Vibrational branching rations and asymmetry parameters in the photoionization of [CO.sup.2] in the region between 650 a 840 A

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Date: Sept. 2001
Publisher: National Institute of Standards and Technology
Document Type: Article
Length: 7,173 words

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The vibrational branching ratios and asymmetry parameters for [CO.sub.2] have been determined in the wavelength region of 650 A to near the ionization onset at about 840 A. The study was performed using synchrotron radiation from the Daresbury storage ring that was dispersed with a 5 m grating monochomator that afforded resolution of 0.1 A to 0.2 A. This resolution allowed the study of the branching ratios and asymmetry parameters with enough detail to see the changes in the parameters within the pronounced autoionization structure in [CO.sub.2] in this wavelength region. While the electron spectrometer resolution was not sufficient to resolve the spin orbit and Renner-Teller splitting in the photoelectron spectra, we are able to fit the data with a model that identifies the major structure in terms of the symmetric stretch and elements of the asymmetric stretch and bending modes. A calculation of the expected relative vibrational excitations based upon the Frankck-Condon principle clearly showed non-Franek-Condon behavior in some of the vibrational-electronic transitions.

Key words: asymmetry; autoionization; branching ratios; parameters; photoelectron spectra.

Accepted: July 5, 2001

Available online: http;//www.nist.gov./jres

1. Introduction

The study of [Co.sub.2] photoionization has historically attracted much attention due to importance of the ionization of [Co.sup.2] in the photophysics of planetary atmospheres, including the Earth's atmosphere (1,2). Additionally, [Co.sub.2] is an integral part of the carbon cycle for plant life and, as a consequence, the photochemistry and photophysics of this molecule has been the subject of considerable interest by a number of scientists over the last several decades. The ready availability of high purity samples of [CO.sub.2] and its ease of use in various experimental arrangements have produced many experimental efforts studying its various properties (3-7).

The ionization onset region in the photoabsorption of [CO.sub.2] contains a number of strong autoionization features that converge on the A [[blank.sup.2][pi.sub.u] and [[blank].sup.2][Summation over(+/u)]. states seen in a photoionization mass spectrometric study by McCulloh (8) who tabulated them and gave the various series identifying labels which we will use in this work. Hubin-Franskin et al. (9) and Holland and Hayes (10) have used photoionization techniques coupled with synchrotron radiation sources to study the branching ratios or vibrational transitions in the wavelength region reported in this study (9,10). Baer and Guyon (11) used synchrotron radiation to study autoionization and isotope effects in the photoionization in the 620 A to 900 A wavelength region. These researchers used a threshold photoelectron spectrometer system and identified a number of vibrational transitions that are ordinarily weak or forbidden by selection rules.

Recently Buenker et al. (12) have performed calculations and reviewed the status of the studies on [CO.sub.2] electron energy loss spectroscopy. This technique can give results for cross sections and oscillator strengths that are similar to those obtained by photoionization and can be a direct test of calculational techniques. Takeshita et al. (13) recently calculated the ionization energies and the Franck-Condon factors for [CO.sub.2] photoionization. Johnson and Rostas (14) reviewed the spectroscopic literature for the vibronic structure of the ground state and first few...

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Gale Document Number: GALE|A83448415