Hyperfine Suppression of a $2\;^3$S$_1$ -- $3\;^3$P$_1$ Radiative Transition in Helium

Marc Smiciklas, David Charles Shiner

Chien-ping Lin

A. S. Elkhovskii, Iosif B. Khriplovich

Xiaoyan Shen, Kang Dai, Yifan Shen

The effective radiation rate for Cs (6P3/2) resonance level in the presence of helium was determined. A cell filled with Cs metal and He at pressure P(He) (0-500 Pa) was established at T = 300 K. The cell was heated in the temperature range between 330 and 370 K, which produced Cs number densities approximately in the range between 10(12) and 10(13) cm(-3). Cs atoms were excited to the 6P3/2 state using a single-mode diode laser(pump laser). The transmission of the medium at the center of the Doppler envelope of the strong h. f. component of the CsD2 line due to hyperfine pumping alone amounts to approximately 5%. The assumption that has been made is that the lower-state hyperfine levels are populated in a statistical ratio. The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode diode laser beam (probe laser) tuned to the 6P3/2 --> 8S1/2 transition which could be translated parallel to the pump beam. In the presence of radiation trapping, the spontaneous radiation rate is multiplied by the transmission factor T6P3/2 -->1/2, which describes the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed. The T6P3/2 --> 6S1/2 is related to the frequency dependent absorption cross section and the density and spatial distribution of atoms at the level of the transition. Position dependent 6P3/2 state densities were combined with the collisional broadening rate of 6P3/2 <-- 6S1/2 line due to the perturbation of both helium and cesium to yield T6P3/2 --> 6S1/2. The effective radiation rates of the Cs D2 line as a function of the He pressure P(He) were obtained. The helium caused line broadening and therefore increased the effective radiation rate. The fluorescence intensity I852 of the 6P3/2 --> 6S1/2 emission was measured simultaneously. The measured fluorescence ratios determined the ratios of the effective radiation rates at different He density. These ratios are in agreement with theoretical evaluation.

G. W. F. Drake, Jacob Manalo

GREGORY A. BREIT, Edward Teller

The mean life of the 2S state of hydrogen in the absence of collisions is primarily determined by the simultaneous emission of two photons. Owing to this cause the mean life is `-*4 sec. It is expected that the mean life of is 2S `S of He i is of the same order of magnitude and is much shorter than that of is 2S 3S of He i. The non-adiabatic collisions with electrons are found to be more important than the static effects at small electron densities, leading to a transition probability `-*-`~h sec ` for an electron temper- ature `-.--`io,ooo° C and an electron density 30 cm3. The effect of static electric fields on the 2S state of hydrogen is worked out in more detail than previously by taking into account the hyperfine structure. The mean life in static electric fields which are strong enough to make double emission of little importance and yet weak enough to give a mean life > io7 sec is found to be the sum of two lives: (i) the mean life calculated neglecting hyperfine structure and (2) the mean life calculated neglecting radiation coupling. In both (i) and (2) the mixing of 2~ into 2S by the Stark effect is taken into account. Radiations due to quadrupole and magnetic dipole effects are considered qualita- tively for is 2S 3S of helium and quantitatively for 2S of hydrogen. For the latter there exists a small radiative magnetic dipole effect on Dirac's theory of the electron which is absent in the ordinary spin model