New
experimental and theoretical aspects of overtone spectroscopy
Lauri
Halonen
Laboratory of Physical Chemistry,
P.O. Box 55 (A. I. Virtasen aukio 1), FIN-00014 University of Helsinki,
FINLAND
The laser-induced dispersed fluorescence method has been
developed to investigate vibration-rotation states of gaseous polyatomic
molecules in the ground electronic state [1 - 3]. A specific molecular
rovibrational state in the wave number region 10 500 - 14 000 cm-1
is pumped by a high-resolution Titanium:Sapphire ring laser. The sample
cell with a parabolic mirror is placed inside the ring-laser cavity. The
resulting fluorescence from the sample is collected and then dispersed
by an interferometer. We have applied this technique to acetylene and water,
in which collision-induced spectra are observed. The strongest fluorescence
transitions correspond to the Delta(v) = 1 change in the hydrogen stretching
vibrational quantum number. In C2H2,
we have observed a number of symmetrical states ([40+]0, [30+]0, and [20+]1,
where the first two numbers refer to the CH oscillators and the last one
refers to the CC oscillator). These would not be accessible by one-photon
transitions from the ground vibrational state. Neither in acetylene nor
in water changes in nuclear spin states due to molecular collisions have
been detected. In water, we have verified the experimental fluorescence
intensity patterns using the discrete variable method with an exact kinetic
energy operator and an Eckart frame dipole surface.
Overtone Hamiltonians have been developed and applied
to molecular systems involving large amplitude motions [4 - 6]. Our general
strategy is to assume that both the kinetic and potential energy parameters
are functions of the large amplitude coordinates. In practice, these functional
dependencies can be determined by electronic structure calculations. We
have applied our methods particularly to methanol and ammonia, which include
a large amplitude torsional and inversion mode, respectively.
[1] P. Jungner and L. Halonen,
Laser-induced vibration-rotation fluorescence and infrared forbidden transitions
in acetylene, J. Chem. Phys. 107, 1680 - 1682 (1997).
[2] M. Saarinen, D. Permogorov,
and L. Halonen, Collision-induced vibration-rotation fluorescence spectra
and rovibrational symmetry changes in acetylene,
J. Chem. Phys. 110,
1424 - 1428 (1999).
[3] M. Nela, D. Permogorov,
A. Miani, and L. Halonen, Vibration-rotation fluorescence spectra of water
in the ground electronic state, J. Chem. Phys., accepted for publication.
[4] L. Halonen, Theoretical
study of overtone spectroscopy and dynamics of methanol,
J. Chem. Phys.
106, 7931 - 7945 (1997).
[5] V. Haenninen, M. Horn,
and L. Halonen, Torsional motion and vibrational overtone spectroscopy
of methanol,
J. Chem. Phys. 111, 3018 - 3026 (1999).
[6] A. Miani, V. Haenninen,
M. Horn, and L. Halonen, Anharmonic force field of methanol, Mol. Phys.,
accepted for publication.