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Journal of Physics B Atomic and Molecular Physics

Noble-gas broadening of the sodium D lines measured by photon echoes

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

Published Jun 28, 1980• 26 citations
Publisher

Abstract

Noble-gas-induced relaxation of photon echo signals is used to determine the broadening rates of the Na D lines. Since echo signals are 'Doppler free', precision measurements are possible at low perturber-gas pressures (<or=1 Torr), and are sensitive to the previously inaccessible central lineshape. Interestingly, the authors results disagree with other recently published broadening rates. The equivalence of linewidth measurements and echo measurements on transitions between states of low electronic angular momentum is discussed.

DOI
10.1088/0022-3700/13/12/002

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AIP conference proceedings•Jan 1, 1986•1 citation
Noble-gas-induced relaxation of photon echo signals is used to determine the broadening rates of the Na D lines. Since echo signals are 'Doppler free', precision measurements are possible at low perturber-gas pressures (<or=1 Torr), and are sensitive to the previously inaccessible central lineshape. Interestingly, the authors results disagree with other recently published broadening rates. The equivalence of linewidth measurements and echo measurements on transitions between states of low electronic angular momentum is discussed.
Photon echoes have been employed to measure the relative broadening rates of the lithium DDD lines perturbed by the noble gases. We find that, for each noble-gas perturber, the D1{D}_{1}D1​-line and D2{D}_{2}D2​-line photon-echo-relaxation rates are identical. Our results are compared with those obtained in line-shape studies of pressure broadening.
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 $3S\ensuremath{-}\mathrm{nS} (\mathrm{nD})$ 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 ${10}^{\ensuremath{-}6}$ 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 $3S\ensuremath{-}\mathrm{nS}$ 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 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 $3S\ensuremath{-}\mathrm{nS}$ 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 $3S\ensuremath{-}\mathrm{nD}$ 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-nnn $3S\ensuremath{-}\mathrm{nD}$ 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.
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 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.