The B1Πu–X1Σg+ band system of the 7Li2 molecule

Abstract

The absorption and laser induced fluorescence spectra of the B¹Π_u–X¹Σ_g⁺ band system of the ⁷Li₂ molecule has been photographed at high resolution (360 000) and high dispersion (0.4 Å/mm). Over 14 000 spectral lines have been assigned to this band system for a wide range of rotational quantum numbers (J=0−80) and vibrational quantum numbers (v′=0−13 and v^″=0−18). Using a Dunham type analysis, we have obtained the molecular constants for both states and have fitted 8358 nonoverlapping lines with an rms error of 0.012 cm⁻¹. The B¹Π_u lambda doubling constants, including the centrifugal distortion lambda doublet constant, have been determined. Quantum mechanical potential curves have been generated for this system using a novel variational procedure and Franck–Condon factors for different rotational quantum numbers have been determined over the range of vibrational levels observed. Excellent agreement has been achieved for the centrifugal distortion terms D_v, H_v, and L_V which were obtained from the least squares fit of the assigned lines and those calculated from the quantum mechanical rotationless potential. Levels above the 2 ²P+2 ²S_1/2 dissociation limit have been observed and a maximum for the barrier in the B ¹Π_u potential of 450±50 cm⁻¹ above this atomic level was found. The dissociation energy, D_e′ of the X ¹Σ_g state was found to be 8600±150 cm⁻¹. A review of the literature on the Li₂ molecule through 1977 is given.