Article pubs.acs.org/JPCA
Bichromatic Laser-Induced Quadrupole−Dipole Collisional Energy Transfer in Ca−Sr Zhenzhong Lu,* Yanling Sun, Lin Ma, and Jifang Liu School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China ABSTRACT: We consider the response of a laser-induced quadrupole−dipole collisional system driven by a strong dressing laser field with the aim of calculating the collisional cross section of a weak inducing laser probe. The addition of a second driving field to the traditional arrangement will cause magnitude changes of the spectra and modify the profile. The calculation results show that the bichromatic laser-induced collisional energy-transfer process can be an efficient way to probe Stark splitting of both the final state and intermediate state. The magnitude and position of the splitting spectral lines are strongly dependent on the intensity of the dressing laser field. The peak cross section almost reduces by a factor of 2 with the presence of the dressing laser. Also, in the antistatic wing, bright and dark lines are periodic, appearing with the increasing of the dressing laser intensity.
1. INTRODUCTION The particle collision energy-transfer process has been the hot topic for many years. However, the energy-transfer process is severely limited by the energy defect between the collisional particles, especially in the low-collisioanl-energy regime. The laser-induced collision process can overcome the energy defect factor, and the cross section of the collision process will be increased greatly. The reaction of a laser-induced energytransfer process (LICET) can be expressed by eq 1 A*(i) + B(i′) + ℏω → A(f) + B**(f′)
2. THEORY Figure 1 shows the bichromatic LICET process between two different atoms.
(1) 1−5
The laser-induced collision was proposed in the 1970s, and researchers have made many experimental and theoretical studies.6−12 In the past 40 years, research on LICET mainly is the study of the variation of the cross section with the inducing laser frequency. The energy-transfer process between different atoms is realized by a monochromatic laser field.13−20 In this paper, we added a strong dressing laser field to the traditional monochromatic LICET configuration. During the collision between two different atoms, a strong dressing laser field is used to drive the ac Stark splitting of the intermediate state and the final state, and a weak inducing laser field is used to induce the interparticle transition, with the result that the excitation profile will be modified greatly. So far, experimental results of LICET are very limited, which is due to the complexity of the problem itself. In the present collision process, the addition of the dressing laser field not only can be used to study the ac Stark effect of atomic levels but also can be used to realize the efficient population transfer, which makes this theoretical system very promising for experimental verification. Moreover, our study indicates that the bichromatic laserinduced collision configuration can be an efficient way to probe the ac Stark effect of the nonradiative excited state. © 2015 American Chemical Society
Figure 1. Coordinate system formed by two colliding atoms.
In the collisional energy-transfer process, the projectile is a straight line R(t) = (b2 + v2t2)1/2, with collisional velocity v and impact parameter b. The bichromatic LICET process includes two laser field interactions and the multipole interaction, and assuming that the E(t) = E0ay cos(ωt) and Ed(t) = Eday cos(ωdt) are the inducing laser field and dressing laser field with frequency ω and ωd, respectively, the interaction Hamiltonian takes the form Received: January 23, 2015 Published: February 25, 2015 1957
DOI: 10.1021/acs.jpca.5b00722 J. Phys. Chem. A 2015, 119, 1957−1963
Article
The Journal of Physical Chemistry A
where the collisional interaction is VAB = ∑j=3 VjR(t)−j, with
H(t ) = −eyA E0 cos ωt − eyB E0 cos ωt − eyA Ed cos ωdt − eyB Ed cos ωdt + VAB
Vj =
=
(2)
⎡ ⎤1/2 (2j − 2)! j−l−1 − ( 1) ⎢ ⎥ C(j − l − 1, l , j − 1; m , −m , 0)Q lm(A)Q j−−ml − 1(B) ∑ ∑ − − ! ! (2 j 2 l 2) (2 l ) ⎣ ⎦ l = 1 m =−1 < j−2
l
