The new investigation of the b3Σ--a3Π system of AlH

The b(3)Sigma(-)-a(3)Pi visible system of AlH was observed at high resolution by using a high accuracy, dispersive optical spectroscopy technique. The emission spectrum was excited in an aluminum hollow-cathode lamp with two anodes, filled with a static Ne/NH3 gas mixture. In the 25, 900-26, 500 cm(-1) spectral region, the rotational structure of the two overlapped 0-0 and 1-1 bands was clearly observed and precisely measured. In total, 260 transition wavenumbers have been assigned with an estimated accuracy of about 0.005 cm(-1). The open rotational structure of the Q branches in both bands has been measured for the first time, and the A-doubling in the a(3)Pi,v = 0,1 levels has been described by o,p and q parameters. For example, the values for the v = 0 level are P-0 = 1.754(14) x 10(-2) cm(-1), q(0) = 3.264(28) x 10(-3) cm(-1) and o(0) = 9.34(20) x 10(-2) cm(-1). Moreover, the spin-orbit interaction constants for the a(3)Pi state have been obtained experimentally as follows: A(0) = 40.6040(42) cm(-1) and A(1) = 40.419(59) cm(-1). The a(3)Pi, v = 0,1 levels are considered as regular, while for the b(3)Sigma(-), v = 0,1 levels considerable perturbations in the rotational structure have been observed. Consequently, these states have been represented in a different way in our least-squares treatment: a(3)Pi state by molecular constants and b(3)Sigma(-) state by term values. The observed irregularities in the b(3)Sigma(-) state have been graphically described by plotting experimental minus calculated term values versus quantum number N for the v = 0 level, as well as by plotting reduced term values versus N(N + 1) for the v = 1 level. (C) 2016 Elsevier Ltd. All rights reserved.