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An experimental study of the two-photon spectra of high members of the Sr I 5snd 1,3 D 2 Rydberg series is described. We use transverse excitation of an atomic beam with a narrow-bandwidth tuneable dye laser within a heat pipe equipped for field ionization detection. The presence of weak collisions has revealed several anomalies. Strong spin-orbit interaction and the breakdown of parity and selection rules combined with 'l' mixing are observed. The breakdown is due to the Stark field present in the interaction region. Singlephoton transitions originating from the 5snp 1 P 1 and 5snp 3 P J states populated by electron impact excitations are also observed. They create highly compact doubly excited 4d 2 3 P J , 5p 2 1 D 2 and 4d5p 3 P J states, which act as dominant intruders in the observed spectrum. Such states with highly localized orbitals survive collision and field effects, whereas Rydberg states are highly sensitive to external perturbations. Also, we have observed l-changing collisional processes like Ar + Sr (5snd 1,3 D 2 ) → Sr (5snl, l 3). Several members of a new series which begin to appear at large n are observed and are tentatively designated as the 5snf 1 F 3 Rydberg series. Furthermore, we have observed and identified several members of the Sr I 5snp 1 P 1 and 5snp 3 P 2 Rydberg series by twophoton absorption from the ground state. They are excited via the breakdown of parity and other selection rules, owing to the presence of external fields. New values of energy levels of the Sr I 5snd 1,3 D 2 Rydberg series are presented: n = 25-73 for singlet and n = 25-47 for triplet members, obtained from twophoton spectra. Comparison of these values with those given by other authors who used different excitation schemes shows good agreement. The spectrum of fluorescence emission from a low-pressure discharge around the field ionization probe was obtained using a monochromator-photomultiplier detection system. The emission lines confirm the collisional excitations observed in the spectra of the Rydberg series. An energy level diagram summarizing the various transitions observed in our study is also presented.

A conventional heat pipe is modified to operate as a novel atomic jet setup with unprecedented advantages for multiphoton spectroscopy of alkaline-earth atoms especially to study the effect of external electric field, controlled excitation of forbidden transitions and for the study of collisional broadening and shift of excited states, and time evolution of Rydberg atoms.

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