Assessment of a Rubidium ESFADOF Edge-Filter as ... - tuprints

120 5 Discussion of the Experimental Results
• Energy pooling:
Rb(5S 1/2 )+hν P → Rb * (5P 3/2 ) (5.11)
Rb * (5P 3/2 )+Rb * (5P 3/2 )+∆E → Rb ** (6P 1/2,3/2 )+Rb(5S 1/2 ),
∆E = -0.2118 eV (5.12)
Rb * (5P 3/2 )+Rb * (5P 3/2 )+∆E → Rb ** (5D 3/2,5/2 )+Rb(5S 1/2 ),
∆E = 0.0283 eV (5.13)
Rb * (5P 3/2 )+Rb * (5P 3/2 )+∆E → Rb ** (6S 1/2 )+Rb(5S 1/2 ),
∆E = 0.1034 eV (5.14)
Negative values for ∆E represent an energy excess, whereas positive values stand
for energy defects. The energy gap is compensated by the thermal bath, as the
mean kinetic energy of the Rb atoms, 3 2kT, lies between 55–61 meV for the vapor
temperatures of interest, T ESFADOF = 150 − 200 ◦ C. Ban et al. investigated the
emitted spectral intensity for different pump intensities [201]. They found, that
in particular the blue fluorescence line 4 (420.2 nm, cf. Fig. 5.17 and Tab. 5.1)
exhibits a quadratic dependency on the injected pump intensity. From that observation
they conclude that the energy-pooling process is the dominant population
mechanism of the Rb(6P 3/2 ) state (marked by a broken arrow in Fig. 5.18). Additionally,
their quoted laser induced fluorescence spectrum (Fig. 3 in Ref. [201])
does not show the same pronounced spectral lines emerging from high lying excited
states. Thus, they operate below the plasma formation threshold. Their
far detuned pump laser, which operates at 782 nm and the presence of additional
buffer gas (52 Torr of Argon) corroborate this observation. Hence, they
exclude three photon ionization and subsequent recombination as the effective
population mechanism for the Rb(6P 3/2 ) level. This observation agrees well with
Fig. 5.17b, which has been recorded below the plasma formation threshold. There,
a pronounced peak at 420 nm confirms the population of the Rb(6P) levels by
the energy-pooling process. The absence of the majority of all other transition
lines, which mainly emerge from high lying Rydberg-states, discard ionization
processes and subsequent recombination into Rydberg-states.
The case changes dramatically as soon as the ESFADOF spectra collapse (cf.
Fig. 5.17a). A large amount of transition lines appear. They emerge from high
lying Rydberg-states. In particular line 7 starts from the 11S 1/2 state, which lies
only 0.2195 eV below the Rb ionization limit. As the above discussion showed,
these high lying states can not be populated by the energy-pooling process. Thus,
only ionization processes and subsequent recombination into Rydberg-states is
able to explain the fluorescence spectrum of Fig. 5.17a. In addition, the entire
vapor cell volume shows similar emission spectra, as soon as the breakdown of
the ESFADOF spectra occurs. In conclusion, a plasma is induced and maintained
by the pump laser, once the mentioned thresholds have been exceeded [189, 190].
These observations agree well with the observations of Saha et al., which characterized
the violet emission from heavily optically pumped Rb vapor [200].