Photoionization of CO2 - Introduction

The study of CO₂ photoionization has historically attracted much attention due to the importance of the ionization of CO₂ in the photophysics of planetary atmospheres, including the Earth's atmosphere [1,2]. Additionally, CO₂ 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 CO₂ 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 CO₂ contains a number of strong autoionization features that converge on the A ²Π_u and B ²Σ_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 Å to 900 Å 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 CO₂ 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 CO₂ photoionization. Johnson and Rostas [14] reviewed the spectroscopic literature for the vibronic structure of the ground state and first few excited electronic states of CO₂. The spectroscopic results given by these authors were most useful in developing a method of analyzing the data reported in this work.

In a previous publication [15], we gave results for the branching ratios and asymmetry parameters for CO₂ in the wavelength region of 685 Å to 795 Å for some of the autoionizing lines in the Tanaka-Ogawa (TO) series, the Henning sharp (s) and diffuse (d) series. However, we did not publish the extensive additional data we had obtained as a result of analyzing the photoelectron spectra taken in this wavelength region. The results reported here are a complete analysis of all our data, providing extensive information on the photoionization behavior of CO₂ in this wavelength region. Another publication from our collaboration used the angular distribution information to determine the electronic structure, symmetry, and decay dynamics for members of the TO series [16]. It was noted in these publications, as well as in preliminary work done in the early 1980's, that the strength of the vibrational progressions were not described by a simple Franck-Condon model of photoionization [17]. We also explored the effects of vibronic coupling on the intensity distribution in the 4σ_g photoionization channel of CO₂ [18]. Hubin-Franskin et al. studied CO₂ autoionization and pointed out that the use of the method worked out by Smith and modified by Eland to apply the Fano-Mies configuration interaction theory to molecules could be used to explain portions of the non-Franck-Condon behavior seen in the photoionization of CO₂ [19,23]. These papers pointed out the difficulty of completely accounting for the vibrational-electronic transition intensities due to configuration mixing of electronic states, vibronic and rotational interactions, and the competition between the many paths available for an excited state neutral molecule to decay. The Smith formalism requires a knowledge of the Franck-Condon factors for all the transitions involved, ground state to autoionizing level and autoionizing level to all the possible final states. While this calculation is possible with diatomic molecules, it is much more difficult with larger molecules with additional degrees of freedom. In the case of CO₂, the situation is even more complicated due to Renner-Teller splitting of the electronic levels when the bending mode is excited. As a result, there has been no systematic theoretical exploration of the effects of autoionization on the vibrational branching ratios and asymmetry parameters for CO₂.

In our previous publication concerning the branching ratios and asymmetry parameters in this wavelength region we presented only a small subset of the data we have on CO₂ in this wavelength region. We present here a detailed explanation of the data accumulation techniques and a complete discussion of the analysis of the data that was not possible in the shorter summary of this work [15]. To be consistent with the earlier NIST publications on this topic, we have used the unit of the ångström to describe the wavelength scale used for these experiments even though this unit is no longer sanctioned as an acceptable metric system unit.