Collision radiative

Specifically, the collision-induced absorption and emission coefficients for electric-dipole forbidden atomic transitions were calculated for weak radiation fields and photon energies Ha> near the atomic transition frequencies, utilizing the concepts and methods of the traditional theory of line shapes for dipole-allowed transitions. The example of the S-D transition induced by a spherically symmetric perturber (e.g., a rare gas atom) is treated in detail and compared with measurements. The case of the radiative collision, i.e., a collision in which both colliding atoms change their state, was also considered. 

Fig. 15.1 (a) Energy levels and dipole matrix elements for the resonant collision of two atoms in the s and s states resulting in the production of two atoms in the p and p states, (b) Energy levels and matrix elements for the radiative collision in which an s and an s atom collide to produce atoms in the p and d states. The production of the d state is via the virtual p state which is detuned from the real p state by an energy A. 

Evaluating Eq. (15.4) at n = 20 leads to a field of 50 V/cm, or a power of 6 W/cm2, a power roughly six orders of magnitude lower than those used in optical radiative collision experiments with low lying atomic states.4"7 These powers are not only readily obtained, but are easily exceeded by orders of magnitude, so that... 

The Na ns + Na ns — Na np + Na (n - l)p collisions in combined static and microwave fields were the first Rydberg atom radiative collisions studied. Specifically, the process... 

The experimental data shown in Figs. 15.3 and 15.5 were obtained with a microwave frequency oj/2ji > lit where r is the time or duration of the collision and 1/r is the linewidth. In this case the resonances corresponding to the absorption or emission of different numbers of photons are resolved. Here we describe radiative collisions starting from the high frequency regime, oj/2ji > 1/r and progressing to the low frequency regime, a> 2n < 1/r. 

Fig. 15.6 Energy levels of the Na 17p, 18s, and 18p states showing the first upper and lower sideband states of the p states. The numbers +2,..., -2 refer to the net number of photons emitted in a radiative collision at these tunings of the field. Note that there are several processes which lead to the net emission of, for example, zero photons (from ref. 3).

All radiative collision experiments with Rydberg atoms have been done with the collision velocity perpendicular to the static and microwave fields. Thus the results obtained are an average over spatial orientations, and it makes little sense to use a detailed model of the interaction. Accordingly, we assume that... 

A radiative collision with the emission of no photons in the absence of a microwave field is simply a resonant collision. Since 7o(0) = 1, we can express the radiative collision cross section of Eq. (15.28) as... 

Fig. 15.7 Collisions in the presence of a low frequency microwave field (a) the resonant collision profile without the microwaves (b) one period of microwave field showing both the amplitude and the time weighting of the extreme field values (c) typical time weighted radiative collision lineshape when the microwave period becomes long compared to the duration of one collision (from ref. 3).

Fig. 15.10 Cross sections as a function of rf field strength for the first four orders of sideband resonances of the K 29s + K 27d radiative collisions in a 4 MHz rf field, (a) The zero-photon resonant collision cross section, (b) the +1 sideband resonance, (c) the —2 sideband resonance, and (d) the +3 sideband resonance. The solid line shows the experimental data, the bold line indicates the prediction the Floquet theory, and the dashed fine is the result of numerical integration of the transition probability (from ref. 17).

R. Buffa, Laser-induced resonances in ionizing radiative collisions. Opt. Commun. 128 (1996) 30. 

V. S. Dubov, L. I. Gudzerko, L. V, Gurvich, and S. I. lakovlenko. Experimental detection of chemical radiative collisions, Chem. Phys. Lett. 53 170 (1978) and earlier work cited therein. 

---