We have observed resonant-energy-transfer collisions between K Rydberg atoms. We have tuned the
ns1/2+(n-2)
dj\ensuremath{\rightarrow}${\mathrm{np}}_{\mathrm{j}\ensuremath{'}}$+(n-1)${\mathrm{p}}_{\mathrm{j}\mathrm{\ensuremath{''}}}$ collisional process into resonance with a static electric field. The measured resonance field positions from n=24 to 29 agree well with the calculated positions. We show that the dipole-dipole selection rule, \ensuremath{\Delta}
mj=0,\ifmmode\pm\else\textpm\fi{}1, is valid for these resonances but that \ensuremath{\Delta}J=0,\ifmmode\pm\else\textpm\fi{}1 is not, due to the presence of the electric field. In addition, the polarization of the lasers used to excite the Rydberg states can suppress many of the resonances.
We have observed the sharply resonant enhancement of the cross section of the collision process $ns+ns\ensuremath{\rightarrow}np+(n\ensuremath{-}1)p$ between two
Nans (
16< n< 27) atoms which occurs when the levels are shifted by an electric field so that the
ns level lies midway between the two
p states. The widths of the collisional resonances scale as ${n}^{\ensuremath{-}2}$ and the magnitudes of the cross sections as
n4, with values of 0.5 GHz and 8\ifmmode\times\else\texttimes\fi{}
108 A˚2 for the
20s state. These results are in good agreement with calculations based on the long-range dipole-dipole interaction between the two atoms.
The experimental investigation of the resonant collision process $\mathrm{Na}ns+\mathrm{Na}\mathrm{ns}\ensuremath{\rightarrow}\mathrm{Na}np+\mathrm{Na}(n\ensuremath{-}1)p$ which occurs when the levels are tuned with an electric field so that the
ns level lies midway between the two
p levels is described in detail. The observations are shown to be in good agreement with a long-range resonant dipole-dipole interaction.
The effects of weak electric fields on resonant energy transfer between
NH3 in the
X 1A1 ground electronic state and Rydberg He atoms in triplet states with principal quantum numbers
n=36--41 have been studied in a crossed-beam apparatus. For these values of
n, electric dipole transitions between the Rydberg states that evolve adiabatically to the $|ns\ensuremath{\rangle}$ and $|np\ensuremath{\rangle}$ states in zero electric field can be tuned into resonance with the ground-state inversion transitions in
NH3 using electric fields, with energy transfer occurring via F\"orster resonance. In the experiments the Rydberg He atoms, traveling in pulsed supersonic beams, were prepared by resonant two-photon excitation from the metastable
1s2s3S1 level and crossed an effusive beam of
NH3 before being detected by state-selective pulsed-electric-field ionization. The resonant-energy-transfer process was identified by monitoring changes in the ionization signal from the $|ns\ensuremath{\rangle}$ and $|np\ensuremath{\rangle}$ Rydberg states for each value of
n. The electric-field dependence of the experimental data is in good agreement with the results of calculations in which the resonant dipole-dipole coupling between the collision partners was accounted for.
We have developed a simple analytical model describing multiatom signals that are measured in experiments on dipole-dipole interaction at resonant collisions of a few Rydberg atoms. It has been shown that finite efficiency of the selective field-ionization detector leads to the mixing up of the spectra of resonant collisions registered for various numbers of Rydberg atoms. The formulas which help to estimate an appropriate mean Rydberg atom number for a given detection efficiency are presented. We have found that a measurement of the relation between the amplitudes of collisional resonances observed in the one- and two-atom signals provides a straightforward determination of the absolute detection efficiency and mean Rydberg atom number. We also performed a testing experiment on resonant collisions in a small excitation volume of a sodium atomic beam. The resonances observed for 1--4 detected Rydberg atoms have been analyzed and compared with theory.
We have studied the collisional energy transfer process K (ns) +K (n-2) d {yields} K (np) +K (n-1) p which is tuned into resonance with an applied static electric field. The initial Rydberg states are excited stepwise with two dye laser pulses. The resonant energy transfer is detected by selectively field ionizing the final K(np) state as the static field is swept over many laser shots. Due to fine structure splitting of the d and p levels 45 resonances are energetically allowed at n{sub s}=25. The actual number of resonances that are observed depends on the polarization of the laser light, the dipole-dipole selection rules, and the n state excited. Above n{sub s}=29 all resonances are energically forbidden. We have measured the position of the resonances in electric field from n{sub s}=25 to n{sub s}=29 and found them to be in good agreement with calculated positions.