Molecular beam laser stark spectroscopy of highly vibrationally excited molecules
Date
2017-08-04
Authors
Stoer, Marcell
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Abstract
The Stark field perturbed spectra of near infrared vibrational
overtones of hydrogen fluoride and acetylene have been measured with a
high resolution molecular beam laser spectrometer. A high performance
laser power build-up cavity (optical resonator) was constructed to
measured the weak ro-vibrational transitions of the v₂ + 3v₃ vibrational
combination band of acetylene. The measured gain of the build-up
cavity was found to be at least 300 out of a potential 2000. The
primary reason for the lower than expected gain was attributed to
losses induced by the extreme heat build-up on the mirror surfaces.
The electric dipole moment for the v = 3 vibrational overtone of
hydrogen fluoride was determined to be 1.9614 ± 0.0021 Debye. This
result was compared with predictions from the available theoretical
models and some theoretical constants were revised based on the
current contribution to dipole moment function of hydrogen fluoride.
The Stark field perturbed spectra of the v₁ + 3v₃ and v₂ + 3v₃
vibrational combination bands of acetylene were analysed for their
polarisability tensors. In order to complete the study, the ground
electronic state static polarisability and anisotropy of the
polarisability were also determined. They were found to be 3.96A³ and
1.071 ± 0.014A³, respectively. The |1030⁰0⁰> state (v₁ + 3v₃) was
observed to be coupled with the |0040⁰0⁰> infrared forbidden state (4v₃)
in the presence of the Stark electric field. The resultant analysis
produced values of 4.62 ± 0.09A³ for the polarisability and 1.15 ±
0.03A³ for the polarisability anisotropy of the |1030⁰0⁰> state. The
difference in energy between |1030⁰0⁰> and |0040⁰0⁰> was determined to be
4.133 cm⁻¹, which compares well with local mode calculations.
The measurements of the v₂ + 3v₃ band indicated that the |0130⁰0⁰> state
was strongly coupled with another infrared allowed, unidentified
(rogue), state in the absence of the Stark field as well as with the
infrared forbidden, |1120⁰0⁰> state in the presence of the Stark field.
The previously unobserved J = 5 ← 4 transition of the infrared allowed
rogue state was recorded here for the first time. The Stark field
perturbed spectra of the R(3) and R(5) ro-vibrational transitions of
the v₂ + 3v₃ band also showed evidence of rogue transitions. The ensuing
analysis determined that the |0130°0°) state has a polarisability of
3.5 ± 0.3A³ and a polarisability anisotropy of 5.6 ± 1.8A³. The Stark
field perturbed spectra of the R(3) and R(5) transitions were fit to a
non-crossing model and the energy levels of the rogue J = 4 and J = 6
states were determined. The energy level difference between |0130°0°)
and |1120°0°) was determined to be -11.88±0.22 cm⁻¹. This does not
compare well with local mode calculations and it is possible that the
perturbations due to the presence of the rogue state impeded the
accurate determination of the energy level difference. The identity of
the rogue vibrational state could not be determined from the data
presented in this thesis alone. However, collaborative work with
another research group suggests that the rogue vibrational state is |0306°3¹) (see Chapter 7).
Description
Keywords
Molecular spectroscopy, Laser beams