Perturbations i have Known and Loved

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Mini-Symposium: Spectroscopic Perturbations

Scientific paper

A spectroscopic perturbation is a disruption
of a ^1Σ-^1Σ-like
regular pattern
that can embody level-shifts,
extra lines,
and intensity anomalies. Once upon a time,
when a band was labeled ``perturbed,''
it was considered worthless
because it could at best yield molecular constants
unsuited for archival tables. Nevertheless,
a few brave spectroscopists,
notably Albin Lagerqvist and Richard Barrow,
collected perturbations because they knew
that the pattern of multiple perturbations
formed an intricate puzzle
that would eventually reveal the presence
and electronic symmetry
of otherwise unobservable electronic states. There are many kinds of patterns of broken patterns. In my PhD thesis I showed how to determine absolute vibrational assignments
for the perturber from
patterns among the observed values of perturbation matrix elements. When a ^3Π state is perturbed,
its six (Ω, parity) components
capture a
pattern of level shifts and intensity anomalies
that reveals more about the nature of the perturber
than a simple perturbation of the single component of a ^1Σ state. In perturbation-facilitated OODR,
a perturbed singlet level acts as a spectroscopic doorway
through which the entire triplet manifold
may be systematically explored. For polyatomic molecule vibrations,
a vibrational polyad
(a group of mutually perturbing vibrational levels,
among which the perturbation matrix elements
are expected to follow harmonic oscillator scaling rules)
can contain more components than a ^3Π state
and
intrapolyad patterns can be exquisitely sensitive
not merely to the nature of an interloper within the polyad
but also to the eigenvector character of the vibronic state
from which the polyad is viewed. Variation of scaled polyad interaction parameters
from one polyad to the next,
a pattern of patterns,
can signal proximity to an isomerization barrier. Everything in Rydberg-land seems to scale as N⋆-3,
yet a trespassing valence state causes all scaling
and propensity rules go out the window.

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