Doppler-free optical double-resonance spectroscopy is used to study the   s p p 5 5 6 1 2 3 2 3 2 excitation sequence in room-temperature rubidium atoms. This involves a  s p 5 5 F or ±2 electric quadrupole transitions.

Direct evidence of excitation of the 5p 3/2 → 6p 3/2 electric dipole forbidden transition in atomic rubidium is presented. The experiments were performed in a room temperature rubidium cell with continuous wave extended cavity diode lasers. Optical-optical double resonance spectroscopy with counterpropagating beams allows the detection of the non-dipole transition free of Doppler broadening. The 5p 3/2 state is prepared by excitation with a laser locked to the maximum F cyclic transition of the D2 line, and the forbidden transition is produced by excitation with a 911 nm laser. Production of the forbidden transition is monitored by detection of the 420 nm fluorescence that results from decay of the 6p 3/2 state. Spectra with three narrow lines (≈ 13 MHz FWHM) with the characteristic F − 1, F and F + 1 splitting of the 6p 3/2 hyperfine structure in both rubidium isotopes were obtained. The results are in very good agreement with a direct calculation that takes into account the 5s → 5p 3/2 preparation dynamics, the 5p 3/2 → 6p 3/2 non-dipole excitation geometry and the 6p 3/2 → 5s 1/2 decay. The comparison also shows that the electric dipole forbidden transition is a very sensitive probe of the preparation dynamics.PACS numbers: 32.70. Cs,32.70.Fw While the electric dipole approximation is a cornerstone in the study of the interaction between optical radiation fields and atoms, transitions induced by optical fields beyond this approximation have also become important tools in basic and applied studies of atoms. These so called "forbidden transitions" have been traditionally used in astrophysical and plasma studies [1]. They now play a fundamental role in metrology [2] and have also been used in experiments testing parity nonconserving interactions in atoms [3].In early studies of forbidden transitions, Sayer et al. [4] determined transition probabilities of electric quadrupole (E2) transitions using a tungsten lamp. The first direct observation of electric quadrupole effects in multiphoton ionization dates back to the work of Lambropoulos et al.[5]. Electric-dipole-forbidden transitions were exploited in three-wave-mixing experiments for optical sum and difference frequency generation in [6].The use of intense continuous-wave or pulsed laser sources has facilitated the observation of weak absorption lines. For instance, Tojo et al.[7] reported a determination of the oscillator strength of a E2 transition with a temperature-controlled cell and an extended cavity diode laser. Also, the study of strongly forbidden J = 0 → J = 0 transitions via single-photon excitation is presented in [8]. Excitation of forbidden transitions involving states with nonzero angular momentum in alkali atoms have also been studied over the last few years [9][10][11][12][13]. The coherent mixing of waves is theoretically studied in [9] for n 1 2 P − n 2 2 P transitions. The excitation of the 5p → 8p forbidden transition in thermal rubidium atoms was produced with a pulsed laser in [10] and using cold atoms in [12]. The experiment with co...

We present the first evidence of excitation of the 5p 3/2 → 6p 1/2 electric dipole-forbidden transition in atomic rubidium. The experiments were carried out in a rubidium vapor cell using Doppler-free optical-optical double-resonance spectroscopy with counter-propagating beams. A 5s 1/2 → 5p 3/2 electric dipole preparation step using a diode laser locked to the F = 3 → 4 cyclic transition of the D2 line in 85 Rb is used to prepare the atoms in the first excited state. This is then followed by the 5p 3/2 F 2 = 4 → 6p 1/2 F 3 dipole-forbidden excitation (λ ≈ 917.5 nm) to establish a two-photon ladder (Ξ) excitation scheme. Production of atoms in the 6p 1/2 excited state is verified by detection of the 421 nm fluorescence that results from direct decay into the 5s 1/2 ground state.The polarization dependence of the relative intensities of the lines of the decay fluorescence is also investigated. Experimental data for different polarization configurations of the light beams used in this two-photon spectroscopy are compared with the results of calculations that consider a strong atom-field coupling in the preparation step, followed by a weak electric quadrupole excitation and the blue fluorescence decay emission. Good agreement between experiment and this three-step model is found in the case of linear-linear polarizations.

An advanced undergraduate experiment to study the 5P3/2→6P3/2 electric quadrupole transition in rubidium atoms is presented. The experiment uses two external cavity diode lasers, one operating at the D2 rubidium resonance line and the other built with commercial parts to emit at 911 nm. The lasers produce the 5s→5p→6p excitation sequence in which the second step is the forbidden transition. Production of atoms in the 6P3/2 state is observed by detection of the 420 nm fluorescence that results from electric dipole decay into the ground state. Lines whose widths are significantly narrower than the Doppler width are used to study the hyperfine structure of the 6P3/2 state in rubidium. The spectra illustrate characteristics unique to electric dipole forbidden transitions, like the electric quadrupole selection rules; they are also used to show general aspects of two-color laser spectroscopy such as velocity selection and hyperfine pumping.

Polarized velocity selective spectra for rubidium atoms in a room temperature cell are presented. The experiments were performed in the lambda configuration (D2 manifold) and in the → → 5s 5p 5d j 3 2 ladder configuration. For the lambda configuration the effect of the probe beam intensity in the absorption and polarization spectra are compared with results of a rate equation approximation. Good overall agreement between experiment and theory is found. The results indicate different saturation rates for each of the atomic transitions. Distinctive polarization signals with hyperfine-resolved components are found for the ladder 5d 3 2 and 5d 5 2 upper states. Fluorescence detection of the 420 nm that results from the second step in the cascade decay → → ′ 5d 6p 5swas used in the ladder experiments. This fluorescence was also used for the detection of the → 5p 6p3 2 3 2 electric dipole forbidden transition in atomic rubidium that occurs at 911 nm. The 6p 3 2 hyperfine structure was resolved in this continuous wave, non-dipole excitation.