Characterization of metal-carbon containing species using a mass-selective multiphoton ionization technique

This thesis describes the experimental measurements and spectral analysis of large
polyacetylenic chains and metal-capped carbon chains. The experimental method
employed is a resonance enhanced multiphoton ionization technique (REMPI).
The transient species were produced in a discharge or laser vaporization source.
The main advantage of the REMPI technique is its mass selectivity making the
assignment of the spectral carriers unambiguous.
The 1§+
u à X1§+
g electronic transitions of the HC2nH (n=5-7) chains have
been measured for the �rst time in the gas phase. The origin band shifts to the red
with the increase in the chain size. The spectrum is dominated by a progression
in the acetylenic stretching mode. It is the only progression and therefore the
only signature of a change in geometry for all these chains. The ground state is
characterized by a bond alternation between the single and triple bonds. Bond
lengths in the excited state have not been calculated. The trend can however be
derived from the vibrational pattern of the electronic spectra. Because the C´C
bond length increases in the excited state, the single bonds must be shorter. Thus
bond alternation in the excited state is less pronounced and the ¼¤ orbital is more
delocalized. The individual vibronic bands can not be rotationally resolved due to
the lifetime broadening. Internal conversion is responsible for the subpicosecond
lifetime of the 1§+
u excited state.
In order to understand the e�ect of a metal atom attached to the carbon chains,
an appropriate system to study is the metal-capped acetylene. In this direction,
the electronic spectrum of the AlCCH molecule has been measured in the gas phase
for the �rst time. The complicated vibronic structure is due to the Renner-Teller
e�ect involving two vibrational modes. The origin band has been rotationally resolved and the linear structure of the molecule has been con�rmed. The Al ¡ C
bond length decreases in the excited state leading to the conclusion that the electronic transition is dominated by the promotion of a ¾-electron localized on
the Al atom to a ¼ orbital with bonding character.
For studying the chain size e�ect on these metal-capped chains, the increase
in the number of carbon atoms is important. Therefore, the properties of longer
metal-capped chains have been studied by measuring the electronic spectra of the
MgC2H, MgC4H and MgC6H molecules. It has been observed that the oscillator
strength of the allowed electronic transitions decreases with the increase in the
chain size while the dipole moment increases. Therefore, the longer the chain the
more di�cult it is to record their electronic spectrum and the easier to measure
their microwave spectrum. In comparison to the bare carbon chains, the nature
of the electronic transition in the metal-capped chains is di�erent. The latter
involves excitation of an electron localized on the metal atom in contrast to the
carbon chains where the delocalized ¼-electron is excited.
When the shortest metal-capped chain looses the other hydrogen atom the deviation
from the linear structure of the molecule is observed. Thus, the T-shaped
structure of the AlC2 molecule has been con�rmed by measuring its electronic
spectrum in the gas phase. Two electronic systems have been recorded and analyzed.
Changes in the excited state geometries relative to the ground state have
been noticed and the molecular parameters for both ground and excited states
have been determined. It is to be noted that the electronic transition shifts to
the visible region of the electromagnetic spectrum with the loss of the hydrogen
atom.
Another molecule of interest, based on its astrophysical relevance and also technological
applications is titanium dioxide, TiO2. The most adequate way to investigate
the structure of such a molecule is to measure its electronic spectrum in the gas-phase, free of any interactions with the surrounding host lattice. The
electronic spectrum of titanium dioxide has been measured for the �rst time in the
gas phase and reveals a complicated vibronic pattern. The preliminary analysis
indicates that few vibronic bands can be assigned to the excitation of totally symmetric
modes (º1 and º2). From the rotational analysis of two vibronic bands the
excited state geometry could be inferred, indicating a lengthening of the Ti-O bond
and a decrease in the O=Ti=O angle. A complete analysis of the vibronic spectrum
awaits dispersed �uorescence measurements of more of the vibronic bands in the lower frequency region of the spectrum and elaborated calculations to take
into account perturbations due to other electronic states.
The purpose of measuring the electronic spectra of these species is not only
the fundamental understanding of their electronic properties but also their astrophysical
relevance. The carbon chains have been proposed for a long time as
carriers of the di�use interstellar bands and have also been detected in di�erent
environments such as circumstellar shells of the carbon-rich stars. Metals and
metal-containing species have been observed in the same environments based on
their sub-millimeter/millimeter spectrum. As the species mentioned in this thesis
have been measured for the �rst time in the gas phase, the molecular parameters
deduced here provide the basis for the search of their millimeter spectra in the
laboratory leading to searches for their detection in space.