociation processes are considered at a time scale shorter than the radiative lifetime, so that spontaneous emission does not have to be considered. The characteristic times are then the vibrational period in the excited state, which for alkali-metal dimers can be estimated of the order of a few hundreds of picoseconds, and the Rabi period, depending upon the laser intensity. Numerical calculations using wave-packet propagation are performed for the coupling of the vibrational motion in the Cs21g(6s+6p3/2) and ${a}^{3}{\ensuremath{\Sigma}}_{u}^{+}(6s+6s)$ channels by a cw laser slightly red detuned relative to the D2 resonance line. The results show Rabi oscillations in the populations of the two channels during time intervals when the vibrational motion is stopped at the outer turning point. At intensities of $\ensuremath{\approx}250\mathrm{kW}{\mathrm{cm}}^{\ensuremath{-}2},$ a new characteristic time appears, a factor of 2 larger than the classical vibrational period, which corresponds to vibrational motion in the upper adiabatic potential created by the coupling. Such an effect modifies the scattering length for collisions in the lower state, and it clearly opens a possibility of control by tuning the laser intensity.