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Physical Review

Anomalous Population Distributions in an Optically Excited Metastable Level in Ca F2 : Tm2+

Charles H. Anderson, Edward S. Sabisky

Published Feb 10, 1969• 20 citations
Publisher

Abstract

The population distribution among the magnetic sublevels of the metastable state of Ca${\mathrm{F}}_{2}$: ${\mathrm{Tm}}^{2+}$ is shown to be strongly influenced by spin orientation memory from the ground state. This memory is dependent on the wavelength and polarization of the pump radiation, being quite strong at 5790 \AA{}. The population is also influenced by an optical cross-relaxation or excitation transfer between neighboring ions, particularly so because the hyperfine splittings in the ground and excited states are almost identical. Also, it is shown that there is a high degree of nuclear spin orientation memory.

DOI
10.1103/physrev.178.547

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A crystal of Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ was operated as a pulsed optical maser at temperatures close to 4.2\ifmmode^\circ\else\textdegree\fi{}K. The 6969 \AA{} output corresponds to the wavelength of an exceedingly sharp line ($\ensuremath{\Delta}\ensuremath{\nu}<0.6$ ${\mathrm{cm}}^{\ensuremath{-}1}$) in which almost all of the low-temperature fluorescence is concentrated. In contrast to what prevails in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{+2}$ masers, the output is characterized by strong relaxation oscillations. This may be explained by the large difference in the fluorescent lifetimes in the two systems. While in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ the value of $\ensuremath{\tau}$ is approximately 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ sec, in Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ it has a value of 15 msec. The reason for the large difference in $\ensuremath{\tau}$ values, in turn, depends upon the relative proximity of the long-wavelength edge of the red band to the position of the metastable $^{5}D_{0}$ level from which fluorescence occurs. Absorption measurements taken at helium temperatures on Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ show that the lowest vibrational level of the lowest lying $4f\ensuremath{-}5d$ band lies \ensuremath{\sim}600 ${\mathrm{cm}}^{\ensuremath{-}1}$ above the position of the $^{5}D_{0}$ level while in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ there is near coincidence of the corresponding levels and consequently strong configurational mixing. Crystals of Sr${\mathrm{Cl}}_{2}$: ${\mathrm{Sm}}^{2+}$ were also studied in this regard and display an expected close similarity in spectral characteristics to Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$. The vibrational structure on the $4f\ensuremath{-}5d$ bands is much more pronounced, however, than in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ and Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$, indicating weaker coupling to the lattice in the excited $5d$ state. A tentative interpretation for the splitting of the red band seen in the absorption spectra of ${\mathrm{Sm}}^{2+}$ ions is given.

Physical Review•Jul 15, 1962•66 citations
The population distribution among the magnetic sublevels of the metastable state of CaF2{\mathrm{F}}_{2}F2​: Tm2+{\mathrm{Tm}}^{2+}Tm2+ is shown to be strongly influenced by spin orientation memory from the ground state. This memory is dependent on the wavelength and polarization of the pump radiation, being quite strong at 5790 \AA{}. The population is also influenced by an optical cross-relaxation or excitation transfer between neighboring ions, particularly so because the hyperfine splittings in the ground and excited states are almost identical. Also, it is shown that there is a high degree of nuclear spin orientation memory.
ESR signals of Tm2+{\mathrm{Tm}}^{2+}Tm2+:SrF2{\mathrm{SrF}}_{2}SrF2​ are observed in the ground (2{(}^{2}(2F7/2{\mathrm{F}}_{7/2}F7/2​,E5/2{E}_{5/2}E5/2​) and metastable (2{(}^{2}(2F5/2{\mathrm{F}}_{5/2}F5/2​,E5/2{E}_{5/2}E5/2​) states in low magnetic fields (0--300 Oe) at liquid-helium temperatures. We achieved a very high detection sensitivity (\ensuremath{\sim}109{10}^{9}109 spins in the ground state) using optical means; the population differences are created by circular dichroism and spin memory, and the change of the Faraday rotation is detected by a polarimeter. Several interesting phenomena were observed. The signals in the ground and metastable states appeared with opposite polarities. The preferential pumping from the metastable state was examined by a double pumping experiment. We also measured the Faraday rotation between 4000 and 7000 A\r{}. The experimental results suggest that the magnetization in the metastable state created by the preferential pumping from the ground state followed by the spin memory is in the opposite direction to that in the ground state. Anomalous behavior of the metastable-state signals was observed both in frequency and time domains. These are explained as an effect of the metastable-state--ground-state cross relaxation enhanced by the ground-state--ground-state cross relaxation.
Nuclear-spin states of gaseous-state Cs atoms in the ground state are optically manipulated using a Ti:sapphire laser in a magnetic field of 1.516 T, in which optical coupling of the nuclear-spin states is achieved through hyperfine interactions between electrons and nuclei. The steady-state population distribution in the hyperfine Zeeman sublevels of the ground state is detected by using a tunable diode laser. Furthermore, the state population transfer among the hyperfine Zeeman sublevels, which results from the collision-induced modification δ a ( S · I ) of the hyperfine interaction of Cs in the ground state due to stochastic collisions between Cs atoms and buffer-gas molecules, is studied at different buffer-gas pressures. The experimental results show that high-field optical pumping and the small change δ a ( S · I ) of the hyperfine interaction can strongly cause the state population transfer and spin-state interchange among the hyperfine Zeeman sublevels. The calculated results maybe explain the steady-state population in hyperfine Zeeman sublevels in terms of rates of optical-pumping, electron-spin flip, nuclear spin flip, and electron-nuclear spin flip-flop transitions among the hyperfine Zeeman sublevels of the ground state of Cs atoms. This method may be applied to the nuclear-spin-based solid-state quantum computation.
Spectral hole burning on the $4{f}^{7}(^{8}S_{\frac{7}{2}})\ensuremath{\rightarrow}4{f}^{6}5d$ transition of Eu2+{\mathrm{Eu}}^{2+}Eu2+ is observed to occur by two mechanisms in CaF2{\mathrm{F}}_{2}F2​:Eu2+{\mathrm{Eu}}^{2+}Eu2+. At zero magnetic field, persistent spectral hole burning occurs by two-step photoionization where the hole burning survives cycling to room temperature. The observed hole spectrum is compared with a calculation which considers the small Eu2+{\mathrm{Eu}}^{2+}Eu2+ ground-state splitting and both the ground- and excited-state hyperfine interactions. In a magnetic field above 1 T, transient spectral hole burning occurs by population redistribution among the ground-state hyperfine levels. Long-lived (minutes) holes are observed for a half integral spin system. The resulting hole spectrum consists of holes, antiholes, and a nuclear spin-flip sideband and can be explained from the hyperfine interactions in the ground and excited states. The dominant hole decay occurs by phonon-induced transitions among hyperfine levels of the ${M}_{S}=\ensuremath{-}\frac{7}{2}$ and ${M}_{S}=\ensuremath{-}\frac{5}{2}$ electron-spin sublevels. Central hole linewidths of 200 MHz are observed at zero magnetic field. Hole linewidths at high magnetic fields are as narrow as 40 MHz but these are inhomogenously broadened by superhyperfine interactions with the FFF nuclear spins. Estimates of hole-burning quantum efficiency are also obtained.
We have measured changes in the ground-state populations of Cs vapor induced by optical pumping at high magnetic field. The 2.72.72.7-T field of our experiments is strong enough to decouple the nuclear and electronic spins, allowing us to independently measure each population. The spatial dependence of the Cs populations in small amounts of buffer gas obeys a simple coupled diffusion model and the relative populations reveal the details of relaxation within the vapor cell. Optical pumping can produce high nuclear polarization in the Cs vapor due to perturbations of the hyperfine interaction during collisions with buffer-gas particles and depending on the pumping transition, radiation trapping can strongly influence the electronic and nuclear polarizations in the vapor.
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.
A crystal of SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ was operated as a pulsed optical maser at temperatures close to 4.2\ifmmode^\circ\else\textdegree\fi{}K. The 6969 \AA{} output corresponds to the wavelength of an exceedingly sharp line ($\ensuremath{\Delta}\ensuremath{\nu}<0.6$ ${\mathrm{cm}}^{\ensuremath{-}1}$) in which almost all of the low-temperature fluorescence is concentrated. In contrast to what prevails in CaF2{\mathrm{F}}_{2}F2​: Sm+2{\mathrm{Sm}}^{+2}Sm+2 masers, the output is characterized by strong relaxation oscillations. This may be explained by the large difference in the fluorescent lifetimes in the two systems. While in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ the value of $\ensuremath{\tau}$ is approximately 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ sec, in SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ it has a value of 15 msec. The reason for the large difference in $\ensuremath{\tau}$ values, in turn, depends upon the relative proximity of the long-wavelength edge of the red band to the position of the metastable 5D0^{5}D_{0}5D0​ level from which fluorescence occurs. Absorption measurements taken at helium temperatures on SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ show that the lowest vibrational level of the lowest lying $4f\ensuremath{-}5d$ band lies \ensuremath{\sim}600 ${\mathrm{cm}}^{\ensuremath{-}1}$ above the position of the 5D0^{5}D_{0}5D0​ level while in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ there is near coincidence of the corresponding levels and consequently strong configurational mixing. Crystals of SrCl2{\mathrm{Cl}}_{2}Cl2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ were also studied in this regard and display an expected close similarity in spectral characteristics to SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+. The vibrational structure on the $4f\ensuremath{-}5d$ bands is much more pronounced, however, than in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ and SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+, indicating weaker coupling to the lattice in the excited 5d5d5d state. A tentative interpretation for the splitting of the red band seen in the absorption spectra of Sm2+{\mathrm{Sm}}^{2+}Sm2+ ions is given.