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Physical Review A

N2 - and CO-induced collisional broadening of forbidden ground-to-Rydberg-state transitions in sodium as measured by trilevel echoes

Ravinder Kachru, THOMAS W. MOSSBERG, Sven R. Hartmann

Published Nov 1, 1980• 9 citations
Publisher

Abstract

We report the first study of molecular-perturber-induced broadening of forbidden ground-to-Rydberg-state transitions. The ${\mathrm{N}}_{2}$- and CO-induced-broadening cross sections for the $3S\ensuremath{-}\mathrm{nS} (\mathrm{nD})$ transitions of sodium have been measured for values of $n$, the principal quantum number of the upper state, ranging from 4 to 41. These measurements, performed using the trilevel echo technique, were accomplished at sodium and perturber densities of ${10}^{\ensuremath{-}6}$ and 1 torr, respectively. The broadening rates, observed in the present experiment, differ in some respects from those observed previously with noble-gas perturbers. These differences, which presumably arise because of the presence of accessible internal degrees of freedom in the ${\mathrm{N}}_{2}$ and CO perturbers (and in the case of CO also because of its permanent electric-dipole moment), are most clearly displayed in $3S\ensuremath{-}\mathrm{nS}$ transitions. Previous work has shown that with noble-gas perturbers the $3S\ensuremath{-}\mathrm{nS}$ broadening cross sections display a dramatic increase at low values of $n$; they then decline at intermediate values of $n$, and at high $n$ they approach a limiting value. In the present experiment we find that the initial increase in $3S\ensuremath{-}\mathrm{nS}$ broadening cross sections with $n$ is relatively small and more gradual. For high $n$, the broadening cross sections become approximately constant. As is the case with noble-gas perturbers, the broadening of $3S\ensuremath{-}\mathrm{nD}$ transitions is generally larger than the broadening of the corresponding $3S\ensuremath{-}\mathrm{nS}$ transitions. With CO as a perturber, an unexplained irreproducibility in the broadening cross sections of high-$n$ $3S\ensuremath{-}\mathrm{nD}$ transitions is observed. One explanation has to do with the CO-induced frequency shifts being unusually large and exceeding the spectral width of our excitation lasers.

DOI
10.1103/physreva.22.1953

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Level broadening cross sections for nP1/2Rydberg states of Na, K and Rb atoms

Leszek Sirko, K. Rosiński

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Journal of Physics B Atomic Molecular and Optical Physics•Jul 28, 1988•0 citations
We report the first study of molecular-perturber-induced broadening of forbidden ground-to-Rydberg-state transitions. The N2{\mathrm{N}}_{2}N2​- and CO-induced-broadening cross sections for the 3S\ensuremath−nS(nD)3S\ensuremath{-}\mathrm{nS} (\mathrm{nD})3S\ensuremath−nS(nD) transitions of sodium have been measured for values of nnn, the principal quantum number of the upper state, ranging from 4 to 41. These measurements, performed using the trilevel echo technique, were accomplished at sodium and perturber densities of 10\ensuremath−6{10}^{\ensuremath{-}6}10\ensuremath−6 and 1 torr, respectively. The broadening rates, observed in the present experiment, differ in some respects from those observed previously with noble-gas perturbers. These differences, which presumably arise because of the presence of accessible internal degrees of freedom in the N2{\mathrm{N}}_{2}N2​ and CO perturbers (and in the case of CO also because of its permanent electric-dipole moment), are most clearly displayed in 3S\ensuremath−nS3S\ensuremath{-}\mathrm{nS}3S\ensuremath−nS transitions. Previous work has shown that with noble-gas perturbers the 3S\ensuremath−nS3S\ensuremath{-}\mathrm{nS}3S\ensuremath−nS broadening cross sections display a dramatic increase at low values of nnn; they then decline at intermediate values of nnn, and at high nnn they approach a limiting value. In the present experiment we find that the initial increase in 3S\ensuremath−nS3S\ensuremath{-}\mathrm{nS}3S\ensuremath−nS broadening cross sections with nnn is relatively small and more gradual. For high nnn, the broadening cross sections become approximately constant. As is the case with noble-gas perturbers, the broadening of 3S\ensuremath−nD3S\ensuremath{-}\mathrm{nD}3S\ensuremath−nD transitions is generally larger than the broadening of the corresponding 3S\ensuremath−nS3S\ensuremath{-}\mathrm{nS}3S\ensuremath−nS transitions. With CO as a perturber, an unexplained irreproducibility in the broadening cross sections of high-nnn 3S\ensuremath−nD3S\ensuremath{-}\mathrm{nD}3S\ensuremath−nD transitions is observed. One explanation has to do with the CO-induced frequency shifts being unusually large and exceeding the spectral width of our excitation lasers.
We have made comprehensive measurements of the noble-gas-induced (He, Ne, Ar, Kr, and Xe) broadening cross sections of the Na $3S\ensuremath{-}nS$ ($3S\ensuremath{-}nD$) transitions, from n=5to38n=5 \mathrm{to} 38n=5to38 (n=4to40n=4 \mathrm{to} 40n=4to40). These measurements were made using the trilevel-echo technique and are an extension and refinement of our earlier work. We find that the cross sections increase as the geometric cross section at low values of nnn, and then, at a value of nnn which depends on the particular noble gas, begin to decrease and eventually reach an asymptotic value at high nnn. The broadening cross sections for the nearly degenerate DDD states are significantly larger than those for the SSS states and clearly show the effect of lll-changing collisions. We also compare our measured SSS-state broadening cross sections with the cross sections calculated using the theory of Omont and find good qualitative agreement.
A variant of the trilevel-echo effect is observed and utilized to measure the effective cross section for broadening of the $3{P}_{\frac{1}{2}}\ensuremath{-}3{P}_{\frac{3}{2}}$ transition of sodium by the noble gases. The cross section measured here should be the same as the broadening cross section obtained from a direct measurement of the collisionally broadened $3{P}_{\frac{1}{2}}\ensuremath{-}3{P}_{\frac{3}{2}}$ transition linewidth (if such a measurement were possible). The new echo, the "inverted-difference-frequency" (IDF) trilevel echo, is well suited to the study of transitions between excited states of the same parity. At 400 K the measured broadening cross sections are He 115(12) A˚2{\mathrm{\AA{}}}^{2}A˚2, Ne 120(12) A˚2{\mathrm{\AA{}}}^{2}A˚2, Ar 234(23) A˚2{\mathrm{\AA{}}}^{2}A˚2, Kr 266(27) A˚2{\mathrm{\AA{}}}^{2}A˚2, and Xe 311(31) A˚2{\mathrm{\AA{}}}^{2}A˚2. With He as the perturber, the cross section for broadening of the $3{P}_{\frac{1}{2}}\ensuremath{-}3{P}_{\frac{3}{2}}$ transition can be calculated from measured depolarization and fine-structure-changing collision cross sections. With the other perturbers, however, collisional phase changes appear to be important. An intuitive diagrammatic technique for the analysis of echoes is applied to the IDF trilevel echo.
We have measured the He-, Ne-, and Ar-induced relaxation properties of the $3S\ensuremath{-}nS$ ($3S\ensuremath{-}nD$) superposition states of atomic sodium, where nnn ranges from 5 to 20 (4 to 34). The measurements were made using the trilevel echo effect. We find that the effective collisional cross section first increases (as a function of nnn) at low nnn, and then decreases to a lower asymptotic limit at high nnn. This behavior is similar to that previously observed for the $3S\ensuremath{-}nP$ series. We also find that the difference between the collisional cross sections for SSS and DDD series superpositions closely approximates the inelastic collision cross sections measured elsewhere for the ddd states. A theoretical description of the trilevel echo effect used to make the measurements is presented.
The impact broadening of Na(nP1/2) (n=6-24), K(nP1/2) (n=8-24) and Rb(nP12/) (n=10-24) levels by noble-gas atoms, N2 molecules and alkali atoms in the ground state has been investigated. Broadening cross sections have been evaluated on the grounds of contributions coming from Rydberg-electron-perturber and ionic core-perturber scattering. Broadening cross sections for Cs(nD3/2)+Cs(6S) collisions for n=11-23 have also been calculated to compare with the experimental ones. Theoretically evaluated broadening cross sections for K(nP12/)+K(4S) and Cs(nD3/2)+Cs(6S) collisions appeared to be in good agreement with existing experimental data.