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<jats:p>A skeptical physicist charges that his field has been wandering in a philosophical wilderness for 80 years. The good news: He thinks he knows the way out.</jats:p>

<jats:p> Chakraborty <jats:italic>et al</jats:italic> . (Reports, 5 September 2008, p. 1328) concluded that an anomalously enriched atomic oxygen reservoir can be generated through carbon monoxide photodissociation without self-shielding. We show that this conclusion is based on the incorrect assumption that the spectral shifts of the 97.03-nanometers and 107.61-nanometers vibrational bands for C <jats:sup>16</jats:sup> O, C <jats:sup>17</jats:sup> O, and C <jats:sup>18</jats:sup> O are negligible and point out shortcomings of the low-resolution light source used in their experiments. </jats:p>

<jats:p> Chakraborty <jats:italic>et al</jats:italic> . (Reports, 5 September 2008, p. 1328) demonstrated very large, wavelength-dependent mass-independent isotopic effects during carbon monoxide (CO) photodissociation and argued that self-shielding in CO was not responsible. We suggest that variations in band oscillator strengths and linewidths among CO isotopologs are responsible for most of the wavelength dependence observed and that the reported experiments confirm the importance of self-shielding during CO photodissociation. </jats:p>

<jats:p> Chakraborty <jats:italic>et al</jats:italic> . (Reports, 5 September 2008, p. 1328) suggested that experimental results provide support for CO photodissociation having caused the oxygen isotope ratio associated with the early solar nebula. We point out that further analysis is required before other mechanisms, such as self-shielding, are shown to be of little importance. </jats:p>

<jats:p> We address the comments by Lyons <jats:italic>et al</jats:italic> ., Federman and Young, and Yin <jats:italic>et al</jats:italic> . regarding the interpretation of our carbon monoxide photodissociation experiments and provide further experimental data analysis, including measured synchrotron beam profiles. The experimental data do not support existing self-shielding models that attempt to explain observed meteoritic oxygen isotopic compositions because they rely on previously untested theoretical assumptions. </jats:p>