Two-step optogalvanic spectroscopy of neutral barium: Observations and interpretation of the even levels below the6sionization limit withJ=1, 3, 4, and 5

Even-parity levels of neutral barium located in the vicinity of the first ionization limit

6 s

have been investigated by using a two-step pulsed-laser excitation with optogalvanic detection. From the metastable states

5 d 6 s

populated by a dc discharge in a heat-pipe vapor, we reach the

6 s n l

even-Rydberg series with

J

equal to 0 to 5 and all the levels of the

5 d 6 d

and

5 d 7 d

configurations via the intermediate

5 d 6 p

levels. The

6 s n s {}^{3}S_{1}

(

13 \leq n \leq 47

) and

6 s n d {}^{3}D_{1}

(

12 \leq n \leq 38

) series are observed for the first time with five perturbers assigned to

5 d 8 s {}^{3}D_{1}

5 d 7 d {}^{3}D_{1}

{}^{3}S_{1}

{}^{1}P_{1}

, and

{}^{3}P_{1}

. The

6 s n d {}^{3}D_{3}

(

11 \leq n \leq 24

) series is perturbed by five levels

5 d 8 s {}^{3}D_{3}

5 d 7 d {}^{3}G_{3}

{}^{1}F_{3}

{}^{3}D_{3}

, and

{}^{3}F_{3}

. The fine structure of the

6 d n g {}^{1, 3}G

series is partially resolved in the vicinity of the perturbing

5 d 7 d J = 3, 4, and 5

levels. Semiempirical calculations using Racah's method have allowed us to understand the observed energies of the levels belonging to

5 d 6 d

and

5 d 7 d

configurations and to remove the ambiguity which was subsisting in the literature for the energy position of the

6 p^{2} {}^{1}S_{0}

level.

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