Highly Excited Triplet States of Carbon Monoxide

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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 well-knon 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 spin-forbidden.

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) spin-allowed 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 multi-step 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).
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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)

spin-orbit interaction A 29.953(14)

spin-spin interaction lambda -0.077(32) 0.022(11)

spin-rotation interaction gamma -0.091(21)

lambda doubling p 0.0

lambda doubling q 0.0

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)
spin-orbit interaction A		29.953(14)
spin-spin interaction lambda	-0.077(32)	0.022(11)
spin-rotation 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, J-dependent 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

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Last change: 28 June 1997