EGAS41 - Swansea University

41 st EGAS CP 207 Gdańsk 2009
Perturbation in the A 1 Π (v = 1, J = 5) state of AlH
W. Szajna ∗ , M. Zachwieja, R. Hakalla, R. Kȩpa
Atomic and Molecular Physics Laboratory, Institute of Physics, University of Rzeszów,
35-959 Rzeszów, Poland
∗ Corresponding author: szajna@univ.rzeszow.pl
The high-resolution emission spectrum of the A 1 Π−X 1 Σ + transition of AlH was observed
in the 18000 - 25000 cm −1 spectral region using a conventional spectroscopic technique.
The AlH molecules were excited in an Al hollow-cathode lamp filled with a mixture of Ne
carried gas and a trace amount of NH 3 . The emission from the discharge was observed
with plane grating spectrograph and recorded by a photomultiplier tube. In total 163
transitions wavenumbers belonging to six bands (0 − 0, 1 and 1 − 0, 1, 2, 3) were precisely
measured and rotationally analyzed. The four off-diagonal (0−1, 1−0, 1−2, 1−3) bands
have been recorded for the first time since 1954 [1].
So far not pointed out [2] a very slight rotational perturbation has been discovered
in the v = 1 (J = 5) vibrational level of the excited A 1 Π state. This perturbation
affects all branches of the 1 −v ′′ progression bands: R(4), Q(5), and P(6) respectively. It
confirms that both e and f components of the A 1 Π state have been perturbed (see Figure
1) and indicates that the perturbing state is not X 1 Σ + . Shift of the f components:
∆E F1f = 0.0228 cm −1 is about four times bigger than e one: ∆E F1e = −0.0051 cm −1 .
Also, the fact that the perturbation appeared to be sharp, suggests that the perturbing
state has rotational constants quite different from those of the perturbed A 1 Π state. On
basis of above mentioned conclusions, we suggest that the a 3 Π state (B e = 6.704 cm −1 ),
which lies about 9000 cm −1 lower is responsible for perturbation in the A 1 Π(v = 1) state.
The exact assignment of the perturbing vibrational level was impossible, due to the lack
of information about vibrational constants of the a 3 Π state (only uncertain ω e = 1688
cm −1 value is known [3]). In order to get more precise estimation of the perturbation
Figure 1: Calculated terms energy shifts of A 1 Π,v = 1 (△ for f component, ○ for e component).
The perturbation at J = 5 is clearly shown.
value, we calculated true (perturbed) values of the rotational terms for the A 1 Π(v = 1)
state, by means of the method introduced by Curl and Dane [4] and Watson [5]. The
obtained terms values have been compared with those derived from molecular constants
for upper (v ′ = 1) and lower (v ′′ = 0 − 3) vibrational levels obtained by using traditional
hamiltonian method. The terms differences are plotted in Figure 1.
References
[1] P.B. Zeeman, G.J. Ritter, Can. J. Phys. 32, 555 (1954)
[2] R.S. Ram, P.F. Bernath, Appl. Opt. 35, 2879 (1996)
[3] G. Herzberg, Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules
(Van Nostrand, Princeton 1950)
[4] R.F. Curl, C.B. Dane, J. Mol. Spectrosc. 128, 406 (1988)
[5] J.K.G. Watson, J. Mol. Spectrosc. 138, 302 (1989)
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