The effect of excited-state mixing on the optical pumping of alkali-metal vapors is considered for the case where the energy separation between the
2P21 and
2P23 first excited states is relatively large, allowing the isolation of mixing effects within a particular
J level. Two models for the mixing process are proposed, one a random reorientation of the total electronic angular momentum J, and the other a reorientation of J subject to the selection rule $\ensuremath{\Delta}{m}_{J}=0, \ifmmode\pm\else\textpm\fi{}2$. The probabilities for mixing transitions among
J-state sublevels have been calculated for an alkali atom of nuclear spin
23, and can be used to calculate the optical-pumping transition probabilities for a vapor subject to any degree of excited-state mixing. Several possible experiments suggested by these calculations are discussed.
Collisions of alkali-metal atoms in the ground state with the electron spin S = 1/2 are accompanied by exchange of electron coordinates between colliding particles, which leads to polarization transfer between them (i.e., to the well-known phenomenon of spin exchange). In addition, along with the polarization transfer from one partner to another, the magnetic resonance lines of colliding atoms are broadened and shifted in spin-exchange collisions. In particular the last two processes depend on the complex spin-exchange cross section. The real part of the cross section determines the so-called "spin-exchange cross section," which is responsible for the broadening of magnetic resonance lines, while the imaginary part determines the shift cross section, which governs the frequency shift of magnetic resonance. In order to describe the spin-exchange process one have to know complex spin-exchange cross sections. Complex spin-exchange cross sections are calculated on the basis of the data on the singlet (X 1 Σ + ) and triplet (a 3 Σ + ) potentials describing the interaction alkali-metal atoms in the ground state. Collisions of alkali-metal atoms Cs, Rb, K, Na in the ground state are considered in the energy interval of 10 −4 −10 −2 au.
Excitation transfer between the
62P21 and
62P23 resonance states in cesium, induced by Cs-
N2 and Cs-
H2 collisions, was observed in the first experiment of its kind, in which techniques of sensitized fluorescence are used to investigate collisional mixing between resonance states of alkali atoms in mixtures with molecular gases. The cesium-vapor-gas system, in which the cesium vapor pressure was low enough to avoid radiation trapping, was irradiated with a single component of the resonance doublet. The fluorescent light, which contained both components of the doublet, was monitored at right angles to the direction of excitation. Measurements of relative intensities of the fluorescent components yielded the following cross sections for excitation transfer and for quenching of resonance radiation. For Cs-
N2 collisions: ${Q}_{12}(^{2}P_{\frac{1}{2}}\ensuremath{\rightarrow}^{2}P_{\frac{3}{2}})=3.6$
A˚2, ${Q}_{21}(^{2}P_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=16.2$
A˚2, ${Q}_{10}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{1}{2}})=55.2$
A˚2, ${Q}_{20}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=63.5$
A˚2. For Cs-
H2 collisions:
Q12=6.7 A˚2,
Q21=29.6 A˚2,
Q10=11.7 A˚2,
Q20=6.7 A˚2. The large sizes of the excitation-transfer cross sections, which are comparable to the corresponding Cs-Cs cross sections, are attributed to resonances with transitions in the molecules.
Na atoms in the magnetic field have been selectively excited to each Zeeman sub-level of either the 3p2P1/2 or 3p 2P3/2 state by a tunable dye laser and the Zeeman spectra of the fluorescence from the collisional populated 3p 2P levels have been measured. From the intensities of the Zeeman lines, the selectivity in the collisional transitions between the Zeeman sub-levels of the 3p 2P1/2 and 3p 2P3/2 states has been directly specified. The time evolution of the σ-polarized fluorescence intensity of the Na(3p 2P1/2→ 3s 2S1/2) transition has been measured following the Na(3p 2P3/2â†� 3s 2S1/2) excitation by a σ-polarized short light pulse at H= 129 G. An amplitude modulation was found in the fluorescence from the collisional energy transferred state Na(3p 2P1/2). This is the first observation of quantum beats in collision-induced fluorescence. The frequency of the modulation coincides very well with that of the quantum beats of the resonance fluorescence Na(3p 2P3/2→ 3s 2S1/2), but the phases are opposite. These results are explained by assuming the conservation of coherence during collisional energy transfer.
The distribution of population in the ground-state sublevels of an optically pumped alkali-metal vapor has been found to be strongly dependent on the mechanism assumed responsible for relaxation. Two modes of optical pumping zero mixing and complete mixing in the excited state, and three modes of alkali relaxation, uniform, Zeeman, and electron randomization, are considered. Calculations of the sublevel relative populations are made in terms of experimentally measurable parameters for all six combinations of pumping and relaxation modes. Optical-pumping transient signals are then used to demonstrate that alkali relaxation due to wall collisions is of the low-frequency "Zeeman" variety, while relaxation due to alkali-rare-gas collisions takes place through randomization of the spin of the alkali valence electron. Equations for the rate of change of the electronic spin polarization are derived for all cases, and are found to be nonexponential, in contrast with the case of an alkali of zero nuclear spin.
Sensitized fluorescence in rubidium vapor has been investigated in order to determine total cross sections for collisions of the second kind between rubidium atoms, leading to the transfer of excitation between the 5 2P1/2 and 5 2P3/2 resonance levels. The experiments were carried out at vapor pressures below 10−5 mm Hg, where there is virtually no imprisonment of resonance radiation. The exceedingly low fluorescent intensities were registered in an automatically programmed sequence of measurements, using photon counting techniques. The cross sections Q1(5 2P1/2 → 52P3/2) and Q2(5 2P1/2 ← 5 2P3/2) equal 53 Å2 and 68 Å2 respectively at 87 °C and 60 Å2 and 72 Å2 respectively at 107 °C. At both temperatures the ratios Q1/Q2 are in agreement with the values predicted by the principle of detailed balancing.