Rydberg and ion-pair state mixing in the 1 1Σ+u and 2 3Πu vibronic states of Cl2 as studied by vacuum ultraviolet laser spectroscopy

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Abstract
Fluorescence excitation and resonantly enhanced photoionization spectra of jet‐cooled Cl2 in the 71 000–77 500 cm−1 region are measured using a tunable vacuum ultraviolet light generated by four‐wave difference frequency mixing of two lasers in Kr. For the 1 1Σ+u state, vibrational term values for 35Cl2, 35Cl37Cl, and 37Cl2 and rotational constants for 35Cl2 and 35Cl37Cl are determined with high precision by the measurement of the 1 1Σ+u –X 1Σ+g transitions. It is shown clearly that the v’=39 vibrational wave functions of the 1 1Σ+u state of 35Cl2 and 35Cl37Cl are localized in the Rydberg well while the vibrational wave functions for the v’v’>39 levels are between the inner Rydberg wall and the outer ion‐pair wall. A double minimum potential composed of the Rydberg and the ion‐pair states explains well the observed vibrational and rotational structures. The molecular constants for the Rydberg well of 2 3Π(0+u) determined from the rovibrational structures of the 2 3Π(0+u)–X 1Σ+g transitions are ωe=647.3(33) cm−1, ωexe=3.37(59) cm−1 and re=1.833(20) Å, respectively.