Spin-Forbidden Transitions in the Vicinity of the 2 ¹Π_u ← X ¹Σ_g⁺ Band System of Rb₂

Abstract

We have investigated the Rb2 475 nm system by resonance enhanced two-photon ionization spectroscopy in a pulsed molecular beam. Strong extra bands accompanying the 2 (1)Pi(u) v' = 5 - 8 <-- X (1)Sigma(g)(+) v'' = 0 bands were newly observed. Rotational analysis of the main and extra bands reveals that the 2 (1)Pi(u) v' = 5 - 8 levels are significantly perturbed, mainly by the 3 (3)Sigma(u)(+)(1 u) state and also by the 2 (3)Pi(u)(1 u) state. For the major perturber, 3 (3)Sigma(u)(+)(1 u), the intensity borrowing has been found to be facilitated by the 2 (1)Pi(u)-3 (3)Sigma(u)(+)(1 u) potential energy curve crossing near 21,100 cm(-1). For the vibronic-band intensities of the 2 (3)Pi(u)(1 u) v' <-- X (1)Sigma(g)(+) v'' = 0 transitions observed in this spectral region, intensity borrowing was most effective when the 2 (3)Pi(u)(1 u) levels were close to the 3 (3)Sigma(u)(+)(1 u) levels. A deperturbation fit for the perturbing bands has provided the 2 (1)Pi(u)-3 (3)Sigma(u)(+)(1 u) coupling constants.

Theoretical study of spin–orbit and Coriolis coupling among the low-lying states of Rb2 and Cs2

“…The SO splitting of the Rb 2 2 3 Π u state (figure 4b) is calculated to be about 3 cm −1 near the equilibrium distance, which is somewhat larger than 1.5-2.6 cm −1 experimentally determined for the lowest vibrational levels from the analysis of the spin-forbidden 2 3 Π Ωu=0,1 − X 1 Σ + g band transitions [40]. The present 2 1 Π u -3 3 Σ + u SO functions (see figure 4b) estimated near the crossing point of the interacted states is about an order less than its empirical counterpart [42] ξ so = 118 ± 60 cm −1 obtained by deperturbation analysis of the locally perturbed 2 1 Π u ←X 1 Σ + g bands. This large discrepancy could be attributed to high sensitivity of the empirical vibronic matrix elements ξ so values to the small Franck-Condon factors used [42].…”

“…The present 2 1 Π u -3 3 Σ + u SO functions (see figure 4b) estimated near the crossing point of the interacted states is about an order less than its empirical counterpart [42] ξ so = 118 ± 60 cm −1 obtained by deperturbation analysis of the locally perturbed 2 1 Π u ←X 1 Σ + g bands. This large discrepancy could be attributed to high sensitivity of the empirical vibronic matrix elements ξ so values to the small Franck-Condon factors used [42].…”

“…The solid symbols -present work, open symbols -empirical and semi-empirical data borrowed from Refs [9,40,42]…”

Unravelling Complex Spectra of a Simple Molecule: REMPI Study of the 420 nm Band System of KRb

“…Alkali metal diatomic molecules consist of two alkali group elements and have only two valence electrons. In spite of their simple structures, their high density of electronic states in the high energy region, [1][2][3][4][5][6][7][8][9][10][11][12] their complex interactions, [13][14][15][16][17][18][19][20][21][22][23][24] and contributions from several isotopomers of alkali dimers make their excitation spectra extremely complex, and the interpretation of the spectra becomes quite challenging. Fortunately, the small numbers of nuclei and valence electrons enable accurate prediction of electronic structures of alkali dimers by ab initio calculations.…”

“…Mass-resolved resonance-enhanced multi-photon ionization (REMPI) in a very cold molecular beam has greatly simplified the complex excitation spectra of alkali dimers overlapped by contributions from several isotopomers by extensive cooling in a pulsed molecular beam and separating the contributions from individual isotopomers. [13][14][15][16][17][18] The vibrational quantum numbers (v') of excited levels can be conveniently and accurately assigned by spectra from different isotopomers. [25] In addition, most of the excited states of alkali dimers can be reached by visible light and absorbing another photon with the same wavelength easily ionizes the alkali molecules, enabling the mass-separated detection.…”