Highly Excited Triplet States of Carbon Monoxide
1. Introduction
Carbon monoxide is the second most abundant molecule in interstellar space
and therefore of key interest in astrophysics. In particular, the rate and
mechanism of photodissociation by vacuum ultraviolet (VUV) radiation have
been the subject of intensive spectroscopic investigations over the past
years. It is now a wellknon fact that CO is photodissociated by absorption
of VUV photons on discrete lines. The excited (Rydberg) states are mostly
predissociated through the interaction with continuum states.
While considerable data has been gathered on highly excited singlet states,
very little was known about their triplet counterparts. The reason, of
course, is that conventional absorption spectroscopy is of limited value for
the study of triplet states since the transitions from the
X 1Sigma+ ground state are spinforbidden.
2. Experiment
A multistep laser excitation is one way of getting access to the
highly excited triplet states. Several triplet valence
states which are perturbed by the A 1Pi state can be populated with the
tunable, coherent VUV source
of our laboratory. Through a second (and possibly third) spinallowed
transition at visible wavelengths, levels up to the ionization limit
at 113 027 cm^{1} can be investigated at a resolution
of 0.03 cm^{1}.
The multistep excitation scheme
The experimental setup
3. Results
3.1 New states near the dissociation threshold
Two new states were found near the dissociation threshold: the
j 3Sigma+ Rydberg state and the k 3Pi valence state (the latter was
simultaneously discovered by a group at Meudon/France).
Click here for publication info.
Rotational constants (in cm^{1)
}
 j 3Sigma+ (v=0)  k 3Pi


electronic and vibrational energy T  90833.233(32)  91959.354(24)

rotational constant B  1.9381(12)  1.2805(16)

spinorbit interaction A   29.953(14)

spinspin interaction lambda  0.077(32)  0.022(11)

spinrotation interaction gamma   0.091(21)

lambda doubling p   0.0

lambda doubling q   0.0

RKR potential curves of CO near the dissociation threshold
3.2 Triplet Rydberg states (p, d and f series, n>=4)
Selected highlights:
 Rotational parameters obtained for the following states:
5p\sigma...8p\sigma, 4d\sigma...9d\sigma, 4d\delta...9d\delta, 4f...7f
 Interaction witht the k 3Pi states results in strong, Jdependent
predissociation of the p\sigma levels (linewidth > 2 cm^{1})
 Strongly broadened f levels (linewidth <= 10 cm^{1}) with
anomalous sequence of rotational levels
 Multichannel quantum defect theory gives unified description
of s, p and d levels in the range n=4...9
Publication on the np series
Rydberg levels of carbon monoxide
© 1995
C.R.Vidal; Email:
crv@mpegarching.mpg.de
Last change: 28 June 1997