An ab initio Study of Electronic States and Transitions of Tellurium Monohalides
Rai
Vidisha
Rai, Vidisha
aut
2003-02-20
2002-11-20
2018-01-22
en
Ab initio multireference configuration interaction calculations including spin-orbit coupling are carried out for the first time for valence electronic states of the TeX (X = F, Cl, Br, I) radicals. The calculated spectroscopic constants are in good agreement with experimental data in the rare cases when the latter are available. It is shown that the <i>X</i><sup>2</sup>Π(σ<sup>2</sup>Π<sup>4</sup>Π<sup>*3</sup>) ground state bonding becomes consistently weaker down the TeX group (calc. <i>D<sub>e</sub></i> = 25480<i>cm</i><sup>-1</sup> for TeF, 12100r<i>cm</i><sup>-1</sup> for TeI) due to the more covalent character of bonding in the heavier radicals. The first excited state, <i>A</i><sup>4</sup>σ<sup>-</sup>(Π<sup>*</sup> → σ<sup>*</sup>), is calculated to be bound in all systems. It is split into Ω 1/2 and 3/2 components, with regular ordering in the Franck-Condon region, opposite to that of the ground <i>X</i><sup>2</sup>Π state. At larger internuclear distances, the <i>A</i><sub>1</sub> <sup>4</sup>σ<sup>-</sup><sub>1/2</sub> state undergoes an avoided crossing with <i>X</i><sub>2</sub><sup>2</sup>Π<sub>1/2</sub>, which causes a shoulder in the <i>X</i><sub>2</sub> potential curve and also leads to a crossing between the <i>A</i><sub>1</sub> and <i>A</i><sub>2</sub> potential curves and large distinctions in their vibrational frequencies. The Π<sup>*</sup> → σ<sup>*</sup> <i>B</i><sup>2</sup>σ<sup>-</sup>, <i>C</i><sup>2</sup>Δ, and 1<sup>2</sup>σ<sup>+</sup> states are calculated to lie next in energy. They are all bound in the lightest of the TeX radicals, TeF, but successively lose their bonding character down the group. In contrast to oxygen monohalides, the 2<sup>2</sup>Π(σ<sup>2</sup>Π<sup>3</sup>Π<sup>*4</sup>) state has a repulsive potential curve. The <i>A</i><sub>1,2</sub><sup>4</sup>σ<sup>-</sup><sub>1/2,3/2</sub> and <i>B</i><sup>2</sup>σ<sup>-</sup><sub>1/2</sub> states of TeX (X = Cl, Br, I) still await their experimental observation.
<br><br>For all the monohalides of tellurium, the <i>X</i><sub>1</sub> - <i>X</i><sub>2</sub> fine structure transition is quite weak. The radiative lifetimes for transitions from the various excited states to the X<sub>1,2</sub> states have been calculated in all cases. Most of them are found to be quite weak. The strongest transition in TeF is <i>B</i><sup>2</sup>σ<sup>-</sup> → X<sub>1</sub> <sup>2</sup>Π<sub>3/2</sub> with a lifetime of 9.5 μs. The calculations indicate that the <i>C</i><sub>1,2</sub> - <i>X</i><sub>1,2</sub> are only slightly weaker than the <i>B</i> - <i>X</i><sub>1,2</sub> transitions and should be spectroscopically observable. The parallel transitions among the low-energy states are found to be quite weak. The radiative lifetimes for transitions from the various excited states to the <i>X</i><sub>1,2</sub> states in TeCl have been calculated and it is observed that as opposed to TeF the parallel transitions are comparatively more dominant than the perpendicular ones in TeCl. The strongest transition is <i>A</i><sub>2</sub><sup>4</sup>σ<sup>-</sup><sub>3/2</sub> → <i>X</i><sub>1</sub><sup>2</sup>Π<sub>3/2</sub> with a lifetime of 29.4 μs. The <i>B</i> → <i>X</i><sub>1,2</sub> transitions behave different from that in TeF due to the fact that in TeF the SO mixing of these two states occurs much closer to the dissociation limit. Hence it does not influence the transitions so much as in TeCl where this mixing starts closer to the equilibrium distance of the B state. The radiative lifetimes calculated for TeBr are comparatively longer than in TeF and again the dominance of parallel transitions over the perpendicular is obvious. The strongest transition here is the <i>A</i><sub>2</sub><sup>4</sup>σ<sup>-</sup><sub>3/2</sub> → <i>X</i><sub>1</sub><sup>2</sup>Π<sub>3/2</sub>, as in TeCl, with a lifetime of 17.3μs. Of the allowed transitions, namely <i>B</i><sup>2</sup>σ<sup>-</sup>,<i>C</i><sup>2</sup>Δ → <i>X</i><sup>2</sup>Π, at the Λ-S level, only the <i>B</i> → <i>X</i><sub>1,2</sub> lead to discrete spectra. For TeI the <i>A</i><sub>1</sub> - <i>X</i><sub>2</sub> and <i>A</i><sub>2</sub> - <i>X</i><sub>1</sub> parallel transitions are of comparable strength as in TeCl and TeBr with radiative lifetimes of 302μs and 25.6μs, respectively. Since other states in TeI do not possess potential well deep enough to support any vibrational states, other transitions possible for the lighter monohalides should not be observed in this case.
urn:nbn:de:hbz:468-20030065
2018-01-22T10:48:40.857Z
2018-01-22T12:20:01.276Z
published
Diss
fb09/diss2002/rai